2623 — Medical Vocabulary
7 questions — Aligned to EAP objectives — Martini Chapter 7
Q1MC
The root "sarco-" means flesh. Which term correctly uses this root to name a structure of a muscle fiber?
Sarcolemma — the plasma membrane of a muscle fiber
Myosin — the thick protein filament of a sarcomere
Endomysium — the deepest connective tissue wrapping
Perimysium — the connective tissue surrounding a fascicle
Q2MC
The prefix "epi-" means on or over. Based on this, the epimysium is the connective tissue layer that:
Surrounds individual muscle fibers within a fascicle
Binds groups of muscle fibers into a fascicle
Wraps the entire skeletal muscle on the outside
Attaches muscle to bone at both ends
Q3MC
The root "meros" means part. This root appears in which muscle anatomy term?
Myosin — the thick filament protein
Perimysium — the fascicle connective tissue wrap
Sarcomere — the repeating contractile unit of a myofibril
Isometric — constant-length contraction
Q4FITB
A bundle of muscle fibers wrapped together by the perimysium is called a ___.
Q5MC
The root "tetanos" means stiff or rigid. In a physiological context, tetanus refers to:
A single brief contraction-relaxation cycle in response to one action potential
Progressive increase in twitch force from repeated identical stimuli
A sustained maximal contraction from very rapid repeated stimulation
The resting state of a muscle maintaining baseline tone
Q6MC
The root "-trophy" means nourishment or development. Which statement correctly applies this root?
Atrophy means increased muscle fiber size from resistance training
Hypertrophy means above-normal development — individual fibers enlarge
Dystrophy means normal development in a well-trained athlete
Tetanus means sustained muscle stiffness from rapid stimulation
Q7MC
The roots "syn-" (together) and "ergon" (work) combine to form synergist. A synergist muscle is one that:
Directly opposes the prime mover to limit range of motion
Assists the prime mover and reduces unnecessary movement
Converts chemical energy to mechanical energy at the molecular level
Serves as the primary muscle generating force for a movement
2624 — Functions of Skeletal Muscle
7 questions — Aligned to EAP objectives — Martini Chapter 7
Q8MC
The human body contains approximately how many skeletal muscles?
About 100
About 350
About 700
About 1,500
Q9MC
The abdominal wall and pelvic floor muscles prevent visceral organs from dropping downward. This represents which function of skeletal muscle?
Maintaining posture and body position
Guarding body entrances and exits
Supporting soft tissues
Maintaining body temperature
Q10MC
Skeletal muscle contributes to temperature maintenance primarily by:
Absorbing heat from blood and releasing it through the skin
Generating heat as a byproduct of metabolic reactions, both at rest and during activity
Contracting only during extreme cold to directly warm circulating blood
Directing blood flow from the core to the skin for heat dissipation
Q11MC
Because muscles can only pull (shorten) and cannot push, movement in opposite directions at a joint requires:
Tendon recoil after muscle relaxation
Antagonistic muscle pairs that pull in opposite directions
A single large muscle with two distinct attachment points
Cartilaginous springs at joint surfaces
Q12SATA
Select ALL five major functions of skeletal muscle as listed in the Martini textbook.
Q13MC
The external urethral and anal sphincters are skeletal muscles that guard body openings. Their voluntary control is provided by:
The autonomic nervous system acting on smooth muscle sphincters
Cardiac pacemaker signals coordinating sphincter timing with heart rate
The somatic nervous system — these are voluntary skeletal muscles
Hormonal signals that open and close the sphincters automatically
Q14MC
Each skeletal muscle functions as an organ. Which combination of tissues makes up a typical skeletal muscle organ?
Muscle tissue and nervous tissue only
Muscle tissue, connective tissue, blood vessels, and nerves
Muscle tissue and connective tissue only, with no direct nerve supply
Muscle tissue, cartilage, and bone at the tendon attachments
2625 — Organization of Skeletal Muscle
8 questions — Aligned to EAP objectives — Martini Chapter 7
Q15MC
The connective tissue layers of a skeletal muscle from outermost to innermost are:
Endomysium → perimysium → epimysium
Perimysium → epimysium → endomysium
Epimysium → perimysium → endomysium
Epimysium → endomysium → perimysium
Q16FITB
The outermost connective tissue layer that wraps an entire skeletal muscle and blends with its tendons is the ___.
Q17MC
Transverse tubules (T tubules) are:
Modified collagen fibers transmitting tension from myofibrils to tendons
Channels connecting terminal cisternae to the sarcoplasmic reticulum lumen
Extensions of the sarcolemma that conduct action potentials deep into the fiber interior
Protein bridges connecting thick and thin filaments in the zone of overlap
Q18MC
A triad is the functional unit for excitation-contraction coupling. It consists of:
Three myofibrils wrapped by a single loop of sarcoplasmic reticulum
One T tubule flanked on each side by a terminal cisterna
Three sarcomeres connected by a shared Z line
Two T tubules surrounding one terminal cisterna
Q19MC
The terminal cisternae of the sarcoplasmic reticulum store and release:
Glucose for immediate ATP synthesis during contraction
Pre-energized myosin heads ready to form cross-bridges
Calcium ions (Ca²⁺) that trigger contraction when released
ATP reserves that fuel the cross-bridge cycle directly
Q20MC
The sarcolemma of a muscle fiber is:
The cytoplasm of a muscle fiber containing organelles and myofibrils
The plasma membrane of a muscle fiber
The connective tissue wrapping around each individual muscle fiber
The specialized smooth ER network storing calcium ions
Q21MC
The sarcoplasm of a muscle fiber differs from typical cytoplasm in that it contains large amounts of:
Hemoglobin for oxygen transport to neighboring fibers
Glycogen granules and myoglobin in addition to normal cytoplasmic contents
Only water and dissolved proteins, lacking organelles
No mitochondria — skeletal muscle relies entirely on anaerobic energy
Q22MC
Satellite cells are stem cells found in the endomysium. Their primary function is to:
Produce the collagen fibers of the endomysium
Secrete hormones that regulate muscle hypertrophy systemically
Divide and differentiate to repair or replace damaged muscle fibers
Store glycogen reserves adjacent to myofibrils
2626 — Sarcomere Structure
8 questions — Aligned to EAP objectives — Martini Chapter 7
Q23MC
The A band of a sarcomere is defined by the length of the thick (myosin) filaments. During muscle contraction, the A band:
Narrows as thick filaments shorten during the power stroke
Disappears completely at maximal contraction
Remains constant in width because myosin filament length never changes
Widens as thin filaments are drawn deeper into the thick filament array
Q24MC
The I band (the thin-filament-only zone at each end of the sarcomere) during contraction:
Widens as thin filaments are pulled apart during the power stroke
Stays constant in width like the A band
Narrows as thin filaments slide toward the M line
Disappears immediately at the onset of any contraction
Q25FITB
The boundary structures that define each sarcomere — the protein discs where thin filaments are anchored — are the ___ lines.
Q26MC
In a resting muscle, tropomyosin's role is to:
Connect myosin heads to actin in preparation for contraction
Anchor thin filaments to the Z lines
Cover the active sites on actin, blocking myosin cross-bridge formation
Store calcium ions until an action potential triggers their release
Q27MC
When calcium ions (Ca²⁺) bind to troponin, the result is:
Immediate ATP hydrolysis to energize the myosin heads
Cross-bridge formation between myosin and the Z line proteins
A conformational change that moves tropomyosin away from the actin active sites
Calcium directly attaches to myosin heads, enabling them to bind actin
Q28MC
The H band is:
The thin-filament-only zone at each end of the sarcomere
The protein disc at the center of the A band that anchors thick filaments
The thick-filament-only zone in the center of the A band where no thin filaments overlap
The region where thick and thin filaments overlap to form cross-bridges
Q29MC
During muscle contraction, the zone of overlap between thick and thin filaments:
Decreases as thick filaments shorten
Stays constant because both bands narrow equally
Disappears in any normal contraction
Increases as thin filaments slide deeper into the thick filament array
Q30MC
Before contraction begins, myosin heads are in a "cocked" pre-energized state because they are already bound to:
Calcium ions from the terminal cisternae
Actin active sites at the zone of overlap
ADP and inorganic phosphate (Pi), having already hydrolyzed an ATP molecule
Troponin molecules along the thin filament
2627 — Skeletal Muscle Contraction
10 questions — Aligned to EAP objectives — Martini Chapter 7
Q31MC
Acetylcholine (ACh) is released from the axon terminal into the synaptic cleft by:
Active transport pumps in the axon terminal membrane
Passive diffusion down its concentration gradient
Exocytosis — synaptic vesicles fuse with the membrane and release their contents
Voltage-gated potassium channels opening when the action potential arrives
Q32MC
Acetylcholinesterase (AChE) breaks down ACh in the synaptic cleft after each stimulus. Without AChE:
No action potential would be generated in the muscle fiber
Calcium would not be released from the sarcoplasmic reticulum
The muscle would be continuously stimulated and unable to relax
The muscle fiber would fail to repolarize after its action potential
Q33MC
ACh binding to motor end plate receptors increases sodium permeability, causing Na⁺ to rush in. This generates:
Direct cross-bridge formation between myosin and actin
Immediate calcium release from the terminal cisternae
An action potential in the muscle fiber sarcolemma
The power stroke of myosin cross-bridges
Q34MC
When the action potential reaches the triads (T tubule + terminal cisternae), the immediate result is:
Cross-bridge formation between myosin heads and actin
ATP hydrolysis to energize the myosin heads
Massive release of Ca²⁺ from the terminal cisternae into the sarcoplasm
Troponin conformational change that moves tropomyosin away from actin
Q35MC
After Ca²⁺ exposes the active sites on actin, the pre-energized myosin head binds and then:
Hydrolyzes a new ATP molecule to begin the power stroke
Releases ADP + Pi, completing the power stroke and pulling the thin filament toward the M line
Immediately detaches from actin and waits for a new ATP
Binds calcium directly to confirm the active site is accessible
Q36FITB
The binding of a new ___ molecule to the myosin head causes cross-bridge detachment from actin.
Q37MC
Botulinum toxin causes flaccid paralysis because it:
Blocks ACh receptors on the motor end plate
Destroys acetylcholinesterase, causing continuous muscle stimulation
Prevents ACh release from the axon terminal into the synaptic cleft
Directly prevents calcium from binding to troponin
Q38MC
Myasthenia gravis is an autoimmune disease in which the immune system destroys:
Myosin heads of the thick filaments
Acetylcholinesterase enzymes in the synaptic cleft
ACh receptors on the motor end plate
Terminal cisternae of the sarcoplasmic reticulum
Q39MC
Rigor mortis occurs after death because:
Calcium floods all cells simultaneously, triggering a final maximal contraction
Acetylcholinesterase remains active and continuously stimulates all muscles
ATP production ceases, so myosin heads cannot detach from actin
Sarcomeres lock at maximum overlap as the cell membrane fails
Q40SATA
Select ALL steps that are part of initiating a skeletal muscle contraction (from ACh release to the power stroke).
2628 — Contraction Types & Motor Units
8 questions — Aligned to EAP objectives — Martini Chapter 7
Q41MC
A muscle twitch is:
The maximum force a motor unit generates over time
A sustained contraction from very rapid stimulation
A single contraction-relaxation cycle in response to one action potential
Progressive increase in twitch force from repeated identical stimuli
Q42MC
During the latent period of a muscle twitch, what is happening?
Cross-bridges are rapidly cycling but generating no net force
Calcium is being pumped back into the SR in preparation for relaxation
The action potential is spreading and Ca²⁺ is being released, but no tension has developed yet
Myosin heads are detaching from actin in preparation for the contraction phase
Q43MC
When a second stimulus arrives before the muscle fully relaxes from the first twitch, tensions add together. This is called:
Complete tetanus
Treppe
Wave summation
Motor unit recruitment
Q44MC
An isometric contraction differs from an isotonic contraction in that during an isometric contraction:
Concentric shortening occurs against a constant load
No cross-bridge activity takes place
Tension increases but muscle length does not change
The muscle requires less ATP per unit time
Q45MC
A concentric isotonic contraction is one in which:
The muscle lengthens under tension while resisting an external load
The muscle generates tension but neither shortens nor lengthens
The muscle shortens while maintaining relatively constant tension
The twitch is too brief to move the attached bone
Q46MC
Eccentric contractions occur when:
The muscle shortens while moving a load in the direction of contraction
Multiple motor units are activated simultaneously to increase force
The muscle lengthens while still generating tension to control movement
The muscle is at rest and generating only resting tone
Q47MC
Motor unit recruitment increases force production by:
Increasing the rate of stimulation to a single motor unit
Activating additional motor units — small ones first, then progressively larger ones
Increasing the calcium released from the SR per action potential
Lengthening individual muscle fibers to increase their contractile range
Q48MC
A motor unit consists of:
All the motor neurons innervating a single skeletal muscle
A single muscle fiber and all the sarcomeres within it
One motor neuron and all the muscle fibers it innervates
A group of slow and fast fibers that always contract together
2629 — Muscle Energy
10 questions — Aligned to EAP objectives — Martini Chapter 7
Q49MC
A resting muscle fiber contains only enough stored ATP for:
About 15 seconds of maximal contraction
About 30 seconds of moderate activity
A few seconds of maximal contraction
About 5 minutes of low-intensity activity
Q50MC
Creatine phosphate (CP) reserves can regenerate ATP for approximately how long during maximal activity?
5–10 seconds
About 15 seconds
About 2 minutes
About 15 minutes
Q51MC
At resting levels of activity, aerobic metabolism provides approximately what percentage of the muscle's ATP?
About 10%
About 40%
About 95%
About 60%
Q52MC
Each glucose molecule processed by anaerobic glycolysis alone yields a net of:
36–38 ATP molecules
About 100 ATP molecules
2 ATP molecules
4 ATP molecules gross, 2 ATP net
Q53MC
During intense exercise, anaerobic glycolysis must supplement aerobic metabolism because:
Aerobic metabolism shuts down completely during strenuous activity
Glycolysis produces more ATP per glucose molecule than aerobic respiration
Aerobic ATP production cannot increase fast enough to meet the full ATP demand of peak activity
Mitochondria are damaged by the heat generated during intense contractions
Q54FITB
The additional oxygen consumed after exercise ends — used to replenish CP, restore glycogen, and clear lactate — is called the ___ debt.
Q55MC
Muscle fatigue occurs primarily due to:
The motor neuron stopping firing despite adequate ATP in the muscle
Lactic acid directly destroying ATP molecules needed for cross-bridge cycling
Energy reserve depletion combined with pH drop from H⁺ accumulation, impairing contraction
Complete motor unit recruitment having been reached
Q56SATA
Select ALL energy sources that contribute to ATP production in skeletal muscle — from fastest but most limited to slowest but most sustained.
Q57MC
Resting skeletal muscle primarily uses which fuel source for aerobic ATP production?
Glucose from liver glycogen stores
Amino acids from protein breakdown
Fatty acids from circulating lipids
Lactate recycled from previous contractions
Q58MC
Lactic acid dissociates into lactate and H⁺ ions. The H⁺ contributes to muscle fatigue by:
Directly destroying ATP molecules needed for cross-bridge cycling
Preventing calcium release from the SR by blocking T tubule function
Lowering intracellular pH, inhibiting enzymes involved in contraction
Blocking ACh receptors at the motor end plate
2630 — Muscle Fiber Types
8 questions — Aligned to EAP objectives — Martini Chapter 7
Q59MC
Fast (type II) muscle fibers reach peak twitch tension in:
About 40–100 milliseconds
10 milliseconds or less
About 3–4 seconds
About 1 second
Q60MC
Compared to fast (type II) fibers, slow (type I) fibers are:
Larger in diameter and generate more force
Higher in glycolytic enzyme activity and fatigue faster
Smaller in diameter and more resistant to fatigue
Lacking mitochondria and reliant entirely on glycolysis
Q61MC
The high fatigue resistance of slow (type I) fibers is supported by:
High glycolytic enzyme activity for rapid ATP production
Large amounts of glycogen stored within each fiber
High myoglobin content, rich capillary supply, and abundant mitochondria
Large diameter and high myosin concentration
Q62MC
A soldier is running a 400-meter sprint at maximum effort. The contracting muscles are primarily utilizing:
Type I slow oxidative fibers
A 50/50 mix of fast and slow fibers
Type II fast glycolytic fibers
Cardiac muscle fibers recruited through the nervous system
Q63MC
Hypertrophy of skeletal muscle refers specifically to:
An increase in the number of muscle fibers from stem cell differentiation
Conversion of slow fibers to fast fibers from high-intensity training
An increase in the diameter of existing individual muscle fibers
Addition of entirely new muscle organ units
Q64FITB
The duration of contractions that can be sustained using only glycolysis and ATP/CP — without relying on aerobic metabolism — is called ___ endurance.
Q65MC
Aerobic endurance training (e.g., distance running) improves performance primarily by:
Increasing the diameter of fast fibers to generate more force
Converting fast fibers to slow fibers to resist fatigue
Improving cardiovascular delivery and aerobic metabolic capacity without significant hypertrophy
Increasing the number of motor units available for recruitment
Q66MC
An individual's ratio of slow to fast muscle fibers is:
Determined primarily by training — endurance athletes develop more slow fibers over time
Fixed at birth by genetic factors and does not change with training
Determined by diet — high-protein diets promote fast fiber development
Altered by aging — fast fibers gradually convert to slow fibers after age 40
2631 — Comparison of Muscle Tissue Types
8 questions — Aligned to EAP objectives — Martini Chapter 7
Q67MC
Cardiac muscle is found exclusively in:
The diaphragm and accessory breathing muscles
The walls of large arteries and veins
The walls of the heart (myocardium)
The sphincters at cardiac junctions of the GI tract
Q68MC
Intercalated discs are unique to cardiac muscle. The gap junctions within them:
Anchor the cell to the basement membrane for structural support
Allow calcium to be stored in compartments between adjacent cells
Enable electrical signals to pass directly between adjacent cardiac cells, synchronizing contraction
Prevent action potentials from spreading between cells for independent control
Q69MC
Cardiac muscle exhibits automaticity, meaning it:
Automatically converts to a faster fiber type under high cardiac demand
Can regulate its contraction speed based on local oxygen availability
Can generate action potentials and contract without any neural input
Automatically recruits additional muscle layers when load increases
Q70MC
Cardiac muscle cannot undergo tetanic contractions. This is physiologically essential because:
Tetanus would convert cardiac fibers to slow oxidative type
A tetanized heart cannot relax to fill with blood, making effective pumping impossible
Tetanus would exhaust all cardiac glycogen within minutes
Sustained contraction would prevent intercalated discs from opening
Q71MC
Smooth muscle differs from skeletal muscle in that smooth muscle:
Contains sarcomeres and shows cross-striations under a microscope
Uses calcium ions in its contraction mechanism
Lacks sarcomeres and striations — thick filaments are scattered irregularly throughout the cell
Cannot generate sustained force like skeletal muscle
Q72MC
Smooth muscle can contract over a much wider range of lengths than skeletal or cardiac muscle. This is functionally important because:
Smooth muscle must generate more peak force than skeletal muscle
Smooth muscle must contract faster than skeletal muscle
Smooth muscle lines hollow organs (bladder, stomach, uterus) that change volume dramatically
Smooth muscle is under voluntary control requiring a wide range of positions
Q73SATA
Select ALL features that are unique to cardiac muscle and NOT shared with skeletal or smooth muscle.
Q74MC
In smooth muscle, calcium ions that trigger contraction come primarily from:
The sarcoplasmic reticulum, identical to the mechanism in skeletal muscle
Mitochondria within the smooth muscle cell
The extracellular fluid, entering through the plasma membrane
Glycogen granules that release calcium as they are metabolized
2632 — Axial Muscles
10 questions — Aligned to EAP objectives — Martini Chapter 7
Q75MC
The masseter is a primary chewing muscle. Its origin is the:
Mandible
Temporal lines of the skull
Zygomatic arch
Styloid process of the temporal bone
Q76MC
When both sternocleidomastoid (SCM) muscles contract simultaneously, the result is:
Rotation of the head to the right
Extension of the neck, head tilting backward
Flexion of the neck — the chin moves toward the chest
Elevation of the sternum and clavicle
Q77MC
When only the right sternocleidomastoid muscle contracts, the result is:
Neck flexion with the chin moving directly forward
Extension and rotation of the head to the right
The head tilts toward the right shoulder and the face rotates to the left
Depression of the right clavicle
Q78MC
The external intercostal muscles' primary role during breathing is:
Depressing the ribs to reduce thoracic volume during expiration
Elevating the ribs to expand the thoracic cavity during inspiration
Stabilizing the ribs laterally during coughing
Rotating the trunk by pulling the rib cage toward the pelvis
Q79MC
The diaphragm is the primary muscle of quiet breathing. When it contracts, it:
Compresses the thoracic cavity to force air out
Elevates the ribs by pulling on the costal cartilages
Flattens and moves downward, increasing thoracic volume and drawing air in
Constricts the esophageal opening to prevent acid reflux
Q80MC
Unlike the oblique muscles, the transversus abdominis:
Flexes the vertebral column when both sides contract
Rotates the trunk when one side contracts
Compresses the abdominopelvic cavity without producing rotation
Elevates the ribs during deep inspiration
Q81FITB
The vertical midline band of dense connective tissue separating the left and right rectus abdominis muscles is called the ___ ___.
Q82MC
Bilateral contraction of the erector spinae group produces:
Lateral flexion of the vertebral column to the same side
Extension of the vertebral column and maintenance of upright posture
Trunk rotation toward the contracting side
Compression of the abdominopelvic cavity
Q83SATA
Select ALL muscles whose primary actions include compressing the abdominopelvic cavity.
Q84MC
The external urethral and anal sphincters are skeletal muscles. This means they are under:
Involuntary autonomic control — same as the internal sphincters
Cardiac pacemaker timing to coordinate with abdominal pressure waves
Voluntary somatic control — allowing conscious deferral of urination and defecation
Hormonal control that coordinates with GI peristaltic activity
2633 — Appendicular Muscles
10 questions — Aligned to EAP objectives — Martini Chapter 7
Q85MC
The deltoid originates from the acromion, scapular spine, and lateral clavicle. Its primary action is:
Medial rotation of the humerus
Extension of the shoulder
Abduction of the shoulder
Adduction of the shoulder
Q86MC
The triceps brachii is notable because it is:
The only muscle capable of shoulder flexion in the upper limb
The strongest supinator of the forearm
The only muscle that extends the elbow
The primary wrist flexor
Q87MC
The biceps brachii performs which combination of actions?
Elbow extension and pronation
Shoulder extension and elbow flexion
Elbow flexion and supination of the forearm
Wrist flexion and shoulder abduction
Q88MC
The four muscles of the rotator cuff that stabilize the glenohumeral joint are:
Deltoid, biceps brachii, coracobrachialis, and brachialis
Supraspinatus, infraspinatus, teres minor, and subscapularis
Pectoralis major, latissimus dorsi, teres major, and coracobrachialis
Rhomboid major, rhomboid minor, trapezius, and serratus anterior
Q89MC
All three hamstring muscles (biceps femoris, semitendinosus, semimembranosus) share a proximal origin on the:
Posterior femur (linea aspera)
Anterior superior iliac spine
Ischial tuberosity
Greater trochanter of the femur
Q90MC
Of the four quadriceps muscles, which one also crosses the hip joint?
Vastus lateralis
Vastus medialis
Vastus intermedius
Rectus femoris
Q91MC
Both gastrocnemius and soleus plantar flex the foot via the Achilles tendon. However, only the gastrocnemius can also flex the knee because:
The soleus attaches to the fibula only, mechanically excluding knee action
The gastrocnemius has a larger pennation angle enabling multi-joint action
The gastrocnemius originates on the femoral condyles, above the knee joint
The soleus lacks a tendon long enough to reach the knee
Q92MC
The gluteus maximus is the largest muscle in the body. Its primary actions are:
Hip abduction and medial rotation
Hip adduction and flexion
Hip extension and lateral rotation
Knee extension and hip abduction
Q93FITB
The three hamstring muscles all originate on the ___ tuberosity of the pelvis.
Q94MC
The preferred intramuscular injection site in infants and young children is the:
Deltoid — the largest accessible upper limb muscle
Gluteus medius — the standard adult injection site
Rectus femoris — on the anterior thigh
Vastus lateralis — well-developed in infants with minimal adjacent neurovascular structures
2634 — Effects of Aging on Skeletal Muscle
4 questions — Aligned to EAP objectives — Martini Chapter 7
Q95MC
With aging, skeletal muscle mass declines (sarcopenia) primarily because:
Fiber type conversion from slow to fast fibers reduces endurance capacity
Motor nerve supply to peripheral muscles is progressively eliminated
Muscle fiber diameter decreases as the number of myofibrils per fiber declines
Glycogen and creatine phosphate stores are permanently depleted
Q96MC
With aging, fibrosis of skeletal muscle means:
Enlargement of remaining fast fibers to compensate for atrophied slow fibers
Increased satellite cell activity replacing lost fibers
Functional muscle tissue is replaced by fibrous connective tissue
Conversion of aerobic to anaerobic fiber metabolism with aging
Q97MC
According to the Martini textbook, the rate of age-related muscle function decline:
Is significantly slower in individuals who exercise regularly
Is faster in males than females due to testosterone-dependent maintenance
Is the same for everyone — exercise builds a higher baseline, not a slower slope
Stops at age 70 when fiber loss reaches a stable plateau
Q98MC
Elderly individuals are at increased risk for heat-related illness during exertion because:
Myoglobin loss reduces aerobic capacity and increases metabolic heat production
Blood supply to the skin is reduced while sweat gland activity also decreases
Decreased fiber diameter reduces heat generation but paradoxically increases core temperature
Fibrosis impedes blood flow to active muscles, trapping metabolic heat within the fibers
2635 — System Relationships
2 questions — Aligned to EAP objectives — Martini Chapter 7
Q99MC
During sustained skeletal muscle activity, the cardiovascular system responds by:
Constricting vessels in active muscles to prevent excessive blood pressure
Decreasing heart rate to prevent cardiac fatigue
Increasing cardiac output and dilating vessels in active muscles to deliver more O₂
Redistributing all blood from visceral organs to the skin only
Q100MC
The diaphragm and external intercostal muscles directly link the muscular system to the:
Cardiovascular system — their contractions increase venous return
Digestive system — they increase abdominal pressure during swallowing
Respiratory system — their contractions perform the mechanical work of breathing
Nervous system — the phrenic nerve is the only neural connection involved
SOMAPL17 Practice Test — Complete
The Muscular System — Objectives 2623–2635
-- correct-- incorrect-- total
SOMAPL17 — The Muscular System
Study guide aligned to objectives 2623–2635. Covers medical vocabulary, skeletal muscle functions, tissue organization, sarcomere structure, contraction mechanics, energy systems, fiber types, muscle tissue comparison, axial and appendicular muscles, aging, and systemic relationships.
OBJ Click any card to expand
TRAP Exam traps flagged in red
TIP Clinical notes in green
Objectives 2623–2635
2623
Medical Vocabulary — Latin/Greek roots
►
These word roots from the textbook Vocabulary Development section appear throughout Chapter 7. Knowing them decodes unfamiliar muscular system terms on the exam.
sarkosflesh → sarcolemma, sarcoplasm, sarcomere, sarcoplasmic reticulum
mys / myo-muscle → epimysium, perimysium, endomysium, myofibril, myofilament
lemmahusk → sarcolemma = plasma membrane of a muscle fiber
merospart → sarcomere = smallest contractile unit of a muscle fiber
epi-on → epimysium (outermost connective tissue layer)
peri-around → perimysium (wraps fascicles)
endo-inside → endomysium (wraps individual fibers)
iso- / tonosequal / tension → isotonic contraction (tension constant, length changes)
metronmeasure → isometric contraction (length constant, tension varies)
aer / an-air / not → aerobic (O2 required), anaerobic (no O2)
tetanosconvulsive tension → tetanus (sustained maximal contraction)
tropeturning → tropomyosin (turns away to expose active sites)
-trophynourishing → atrophy (wasting), hypertrophy (enlargement)
fasciculusbundle → fascicle = bundle of muscle fibers within perimysium
syn- / ergontogether / work → synergist helps a prime mover work efficiently
bi / caputtwo / head → biceps (two tendons of origin)
Note
Tetanus the disease (Clostridium tetani) and tetanus the muscle response share a name but are unrelated mechanisms. The disease suppresses motor neuron inhibition. The muscle response is normal physiology produced by high-frequency stimulation.
2624
Functions of Skeletal Muscle Tissue — five primary functions
►
Skeletal muscles are organs composed primarily of skeletal muscle tissue but also containing connective tissues, nerves, and blood vessels. The muscular system includes approximately 700 skeletal muscles performing five primary functions.
1. Produce Movement of the Skeleton
Skeletal muscle contractions pull on tendons and thereby move the bones. These contractions may produce a simple motion, such as extending the arm, or the highly coordinated movements of swimming, skiing, or typing.
2. Maintain Posture and Body Position
Continuous muscle contractions maintain body posture. Without this constant action, you could not sit upright without collapsing, or stand without toppling over.
3. Support Soft Tissues
The abdominal wall and floor of the pelvic cavity consist of layers of skeletal muscle. These muscles support the weight of our visceral organs and shield our internal tissues from injury.
4. Guard Entrances and Exits
Skeletal muscles encircle openings of the digestive and urinary tracts. These muscles provide voluntary control over swallowing, defecation, and urination.
5. Maintain Body Temperature
Muscle contractions require energy, and whenever energy is used in the body, some of it is converted to heat. Heat from working muscles keeps body temperature in the range required for normal functioning. Shivering is an extreme example.
Exam Trap
The answer is FIVE functions. Skeletal muscles can only PULL (generate tension) — they cannot push. Movement results from tension transmitted through tendons to bone. The diaphragm is a skeletal muscle that normally functions subconsciously but can be voluntarily controlled.
Clinical
Voluntary control over defecation and urination depends on skeletal muscle sphincters (external urethral and external anal sphincters). Spinal cord injury above sacral level eliminates voluntary external sphincter control even though smooth muscle internal sphincters remain intact.
2625
Organization of Muscle at the Tissue Level — layers, fibers, organelles
►
Skeletal muscle has a precise hierarchical organization from the whole organ down to the individual protein filament. Three connective tissue layers wrap each level of organization and converge at each end to form the tendon or aponeurosis.
1
EpimysiumLayer of collagen fibers surrounding the entire muscle. Separates the muscle from surrounding tissues and organs. (epi = on)
2
PerimysiumConnective tissue dividing skeletal muscle into compartments. Each compartment contains a bundle of muscle fibers called a fascicle. Contains blood vessels and nerves supplying the fascicles. (peri = around)
Fascicle = fasciculus = a bundle of muscle fibers
3
EndomysiumSurrounds each individual skeletal muscle fiber and ties adjacent fibers together. Contains capillaries, nerve fibers, and stem cells (satellite cells) for repair. (endo = inside)
4
Tendon or AponeurosisAt each end of the muscle, collagen fibers of all three layers converge. A cord = tendon. A flat sheet = aponeurosis. Tendon fibers interweave with bone periosteum. Every contraction pulls on the tendon, which pulls on the bone.
5
Sarcolemma and T TubulesThe plasma membrane of a muscle fiber (sarcolemma) surrounds the cytoplasm (sarcoplasm). Openings across the sarcolemma lead into T tubules (transverse tubules) filled with extracellular fluid. T tubules carry action potentials deep into the fiber, coordinating simultaneous contraction of all sarcomeres.
6
Sarcoplasmic Reticulum and TriadsSpecialized smooth ER forming a tubular network around each myofibril. Stores calcium ions in terminal cisternae. A T tubule sandwiched between two terminal cisternae = a triad. The T tubule signal triggers Ca2+ release from the cisternae to start contraction.
7
Myofibrils and MyofilamentsCylindrical myofibrils (1-2 micrometers diameter, as long as the fiber) responsible for contraction. Each fiber contains hundreds to thousands of myofibrils. Myofibrils contain thick (myosin) and thin (actin) myofilaments organized into sarcomeres. Each muscle fiber is multinucleate (hundreds of nuclei beneath the sarcolemma).
Exam Trap
T tubules do NOT store calcium. The sarcoplasmic reticulum (specifically the terminal cisternae) stores Ca2+. T tubules only carry the electrical signal. A triad = 1 T tubule flanked by 2 terminal cisternae. Confusing T tubules with SR is a classic exam error.
2626
Structural Components of a Sarcomere — bands, lines, zones, filaments
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The sarcomere is the smallest functional unit of the muscle fiber. Each myofibril contains approximately 10,000 sarcomeres end to end. Resting length is about 2 micrometers. Differences in size and density of thick and thin filaments account for the banded (striated) appearance.
| Structure | Description | Change During Contraction |
|---|---|---|
| Z lines | Protein discs marking the boundaries of each sarcomere. Thin (actin) filaments anchor here. Strands extend from Z lines to ends of thick filaments to maintain alignment. | Move CLOSER together |
| M line | Proteins at center connecting central portions of adjacent thick filaments to each other. | Stays centered |
| A band | DARK band; spans the full length of the thick (myosin) filaments. Zone of overlap lies within the A band. | Width UNCHANGED |
| I band | LIGHT band; region of thin filaments only, between two successive A bands, including the Z line. | Gets SMALLER |
| H band | Lighter central zone of A band; thick filaments only, no overlap with thin filaments. | Gets SMALLER |
| Zone of overlap | Region within A band where thick and thin filaments interdigitate. Cross-bridges form here during contraction. | Gets LARGER |
Each thin filament is a twisted strand of actin molecules, each with an active site capable of binding myosin. In a resting muscle, active sites are covered by tropomyosin strands held in position by troponin bound to the actin strand. Calcium is the key that unlocks active sites: Ca2+ binds troponin, troponin changes shape, tropomyosin swings away, active sites are exposed.
Thick filaments are composed of myosin molecules, each with a tail and globular head. Myosin heads project outward from the M line. In the resting sarcomere each myosin head is already energized (cocked) with ADP and phosphate bound. When active sites are exposed, heads bind to form cross-bridges and pivot toward the M line (power stroke).
Exam Trap
The A band width stays CONSTANT during contraction — it equals the thick filament length and never changes. Everything else changes. A band = dArk = constant. I band = lIght = narrows. H band = Hole in the middle = shrinks. Z lines move closer together. Zone of overlap increases.
2627
Key Steps in Muscle Fiber Contraction — NMJ through sliding filament
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Skeletal muscle fibers contract only under nervous system control. Each fiber is controlled by a motor neuron at a neuromuscular junction (NMJ). The axon terminal contains vesicles filled with acetylcholine (ACh). A narrow synaptic cleft separates the axon terminal from the motor end plate on the sarcolemma. Both the cleft and motor end plate contain acetylcholinesterase (AChE) which breaks down ACh.
1
ACh Released at NMJAction potential arrives at axon terminal. Vesicles fuse with the membrane. ACh released by exocytosis into the synaptic cleft.
2
Action Potential Generated in SarcolemmaACh binds to receptors on motor end plate. Na+ permeability increases. Na+ rushes into sarcoplasm. Action potential generated and sweeps across the entire sarcolemma surface.
3
AChE Breaks Down AChAChE quickly breaks down ACh in the synaptic cleft and motor end plate, inactivating the ACh receptors. No further stimulus until the next action potential arrives at the axon terminal.
4
Action Potential Travels Down T TubulesAction potential spreads across sarcolemma and down T tubules deep into the fiber interior, reaching the triads.
5
Calcium Released from SRT tubule signal triggers massive release of Ca2+ from the terminal cisternae of the sarcoplasmic reticulum into the sarcoplasm around the sarcomeres.
6
Active Sites Exposed on ActinCa2+ binds to troponin. Troponin changes shape. Tropomyosin swings away from active sites on actin molecules of the thin filaments.
7
Cross-Bridge FormationEnergized myosin heads (cocked with ADP + Pi bound) bind to exposed active sites, forming cross-bridges.
8
Power StrokeStored energy used to pivot myosin head toward M line. Thin filament pulled toward center of sarcomere. Bound ADP and phosphate group released. Sarcomere shortens.
9
Cross-Bridge Detachment and RecyclingNew ATP binds to the myosin head. Link between head and active site is broken. Free myosin head splits ATP into ADP + Pi. Energy released recocks the head. Cycle repeats as long as Ca2+ and ATP are available.
Relaxation: action potentials stop, AChE eliminates ACh, SR calcium pumps recapture Ca2+, troponin-tropomyosin return to resting positions covering active sites, cross-bridge cycling stops, muscle returns passively to resting length. Contraction is active; elongation is entirely passive.
Exam Traps
Botulism: bacterial toxin prevents ACh RELEASE from axon terminal = flaccid paralysis. Myasthenia gravis: autoimmune destruction of ACh RECEPTORS at motor end plate = progressive weakness. Rigor mortis: ATP depleted after death = SR cannot pump Ca2+ = cross-bridges cannot detach = muscles lock contracted (begins 2-7 hours after death, resolves 1-6 days later). Organophosphates inhibit AChE = ACh accumulates = sustained depolarization = convulsions then paralysis.
2628
Types of Muscle Contractions — twitch, tetanus, isotonic, isometric
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A twitch is a single stimulus-contraction-relaxation sequence. A myogram is a graph of tension development during a twitch. Three phases in the gastrocnemius twitch (total ~40 msec): latent period (~2 msec, Ca2+ released, no tension yet); contraction phase (~15 msec, tension rises to peak); relaxation phase (~25 msec, Ca2+ recaptured, tension falls).
INCOMPLETE TETANUS
Second stimulus arrives before relaxation phase ends. Twitches add together (summation). Tension rises to a peak during rapid cycles of contraction and relaxation. Brief relaxation periods still occur.
Virtually all normal muscular contractions involve incomplete tetanus of the participating muscle fibers.
COMPLETE TETANUS
Stimulus rate so high that SR cannot recapture Ca2+. High Ca2+ concentration prolongs contraction making it continuous. Relaxation phase completely eliminated. Maximum tension produced and sustained.
Tension plateaus at maximal levels. Cannot occur in cardiac muscle.
A motor unit = one motor neuron + all the muscle fibers it innervates. Eye muscles: 2-3 fibers per unit (fine control). Leg muscles: up to 2,000 fibers per unit (power). Recruitment = activating progressively more motor units to produce a smooth, steady increase in muscular tension. Motor unit fibers are intermingled with those of other units so pull direction stays constant regardless of which units are active.
Isotonic Contraction
Tension rises, then the skeletal muscle length changes while tension remains constant until relaxation occurs. Lifting an object off a desk, walking, and running involve isotonic contractions. (iso = equal, tonos = tension)
Isometric Contraction
The muscle as a whole does not change length and the tension produced never exceeds the load. Examples: pushing against a closed door, trying to pick up a car. Many everyday reflexive postural contractions opposing gravity are isometric. (metron = measure)
Muscle Tone and Atrophy
Some motor units are always active even when the whole muscle is not contracting. Their contractions do not produce enough tension for gross movement but they tense and firm the muscle. This resting tension = muscle tone. A skeletal muscle not regularly stimulated will atrophy — fibers become smaller and weaker. Initially reversible; dying fibers replaced by fibrous tissue permanently.
Exam Trap
Cardiac muscle CANNOT undergo tetanus — its plasma membrane properties differ from skeletal muscle and the SR cannot stay open long enough. A tetanic heart could not pump blood between beats. Smooth muscle CAN tetanize; skeletal CAN tetanize; cardiac CANNOT.
2629
Energy Mechanisms for Muscle Contraction — ATP, aerobic, glycolysis, fatigue, recovery
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Muscle contraction requires large amounts of energy. An active skeletal muscle fiber may require 600 trillion ATP molecules per second. A resting fiber contains only enough ATP to sustain a contraction for a few seconds. Three systems generate ATP to meet demand throughout the duration of activity.
ATP + Creatine Phosphate (Immediate)
Fastest; active in the first ~15 seconds
At rest, surplus ATP transfers energy to creatine forming creatine phosphate (CP). During contraction: CP + ADP yields ATP + creatine. Regulated by creatine phosphokinase (CPK/CK). Resting muscle has ~6x more CP than ATP. Elevated blood CPK = muscle cell damage.
Aerobic Metabolism (Mitochondria)
Sustained moderate activity; requires O2
Provides 95% of resting ATP. Citric acid cycle in mitochondria. Each pyruvate yields 17 ATP. Resting muscle burns fatty acids. When activity begins, switches to pyruvate from glycolysis. Limited by O2 delivery: mitochondrial output can increase 40x resting, but peak demand = 120x resting.
Glycolysis (Anaerobic)
Peak activity; occurs in cytoplasm; no O2 needed
Breakdown of glucose to pyruvate in cytoplasm. Glucose from glycogen granules stored in sarcoplasm. Net yield: 2 ATP per glucose (vs. 34 more ATP from aerobic completion). At peak activity: glycolysis provides ~2/3 of ATP; mitochondria ~1/3. By-product: lactic acid = lactate + H+ = pH drop = fatigue.
Fatigue and Recovery
Inability to contract despite continued stimulation
Caused by: (1) exhaustion of energy reserves, (2) pH drop from lactic acid. Recovery: SR pumps Ca2+ back in; liver converts lactate to glucose to glycogen; CP and ATP stores replenished. Extra O2 consumed during recovery above resting = oxygen debt. Explains continued heavy breathing after exercise stops.
Activity Level Summary
Resting: fatty acids oxidized aerobically; surplus ATP builds CP and glycogen. Moderate: aerobic breakdown of glucose and fatty acids; mitochondria meet demand. Peak: glycolysis dominant (~2/3 ATP); lactate accumulates; mitochondria provide only ~1/3.
Clinical
Elevated blood CPK (creatine phosphokinase) indicates muscle cell membrane damage allowing the enzyme to leak into circulation. Used clinically to diagnose rhabdomyolysis, myocardial infarction, and other muscle damage conditions.
2630
Muscle Fiber Types and Performance — fast vs. slow, aerobic vs. anaerobic endurance
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Muscle performance = force (maximum tension produced) and endurance (time activity can be sustained). Two major factors: fiber type and physical conditioning. The human body contains two contrasting types of skeletal muscle fibers.
| Property | Fast Fibers (Fast-Twitch) | Slow Fibers (Slow-Twitch) |
|---|---|---|
| Peak tension time | 0.01 sec or less | ~3x longer than fast |
| Diameter | Large | About half of fast fibers |
| Myofibrils | Densely packed; more power | Less dense |
| Mitochondria | Few | Many (aerobic capacity) |
| Glycogen reserves | Large | Smaller |
| Capillary supply | Standard | Extensive network |
| Myoglobin | Little — pale/white appearance | Abundant — red appearance |
| Primary energy | Glycolysis (anaerobic) | Aerobic metabolism |
| Fatigue rate | Rapid | Resistant to fatigue |
| Best suited for | Short, powerful bursts | Sustained activity |
Most human muscles appear pink (mixed fiber types). Eye and hand muscles: no slow fibers (swift, brief contractions needed). Back and calf muscles: dominated by slow fibers (continuous postural contraction). The percentage of fast vs. slow fibers is genetically determined. Training cannot convert fiber type but can increase fatigue resistance of fast fibers.
ANAEROBIC ENDURANCE
Length of time contractions can be supported by glycolysis + ATP/CP reserves. Involves fast muscle fibers.
Training: frequent, brief, intense workouts. Result: hypertrophy — each fiber increases in diameter. NUMBER of fibers does NOT increase. Examples: sprinting, pole vault, weightlifting.
AEROBIC ENDURANCE
Length of time contractions can be sustained by mitochondrial activities. Determined by substrate availability.
Training: sustained low-level activity (jogging, distance swimming). Aerobic training does NOT produce hypertrophy. Endurance athletes carboload (high-carb diet 3 days before event) to maximize glycogen stores.
Exam Trap
Hypertrophy = individual fibers get LARGER. The NUMBER of muscle fibers does NOT increase with training. Aerobic training does NOT produce hypertrophy. Only anaerobic/resistance training causes hypertrophy. Myoglobin stores O2 within the fiber for use during contraction — it is structurally related to hemoglobin but is NOT the same molecule.
2631
Skeletal vs. Cardiac vs. Smooth Muscle — structure and function comparison
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Actin and myosin are present in all three muscle types. However, their internal organization, control mechanisms, calcium sources, and functional properties differ significantly.
| Property | Skeletal Muscle Fiber | Cardiac Muscle Cell | Smooth Muscle Cell |
|---|---|---|---|
| Location | Attached to skeleton | Heart only | Walls of most organs |
| Shape | Long cylinders | Branched cells | Spindle-shaped |
| Nuclei | Multiple, near sarcolemma | Usually single, central | Single, centrally located |
| Striations | Yes | Yes | No |
| Sarcomeres | Yes, along myofibrils | Yes, along myofibrils | No; filaments scattered in sarcoplasm |
| Special junctions | None | Intercalated discs with gap junctions | Anchoring sites transmit contractile forces |
| Control | Neural, single NMJ; voluntary | Automaticity (pacemaker cells); involuntary | Automaticity (pacesetter cells); neural or hormonal; involuntary |
| Ca2+ source | SR only | SR + extracellular fluid | Extracellular fluid + SR |
| Tetanus possible? | Yes | NO | Yes |
| Fatigue | Rapid fatigue | Resistant to fatigue | Resistant to fatigue |
| Energy source | Aerobic (moderate); glycolysis at peak | Aerobic; lipid or carbohydrate | Primarily aerobic |
Cardiac unique features: Intercalated discs contain gap junctions allowing action potentials to pass directly cell-to-cell, producing simultaneous contraction of entire chambers. Automaticity = contracts without neural input. Contractions last ~10x longer than skeletal. Cannot tetanize — heart must relax between beats to fill with blood.
Smooth unique features: No sarcomeres, no striations. Thick filaments scattered in sarcoplasm; thin filaments anchored to sarcolemma and cytoplasmic dense bodies. Contraction causes a corkscrew twist. Can contract over a wider range of lengths — critical for hollow organs (bladder, stomach) undergoing large volume changes. Many cells respond to hormones and pacesetter cells without direct motor neuron input. Sphincters = rings of smooth muscle in digestive and urinary systems.
Exam Traps
Cardiac muscle has automaticity AND cannot tetanize — both are unique to cardiac muscle. Smooth muscle CAN tetanize. Skeletal cannot achieve automaticity. Extracellular Ca2+ entry is important for BOTH cardiac AND smooth muscle contraction, but NOT for skeletal muscle.
2632
Main Axial Muscles and Their Actions — head, neck, spine, trunk, pelvic floor
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Axial muscles arise on the axial skeleton, position the head and spinal column, and move the rib cage. They encompass roughly 60% of all skeletal muscles and do NOT move the pectoral or pelvic girdles. A prime mover (agonist) is chiefly responsible for a particular movement. Antagonists oppose that movement. Synergists help the prime mover work efficiently. Fixators stabilize the origin of a prime mover.
| Muscle | Primary Action(s) | Key Origin / Insertion / Note |
|---|---|---|
| Masseter | Elevates mandible (closes jaw) | Origin: zygomatic arch; Insertion: mandible. Primary chewing muscle. |
| Temporalis | Elevates mandible | Along temporal lines of skull to coronoid process of mandible. |
| Pterygoids | Elevate, protract, and move mandible side to side | Grinding motion; inferior processes of sphenoid. |
| Buccinator | Compresses cheeks; moves food back onto teeth | "Trumpeter" muscle; produces suckling suction in infants. |
| Orbicularis oris | Compresses and purses lips | Kissing, whistling, speech articulation. |
| Orbicularis oculi | Closes eye (blinking, winking) | Origin: medial margin of orbit. |
| Frontalis | Raises eyebrows; wrinkles forehead | Anterior part of occipitofrontalis. |
| Zygomaticus | Draws corner of mouth back and up | Smiling muscle; origin: zygomatic bone. |
| Platysma | Tenses neck skin; depresses mandible | Flat; covers ventral neck surface; from second rib to mandible. |
| Sternocleidomastoid (SCM) | Bilateral: flex neck. Unilateral: tilt head to same side, rotate face to opposite side | Origin: sternum + clavicle; Insertion: mastoid process of skull. |
| Digastric | Depresses mandible (opens mouth); elevates larynx | Two bellies (di = two, gaster = stomach). |
| Mylohyoid | Elevates floor of mouth and hyoid; depresses mandible | Flat muscular floor of mouth; supports tongue. |
| Splenius capitis | Bilateral: extend neck. Unilateral: rotate and laterally flex head to that side | Posterior neck; superficial spinal muscle. |
| Erector spinae (Spinalis, Longissimus, Iliocostalis) | Bilateral: extend vertebral column. Unilateral: lateral flexion of spine | More extensors than flexors because body weight anterior to spine pulls spine into flexion — gravity already does the flexor's job. |
| Quadratus lumborum | Together: depress ribs, flex vertebral column. One side: lateral flexion | Iliac crest to last rib and lumbar vertebrae. |
| External intercostals | Elevate ribs (inspiration) | Origin: inferior border of each rib; Insertion: superior border of next rib. |
| Internal intercostals | Depress ribs (forced expiration) | Origin: superior border of each rib. |
| Diaphragm | Expands thoracic cavity (inspiration); compresses abdominopelvic cavity | Origin: xiphoid, ribs 4-10 cartilages, lumbar vertebrae; Insertion: central tendon. |
| External oblique | Compresses abdomen; depresses ribs; flexes or laterally flexes vertebral column | Lower 8 ribs to linea alba and iliac crest. |
| Internal oblique | Compresses abdomen; depresses ribs; flexes or laterally flexes vertebral column | Iliac crest to lower ribs; rotates in opposite direction from external oblique. |
| Transversus abdominis | Compresses abdomen only (no rotation) | Deepest abdominal layer. |
| Rectus abdominis | Depresses ribs; flexes vertebral column | Pubis to ribs 5-7 and xiphoid; separated by linea alba (midline connective tissue band). |
| Levator ani | Tenses pelvic floor; supports pelvic organs; flexes coccyx | Main pelvic floor muscle of the perineum. |
| External urethral sphincter | Closes urethra voluntarily | Skeletal muscle; voluntary control of urination. |
| External anal sphincter | Closes anal opening voluntarily | Skeletal muscle; voluntary control of defecation. |
Exam Trap
Voluntary control of urination and defecation = SKELETAL muscle (external sphincters). Internal sphincters are smooth muscle and are involuntary. Axial muscles do NOT move the pectoral or pelvic girdles — that is the role of the appendicular muscle group.
2633
Main Appendicular Muscles and Their Actions — shoulders, upper limbs, pelvic girdle, lower limbs
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Appendicular muscles stabilize or move components of the appendicular skeleton. The pectoral girdle connection to the axial skeleton must act as a shock absorber while allowing upper limb mobility. The pelvic girdle connection transfers weight from the axial to the appendicular skeleton with emphasis on power over mobility.
| Muscle | Primary Action(s) | Key Origin / Insertion / Note |
|---|---|---|
| Trapezius | Elevates, adducts, depresses, or rotates scapula (depending on active region); can extend/hyperextend neck | Occipital bone and spinous processes of thoracic vertebrae to clavicle and scapula. |
| Levator scapulae | Elevates scapula (shrugging) | Covered by trapezius; transverse processes of C1-C4. |
| Rhomboid muscles | Adducts (retracts) scapula toward midline | Spinous processes of lower cervical and upper thoracic vertebrae. |
| Serratus anterior | Protracts shoulder; abducts and medially rotates scapula | Origin: ribs 1-9; Insertion: vertebral border of scapula. |
| Pectoralis minor | Depresses and protracts shoulder | Origin: ribs 3-5; Insertion: coracoid process of scapula. |
| Deltoid | Abduction at shoulder | Major abductor of the arm; IM injection site (2.5 cm distal to acromion). |
| Pectoralis major | Flexion, adduction, and medial rotation at shoulder | Large anterior chest muscle; cartilages of ribs 2-6, sternum, clavicle to greater tubercle of humerus. |
| Latissimus dorsi | Extension, adduction, and medial rotation at shoulder | Lower thoracic vertebrae, ribs, lumbar vertebrae to intertubercular groove of humerus. |
| Supraspinatus* | Abduction at shoulder | *Rotator cuff (SITS). Tendons blend with shoulder joint capsule for support. Common throwing-sport injury site. |
| Infraspinatus* | Lateral rotation at shoulder | *Rotator cuff. Infraspinous fossa of scapula to greater tubercle. |
| Teres minor* | Lateral rotation at shoulder | *Rotator cuff. Lateral border of scapula to greater tubercle. |
| Subscapularis* | Medial rotation at shoulder | *Rotator cuff. Subscapular fossa of scapula to lesser tubercle. |
| Biceps brachii | Flexion at shoulder and elbow; supination | Short head from coracoid process; long head from supraglenoid tubercle; inserts on radial tuberosity. Strongest when forearm supinated. |
| Triceps brachii | Extension at elbow | ONLY elbow extensor. Long head from scapula; inserts on olecranon of ulna. |
| Brachialis | Flexion at elbow | Pure elbow flexor; anterior distal humerus to ulnar tuberosity. |
| Brachioradialis | Flexion at elbow | Lateral epicondyle of humerus; forms lateral forearm bulge. |
| Pronator teres | Pronation | Medial epicondyle of humerus to lateral radius. With pronator quadratus. |
| Supinator | Supination | Works with biceps brachii; lateral epicondyle of humerus. |
| Flexor carpi radialis | Flexion and abduction at wrist | Medial epicondyle to 2nd and 3rd metacarpal bases. |
| Flexor carpi ulnaris | Flexion and adduction at wrist | Opposite direction from flexor carpi radialis. |
| Palmaris longus | Flexion at wrist | Tendinous sheet on the palm. |
| Extensor carpi radialis | Extension and abduction at wrist | Posterior forearm; distal lateral humerus. |
| Extensor carpi ulnaris | Extension and adduction at wrist | Lateral epicondyle of humerus and ulna; base of 5th metacarpal. |
| Gluteus maximus | Extension and lateral rotation at hip | Largest muscle in body; primary power for running and climbing. Iliac crest, sacrum, coccyx to iliotibial tract and gluteal tuberosity. |
| Gluteus medius | Abduction and medial rotation at hip | Preferred IM injection site (posterior/lateral superior quadrant). |
| Gluteus minimus | Abduction and medial rotation at hip | Deep to gluteus medius; lateral surface of ilium. |
| Iliopsoas (Psoas major + Iliacus) | Flexion at hip and/or lumbar intervertebral joints | Largest hip flexor; inserts at lesser trochanter of femur. Two muscles sharing a common insertion. |
| Adductor group (magnus, longus, brevis, pectineus, gracilis) | Adduction at hip; flexion and medial rotation (varies by muscle) | Strain of one of these = "pulled groin." |
| Biceps femoris | Flexion at knee; extension and lateral rotation at hip | Hamstring. Origin: ischial tuberosity and linea aspera of femur. |
| Semitendinosus | Flexion at knee; extension and medial rotation at hip | Hamstring. Origin: ischial tuberosity. |
| Semimembranosus | Flexion at knee; extension and medial rotation at hip | Hamstring. Origin: ischial tuberosity. Pulled hamstring = strain in one of these three. |
| Sartorius | Flexion at knee; flexion and lateral rotation at hip | Longest muscle in the body; "tailor's muscle" (crossing-legs position). Anterior superior iliac spine to medial tibia. |
| Rectus femoris | Extension at knee; flexion at hip | Quadriceps. ONLY quadriceps muscle crossing the hip joint. Anterior inferior iliac spine to tibial tuberosity via patellar ligament. |
| Vastus lateralis | Extension at knee | Quadriceps. Preferred IM site in infants and young children. |
| Vastus medialis | Extension at knee | Quadriceps. Entire length of linea aspera of femur. |
| Vastus intermedius | Extension at knee | Quadriceps. Lies beneath the other three; not visible in surface view. |
| Popliteus | Rotates tibia medially; flexion at knee | Unlocks the extended/locked knee. Lateral condyle of femur to posterior proximal tibial shaft. |
| Gastrocnemius | Plantar flexion at ankle; flexion at knee | Origin: femoral condyles (crosses knee joint). Shares calcaneal (Achilles) tendon with soleus. |
| Soleus | Plantar flexion at ankle ONLY | No femoral origin = does NOT cross or flex knee. Head and proximal shaft of fibula to calcaneus via calcaneal tendon. |
| Tibialis anterior | Dorsiflexion at ankle; inversion of foot | Opposes gastrocnemius. Lateral condyle and proximal shaft of tibia to 1st metatarsal base. |
| Fibularis (peroneus) muscles | Eversion of foot; plantar flexion at ankle | Fibula and lateral condyle of tibia to metatarsal bases. |
Exam Traps
Triceps brachii is the ONLY elbow extensor. Biceps brachii produces supination AND flexion — loses mechanical advantage with forearm pronated. Rectus femoris is the ONLY quadriceps muscle crossing the hip joint. Soleus does NOT flex the knee because it has no femoral origin. Gastrocnemius DOES flex the knee because it originates on the femoral condyles.
Clinical — IM Injection Sites
Gluteus medius (posterior/lateral superior quadrant): large, few vessels and nerves, up to 5 mL. Deltoid (2.5 cm distal to acromion): accessible but limited volume. Vastus lateralis: preferred in infants, young children, and elderly patients with atrophied gluteal and deltoid muscles — thick muscle, no large vessels or nerves encountered.
2634
Effects of Aging on Muscle Tissue — four changes and clinical implications
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Aging degrades every aspect of muscular performance. The rate of decline is the same in all people regardless of exercise patterns or lifestyle. To be in good shape late in life, an individual must be in very good shape early in life. Regular exercise helps control body weight, strengthens bones, and generally improves the quality of life at all ages.
1. Skeletal Muscle Fibers Become Smaller in Diameter
Reduction in size reflects a decrease in the number of myofibrils. Fibers contain smaller ATP, CP, and glycogen reserves and less myoglobin. Overall effects: reduced muscle strength, reduced endurance, tendency to fatigue rapidly. Cardiovascular performance also decreases — blood flow to active muscles does not increase with exercise as rapidly as in younger people.
2. Skeletal Muscles Become Less Elastic (Fibrosis)
Aging skeletal muscles develop increasing amounts of fibrous connective tissue — a process called fibrosis. Fibrosis makes the muscle less flexible and less responsive. Collagen fibers restrict movement and circulation.
3. Tolerance for Exercise Decreases
Lower tolerance results from the tendency to tire quickly and from reduced thermoregulatory capacity. Individuals over age 65 cannot eliminate muscle-generated heat as effectively as younger people, leading to risk of overheating during exercise.
4. Ability to Recover from Muscular Injuries Decreases
When an injury occurs, repair capabilities are limited and scar tissue formation is the usual result. Dying muscle fibers are not replaced. In extreme atrophy the functional losses are permanent. This is why physical therapy is critical for patients who are temporarily unable to move normally.
Key Principle
The rate of muscular decline is universal — regular exercise does not slow the rate of decline itself. Exercise preserves function only by maintaining a higher starting point. Extremely demanding exercise in the elderly can damage tendons, bones, and joints. Regular moderate exercise — not extreme exercise — is the goal at all ages.
Clinical
Physical therapy is critical for patients who are temporarily unable to move normally. Muscle atrophy is initially reversible, but dying fibers are not replaced. Early intervention prevents permanent functional loss. Even brief periods of immobilization following injury or surgery require active rehabilitation to prevent significant long-term deficits.
2635
Functional Relationships with Other Body Systems — systems integrator
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Even when the body is at rest, the muscular system is interacting with other organ systems. The changes that occur during exercise provide a good example of such interactions. Active muscles consume oxygen and generate carbon dioxide and heat, requiring coordinated responses from every other system.
Integumentary System
Removes excess body heat generated by contracting muscles
Blood vessels dilate and sweat gland secretion increases — promotes evaporation and heat removal from skin surface. Synthesizes vitamin D3 for calcium and phosphate absorption needed by muscle. Skeletal muscles pulling on dermis of face produce facial expressions.
Skeletal System
Provides sites of attachment and the lever system
Provides movement and support. Mechanical stress from tendons maintains bone mass. Muscles stabilize bones and joints. Bone provides mineral reserve (calcium, phosphate) for maintaining normal levels in body fluids required for muscle contraction.
Nervous System
Directs immediate responses; coordinates all systems during exercise
Motor neurons provide mandatory stimulation for skeletal muscle contraction. Directs heart rate, respiratory rate, sweat gland activity, and release of stored energy reserves. Coordinates all organ system responses during exercise.
Endocrine System
Directs long-term changes in response to activity
Hormones adjust metabolic rate and mobilize fuel reserves. Epinephrine mobilizes glucose. Works with nervous system to direct responses of other systems during exercise and recovery.
Cardiovascular System
Delivers O2 and nutrients; removes CO2 and heat
During exercise: blood vessels in active muscles and in the skin dilate, heart rate increases. Speeds O2 delivery and CO2 removal. Brings heat to skin for radiation. Elevated blood CPK indicates skeletal muscle damage.
Respiratory System
Increases O2 delivery; removes excess CO2
Rate and depth of respiration increase during exercise, keeping pace with increased blood flow through the lungs. Diaphragm and intercostals are skeletal muscles that power breathing — voluntarily controllable and automatically rhythmic.
Digestive System
Provides fuel for muscular activity
Provides nutrients: glucose, fatty acids, amino acids. Abdominal wall muscles assist digestion by compressing digestive organs. Smooth muscle in digestive walls performs peristalsis. Skeletal muscle sphincters control defecation.
Urinary System
Eliminates metabolic wastes from muscle activity
Removes excess water, salts, and waste products generated by active skeletal muscles. External urethral sphincter (skeletal muscle) provides voluntary control of urination.
The Big Picture
During vigorous exercise, every organ system responds simultaneously. Cardiovascular: vessels dilate, heart rate increases. Respiratory: rate and depth increase. Integumentary: vessels dilate, sweating increases. Nervous and endocrine: coordinate all responses and release stored energy. The muscular system is the primary driver of these whole-body responses.