Muscle Tissue Structure, Physiology, and Neuromuscular Function

Comprehensive practice flashcards defining the histology, hierarchical structural organization, neuromuscular transmission, excitation-contraction coupling, cross-bridge cycling, and clinical conditions associated with skeletal, cardiac, and smooth muscle tissues.

Comparison of Muscle Tissue Types

The structural and functional differentiation of skeletal (striated, voluntary, multi-nucleated), cardiac (striated, branched, involuntary with intercalated discs), and smooth (non-striated, spindle-shaped, involuntary) tissues.

Skeletal Muscle

Striated muscle composed of long, cylindrical, unbranched fibers with multiple peripheral nuclei under voluntary control (somatic nervous system), attached to bones via tendons, and specialized for body movement, posture, and heat production.

Cardiac Muscle

Striated, involuntary muscle tissue found only in the heart (forming the myocardium), consisting of short, branched fibers with a single central nucleus and intercalated discs that contracts rhythmically and continuously to pump blood.

Smooth Muscle

Non-striated, involuntary muscle tissue composed of spindle-shaped (fusiform) fibers arranged in layers with a single central nucleus, found in walls of hollow organs to move substances and regulate organ diameter.

Intercalated Discs

Specialized cell junctions connecting adjacent cardiac muscle cells that contain gap junctions and desmosomes to enable electrical and mechanical communication.

Hierarchical Organization of Skeletal Muscle

The structural arrangement of skeletal muscle from the whole muscle, to fascicles, to muscle fibers (cells), to myofibrils, and down to repeating sarcomeres.

Epimysium

The outer connective tissue layer that envelops the entire skeletal muscle and connects to tendons.

Perimysium

The connective tissue sheath that surrounds each muscle fascicle and carries nerves, blood vessels, and connective tissue.

Endomysium

The fine connective tissue layer that encloses each individual muscle fiber.

Fascicle

A bundle of skeletal muscle fibers enclosed by the perimysium.

Sarcolemma

The plasma cell membrane surrounding an individual skeletal muscle fiber.

Sarcoplasm

The specialized cytoplasm of a muscle fiber containing myofibrils, multiple peripheral nuclei, and abundant mitochondria.

Myofibril

A rod-like contractile structure within a muscle fiber composed of repeating sarcomeres made up of actin and myosin filaments.

Sarcomere

The basic, microscopic contractile unit of striated muscle that extends from one Z line (disc) to the next Z line (disc).

Z Disc

The structural boundary of each sarcomere that anchors thin (actin) filaments and moves closer to adjacent boundaries during contraction.

M Line

The central structural line in the middle of a sarcomere that anchors and holds thick (myosin) filaments in place.

I Band

The light region of the sarcomere containing thin filaments (actin) only, which shortens during muscle contraction.

A Band

The dark region across the entire length of thick filaments (myosin), which remains constant in length during contraction.

H Zone

The central section of the A band containing only thick filaments (myosin), which shortens or disappears when the muscle contracts.

Tropomyosin

A regulatory filament that blocks myosin-binding sites on actin monomers when a muscle fiber is at rest.

Troponin

A regulatory protein complex that binds $Ca^{2+}$ (specifically troponin C), inducing a conformational shift that pulls tropomyosin off the myosin-binding sites on actin.

Sliding Filament Theory

The model of muscle contraction stating that actin and myosin filaments do not shorten, but instead slide past one another to shorten the sarcomere.

Neuromuscular Junction (NMJ)

A chemical synapse formed between the axon terminal of a somatic motor neuron and the motor end plate of a skeletal muscle fiber.

Synaptic Cleft

The narrow gap of approximately $\sim 50\,\text{nm}$ between the motor neuron axon terminal and the motor end plate across which acetylcholine diffuses.

End Plate Potential (EPP)

A graded depolarization of the motor end plate produced by $Na^+$ influx (greater than $K^+$ efflux) through ligand-gated nicotinic acetylcholine receptors.

Acetylcholinesterase (AChE)

An enzyme in the synaptic cleft that terminates neurotransmission by rapidly breaking down acetylcholine into choline (recycled back into the neuron) and acetate.

Myasthenia Gravis

An autoimmune disorder characterized by autoantibodies blocking nicotinic acetylcholine receptors at the neuromuscular junction, causing muscle weakness and fatigue.

Botulinum Toxin (Botox)

A neurotoxin that prevents the release of acetylcholine from synaptic vesicles at the neuromuscular junction, producing muscle paralysis.

Organophosphates

Compounds used in nerve agents and insecticides that inhibit acetylcholinesterase, leading to excessive muscle stimulation, weakness, paralysis, or death.

Excitation-Contraction Coupling

The physiological mechanism by which an electrical action potential at the sarcolemma triggers intracellular $Ca^{2+}$ release and subsequent sarcomere contraction.

T-Tubules (Transverse Tubules)

Invaginations of the sarcolemma that conduct electrical action potentials deep into the interior of the muscle fiber to reach the sarcoplasmic reticulum.

DHP Receptor

A voltage sensor embedded in the T-tubule membrane that detects electrical signals and mechanically opens adjacent ryanodine receptors on the sarcoplasmic reticulum.

Ryanodine Receptor

A calcium release channel located in the sarcoplasmic reticulum membrane that releases stored $Ca^{2+}$ into the sarcoplasm when mechanically activated by DHP receptors.

SERCA Pump

An active transporter that utilizes ATP to pump $Ca^{2+}$ from the sarcoplasm back into the sarcoplasmic reticulum, permitting muscle relaxation.

Power Stroke

The pivoting motion of the myosin head that pulls the actin filament toward the M line, triggered by the release of ADP and $\text{P}_i$.

Cross-Bridge Detachment

The step in the cross-bridge cycle where a newly bound ATP molecule causes the myosin head to release its grip on the actin filament.

Rigor Mortis

A postmortem state of sustained muscular stiffness occurring because ATP depletion prevents myosin heads from detaching from actin filaments.

Malignant Hyperthermia

A genetic condition causing excessive and uncontrolled $Ca^{2+}$ release from the sarcoplasmic reticulum, resulting in sustained contraction and elevated body temperature.

Dantrolene

A therapeutic drug that treats malignant hyperthermia by inhibiting $Ca^{2+}$ release from the sarcoplasmic reticulum.

Muscle Tissue Types

The three primary contractile tissues in the human body—skeletal, cardiac, and smooth muscle—each exhibiting distinct structural specializations, regulatory mechanisms, and physiological roles.

Skeletal Muscle

A striated, voluntarily controlled muscle tissue composed of long, cylindrical, unbranched fibers with multiple peripheral nuclei; anchored to bones via tendons to produce locomotion, support posture, and generate body heat.

Cardiac Muscle

An involuntary, striated muscle tissue found exclusively in the heart wall (myocardium), characterized by short, branched fibers, a single centrally located nucleus, and intercalated discs.

Smooth Muscle

A non-striated, involuntary muscle tissue consisting of spindle-shaped (fusiform) fibers arranged in sheets with a single central nucleus; found in hollow organ walls (e.g., blood vessels, digestive tract, bladder) to regulate diameter and transport substances.

Intercalated Discs

Specialized cell junctions connecting adjacent cardiac muscle cells that contain desmosomes for mechanical adhesion and gap junctions for rapid electrical signal transmission.

Hierarchical Organization of Skeletal Muscle

The architectural structural arrangement in skeletal muscle extending from the whole muscle down through fascicles, individual muscle fibers (cells), myofibrils, and myofilaments within sarcomeres.

Epimysium

The outer sheath of dense connective tissue that encloses the entire skeletal muscle and connects to tendons anchored to bone.

Perimysium

The intermediate connective tissue layer that groups individual muscle fibers into bundles called fascicles, containing blood vessels and nerves.

Endomysium

The delicate connective tissue layer that surrounds each individual skeletal muscle fiber.

Fascicle

A discrete bundle of skeletal muscle fibers bounded by the perimysium.

Muscle Fiber

An individual, multinucleated, cylindrical skeletal muscle cell packed with myofibrils and enveloped by the sarcolemma and endomysium.

Sarcolemma

The specialized plasma membrane surrounding a muscle fiber.

Sarcoplasm

The specialized cytoplasm of a muscle fiber containing myofibrils, glycogen stores, and abundant mitochondria.

Myofibril

A rod-like intracellular organelle within a muscle fiber composed of repeating, end-to-end sarcomeric units containing actin and myosin filaments.

Sarcomere

The fundamental contractile unit of striated muscle, bounded at each end by a Z disc and containing organized thin and thick myofilaments.

Z Disc

A protein boundary structure anchoring the thin (actin) filaments at both lateral ends of a sarcomere.

M Line

A central structural protein line in the middle of the sarcomere that stabilizes and anchors thick (myosin) filaments.

I Band

The light microscopic region of the sarcomere containing thin (actin) filaments only, which shortens during muscle contraction.

A Band

The dark band spanning the entire length of the thick (myosin) filaments; its physical length remains constant throughout muscle contraction.

H Zone

The central sector of the A band consisting solely of thick (myosin) filaments, which narrows or disappears during contraction as actin filaments slide inward.

G-Actin and F-Actin

Globular actin monomers (G-actin) that polymerize into long filamentous polymers (F-actin), forming the backbone of the thin myofilament.

Tropomyosin

A filamentous protein that winds along actin strands, physically obstructing myosin-binding sites when a muscle fiber is at rest.

Troponin

A regulatory protein complex associated with tropomyosin; the troponin C subunit binds $\text{Ca}^{2+}$, causing a conformational shift that pulls tropomyosin off the active binding sites of actin.

Sliding Filament Theory

The mechanism of muscle contraction whereby thick and thin filaments slide past one another toward the M line without altering their individual lengths, pulling the Z discs closer together and shortening the sarcomere.

Neuromuscular Junction (NMJ)

The chemical synapse formed between the terminal axon of a somatic motor neuron and the motor end plate of a skeletal muscle fiber.

Motor End Plate

The specialized, folded portion of the sarcolemma directly facing the synaptic cleft, densely populated with ligand-gated nicotinic acetylcholine receptors.

Synaptic Cleft

The narrow extracellular gap of approximately $\sim 50\,\text{nm}$ separating the presynaptic motor neuron terminal from the postsynaptic motor end plate.

Acetylcholine (ACh)

The primary neurotransmitter released via exocytosis from presynaptic motor neuron vesicles across the synaptic cleft to initiate skeletal muscle excitation.

End Plate Potential (EPP)

The localized depolarization produced at the motor end plate by the net inward flow of $\text{Na}^+$ ions through opened nicotinic acetylcholine receptors; triggers a muscle action potential if threshold is reached.

Acetylcholinesterase (AChE)

An enzyme present within the synaptic cleft that rapidly hydrolyzes acetylcholine into choline and acetate, terminating synaptic transmission and preventing continuous depolarization.

Myasthenia Gravis

An autoimmune disorder characterized by autoantibodies targeting and blocking nicotinic acetylcholine receptors at the neuromuscular junction, leading to progressive muscle weakness and fatigue.

Botulinum Toxin (Botox)

A bacterial toxin that blocks acetylcholine release from motor neuron synaptic vesicles, producing flaccid paralysis that can be used therapeutically.

Organophosphates

Chemical compounds found in nerve agents and certain insecticides that irreversibly inhibit acetylcholinesterase, leading to excessive muscle stimulation, weakness, paralysis, or death.

Excitation-Contraction Coupling

The physiological cascade connecting an electrical impulse (action potential) along the sarcolemma and T-tubules to intracellular calcium release and the mechanical initiation of cross-bridge cycling.

Transverse Tubules (T-tubules)

Deep, periodic invaginations of the sarcolemma that propagate action potentials from the fiber surface into the interior to trigger calcium release from the sarcoplasmic reticulum.

Sarcoplasmic Reticulum (SR)

A specialized smooth endoplasmic reticulum network enveloping myofibrils that sequesters, stores, and releases $\text{Ca}^{2+}$ ions into the sarcoplasm.

DHP Receptors

Dihydropyridine receptors; voltage sensors located in the T-tubule membrane that undergo conformational changes upon depolarization to mechanically activate ryanodine receptors.

Ryanodine Receptors

$\text{Ca}^{2+}$ release channels embedded in the sarcoplasmic reticulum membrane that open in response to mechanical interaction with activated DHP receptors.

SERCA Pump

Sarcoplasmic/endoplasmic reticulum calcium ATPase; an ATP-powered active transporter that pumps $\text{Ca}^{2+}$ back into the sarcoplasmic reticulum lumen to bring about muscle relaxation.

Cross-Bridge Formation

The physical attachment of an activated myosin head (carrying ADP and $\text{P}_i$) to an exposed binding site on an actin filament.

Power Stroke

The conformational pivot of the myosin head that pulls the actin filament toward the M line upon the dissociation of ADP and $\text{P}_i$, thereby generating mechanical force.

Cross-Bridge Detachment

The dissociation of the myosin head from the actin binding site, triggered by the binding of a new ATP molecule to the myosin head.

Cross-Bridge Re-cocking

The resetting of the myosin head into its energized, high-affinity conformation through the enzymatic hydrolysis of ATP into ADP and $\text{P}_i$.

Rigor Mortis

The sustained post-mortem muscular stiffness that develops because depleted ATP supplies prevent myosin head detachment from actin filaments.

Malignant Hyperthermia

A pharmacogenetic condition characterized by abnormal, excessive $\text{Ca}^{2+}$ release from the sarcoplasmic reticulum, resulting in sustained muscular hypercontraction and severe hyperthermia.

Dantrolene

A clinical medication used to treat malignant hyperthermia by directly antagonizing ryanodine receptors to inhibit pathological $\text{Ca}^{2+}$ release from the sarcoplasmic reticulum.