Skeletal Muscle Anatomy, NMJ, and Contraction Physiology

excitability

in muscles, the ability to contract or relax based on electrical properties of the plasma membrane.

elasticity

the ability to recoil back to original length due to the presence of elastic fibers.

extensibility

the ability to stretch or extend.

Contractility

the ability to shorten with force by pulling on attachment points.

epimysium

sheath of dense, irregular connective tissue that surrounds an entire skeletal muscle.

fascia

a layer of dense (fibrous) connective tissue that closely surrounds an entire skeletal muscle and lies external to the epimysium.

aponeurosis

broad, tendonlike sheet of connective tissue that may fuse with the epimysium, connecting to another skeletal muscle or to a bone.

fascicle

1) a bundled group of skeletal muscle fibers wrapped by the perimysium; 2) bundles of axons within a nerve, surrounded by a layer of connective tissue called the perineurium.

perimysium

layer of connective tissue that closely surrounds a fascicle.

endomysium

thin layer of collagen and reticular fibers that surrounds each muscle fiber within the fascicle; contains extracellular fluid and nutrients.

sarcolemma

the plasma membrane of a skeletal muscle cell.

sarcoplasm

the cytoplasm of a skeletal muscle cell.

sarcoplasmic reticulum (SR)

specialized smooth endoplasmic reticulum of a skeletal muscle cell, which stores and releases calcium ions for muscle contraction.

T-tubules

deep invaginations in the sarcolemma that transmit electrical signals into the sarcoplasmic reticulum of muscle cells

myofibril

the contractile machinery of muscle cells; long cylinders of contractile proteins that shorten during muscle contraction.

myosin

protein subunit that makes up much of the thick myofilaments within a sarcomere.

actin

protein subunit that makes up much of the thin myofilaments in a sarcomere.

myofilaments

protein subunits that make up the myofibrils in muscle cells; composed mainly of actin or myosin.

troponin

regulatory protein that makes up part of the thin myofilament; binds to actin, tropomyosin, and calcium; changes the shape of the tropomyosin molecule to allow myosin to bind to actin during muscle contraction.

tropomyosin

regulatory protein that makes up part of the thin myofilament; blocks myosin-binding sites to prevent actin from binding to myosin when a muscle cell is relaxed.

thin filament

strand of actin and a troponin-tropomyosin complex; projects from the Z disc toward the center of the sarcomere.

thick filament

myosin strand made up of multiple heads; projects from the center of the sarcomere toward the Z disk

Z disc

a disc of proteins that forms the outer boundary of a sarcomere; anchors the thin filaments in place, and indirectly anchors the thick filaments in place via titins.

sarcomere

repeating contractile unit of skeletal muscle; consists of overlapping thick and thin myofilaments.

A band

the portion of the sarcomere that contains thick filaments; in some portions, the thick filaments overlap with the thin filaments; appears as dark band under a light microscope.

I bands

the portion of the sarcomere in which only thin filaments are found; lies between adjacent A bands, and contains the Z disks.

M line

A line in the center of the sarcomere, consisting of proteins that anchor the centers of the thick filaments in place.

H zone

the area in the center of the A band of a sarcomere, in which only thick filaments are found.

motor neurons

nerve fibers that carry signals that emanate from the central nervous system and connect to effectors.

neuromuscular junction (NMJ)

point of contact between the skeletal muscle and the controlling neuron; site of muscle stimulation.

motor end plate

region of sarcolemma located at the neuromuscular junction of a skeletal muscle cell.

membrane potential

The electrical charge difference between the inside and the outside of a cell membrane.

resting membrane potential

the electrical charge difference between the inside and outside of a cell membrane when the cell is at rest (not conducting action potentials).

action potential

electrical signal unique to neurons and muscle fibers; caused by a change in voltage across a cell membrane in response to a stimulus. These electrochemical impulses allow the neurons to communicate throughout the body.

neurotransmitter

chemical messenger released by nerve terminals; binds to and activates receptors on target cells.

acetylcholine (ACh)

a neurotransmitter used in the central nervous system and in neuromuscular junctions; binds to receptors on the motor end-plate to trigger depolarization.

synaptic cleft

1) small space between a nerve terminal and the motor end plate of a muscle cell; 2) the small gap between two cells at a chemical synapse.

polarized

a state in which a charge difference exists between the inside and outside of a cell membrane.

depolarize

to make the membrane potential of a cell less negative.

voltage-gated channels

ion channel in the cell membrane of a neuron or muscle that opens because of a change in the electrical properties of the membrane (membrane potential) where it is located.

voltage-gated sodium channels

ion channels in the cell membrane of a neuron or muscle cell that open to allow Na+ ions to enter the cell in response to a change in the membrane potential; open during the depolarization phase of an action potential.

voltage-gated potassium channels

ion channels in the cell membrane of a neuron or muscle cell that open to allow K+ ions to leave the cell in response to a change in the membrane potential; open during the repolarization phase of an action potential.

repolarizes

increases the negativity inside an excitable cell, after the depolarization phase of an action potential; returns the cell to its resting potential.

acetylcholinesterase (AChE)

the enzyme that decomposes acetylcholine (Ach) in the synaptic cleft after Ach has transmitted its message to the muscle cell or postsynaptic neuron.

excitation-contraction coupling

the union of events beginning with motor neuron signaling to a skeletal muscle fiber and ending with contraction of the fiber's sarcomeres.

power stroke

a change in shape of the myosin head (cross-bridge) due to its binding to actin.

Creatine phosphate

molecule that regenerates ATP from ADP in muscle cells, by donating a phosphate group to ADP.

Glycolysis

an anaerobic sequence of metabolic reactions that decomposes glucose into two molecules of pyruvate and results in a net gain of two molecules of ATP.

lactic acid

product of anaerobic breakdown of pyruvate.

aerobic respiration

breakdown of pyruvate in the presence of oxygen; results in ATP formation.

Slow oxidative (SO)

muscle fibers that contract relatively slowly and use aerobic respiration to produce ATP.

Fast glycolytic (FG)

muscle fibers that contract relatively quickly and use glycolysis to produce ATP.

Fast oxidative (FO)

muscle fibers that contract relatively quickly and use a mixture of aerobic respiration and glycolysis to produce ATP.

muscle tension

the force generated by the contraction of a muscle.

isometric contraction

muscle contraction that does not move a load or result in a change in muscle length.

isotonic contraction

muscle contraction that moves a load or involves a change in muscle length.

concentric contraction

shortening of muscle to move a load.

eccentric contraction

contraction characterized by a decrease in muscle tension as the muscle stretches to a length greater than its resting length.

twitch

the contraction of a single muscle fiber caused by a single action potential.

myogram

a graph that represents muscle tension produced over time.

latent period

phase of muscle contraction during which the action potential is propagated along the sarcolemma and calcium ions are released.

contraction phase

phase of muscle contraction during which calcium ions have bound to troponin, and sarcomeres are shortening.

relaxation phase

phase of muscle contraction during which contraction stops, and the sarcomeres return to their resting length.

summation

an increase in muscle tension, due to the additive effects of several action potentials.

tetanus

continuous muscle contraction.

muscle tone

baseline level of muscle contraction; occurs when the muscle is not producing movement.

motor unit

group of muscle fibers innervated by a single motor neuron.

recruitment

an increase in muscle contraction as more motor units are employed.

intercalated disc

physical junction between adjacent cardiac muscle cells; consisting of desmosomes, specialized linking proteoglycans, and gap junctions that allow passage of ions between the two cells.

dense body

smooth muscle structure that attaches the thin filaments to the sarcolemma; shortens the muscle fiber as thin filaments slide past thick filaments.

caveolae

small indentations in the cell membranes of smooth muscle cells, which help in intercellular communication, endocytosis, and exocytosis.

calmodulin

regulatory protein that controls cross-bridge formation and facilitates contraction in smooth muscles; activated by the binding of calcium ions.

varicosity

structure in some autonomic neurons resembling a string of swellings along the length of the fiber; releases neurotransmitter molecules to exert control over various target organs

stress-relaxation response

contraction and relaxation of smooth muscle cells stimulated by the stretching of the muscle.

origin

fixed (unmovable) end of a muscle; generally attached to a bone.

insertion

moveable end of a muscle; attaches to the structure being pulled during muscle contraction.

prime mover

principal muscle responsible for a movement.

synergists

muscle that contributes to the movement of a prime mover; often by flexing a joint or stabilizing the insertion.

fixator

synergist that works by stabilizing the insertion of the prime mover.

agonist

prime mover whose action is responsible for a particular movement.

antagonist

muscle whose action is the opposite of an agonist.

Parallel

muscles whose fibers are arranged in the same direction as the fascicles.

belly

the plump center of a muscle.

fusiform

spindle-shaped parallel muscles with a belly.

Circular

muscles whose fascicles are arranged in a circle.

sphincter

a ring-shaped muscle that encircles a passageway in the body; it constricts or relaxes to regulate the flow of a substance (such as urine or ingested food) through the passageway.

convergent

a muscle with a widespread origin over a single area whose fascicles come to a single, common insertion point.

Pennate

a muscle whose fibers blend into a tendon that runs through the central region of the muscle.

unipennate

a pennate muscle whose fasicles are located on one side of the tendon.

bipennate

a pennate muscle with fascicles on both sides of the tendon.

multipennate

a pennate muscle in which the tendon branches within the muscle.

lateralis

located laterally to.

medialis

located medially to.

rectus

referring to the direction of muscle fibers; straight.

oblique

referring to the direction of muscle fibers; at an angle.

axial

referring to the trunk and head.

appendicular

referring to the arms and legs.

orbicularis oris

circular muscle surrounding the mouth.

orbicularis oculi

circular muscle responsible for closing the eye.

occipitofrontalis

muscle that moves the scalp and eyebrows; consists of the frontal belly and occipital belly.