Physiology of Muscle Contraction

Muscle

A contractile tissue that accomplishes diverse functions by shortening and pulling on other structures.

Excitability

The capacity to receive and respond to a stimulus.

Extensibility

The ability to be stretched.

Elasticity

The ability to return to its original shape after being stretched.

Skeletal muscle

Striated, tubular, multinucleated, voluntary muscle attached to the skeleton.

Cardiac muscle

Striated, branched, uninucleated, involuntary muscle covering the walls of the heart.

Smooth muscle

Non-striated, spindle-shaped, uninucleated, involuntary muscle covering the walls of internal organs.

Sarcolemma

The cell membrane of the muscle fiber filled with intracellular fluid.

Sarcomere

The repetitive unit containing actin and myosin filaments that gives rise to striations.

Actin filaments

Thin filaments found in the sarcomere.

Myosin filaments

Thick filaments located centrally within each sarcomere.

I band

Isotropic band consisting of actin of two sarcomeres common to the same Z line.

A band

Anisotropic dark band formed by centrally located myosin filaments.

Z discs

Located at each end of the sarcomere and composed of filamentous proteins different from actin and myosin.

Sarcoplasmic reticulum

Tubules arranged parallel to and encircle the myofibrils.

T-tubules

Tubular extensions of the sarcolemma extending deep into the myofibrils at the Z-line or A-I junction depending on muscle type and species.

Red fiber (slow twitch fiber)

Rich in myoglobin, contains numerous mitochondria and capillaries, and is associated with sustained activity.

White fiber (fast twitch fiber)

Contracts rapidly with short duration, has extensive sarcoplasmic reticulum, fewer mitochondria, and produces strong contractions.

Thin filament proteins

Actin, tropomyosin, and troponin.

Actin

Contains active sites where myosin cross-bridges attach.

Tropomyosin

Guards the actin filaments by covering the myosin-binding sites.

Troponin T

Binds the other two troponin units with tropomyosin.

Troponin C

Calcium acceptor protein.

Troponin I

Induces the inhibitory conformation of the actin-myosin filaments.

Myosin head

Globular polypeptide structure responsible for cross-bridge formation.

Light chain

Helps control the function of the myosin head during contraction.

Cross-bridges

Temporary connections formed between myosin heads and actin that generate force.

Stages of skeletal muscle contraction

Excitation-contraction coupling, exposure of binding sites, cross-bridge cycle, and detachment/reset.

Motor unit

A motor neuron and all the muscle fibers it innervates.

Motor end plate

Specific area of the sarcolemma directly facing the neuron's terminal branches.

Neuromuscular junction (NMJ)

Specialized junction where a motor neuron contacts a muscle fiber.

Acetylcholine (ACh)

Neurotransmitter stored in membrane-bound vesicles in the terminal branch.

Excitation-contraction coupling

Mechanism in which an action potential increases intracellular calcium, triggering muscle contraction.

Walk-along mechanism

Repeated movement of myosin heads along actin filaments during contraction.

Dense bodies

Attachment points for actin filaments in smooth muscle.

Caveolae

Invaginations that replace T-tubules in smooth muscle and serve as calcium entry points.

Plasticity

Characteristic property of smooth muscle.

Stress relaxation response

Characteristic property of smooth muscle.

Reverse stress-relaxation response

Characteristic property of smooth muscle.

Tone

State of sustained smooth muscle contraction.

Single-unit smooth muscle

Large masses of muscle fibers contract simultaneously; example: peristalsis.

Multiunit smooth muscle

Each muscle fiber acts independently under nerve control.

Calmodulin

Calcium-binding protein that activates MLCK.

Myosin light-chain kinase (MLCK)

Phosphorylates the myosin light chain to initiate smooth muscle contraction.

Cardiac muscle

Striated, branched, centrally uninucleated muscle with intercalated disks.

Intercalated disks

Dense cross-bands connecting cardiac muscle fibers.

Pacemaker cells

Specialized cardiac muscle cells that spontaneously generate electrical impulses.

SA node

Primary pacemaker of the heart.

AV node

Electrical relay station of the heart.

Purkinje fibers (Bundle of His)

Final emergency backup conduction pathway.

Funny channel

Allows Na⁺ entry into pacemaker cells at threshold potential (-40 mV).

Pacemaker cells

Have no true resting membrane potential.

Contractile myocyte resting potential

-90 mV.

Contractile myocyte threshold potential

-70 mV.

Channels in contractile myocytes

Fast Na⁺ channels, slow Ca²⁺ (Ca-Na) channels, and K⁺ channels