Module 8: Skeletal Muscle

Skeletal Muscle

Muscle attached to bones and skin, striated, requiring nervous system stimulation, and is voluntary.

Cardiac Muscle

Muscle found exclusively in the heart, striated, can contract without nervous system stimulation, and is involuntary.

Smooth Muscle

Muscle found in the walls of hollow organs, not striated, can contract without nervous system stimulation, and is involuntary.

Excitability

The ability of muscle tissue to receive and respond to stimuli.

Contractility

The ability of muscle tissue to shorten forcibly when stimulated.

Extensibility

The ability of muscle tissue to be stretched.

Elasticity

The ability of muscle tissue to recoil to resting length.

Endomysium

Fine connective tissue surrounding each individual muscle fiber.

Perimysium

Fibrous connective tissue surrounding fascicles (groups of muscle fibers) which includes nerves and blood vessels.

Epimysium

Dense irregular connective tissue surrounding the entire muscle.

Myofibril

Densely packed, rod-like elements in muscle cells, comprising ~80% of cell volume. Contain sarcomeres. Exhibit striations

Sarcoplasmic Reticulum

Modified endoplasmic reticulum that wraps around myofibrils and sequesters Ca2+.

Transverse Tubules (T-tubules)

Invaginations into the muscle fiber to allow for deeper penetration of an action potential. Increase surface area

Actin

Thin filaments in the sarcomere that interact with myosin for muscle contraction.

Myosin

Thick filaments in the sarcomere that bind to actin to facilitate muscle contraction.

Sarcomere

The contractile unit of a muscle fiber, consisting of myofilaments.

Calcium Pumps

Membrane proteins that transport calcium ions back into the sarcoplasmic reticulum during muscle relaxation.

Motor Unit

A single motor neuron and all the muscle fibers it controls; controls muscle contraction.

Isometric Contraction

Type of contraction where muscle tension increases without shortening.

Isotonic Contraction

Type of contraction where muscle shortens because tension exceeds load.

Muscle Twitch

Response to a single action potential of its motor neuron

Muscle Fatigue

Physiological inability of muscles to contract despite continued neural stimulation.

Aerobic Respiration

Energy production process using oxygen, yielding more ATP than anaerobic pathways.

Duchenne’s Muscular Dystrophy

A genetic disorder resulting from mutation affecting dystrophin, connecting muscle cytoskeleton to the extracellular matrix.

McArdle’s Disease

A genetic disorder caused by myophosphorylase deficiency, preventing glycogen conversion to glucose-6-phosphate.

Sarcolemma

The cell membrane of a muscle fiber

Sarcoplasm

The cytoplasm of the muscle fiber

muscle fibers

A collection of muscle cells in a skeletal muscle. Long cylinder cell, Multiple peripheral nuclei. The largest cells in the body, surrounded by stem cells

Fasicicles

Groups of muscle fibers

Muscle proper

Groups of muscle fascicles

Myoglobin

Temporary oxygen storage by the muscle

Crossbridges

Form and break several times, ratcheting thin filaments toward the center of the sarcomere (M line)

3 components of the neuromuscular junction (NMJ)

The axon terminus, the synapse, and the motor end plate

Troponin

A calcium-binding complex of three proteins

Tropomyosin

Blocks actin/myosin interaction

Titin

Large protein that coils at the end (shock absorber). Anchor thick filaments to Z disc

Events at the neuromuscular junction

Convert an acetylcholine signal from a somatic motor neuron into an electrical signal in the muscle fiber

Excitation-contraction (E-C) coupling

The process in which muscle action potentials are translated into calcium signals. Calcium signals in turn initiate a contraction-relaxation cycle

Contraction-relaxation cycle

Can be explained by the sliding theory of contraction. Begins when intracellular Ca2+ levels begin to fall due the activity of Ca2+ pumps (ATPase)

Contraction

The creation of tension in a muscle

Muscle tension

The force created by contracting muscle

Satellite cells

Bandage microtears. Add their nucleus to muscle

Wave summation

Increased stimulus frequency where the muscle is not allowed to completely relax between stimuli resulting in a stronger second contraction

Incomplete tetanus

stimuli are far enough apart to allow muscle to relax slightly between stimuli. Partial relaxation

Fused tetanus

Muscle reaches steady tension. If muscle fatigues, tension decreases rapidly. Preventing all relaxation

Muscle tonus

Constant, slightly contracted state of all muscles. Keeps muscles firm, healthy, and ready to respond

Recruitment

Multiple motor unit summation controls forces of contraction

Subthreshold stimulus

Stimulus strength causing first observable muscle contraction

Maximal stimulus

Strongest stimulus that increases contractile force

Hyperplasia (Smooth)

An increase in the number of cells in a tissue or organ, often resulting in its enlargement.

Hypertrophy (Skeletal)

The muscle gets bigger but you’re not adding more cells