Study this Muscle Anatomy and Physiology Overview

Sarcolemma

plasma membrane of muscle cell

Sarcoplasm

cytoplasm of muscle cell

Sarcoplasmic reticulum

endoplasmic reticulum of muscle cell

Muscle fiber

muscle cell

Tendons

connective tissues that attach muscle to bone

Aponeuroses

flat sheets of connective tissue that attach muscles to each other or to bones

Origin

immovable attachment point of a muscle

Insertion

movable attachment point of a muscle

Functions of skeletal muscle

Movement, support, posture, thermoregulation, and communication

Excitability

can receive and respond to stimuli

Contractability

when stimulated, generates force

Extensibility

can stretch beyond the resting position

Elasticity

recoils to resting position after being stretched

Epimysium

sheath of dense irregular connective tissue that surrounds the entire muscle

Perimysium

connective tissue surrounding a bundle of muscle fibers called a fascicle

Endomysium

thin connective tissue layer surrounding each muscle fiber; contains capillaries and nerves

Muscle fibers

elongated, cylindrical cells

Myofibrils

packed structures within the sarcoplasm made up of myofilaments

Striations

due to repeating A bands (dark) and I bands (light)

A band

thick (myosin) filaments

I band

thin (actin) filaments

Z line

anchors sequential sarcomeres

H zone

lighter region in middle of A band (no overlap of fibers)

M line

center of sarcomere

Neuromuscular junction

synapse between a motor neuron and a muscle fiber

Sequence of events in muscle contraction

1. AP arrives at axon terminal. 2. Voltage-gated Ca2+ channels open; Ca2+ diffuses into cell. 3. Ca2+ triggers acetylcholine (ACh) release by exocytosis. 4. ACh diffuses across synaptic cleft and binds to receptors (ligand-gated ion channels) on the sarcolemma. 5. Ion channels open; sarcolemma is depolarized. 6. Stimulation is ended when acetylcholinesterase breaks down ACh.

General features of muscle tissue

Highly cellular, Well vascularized, Elongated cells, Cells possess myofilaments

Sarcoplasm

packed full of myofibrils

Myofibrils

made up of myofilaments (thick and thin)

Myosin

has binding sites for both actin and ATP

Actin

has binding site for myosin

Tropomyosin

covers myosin binding site

Troponin

interacts with Ca2+ to remove tropomyosin interference

Action Potential

An electrical message on the surface of the muscle cell that triggers contraction.

Sarcolemma

The cell membrane of a muscle cell where the action potential travels.

Transverse Tubules (T-tubules)

Special tunnels that go deep inside the muscle cell, carrying the action potential.

Sarcoplasmic Reticulum (SR)

A storage tank inside the muscle cell that holds calcium, essential for muscle contraction.

Calcium Release Channels

Special doors in the sarcoplasmic reticulum that open in response to an action potential.

Troponin

A protein that holds tropomyosin in place and changes shape when calcium binds to it.

Tropomyosin

A protein that blocks myosin from binding to actin, acting as a guardrail.

Actin

Thin protein ropes inside the muscle cell that interact with myosin during contraction.

Myosin

Thicker protein ropes that pull on actin filaments during muscle contraction.

Cross-Bridge Cycle

The process where myosin heads attach to actin, pull, detach, and reattach, causing muscle contraction.

Power Stroke

The pivoting action of the myosin head that pulls the actin filament towards the center of the muscle fiber.

ATP

The energy currency of the cell used to re-energize the myosin head for another cycle.

Muscle Relaxation

The process where calcium ions are removed from the sarcoplasm, allowing troponin and tropomyosin to block actin again.

Cessation of Neural Stimulation

The stopping of action potentials from the motor neuron, leading to the end of muscle contraction.

Calcium Ions (Ca2+)

Ions that must be removed from the sarcoplasm for muscle relaxation and to end the cross-bridge cycle.

Neuromuscular Junction

The synapse between a motor neuron and a muscle fiber where acetylcholine is released.

Acetylcholine (ACh)

A neurotransmitter released at the neuromuscular junction that initiates muscle contraction.

Sliding Filament Mechanism

The theory explaining how actin and myosin filaments slide past each other to cause muscle contraction.

Muscle Fiber

A single muscle cell that contracts in response to action potentials.

Electrical Signal

The signal that triggers the release of calcium from the sarcoplasmic reticulum.

Calcium Flooding

The influx of calcium ions into the muscle cell that initiates contraction.

Shape Change of Troponin

The alteration in troponin's structure upon binding with calcium, allowing muscle contraction to occur.

ACh Breakdown

Acetylcholinesterase breaks down the ACh in the synaptic cleft, removing the stimulus for depolarization of the sarcolemma.

Repolarization of Sarcolemma

The muscle fiber's membrane potential returns to its resting state.

Closure of SR Calcium Channels

The voltage-sensitive proteins in the T-tubules that trigger calcium release from the sarcoplasmic reticulum (SR) return to their resting conformation, closing the calcium release channels in the SR membrane.

Active Transport of Ca2+ into the SR

ATP-dependent calcium pumps in the SR membrane actively transport calcium ions from the sarcoplasm back into the SR. This lowers the calcium concentration in the sarcoplasm.

Calcium Detaches from Troponin

As calcium levels in the sarcoplasm fall, calcium ions detach from troponin.

Tropomyosin Covers Myosin-Binding Sites

Without calcium bound to troponin, tropomyosin shifts back to its original position, blocking the myosin-binding sites on the actin filaments.

Cessation of Cross-Bridge Cycling

Myosin heads can no longer bind to actin, and the cross-bridges detach.

Muscle Returns to Resting Length

The muscle passively returns to its resting length due to its elasticity.

Motor Unit

A motor unit consists of one motor neuron and all the skeletal muscle fibers it innervates. When that motor neuron fires an action potential, all the muscle fibers connected to it contract.

Muscle Twitch

A muscle twitch is the mechanical response of a single muscle fiber or a motor unit to a single action potential.

Latent Period

The brief delay between the stimulus and the onset of contraction. During this time, the action potential travels along the sarcolemma and T-tubules, and calcium ions are released from the SR.

Period of Contraction

The muscle fibers shorten as cross-bridges form, cycle, and generate tension.

Period of Relaxation

Calcium ions are actively transported back into the SR, cross-bridge cycling stops, and the muscle fiber returns to its original length.

Wave Summation

If a second action potential arrives before a muscle fiber has completely relaxed from the first twitch, the second contraction will be added to the first, resulting in a greater tension.

Tetanus

If action potentials are delivered to the muscle fiber at a very high frequency, the muscle fiber does not have time to relax between stimuli. The contractions fuse into a smooth, sustained contraction.

Incomplete Tetanus (Unfused Tetanus)

The muscle fiber relaxes only partially between stimuli, resulting in a jerky, sawtooth-like pattern of tension.

Complete Tetanus (Fused Tetanus)

At even higher frequencies, there is no relaxation between stimuli, and the tension plateaus at a maximal, smooth level.

Muscle Tone

Muscle tone is a state of sustained, partial contraction of skeletal muscles, even when the muscle is not actively being used.

Importance of Muscle Tone

Helps keep the body upright against gravity, stabilizes joints by keeping the muscles around them slightly taut, and makes muscles more responsive to sudden demands.

Contraction

Physiologically, contraction refers to the activation of the cross-bridges (the force-generating sites) within the muscle fibers. This activation may or may not result in muscle shortening.

Isotonic Contraction

Muscle tension remains relatively constant during the contraction, and the muscle changes in length.

Concentric Contraction

The muscle shortens and does work (e.g., lifting a weight).

Eccentric Contraction

The muscle lengthens while resisting a load (e.g., lowering a weight slowly).

Isometric Contraction

Muscle tension develops, but the muscle length remains relatively constant. No work is done (e.g., pushing against an immovable wall).

Direct Phosphorylation

Creatine phosphate (CP) donates a phosphate group to ADP to quickly form ATP. This provides energy for a very short duration (about 15 seconds).

Anaerobic Glycolysis

Glucose is broken down into pyruvic acid, producing a small amount of ATP (2 ATP per glucose). If oxygen is limited, pyruvic acid is converted to lactic acid. This provides energy for a short to moderate duration (30-60 seconds).

Aerobic Respiration

Glucose, pyruvic acid, fatty acids, and amino acids are broken down in the mitochondria in the presence of oxygen to produce a large amount of ATP (about 32 ATP per glucose). This is the primary source of ATP for prolonged activity.

Slow Oxidative (Type I) Fibers

Contraction Speed: Slow; Fatigue Resistance: High; Primary ATP Synthesis: Aerobic Respiration; Mitochondria: Many; Capillaries: Rich; Myoglobin Content: High (red); Glycogen Stores: Low; Fiber Diameter: Small; Force Production: Low.

Fast Oxidative (Type IIa) Fibers

Contraction Speed: Fast; Fatigue Resistance: Intermediate; Primary ATP Synthesis: Aerobic Respiration (some anaerobic); Mitochondria: Many; Capillaries: Rich; Myoglobin Content: High (red to pink); Glycogen Stores: Intermediate; Fiber Diameter: Intermediate; Force Production: Intermediate.

Fast Glycolytic (Type IIx or IIb) Fibers

Contraction Speed: Fast; Fatigue Resistance: Low; Primary ATP Synthesis: Anaerobic Glycolysis; Mitochondria: Few; Capillaries: Sparse; Myoglobin Content: Low (pale); Glycogen Stores: High; Fiber Diameter: Large; Force Production: High.

Skeletal Muscle

Structure: Long, cylindrical, multinucleated fibers, striated; Location: Attached to bones; Control: Voluntary; Contraction Speed: Fast to slow; Fatigue Resistance: Low to high (varies by fiber type); Cellular Connections: Independent fibers; Initiation of Contraction: Nerve stimulation at NMJ.

Cardiac Muscle

Structure: Branched, uninucleated fibers, striated, intercalated discs; Location: Walls of the heart; Control: Involuntary; Contraction Speed: Intermediate; Fatigue Resistance: High; Cellular Connections: Intercalated discs (gap junctions and desmosomes); Initiation of Contraction: Autorhythmic, influenced by nerves and hormones.

Smooth Muscle

Structure: Spindle-shaped, uninucleated cells, no striations; Location: Walls of hollow organs (e.g., blood vessels, digestive tract, bladder); Control: Involuntary; Contraction Speed: Slow; Fatigue Resistance: High; Cellular Connections: Gap junctions in some types; Initiation of Contraction: Various stimuli (nerves, hormones, local factors, stretch).