Muscle Tissue: Types, Functions, and Contraction Mechanisms

Five main functions of muscle tissue

Movement, stability (posture & joints), control of body openings/passages, heat production (thermogenesis), glycemic control (glucose regulation).

Four key properties of muscle tissue

Excitability (respond to stimuli), Conductivity (propagate signals), Contractility (shorten & do work), Extensibility (stretch), Elasticity (return to original length).

Three types of muscle tissue

Skeletal (striated, voluntary, multinucleate), Cardiac (striated, involuntary, intercalated discs), Smooth (non-striated, involuntary, spindle-shaped).

Muscle type with autorhythmic cells and gap junctions

Cardiac muscle.

Muscle type capable of maintaining tone for long periods

Smooth muscle (latch-bridge mechanism).

Connective tissue layers of a skeletal muscle

Endomysium (around fibers), Perimysium (around fascicles), Epimysium (around entire muscle).

Fascia

A sheet of connective tissue separating muscles; deep fascia surrounds muscles, superficial fascia connects to skin.

Connects muscle to bone

Tendons (rope-like) or aponeuroses (sheet-like).

Sarcolemma

The plasma membrane of a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle cell, containing glycogen, myoglobin, and organelles.

Myofibril

Long protein bundles inside a muscle fiber, composed of myofilaments.

Organelle that stores calcium for contraction

Sarcoplasmic reticulum (SR).

T-tubules

Invaginations of the sarcolemma that transmit action potentials deep into the fiber.

Triad in muscle fibers

One T-tubule + two terminal cisternae of SR.

Proteins that make up thick filaments

Myosin molecules with heads that bind actin and ATP.

Proteins that make up thin filaments

Actin (binding sites), tropomyosin (blocks sites), troponin (binds Ca²⁺ to move tropomyosin).

Functional unit of muscle contraction

The sarcomere, from Z-disc to Z-disc.

Regions of a sarcomere

A-band (thick filaments), I-band (thin only), H-zone (thick only), M-line (middle), Z-disc (boundary).

NMJ

The synapse between a motor neuron and a muscle fiber.

Found in the synaptic knob

Synaptic vesicles filled with acetylcholine (ACh).

Motor end plate

The specialized region of the sarcolemma with ACh receptors.

Enzyme that breaks down ACh

Acetylcholinesterase (AChE).

Four stages of muscle contraction

Excitation, Excitation-contraction coupling, Contraction, Relaxation.

Excitation-contraction coupling

Action potential travels through T-tubules → Ca²⁺ released from SR → Ca²⁺ binds troponin → moves tropomyosin → exposes actin binding sites.

Contraction

Myosin binds actin → power stroke → ATP binds to release → ATP hydrolysis resets head → cycle repeats (sliding filament theory).

Relaxation

ACh broken down, Ca²⁺ pumped back into SR, troponin-tropomyosin block re-covers actin.

Energy systems for muscle contraction

Immediate (phosphagen system: creatine phosphate), Short-term (anaerobic glycolysis, lactic acid), Long-term (aerobic respiration).

Phosphagen system

Provides ATP for sprinting <10 seconds.

Anaerobic glycolysis

Supports activities up to ~1 min.

Aerobic respiration

Supports endurance exercise (mitochondria, O₂, myoglobin).

Oxygen debt

Extra O₂ needed post-exercise to restore ATP, CP, and myoglobin, and clear lactic acid.

Muscle twitch phases

Latent period, Contraction phase, Relaxation phase.

Wave summation

Increased force from repeated stimuli before full relaxation.

Tetanus

Sustained contraction: incomplete (partial relaxation) or complete (no relaxation).

Isometric contraction

Tension increases but muscle length does not change.

Isotonic contraction

Muscle changes length while maintaining tension (concentric = shortening, eccentric = lengthening).

Motor unit

One motor neuron and all the muscle fibers it innervates.

Small vs. large motor units

Small (few fibers per neuron) → precise control (eye muscles); Large (many fibers) → strength (thigh muscles).

Recruitment

Increasing force by activating more motor units.

Types of muscle fibers

Slow oxidative (Type I), Fast oxidative (Type IIa), Fast glycolytic (Type IIb).

Characteristics of Type I fibers

Red, high myoglobin, many mitochondria, fatigue-resistant, endurance.

Characteristics of Type IIa fibers

Intermediate fatigue resistance, aerobic + anaerobic capacity, fast contractions.

Characteristics of Type IIb fibers

White, low myoglobin, rely on glycolysis, fast fatigue, explosive power.

Smooth muscle contractions

No sarcomeres, Ca²⁺ binds calmodulin (not troponin), slower, sustained contractions, can be autorhythmic.

Cardiac muscle differences

Involuntary, autorhythmic pacemaker cells, intercalated discs with gap junctions, long refractory period to prevent tetanus.