Muscles and Muscle Tissue: Structure, Function, and Contraction

Muscle Tissue: Overview and Types

Muscle Tissue Terminology and Characteristics

Muscle tissue is essential for movement, posture, joint stability, and heat generation. The prefixes myo-, mys-, and sarco- refer to muscle. Sarcoplasm is the cytoplasm of a muscle cell, and myocardium refers to heart muscle. Muscle fibers are elongated cells found in skeletal and smooth muscle, but not in cardiac muscle, which is branched. Muscle fibers are multinucleate due to the fusion of myoblasts during development.

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched and relaxed.

• Elasticity: Ability to recoil to resting length.

Muscle tissue functions include producing movement, maintaining posture, stabilizing joints, and generating heat.

Types of Muscle Tissue

There are three types of muscle tissue: skeletal, cardiac, and smooth. Each type has distinct structural and functional characteristics.

Characteristic	Skeletal	Cardiac	Smooth
Body Location	Attached to bones or skin	Walls of the heart	Walls of hollow organs (except heart)
Cell Shape & Appearance	Long, cylindrical, multinucleate, striated	Branching chains, uni- or binucleate, striated	Spindle-shaped, uninucleate, no striations

Structure of Skeletal Muscle

Connective Tissue Sheaths

Skeletal muscle is organized into bundles and surrounded by connective tissue sheaths:

• Epimysium: Dense irregular connective tissue, surrounds the entire muscle.

• Perimysium: Dense irregular connective tissue, surrounds fascicles (bundles of muscle fibers).

• Endomysium: Areolar connective tissue, surrounds individual muscle fibers.

Muscle Attachments

Muscles attach to bones at two points:

• Origin: Attachment to immovable or less movable bone.

• Insertion: Attachment to movable bone.

Attachments can be direct (muscle to bone) or indirect (via tendons or aponeuroses).

Skeletal Muscle Fiber Structure

Skeletal muscle fibers are long, cylindrical, multinucleate cells. The sarcolemma is the plasma membrane, and the sarcoplasm is the cytoplasm, containing glycosomes (glycogen storage) and myoglobin (oxygen storage). Specialized structures include myofibrils, sarcoplasmic reticulum, and T tubules.

Myofibrils and Sarcomeres

Myofibril Structure

Myofibrils are rod-like elements that make up most of the muscle cell volume. They contain repeating units called sarcomeres, the smallest contractile units of muscle. Striations are due to the arrangement of thick (myosin) and thin (actin) filaments.

Sarcomere Organization

A sarcomere extends from one Z disc to the next. Thick filaments are anchored to the M line and indirectly to Z discs by titin protein.

Myofilaments: Actin and Myosin

Myofilaments are organized within the sarcomere:

• Actin: Thin filaments

• Myosin: Thick filaments

Sarcoplasmic Reticulum and T Tubules

The sarcoplasmic reticulum (SR) is a network of smooth endoplasmic reticulum surrounding each myofibril, storing calcium ions. T tubules are invaginations of the sarcolemma that conduct impulses deep into the muscle fiber. The triad consists of a T tubule and two terminal cisterns of the SR.

Sliding Filament Model of Contraction

Mechanism of Contraction

During contraction, thin filaments slide past thick filaments, increasing their overlap but not changing their length. This sliding shortens the muscle fiber.

• I bands shorten

• Z discs move closer together

• H zones disappear

• A bands move closer together

Excitation-Contraction Coupling

Ion Channels in Muscle Contraction

Muscle contraction is regulated by ion channels:

• Chemically gated ion channels: Open in response to neurotransmitters (e.g., acetylcholine).

• Voltage-gated ion channels: Open in response to changes in membrane potential.

Neuromuscular Junction (NMJ)

The NMJ is where the motor neuron meets the skeletal muscle fiber. The axon terminal releases acetylcholine (ACh) into the synaptic cleft, which binds to receptors on the sarcolemma, initiating muscle contraction.

Steps of Skeletal Muscle Contraction

1. Events at the neuromuscular junction: Action potential arrives, Ca2+ channels open, ACh is released, binds to receptors, and triggers end plate potential.

2. Muscle fiber excitation: End plate potential triggers an action potential across the sarcolemma.

3. Excitation-contraction coupling: Action potential travels down T tubules, causing Ca2+ release from SR, which binds to troponin, exposing myosin binding sites.

4. Cross bridge cycle: Myosin heads bind to actin, perform power stroke, detach with ATP, and reset.

Muscle Twitch and Graded Contractions

Muscle Twitch

A muscle twitch is the response of a muscle fiber to a single action potential. It consists of three phases:

• Latent period: Excitation-contraction coupling

• Contraction period: Cross bridge formation

• Relaxation period: Ca2+ reentry into SR

Graded Muscle Contractions

Muscle contractions are graded by changing the frequency and strength of stimulation. Temporal (wave) summation occurs when stimuli are delivered in rapid succession, leading to increased force. Sustained contractions can result in unfused (incomplete) or fused (complete) tetanus.

Muscle Fiber Types

Types of Skeletal Muscle Fibers

Characteristic	Slow Oxidative	Fast Oxidative	Fast Glycolytic
Speed of contraction	Slow	Fast	Fast
ATP synthesis	Aerobic	Aerobic/anaerobic	Anaerobic glycolysis
Myoglobin	High	High	Low
Fatigue resistance	High	Intermediate	Low
Best for	Endurance	Sprinting/walking	Short, powerful movements

Smooth Muscle

Structure and Function

Smooth muscle is found in the walls of hollow organs. It has two layers: longitudinal (shortens organ) and circular (constricts lumen). Smooth muscle cells are spindle-shaped, uninucleate, and lack striations.

Comparison of Skeletal and Smooth Muscle

Characteristic	Skeletal	Smooth
Connective tissue	Epimysium, perimysium, endomysium	Endomysium only
Myofibrils/sarcomeres	Yes	No (dense bodies instead)
T tubules	Yes	No (caveolae instead)
SR	Elaborate	Less developed
Gap junctions	No	Yes (unitary muscle)
Regulation	Voluntary (somatic NS)	Involuntary (autonomic NS, hormones, stretch)
Calcium source	SR	Extracellular fluid and SR
Calcium regulation	Troponin	Calmodulin
Contraction speed	Fast	Very slow