Muscle Tissue and Physiology

Muscle Tissue: Structure and Types

Muscle tissue is specialized for contraction and is essential for movement, posture, and many physiological processes. There are three main types of muscle tissue, each with unique structural and functional characteristics.

• Myocyte: A muscle cell; the basic cellular unit of muscle tissue, capable of contraction.

• Excitable Cell: A cell that can respond to electrical stimuli by generating action potentials.

• Myofilament: Protein filaments (actin and myosin) within muscle cells responsible for contraction.

• Striated Muscle Cell: Muscle cells with a banded appearance due to the arrangement of myofilaments (found in skeletal and cardiac muscle).

• Striation: The alternating light and dark bands seen in striated muscle cells under a microscope.

• Smooth Muscle Cell: Non-striated, spindle-shaped muscle cells found in the walls of hollow organs.

Types of Muscle Tissue

• Skeletal Muscle Tissue: Voluntary, striated muscle attached to bones; responsible for body movement.

• Muscle Fiber: Another term for a skeletal muscle cell; long, cylindrical, multinucleated.

• Cardiac Muscle Tissue: Involuntary, striated muscle found only in the heart.

• Cardiac Muscle Cell: Branched, striated cells with one or two nuclei; interconnected by intercalated discs.

• Intercalated Disc: Specialized junctions between cardiac muscle cells that allow rapid transmission of electrical impulses and mechanical connection.

• Smooth Muscle Tissue: Involuntary, non-striated muscle found in the walls of hollow organs (e.g., intestines, blood vessels).

Properties of Muscle Cells

Muscle cells possess unique properties that enable their function:

• Contractility: The ability to shorten forcefully when stimulated.

• Excitability: The ability to respond to stimuli by generating action potentials.

• Conductivity: The ability to conduct electrical signals along the cell membrane.

• Distensibility: The ability to stretch without being damaged.

• Elasticity: The ability to return to original length after stretching or contracting.

Skeletal Muscle Structure

Skeletal muscle is organized into bundles and connective tissue layers for strength and function.

• Muscle Fascia: Dense connective tissue that surrounds and separates muscles.

• Epimysium: Outer layer of connective tissue surrounding the entire muscle.

• Fascicle: A bundle of muscle fibers within a muscle.

• Perimysium: Connective tissue surrounding each fascicle.

• Endomysium: Thin connective tissue surrounding each muscle fiber.

Skeletal Muscle Fibers: Internal Structure

• Sarcolemma: The plasma membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and myofibrils.

• Myofibril: Long, cylindrical organelles composed of myofilaments; responsible for muscle contraction.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions.

• Transverse (T-) Tubule: Invaginations of the sarcolemma that conduct action potentials into the muscle fiber.

• Terminal Cisternae: Enlarged areas of the SR adjacent to T-tubules; store calcium ions.

• Triad: A structure formed by a T-tubule and two adjacent terminal cisternae; critical for excitation-contraction coupling.

Myofilaments: Types and Structure

• Thick Filament: Composed mainly of myosin molecules.

• Myosin: Motor protein with a head, neck, tail, and hinge region; binds to actin and hydrolyzes ATP.

• Myosin Head: Binds to actin and ATP; performs the power stroke during contraction.

• Myosin Tail: Forms the central part of the thick filament.

• Myosin Neck: Connects the head and tail; acts as a lever during contraction.

• Myosin Hinge: Flexible region allowing movement of the head.

• Thin Filament: Composed mainly of actin, along with tropomyosin and troponin.

• Actin: Globular protein forming a double helix; contains active sites for myosin binding.

• Active Site: Region on actin where myosin heads bind.

• Tropomyosin: Regulatory protein that covers active sites on actin in resting muscle.

• Troponin: Regulatory protein complex that binds calcium and moves tropomyosin off active sites.

• Elastic Filament: Composed of titin; provides elasticity and stabilizes the thick filament.

• Titin: Large protein that anchors thick filaments to the Z-disc and allows recoil after stretching.

Sarcomeres: Functional Unit of Muscle

The sarcomere is the basic contractile unit of striated muscle, defined by the arrangement of myofilaments.

• Sarcomere: The segment between two Z-discs; the functional unit of muscle contraction.

• I Band: Light band containing only thin filaments (actin).

• A Band: Dark band containing the entire length of thick filaments (myosin), including overlapping thin filaments.

• Z-Disc: Boundary of the sarcomere; anchors thin filaments.

• H Zone: Central region of the A band with only thick filaments.

• M Line: Center of the sarcomere; holds thick filaments together.

• Zone of Overlap: Area where thick and thin filaments overlap, allowing crossbridge formation.

• Sliding Filament Mechanism: Model describing how muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere.

Example: During contraction, the I band and H zone decrease in width, while the A band remains constant.

Membrane Potentials and Action Potentials

Muscle contraction is initiated by changes in the electrical charge across the muscle cell membrane.

• Membrane Potential: The voltage difference across a cell membrane.

• Leak Channel: Ion channel that is always open, allowing ions to move according to their gradients.

• Resting Membrane Potential: The stable, negative charge inside a resting cell, typically around -70 mV in muscle fibers.

• Concentration Gradient: Difference in the concentration of a substance across a membrane.

• Electrical Gradient: Difference in charge across a membrane.

• Electrochemical Gradient: Combined effect of concentration and electrical gradients on ion movement.

• Action Potential: Rapid, temporary reversal of membrane potential that propagates along the cell membrane.

• Depolarization Stage: Membrane potential becomes less negative due to sodium influx.

• Repolarization Stage: Return to resting membrane potential, usually by potassium efflux.

• Sodium-Potassium Pump: Active transport protein that restores ion gradients by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed.

Equation:

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a skeletal muscle fiber, where nerve impulses trigger muscle contraction.

• Axon Terminal (of Motor Neuron): The end of a motor neuron that releases neurotransmitters.

• Synaptic Vesicle: Membrane-bound sacs in the axon terminal containing acetylcholine (ACh).

• Acetylcholine (ACh): Neurotransmitter that binds to receptors on the muscle fiber, initiating contraction.

• Voltage-Gated Calcium Channel: Opens in response to action potential, allowing Ca2+ influx into the axon terminal.

• Synaptic Cleft: Small gap between the axon terminal and the muscle fiber membrane.

• Acetylcholinesterase: Enzyme that breaks down ACh in the synaptic cleft, terminating the signal.

• Motor End Plate (of Muscle Fiber): Specialized region of the sarcolemma with ACh receptors.

• Ligand-Gated Ion Channel: Opens when ACh binds, allowing Na+ influx and depolarization.

Skeletal Muscle Contraction: Phases

Muscle contraction occurs in several coordinated phases:

• Excitation Phase: Action potential from the neuron triggers ACh release and depolarization of the muscle fiber.

• Excitation-Contraction Coupling: The process linking muscle fiber excitation to Ca2+ release and contraction.

• Contraction Phase: Crossbridge cycling occurs, leading to sarcomere shortening.

• Crossbridge: The connection formed when a myosin head binds to actin.

• Power Stroke: The myosin head pivots, pulling actin filaments toward the center of the sarcomere.

• Muscle Relaxation: Ca2+ is pumped back into the SR, crossbridges detach, and the muscle returns to resting length.

Twitch Contraction and Muscle Fiber Types

A muscle twitch is a single contraction in response to a single stimulus. Muscle fibers vary in their contraction speed and fatigue resistance.

• Muscle Twitch: The response of a muscle to a single brief stimulus.

• Refractory Period: Time after an action potential when the muscle cannot respond to another stimulus.

• Latent Period: Delay between stimulus and contraction onset.

• Contraction Period: Time during which the muscle shortens.

• Relaxation Period: Time during which the muscle returns to resting length.

• Fused Tetanus: Sustained contraction with no relaxation between stimuli.

• Unfused Tetanus: Partial relaxation between stimuli; contractions are not fully fused.

• Slow-Twitch (Class I) Fibers: Contract slowly, resist fatigue, rely on aerobic metabolism.

• Fast-Twitch (Class IIa) Fibers: Intermediate speed and fatigue resistance; use both aerobic and anaerobic metabolism.

• Fast-Twitch (Class IIx) Fibers: Contract rapidly, fatigue quickly, rely on anaerobic metabolism.

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• Recruitment: Increasing the number of active motor units to generate more force.

Types of Muscle Contractions

Muscle contractions can be classified based on whether the muscle changes length during contraction.

• Isotonic Concentric Contraction: Muscle shortens as it contracts (e.g., lifting a weight).

• Isotonic Eccentric Contraction: Muscle lengthens while contracting (e.g., lowering a weight).

• Isometric Contraction: Muscle develops tension without changing length (e.g., holding a weight steady).

Smooth Muscle: Structure and Function

Smooth muscle is found in the walls of hollow organs and is responsible for involuntary movements such as peristalsis and flow regulation.

• Smooth Muscle: Non-striated, involuntary muscle tissue found in organs like the intestines and blood vessels.

• Peristalsis: Wave-like contractions that move substances through hollow organs.

• Sphincter: Circular muscle that controls the passage of substances by opening or closing a passageway.

• Flow Regulation: Smooth muscle contraction regulates the flow of substances (e.g., blood, food) through organs.

Table: Comparison of Muscle Tissue Types

Additional info: This guide expands on the vocabulary list by providing definitions, explanations, and examples for each term, as well as a comparative table for muscle tissue types. For exam preparation, students should be able to label diagrams, explain the function of each structure, and describe the physiological processes involved in muscle contraction and relaxation.