Chapter 11: Muscular Tissue – Structure, Function, and Physiology

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Muscle Cells

Types and Characteristics of Muscular Tissue

Muscle tissue is essential for movement in all living organisms. Muscle cells convert the chemical energy of ATP into mechanical energy, enabling contraction and movement. There are three main types of muscle tissue: skeletal, cardiac, and smooth muscle.

Skeletal muscle: Voluntary, striated, attached to bones.
Cardiac muscle: Involuntary, striated, found only in the heart.
Smooth muscle: Involuntary, non-striated, found in walls of hollow organs.

The physiology of skeletal muscle underlies warm-up, quickness, strength, endurance, and fatigue.

Characteristics of Muscle

Responsiveness (Excitability): Ability to respond to chemical signals, stretch, and electrical changes across the plasma membrane.
Conductivity: Local electrical change triggers a wave of excitation that travels along the muscle fiber.
Contractility: Muscle shortens when stimulated.
Extensibility: Muscle can be stretched between contractions.
Elasticity: Muscle returns to its original resting length after being stretched.

Skeletal Muscle

Microscopic Anatomy of Skeletal Muscle

Skeletal muscle is composed of long, cylindrical cells called muscle fibers (or myofibers), which can be up to 30 cm in length. These fibers exhibit striations, which are alternating light and dark transverse bands resulting from the overlapping of internal contractile proteins.

Voluntary control: Skeletal muscle is usually subject to conscious control.

Connective Tissue Elements

Connective tissues organize and support muscle fibers:

Tendons: Attach muscle to bone matrix.
Endomysium: Surrounds individual muscle cells.
Perimysium: Surrounds muscle fascicles (bundles of fibers).
Epimysium: Surrounds the entire muscle.
Collagen: Provides extensibility and elasticity, resists excessive stretching, and contributes to muscle efficiency.

Table: Structure and Organization of Skeletal Muscle

Structure & Organizational Level	Description	Connective Tissue Wrapping
Muscle (organ)	Consists of hundreds to thousands of muscle cells, plus connective tissue, blood vessels, and nerve fibers	Epimysium
Fascicle	Discrete bundle of muscle cells, segregated from the rest of the muscle by a connective tissue sheath	Perimysium
Muscle fiber (cell)	Elongated multinucleate cell; has a banded (striated) appearance	Endomysium

Structure of a Skeletal Muscle Fiber

Sarcolemma: Plasma membrane of a muscle fiber.
Sarcoplasm: Cytoplasm of a muscle fiber, contains glycogen and myoglobin.
Myofibrils: Long protein bundles that occupy most of the sarcoplasm.
Multiple nuclei: Flattened nuclei pressed against the inside of the sarcolemma.
Mitochondria: Packed between myofibrils, provide energy.
Sarcoplasmic reticulum (SR): Smooth ER that forms a network around each myofibril, stores calcium.
Terminal cisternae: Dilated end-sacs of SR, cross muscle fiber from one side to the other.
T tubules: Tubular infoldings of the sarcolemma, penetrate through the cell and emerge on the other side.
Triad: A T tubule and two terminal cisterns.

Myofilaments

Thick Myofilaments

Composed of several hundred myosin molecules.
Myosin is shaped like a golf club, with two intertwined chains forming a shaft-like tail and a double globular head.
Heads are directed outward in a helical array around the bundle, with a bare zone in the middle.

Thin Myofilaments

Fibrous (F) actin: Two intertwined strands; each strand is a string of globular (G) actin subunits with an active site for myosin binding.
Tropomyosin: Blocks 6 or 7 active sites on G actin subunits.
Troponin: Small, calcium-binding protein on each tropomyosin molecule.

Regulatory and Contractile Proteins

Contractile proteins: Myosin and actin, responsible for muscle contraction.
Regulatory proteins: Tropomyosin and troponin, determine when a fiber can contract.
Contraction is activated by release of calcium into sarcoplasm and its binding to troponin, which changes shape and moves tropomyosin off the active sites on actin.

Overlap of Thick and Thin Filaments

Thick and thin filaments overlap in a sarcomere, with a bare zone in the middle of the thick filament where no heads are present.

Accessory Proteins

Accessory proteins anchor myofilaments, regulate their length, and align them for maximum effectiveness.
Dystrophin: Links actin in outermost myofilaments to transmembrane proteins and the endomysium; transfers force of muscle contraction to connective tissue. Genetic defects in dystrophin cause muscular dystrophy.

Striations and Sarcomeres

Striations

Myosin and actin are proteins found in all cells, but are organized in a precise way in skeletal and cardiac muscle.
A band: Dark band where thick and thin filaments overlap.
I band: Light band, contains only thin filaments.
Z disc: Provides anchorage for thin and elastic filaments.
Sarcomere: Segment from one Z disc to the next; functional contractile unit of muscle fiber.

Sarcomeres

Muscle cells shorten because their individual sarcomeres shorten.
Z discs are pulled closer together as thick and thin filaments slide past each other.
Neither thick nor thin filaments change length during shortening; only the amount of overlap changes.
Dystrophin and linking proteins also pull on extracellular proteins during shortening, transferring pull to extracellular tissue.

Nerve-Muscle Relationship

Motor Units

Motor unit: One nerve fiber and all the muscle fibers it innervates.
Muscle fibers of one motor unit are dispersed throughout the muscle and contract in unison.
Average motor unit contains about 200 muscle fibers.
Small motor units (3-6 fibers per neuron) allow fine control (e.g., eye and hand muscles).
Large motor units (up to 1000 fibers per neuron) provide more strength than control (e.g., gastrocnemius).

The Neuromuscular Junction

Synapse: Point where a nerve fiber meets its target cell.
Neuromuscular junction (NMJ): Synapse where the target cell is a muscle fiber.
Each terminal branch of the nerve fiber within the NMJ forms a separate synapse with the muscle fiber.
One nerve fiber stimulates the muscle fiber at several points within the NMJ.

Components of Neuromuscular Junction

Synaptic knob: Swollen end of nerve fiber, contains synaptic vesicles filled with acetylcholine (ACh).
Synaptic cleft: Tiny gap between synaptic knob and muscle sarcolemma.
Schwann cell: Envelops and isolates all of the NMJ from surrounding tissue fluid.
Synaptic vesicles undergo exocytosis, releasing ACh into synaptic cleft.
50 million ACh receptors are incorporated into muscle cell plasma membrane, especially in junctional folds beneath the synaptic knob.
Basal lamina: Thin layer of collagen and glycoprotein separating Schwann cell and entire muscle cell from surrounding tissues; contains acetylcholinesterase (AChE) that breaks down ACh after contraction, causing relaxation.

Neuromuscular Toxins

Toxins that interfere with synaptic function can paralyze muscles.
Cholinesterase inhibitors: Found in some pesticides, prevent breakdown of ACh, causing spastic paralysis.
Tetanus: Caused by Clostridium tetani toxin, blocks glycine release in spinal cord, leading to spastic paralysis.
Flaccid paralysis: Muscles are limp and cannot contract; caused by curare (competes with ACh for receptor sites) and botulism (Clostridium botulinum toxin blocks ACh release).
Botox: Cosmetic injections use botulinum toxin to cause localized muscle relaxation.

Electrophysiology of Muscle Cells

Electrically Excitable Cells

Muscle fibers and neurons are electrically excitable; their plasma membranes exhibit voltage changes in response to stimulation.
Electrophysiology: Study of electrical activity of cells.
In an unstimulated (resting) cell:
- Plasma membrane is polarized (charged).
- Excess sodium ions (Na+) in extracellular fluid (ECF).
- Excess potassium ions (K+) in intracellular fluid (ICF).
- Resting membrane potential (RMP) is about -90 mV, maintained by sodium-potassium pump.

Action Potentials

When stimulated, ion gates open in the plasma membrane.
Na+ diffuses into the cell, depolarizing the membrane.
Na+ gates close and K+ gates open; K+ rushes out, repolarizing the membrane.
Action potential: Quick up-and-down voltage shift from negative RMP to positive value and back.
Action potentials propagate along the plasma membrane, triggering subsequent action potentials.

Excitation of a Muscle Fiber

Steps of Excitation

Step 1: Nerve signal opens voltage-gated calcium channels in synaptic knob.
Step 2: Calcium stimulates exocytosis of ACh from synaptic vesicles; ACh released into synaptic cleft.
Step 3: Two ACh molecules bind to each receptor protein, opening Na+ and K+ channels.
Step 4: Na+ enters, shifting RMP from -90 mV to +75 mV; K+ exits, RMP returns to -90 mV. This quick voltage shift is called an end-plate potential (EPP).
Step 5: Voltage change (EPP) in end-plate region opens nearby voltage-gated channels, producing an action potential that spreads over the muscle surface.

Equation: Resting Membrane Potential

The resting membrane potential is maintained by the sodium-potassium pump:

Example: Neuromuscular Junction

At the neuromuscular junction, the arrival of a nerve impulse leads to the release of ACh, which binds to receptors on the muscle fiber, initiating a sequence of electrical events that result in muscle contraction.

Additional info:

Further steps in muscle contraction include excitation-contraction coupling, contraction, and relaxation, involving calcium release, cross-bridge cycling, and ATP utilization.