CHAPTER 8: MUSCULAR SYSTEM

These flashcards cover key terms and definitions related to the muscular system, including muscle types, structures, and functions.

Muscle

An organ composed of muscle tissue, connective tissue, nerves, and blood vessels.

Functions of the muscular system

Movement, maintaining posture, stabilizing joints, generating heat, and protecting internal organs.

Elasticity

The quality of muscle tissue that allows it to return to its original length after contraction.

Extensibility

The ability of muscle tissue to stretch or extend.

Contractility

The ability of muscle tissue to pull on attachment points and shorten with force.

Excitability

The capacity of muscle tissue to receive and respond to electrical stimuli.

Skeletal Muscle

Muscle primarily associated with and attaches to bones, appearing long, cylindrical, and with striations, under voluntary control.

Cardiac Muscle

Muscle found exclusively in the heart, contains striated, branched cells with intercalated discs, and is involuntary.

Smooth Muscle

Muscle found in walls of hollow internal organs, spindle-shaped, lacks striations, and is involuntary.

Connective Tissue Coverings

These layers provide support, protect, and transmit tension

Epimysium

The outermost connective tissue layer that surrounds the entire muscle.

Perimysium

Connective tissue that divides the muscle into bundles of muscle fibers called fascicles.

Endomysium

The innermost layer that surrounds individual muscle fibers within a fascicle.

Tendons

Connective tissue structures that attach muscle to bone and are continuous with the periosteum.

Muscle fiber

A single, elongated skeletal muscle cell.

Sarcolemma

The plasma membrane of the muscle fiber. Contains invaginations called Ttubules (transverse tubules) that extend deep into the cell, carrying electrical signals (action potentials).

Sarcoplasm

The cytoplasm of the muscle fiber. It contains abundant glycogen (for glucose storage) and myoglobin (for oxygen storage).

Sarcoplasmic Reticulum (SR)

A specialized smooth endoplasmic reticulum. It surrounds each myofibril and functions as the primary storage and release site for calcium ions (Ca²⁺)

Sarcomere

The fundamental functional unit of skeletal muscle contraction, extending from one Z-line to the next.

Myofibrils

Rod-like structures within muscle fibers containing sarcomeres.

A-band

The dark central region of the sarcomere containing thick (myosin) filaments.

I-band

The light band of the sarcomere that contains only thin (actin) filaments.

H-zone

The central part of the A-band, containing only thick filaments (no overlap with thin filaments) when the muscle is relaxed. It disappears during contraction.

M-line

A protein line in the middle of the H-zone, anchors thick filaments.

Z-line

Dark line in the middle of the I-band that anchors thin filaments.

Thick filaments

Composed primarily of the protein myosin. Myosin heads form cross bridges during contraction. They are thicker than thin filaments.

Thin filaments

Composed of three proteins: actin, troponin, and tropomyosin.

Actin

provides the binding sites for myosin heads

Tropomyosin

covers the myosin-binding sites on actin in a relaxed muscle

Muscle contraction

is initiated by a nerve signal

Motor Neuron

A nerve cell that stimulates skeletal muscle fibers.

Neuromuscular Junction (NMJ)

The specialized synapse between a motor neuron and a skeletal muscle fiber.

Presynaptic terminal (axon terminal)

The end of the motor neuron axon where neurotransmitters are released.

Synaptic cleft

The space between the neuron terminal and the muscle fiber

Motor end plate

A specialized region of the sarcolemma that contains receptors for neurotransmitters.

Acetylcholine (ACh)

When an action potential reaches the motor neuron terminal, ACh is released into the synaptic cleft.

Membrane Potential

The difference in electrical charge across the plasma membrane of a cell.

Resting Membrane Potential

In a resting muscle fiber, the inside of the sarcolemma is more negative than the outside, typically -70 to -90 mV. This is maintained by the Na⁺/K⁺ pump.

Action Potential (AP)

A rapid, brief reversal of membrane potential that propagates along the sarcolemma.

Depolarization

When ACh binds to receptors on the motor end plate, voltage-gated Na⁺ channels open, allowing Na⁺ ions to rush into the cell

Repolarization

Immediately after depolarization, voltage-gated K⁺ channels open, and K⁺ ions rush out of the cell, restoring the negative charge inside

Cross-Bridge Formation

An energized myosin head (containing ADP and Pi from previous ATP hydrolysis) binds to an exposed active site on the actin filament, forming a cross-bridge.

Power Stroke

The release of ADP and inorganic phosphate (Pi) from the myosin head triggers a conformational change

Cross-Bridge Detachment

A new molecule of ATP binds to the myosin head. This binding causes the myosin head to detach from the actin filament.

Reactivation of Myosin Head ("Cocking")

The newly bound ATP is immediately hydrolyzed into ADP and Pi by the ATPase enzyme on the myosin head.

Myosin Head Detachment

ATP binding is required for the myosin head to detach from actin after a power stroke. Without new ATP (e.g., in rigor mortis), myosin heads remain locked to actin.

Myosin Head Re-energization/Cocking

The hydrolysis of ATP (into ADP + Pi) provides the energy to "cock" or re-energize the myosin head, preparing it for the next cross bridge formation and power stroke.

Calcium Reuptake

ATP is also required by the Ca²⁺ pumps in the sarcoplasmic reticulum to actively transport Ca²⁺ back into the SR, which is necessary for muscle relaxation

Sarcomere Shortening

The repeated cycles of cross-bridge formation, power stroke, detachment, and re-cocking cause the thin (actin) filaments to slide past the thick (myosin) filaments. This sliding filament model leads to the shortening of the sarcomeres.

Oxygen Debt (EPOC)

The extra oxygen required post-exercise to restore metabolic conditions to pre-exercise levels.

This extra oxygen is used to: Replenish ATP and creatine phosphate stores. Convert lactic acid back into pyruvate or glucose in the liver. Replenish oxygen stores in myoglobin.

Myoglobin

An oxygen-binding protein found in muscle cells. It stores oxygen within the muscle fiber, providing an immediate oxygen reserve for aerobic respiration. Muscles rich in myoglobin appear red.

Muscle Twitch

A single, brief contraction of a muscle fiber or a whole muscle in response to an adequate stimulus.

CARDIAC MUSCLE TISSUE

Striated: Like skeletal muscle, it has a banded appearance due to sarcomeres.

Branched cells: Individual cells are often branched, forming a network.

Single nucleus: Typically, each cell has one central nucleus.

Intercalated Discs: Unique to cardiac muscle, these specialized cell junctions connect adjacent cardiac muscle cells.

CARDIAC MUSCLE TISSUE

Striated: Like skeletal muscle, it has a banded appearance due to sarcomeres.

Branched cells: Individual cells are often branched, forming a network.

Single nucleus: Typically, each cell has one central nucleus. Intercalated Discs: Unique to cardiac muscle, these specialized cell junctions connect adjacent cardiac muscle cells.

Involuntary Control: Regulated by the autonomic nervous system and intrinsic pacemaker cells. Pacemaker cells initiate their own action potentials.

SMOOTH MUSCLE TISSUE

Non-striated ("Smooth"): Lacks the regular arrangement of sarcomeres, so it does not appear banded.

Spindle-shaped cells: Elongated with pointed ends. Single central nucleus.

No T-tubules: Instead, it has caveolae (pouch-like invaginations of the sarcolemma) that hold Ca²⁺ ions. Contraction Mechanism: Still uses actin and myosin, but they are arranged diagonally, allowing for a corkscrew-like contraction that shortens and twists the cell.

Involuntary Control: Regulated by the autonomic nervous system, hormones, and local chemical factors.

Slow, Sustained Contractions: Can maintain muscle tone for long periods without fatigue due to the low ATP consumption.

Stretches and Relaxes: Can stretch considerably and still generate effective contractions exhibiting stress-relaxation response.

Sliding and Shortening: Once the myosin heads are activated, they interact with the actin filaments, causing the thin and thick filaments to slide past each other, similar to the sliding filament model, but without the defined sarcomere structure.

Pacemaker Cells

Cells that initiate their own action potentials, regulating the heart's rhythm.

Fascicle

A bundle of muscle fibers.

Prime Mover (Agonist)

The muscle that has the primary responsibility for producing a specific movement.

Antagonist

A muscle that opposes or reverses a movement produced by the prime mover.

Synergist

A muscle that assists the prime mover in performing a movement.

Parallel

Fascicles run parallel to the long axis of the muscle.

Fusiform

A type of parallel arrangement where the muscle has an expanded central belly and tapers at the ends (e.g., biceps brachii).

Circular

Fascicles are arranged in concentric rings, forming sphincters that close openings.

Convergent

Broad origin, fascicles converge toward a single tendon of insertion, giving the muscle a triangular or fan shape.