Muscular System Functions and Types

Muscular System Functions

Movement of the body
Maintaining posture and body position
Respiration, including breathing
Production of heat to regulate body temperature
Communication, through facial expressions and other movements
Heart beat, powered by cardiac muscle
Contraction of internal organs and vessels, such as the intestines and blood vessels

Types of Muscles

Skeletal Muscle: Attached to bones, allowing for voluntary movement.
Cardiac Muscle: Found only in the heart, responsible for involuntary heart contractions.
Smooth Muscle: Lines internal organs and blood vessels, responsible for involuntary movements like digestion and blood vessel constriction.

Abilities of Skeletal Muscles

Contractility: The ability to shorten and generate force.
Excitability: The ability to respond to stimuli, such as nerve impulses.
Extensibility: The ability to stretch or lengthen.
Elasticity: The ability to return to its original shape after stretching.

Skeletal Muscle Characteristics

Makes up 40% of body weight.
Attached to bones, therefore named "skeletal".
Contains multiple nuclei per cell, located near the periphery.
Possesses a striated appearance due to the arrangement of protein filaments.
Longest of the muscle types.

Skeletal Muscle Structures - Connective Tissue Coverings

Epimysium: A layer of connective tissue surrounding the entire skeletal muscle, forming the outermost covering.
Muscle Fasciculus: A bundle of muscle fibers, held together by perimysium.
Perimysium: Connective tissue that surrounds individual muscle fasciculi.
Muscle Fiber: A single skeletal muscle cell, containing multiple nuclei.
Endomysium: Connective tissue that surrounds each individual muscle fiber.

Skeletal Muscle Structures - Muscle Fiber Structure

Myofibril: Thread-like protein structures that make up muscle fibers.
Myofilament: Protein filaments found within myofibrils, such as actin and myosin.
Sarcoplasm: The cytoplasm of a muscle fiber.
Sarcolemma: The cell membrane of a muscle fiber, containing t-tubules.
T-tubules (transverse tubules): Extensions of the sarcolemma that wrap around sarcomeres at the A band, connected to the sarcoplasmic reticulum.
Sarcoplasmic reticulum: A type of smooth endoplasmic reticulum, surrounding myosin filaments, storing and releasing calcium ions (Ca2+).

Skeletal Muscle Structures - Actin and Myosin Myofilaments

Actin: Thin myofilament, resembling two strands of pearls.
Myosin: Thick myofilament, resembling golf clubs.
Troponin: Protein that attaches to actin, serving as the binding site for calcium ions.
Tropomyosin: A filament located in the grooves of actin, providing the attachment site for myosin.

Skeletal Muscle Structures - Sarcomeres

Sarcomere: The functional unit of a muscle fiber responsible for contraction, containing actin and myosin filaments.
Z-disk: Protein fibers that serve as attachment sites for actin filaments.
H-zone: The center of a sarcomere, containing only myosin filaments.
I-band: Region of a sarcomere containing only actin filaments.
A-band: The region where actin and myosin filaments overlap.
M-line: The central point where myosin filaments are anchored.

Resting Membrane Potential

Outside the cell: High concentration of sodium ions (Na+) and a positive charge.
Inside the cell: High concentration of potassium ions (K+) and a negative charge.
Sodium channels are closed: Preventing Na+ from entering the cell.
Potassium channels are partially open: Allowing K+ to diffuse out of the cell.
The inside is negative: Large negative molecules within the cell cannot easily pass through the membrane, resulting in a net negative charge inside the cell.

Action Potential

A rapid depolarization and repolarization of the cell membrane, responsible for muscle contraction.
Occurs due to the changes in ion permeability across the membrane.

Depolarization

A change in charges across the cell membrane.
The inside becomes more positive and the outside becomes more negative.
Sodium channels open, allowing Na+ to rush into the cell.

Repolarization

Sodium channels close, stopping the influx of Na+.
The cell membrane returns to its resting potential.

Sodium-Potassium Pump

Active transport mechanism that pumps Na+ out of the cell and transports K+ into the cell.
Restores the balance of ions across the cell membrane after depolarization and repolarization.

Nerve Supply

Motor neuron: Nerve cells that carry action potentials to stimulate muscle fibers.
Neuromuscular junction (synapse): The specialized site where a motor neuron and muscle fiber meet.
Presynaptic terminal: The end of a motor neuron's axon, responsible for releasing neurotransmitters.
Postsynaptic membrane: The membrane of the muscle fiber, containing receptors for neurotransmitters.
Synaptic cleft: The narrow gap between the presynaptic terminal and the postsynaptic membrane.
Synaptic vesicle: Small sacs within the presynaptic terminal that store and release neurotransmitters.

Neurotransmitter

Chemicals released by nerve endings to stimulate or inhibit muscle fibers.
Acetylcholine is a primary neurotransmitter at the neuromuscular junction.

Motor Unit

A group of muscle fibers innervated by a single motor neuron.

Steps in a Muscle Contraction (Sliding Filament Theory)

Action potential travels down motor neuron: The action potential reaches the presynaptic terminal of the motor neuron.
Calcium channels open: The action potential triggers the opening of calcium ion (Ca2+) channels in the presynaptic terminal.
Synaptic vesicles release acetylcholine: Ca2+ influx causes synaptic vesicles to release acetylcholine into the synaptic cleft.
Acetylcholine binds to receptors: Acetylcholine binds to receptors on the postsynaptic membrane of the muscle fiber, opening sodium channels.
Sodium enters muscle fiber: Sodium ions (Na+) rush into the muscle fiber, causing depolarization of the sarcolemma.
Depolarization spreads through T-tubules: The depolarization spreads through the t-tubules, reaching the sarcoplasmic reticulum.
Calcium released from sarcoplasmic reticulum: Depolarization triggers the release of Ca2+ from the sarcoplasmic reticulum.
Calcium binds to troponin: Released Ca2+ binds to troponin, causing a conformational change.
Tropomyosin moves: The change in troponin shifts tropomyosin, exposing the binding sites for myosin on actin filaments.
Myosin binds to actin: Myosin heads attach to actin filaments.
Power stroke: Myosin heads pivot and pull on actin filaments, sliding them towards the center of the sarcomere.
ATP hydrolysis: The movement of the myosin heads requires the hydrolysis of ATP, providing energy for the power stroke.
Sarcomere shortens: The sliding of actin filaments over myosin filaments causes sarcomeres to shorten, resulting in muscle contraction.
Calcium is reabsorbed: Ca2+ is actively reabsorbed back into the sarcoplasmic reticulum.
Tropomyosin blocks binding sites: As Ca2+ concentration decreases, tropomyosin returns to its blocking position on actin filaments, preventing myosin binding.
Muscle relaxes: Without myosin binding, the sarcomere lengthens, and the muscle relaxes.