Muscle Function Overview

Muscle Function

• Skeletal Muscle: Attached to bones, voluntary control, responsible for movement, posture, respiration
• Smooth Muscle: Found in the walls of hollow organs and blood vessels, involuntary control, responsible for movement of substances and blood flow regulation
• Cardiac Muscle: Located in the heart, involuntary control, responsible for pumping blood

Muscle System Functions

• Body Movement: Voluntary movements like walking and running
• Posture Maintenance: Maintains muscle tone for stability
• Respiration: Muscles in the thorax assist with breathing
• Heat Production: Muscle contractions generate heat for temperature regulation
• Communication: Facial expressions and speech
• Organ Constriction: Smooth muscles control movement within organs and blood vessels
• Heartbeat Regulation: Cardiac muscle drives heartbeats

Neuromuscular Junction (NMJ)

• Connects a motor neuron to a muscle fiber
• Composed of the presynaptic terminal, synaptic cleft, and motor end-plate
• Neurotransmitter Release: Acetylcholine (ACh) is released into the synaptic cleft, binding to receptors on the muscle fiber, causing depolarization
• Acetylcholinesterase: Breaks down ACh to prevent continuous stimulation

Excitation-Contraction Coupling

• Action Potential Propagation: Action potentials travel along the sarcolemma and T tubules, triggering Ca2+ release from the sarcoplasmic reticulum (SR)
• Calcium Binding: Ca2+ binds to troponin on actin, moving the troponin-tropomyosin complex to expose myosin binding sites

Cross-Bridge Cycle

• Formation and Movement: Myosin heads attach to actin, perform a power stroke, pulling filaments towards the sarcomere center, and release. ATP is needed for each cycle to detach the myosin head and reset it
• Energy Use: ATP is essential for cross-bridge formation and movement. It also allows the myosin head to return to its original position

Muscle Relaxation

• Calcium Reuptake: Ca2+ is actively transported back into the SR, decreasing its concentration in the sarcoplasm
• Troponin-Tropomyosin Complex: Returns to its original position, covering the myosin binding sites on actin, resulting in muscle relaxation due to decreased cross-bridge cycling