physiology of muscles

PHYSIOLOGY OF MUSCLE

Muscle Function
Skeletal muscles are responsible for all locomotion Cardiac muscle is responsible for coursing the blood through the body Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs Muscles also maintain posture, stabilize joints, and generate heat

Types of Muscles

Action Potential: Electrical Conditions of a Polarized Sarcolemma
The outside (extracellular) face is positive, while the inside face is negative This difference in charge is the resting membrane potential

Action Potential: Electrical Conditions of a Polarized Sarcolemma

The predominant extracellular ion is Na+ The predominant intracellular ion is K+ The sarcolemma is relatively impermeable to both ions

Action Potential: Depolarization and Generation of the Action Potential

An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become permeable to Na+ (sodium channels open)

Action Potential: Depolarization and Generation of the Action Potential

Na+ enters the cell, and the resting potential is decreased (depolarization occurs) If the stimulus is strong enough, an action potential is initiated

Action Potential: Propagation of the Action Potential

Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch Voltage-regulated Na+ channels now open in the adjacent patch causing it to depolarize

Action Potential: Propagation of the Action Potential

Thus, the action potential travels rapidly along the sarcolemma Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle

Action Potential: Repolarization

Immediately after the depolarization wave passes, the sarcolemma permeability changes Na+ channels close and K+ channels open K+ diffuses from the cell, restoring the electrical polarity of the sarcolemma

Motor Unit: The Nerve-Muscle

Contraction of Sarcomere

MUSCLE PHYSIOLOGY

SKELETAL MUSCLE Summary of Contraction Sequence (cont) Ca++ binds to troponin-tropomyosin Myosin heads bind to actin Myosin-ATPase activated Cross-bridges attach & detach Myosin & actin slide past each other Sarcoplasmic reticulum pumps Ca++ back into lumen Ca++ removed from tropin-tropomyosin ` complex & actin-myosin interaction inhihited

MUSCLE PHYSIOLOGY

SKELETAL MUSCLESingle Action Potential  muscle twitch, brief contraction followed by relaxationTwitch starts 2 msec after depolarization of membrane begins, i.e. during re-polarization. Action Potential
Muscle Twitch
Contraction Time
Relative Timing of AP & Muscle Contraction
Start of Electrical Response
Peak of Contraction

MUSCLE PHYSIOLOGY

SKELETAL MUSCLE Summary of Contraction SequenceAP get to end of axon ACh released at neuromuscular junction ACh diffuse across gap Nicotinic ACh receptor at end-plates react with AChMuscle cell membrane depolarized AP travel along muscle cell membrane T tubule depolarization, travel to sarcoplasmic reticulum  Ca++ release into cytoplasm

MUSCLE PHYSIOLOGY

Tetanus
Unfused Tetanus (clonus)
Response to 2 stimuli
Single Twitch
0.5
1.0
1.5
2
Peak Tension (%)
25
50
Time
Tetanus
Twitch Amplitude & Relative Timing & Amplitude for Force Generated
100

Types of Contractions

Muscles of Mastication

Muscles of Mastication

MUSCLE PHYSIOLOGY
Tetanus Summation (fusion) of Contractions Result from high frequency neural stimulation over short period of time Cause partly because elastic elements have been fully stretched from early contractions hence maximum tension develop with no time for relaxation of fibers

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