Action potential Motor neuron- releases acetylcholine at neuromuscular junction ◦ ACh depolarizes the postsynaptic muscle cell creating the action potential ◦ The action potential spreads to the T (transverse) tubule ◦ T tubule is connected to the sarcoplasmic reticulum ◦ Efflux of calcium ions is caused by a change in the permeability of sarcoplasmic reticulum due to the action potential
Overview of muscle contraction Synaptic terminal of motor neuron releases Ach ACh depolarizes postsynaptic muscle cell Action potential is generated in the muscle cell Action potential spreads through T tubules Sarcoplasmic reticulum becomes permeable to calcium ions Calcium ions bind to troponin Troponin releases the active site of actin by moving the tropomyosin
Overview of muscle contraction Myosin head with attached ATP is under low energy state ATP is hydrolyzed creating a high energy state myosin head Cross-bridge is formed Release of energy relaxes the myosin molecule Another ATP molecule detaches the myosin from actin Contraction occurs as the myosin head walks on the thin filament
End of contraction • End of contraction occurs if calcium ion concentration in the cytosol falls leading to the blockage of the active site of actin
The all-or-none response At the cellular level, contraction is an all- or-none response Frequency of action potential creates the graded contraction Summation of action potential occurs Tetanus- a smooth and sustained contraction
Muscle tone • Muscular contractions do not always cause actual movement. • A small fraction of the total number of fibers in most muscles are usually contracting. • This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. • This slight continuous contraction is called muscle tone.
Muscle fatigue • ATP -energy • Oxygen - respiration • Anaerobic respiration- absence oxygen • Lactic acid accumulation - fatigue • Muscle recovery – removal of lactic acid and conversion to pyruvic acid
Action of skeletal muscle • Antagonistic • Isotonic contraction- uniform contraction with movement of body • Isometric contraction- contraction of muscle without shortening of muscle
Answer the Questions 1. Describe the three types of muscles. Draw labeled diagrams. 5 marks 2. Explain Sliding filament theory of muscle contraction. 5 marks Write notes on :- a) Neuromuscular junction b) Muscle tone c) Muscle fatigue
• d) Action of skeletal muscles • e) Muscle origin and insertion
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