Excitation Contraction Coupling in Skeletal Muscle

Throughout Part II of the book, we used the patellar reflex as an example system to explore the cellular signals underlying nervous system function. The final stage of the patellar reflex is the contraction of the quadriceps muscle brought about by the activity of the motor neurons making excitatory synaptic connections with that muscle. The arrival of an action potential in the synaptic terminal of the presynaptic motor neuron causes release of the chemical neurotransmitter, ACh. The ACh in turn depolarizes the end-plate region of the postsynaptic muscle cell, initiating an action potential in the muscle cell. This action potential propagates along the long, thin muscle cell just as the neuron action potential propagates along nerve fibers. The muscle action potential serves as the trigger for contraction of the muscle cell. This chapter will examine the events that intervene between the occurrence of the action potential in the plasma membrane of the muscle cell and the activation of the contraction: the process of excitation-contraction coupling. Then we will move on in Chapter 11 to look at how the motor nervous system is organized to integrate the twitch-like contractions of individual muscle fibers into the smooth and graded contractions of a muscle as a whole.

To begin, it will be useful to examine the structure of the muscle cells at the level of both the light and electron microscopes. We will then consider the molecular makeup of the contractile apparatus and discuss the biochemical mechanisms that control the action of that apparatus. The chapter will conclude with a discussion of how the action potential of the muscle cell is coupled to the contractile machinery to produce the muscle contraction.

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