We will now look more closely at a muscle fiber, keeping in mind that there are thousands of these cylindrical cells in one muscle. Each muscle fiber has its own motor nerve ending; the neuromuscular junction is where the motor neuron terminates on the muscle fiber. The axon terminal is the enlarged tip of the motor neuron; it contains sacs of the neurotransmitter acetylcholine (ACh). The membrane of the muscle fiber is the sarcolemma, which contains receptor sites for acetylcholine, and an inactivator called cholinesterase. The synapse (or synaptic cleft) is the small space between the axon terminal and the sarcolemma.

Within the muscle fiber are thousands of individual contracting units called sarcomeres, which are arranged end to end in cylinders called myofibrils. The Z lines are the end boundaries of a sarcomere. Filaments of the protein myosin are in the center of the sarcomere, and filaments of the protein actin are at the ends, attached to the Z lines. Myosin filaments are anchored to the Z lines by the protein titin.

Myosin and actin are the contractile proteins of a muscle fiber. Their interactions produce muscle contraction. Also present are two inhibitory proteins, troponin and tropomyosin, which are part of the actin filaments and prevent the sliding of actin and myosin when the muscle fiber is relaxed.

Surrounding the sarcomeres is the sarcoplasmic reticulum, the endoplasmic reticulum of muscle cells. The sarcoplasmic reticulum is a reservoir for calcium ions (Ca+2), which are essential for the contraction process.

All of these parts of a muscle fiber are involved in the contraction process. Contraction begins when a nerve impulse arrives at the axon terminal and stimulates the release of acetylcholine. Acetylcholine generates electrical changes (the movement of ions) at the sarcolemma of the muscle fiber. These electrical changes initiate a sequence of events within the muscle fiber that is called the sliding filament mechanism of muscle contraction. We will begin our discussion with the sarcolemma.

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