V4

10 msec

Stimulator

Motor Axon

Muscle Cell

FIGURE 6 Propagation of the EPP. A muscle cell is impaled at multiple points from the end plate, and the motor axon is stimulated. The amplitude of the EPP is maximal at the motor end plate. As recordings are made more distant from the end plate, the amplitude of the EPP becomes smaller. (Modified from Fatt P, Katz B. J Physiol 1951; 115:320-370.)

Motor Axon

Muscle Cell

FIGURE 6 Propagation of the EPP. A muscle cell is impaled at multiple points from the end plate, and the motor axon is stimulated. The amplitude of the EPP is maximal at the motor end plate. As recordings are made more distant from the end plate, the amplitude of the EPP becomes smaller. (Modified from Fatt P, Katz B. J Physiol 1951; 115:320-370.)

1936, Dale and his colleagues found that electrical stimulation of motor axons led to an increase in the concentration of a substance in the perfusing solution, which they identified as ACh. When the isolated ACh was injected into the arterial supply it was capable of producing large muscular contractions. Subsequent studies on the transmitter substance used by the neuromuscular junction have confirmed and greatly extended the original observations. As a result, it is now possible to describe the total sequence of events underlying synaptic transmission at the neuromuscular junction (Fig. 7).

ACh is synthesized and stored in the presynaptic terminals of motor axons. As a result of a nerve action potential that invades the presynaptic terminal, ACh is released into the synaptic cleft. Acetylcholine diffuses across the synaptic cleft and combines with receptors on the postsynaptic or the postjunctional membrane. When ACh binds with these receptors, there is an increase in Na+ and K+ permeabilities. The increase in Na+ and K+ permeabilities depolarizes the postjunctional, or the postsynaptic, membrane. This depolarization is known as the EPP. Normally, the EPP is approximately 50 mV in amplitude, so the threshold level of the muscle is easily reached and an action potential is triggered. The action potential in the muscle cell leads to muscular contraction. The EPP is a transient event that persists for about 10 msec. There are two reasons for its transient nature. First, ACh diffuses away from the synaptic cleft and produces no further permeability changes. Second, there is a substance in the synaptic cleft known as AChE. AChE hydrolyzes ACh into inactive substances. In the following sections, this sequence of events is examined in greater detail.

Synthesis and storage of ACh in motoneurons

Nerve action potential that depolarizes presynaptic terminal

Removal by diffusion

Hydrolysis of ACh by AChE

ACh release and _ diffusion into synaptic cleft

ACh combines with receptor sites on postjunctional membrane (PJM)

Increase in Na+ and K+ i permeability of PJM

w Depolarization of PJM (EPP)

Initiation of muscle ■ action potential

Excitation-Contraction ^^^^^^ Tension coupling ^^^^^^ development

FIGURE 7 Summary of the sequence of events underlying synaptic transmission at the skeletal neuromuscular junction. PJM, postjunctional membrane; EPP, end-plate potential; ACh, acetylcholine; AChE, acetylcholinesterase.

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