Endplate Potential

It is possible to study various aspects of chemical synaptic transmission in a reduced preparation. One can dissect a skeletal muscle cell (with its intact neural innervation) from an animal and place it in an experimental solution, where it can remain viable for long periods of time (Fig. 4). The postsynaptic muscle cell is impaled with a microelectrode to record potentials in the muscle cell, and the motor axon is stimulated to initiate action potentials in the axon.

Figure 5A illustrates a typical result. At the arrow, an electrical stimulus is delivered to the motor axon. This stimulus elicits an action potential in the axon, which then propagates down the axon, invades the synaptic terminal, and leads to the release of chemical transmitter. The transmitter diffuses across the cleft and binds with receptor sites on the postsynaptic membrane to trigger the illustrated sequence of potential changes. Note that there is a distinct delay between the application of the electrical stimulus and the production of any potential change in the muscle cell. The delay is due to two factors: (1) it takes time for the action potential to propagate from its site of initiation down the motor axon, and (2) there is a delay due to the time necessary

FIGURE 3 Electron micrograph of a synaptic contact at the neuromuscular junction. The presynaptic terminal (upper portion) contains many small vesicles and larger mitochondria. There is a distinct separation (the synaptic cleft) between the membranes of the pre- and postsynaptic cells. (Micrograph produced by Dr. John E. Heuser of Washington University, St. Louis, MO.)

FIGURE 3 Electron micrograph of a synaptic contact at the neuromuscular junction. The presynaptic terminal (upper portion) contains many small vesicles and larger mitochondria. There is a distinct separation (the synaptic cleft) between the membranes of the pre- and postsynaptic cells. (Micrograph produced by Dr. John E. Heuser of Washington University, St. Louis, MO.)

for the chemical transmitter substance to be released from the presynaptic terminal, diffuse across the synap-tic cleft, and produce the permeability changes that trigger the potential changes recorded in the postsynap-tic muscle cell.

There are two components to the potential changes in the muscle that are produced as a result of stimulating the motor axon. At first, there is a relatively slow rising potential. At a potential of about —50 mV, there is a sharp inflection at which a second potential is triggered. This second potential, the action potential, quickly reaches a peak value and then rapidly decays back to the resting potential. In the discussion here, the major focus is not the action potential but the somewhat slower initial underlying event that triggers the action potential.

An important chemical substance that has facilitated the analysis of synaptic transmission at the neuromus-cular junction is curare. (Curare is derived from plants and is used by some South American Indians for arrow poison.) If a low dose of curare is added (Fig. 5B) and the motor axon is again stimulated, the slower underlying event is reduced in amplitude but is still capable of depolarizing the muscle cell to threshold and initiating an action potential (assume that threshold in this cell is about —50 mV). If a somewhat higher

6. Neuromuscular and Synaptic Transmission

Chamber

FIGURE 4 Schematic diagram of the preparation used to study features of synaptic transmission at the skeletal neuromuscular junction.

Chamber

FIGURE 4 Schematic diagram of the preparation used to study features of synaptic transmission at the skeletal neuromuscular junction.

dose of curare is added, the slower underlying event is reduced further (Fig. 5C) and now fails to reach threshold. This underlying postsynaptic potential that is produced as a result of stimulation of the presynap-tic motor axon and release of chemical transmitter substance is known as the end-plate potential (EPP). It is called the end-plate potential because it is the potential that is recorded at the motor end plate. One of the clear results of this experiment is that curare reduces the amplitude of the EPP. If sufficient curare is added, the EPP is completely abolished. A person poisoned with curare will asphyxiate because curare blocks neuromuscular transmission at the respiratory muscles. Note that if the muscle cell is artificially depolarized to threshold in the presence of a high dose of curare, an action potential can still be initiated that is indistinguishable from an action potential produced artificially in the absence of curare. Thus, although curare is effective in blocking the EPP, it has no direct effect on the action potential. Because curare does not affect the action potential, it does not affect the voltage-dependent Na+ and K+ channels that underlie the action potential.

The EPP is the critical event underlying the initiation of an action potential in a muscle cell. For this reason, we will explore some of the properties of EPPs in considerable detail.

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