18 Synapse

Learning Objectives

After reading this section, you should be able to-

  • Define a synapse and explain the difference between an electrical synapse and a chemical synapse.
  • Describe the structures involved in a typical chemical synapse (e.g., axon terminal [synaptic knob], voltage-gated calcium channels, synaptic vesicles of presynaptic cell, synaptic cleft, neurotransmitter receptors of the postsynaptic cell).
  • Describe the events of synaptic transmission in proper chronological order from the release of neurotransmitter by synaptic vesicles to the effect of the neurotransmitter on the postsynaptic cell.
  • Describe the different mechanisms (e.g., reuptake, enzymatic breakdown, diffusion) by which neurotransmitter activity at a synapse can be terminated
The electrical changes taking place within a neuron, as described in the previous section, are similar to a light switch being turned on. A stimulus starts the depolarization (hand on switch), but the action potential runs on its own once a threshold has been reached (electricity moving through the wires to the light). The question is now, “What flips the light switch on?” Temporary changes to a neuron’s cell membrane voltage can result from stimuli in the environment, or from the action of one neuron on another. These temporary changes in membrane potential influence a neuron and determine whether an action potential will or will not occur.

Synapses

A synapse is the site of communication between a neuron and another cell. There are two types of synapses: chemical synapses and electrical synapses. In a chemical synapse, a chemical signal, which is called a neurotransmitter, is released from the neuron and binds to a receptor on the other cell. In an electrical synapse, the membranes of two cells directly connect through a gap junction so that ions can pass directly from one cell to the next, transmitting a signal. Both types of synapses occur in the nervous system, though chemical synapses are more common.

An example of a chemical synapse is the neuromuscular junction (NMJ) , which will be described in the chapter on muscle tissue. In the nervous system, there are many additional synapses that utilize the same mechanisms as the NMJ. All chemical synapses have common characteristics, which can be summarized in Table 18.1:

Example Chemical Synapse (Table 18.1)
Common Chemical Synapse Element Specific element in a Skeletal Muscle Neuromuscular Junction
presynaptic element somatic motor neuron axon terminal
neurotransmitter (packaged in vesicles) acetylcholine
synaptic cleft space between somatic motor neuron and muscle cell membrane
receptor proteins nicotinic acetylcholine (cholinergic) receptor
postsynaptic element motor end plate of the sarcolemma
neurotransmitter elimination/re-uptake degrading enzyme: acetylcholinesterase

Neurotransmitter Release & the Synaptic Cleft

When an action potential reaches the axon terminals, voltage-gated Ca2+ channels in the membrane of the synaptic end bulb open. Ca2+ diffuses down its concentration gradient and enters into the presynaptic neuron axon terminal (end bulb). Once Ca2+ is inside the presynaptic end bulb, it associates with proteins to trigger exocytosis (e.g., movement of vesicles to the cell membrane and release of contents to the outside of the cell) of neurotransmitter vesicles. The released neurotransmitter moves into the small gap between the cells, called the synaptic cleft.

Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane and can bind to neurotransmitter receptors. Receptors are specific for the neurotransmitter, and the two fit together like a lock and key, and so a neurotransmitter will not bind to receptors for other neurotransmitters (Figure 18.1).

This diagram shows a postsynaptic neuron. An axon from a presynaptic neuron is synapsing with the dendrites on the post synaptic neuron. The axon of the presynaptic neuron branches into several club shaped axon terminals. A magnified view of one of the synapses reveals that the axon terminal does not contact the dendrite of the postsynaptic neuron. Instead, there is a small space between the two structures, called the synaptic cleft. The axon terminal of the presynaptic neuron contains several synaptic vesicles, each holding about a dozen neurotransmitter particles. The synaptic vesicles travel to the edge of the axon terminal and release their neurotransmitters into the synaptic clefts The neurotransmitters travel through the synaptic cleft and bind to carrier proteins on the postsynaptic neuron that contain receptors foe neurotransmitters.
Figure 18.1 – The Synapse: The synapse is a connection between a neuron and its target cell (which is not necessarily a neuron). The presynaptic element is the synaptic end bulb of the axon where Ca2+ enters the bulb to cause vesicle fusion and neurotransmitter release. The neurotransmitter diffuses across the synaptic cleft to bind to its receptor. The neurotransmitter is cleared from the synapse either by enzymatic degradation, neuronal reuptake, or glial reuptake.
Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 12.
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