Chemical and Electrical Synapses

The point where activity is transmitted from one nerve cell to another or from a motor neuron to a muscle cell is called a synapse. In the patellar reflex, there are two synapses: one between the sensory neuron and the motor neuron in the spinal cord, and another between the motor neuron and the cells of the quadriceps muscle. There are two general classes of synapse: electrical synapses and chemical synapses. In both types, special membrane structures exist at the point where the input cell (called the presynaptic cell) comes into contact with the output cell (called the postsynaptic cell).

At a chemical synapse, an action potential in the presynaptic cell causes it to release a chemical substance (called a neurotransmitter), which diffuses through the extracellular space and changes the membrane potential of the postsynaptic cell. At an electrical synapse, a change in membrane potential (such as the depolarization during an action potential) in the presynaptic cell spreads directly to the postsynaptic cell without the action of an intermediary chemical. Both synapses in the patellar reflex, are chemical synapses. At a chemical synapse, the membranes of the presynaptic and postsynaptic cells come close to each other but are still separated by a small gap of extracellular space. At an electrical synapse, the presynaptic and postsynaptic membranes touch and the cell interiors are directly interconnected by means of special ion channels called gap junctions that allow flow of electrical current from one cell to another. We will concentrate in this chapter on chemical synaptic transmission. Electrical synaptic transmission will be described in more detail in Chapter 12.

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