Excitatory and Inhibitory Synapses

At the neuromuscular junction, ACh depolarizes the muscle cell, causing it to fire an action potential. Synapses of this type are called excitatory synapses because the neurotransmitter brings the membrane potential of the postsynaptic cell toward the threshold for firing an action potential and thus tends to "excite" the postsynaptic cell. The synapse between the sensory neuron and the quadriceps motor neuron in the patellar reflex is an example of an excitatory synapse between two neurons. Synapses between neurons are not always excitatory, however. At inhibitory synapses, the postsynaptic effect of the neurotransmitter tends to prevent the postsynaptic cell from firing an action potential, by keeping the membrane potential of the postsynaptic cell more negative than the threshold potential. Thus, the postsynaptic cell is "inhibited" by the release of the inhibitory neurotransmitter. One major difference between synaptic transmission at the neuromuscular junction and synaptic transmission in the nervous system in general is that transmission at the neuromuscular junction is always excitatory, whereas transmission in the nervous system may be either excitatory or inhibitory.

We will return to a discussion of inhibitory synapses later in this chapter. At this point, the discussion will center on the properties of excitatory synaptic transmission between neurons in the nervous system.

0 0

Post a comment