Characteristics ofInhibitory Synaptic Transmission

We will now consider some of the properties of postsynaptic responses at an inhibitory synapse and then discuss the underlying mechanisms in the post-synaptic membrane. Figure 9-7 shows schematically an experimental arrangement to examine the inhibition of the antagonistic motor neuron in the patellar reflex. An intracellular microelectrode monitors the membrane potential of the motor neuron, while the inhibitory presynaptic neuron is stimulated electrically to fire action potentials.

Release of neurotransmitter at the inhibitory synapse follows the same basic scheme as at other chemical synapses: depolarization produced by the presynaptic action potential stimulates calcium entry through voltage-sensitive calcium channels, inducing synaptic vesicles containing neuro-transmitter to fuse with the membrane and release their contents. On the postsynaptic side, however, the effect of the transmitter is very different from that of ACh at the neuromuscular junction, as shown in Figure 9-7b. An action potential in the presynaptic cell is followed by a transient increase in the post-synaptic membrane potential. When the membrane potential becomes more negative, the cell is said to be hyperpolarized. Because hyperpolarization moves the membrane potential away from the threshold for firing an action potential, it is less likely that an excitatory input will be able to trigger an action potential, and the postsynaptic cell is inhibited. The hyperpolarization of the postsynaptic cell caused by inhibitory neurotransmitter is called an inhibitory postsynaptic potential (i.p.s.p.).

Membrane Potential Neuron And Muscle

Figure 9-7 Inhibitory synaptic transmission between two neurons in the circuit of Figure 9-6. An action potential in the presynaptic neuron releases a neurotransmitter that hyperpolarizes the postsynaptic neuron.

(a) A diagram of the synaptic circuitry and recording arrangement.

(b) Postsynaptic response of the motor neuron.

Stimulate presynaptic neuron

Stimulate presynaptic neuron

Figure 9-7 Inhibitory synaptic transmission between two neurons in the circuit of Figure 9-6. An action potential in the presynaptic neuron releases a neurotransmitter that hyperpolarizes the postsynaptic neuron.

(a) A diagram of the synaptic circuitry and recording arrangement.

(b) Postsynaptic response of the motor neuron.

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