Inhibitory Synapses

The primary inhibitory neurotransmitter in the CNS is gamma-aminobutyric acid (GABA). These receptors function much like the AChR with one crucial difference. Instead of the channel being permeable to cations the channel is permeable to chloride ions. This means that when the receptor is activated it tends to either hyperpolarize the membrane potential or at least stabilize it close to the resting membrane potential.

In many neurons active chloride transport maintains the chloride equilibrium potential, E_Cl, more negative than the resting membrane potential. When the GABA receptor channels open in these cells the membrane is hyperpolarized. This is known as an inhibitory postsynaptic potential, ipsp. If an excitatory and an inhibitory input occur close together in time they tend to cancel each other out Figure 1.

In a typical neuron, whether or not the cell fires an action potential will depend upon the balance of excitatory and inhibitory synaptic inputs that the cell is receiving at any particular point in time. Normally the neuron is receiving a constant barrage of glutaminergic and GABAergic inputs, influencing its excitability millisecond by millisecond. A recording from a neuron in the nervous system looks a lot like random noise (Figure 1). The membrane potential is continuously changing as both excitatory and inhibitory synapses are banging away on the cell and sometimes the cell reaches threshold firing an action potential.

Figure 1 Intracellular recording from a single CNS neuron.

The behavior of single neurons appears anarchic and probabilistic. Individual neurons typically function as part of an ensemble. In general, a single neuron is not solely responsible for anything in particular since there is considerable overlap in function. Rather, the averaged responses of multiple neurons are required to make a decision or produce an action.