Function
The medium spiny neurons are GABAergic neurons and hence have an inhibitory influence on the neurons they project to. Within the basal ganglia, there are several complex circuits of neuronal loops all of which include the medium spiny neurons (for further information see basal ganglia). They send axons to the internal and external segment of the globus pallidus as well as the substantia nigra pars reticulata.
The cortical, thalamic, and brain-stem inputs that arrive at the medium spiny neurons show a vast divergence in that each incoming axon forms contacts with many spiny neurons and each spiny neuron receives a vast amount of input from different incoming axons. Since these inputs are glutamatergic they exhibit an excitatory influence on the inhibitory medium spiny neurons.
There are also a large number of interneurons originating in different areas which regulate the excitability of the medium spiny neurons. The synaptic connections between the spiny neurons and the interneurons are typically close to the spiny neurons' cell soma, or body. Recall that excitatory postsynaptic potentials caused by glutamatergic inputs at the dendrites of the spiny neurons only cause an action potential when the depolarization wave is strong enough upon entering the cell soma. Since the interneurons' influence is located so closely to this critical gate between the dendrites and the soma, they can readily regulate the generation of an action potential. As a result, the excitatory input coming from cortical etc. neurons has to be very strong, or caused by many simultaneously arriving excitations.
In consequence the medium spiny neurons are usually quiet and do not exhibit any spontaneous activity unless sufficiently activated.
Read more about this topic: Medium Spiny Neuron
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