<P> Types of cells in the striatum include: </P> <Ul> <Li> Medium spiny neurons, which are the principal neurons of the striatum . They are GABAergic and, thus, are classified as inhibitory neurons . Medium spiny projection neurons comprise 95% of the total neuronal population of the human striatum . Medium spiny neurons have two primary phenotypes (i.e., characteristic types): D1 - type MSNs of the direct pathway and D2 - type MSNs of the indirect pathway . A subpopulation of MSNs contain both D1 - type and D2 - type receptors, with approximately 40% of striatal MSNs expressing both DRD1 and DRD2 mRNA . </Li> <Li> Cholinergic interneurons release acetylcholine, which has a variety of important effects in the striatum . In humans, other primates, and rodents, these interneurons respond to salient environmental stimuli with stereotyped responses that are temporally aligned with the responses of dopaminergic neurons of the substantia nigra . The large aspiny cholinergic interneurons themselves are affected by dopamine through dopamine receptors D . </Li> <Li> There are many types of GABAergic interneurons . The best known are parvalbumin expressing interneurons, also known as fast - spiking interneurons, which participate in powerful feedforward inhibition of principal neurons . Also, there are GABAergic interneurons that express tyrosine hydroxylase, somatostatin, nitric oxide synthase and neuropeptide - y . Recently, two types of neuropeptide - y expressing GABAergic interneurons have been described in detail, one of which translates synchronous activity of cholinergic interneurons into inhibition of principal neurons . These neurons of the striatum are not distributed evenly . </Li> </Ul> <Li> Medium spiny neurons, which are the principal neurons of the striatum . They are GABAergic and, thus, are classified as inhibitory neurons . Medium spiny projection neurons comprise 95% of the total neuronal population of the human striatum . Medium spiny neurons have two primary phenotypes (i.e., characteristic types): D1 - type MSNs of the direct pathway and D2 - type MSNs of the indirect pathway . A subpopulation of MSNs contain both D1 - type and D2 - type receptors, with approximately 40% of striatal MSNs expressing both DRD1 and DRD2 mRNA . </Li> <Li> Cholinergic interneurons release acetylcholine, which has a variety of important effects in the striatum . In humans, other primates, and rodents, these interneurons respond to salient environmental stimuli with stereotyped responses that are temporally aligned with the responses of dopaminergic neurons of the substantia nigra . The large aspiny cholinergic interneurons themselves are affected by dopamine through dopamine receptors D . </Li>

The corpus striatum is composed of which of the following