The basal ganglia form a major brain system in all vertebrates, but in primates (including humans) there are special differentiating features. The basal ganglia include the striatum, globus pallidus, substantia nigra and subthalamic nucleus. In primates the pallidus is divided into an external and internal globus pallidus, the external globus pallidus is present in other mammals but not the internal globus pallidus. Also in primates, the dorsal striatum is divided by a large nerve tract called the internal capsule into two masses named the caudate nucleus and the putamen. These differences contribute to a complex circuitry of connections between the striatum and cortex that is specific to primates, reflecting different functions in primate cortical areas.
Corticostriatal connection A major output from the cortex, with axons from most of the cortical regions connecting to the striatum, is called the corticostriatal connection, part of the cortico-basal ganglia-thalamo-cortical loop, or basal ganglia loop in short. In the primate most of these axons are thin and unbranched. The striatum does not receive axons from the primary olfactory, visual or auditory cortices. The corticostriatal connection is an excitatory glutamatergic pathway. One small cortical site can project many axon branches to several parts of the striatum.
Striatum
The striatum is the largest structure of the basal ganglia.
Structure
Neuronal constitution Medium spiny neurons (MSN)s, account for up to 95 per cent of the striatal neurons. There are two populations of these projection neurons, MSN1 and MSN2, both of which are inhibitory GABAergic. There are also various groups of GABAergic interneurons and a single group of cholinergic interneurons. These few types are responsible for the reception, processing, and relaying of all the cortical input. Most of the dendritic spines on the medium spiny neurons synapse with cortical afferents and their axons project numerous collaterals to other neurons. The cholinergic interneurons of the primate, are very different from those of non-primates. These are said to be tonically active. The dorsal striatum and the ventral striatum have different populations of the cholinergic interneurons showing a marked difference in shape.
Physiology Unless stimulated by cortical input the striatal neurons are usually inactive.
Levels of organisation The striatum is one mass of grey matter that has two different parts, a ventral and a dorsal part. The dorsal striatum contains the caudate nucleus and the putamen, and the ventral striatum contains the nucleus accumbens and the olfactory tubercle. The internal capsule is seen as dividing the two parts of the dorsal striatum. Sensorimotor input is mostly to the putamen. An associative input goes to the caudate nucleus and possibly to the nucleus accumbens. There are two different components of the striatum differentiated by staining – striosomes and a matrix. Striosomes are located in the matrix of the striatum and these contain μ-opioid receptors and dopamine receptor D1 binding sites. The striatopallidal fibers give a connection from the putamen to the globus pallidus and substantia nigra.
Connectomics Unlike the inhibitory GABAergic neurons in the neocortex that only send local connections, in the striatum these neurons send long axons to targets in the pallidum and substantia nigra. A study in macaques showed that the medium spiny neurons have several targets. Most striatal axons first target the GPe, some of these also target the GPi and both parts of the substantia nigra. There are no single axon projections to either the GPi, or to the SN, or to both of these areas; only connecting as continuing targets via axon collaterals from the striatum to the GPe. The only difference between the axonal connectomes of the striosomes and the axons of those neurons in the matrix, is in the numbers of their branching axons. Striosomal axons cross the extent of the SN, and in macaques emit 4 to 6 vertical collaterals that form vertical columns which enter deep into the SN pars compacta (SNpc); the axons from those in the matrix are more sparsely branched. This pattern of connectivity is problematic. The main mediator of the striatopallidonigral system is GABA and there are also cotransmitters. The GPe stains for met-enkephalin, the GPi stains for either substance P or dynorphin or both, and the SN stains for both. This probably means that a single axon is able to concentrate different co-mediators in different subtrees, depending on the target.
Selectivity of striatal territories for targets A study of the percentage of striatal axons from the sensorimotor (dorsolateral putamen) and associative striatum (caudate nucleus and ventromedial putamen) to the globus pallidus found important differences. The GPe for instance receives a large input of axons from the associative areas. The GPi is strongly sensorimotor connected. The SN is at first associative. This is confirmed by the effects of striatal stimulations. All the projections from the primary somatosensory cortex to the putamen, avoid the striosomes and innervate areas within the matrix.
Pallidonigral set and pacemaker
… excerpt ends here. Continue reading the full article.
