The striatum is the largest collection of neurons in the basal ganglia. Composed of the caudate nucleus and putamen, the basal ganglia, as the name suggests, sits at the base of the cerebrum. It receives input from regions of the cerebral cortex, the limbic system, and the sensorimotor and motivational systems via the thalamus. In addition to the cerebrum, the striatum receives input from the brainstem including the substantia nigra and the raphe nuclei of the reticular formation. The dopamine and serotonin of these two structures serve a modulatory function. Anatomists organise the striatum on the “basis of differential connectivity and distribution of neurochemical markers” (Redgrave, 2007). Processing strong excitatory input, the striatal neural circuits generate a strong inhibitory output, which controls the output of basal ganglia further along in the motor loop.
The major cytology of the striatum is GABAergic medium spiny neurons (MSN), making up about 95% of the total cellular structure. MSNs are organised into two groups based on the peptide they contain, substance P and enkephalin and the proportion of dopamine receptors (D1 or D2) they contain. MSNs create dense networks of axon collaterals. As projection neurons, the MSNs create this dense network by forming axon collaterals with one another. Tunstall et al, 2002 found that almost 30% form an axon collateral with a neighbouring MSN. Research has shown that the function of these collaterals is in cellular recognition and “classification of cortical patterns” (Blomeley, et al. , 2009).
The striatum is a vital part of the basal ganglia, and all pathways run through it. From the striatum onwards, the pathway either becomes direct or indirect. As shown in the figure below.
Blomeley, C. P., Kehoe, L. A., & Bracci, E. (2009). Substance P mediates excitatory interactions between striatal projection neurons. The Journal of Neuroscience, 29(15), 4953-4963.
Redgrave, P. (2007). Basal ganglia. Scholarpedia, 2(6): 1825.