Medium spiny neuron
[2] The dorsal striatal MSNs play a key role in initiating and controlling movements of the body, limbs, and eyes.
Dorsal and ventral medium spiny neuron subtypes (i.e., direct D1-type and indirect D2-type) are identical phenotypes, but their output connections differ.
In the indirect pathway the MSNs ultimately project to these two structures via an intermediate connection to the globus pallidus external (GPe) and ventral pallidum (VP).
Since the fast-spiking 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.
[13][14] The direct pathway within the basal ganglia receives excitatory input from the cortex, thalamus, and other brain regions.
In the direct pathway, medium spiny neurons project to the internal division of the globus pallidus (GPi) or the substantia nigra pars reticula (SNpr or SNr).
[16] The SNr and GPi outputs are both tonically active inhibitory nuclei and are thus constantly inhibiting the thalamus (and thus motor cortex).
However, transient activity in (inhibitory) direct pathway medium spiny neurons ultimately disinhibits thalamus projections to the motor cortex and enables movement.
Indirect pathway medium spiny neurons project to the external segment of the globus pallidus (GPe).
[24] Despite the abundance of evidence for the initiation/termination model, recent evidence using transgenic mice expressing calcium indicators in either the direct or indirect pathway demonstrated that both pathways are active at action initiation, but neither are active during inactivity,[25] a finding which has been replicated using simultaneous two-channel calcium imaging.
[32] Recent experiments have established that the direct and indirect pathways of the dorsal striatum are not solely involved in movement.
The direct pathway of the ventral striatum within the basal ganglia mediates reward-based learning and appetitive incentive salience, which is assigned to rewarding stimuli.
