Pars reticulata

Infobox Brain
Name = PAGENAME
Latin =
GraySubject = 188
GrayPage = 802


Caption =


Caption2 =
IsPartOf =
Components =
Artery =
Vein =
BrainInfoType = hier
BrainInfoNumber = 529
MeshName =
MeshNumber =
The pars reticulata is a portion of the substantia nigra.

Anatomy

Neurons in the pars reticulata are much less densely packed than those in the compacta (they were sometimes named pars diffusa). They are smaller and thinner than the dopaminergic neurons and conversely identical and morphologically similar to the pallidal neurons (see primate basal ganglia). Their dendrites as well as the pallidal are preferentially perpendicular to the striatal afferents. [cite journal | author=François, C.; Yelnik, J. & Percheron, G. | title=Golgi study of the primate substantia nigra. II. Spatial organization of dendritic arborizations in relation to the cytoarchitectonic boundaries and to the striatonigral bundle | journal=Journal of Comparative Neurology | year=1987 | pages=473–493 | volume=265 | issue=4|pmid=3123530 | doi=10.1002/cne.902650403] The massive striatal afferences corresponds to the medial end of the striato-pallidonigral bundle. Nigral neurons have the same peculiar synaptology with the striatal axonal endings. They make connections with the dopamine neurons of the pars compacta whose long dendrites plunge deeply in the pars reticulata. The neurons of the pars reticulata and lateralis produce the neurotransmitter gamma-aminobutyric acid (GABA).The neurons of the pars reticulata through the nigrothalamic bundle send axons to a particular part of the motor thalamus. The nigral territory corresponds to the nucleus ventralis anterior VA (different from the pallidal VO). VA is the origin of one output of the basal ganglia system. It sends axons to the frontal and oculomotor cortex. In addition the pars reticulata sends neurons to the pars parafascicularis of the central region of the thalamus and to the pedunculopontine complex).The particularity of the pars lateralis is to send its axons to the superior colliculus, [cite journal | author=François, C.; Percheron, G. & Yelnik, J. | title=Localization of nigrostriatal, nigrothalamic and nigrotectal neurons in ventricular coordinates in macaques | journal=Neuroscience | year=1984 | pages=61–76 | volume=13 | issue=1 | pmid=6387531 | doi=10.1016/0306-4522(84)90259-8] which is a too minimized output of the basal ganglia system.

Function

The neurons of the pars reticulata are fast-spiking pacemakers, generating action potentials in the absence of synaptic input. [cite journal | author=Atherton, J.F. & Bevan, M.D. | title=Ionic mechanisms underlying autonomous action potential generation in the somata and dendrites of GABAergic substantia nigra pars reticulata neurons "in vitro" | journal=Journal of Neuroscience | year=2005 | pages=8272–8281 | volume=25 | issue=36 | pmid=16148235 | doi=10.1523/JNEUROSCI.1475-05.2005] In primates they discharge at a mean rate of 68Hz in contrast to dopaminergic neurons (below 8Hz). [cite journal | author=Schultz, W. | title=Activity of pars reticulata neurons of monkey substantia nigra in relation to motor, sensory and complex events | journal=Journal of Neurophysiology | year=1986 | pages=660–677 | volume=55 | issue=4 | pmid=3701399] They receive abundant afferrences from the striatum (mainly from the associative striatum) with the same very peculiar synaptology as the pallidum. It receives axons from the subthalamic nucleus and a dopaminergic innervation from the dopaminergic ensemble.

The pars reticulata is one of the two primary output nuclei of the basal ganglia system to the motor thalamus (the other output is the internal segment of the globus pallidus).The nigral neurons have their own territory distinct from the cerebellar and the pallidal in the nucleus ventralis anterior VA. This sends axons to the frontal and oculomotor cortex. Hikosaka and Wurtz [cite journal | author=Hikosaka, O. & Wurtz, R.H. | title=Visual and oculomotor functions of monkey substantia nigra pars reticulata. I to IV | journal=Journal of Neurophysiology | year=1983 | volume=49 | issue=5 | pages=1230–1301|pmid=6864248, PMID 6864249, PMID 6864250 & PMID 6306173] devoted four papers to "the visual and oculomotor functions of the monkey substantia nigra pars reticulata". This is largely involved in orientation and the control of eye movements in stabilisation of gaze and in saccades.

Pathology

The function of the neurons of the pars reticulata is profoundly changed in parkinsonism and epilepsy. These changes are thought to be mostly secondary to pathology elsewhere in the brain, but may be crucial to understanding the generation of the symptoms of these disorders.

References