Anatomy

Neurons in the pars reticulata are much less densely packed than those in the pars compacta

Pars compacta

-Anatomy:In humans, the nerve cell bodies of the pars compacta are coloured black by the pigment neuromelanin. The degree of pigmentation increases with age. This pigmentation is visible as a distinctive black stripe in brain sections and is the origin of the name given to this area. The neurons...

 (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. 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 pars lateralis produce the neurotransmitter gamma-aminobutyric acid

Gamma-aminobutyric acid

γ-Aminobutyric acid is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays a role in regulating neuronal excitability throughout the nervous system...

 (GABA

Gabâ

Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...

).
The neurons of the pars reticulata through the nigrothalamic bundle send axons to a particular part of the motor thalamus

Thalamus

The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...

. The nigral territory corresponds to the nucleus ventralis anterior

Ventral anterior nucleus

-Inputs and outputs:The ventral anterior nucleus receives neuronal inputs from the basal ganglia which includes the substantia nigra and the globus pallidus. It also has inputs from the cerebellum. It sends out neuronal input to the supplementary motor cortex and to primary motor...

 (VA

Ventral anterior nucleus

-Inputs and outputs:The ventral anterior nucleus receives neuronal inputs from the basal ganglia which includes the substantia nigra and the globus pallidus. It also has inputs from the cerebellum. It sends out neuronal input to the supplementary motor cortex and to primary motor...

) (see also List of thalamic nuclei) (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, which is a too minimized output of the basal ganglia system.

Function

The neurons of the pars reticulata are fast-spiking pacemakers, generating action potential

Action potential

In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

s in the absence of synaptic input. In primates they discharge at a mean rate of 68Hz in contrast to dopaminergic neurons (below 8Hz). They receive abundant afferrences from the striatum

Striatum

The striatum, also known as the neostriatum or striate nucleus, is a subcortical part of the forebrain. It is the major input station of the basal ganglia system. The striatum, in turn, gets input from the cerebral cortex...

 (mainly from the associative striatum) with the same very peculiar synaptology as the pallidum. It receives axons from the subthalamic nucleus

Subthalamic nucleus

The subthalamic nucleus is a small lens-shaped nucleus in the brain where it is, from a functional point of view, part of the basal ganglia system. Anatomically, it is the major part of subthalamus. As suggested by its name, the subthalamic nucleus is located ventral to the thalamus. It is also...

 and a dopaminergic innervation from the dopaminergic ensemble.

The pars reticulata is one of the two primary output nuclei

Nucleus (neuroanatomy)

In neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex. In anatomical sections, a nucleus shows up...

 of the basal ganglia

Basal ganglia

The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...

 system to the motor thalamus (the other output is the internal segment of the globus pallidus

Globus pallidus

The globus pallidus also known as paleostriatum, is a sub-cortical structure of the brain. Topographically, it is part of the telencephalon, but retains close functional ties with the subthalamus - both of which are part of the extrapyramidal motor system...

).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 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

Human eye

The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 movements in stabilisation of gaze and in saccades.

Pathology

The function of the neurons of the pars compacta (not reticulata) is profoundly changed (60% of Dopamine secreting neurons, 80% decrease in dopamine in striatum) in parkinsonism

Parkinson's disease

Parkinson's disease is a degenerative disorder of the central nervous system...

 and epilepsy

Epilepsy

Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...

. 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.