The pars reticulata (SNpr) is a portion of the

substantia nigra The substantia nigra (SN) is a basal ganglia structure located in the midbrain that plays an important role in reward and movement. ''Substantia nigra'' is Latin for "black substance", reflecting the fact that parts of the substantia nigra app ...

and is located lateral to the

pars compacta The pars compacta (SNpc) is a portion of the ''substantia nigra'', located in the midbrain. It is formed by dopaminergic neurons and located medial to the pars reticulata. Parkinson's disease is characterized by the death of dopaminergic neurons ...

. Most of the neurons that project out of the pars reticulata are inhibitory

GABAergic In molecular biology and physiology, something is GABAergic or GABAnergic if it pertains to or affects the neurotransmitter GABA. For example, a synapse is GABAergic if it uses GABA as its neurotransmitter, and a GABAergic neuron produces GABA. A ...

neurons (i.e., these neurons release GABA, which is an inhibitory

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neuro ...

Anatomy

Neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

s in the pars reticulata are much less densely packed than those in the

(they were sometimes named pars diffusa). They are smaller and thinner than the

dopaminergic Dopaminergic means "related to dopamine" (literally, "working on dopamine"), dopamine being a common neurotransmitter. Dopaminergic substances or actions increase dopamine-related activity in the brain. Dopaminergic brain pathways facilitate d ...

neurons and conversely identical and morphologically similar to the

pallidal The globus pallidus (GP), also known as paleostriatum or dorsal pallidum, is a subcortical structure of the brain. It consists of two adjacent segments, one external, known in rodents simply as the globus pallidus, and one internal, known in rod ...

neurons (see

primate basal ganglia The basal ganglia form a major brain system in all species of vertebrates, but in primates (including humans) there are special features that justify a separate consideration. As in other vertebrates, the primate basal ganglia can be divided into ...

). Their dendrites as well as the pallidal are preferentially perpendicular to the striatal afferents. The massive striatal afferents correspond to the medial end of the nigrostriatal 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 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 thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all directions, ...

. The nigral territory corresponds to the nucleus ventralis anterior ( VA) (see also

List of thalamic nuclei This traditional list does not accord strictly with human thalamic anatomy. Nuclear groups of the thalamus include: *anterior nuclear group ** anteroventral nucleus ** anterodorsal nucleus ** anteromedial nucleus **superficial ("lateral dorsal") ...

) (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 pedunculopontine nucleus (PPN) or pedunculopontine tegmental nucleus (PPT or PPTg) is a collection of neurons located in the upper pons in the brainstem. It lies caudal to the substantia nigra and adjacent to the superior cerebellar pedunc ...

). 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 An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

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

striatum The striatum, or corpus striatum (also called the striate nucleus), is a nucleus (a cluster of neurons) in the subcortical basal ganglia of the forebrain. The striatum is a critical component of the motor and reward systems; receives glutamate ...

(mainly from the associative striatum) with the same very peculiar synaptology as the

pallidum The globus pallidus (GP), also known as paleostriatum or dorsal pallidum, is a subcortical structure of the brain. It consists of two adjacent segments, one external, known in rodents simply as the globus pallidus, and one internal, known in rod ...

. It receives axons from the

subthalamic nucleus The subthalamic nucleus (STN) is a small lens-shaped nucleus in the brain where it is, from a functional point of view, part of the basal ganglia system. In terms of anatomy, it is the major part of the subthalamus. As suggested by its name, the ...

and a dopaminergic innervation from the dopaminergic ensemble. The pars reticulata is one of the two primary output nuclei of the

basal ganglia The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an extern ...

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

globus pallidus The globus pallidus (GP), also known as paleostriatum or dorsal pallidum, is a subcortical structure of the brain. It consists of two adjacent segments, one external, known in rodents simply as the globus pallidus, and one internal, known in rod ...

). 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 movements in stabilisation of gaze and in

saccade A saccade ( , French for ''jerk'') is a quick, simultaneous movement of both eyes between two or more phases of fixation in the same direction.Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishi ...

s. Pars reticulata sends inhibitory projections to the

superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form ''tectal'' is commonly ...

, inhibiting eye movement, and this inhibition is 'lifted', i.e. the projecting neurons cease firing, during 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 Parkinsonism is a clinical syndrome characterized by tremor, bradykinesia (slowed movements), rigidity, and postural instability. These are the four motor symptoms found in Parkinson's disease (PD), after which it is named, dementia with Lewy bo ...

and

epilepsy Epilepsy is a group of non-communicable neurological disorders characterized by recurrent epileptic seizures. Epileptic seizures can vary from brief and nearly undetectable periods to long periods of vigorous shaking due to abnormal electrical ...

. 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

{{Authority control Basal ganglia Midbrain