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 a ...

and is located lateral to the pars compacta. Most of the neurons that project out of the pars reticulata are inhibitory GABAergic neurons (i.e., these neurons release

GABA GABA (gamma-aminobutyric acid, γ-aminobutyric acid) is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervous system. GA ...

, which is an inhibitory

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

Anatomy

Neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

s in the pars reticulata are much less densely packed than those in the pars compacta (they were sometimes named pars diffusa). They are smaller and thinner than the

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

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 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 (gamma-aminobutyric acid, γ-aminobutyric acid) is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervous system. GA ...

(

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

thalamus The thalamus (: thalami; from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter on the lateral wall of the third ventricle forming the wikt:dorsal, dorsal part of the diencephalon (a division of ...

. The nigral territory corresponds to the nucleus ventralis anterior ( VA) (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 An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

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

striatum The striatum (: striata) or corpus striatum is a cluster of interconnected nuclei that make up the largest structure of the subcortical basal ganglia. The striatum is a critical component of the motor and reward systems; receives glutamat ...

(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 The basal ganglia (BG) or basal nuclei are a group of subcortical Nucleus (neuroanatomy), nuclei found in the brains of vertebrates. In humans and other primates, differences exist, primarily in the division of the globus pallidus into externa ...

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 major component of the Cerebral cortex, subcortical basal ganglia in the brain. It consists of two adjacent segments, one external (or lateral), known in rodents simpl ...

). 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 In vision science, a saccade ( ; ; ) is a quick, simultaneous movement of both Eye movement (sensory), eyes between two or more phases of focal points in the same direction. In contrast, in Smooth pursuit, smooth-pursuit movements, the eyes mov ...

s. Pars reticulata sends inhibitory projections to the

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

, 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 and

epilepsy Epilepsy is a group of Non-communicable disease, non-communicable Neurological disorder, neurological disorders characterized by a tendency for recurrent, unprovoked Seizure, seizures. A seizure is a sudden burst of abnormal electrical activit ...

. 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