The pars reticulata is one of two sections in the midbrain structure called the substantia nigra. This structure is one of four nuclei that compose the basal ganglia system, the brain structure responsible for functions like action selection and the pursuit of rewards. The substantia nigra pars reticulata contributes to head and eye movement control and functions as a processing center for the basal ganglia.
The other portion of the substantia nigra is the pars compacta, which takes input from other regions of the basal ganglia and sends information back. The pars reticulata, conversely, receives input as well, but it sends signals beyond the basal ganglia, which is why it can control the head and eyes. The pars compacta is the more known aspect of the substantia nigra due to its association with dopamine production, epilepsy, chorea, and Parkinson's disease, and some researchers believe that this portion of the brain may be involved in schizophrenia as well.
Dysfunction of the pars reticulata exacerbates Parkinson's disease, contributing to the sporadic head and eye movement that is symptomatic of the condition. It may also contribute to such spastic movements characteristic of epileptics, as a normally functioning pars reticulata plays a role in moderating seizures. The structure is primarily composed of spontaneously firing Gamma Aminobutyric Acid (GABA) neurotransmitters, which are involved with movement inhibition. This is what lets a person to sit still or point at a screen without touching it. Being able to manipulate the firing of these neurotransmitters in epileptics could lead to a method of seizure suppression.
Anatomically speaking, the pars reticulata neurons occupy a greater area than those in the pars compacta, making it the larger portion of the substantia nigra. The pars compacta neurons have tree-like structures, called dendrites, which form connections with the pars reticulata, indicating that both areas have some influence on dopamine production. The pars reticulata has more extensive connections with areas of the brain that have strong inhibitory control over physical movements than any other part of the basal ganglia. It is also believed to be connected to parts of the brain that exert control over aspects of behavior.
The production of GABA neurotransmitters, or GABAergic neurons, is an important pars reticulata function, one it shares with the pars lateralis. GABA suppresses hyperactive neuron activity in the nervous system and directly contributes to the regulation of muscle tone in humans. It is the production of GABA that allows the pars reticulata to aid in motor control regulation by providing movement inhibition.
