Reticular formation

The reticular formation is a network of nerve cells and fibers located in the brainstem that connects various parts of the central nervous system. It consists mostly of interneurons and stretches downwards from the midbrain–through the pons and medulla oblongata–to the upper spinal cord, while also projecting upwards to subcortical and cortical brain structures. Embedded within it are several brainstem nuclei such as the red nucleus and the nucleus reticularis tegmenti pontis.

The reticular formation plays a crucial role in integrating functions like arousal, consciousness, sleep, movement coordination, breathing, heartbeat, pain modulation, and habituation. Within the reticular formation, the vasomotor center in the medulla oblongata regulates cardiovascular control, while the pontomedullary junctional region regulates respiratory rhythm. Reticular areas also connect to cranial nerve motor nuclei responsible for respiration-related tasks, including the trigeminal, facial, glossopharyngeal, vagus, and hypoglossal nerves.

The reticular formation can be divided into two fields: lateral and medial tegmental fields. The lateral field influences cranial nerve motor nuclei and the limbic system. It contains premotor neurons that signal and influence spinal cord motor neurons involved in breathing, pressure, urination, and blood pressure. The medial field coordinates eye and head movements alongside other senses.

In the brainstem, the reticular formation is further divided into three columns: raphe nuclei, magnocellular red nucleus, and parvocellular reticular nucleus. These are involved in serotonin production (affecting mood regulation), motor coordination, and regulating exhalation, respectively.

Neuronal projections from the reticular formation reach different parts of the brain through the ascending reticular activating system (ARAS). The ARAS, consisting of dopaminergic, noradrenergic, serotonergic, histaminergic, cholinergic, and glutamatergic pathways, affects alertness, consciousness, sleep, and waking cycles. These projections extend from brainstem nuclei to the thalamus and prefrontal cortex. Additionally, the ARAS helps with habituation by diverting attention from repetitive stimuli to more significant and changing stimuli in the environment.

The reticular formation also sends signals downward to the brainstem and spinal cord through the reticulospinal tracts, which contribute to movement control, balance, posture, and coordination. The descending tracts consist of the lateral system (corticospinal and rubrospinal tracts for fine motor control) and the medial system (reticulospinal and vestibulospinal tracts for posture coordination). The reticulospinal pathway further divides into the medial pontine and lateral medullary reticulospinal tracts, each with its own function. The medial pontine reticulospinal tract controls extensor musculature, while the lateral medullary reticulospinal tract inhibits excitation in axial extensor muscles and regulates autonomic functions of breathing. Damage to these pathways can lead to postural instability and other symptoms. Furthermore, the reticular formation is involved in pain modulation, transmitting pain signals to the brain and blocking their transmission from the spinal cord.

Regarding eye movements and head coordination, the reticular formation includes different regions. The mesencephalic reticular formation controls vertical gaze, the paramedian pontine reticular formation controls horizontal gaze, and the medullary pontine reticular formation coordinates head movements and gaze holding. These regions connect with the extraocular motor nuclei, facilitating saccadic eye movements. They also connect with the spinal cord (through descending reticulospinal neurons) to coordinate posture and neck movements with eye movements.