Reticulospinal tract

Basics

The reticulospinal tract is part of the corticoreticulospinal pathway (system). This pathway participates in the control of motor activity by involving the reticular formation. The corticoreticulospinal system consists of:

• Corticoreticular fibers
• Pontine (medial) reticulospinal tract
• Medullary (lateral) reticulospinal tract

The corticoreticular fibers arise from the premotor cortex and supplementary motor area. These fibers synapse bilaterally with the neurons of the medial zone of the pontine and medullary reticular formation. These medial zone nuclei give rise to the reticulospinal tracts.

In addition to receiving corticoreticular fibers from the motor cortex, the medial zone of the pontine and medullary reticular formation also receives fibers arising from other sources involved in motor function. These sources include the basal ganglia, red nucleus, and substantia nigra via the central tegmental tract, that modulate the activity of the reticulobulbar and reticulospinal neurons.

When discussing reticulospinal tracts, they rise from the neurons of the medial zone of the pontine and medullary reticular formation, descend into the spinal cord and terminate by synapsing with the spinal cord interneurons. In turn, these interneurons synapse with the bodies of the lower motor neurons, where the lateral and medial reticulospinal tracts control locomotion and posture.

Pontine (medial) reticulospinal tract

This tract arises from the medial zone of the pontine reticular formation. This zone consists of the oral pontine reticular nucleus and caudal pontine reticular nucleus that are made of functionally efferent neurons that give rise to this descending motor pathway.
 
The medial reticulospinal tract descends ipsilaterally through the anterior funiculus of the spinal cord. It synapses at all levels of the spinal cord with interneurons that inhibit the flexors and gamma motor neurons that stimulate the extensors of the axial and proximal limb musculature.
 
In addition, some fibers of the medial reticulospinal tract establish inhibitory synaptic contacts with the first order neurons that innervate muscle spindles. In that way, this tract prevents the transmission of afferent input from those muscle spindles, which has an inhibitory effect on stretch reflexes. This is important because it results with refining of voluntary movement by preventing unnecessary contractions that would result with shaking.

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Medullary (lateral) reticulospinal tract

This tract originates from the medial zone of the rostral medullary reticular formation. Specific nuclei whose bodies elongate the fibers of the tract are gigantocellular reticular nucleus and ventral reticular nucleus. This tract also descends in the ipsilateral anterolateral funiculus of the spinal cord. It ends by sending numerous nerve fibers to all levels of the spinal cord, where they synapse primarily with interneurons in the intermediate zone of gray matter.

The effect of this pathway is the opposite to that of the medial reticulospinal tract. This means that it has an inhibitory effect on extensors and an excitatory effect on flexors of the axial and proximal limb musculature.
 
Additionally, some fibers of the lateral reticulospinal tract synapse with the lower motor neurons of the lateral corticospinal tract that innervate the distal muscles of the upper and lower limbs. These synapses have primarily an inhibitory influence on the lower motor neurons that innervate axial extensor muscles, and an excitatory influence on the lower motor neurons that innervate limb flexors. If we remind ourselves that the lateral corticospinal tract functions in the execution of fine, skilled movements of the hands and feet, this means that the lateral reticulospinal tract enhances the finesse of these skilled movements.
 
Both medial and lateral reticulospinal tracts also control muscle tone and reflex activity. To accomplish those tasks, these pathways fit into the pattern of the reciprocal inhibition, which means that during contraction of flexor muscles there is a simultaneous relaxation of the antagonistic extensor muscles. That is why these pathways have opposite functions. 
 
The medial and lateral reticulospinal tracts, with the addition of the lateral vestibulospinal tract, function in maintaining balance and making postural adjustments. Muscle tone, balance maintenance, and postural adjustments form a necessary background upon which voluntary movement is executed, which explains why these pathways have numerous synapses with the lower motor neurons.

Control of eye movement

The nuclei that give rise to the reticulospinal tracts take part in the control of eye movement. Specifically, those are:

• Oral pontine reticular nucleus
• Caudal pontine reticular nucleus
• Gigantocellular reticular nucleus

These nuclei receive sensory information from the visual, auditory, and vestibular systems, and function in head and eye coordination.

Oral and caudal pontine reticular nuclei form a subset of neurons that is called paramedian pontine reticular formation. This subset receives afferent fibers from the frontal eye fields, the superior colliculus, and the vestibular nuclei. Efferent fibers from the paramedian pontine reticular formation terminate:

• In the ipsilateral nucleus of the abducens nerve that innervates the ipsilateral lateral rectus muscle.
• Via the medial longitudinal fasciculus, ending within the contralateral nucleus of the oculomotor nerve to synapse specifically with the motor neurons that supply the contralateral medial rectus muscle.

In this way, the paramedian pontine reticular formation functions in mediating conjugate horizontal eye movements elicited by head movements.

Highlights

• Pontine (medial) reticulospinal tract originates from the oral pontine reticular nucleus and caudal pontine reticular nucleus.
  • It extends through the ipsilateral spinal cord to synapse with interneurons and motor neurons and with the first order muscle spindle afferents.
  • It stimulates the extensor and inhibits the flexor muscles of the trunk and proximal limbs.
  • Prevents the transmission of muscle spindle afferent signals, which has an inhibitory effect on stretch reflexes.
• Medullary (lateral) reticulospinal tract originates from the gigantocellular and ventral reticular nucleus.
  • It extends through the spinal cord (primarily ipsilateral) to synapse with interneurons and motoneurons
  • It inhibits the extensor and stimulates the flexor muscles of the trunk and proximal limbs.