The bulbo-spinal pathways originate in the brainstem and send their axons into the spinal cord. They are pathways from the brainstem to the motoneurones and there are three main groups of neurones involved:

1. Reticulo-spinal tracts
2. Rubrospinal tracts
3. Vestibulo-spinal tracts

Because these pathways do not pass through the pyramids of the medulla, they are sometimes called 'extrapyramidal' by clinicians.

The Descending motor pathways are organized into two major groups:

(a) Pathways that run in the lateral white matter of the cord (including the lateral corticospinal tract and to a lesser extent the rubrospinal tract) control both proximal and distal muscles in the limbs, and are responsible for voluntary movements of arms and legs.

(b) Medial pathways in the anterior funiculi control axial muscles and are responsible for posture, balance, and coarse control of axial and proximal muscles. These include the anterior corticospinal tracts, the vestibulospinal tracts and the reticulospinal tracts. *

The cell bodies of motoneurones in the ventral horn that innervate flexor muscles are located dorsal to those that innervate extensor muscles. Also the perikarya of motoneurones that innervate distal muscles, such as hand or foot muscles are found lateral to those innervating proximal muscles in the limb or trunk.

Brainstem Reticular Formation

The reticulospinal tracts are divided into two- the pontine and medullary reticulospinal pathways. They originate in the brainstem reticular formation, a large, diffuse hotch-potch of neurones that surround the major nuclei and tracts within the brainstem.

These pathways are essentially ipsilateral pathways that project to motoneurones and interneurones at all levels of the spinal cord. The pontine group follow a more medial course through the anterior white matter of the cord, with the medullary projection being more lateral, but still within the anterior columns.

The pontine pathway influences muscle contractions and muscle tone - the resting tension in the muscles -and acts by exciting alpha and gamma motoneurones to extensor muscles. The more lateral medullary pathway tends to inhibitmuscle activity and the normal state depends on the balance between these pathways. Other sits of action of the reticulospinal system are the dorsal horn and the autonomic outflow.

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Rubrospinal tract.

The rubrospinal tract originates in the Red Nucleus of the midbrain; the axons cross the midline and pass down the lateral whiote matter of the spinal cord, close to the lateral corticospinal axons.

Rubrospinal neurones project to all levels of the spinal cord.

The rubrospinal tract is said to be a minor pathway in humans, but that has been disputed. It can convey instructions to the motoneurones and also has a major input from the cerebellum as well as the cortex

Vestibulospinal tracts.

The vestibular nuclei give rise to the medial and lateral vestibulospinal tracts. The medial vestibulospinal tract descends in the anterior funiculus and innervates the upper half of the cord (above T6). It innervates neck muscles and stabilises the head on with respect to the trunk during changes of position of the body. It works in partnership with pathways that control the position of the head and eye movements.

The lateral vestibulospinal tract projects ipsilaterally through the anterior funiculus of the spinal cord to all segments of the spinal cord. It increases the tone of antigravity muscles and is concerned with compensatory adjustments in posture to accommodate tilts and movements of the body.

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The corticospinal tract is a direct pathway from the motor cortex to the ventral horn. It is accompanied by other neurones that have a relay in the brainstem, and in particular the red nucleus of the midbrain. Here the bulbo-spinal system receives a copy of the message sent doen the corticospinal tract, but the cerebellar input to these neurones allows a correction to be made so as to make movements more precise.

The diagram shows the descending pathways in the lateral column of the spinal white matter, and the corticospinal tract is accompanied by the rubrospinal pathway.

It also shows the ventromedial pathways that are concerned with the control of axial muscles, and shows that cortico-bulbar axons make contact with descending pathways in the reticular formation, the superior colliculus and the vestibular nuclei.

PM=premotor cortex; SMA= Supplementary motor area; PPC= posterior parietal cortex; Area 4= Motor cortex; LMNs= lower motoneurones; CPG=central pattern generators.

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