The cerebral cortex and the brainstem are the two main sites in the brain that communicate directly with motoneurones in the spinal cord, and in the cranial nerve nuclei. These pathways are:

The cortex also communicates with the brainstem using several types of cortico-bulbar axons that terminate in:

• motor nuclei of the cranial nerves
• pontine nuclei that project to the cerebellum
• the red nucleus of the midbrain, the origin of the rubro-spinal tract                    }
• the brainstem reticular formation, the origin of the reticulo-spinal pathways     } so these form polysynaptic pathways from cortex to cord.

The planning, initiation, coordination and execution of movements also involve pathways between the:

The main pathways from the brainstem to the motoneurones are:

• Reticulo-spinal tracts
• Rubrospinal tract
• Vestibulo-spinal tracts

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

They are possible alternative routes to the corticospinal tract in the control of movement, since the cortex sends axons to make contact with them. What's more these pathways have potential benefits in that the output of the cerebellum converges on some of them.

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 thoise that innervate extensor muscles. Also the perikearya of motoneurones that innervate distal muscles, such as hand or foots muscles are found lateral to those innervating proximal muscles in the limb or trunk.

Muscle Tone (tonus) is the resting activity of muscle generated by alpha and gamma motoneurones and accounts for the inherent stiffness of muscle, the resistance to movement of a muscle group that is relaxed. Tone can be altered during postural changes, and following certain lesions, and this results from a changes in the balance of excitatory and inhibitory inflences on the motoneurones.

In normal conditions, the brainstem plays a major role in contributing both excitatory and inhibitory pathways that regulate tone. All three major bulbo-spinal pathways, reticulospinal, rubrospinal and vestibulospinal, affect tone; and muscle tone and posture alter when the activity of any of these systems changes.

Thee systems may act not only on the ventral horn, but may modulate transmission in the dorsal horn, changing the excitability of reflexes, such as the flexor reflex, and also have effects on the autonomic system.

Voluntary movements are initiated in the pre-motor and supplemetary motor cortices and are executed by descending pathways such as the executve pathways the corticospinal tract; but there is considerable evidence that other pathways such the rubrospinal tract, and reticulospinal tracts, are involved alongside the corticospinal pathway.

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Brainstem Reticular Formation    Top

The reticulospinal tracts are divided into two - the pontine and medullary reticulospinal pathways. They originate in the brainstem reticular formation, and descend in the anterior white matter of the spinal cord; their projections include motoneurones and interneurones but they also target transmission in the dorsal horn and have effects on the autonomic outflow.

One of the larger nuclei of the reticular formation, the Nucleus gigantocellularis of the medulla, inhibits extensor tone and also has actions in the dorsal horn and autonomic systems.

In contrast the pontine reticulospinal pathways increase extensor tone, so the two parts of the reticulospinal system have antagonistic effects on motoneurones and muscle tone.

By one or other of these mechanisms the reticulospinal pathways can therefore modulate the sensitivity of spinal reflexes, such as the flexion reflex, and be involved in changes in tone and postural responses.

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. The axons make contact with motoneurones and interneurones at all levels of the spinal cord, and the dominant effect is contraction of flexor muscles and inihbition of extensors.

The rubrospinal tract is an alternative pathway by which voluntary motor commands can be sent to the spinal cord. It is sometimes said that it is not of great significance in humans, but this view is being disputed..

The red nucleus is a major relay station on the pathway between the cerebellum and the cerebral cortex, and is able to combine motor commands with cerebellar feedback signals. The cerebellum is believe also to be the site of motor learning and memory, and the rubrospinal system may be involved in modifying cortical commnds by incorporting cerebellar memory traces. It is possible that recovery of some voluntary motor functions after damage to the corticospinal tract may be dependent on the red nucleus. It may play a part in controlling the speed of movements, as rubrospinal lesions can cause a movement to slow down.

The Vestibulospinal Tracts and the Control of Muscle Tone   Top

Depending on the inputs from the vestibular apparatus, compensatory changes in muscle tone throughout the body are transmitted from the vestibular nuclei of the brainstem to the motoneurones in spinal cord using the vestibulospinal tracts. The neurones involved have cell bodies in the vestibular nuclei in the medulla, and long axons that conducts action potentials to the interneurones and motoneurones in the spinal cord.

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.

The vestibulospinal tracts help to maintain balance, as a result of inputs from the vestibular apparatus that signal changes in the position or the rotation of the head. They adjust the tone of the musculature to compensate for movements that would otherwise upset balance, and this is done by modifying the excitation of different motoneurone groups in the spinal cord. Many reflex effects have been described that indicate a role for the vestibulospinal pathways in posture. Only one will be described here:

When the head is tilted to one side, the saccule and utricle change their resting activity due to the change in the direction of gravity affecting the head. The vestibulospinal pathway is responsible for activating extensor muscles on the side of the neck to which the head is tilted, and flexor activity on the other side of the body.