Extrapyramidal System

Introduction

A collection of nerve fibres connecting two masses of grey matter within the central nervous system is referred to as a tract, system or pathway. They are often referred to as fasciculi (bundles) or lemnisci (ribbons). Tracts may be ascending or descending, and are usually named after the masses of grey matter they interconnect. Therefore, a pathway that originates in the cerebral cortex and descends to terminate in the spinal cord is referred to as a corticospinal tract, while a tract ascending from the spinal cord to the thalamus is called the spinothalamic tract.

The extrapyramidal pathway or system is an important part of the motor system of the body and can also be described as the descending motor pathway, whose fibres pass through the tegmentum rather than the medullary pyramid. The extrapyramidal pathway is actively involved in the initiation and selective activation of movements, along with their coordination. Major contribution in the extrapyramidal motor system are the nuclei of the basal ganglia. Other structures which are involved include substantia nigra, red nucleus, subthalamic nucleus, mesencephalic reticular formation and the cerebellum. The extrapyramidal system is also often described as the motor-modulation system. The term “extrapyramidal” is to distinguish between the effects of basal ganglia diseases and those of damage to the “pyramidal” system, even though there is an intertwine of a functional relationship between the two systems. Extrapyramidal system is polysynaptic in nature with many synapses within the brainstem.

Components

Most of the descending pathways running from the cerebrum, cerebellum and brainstem towards the spinal cord, without coursing through the pyramids of the medulla, are considered part of the extrapyramidal system. Such tracts include:

• Parts of the rubrospinal tracts
• Tectospinal tracts
• Reticulospinal tracts (lateral and medial)
• Lateral and medial vestibulospinal tracts

Rubrospinal tracts

Fibres originating from the red nucleus (which lies in the upper part of the midbrain) course to the spinal cord, as rubrospinal tracts, but decussate to the opposite side in the lower part of the tegmentum of the midbrain. Such crossing fibres constitute what is referred to as the ventral tegmental decussation. The extrapyramidal part of the rubrospinal tracts descend through the pons and medulla to enter the lateral funiculus of the spinal cord, where they terminate by lying immediately anterior to the lateral corticospinal tract (pyramidal tract).

Tectospinal tracts

Another collection of extrapyramidal tracts originates from the superior colliculus. These tracts are referred to as the tectospinal tracts and cross in the posterior tegmental decussation of the midbrain. Fibres of the tract synapse with motor neurons of the cervical muscles and mediate reflex postural movements. They follow the same course of the rubrospinal tracts by passing through the pons and medulla but terminate in the anterior funiculus of the spinal cord.

Reticulospinal tracts

The lateral reticulospinal tracts, which originate in the ventrolateral part of the reticular formation of the pons, cross to the opposite side in the medulla and run inferiorly in the lateral funiculus of the spinal cord. This pathway is associated with motor functions and in the control of pain perception.

In the medial parts of the reticular formations of the pons and medulla, arise the medial reticulospinal tracts. Pontine fibres of these tracts travel in the anterior funiculus and are mainly uncrossed. Medullary fibres travel in the anterior and lateral funiculus as crossed and uncrossed. This tract is mainly concerned with posture.

Vestibulospinal tracts

The medial and lateral vestibulospinal tracts originate in the medial vestibular and lateral vestibular nucleus respectively. The lateral vestibulospinal tracts, are particularly important for the balancing of movements. They lie in relation to the ventral grey column of the spinal cord in the anterior funiculus, where they terminate without crossing as they course through the medulla.

On the other hand, most of the fibres of the medial vestibulospinal tract decussate as the tracts descend through the medulla to terminate in the anterior funiculus within the cervical region of the spinal cord. Thier axons inhibit motor neurons of neck axial muscles.