Extrapyramidal System

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.

Gray matter - ventral view

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

Midbrain - medial view

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

Pons - lateral left view

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

Medial vestibular nucleus - ventral view

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.