The extrapyramidal system describes a number of centers and their associated tracts whose primary function is to coordinate and process motor commands performed at a subconscious level.
The anatomic centers and tracts are separate and apart from the pyramidal system, whose fibers reach their target muscles by traveling through the pyramids of the medulla and carrying their messages via the corticobulbar and corticospinal tracts.
The processing centers of the extrapyramidal system are listed below. These processing centers produce output to a variety of targets, including (1) the primary motor cortex to modulate the activities of the pyramidal system; (2) the cranial nerve nuclei to coordinate reflex activities in response to visual, auditory, and equilibrium input; and (3) descending pathways into the spinal cord including the vestibulospinal tracts, the tectospinal tracts, the rubrospinal tracts, and the reticulospinal tracts whose functions are outlined in Table 2.
The extrapyramidal system.
The Primary Processing Centers of the Extrapyramidal System
| Processing Center | Location | Primary Function |
|---|---|---|
| Vestibular nuclei | Pons and medulla oblongata | Processing of balance and control of associated reflexes of equilibrium |
| Superior colliculi | Mesencephalon | Processing of vision and control of associated reflexes |
| Inferior colliculi | Mesencephalon | Processing of hearing and control of associated reflexes |
| Red nucleus | Mesencephalon | Processing and control of skeletal muscle tone |
| Reticular formation | Mesencephalon | Processing of incoming sensory information and outgoing motor commands |
| Cerebral nuclei | Cerebrum | Organization and coordination of extremity and trunk movement |
| Cerebellar nuclei | Cerebellum | Coordination and integration of movement and integration of sensory feedback |
TABLE 2. Extrapyramidal Tracts That Descend Directly into the Spinal Cord
| Extrapyramidal Tracts | Function |
|---|---|
| Vestibulospinal tracts | Transmit balance information directly into the spinal cord from the vestibular nuclei |
| Tectospinal tracts | Transmit commands to change the position of the head, neck, eyes, and arms in response to sudden movements, loud noises, and/or bright lights |
| Rubrospinal tracts | Transmit motor commands to spinal motor neurons to maintain muscle tone |
| Reticulospinal tracts |
Magnetic resonance image of a coronal section through the head at the level of the lentiform nucleus.
Magnetic resonance image of an axial section through the head at the level of the lentiform nucleus.
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