Extrapyramidal system

Structure and components

The extrapyramidal system is composed of a cluster of interconnected nuclei that are located deep within the white matter of the brain. Broadly speaking, these nuclei receive the nerve impulses from the cerebral cortex and send projections to the brainstem and spinal cord.

The extrapyramidal nuclei are the following:

• The basal ganglia
• Deep nuclei of cerebellum
• Nuclei of the brainstem: red nucleus, substantia nigra, vestibular nuclei and reticular formation

These structures communicate with each other via many neural circuits. The three of the most important circuits begin within the basal ganglia, and are called the direct, indirect and hyperdirect pathways of the basal ganglia. These three pathways connect the basal ganglia with the cerebral cortex, thalamus, subthalamus and brainstem and regulate the initiation and termination of movements, as well as the selection of appropriate movements and the suppression of inappropriate ones.

The next major component is the cerebellum. The cerebellum has numerous connections with the brainstem, especially with the reticular and vestibular nuclei, and with the thalamus. It is mainly in charge of the coordination of movement.

If you’re looking for comprehensive study materials about the basal ganglia and cerebellum, check out these study units:

After the nuclei of the extrapyramidal system finish the processing of neural impulses, their outputs are conveyed via four major pathways. These pathways descend from the brainstem to the spinal cord and carry the “extrapyramidal messages” down the road to be executed. Namely, they are the:

• Reticulospinal tract
• Vestibulospinal tract
• Rubrospinal tract
• Tectospinal tract

Reticulospinal tract

The reticulospinal tract is one of the pathways of the corticoreticulospinal system. This system gathers all the pathways that receive the impulses from the cerebral cortex, start within the reticular formation of the brainstem and terminate within the spinal cord. Put simply, this tract controls body movements by involving the reticular formation.

The reticulospinal tract originates from the reticular nuclei of inferior pons and medulla oblongata. It is divided into two tracts; the medial (pontine) reticulospinal tract and the lateral (medullary) reticulospinal tract.

The medial reticulospinal tract originates from the pontine reticular formation. Its fibers descend ipsilaterally through the brainstem and terminate within the ventromedial spinal cord (ventral funiculus) by synapsing with the neurons of this region. This portion of the spinal cord contains alpha and gamma motor neurons which innervate the extensors of the trunk and upper limb. That said, the activity of the medial reticulospinal tract fine tunes the voluntary movements of these muscles by synapsing with alpha motor neurons that innervate them, but also influences their reflexes and muscle tone by controlling the gamma motor neurons.

The lateral reticulospinal tract originates from the medullary reticular formation. A part of its fibers decussate and descend through the contralateral side, while the remainder continues descending ipsilaterally. Its fibers terminate in the ventral funiculus and ventral portion of the lateral funiculus along the entire length of the spinal cord. It synapses with the interneurons in the intermediate zone of gray matter, which are known as the neurons that inhibit the activity of the alpha and gamma motor neurons. For this reason, the action of the lateral reticulospinal tract is almost completely opposite to the action of the medial reticulospinal tract, as it inhibits the activity of the extensors of the trunk and the upper limb, while it facilitates the activity of the flexors of these regions.

The exact functions of the reticulospinal tracts are still under discussion, however according to the majority of research, it is believed that they mainly influence the tone of the postural musculature. In other words, these tracts send motor impulses to regulate the stiffness of the antigravitational muscles. Some authors believe that this tract also controls respiration through the action of motor neurons of phrenic and intercostal nerves. Furthermore, it possibly affects sweating and other autonomic responses.

Vestibulospinal tract

The vestibulospinal tract also originates from the medulla and pons. Depending on its nucleus of origin, it is divided into medial and lateral vestibulospinal tracts.

• The medial vestibulospinal tract originates from the medial vestibular nuclei (Schwalbe's nucleus) located in the medulla. It projects into the spinal cord, sending efferent impulses to the motor neurons that innervate the muscles of the neck and upper limbs.
• The lateral vestibulospinal tract originates from the pons, more specifically from the lateral vestibular nuclei or the Deiter's nucleus. From here, it sends efferent fibers ipsilaterally to the laminae VII and VIII of the spinal cord. These laminae contain motor neurons and interneurons that control the extensor musculature and enable the erect body posture.

In general, the main function of both of these tracts is to regulate posture and balance. More specifically, they are in charge of maintaining the balance while a person is preparing for a certain movement. Moreover, when a movement starts, these tracts are continuously sending impulses to synchronise the ongoing movement with adequate posture.

Rubrospinal tract

The rubrospinal tract originates from neurons of the caudal part of the red nucleus, located centrally in the midbrain tegmentum. This tract mainly transmits signals that arrive in the red nucleus from the motor centers in the cortex and cerebellum. The rubrospinal tract then crosses contralaterally and descends into the ventrolateral part of the spinal cord. The tract terminates with the laminae V to VII of the spinal cord, where it synapses with motor neurons that innervate flexors of the body.

The main function of the rubrospinal tract is to maintain the muscle tone of these muscles and to modulate their movements that are directed by the pyramidal system.

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Tectospinal tract

The tectospinal tract (colliculospinal tract) originates from the superior colliculus located in the dorsal midbrain. It projects inferiorly to the cervical and upper thoracic portions of the spinal cord. Its fibers terminate in the contralateral anterior gray horn, specifically within the laminae VI to VIII. Through the superior colliculus, the tectospinal tract receives auditory stimuli from the auditory pathway, as well as visual impulses directly from the retina and the visual cortical association centers.

For that reason, the tectospinal tract represents the crucial link between the visual and auditory stimuli and muscle movements. In other words, this tract is usually activated by a sudden loud noise, movements or bright lights appearing in the visual field. When activated, it sends efferent signals to activate the muscles of the head, neck and upper extremities which adjust the body position and reaction in accordance with the external stimuli. This is why this tract is crucial for protection and tasks that require visual (eye-hand) and auditory (ear-hand) guidance.

For more details about the spinal cord tracts, including the extrapyramidal ones, take a look at our study unit about the cross-section of the spinal cord.

Functions

The extrapyramidal system has a major role in adjusting and fine-tuning the voluntary movements directed by the pyramidal system, as well as in the control of involuntary movements and postural tone. In summary, the roles of the EPS are as follows:

• Posture maintenance and adjustment
• Planning and preparation of the involuntary movements
• Fine-tuning and adjustment of the voluntary movements making them more precise and correct
• Control of the reflex reactions
• Control of the automatic voluntary movements (e.g. walking, riding a bicycle)
  Inhibition of involuntary movements