Extrapyramidal Motor Pathways

The extrapyramidal pathways are those motor pathways that do not pass through the pyramids of the medulla oblongata. They consist of central pathways that modulate CNS motor areas in cerebral cortex, cerebellum, the brain stem, and spinal cord. The primary function of the extrapyramidal system is the 'fine-tuning' of voluntary movement to render it amenable to higher levels of conscious control. The absence of such fine-tuning becomes obvious in conditions such as parkinsonism (see p. 370), when voluntary movement is hampered through the presence of uncontrollable tremor in the hands, for example.

Extrapyramidal fibers may originate in the frontal or parietal cortex, and travel to the cerebellum, or to other major extrapyramidal sites such as the striatum, the substantia nigra, reticular formation, tegmental nuclei, and the red nucleus. The corticopontocerebellar tracts, for example, connect the cerebral cortex with the contralateral cerebellum. Since the cerebellum also receives afferent inputs from the peripheral musculature, it is well placed to integrate this information and produce responses designed to maintain posture and purposeful movement.

From these central structures, second-, or third-order neurons, or both, project downwards through the spinal cord in various pathways, and at different levels and give off branches that synapse with motoneurons or interneurons in the gray matter of the spinal cord.

The reticulospinal pathways originate in the reticular formation of the pons and medulla. The pathways from the pons lie medially in the cord as the ipsilateral pon-tine reticulospinal tract, while those that arise in the medulla travel as the lateral or medullary reticulospinal tract. These pathways modulate a- and ^-motoneuron activity, particularly in the control of breath ing, and circulatory pressor and depressor activity.

The rubrospinal tract originates in the red nucleus in the midbrain tegmentum, decussates in the ventral tegmental decus-sation, and travels down the cord partly mingled with the lateral corticospinal tract. The pathway is excitatory to moto-neurons that contract limb flexor muscles.

The tectospinal tract fibers arise in the superior colliculus of the midbrain and decussate in the dorsal tegmental decussa-tion. The pathway descends the cord proximal to the ventral median fissure, and most fibers terminate in cervical segments. The tract is believed to carry motor responses to visual inputs received in the superior colliculus.

The vestibulospinal tracts originate in the vestibular nuclei, which receive inputs from the labyrinthine system of the ear via the cerebellum and the vestibular nerve. The medial vestibular nucleus gives rise to the ipsilateral medial longitudinal fasciculus, or medial vestibulospi-nal tract, while the lateral vestibular nucleus gives rise to the lateral vestibulospi-nal tract, which excites spinal moto-neurons which contract extensor muscles involved in antigravity maintenance of posture.

It is important to note that both pyramidal and extrapyramidal descending influences ultimately meet at the moto-neuron to modulate its activity. The clinical implications of damage to both extrapyramidal and pyramidal tracts are dealt with elsewhere (see pp. 42,370), but it should be appreciated that damage to only the pyramidal system will result in flaccid paralysis, whereas damage to extrapyramidal pathways as well results in spastic paralysis.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • Kristiina
    Is premotor cortex extrapyramidal?
    7 years ago
  • tesfalem
    What is the extrapyramidal system?
    2 years ago

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