The Human Brain : From Neurone to Nervous System

Bulbo-Spinal Pathways - the Extrapyramidal System.

  1. Apart from the corticospinal (pyramidal) tract, there are several other pathways that originate in the brain and project to motoneurones. These arise in the brainstem and are of importance in the control of muscle tone; they are sometimes called 'extra-pyramidal' pathways.

  2. Some corticobulbar axons from the premotor and motor cortices terminate in the brainstem reticular formation, where they synapse on:
    • reticulo-spinal neurones concerned with movement and the tone of the body musculature - the stiffness of muscles (which is generated by tonic activity in alpha and gamma motoneurones), and

    • the rubro-spinal tract. The pathway originates in the red nucleus of the midbrain, and has a similar role to the cortico-spinal tract, except that there is a synapse in the red nucleus, which also receives an input from the cerebellum.

  3. There is a hierarchy of circuits controlling motoneurones: at the simplest level are the spinal reflexes; the development of the brainstem vestibular pathways and the flocculonodular lobe of the cerebellum added the sense of balance and the regulation of posture as well as eye movements. The reticulo- and rubro- spinal tracts provide some regulation of the stiffness or tone of somatic muscles.

  4. Key Words: Rubro-spinal, reticulospinal, vestibulospinal pathways; skeletal muscle tone; vestibulospinal reflexes