The Human Brain : From Neurone to Nervous System

THE BRAINSTEM


  1. The brainstem is divided into three sections: medulla, pons and midbrain, and consists of networks of neurones known as the Reticular Formation, together with
    • Ascending Pathways that carry information from the Spinal Cord pass through the brainstem to reach their target nuclei in the forebrain.
    • Descending pathways from the brain.
    • The nuclei of origin of many of the cranial nerves

  2. 2. Ascending pathways
    • Dorsal Column-Medial Lemniscal System :
      Sensory neurones with cell bodies in the dorsal root ganglia project through the dorsal column-medial lemniscal pathway, which carries information about touch and vibration to the dorsal column nuclei (cuneate and gracile nuclei); there they synapse with second order neurones that carry this information to the thalamus, which in turn relays the information to the primary receiving ares of the cerebral cortex in the post-central gyrus.

    • Antero-Lateral System:
      Neurones with a cell body in the the spinal cord have long axons that ascend to the thalamus, cerebellum or brainstem, and therefore known as the spino-thalamic, spino-cerebellar or spino-reticular tracts. The anterolateral system carries information about pain and temperature to the thalamus: the spinothalamic tracts are the classical pathways, but the anterolateral system also includes other pathways that synapse in the medulla, pons and midbrain (the spino-reticular and spino-mesencephalic tracts), which also project to the thalamus, and are regarded as alternative pain pathways.

    • Spino-Cerebellar System: The spino-cerebellar tracts carry information about the position of limbs to the cerebellum. This pathway is used in the coordination of movements.

    3. Descending Pathways:
    • Cortico-Spinal Tracts: The main descending pathway from the cortex to the motoneurones is the corticospinal tract, and this consists of bundles of axons that travel in the cerebral peduncles of the midbrain, through the reticular formation of the pons, and then cross the midline in the medulla. The decussation of the pyramids is the cross-over site before these axons enter the lateral columns of the spinal cord.

    4. Reticular formation and its Projections
    • The substance of the brainstem, between the bundles of axons that form the ascending and descending tracts is known as the reticular formation. The reticular formation consists of groups of neurones with less well defined boundaries than the nuclei of the cranial nerves that contain characteristic histochemical properties and neurotransmitters. Examples are the basal forebrain cholinergic neurones and the serotoninergic raphe nuclei. Some of these project to different areas of the brainstem, the forebrain (the ascending reticular formation) or to the spinal cord (the descending reticular formation). Neurones in the ascending reticular formation are concerned with the sleep-waking cycle; and the descending reticular formation controls breathing, the cardiovascular system, muscle tone and nociceptive transmission in the dorsal horn.

    5. Cranial Nerves:
    • Twelve cranial nerves innervate the head, neck and thoracic and abdominal viscera.

    Key Words: Dorsal Columns, Dorsal column nuclei, medial lemniscus, thalamus: dorsal column-medial lemniscal system concerned with touch and vibration.
    Spino-thalamic and Spino-reticular pathways concerned with pain and temperature sensation.
    Spino-cerebellar pathways concerned with transmission of proprioceptive information to the cerebellum.
    Twelve Cranial Nerves


More details on Ascending Pathways, the Corticospinal Pathway, the Reticular formation










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