Neuroscience/Objectives/Lecture 15

From PhysioWiki

Spinal cord somatosensory: Touch and conscious proprioception; unconscious proprioception

Draw and label the three-neuron chain by which sensory information is conveyed within the CNS.

List three possible connections made by dorsal root axons entering the spinal cord and their general function.

1. Interneurons or α-motor neurons
   • Subserve local (segmental) reflexes
2. Dorsal column (gracile and cuneate) nuclei
   • Part of dorsal column/medial lemniscal system
   • Epicritic sensation (conscious proprioception and discriminative touch)
3. Dorsal gray (lamina I especially)
   • Part of spinothalamic (anterolateral) system
   • Protopathic sensation (nociception and temperature)

Diagram the dorsal root entry zone showing the pathways taken by axons in the medial and lateral divisions.

Lateral division

• Thinly myelinated or unmyelinated (groups III and IV or Aδ and C) fibers
• Nociception and itch
• Carried in spinothalamic tract

Medial division

• Heavily myelinated (groups I and II or Aα and Aβ) fibers
• Proprioception and discriminative touch
• Carried in dorsal columns (part of DCML system)

Diagram the major pathway by which information from the body about touch and limb position reaches consciousness.

(H:7-4) These epicritic sensations reach consciousness via the dorsal column/medial lemniscal system. Sensation begins with encapsulated nerve fibers whose cell bodies reside in a dorsal root or cranial nerve ganglion. These first-order neurons have multiple collaterals: some synapse on ipsilateral interneurons and subserve local reflex activity; the remaining collaterals project via the ipsilateral dorsal columns to the dorsal column (cuneate and gracile) nuclei where they synapse on second-order neurons. Axons from these fibers decussate in the caudal medulla, forming internal arcuate fibers that form the contralateral medial lemniscus and ascend to the thalamus, where they synapse on third-order neurons in the ventroposterolateral nucleus. These neurons in turn project to the postcentral gyrus (including BAs 3a, 3b, 1, and 2) to manifest conscious awareness of body position.

Compare the sensory information carried in the medial divisions of the dorsal roots with that in the lateral divisions of the dorsal roots.

Compare each with respect to identifying:

• the type of information carried
• the location of the cell bodies of the first-, second-, and third-order neurons
• where the axons of the first-order neurons synapse within the spinal cord or medulla
• which CNS pathway carries the information to higher levels of the nervous system
• where the axons cross

Describe the topographic relationships between sensation from different regions of the skin (dermatomes) at the following levels of the central nervous system.

Within the spinal cord

The DCML enters the spinal cord via the medial division of DRG fibers. Inferior to T5, sensory information is carried via the gracile fasciculus. Superior to T5, the cuneate fasciculus wedges itself in (cuneatus means "wedge") and carries information from the upper limbs and body.

Within the pathways

The dorsal columns are somatotopically organized as discussed above. After they synapse with second-order neurons in the dorsal column nuclei, fibers remain somatotopically organized in the medial lemniscus (which has its origin in the caudal medulla). Within the medulla, posterior fibers of the medial lemniscus carry information from the upper limbs and body, while anterior fibers carry information from the lower limbs and body. Rostral to the inferior olivary nucleus, the anterior margin of the medial lemniscus begins to migrate laterally, while the posterior area maintains its medial position. By the level of the pons, the medial lemniscus is in a complete medial-lateral orientation, with medial fibers carrying upper body sensations and lateral fibers carrying lower body sensations. This orientation is maintained until the thalamus, at which point the medial lemniscus is absorbed by the VPL.

Within the relay nuclei

Second-order neurons that subserve the lower body are found medially in the gracile nucleus, while second-order neurons for the upper body are found laterally in the cuneate nucleus. In the thalamus, fibers from the gracile nucleus (lower body) terminate on the lateral aspect of the VPL, while fibers from the cuneate nucleus (upper body) terminate on its medial aspect.

Within the cortex

Cortical somatotopy is maintained, with upper body afferents targeting the lateral aspect of the postcentral gyrus and the lower body targeting its medial aspect.

Identify the pathways that transmit unconscious proprioception to the cerebellum.

Unconscious proprioception is transmitted via two pathways to the cerebellum: the dorsal spinocerebellar and the cuneocerebellar pathways.

Dorsal spinocerebellar pathway

The DSP carries proprioceptive information from the leg and lower body. First-order neurons in the DRG synapse on Clarke's nucleus (nucleus dorsalis), which is present from T1-L2. Clarke's nucleus contains second-order neurons, which ascend in the ipsilateral dorsolateral white matter to the brainstem and through the inferior cerebellar peduncle to the cerebellum.

Cuneocerebellar tract

Upper limb and thoracic proprioception is conveyed via first-order neurons in the DRG that ascend via the ipsilateral cuneate fasciculus. These fibers synapse on second-order neurons in the ipsilateral lateral (or external) cuneate nucleus, which project to the cerebellum through the inferior cerebellar peduncle.

Compare the pathways for unconscious proprioceptive sensation from the foot, from the trunk, and from the shoulder.