Tracts of the CNS

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🌱 來自: Huppert’s Notes

Tracts of the CNS🚧 施工中

Tracts of the CNS

The CNS includes the brain and spinal cord. It is composed of gray matter (cell bodies) and white matter (axons). The brain and spinal cord are somatotopically organized. Signals travel on “tracts” – i.e., descending motor tracts and ascending sensory tracts. Here is a summary of the main tracts.

Motor tracts

•   Pyramidal: Corticospinal and corticobulbar tracts (Figure 12.1A)

FIGURE 12.1: A) The main descending motor pathways. Shown is the brain and spinal cord with a depiction of the corticospinal (maroon) and corticobulbar (red) tracts. These tracts carry motor signals from the brain to the periphery (descending). B) The main ascending somatosensory pathways. Shown is the brain and spinal cord with a depiction of the spinothalamic tract (dark blue; pain, thermal sense) and the posterior column-medial lemniscus pathway (light blue; proprioception, vibration). These tracts carry sensory signals form the periphery to the brain (ascending).

-   Corticospinal tracts:

•   Anatomy: Neurons in the primary motor cortex (precentral gyrus), pre-motor and supplemental motor cortex → axons descend through the posterior limb of the internal capsule → enter the brainstem (cerebral peduncle in the midbrain) → basis pontis → medullary pyramids → tract divides into two tracts at the cervicomedullary junction: the contralateral lateral spinal tract and ipsilateral anterior spinal tract → descend and synapse onto the lower motor neuron (LMN).

•   Function: Voluntary control of movement from the neck to the feet

-   Corticobulbar tracts:

•   Anatomy: Same pathway origins, synapse onto the brainstem motor nuclei

•   Function: Voluntary control of face, head and neck movement

•   Extra-pyramidal: Ventromedial bulbospinal and ventrolateral bulbospinal

Sensory Tracts

•   Anterolateral: Anterior and lateral spinothalamic tracts (Figure 12.1B)

•   Anatomy: Smaller afferent fibers of the peripheral nerves enter the dorsal horn of the spinal cord → cross and ascend in the opposite anterior and lateral columns → enter the brainstem → thalamus (VPL nucleus → postcentral gyrus of the parietal cortex)

•   Function: Nociception (pain), temperature sensibility, touch

•   Posterior column-medial lemniscal pathway:

•   Anatomy: Larger afferent fibers of the peripheral nerves enter the spinal cord → ascend in the ipsilateral posterior column → first synapse in the gracile or cuneate nucleus in the lower medulla → cross and ascend in the medial lemniscus (located in the medial medulla and tegmentum of the pons and midbrain) → synapse in the thalamus (VPL nucleus) → parietal cortex

•   Function: Tactile, position sense, and kinesthesia

* Note: Other fibers carrying sensory information about pain, touch, vibration, and proprioception ascend in a diffuse pattern; therefore, a lesion in the posterior column may not result in sensory deficit.