Question

Explain in cleaar detail:

Trigeminal sensory pathways : Discriminative touch pathway, Pain and temperature pathway

Answer 1

Trigeminal nerve sensory supply: most of the face including mouth and eyes (excluding angle of jaw), dura and most of the cranial blood vessels

Most sensory fibers in the trigeminal nerve have their cell bodies in the trigeminal ganglion(located in the lateral wall of the cavernous sinus, immediately lateral to the pituitary gland).

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The trigeminal nerve enters the brain stem through the middle cerebellar peduncle of the pons.

There is a large sensory and a small motor root for this nerve.

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Once inside the pons, sensory fibers segregate into three bands:

-Proprioceptive fibers (largest, most myelinated), extend into the mesencephalon to reach their sensory ganglion cells, located in the mesencephalic nucleus of the trigeminal.

-Sensory axons conveying touch, pressure, and vibration terminate in the chief or principal sensory nucleus of the trigeminal

-The small diameter sensory nerve fibers conveying pain and temperature sensation run caudally along the lateral side of the brain stem through the pons and medulla as the spinal tract of the trigeminal, and they terminate in a long nucleus called the spinal nucleus of the trigeminal.

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From the spinal nucleus, second-order sensory axons decussate in the brain stem to lie in close proximity to the spinothalamic tract. This forms the trigeminal lemniscus or the ventral trigeminothalamic tract which terminates in the contralateral ventral posteromedial nucleus (VPM) of the thalamus.

Second order sensory fibers from the chief sensory nucleus of the trigeminal (conveying touch and pressure sensation) also decussate to follow the ventral trigeminothalamic tract.

However, some fibers from this nucleus also follow the dorsal trigeminal thalamic tract to the ipsilateral VPM of the thalamus. Therefore touch, pressure, and vibration sensory fibers terminate bilaterally in the VPM.

  

The third order neuron from the VPM of the thalamus projects to the face area of the somatosensory cortex on the inferior portion of the postcentral gyrus where initial cortical processing takes place.