Neuro pathways Flashcards
Modalaties of Sensation and Corresponding Tracts
Posterior Column Pathway
FIRST ORDER NEURONS
The first-order neurons in the pathway are located in the dorsal root ganglia at all spinal levels, giving rise to the fasciculus gracilis tract in the lower extremity and the fasciculus cuneatus tract in the upper extremity. The axons comprising these funiculi ascend ipsilaterally to the medulla, where they synapse with the second-order neurons.
SECOND ORDER NEURONS
The second-order neurons are located in the cadual medulla, and their cell bodies form the gracile and cuneate nuclei. Their axons, referred to as internal arcuate fibers, decussate to form the medial lemniscus, which ascends the contralateral brainstem to project to the ventral posterolateral (VPL) nucleus of the thalamus.
THIRD ORDER NEURONS
The third-order VPL neurons send axons through the posterior limb of the internal capsule to the somatosensory cortex (areas 3, 1, 2)
Spinal cord lesions affecting the dorsal column (e.g., vitamin B12 neuropathy, tabes dorsalis) result in ipsilateral sensory deficits below the lesion, because the pathway does not decussate until it is at the level of the medulla.
Spinothalamic tract
The Lateral Spinothalamic Pathway is an ascending spinal tract, carrying sensoryinformation to the brain. It is typically depicted as a chain of three neurons: first-, second-, and third-order neurons.
This pathway mediates sensation of pain and temperature.
The first-order neurons in the pathway are located in the dorsal root ganglia at all spinal levels. Their axons ascend the tract of Lissauer, and synapse with second-order neurons.
The second-order neurons are located in the dorsal horn, and their axons immediately decussate via the ventral white commissure. These axons ascend the lateral funiculus and project to the ventral posterolateral (VPL) nucleus of the thalamus.
Some collaterals are sent to areas involved in arousal, namely the midbrain reticular formation, and the intralaminar nuclei of the thalamus (which then project to the caudatoputamen, and frontal and parietal cortex).
The third-order VPL neurons send axons through the posterior limb of the internal capsule to the somatosensory cortex (areas 3, 1, 2).
LESIONS TO THE LATERAL SPINOTHALAMIC PATHWAY
Spinal cord lesions affecting the Lateral Spinothalamic pathway result in contralateral sensory deficits below the lesion, because the pathway immediately decussates at the second-order neuron level.
VENTRAL SPINOTHALAMIC PATHWAY
There is also a Ventral Spinothalamic Pathway, that carries crude touch sensation. It is organized very similarly to the Lateral Spinothalamic pathway; however, it is less clinically-emphasized since the Dorsal Column Medial Lemniscus pathway is more important for touch sensation. If the Ventral Spinothalamic pathway is lesioned, touch sensation will only be minimally affected, as long as the dorsal column remains intact.
CORTICOSPINAL TRACT
UMN lesion vs LMN lesion
Bell’s Palsy vs Central
OPTIC TRACT LESIONS
CRANIAL NERVES
