SOMATIC SENSORY PATHWAYS

Peripheral  nerve receptive fields
Spinal dorsal root receptive fields:  dermatomes

  Spinal:  Dorsal column-medial lemniscal system Spinal:  Anterolateral system (ALS) Trigeminal:  Touch- pressure Trigeminal: Pain & temperature
Modalities Touch, pressure, vibration, proprioception Temperature (warm, cold), pain, itch, crude touch Touch, pressure, vibration, proprioception Temperature (warm, cold), pain, itch, crude touch
1st order axon A-beta A-delta & C-fiber A-beta A-delta & C-fiber
1st order cell body Dorsal root ganglion Dorsal root ganglion Trigeminal ganglion Trigeminal ganglion (exception: mastication muscle spindle receptors in the Mesencephalic Nucleus)
1st order path in C.N.S. Dorsal columns Synapse shortly after entering spinal cord Synapse shortly after entering pons Spinal tract of c.n. V; descend to medulla and spinal cord, then synapse
1st order synapse/2nd order cell body Medulla, cuneate (from upper body) and gracillis (from lower body) nuclei Spinal dorsal horn Main sensory nucleus of c.n. V Spinal nucleus of c.n. V (particularly caudalis)
2nd order axon Cross in medulla, ascend in medial lemniscus Cross in spinal cord, ascend in anterolateral tracts (spinothalamic and spinoreticular) Cross, ascend in trigeminal (medial) lemniscus Cross, ascend with anterolateral system (trigeminothalamic and trigeminoreticular)
2nd order synapse/3rd order cell body Thalamus VPL nucleus Thalamus VPL (spinothalamic) and intralaminar nuclei (spinoreticular) Thalamus VPM nucleus Thalamus VPM (trigmeinothalamic) and intralaminar nuclei (trigeminoreticular)
3rd order axon Thalamocortical specific projection system Thalamocortical specific projection system and diffuse projection system Thalamocortical specific projection system Thalamocortical specific projection system and diffuse projection system
3rd order synapse Somatosensory cortex layer IV in the corresponding somatosensory area (S1, 1-2-3a-3b); also S2 and M1 Somatosensory cortex layer IV in the corresponding somatosensory area (S1); also S2 and M1, insula (pain), and project widely to cortex Somatosensory cortex layer IV in the face area (S1); also S2 and M1 Somatosensory cortex layer IV in the face area (S1); also S2 and M1, insula (pain), and project widely to cortex
Function of cortical input Convey information about the stimulus Convey information about the stimulus and arousal Convey information about the stimulus Convey information about the stimulus and arousal

Notes:
Pathways involving the reticular formation involve many neurons and many synapses
Separate pathways for visceral sensation and cerebellar input

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EFFECTS OF PERIPHERAL AND SPINAL CORD LESIONS

Site of Lesion Sensory effect of Lesion Motor effect of lesion (skeletal muscle)
Peripheral nerve Anesthesia of peripheral nerves receptive field Flaccid paralysis of muscles innervated by nerve (LMN lesion); no reflex, muscle atrophy, fasciculations
Dorsal root or dorsal root ganglion Anesthesia of corresponding dermatomes Impaired coordination due to reduced proprioceptive feedback
Ventral root or alpha-motoneuron No effect Flaccid paralysis or paresis
Total section of the spinal cord Anesthesia below the level of the section Spastic paralysis below the level of the section (UMN lesion); enhanced reflex & tone, clonus, Babinski sign
Hemisection of the spinal cord (right or left side); note effect of selective lesion in the dorsal quadrant only (sensory effects) or ventral quadrant only (motor effects) Below the level of the section, loss of ipsilateral touch-pressure and contralateral pain-temperature Below the level of the section, ipsilateral paresis or spastic paralysis
Primary somatosensory cortex (S1) Contralateral loss of sensation from corresponding  somatotopic areas  
Primary motor cortex (M1) or corticospinal tract in brain   Contralateral paresis or spastic paralysis in corresponding somatotopic areas

Notes
Cranial nerves and cortico-bulbar lesion variations
Note pyramidal tract medullary decussation (80-90% of axons) and spinal cord decussation (10-20% of axons)