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 |
| 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, 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, 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 |
Note: Pathways involving the reticular formation involve many neurons and many synapses
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EFFECTS OF 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 | No effect | Flaccid paralysis |
| 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) | Below the level of the section, loss of ipsilateral touch-pressure and contralateral pain-temperature | Below the level of the section, paresis or psilateral 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 |