Somatosensory pathways

	1st order neurons	Output pathway	2nd order neurons	Output	3rd order neurons THALAMUS	Output pathway	4th order neurons CORTEX
DORSAL COLUMN-MEDIAL LEMNISCUS (mechano- and proprioception)
BODY	Dorsal root ganglia (DRG)	Dorsal root/dorsal columns	Dorsal column nuclei (medulla)	Medial lemniscus (decussates in medulla)	Ventroposterolateral nucleus (VPL)	Internal capsule	Postcentral gyrus (somatosensory cortex)
HEAD	Cranial nerves V, VII, IX, X	various	Main trigeminal nucleus	Trigeminal lemniscus (decussates in medulla)	Ventroposteromedial nucleus (VPM)	Internal capsule	Postcentral gyrus (somatosensory cortex)
SPINOTHALAMIC TRACT (thermoreception and nociception)
BODY	Dorsal root ganglia (DRG)	Lissauer's tract	Dorsal horn nuclei (spinal cord)	Spinothalamic tract (found in the anterolateral quadrant of the spinal cord)	Ventromedial nucleus (VM) Medialdorsal nucleus (MD) Intralaminar nuclei (IL)	Internal capsule	Many, including: Insula Cingulate gyrus
HEAD	Cranial nerves V, VII, IX, X	Spinal trigeminal tract	Spinal trigeminal nucleus			Internal capsule	Many, including: Insula Cingulate gyrus

from a physiology perspective, there are three key points:
1. the inputs from the body and head that form the medial and trigeminal lemnisci coalesce in the thalamus to form a single body map, which projects to the somatosensory cortex to form the humunculus
2. both pathways are crossed (although they cross at different anatomical levels) - this means that somatosensory stimuli from one side are perceived in the contralateral cortex
3. the spinothalamic tract projects to more diffuse areas of cortex, directly activating non-sensory regions

2007 Sensory Physiology