1. Somatosensory pathway
Domina Petric, MD

2. Introduction Planets Dwarf Planets
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Sensory pathways consist of the chain of neurons,
from receptor organ to cerebral cortex,
that are responsible for the perception of sensations.
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Makemake

3. pseudounipolar neuron parietal lobe of somatosensory cortex
Anatomical features
pseudounipolar neuron
spinal cord
thalamus
parietal lobe of somatosensory cortex
decussation

4. Anatomical features
The first afferent neuron is a pseudounipolar neuron.
Its cell body is located in a peripheral (spinal or cranial) ganglion.

5. Pseudounipolar neuron
It has:
peripheral axon that forms or innervates somatosensory receptors
central process that synapses with second afferent neuron(s) in a spinal cord or brain stem nucleus.

7. Second and third afferent neuron
The second afferent neuron may synapse with third afferent neurons in the spinal cord. OR
They may ascend the neuraxis to synapse with third afferent neurons in the thalamus.

8. Decussation
There is a decussation in each somatosensory pathway below the level of the thalamus.
Decussation means that axons are crossing the midline to the opposite side of the spinal cord or brain stem.

9. Thalamic nucleus
All somatosensory pathways include a thalamic nucleus.
The thalamic neurons send their axons in the posterior limb of the internal capsule to end in the cerebral cortex.
Most somatosensory pathways terminate in the parietal lobe of the cerebral cortex.

10. Peripheral somatosensory axons (first afferent neurons)
Group I
Type C
Type Aδ
Type Aβ
Velocity
form the muscle and tendon receptors
carry body proprioceptive information
the largest diameter and the thickest myelin
form free nerve endings
carry dull pain, deep pain, crude touch or warm/hot information
the smallest 1° afferent axons
unmyelinated
form free nerve endings
carry sharp pain or cool/cold information
thinly myelinated and larger than the Type C 
form encapsulated endings in skin and joints, hair follicle endings or Merkel disks in skin
myelinated
diameter between group I and A delta
The morphology of the peripheral somatosensory axon is also related to the conduction velocity of the action potentials generated by the axon.

11. Conduction velocity
It is determined by electrically stimulating the axon and recording the time (latency) it takes the electrically elicited action potential to reach a recording electrode.

12. Somatotopic organisation
Somatosensory neurons are topographically (spatially) organized so that adjacent neurons represent neighboring regions of the body or face.

13. Somatotopic organisation
It is preserved by a precise point-to-point somatotopic pattern of connections from the spinal cord and brain stem to the thalamus and cortex. 
Within each somatosensory pathway there is a complete map (spatial representation) of the body or face in each of the somatosensory nuclei, tracts and cortex.

14. The sensory information processed by the somatosensory systems travels along different anatomical pathways depending on the information carried. 

15. Medial lemniscal pathway (body discriminative touch and proprioception)
The posterior (dorsal) column or medial lemniscal pathway carries and processes discriminative touch and proprioceptive information from the body.
The afferents carrying discriminative touch information are kept separate from those carrying proprioceptive information up to the level of the cerebral cortex. 

17. It is the cranial homologue of the medial lemniscal pathway.
Main sensory trigeminal pathway (face discriminative touch and proprioception)
The main sensory trigeminal pathway carries and processes discriminative touch and proprioceptive information from the face. 
It is the cranial homologue of the medial lemniscal pathway. 

19. Neospinothalamic pathway (body sharp, pricking, pain and cool, cold)
The neospinothalamic pathway carries and processes sharp, pricking pain and dropping temperature (cool, cold) information from the body.
The pain information is well localized.
The sensations are short lasting: ˝fast˝ or ˝first˝ pain elicited by tissue-damaging cutaneous stimuli.

20. Neospinothalamic pathway
Characterised by somatotopic representation.
Most of the ascending afferents of the spinal pain pathways travel with the neospinothalamic afferents in a fiber tract: the spinothalamic tract or anterolateral spinothalamic tract.

22. It is the cranial homologue of the spinothalamic pathways.
Spinal trigeminal pathway (pain, temperature and crude touch from face)
The spinal trigeminal pathway carries and processes crude touch, pain and temperature information from the face.
It is the cranial homologue of the spinothalamic pathways.

23. https://openi. nlm. nih. gov/detailedresult. php
Nucleus caudalis lesioning: Case report of chronic traumatic headache relief.
Sandwell SE, El-Naggar AO - Surg Neurol Int (2011)

24. Literature
(Neuroscience electronic book, Patrick Dougherty, Ph.D., Department of Anesthesiology and Pain Medicine, MD Anderson Cancer Center)
openi.nlm.nih.gov
what-when-how.com