Somatosensory pathway Domina Petric, MD

Introduction Planets Dwarf Planets Neptune Mercury Jupiter Saturn Uranus Venus Earth Mars Planets Dwarf Planets Sensory pathways consist of the chain of neurons, from receptor organ to cerebral cortex, that are responsible for the perception of sensations. Ceres Pluto Haumea Eris Makemake

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

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

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.

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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.

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.

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.

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.

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.

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

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.

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

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. 

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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. 

http://what-when-how.com/neuroscience/the-cranial-nerves-organization-of-the-central-nervous-system-part-3

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.

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.

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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.

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Literature http://nba.uth.tmc.edu/neuroscience/s2/chapter04.html (Neuroscience electronic book, Patrick Dougherty, Ph.D., Department of Anesthesiology and Pain Medicine, MD Anderson Cancer Center) openi.nlm.nih.gov www.studyblue.com what-when-how.com