Two Major Pathways for Somatosensory Information

Question 1

Spinal-Thalamic

nerves?

Answer 1

Free Nerve Endings enter Dorsal Root of Spinal Cord and synapse there.

• Nerves are small in diameter & mostly unmyelinated so transmit relatively SLOWLY

second order neurons

synapse in spinal cord

Question 2

Second order neurons

Answer 2

cross over in Spinal Cord, ascend on contra-lateral side to synapse in contra-lateral VPN
(Ventral Posterior Nucleus of the Thalamus) So, pathway is named for the sites of its first two synapses.

Question 3

Synapse in Spinal Cord

Answer 3

plays a role in some reflexes

Question 4

Medial Lemniscal

nerves?

Answer 4

• Encapsulated Receptors enter Dorsal Root but, although one collateral of axon synapses there,
  main fiber ascends on the ipsi-lateral side of the Spinal Cord, synapsing first on ipsi-lateral side of the Medulla
• “Second-order” cells
• Nerves are large and myelinated and thus transmit very FAST

Question 5

Longest nerve cells in the body…

Answer 5

…reach from toes to brain!

Question 6

Second-order cells

Answer 6

cross -over in Brain Stem (tract called “Medial Lemniscus”) to synapse in contra-lateral VPN

Question 7

Brown-Sequard Syndrome

Answer 7

Damage to only one side of spinal column (e.g. the right) will result in the:

• reduction/loss of touch and position sense on the ipsi-lateral (right) side below the point of injury and the
• reduction/loss of temperature and pain detection on the contra-lateral (left) side below the point of injury.

Question 8

In Both Pathways:

Parietal Cortex?

Corpus Callosum?

Answer 8

First major synapse occurs at place of cross-over

• From VPN, message continues along contra-lateral side to S, and then S2, both in Parietal Cortex
• After S1, Corpus Callosum exchanges info, so S2 reacts to both sides, altho shows dominance for contra-lateral side
• Note that above are the primary pathways, but there are a variety of other collateral paths!