Sensory Pathways of the Trunk & Limbs

TRUE OR FALSE?

"Free nerve endings in the epidermis contribute

somatotopic specific inputs into the CNS"

• True

• False

Answer explanation

FALSE!!!

“Free nerve endings in the epidermis contribute

modality (pain) specific inputs into the CNS”

Which reflect examples of somatotopic organization of sensory path?

• Crush injury to forearm produces loss of touch sensation in figures on same side

• Nerves from leg enter CNS at lumbar spinal cord

• Physical stimulation of face corresponds with BOLD signal of lateral cortex during fMRI

• All of these examples

• None of these examples

Answer explanation

All of these examples reflect principle of somatotopic organization!

Peripheral processes that innervate adjacent portions of skin form nerve bundles, so if the nerve is damaged, then the entire dermatome is affected!

Nerves enter the spinal cord at levels that approximate the bottom-to-top organization of limbs and trunk (i.e., legs via lumbar spinal levels, trunk via thoracic spinal levels, and arms via cervical spinal levels).

Somatotopy is conserved through and even represented on the brain with the top/face represented by neurons found on the lateral aspect of the somatosensory cortex and moving down the body through the feet which are represented on the medial aspect of the cortex.

How many neurons (usually) does it take make a somatosensory path?

• 1

• 2

• 3

• 4

• 5

Answer explanation

3 (usually)

1st order neuron peripheral process innervates skin, cell body is in the ganglia, and axon projects into spinal cord

2nd order neuron cell body is in the spinal cord (or lower brain stem) and axon ascends to thalamus

3rd order neuron is in the thalamus and axon projects to cortex

... but we will learn about one pathway that will break this "rule"!!!

Cell bodies of 2° neurons that receive information about fine touch from the left arm are housed in:

• Left gracile nucleus

• Right gracile nucleus

• Left cuneate nucleus

• Right cuneate nucleus

• Left dorsal root ganglion

Answer explanation

Left cuneate nucleus

Remember, cuneate nucleus receives ascending, uncrossed (ipsilateral) inputs from "upper body"

Fibers conveying information about somatic pain and crude touch decussate via:

• Dorsolateral tract of Lissauer

• Ventral white commissure

• Arcuate fibers

• Posterior limb of internal capsule

Answer explanation

Ventral White Commissure

In contrast to DC/ML pathway, axons of 2nd order LST/VST neurons cross at spinal cord levels

Where would you find the cell bodies of 2° neurons that convey visceral pain sensations?

• Reticular nucleus of the thalamus

• In dorsal root ganglion of all spinal cord levels

• Ventral part of the lateral fasciculus proprius

• Secondary visceral gray at levels T1-L1/L2 and S2-S4

• Reticular formation of the brainstem

Answer explanation

Secondary visceral gray at levels T1-L1/L2 and S2-S4

1st order neurons are in ganglia

2nd order neurons in secondary visceral gray (i.e., dorsal horn

/substantia gelatinosa of spinal gray)

3rd order neurons in reticular formation (of medulla and pons)

4th order neurons in thalamus