Week 4: Somatosensory Pathways

Question 1

What type of pathways does the somatosensory pathway involve?

Answer 1

ascending axonal pathways

Question 2

What is the function of the Posterior column medial lemniscal pathway”?

Answer 2

sensory (vibration, joint position, fine touch)

Question 3

Shape and receptors of sensory neurons?

Answer 3

A. The first neuron in both somatosensory pathways is a primary sensory neuron that contains specialized receptors in the axon terminal to detect specific stimuli
1)The receptors are ionotropic or metabotropic receptors that depolarize the membrane in response to a sensory stimulation and generate an action potential

B. Sensory neurons have a unique shape:

1) Sensory neurons are pseudounipolar, where they have one axon and no dendrites
2) One end of the axon innervates the sensory organ and the other goes to the CNS

Question 4

Ao(delta) and C axons

Answer 4

are lightly myelinated or not myelinated at all, respectively. They detect temperature and pain primarily. Part of the anterolateral pathways.

Question 5

Aa(alpha) and AB(beta) axons

Answer 5

are large and heavily myelinated and detect proprioception, touch and vibration Part of the posterior leminiscus pathways.

Question 6

The sensory neurons have their cell bodies located in the

Answer 6

dorsal root ganglion, which is located right outside of the spinal cord but within the vertebral column

Question 7

What is a dermatome?

Answer 7

All of the sensory neurons entering the spinal cord at a specific segment:
Innervate a specific region of the body, called a dermatome

Question 8

Dorsal root nerve root and dorsal horn

Answer 8

All of the sensory neurons entering the spinal cord at a specific segment:
Form a dorsal root nerve whose neurons synapse onto neurons in the spinal cord dorsal horn

Question 9

Where do most primary sensory axons go to after they enter the dorsal horn?

Answer 9

Many of the primary sensory axons ascend ipsilaterally up the spinal cord to the posterior column nuclei in the medulla and synapse onto neurons there (neuron 2 in this pathway). There are two distinct axon bundles as they travel to the medulla:
gracile fasciculus + cuneate fasciculus

Question 10

What is the gracile fasciculus?

Answer 10

Gracile fasciculus - axons that innervated the legs and lower trunk. They synapse on the nucleus gracilis in the posterior nuclei of the medulla

Question 11

WHat is the cuneate fasiciculus?

Answer 11

Cuneate fasciculus – axons that innervated the upper trunk, arms and neck. They synapse on the nucleus cuneate in the posterior nuclei of the medulla

Question 12

WHat happens after the axons get to the posterior column nuclei?

Answer 12

The medullary neurons then immediate decussate within the medulla, forming the internal arcuate fibers. Once crossed over to the other side of the medulla, the axons continue travelling up the brainstem and are called medial lemniscus pathways

Question 13

what are the internal arcuate fibers?

Answer 13

The medullary neurons then immediate decussate within the medulla, forming the internal arcuate fibers

Question 14

what are the medial lemniscus pathways?

Answer 14

the crossed over axons that continue to travel up the the brainstem

Question 15

what happens after the axons decussate in the medulla?

Answer 15

The axons as part of the medial lemniscus then synapse onto the 3rd neurons in this pathway in the ventral posterior lateral nucleus (VPL) of the thalamus

Question 16

what happens after the axons go to the vpl?

Answer 16

The axons of the thalamic neurons then traverse through the internal capsule to synapse onto the neurons in the primary somatosensory cortex