Sensory Pathways of the body

Question 1

Sensations

Answer 1

• All sensations of the body start with a Primary Sensory Neuron, whose cell body lies within the Dorsal Root Ganglion
• Almost all sensory information is relayed through the Thalamus
• All sensory information decussates, so that the left side of the brain monitors sensations from the right side of the body and vice versa. (Not all at the same spot).
• All sensations are detected in the Posterior Paracentral Gyrus; Postcentral gyrus (Primary Sensory Cortex) Organized according to the Sensory Homunculus

Receptors vary by function and density

Question 2

Sensory Honmunculus

Answer 2

A distorted figure with body parts sized in proportion to the amount of cerebral cortex devoted to each region.

Notice the extra large hands and lips, indicating a larger part of the cortex is associated with sensation from these regions

Question 3

Sensory Homunculus Locations

Answer 3

• Legs/Feet and Genitals = medial aspect of cortex, extending into lateral fissure
  – Anterior cerebral artery supplies this area.
• Hand and Face are Lateral
  – Middle cerebral artery supplies this
• The gut tube structures are further laterally located, extending into the lateral fissure.

Question 4

Three pathways of sensation

Answer 4

Pain (Nociception) and Temperature
* Better understood as “nociception” and temperature

Pressure and Crude Touch
* No discrimination, just basic awareness of touch

Proprioception, Fine Touch, Light Touch and Vibratory sense
* Proprioception = the ability to sense limb position by monitoring muscle and ligament stretch.
* Fine Touch = the ability to identify an object by touch (stereognosis).
* Light Touch = the ability to sense something brushing lightly against the skin
* Vibratory Sense = the ability to sense vibrations, as from a tuning fork.

Question 5

Spinothalamic Pathway

Answer 5

• Receptor in the skin -> dorsal root ganglion -> dorsal horn of spinal cord
• Synapse with second neuron -> decussation -> ascending as lateral spinothalamic tract -> enters brain stem -> enters Ventral posteriorlateral nucleus of thalamus
• Synapse with a tertiary neuron -> exits thalamus and enters posterior limb of the internal capsule -> leaves and synapes with the postcentral gyrus (primary somatosensory cortex)

Question 6

How do you test the integrity of the spinothalamic pathway?

Answer 6

Sharp/Dull Test

Question 7

Pressure and Crude Touch Pathway

Answer 7

• Receptor in skin -> Dorsal root ganglion -> Dorsal horn
• 2a. Synapse with a secondary neuron and decussate -> Ventral spinothalamic tract -> Ventral posteriorlateral nucleus of thalamus OR
• 2b. Ascend 10 spinal levels -> synapse with secondary neuron then decussate -> ascend as ventral spinothalamic tract -> ventral posteriorlateral nucleus of the thalamus
• Exit thalamus -> Enter posterior limb of the internal capsule -> leave and synapse with the postcentral gyrus (primary somatosensory cortex)

Question 8

Proprioception, Fine Touch, Light Touch and Vibratory Sense Pathway

Answer 8

Pathway for LE (Below T6)
* Receptor on skin -> Dorsal root ganglion -> dorsal horn of spinal cord
* Ascends in the dorsal white column as the fasiculus gracilis
* Reaches Nucleus Gracilis within the medulla (Middle two)
* Decussates -> ascends to brainstem as medial lemniscus -> Ventral posteriorlateral nucleus of the thalamus -> exit thalamus and enter posterior limb of the internal capsule -> Exit and synapse with the postcentral gyrus (primary somatosensory cortex)

Pathway for UE (Above T6)
* Receptor on the skin -> Dorsal root ganglion -> Dorsal horn of spinal cord
* Ascend dorsal white column as fasciculus cuneate -> enter nucleus cuneatus (Outsides) in the medulla
* Decussates -> Ascends as medial lemniscus -> Ventral posteriorlateral nucleus of the thalamus -> exits and enters posterior limb of the internal capsule -> exits and synapse with the postcentral gyrus (primary somatosensory cortex)

Question 9

How to test Proprioception, Fine Touch, Light Touch and Vibratory Sense Pathway

Answer 9

• Two point discrimination
• Monofilament

Question 10

What areas of the brain are important for sensation?

Answer 10

Posterior limb of internal capsule and thalamus

Question 11

What is another name for Proprioception, Fine Touch, Light Touch and Vibratory Sense Pathway

Answer 11

Dorsal Column Lemniscal Pathway

Question 12

Sensation to Perception

Answer 12

• Sensation 1st identified in the Postcentral Gyrus (Primary Sensory Cortex):
  I feel something in my foot
• Unimodal Sensory Association Cortex adds meaning:
  OUCH! I feel sharp pain in my foot
• Heteromodal Sensory Association Cortex adds interpretation:
  OUCH!! I just stepped on a nail

Question 13

Sensory Neurons and Reflex (Flexor Withdrawl)

Answer 13

1. Primary sensory neuron enters dorsal horn
2. Synapse with Internuncial Neurons that travel up and down the spinal cord and may, or may not, decussate
3. Synapse with lower motor neuron in Ventral Horn
4. Move away from the pain! – flexion on stimulated side, extensor pattern on the contralateral