Lecture 5 - Sensory Pathways

Question 1

What does the term Decussate meant? What neurons does it apply to in the sensory pathway?

L5 S6

Answer 1

When axons cross over from one side of the spinal cord to the other.

In the sensory pathways, these are the secondary neurons.

Question 2

What are the two primary ascending sensory pathways?

L5 S7 LO1

Answer 2

• spinothalamic

- medial lemniscal

Question 3

What are the divisions of the spinothalamic tract?
What sensation do the tracts carry?
What do the primary, secondary, and tertiary fibers of each tract do?

L5 S8 LO1

Answer 3

Lateral spinothalamic tract:

• carries sensations of PAIN and TEMPERATURE
• primary fibers ascend 1-2 levels
• secondary fibers decussate and synapse in ventral posterolateral (VPL) nucleus of thalamus
• stimulates wakefulness (via collaterals to reticular formation)
• tertiary fibers terminate in internal capsule

Anterior spinothalamic tract:

• carries sensation of light touch, pressure, tickle and itch
• primary neurons ascend 8-10 levels
• secondary fibers decussate and synapse in VPL of thalamus
• tertiary fiber pass through internal causes and terminate in primary sensory cortex

Question 4

What sensation is carried by the medial lemniscal pathway?
What do the primary, secondary, and tertiary fibers do?

L5 S15 LO1

Answer 4

-carries two point sensation (fine touch), pressure, and vibration

• primary fibers ascend entire spinal cord in fasiculus gracilis (lower extremity) or fasciulus cuneatus (upper extremity)
• primary fiber synapse in nucleus gracilis or nucleus cuneatus respectively
• secondary fibers decussate and synapse in VPL of thalamus
• tertiary fibers pass through internal capsule and terminate in primary sensory cortex

Question 5

What are the main Bordmmann’s areas?

Answer 5

Primary somatosensory area:
-Brodmann’s areas 1, 2, and 3

Somatosensory association area:
-Brodmann’s areas 5 and 7

Question 6

How is the sensory homunculus oriented in the somatosensory area and why are some body parts/features disproportionally large?

L5 S24 LO2

Answer 6

The inferior portion of the body is located more medially and the superior is located more laterally.

The areas that are disproportionally large are because they have more sensory function than other regions.

Question 7

What layer of the somatosensory cortex receives incoming sensory signals?

L5 S27 LO2

Answer 7

Layer IV (4)

Question 8

What are the characteristics of fast pain?

L5 S31 LO3

Answer 8

• felt in 0.1 seconds
• not felt in deep tissues
• elicited by mechanical or thermal stimuli
• carried by Aδ fibers
• terminate in lamina I (lamina marginalis) of dorsal horns
• travel in neospinothalamic tract and synapse in ventrobasal nuclei of thalamus

Question 9

What are the characteristics of slow pain?

L5 S32 LO3

Answer 9

• felt in 1 second and slowly increases
• elicited by mechanical, thermal, and chemical stimuli
• carried on type C fibers
• terminates in layers II or III (substantia gelatinosa) of spinal cord
• travel in paleospinothalamic tract and terminate in brainstem