L03 - Overview of Ascending & Descending Tracts

Question 1

Via which nerve are most sensory signals of the head and neck region conveyed to the brain?

Answer 1

Trigeminal nerve

Question 2

Which side of the brain is responsible for processing sensory information in relation to the receptor?

Answer 2

The side opposite to that of the sensory receptor

Question 3

List the 3 divisions of ascending tracts.

Answer 3

1 - Dorsal columns (fasciculus cuneatus & fasciculus gracilis)

2 - Spinocerebellar tracts (anterior & posterior)

3 - Spinothalamic tracts (anterior & lateral)

Question 4

What is the course of a primary sensory neurone as it enters the spinal cord?

Answer 4

• On reaching the spinal cord, the primary sensory neurone divides to give 3 axonal processes:
• One axonal process terminates in the dorsal horn of its respective spinal segment
• One axonal process sent to dorsal horn of spinal segment below and one to dorsal horn of spinal segment above
• The axonal processes travelling to other spinal segmental levels do so in the posterolateral tract of Lissauer
• The area of the dorsal horn upon which an axon terminates is determined by the modality it represents. The areas are known as laminae

Question 5

Which Rexed laminae exist in the dorsal horns?

What is the function of the neurones that exist in the dorsal horns?

Answer 5

• I - VI

- Neurones here are the targets of primary sensory neurones

Question 6

Which Rexed laminae exist in the lateral horns?

What is the function of the neurones that exist in the lateral horns?

Answer 6

• VII
• Neurones here are the cell bodies of preganglionic sympathetic fibres in T1-L2 and of preganglionic parasympathetic fibres in S1-S5

Question 7

Which Rexed laminae exist in the ventral horns?

What is the function of the neurones that exist in the ventral horns?

Answer 7

• VIII and IX

- Neurones here are motor cell bodies that send their axons to skeletal muscles

Question 8

What type of information is carried through the lateral spinothalamic tract?

Describe the course of a secondary sensory neurone through the lateral spinothalamic tract after having synapsed with its primary sensory neurone at the dorsal horn.

Answer 8

• The lateral spinothalamic tract carries information relating to pain and temperature
• The secondary sensory neurone decussates the cord by passing through the ventral white commissure
• It joins fibres of the lateral spinothalamic tract, which is in the lateral funiculus
• Fibres of the lateral spinothalamic tract travel superiorly (& laterally to the medial lemniscus) to the ventral posterolateral nucleus of the thalamus (terminate on the side of the thalamus opposite to that on which the sensory signal arose)

Question 9

Why is the postcentral gyrus described as showing somatotopic organisation?

Answer 9

• Somatotopy is the point-for-point correspondence of an area of the body to a specific point on the CNS
• The primary somatosensory cortex is typically represented as a sensory homunculus which orients the specific body parts and their respective locations upon the homunculus

Question 10

What is the difference between upper and lower motoneurones?

Answer 10

• UMN – cell bodies in brain

- LMN – cell bodies in spinal cord or cranial nerve motor nuclei

Question 11

What is the difference between pyramidal and extrapyramidal descending tracts?

Answer 11

• Pyramidal tracts – originate in the cerebral cortex, carrying motor fibres to the spinal cord and brainstem. They are responsible for voluntary control of musculature of the body and face
• Extrapyramidal tracts – originate in the brainstem, carrying motor fibres to the spinal cord. They are responsible for involuntary and automatic control of all musculature e.g muscle tone, balance, posture and locomotion