Overview of Ascending and Descending Tracts

Question 1

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

Answer 1

The trigeminal nerve.

Question 2

Which side of the brain is responsible for processing sensory information of the body surface?

Answer 2

The side opposite to that of the sensory receptor.

Question 3

What are ascending tracts?

Answer 3

Nerve fibre tracts of the sensory system by which sensory information is conveyed to the brain.

Question 4

List the 3 divisions of ascending tracts.

Answer 4

1 - Lateral spinothalamic tracts this is the one that the lecture describes the anatomy of, but the functions of all 3 are detailed in the next card.

2 - Dorsal spinocerebellar tracts.

3 - Dorsal corticospinal tracts.

Question 5

Give an overview of the pathways for ascending tracts.

Answer 5

1 - Sensory information is transduced and encoded by sensory receptors.

2 - Sensory information is then conveyed to the spinal cord or, if sensation arises in the head and neck region, the brain stem, by peripheral nerves or the trigeminal nerve.

3 - The spinal cord / brainstem organises the sensory information by sensory modality:

• Lateral spinothalamic tracts carry crude touch, pain and temperature information.
• Dorsal spinocerebellar tracts and dorsal corticospinal tracts carry tactile sensation and proprioceptive information.

4 - Information from dorsal spinocerebellar tracts is processed in the cerebellum.

5 - Information from dorsal corticospinal tracts and lateral spinothalamic tracts are processed in the brainstem and thalamus.

6 - The information is relayed to the primary sensory cortex of the cerebrum.

Question 6

List 3 examples of conscious sensation.

Answer 6

1 - Crude touch.

2 - Pain.

3 - Temperature.

Question 7

List 4 examples of non-conscious sensation.

Answer 7

1 - Tactile sensation.

2 - Muscle length.

3 - Muscle tension.

4 - Proprioception.

Question 8

Which components of the spinal cord are composed of grey matter?

Answer 8

The dorsal, lateral and ventral horns.

Question 9

Describe the course of a primary sensory axon as it enters the spinal cord.

Answer 9

• On reaching the spinal cord, the primary sensory neurone divides to give 3 axonal processes:
• One axonal process terminates in both the dorsal root ganglion and the dorsal horn of its respective spinal segment.
• The axonal process entering the dorsal horn bifurcates into two more axonal processes which terminate in the dorsal horn of the spinal segments directly above and below.
• 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 Rexed laminae.

Question 10

Which Rexed laminae exist in the dorsal horns?

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

Answer 10

• I - VI.

- Neurones here are the targets of primary sensory neurones.

Question 11

Which Rexed laminae exist in the lateral horns?

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

Answer 11

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

Question 12

Which Rexed laminae exist in the ventral horns?

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

Answer 12

• VIII and IX.

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

Question 13

Describe the organisation of white matter of the spinal cord.

Answer 13

• The white matter surrounds the grey matter.
• The white matter can be divided into dorsl, lateral and ventral funiculi.
• The divisions are separated by the dorsal and ventral horns of the grey matter.

Question 14

What type of information is carried through the lateral spinothalamic tract (reminder from previous card)?

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 14

• The lateral spinothalamic tract carries information relating to crude touch, pain and temperature.
• The secondary sensory neurone sends its axon towards the central canal at the midline.
• It then dives under the central canal to emerge on the white matter of the opposite side.
• Here, it joins fibres of the lateral spinothalamic tract, which is in the lateral funiculus.
• Fibres of the lateral spinothalamic tract travel to the ventral posterolateral nucleus of the thalamus (via the brainstem). Ultimately, they will terminate on the side of the thalamus opposite to that on which the sensory signal arose.

Question 15

What is spinal decussation?

What is the significance of spinal decussation?

Answer 15

• The crossing-over of the midline of the spinal cord by secondary sensory neurones.
• Sensory fibres that decussate mean that sensory stimuli are processed on the side of the brain that is opposite to that on which the sensory signal arose.

Question 16

What are the fibres that comprise spinal decussation known as?

Answer 16

Anterior commissures or arcuate fibres.

Question 17

At which spinal levels does spinal decussation occur?

Answer 17

All 31 levels.

Question 18

With which neurones do secondary sensory neurones of the lateral spinothalamic tract synapse?

Where do these neurones synapse?

Answer 18

• Tertiary sensory neurones.

- They ascend further via the internal capsule to terminate in the postcentral gyrus of the cerebral cortex.

Question 19

What is meant when the lateral spinothalamic tract is said to be somatotopically organised?

Answer 19

The spatial mapping of the body’s surface is preserved within the tract.

Question 20

How does the structure of the lateral spinothalamic tract give rise to its somatotopic organisation?

Answer 20

• Fibres arising from the lowest part of the body ascend dorsolaterally within the tract.
• Fibres arising from the highest parts of the body ascend ventromedially.

Question 21

How do the dorsal tracts (dorsal spinocerebellar and dorsal corticospinal) differ structurally from the lateral spinothalamic tract?

Answer 21

• Information in these tracts is carried on the same side of the spinal cord as its origin.
• This information remains ipsilateral until the level of the medulla oblongata.

Question 22

What is contained in the postcentral and precentral gyrus of the brain?

What separates these gyri?

Answer 22

• The postcentral gyrus contains the primary sensory cortex.
• The precentral gyrus contains the primary motor cortex.
• They are separated by the central sulcus.

Question 23

How are neurones of the precentral gyrus arranged?

Answer 23

According to the skeletal muscles they command.

Question 24

Is decussation involved in motor commands?

What is the significance of this?

Answer 24

• Decussation is involved in motor commands.
• This means that efferent motor commands originate from the side of the brain that is opposite to that at which the signal will terminate.

Question 25

Through which tracts does the brain transmit motor commands?

Answer 25

Descending tracts.

Question 26

Give an overview of the pathways for descending tracts.

Answer 26

• Descending tracts are formed of axons of neurones with cell bodies in motor nuclei of the brain.
• There are two classes of descending tracts:

1 - Cortical descending tracts.

2 - Non-cortical descending tracts.

Question 27

What is the difference between cortical descending tracts and non-cortical descending tracts?

Answer 27

• Descending tracts have their primary motoneurones with cell bodies in the primary motor cortex.
• Non-cortical descending tracts have their primary motoneurones with cell bodies in subcortical areas of the brain such as the brainstem.

Question 28

What is the difference between upper and lower motoneurones?

Answer 28

1:

• Upper motoneurones are motoneurones with cell bodies in the brain.
• Lower motoneurones are motoneurones with cell bodies in the spinal cord or cranial nerve nuclei.

2:

• The entire axon of upper motoneurones resides within the CNS.
• The axon of lower motoneurones travels from the CNS into the PNS.

Question 29

List the divisions of upper motoneurones.

What is the difference between these divisions?

Answer 29

1 - Cortical / pyramidal efferents, which have their cell bodies in the cerebral cortex.

2 - Brainstem / bulbar / extrapyramidal efferents, which have their cell bodies in subcortical areas such as the brainstem and basal ganglia.

*These are the distinguishing factors between cortical and non-cortical descending tracts.

Question 30

In which areas of the spinal cord are descending tracts found?

Answer 30

In either ventral (anterior) or lateral funiculi of the white matter, mixed with the ascending tracts.

Question 31

List 5 divisions of descending tracts.

Answer 31

1 - Lateral corticospinal tract.

2 - Lateral reticulospinal tract.

3 - Anterior reticulospinal tract.

4 - Anterior corticospinal tract.

5 - Rubrospinal tract.