Overview of Ascending and Descending Pathways

Question 1

What are the most important motor and sensory tracts of the CNS?

Answer 1

Ascending:

1. Dorsal column pathway - fasciculus gracilis and fasciculus cuneatus
2. Spinothalamic tract
3. Spinocerebellar

Descending:

1. Pyramidal tract - corticospinal and corticobulbar pathways
2. Extrapyramidal tract - rubrospinal, tectospinal, vestibulospinal, reticulospinal

Question 2

Where is the primary motor cortex (M1) located?

Answer 2

In the precentral gyrus of the frontal lobe

Question 3

Where is the premotor cortex located and roughly what is its role?

Answer 3

• Just anterior to the primary motor cortex
• Movement planning and preparation

Question 4

Where is the primary somatosensory cortex (S1) located and what is its role?

Answer 4

• Postcentral gyrus - in the parietal lobe
• It recieves sensory projections for all sensory modalities and is concerned with sensations from the opposite half of the body

Question 5

What is meant by a somatotopic representation, which is a feature of the primary sensory and motor areas?

Answer 5

The orderly point-to-point representation of the opposite half of the body

Question 6

The lower part of the body occupies what part of the cerebral hemisphere somatotopic map?

Answer 6

The medial surface of the hemisphere

Question 7

Describe the somatotopic location of the upper limb and hand areas and the face/tongue areas on the convexity of the hemisphere.

Answer 7

• Convexity = lateral surface of hemisphere
• Upper limb and hand areas - superior
• Face/tongue areas - inferior

Question 8

Describe the 2 components of the primary motor pathway.

Answer 8

1. Corticospinal tract - projects from the motor and premotor areas of the frontal lobe to all levels of the spinal cord. It controls voluntary movements of the contralateral limbs/trunk and consists of approximately 1 million axons on each side.
2. The corticobulbar pathway is the voluntary motor supply to the brain stem (the motor cranial nerve nuclei) and therefore controls movements of the jaw, face, tongue, larynx and pharynx. The word ‘bulb’ is an old-fashioned term for the lower brain stem.

Question 9

Describe the origin and connections of the corticospinal tract fibres.

Answer 9

• 2/3 of fibres originate from the motor and premotor areas of the frontal lobe.
• Their axons synapse with mainly interneurons in the anterior horn but also directly with some motor neurons.
• 1/3 of fibres arise from the parietal lobe.
• These project to the dorsal horn, helping to “filter out” sensations generated by movement.

Question 10

Describe the course of corticospinal tract fibres from their point of origin to the medullary pyramids.

Answer 10

• Fibres leave the cerebral cortex and enter the subcortical white matter.
• They pass through the corona radiata before entering the posterior limb of the internal capsule.
• Fibres then pass through the crus cerebri (anterior midbrain), basilar pons and finally the medullary pyramids.

Question 11

Describe the course of corticospinal tract fibres from the medullary pyramids to their point of termination.

Answer 11

• The tract decussates at the lowermost border of the medulla, at the level of the foramen magnum.
• 90% of fibres pass posteriorly and laterally to enter the lateral column of the spinal cord as the lateral (crossed) corticospinal tract.
• 10% of fibres continue in the anterior part of the cord, on either side of the midline, to become the anterior (uncrossed) corticospinal tract.

Question 12

What are upper motor neurons (UMN)?

Answer 12

UMN are neurons that contribute axons to the corticospinal tract whose cell bodies lie in the motor/premotor cortex. They extend the full length of the spinal cord and synapse with interneurons and lower motor neurons.

Question 13

What are lower motor neurons (LMN)?

Answer 13

LMNs have their cell bodies in the anterior horn of the spinal cord and the cranial nerve motor nuclei, which are analogous. Their axons travel in peripheral nerves and synapse with the target muscle via a neuromuscular junction.

Question 14

What is the “final common pathway”?

Answer 14

Lower motor neuron and its axon - it is responsible for all movements, both voluntary and reflexive

Question 15

Damage to the corticospinal tract in the brain and brain stem leads to weakness or paralysis where?

Answer 15

Contralateral half of the body

Question 16

Damage to the corticospinal tract below what anatomical landmark causes ipsilateral weakness?

Answer 16

Pyramidal decussation

Question 17

What is the main anatomical difference between the spinothalamic and dorsal column pathways?

Answer 17

The position of the second order neuron, and therefore the point at which the nerve fibres cross the midline

Question 18

Describe the position of the first and third order neurons in the dorsal column and spinothalamic pathways.

Answer 18

• First order neuron - dorsal root ganglion
• Third order neuron - ventral posterior nucleus (VPN)

Question 19

The dorsal column pathways is concerned with what sensory stimuli?

Answer 19

• Fine, discriminative touch
• Joint position sense
• Proprioception
• Vibration sense

Question 20

Describe the origin, course and termination of the dorsal column pathway.

Answer 20

• The pathway originates from low-threshold mechanoreceptors, whose cell bodies lie in the dorsal root ganglion
• The central processes of the first order neurons enter the spinal cord via the dorsal root and join the dorsal column
• Fibres from the lower half of the body (below T6) travel in the medial dorsal column - the gracile fasciculus
• Fibres from the upper half of the body (above T6) travel in the lateral dorsal column - the cuneate fasciculus
• These fibres synapse with the second order neurons in the gracile and cuneate nuclei, the dorsal column nuclei
• The second order neurons in these nuclei project axons that cross the midline at the great sensory decussation in the medulla
• The axons then become the medial lemniscus, which terminates on the VP nucleus of the thalamus
• Third order thalamocortical relay neurons project axons that ascend through the posterior limb of the internal capsule before terminating in the primary somatosensory cortex

Question 21

What are the internal arcuate fibres?

Answer 21

• The axons of second order neurons in the dorsal column pathway
• They curve anteriorly and medially through the medulla and so follow an “arching course” - internal arcuate = “arc shaped”

Question 22

What is the trigeminothalamic pathway and where does it terminate?

Answer 22

The trigeminothalamic pathway is an analogous system to the dorsal column and spinothalamic pathways, but for the head and neck structures.

It terminates in the ventral posteromedial nucleus (VPM) of the thalamus.

Question 23

Fibres carrying sensory information from the limbs and trunk synapse where?

Answer 23

The ventral posterior lateral nucleus (VPL) of the thalamus

Question 24

How is the neurological function of the spinothalamic tract tested clinically?

Answer 24

• Sterile neurological examination tips (Neurotips)
• Volatile sprays e.g. ethyl chloride produce a cold sensation

Question 25

The spinothalamic tract is concerned with pain and temperature sensation. Which 2 types of fibre carry these impulses to the brain? Describe their characteristics

Answer 25

Nociceptors:

• Aδ fibres - small diameter, thinly myelinated
• C fibres - small diameter, unmyelinated

Both have slow conduction velocities

Question 26

The second order neurons of the neospinothalamic tract terminate where?

Answer 26

Ventral posterolateral (VPL) nucleus of the thalamus

Question 27

Where are the first order neurons of the spinothalamic tract located?

Answer 27

• Nociceptors - free nerve endings in the skin and viscera
• Cell bodies in the dorsal root ganglion

Question 28

What is the difference between the neo- and paleospinothalamic tracts?

Answer 28

• Paleospinothalamic fibres relay in the reticular formation of the brain stem, which projects in turn to the intralaminar nuclei of the thalamus - these have diffuse cortical targets.
• The paleospinothalamic pathway is evolutionarily older, slower and indirect.
• Neospinothalamic tract fibres run lateral to the medial lemniscus and ascend directly to the VPL nucleus of the thalamus.
• Third order neurons of the neospinothalamic tract project to the sensory strip in the parietal lobe, via the posterior limb of the internal capsule.