Overview of Ascending & Descending Pathways

Question 1

Where is the primary motor cortex located?

Answer 1

The precentral gyrus of frontal lobe, just anterior to central sulcus

Question 2

Where is the primary somatosensory cortex?

Answer 2

Behind motor strip, the postcentral gyrus of the parietal lobe

Question 3

What are two most important somatosensory pathways?

Answer 3

1. Dorsal column
2. Spinothalamic

Question 4

What are dorsal column impulses concerned with?

Answer 4

With fine, precisely-localised (or discriminative) touch, joint position sense, proprioception and vibration sense.

Question 5

What are characteristics of dorsal column fibres?

Answer 5

Originates from low-threshold mechanoreceptors and nerve impulses are transmitted to the brain via large diameter (A-alpha/beta) fibres that are thickly-myelinated and have high conduction velocities, of up to 120m/s

Question 6

Discuss the origin and pathway of the first-order neurone of the dorsal column pathway

Answer 6

The first order neuron has its cell body in the dorsal root ganglion. Its central process enters the spinal cord via the dorsal (sensory) root and joins the dorsal column.

Axons of first order neurons ascend (ipsilaterally) through the dorsal columns to reach the dorsal column nuclei (gracile and cuneate) where they synapse on second-order neurons. This is in the medulla oblongata.

Question 7

Which fibres travel through the gracile and cuneate fasciuli?

Answer 7

Fibres from the lower half of the body (below T6) ascend the cord in the medial part of the dorsal column, referred to as the gracile fasciculus.

Fibres from the upper half of the body (above T6) travel in the lateral part of the dorsal column, within the wedge-shaped cuneate fasciculus.

Question 8

Discuss the origin and pathway of the second and third-order neurones of the dorsal column pathway

Answer 8

The second neuron in the chain has it cells body in the dorsal column nucleus and an axon decussates in the medulla as internal arcuate (arching) fibres.

After decussation the axons become the medial lemniscus. The medial lemniscus terminates on the ventral posterior (VP) nucleus of the thalamus and its constituent fibres synapse on third-order (thalamocortical) relay neurons. This projection ascends through the posterior limb of the internal capsule, just behind to the descending fibres of the corticospinal tract, before terminating in the primary somatosensory cortex.

Question 9

Where does sensory info from limbs and trunk synapse in the thalamus in comparison to sensory info for the head and neck?

Answer 9

Limbs + trunk -> on VPL (lateral part of nucleus)

Head + neck -> VPM (slightly medial part to limbs + trunk), analogous projection called trigeminothalamic pathway, carries info from V1 V2 V3

Question 10

What is the spinothalamic tract concerned with?

Answer 10

Pain and temperature sensation, originates from nociceptors and thermoreceptors. Impulses transmitted to brain via thinly-myelinated A-delta fibres and unmyelinated c-fibres - small diameter and slow conduction.

Question 11

Discuss the origin and pathway of the first-order neurone of the spinothalamic tract

Answer 11

The first order neuron is located in the dorsal root ganglion. Its central process enters the dorsal (sensory) root of the spinal nerve and synapses on the second order neuron at the point of entry.

Question 12

Discuss the origin and pathway of the second-order neurone of the spinothalamic tract

Answer 12

The (neo)spinothalamic second order axon crosses the midline in the most anterior part of the spinal cord (in the ventral white commissure) to reach the opposite side, before turning upwards to ascend the cord.

They continue in an anterolateral position in the medulla, lying just behind the olive, close to the medullary surface. The pathway continues into the pons and comes to lie just lateral to the medial lemniscus. The second-order neurons terminate in the ventral posterolateral (VPL) nucleus of the thalamus.

Question 13

What is the difference between the paleo-spinothalamic and neo-spinothalamic tracts?

Answer 13

A proportion of the spinothalamic tract fibres do not travel directly to the thalamus, but first relay in the reticular formation of the brain stem, which projects in turn to the intralaminar nuclei of the thalamus, which have diffuse cortical targets. This indirect (and slower) pain pathway is referred to as the paleo-spinothalamic tract, in contrast to the evolutionarily more recent neo-spinothalamic tract.

Question 14

Discuss the origin and pathway of the third-order neurones of the spinothalamic tract

Answer 14

Third order (thalamocortical) neurons project in turn to the sensory strip in the parietal lobe, via the posterior limb of the internal capsule

Question 15

What two tracts make up the pyramidal tract?

Answer 15

The corticospinal and corticobulbar tracts

Question 16

What is the corticospinal tract concerned with?

Answer 16

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 (opposite) limbs/trunk and consists of approximately one million axons on each side.

Question 17

What is the corticobulbar pathway concerned with?

Answer 17

It 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 18

2/3rds of the corticospinal tract fibres originate from the motor and premotor areas of the frontal lobe, where does the remaining 1/3rd originate from? And where do each of these axons project towards?

Answer 18

The first 2/3rds project to the anterior horn of the spinal cord grey matter, where they influence spinal motor neurons. This is often indirectly, via interneurons, but a small proportion of axons synapse directly on motor neurones.

The remaining 1/3rd is from the parietal lobe, project to the dorsal horn of the spinal cord.

Question 19

Describe the pathway of the corticospinal tract fibres as from when they leave the cerebral cortex to when they reach the medullary pyramids

Answer 19

• fibres leave the cerebral cortex to enter the subcortical white matter
• passing first through the fan-like corona radiata
• then entering the posterior limb of the internal capsule (passing between the thalamus and the lentiform nucleus of the basal ganglia).
• descend through the anterior part of the brain stem,
• passing through the crus cerebri
• then the basilar pons (breaking up into fascicles at this point and interlacing with the transverse pontine fibres))
• then the pyramids of the medulla (which are located on either side of the midline).

Question 20

What is the clinical importance with the internal capsule in its role of the corticospinal motor tract?

Answer 20

The arrangement of motor fibres within it are compact, so even small areas of damage (eg stroke) can cause complete contralateral paralysis (hemiplegia)

Question 21

What are the crus cerebri?

Answer 21

the most anterior portion of the midbrain, often referred to - in error - as the ‘cerebral peduncle’

Question 22

Where does the corticospinal tract decussate?

Answer 22

At the lowermost border of the medulla, approximately at the level of the foramen magnum.

Question 23

Describe the lateral (crossed) corticospinal tract and what is it important for?

Answer 23

This is when 90% of the fibres (after decussation) pass posteriorly and laterally to enter the lateral column of the spinal cord.

Important for for control of the distal limb flexors and is therefore required for manual dexterity.

Question 24

Describe the anterior (uncrossed) corticospinal tract and its purpose

Answer 24

The 10% of fibres that continue in the anterior part of the cord, on either side of the midline. This pathway is more concerned with proximal/axial musculature and many of the fibres cross the midline close to their point of origin.

Question 25

What are the upper motor neurones (UMN)?

Answer 25

Voluntary movement involves a two-neuron chain between the motor cortex and skeletal muscle.

The first neuron has its cell body in the motor/premotor cortex of the frontal lobe and an axon that contributes to the corticospinal tract.

Upper motor neurons, which make up the corticospinal tract, extend the full length of the spinal cord to synapse on lower motor neurons.

Question 26

What are the lower motor neurones (LMN)?

Answer 26

The cell body in the anterior horn of the spinal cord grey matter (or in a cranial nerve motor nucleus, which is analogous). The axons of lower motor neurons leave the CNS (brain or spinal cord) and travel in a peripheral nerve to reach their target muscle. The lower motor neuron (and its axon) is referred to as the “final common pathway” since it is ultimately responsible for all movements, both voluntary and reflexive.

Question 27

If the damage of the corticospinal tract is in the brain or brain stem, is the weakness on the ipsilateral or contralateral side of the body?

Answer 27

Contralateral (opposite) as decussation of the corticospinal tract occurs at the lowest part of the brainstem.