overview of ascending and descending pathways

Question 1

What are the three important CNS tracts assessed in a neurological exam?
What sides of the body do the pathways provide too?

Answer 1

Pyramidal- corticospinal and corticubulbar.
Dorsal column
Spinothalamic
Provide to the contralateral side of the body.

Question 2

Where is the primary motor centre located?

Answer 2

Precentral gyrus of the frontal lobe. (just anterior to the central sulcus

Question 3

What does the primary motor cortex do?

Answer 3

controls voluntary movement.

Question 4

where is the premotor cortex and what does it d?

Answer 4

Just anterior to the primary motor Cortex - its involved in planning and preparation.

Question 5

Where is the primary somatosensory cortex located?

Answer 5

just posterior to the motor strip- in the post central gyrus of the parietal lobe.

Question 6

What does the primary somatosensory cortex do?

Answer 6

Receives ascending projections for all sensory modalities. Touch, joint pain and temperature.

Question 7

What is the cortical representation of the motor cortex for each body part?

Answer 7

In the sensory strip - the amount of cortex is responsible for tactile sensation.
In the motor areas there is an orderly point the point representation of motor supply - trunk having innervation from medial hemispheres, and limbs from the lateral convexities of the hemispheres.

Question 8

What two tracts are contained within the pyramidal pathway?

Answer 8

Corticospinal tract and corticobulbar tract.

Question 9

What does the corticospinal tract do?

Answer 9

project from the motor and pre motor areas of the frontal lobe to all levels of the spinal cord. Involves voluntary movement of the contralateral limbs and trunk.

Question 10

What does the corticobulbar pathway do?

Answer 10

voluntary motor supply to the brain stem- controls movement of the face, jaw, tongue, larynx and pharynx.

Question 11

Where do the fibres of the corticospinal tract originate and project too?

Answer 11

2/3rds originate in the primary motor and pre motor cortex and project axons to the anterior horn of spinal grey matter- here they act on spinal motor neurones.
The remaining third originate in the parietal lobe and project to the dorsal grey matter- where they filter out sensations.

Question 12

Outline the pathways of the corticospinal tract.

Answer 12

Motor cortex - posterior limb of the internal capsule - crus cerebri - basil pons - medulla pyramid

Question 13

Where does the portico spinal tract desiccate?

Answer 13

Most infererior section of the medulla.

Question 14

Where are upper and lower motor neurones located?

Answer 14

Upper MNs- in the pre motor or primary motor cortex of the frontal lobe.
Lower MNs - in the cell body of the anterior horn of spinal grey matter.

Question 15

What is the final common pathway?

Answer 15

The LMN and peripheral nerve axon.

Question 16

What does damage to the corticospinal tract result in?

Answer 16

weakness or paralysis

Question 17

What are different symptoms of UMN lesions and LMN lesions?

Answer 17

UMN lesion- damaged motor cortex results in less inhibition of descending pathways - resulting in excited muscle spindle fibres- increasing muscle tone - hypertonia.

LMN lesion - damages the peripheral nerve supply to the muscle - therefore no excitation of muscle spindles causing weakness and floppy limbs (paralysis) leading to atonia.

Question 18

What are the two somatosensory pathways?

Answer 18

Dorsal column and spinathalamic.

Question 19

What is the arrangement of the pathways (in terms of neurons)?

Answer 19

Both have three neuron chains between the periphery and the sensory cortex.
First order neurone lies in dorsal root ganglion.
Second order crosses the mideline and ascends to the thalamus
third order lies in the ventral posterior nucleus of the thalamus and projects to the primary sensory cortex.

Question 20

What is the role of the dorsal column pathway?

Answer 20

proprioception and vibration

Question 21

What type of nerve fibres are contained in the dorsal column pathway?

Answer 21

Thick, myelinated alpha or beta fibres with a high conduction speed and diameter.

Question 22

What is the pathway of the dorsal column?

Answer 22

First order neurone originates in the dorsal root ganglia. Enter the spinal cord and ascend in different fascicles.
Below T6 - gracile fasiculus. Above T6 - cuneate fasiculus.
They synapse with second order neurones in the medulla. second order axons cross the midline in the medulla - ‘great sensory decussation’ these fibres become the medial lemniscus.
Fibres synapse on the third order neurone on the ventral posterior nucleus on the thalamus. the third order neurones project to the somatosensory centre from here via the posterior limb of the internal nucleus.

Question 23

What is the role of the spinothalamic pathway?

Answer 23

Pain and temperature sensation.

Question 24

What type of nerve fibres are contained in the spinothalamic pathway?

Answer 24

alpha delta fibres - thin, poor myelination or no myelination.

Question 25

What is the pathways of the spinothalamic pathway?

Answer 25

First order neurones originate in the dorsal root ganglia.
Synapse with the second order neurones then the enter the spinal cord
The second order neurone axons cross the midline - enters pons just lateral od medial lemniscus.
Synapses with the third order neurone as the ventral posterolateral nucleus of the thalamus.
third order axons project to the somatosensory cortex via the posterior limb of the internal capsule.

Question 26

What would be the result of a cut at L1? (think sensory and motor pathways)

Answer 26

No motor innervation below L1 on the ipsilateral side as the cut
No dorsal column sensory innervation below L1 on ipsilateral side.
No spinothalamic sensory innervation below L1 on the contralateral side (as second order axons cross midline as soon as they enter the spinal cord).