Neuroanatomy 2 - ascending and descending pathways

Question 1

Describe the pathway of the anterior and posterior root/rootlets of the spinal nerves emerging from each segment of the spinal cord (verterbral level)

Recall there is 31 spinal nerves

Answer 1

The posterior and anterior roots pass through the subarachnoid space until they reach the interverterbral foramina. As they pass through the foramina the posterior root is enlarged by the posterior (dorsal) root ganglion. The roots fuse to form the mixed spinal nerve (recall only known as spinal nerves when in the intervertebral formaina), which produces posterior and anterior rami.

Question 2

At what vertebral level does the spinal cord terminate at ? and name the structure it terminates as and then continues as

Answer 2

• Terminates at L1/2 vertebral level in a taperd cone-shape called the conus medullaris.
• A thin connective tissue cord called the filum terminale the continues on from the conus medullaris which is anchored to the dorsum of the coccyx.

Question 3

What is the cauda equina and what location does it arise at ?

Answer 3

• ‘horses tail’ formed from the spinal nerves from the lumbosacral enlargement & conus medullaris, it arises at T12/L1 vertebral levels
• The nerves that compose the cauda equina innervate the pelvic organs and lower limbs

Question 4

describe the meninges of the spinal cord

Answer 4

The spinal meninges are continuous with the cranial meninges so have the same 3 layers - dura, arachnoid and pia mater

Note that the subarachnoid space also exists in the spinal cord and is filled with CSF

Question 5

What is the spinal cord suspended in the spinal canal by ?

Answer 5

• A ligament called the denticulate ligament - which is a ribbon of tissue on the lateral aspects of the spinal cord
• This ligament is formed from pia and arachnoid tissue and attachs from the cord to the dura mater at points along the cord

Question 6

Describe the orientation of grey and white mater in the spinal cord and what they contain

Also what is located in the centre of the spinal cord ?

Answer 6

The white mater consists of longituindally (they gp up and down in the spinal cord) orientated nerve fibres (axons), glial cells and blood vessels. White mater is located on the outside part of the spinal cord

Grey mater consists of neuronal soma (cell bodies), cell processes, synapses, glial & blood vessels. Located on the inner aspects of the spinal cord

In the centre of the spinal cord there is the central canal

Question 7

Where does the central canal of the spinal cord run to and from and what is contained within it ?

Answer 7

• Runs from the 4th ventricle and has a blind ending inferiorly
• It contains CSF

Question 8

What is the white mater in the spinal cord conventually described as being made up of ?

Answer 8

• Anterior, posterior and lateral columns (funiculus)
• Fasciculi seen to make up funiculus

Question 9

What 4 parts is the grey mater of the spinal cord divided up into ?

Answer 9

• left and right posterior and anterior horns
• They make up the letter ‘H’. The horizontal part of the ‘H’ represents the dorsal and ventral grey commisures, which surround the central canal.

Question 10

What extra horn in the grey mater of the spinal cord arises between vertebral levels T1 - L2 and what does it contain ?

Answer 10

Lateral horn - it contains presynpatic sympathetic fibres

Question 11

Describe the arterial supply of the spinal cord

Answer 11

1. Three major longitudinal arteries: one anterior and two posterior that originate from the vertebral arteries. These run the length of the cord. (called the anterior and posterior spinal arteries in the pic)
2. Segmental arteries derived from vertebral, intercostal and lumbar arteries
3. Radicular arteries that travel along the dorsal and ventral roots

Question 12

Describe the venous drainage of the spinal cord

Answer 12

It is similar to the arterial supply, it consists of longitudinal and segmental veins

Question 13

What is the main difference between the dura mater inrelation to the spinal cord to that of the dura mater in the cranial cavity ?

Answer 13

Dura mater in the cranial canivty is attached to the periosteum of the cranium

The dura mater in the spinal canal has a space between it and the bone, called the epidural space

Question 14

What is contained within the epidural space and what is its clinical relevance ?

Answer 14

• Adipose tissue and anterior and posterior epidural venous plexuses are contained here
• It is utilised in epidural anaesthesia

Question 15

Where is the primary somatosensory cortex located in the brain ?

Answer 15

The postcentral gyrus - in the parietal lobe

Question 16

How does information reach the primary somatosensory cortex ?

Answer 16

They travel in tracts within the spinal column (except for sensory info from the face and scalp)

Question 17

What are the 2 main ascending pathways of the spinal cord ?

Answer 17

1. Dorsal column medial leminiscus pathway
2. Spinothalamic tract

Question 18

What 2 fasiculi make up the dorsal column ?

Answer 18

Cuneate and gracile fasiculi collectively

Question 19

What is the primary function of the dorsal column?

Answer 19

To convey fine touch, proprioception and vibration to the postcentral gyrus (primary somatosensory cortex)

Question 20

What is a nucleus in the CNS?

Answer 20

it is a discrete collection of cell bodies within the CNS

Question 21

Describe the pathway sensory fibres take in the dorsal column

Answer 21

1. Initially sensations are picked up by receptors
2. Unipolar neurons then transmit these sensations (fine touch and proprioception) to the dorsal column
3. Fibres then ascend in the cuneate and gracile fasiculi (dorsal column) of the spinal cord to reach the medulla
4. Fibres then synapse in the gracile and cuneate nucleus in the medulla
5. Axons/fibres are then projected from these nuclei cross over (decussate) in the medulla to the opposite side of the medulla e.g. R side to L
6. The axons/fibres then continue to ascend now on the opposite side of the brainstem to the thalamus where they synapse to the ventral posterolateral nucleus (these fibres after decussating are now know as the medial leminiscus)
7. The posterolateral nucleus then sends projections (axons/fibres) via the posterior internal capsule to the postcentral gyrus (primary somatosensory cortex)

Question 22

Where do fibres in the dorsal column/medial leminiscus system decussate ?

Answer 22

In the medulla

Question 23

Where do fibres in the dorsal column/medial leminiscus system synapse ?

Answer 23

The thalamus (note also synapse in the medulla but think thalamus one is the big one to know)

Question 24

How many neurons are involved in the dorsal column/medial leminiscus system ?

Answer 24

3

Question 25

Describe the spinothalamic tract

Answer 25

1. The Pathway begins with nerve endings or receptors in the body sending information down the axons of a neuron (the neuron mentioned is a pesudo unipolar neuron like in the dorsal column medial leminiscus system)
2. Spinothalamic tract enters the spinal cord and asecends or descends 1-2 levels within lissauers fasiculus
3. It then synapses onto a cell body in the dorsal horn of the grey mater
4. This cell body (neuron) then sends axons which decussate at the anterior white commissure (cross- segmentally as they decusate in the spinal cord) and then travels via the anterior and lateral spinothalamic fasciculus to the to the brainstem
5. The spinothalamic tract then ascends through the brainstem (the anterior and lateral tracts join in the medulla - now known as the spinal leminiscus)
6. The tract then synapses at the thalamus
7. Thalamus then sends axons to the postcentral gyrus (primary somatosensory cortex)

Question 26

How many neurons are involved in the spinothalamic tract ?

Answer 26

3

Question 27

What is the main function of the spinothalamic tract ?

Answer 27

To convey pain, temp and crude touch (deep pressure) to the postcentral gyrus

Question 28

Where do fibres in the spinothalamic tract decussate ?

Answer 28

They cross segmentally i.e. at the white commissure of spinal cord segment

Question 29

What is the clinical significance of decussations in the ascending and descending pathways ?

Answer 29

• It means that for the sensory pathways the left side of the body is represented on the right cortex and vice versa.
• Motor fibres will cross the midline, thus the right cortex controls muscles on the left side of the body and vice versa.

Question 30

What is the main descending pathway in the spinal cord (motor pathway) ?

ascending pathways are more sensory functions

Answer 30

Corticospinal tract

Question 31

Go over the motor cortex homonculus

Answer 31

Question 32

Describe the corticospinal tract pathway

Answer 32

1. The pathway begins in the precentral gyrus (primary motor cortex)
2. The pathway descends through the internal capsule and then into the brainstem (note that the thalamus is not involved in this pathway)
3. As the pathway enters the medulla it is also known as the pyramidal tract. About 85% of the fibres decussate at the decussation of the pyramids, the fibres which decussate here will innervate the limb muscles and the fibres which don’t decussate and stay on the same side will innervate the axial (body) muscles (important for posture etc)
4. Both sets of fibres/axons then leave the brainstem and run down the 2 corticospinal tracts - the axons which will innervate the limb muscles will run down the lateral corticospinal tract and the axons which will innervate the axial muscles will run down the anterior corticospinal tract
5. When the axons of the anterior corticospinal tract (ones which haven’t yet decussated) reach their target level they decussate through the anterior white commisure and synapse onto a neuron in the anterior horn of the grey mater. The axons of the lateral corticospinal tract have already decussated so once they reach target level the simply synpase onto a neuron in the anterior horn as well
6. The neurons in the anterior horn are known as anterior horn cells and they will produce projections (axons) which will project to the limb and axial muscles

Note irrespective of the level of decussation, left hemisphere controls right musculature and vice versa

Question 33

How many neurons are involved in the corticospinal tract ?

Answer 33

2 - a upper and a lower motor neuron

Question 34

What is the main function of the corticospinal tract ?

Answer 34

• To convey motor control of the body (axial) and limb muscles
• Note the corticospinal tract is sometimes referred to as the pyramidal tract

Question 35

Describe upper and lower motor neurons

Answer 35

• Neurons in the cortex are known as upper motor neurons (UMN) & a lesion anywhere in these fibres from the cortex all the way down to the anterior horn are known as upper motor neuron lesions
• Lower motor neurons (LMN) are the axons from the anterior horn cells to the muscles - a lesion at any point between these points mentioned is known as a LMN lesion

Question 36

Where does the limb pathway and the axial pathway in the corticospinal tract decussate

Answer 36

• Limb pathway decussates at the deucssartion of the pyramids
• Axial pathway decussates through the anterior white commissure in the spinal cord

Question 37

Appreciate this point:

A CVA of the internal capsule can result in a lack of descending control of the corticospinal tract (as well as other effects – the CST is intermingled with other fibres here), which results in a spastic paralysis with hyperflexion of the upper limbs. This is sometimes referred to as decorticate posturing.

Question 38

What are the motor systems outwidth the corticospinal/pyramidal tract known as and what are the motor systems which comprise it called?

Answer 38

Extrapyramidal system

Comprised of:

1. Tectospinal tract
2. Recticulospinal tract
3. Vestibulospinal tract

Question 39

What is the function of each of the following:

1. Tectospinal tract
2. Recticulospinal tract
3. Vestibulospinal tract

Answer 39

1. Tectospinal tract - Thought to mediate reflex head and neck movement due to visual stimuli. Cell bodies reside in the superior colliculus
2. Recticulospinal tract - It has many nuclei and receives input from virtually all parts of the CNS. It has many functions, including the influencing of voluntary movement, such as helps to maintain a standing posture by facilitating contraction of the extensors of the lower limbs. Reticular formation forms the central core of the brainstem
3. Vestibulospinal tract - helps to hold upright and balanced posture by facilitating extensor MNs of antigravity muscles e.g. of the leg. Fibres originate in the vestibular nuclei of pons and medulla

In general: Fibres originating in pons facilitate extensor movements and inhibit flexor movements, while those originating in the medulla do the opposite.

don’t put to much empahsis on knowing these ones

Question 40

Go over the sensory homonculus, and describe what essentially the sensory homonculus shows

Answer 40

The sensory homunculus - shows the relative volume of cortex devoted to each body part is correlated with the density of sensory input from that part (not its size) and hence sensory acuity and importance of sensory input