S3: Overview of Ascending and Descending Pathways

Question 1

Describe the anatomy of M1 ‘motor strip’ and the S1 ‘sensory strip’ in the brain

Answer 1

The primary motor cortex is located in the precentral gyrus of the frontal lobe anterior to the central sulcus and this is M1. It controls voluntary movement of the opposite side of the body. The area in front of the motor strip is the premotor cortex involved in movement, planning and preparation.

Question 2

Describe the anatomy of the S1 ‘sensory strip’ in the brain

Answer 2

The primary somatosensory cortex is just behind the motor strip in the post central gyrus of the parietal lobe. It receives ascending (sensory) projections for all sensory modalities including light touch, joint position, sense, pain, temperature. It is also referred as S1 and it is concerned with sensation from the opposite side of the body.

Question 3

Compare the size of S1 and M1

Answer 3

In both the motor cortex and sensory strip the amount of cortex devoted to each body part is in proportion to the precision of motor control (for motor) and tactile sensitivity (for sensory). In both cases the hands, face and tongue have disproportionately large representations.

Question 4

Describe structure of hemispheric grey matter

Answer 4

There are two types of grey matter: The cerebral cortex of the brain is the outer layer of the cerebrum. Inside the deeper layers are the subcortical nuclei.

Question 5

What is a ‘nucleus’ in neuroanatomy?

Answer 5

It is a cluster of cell bodies of neurones in the CNS located deep within the cerebral hemispheres and brain stem.

Question 6

Why is the Thalamus known as the ‘gateway’?

Answer 6

The thalamus is said to be the gateway to the cerebral cortex (i.e. a relay station). For example, if someone stands on your toe the information has to get to your cortex. The information will go up your spinal cord up to your brain stem and to the thalamus. The thalamus would then relay to information to the sensory strip in the appropriate area.
- The thalamus relays all sensory information except olefactory

Question 7

Describe structure of Thalamus

Answer 7

• We have 2
• Thalami are subdivided into a number of nuclei which possess specialisations allowing them to deal with specific data: there is an anterior, lateral and medial nuclear group.
• Inside the thalamus, the internal medullary lamina is white and’Y’ shaped.
• The two ‘arms’ of Y holds onto the anterior nuclear group. The anterior group is related to memory and emotion.
• The limb of Y gives large lateral nuclear group and smaller mediodorsal nuclear group. The lateral nuclear group gives off many other nucleus’s.
• There are medial and lateral geniculate bodies which relay to primary auditory and visual cortices respectively.

Question 8

What is the ventral posterior (VP) nucleus?

Answer 8

It is part of the lateral nuclear group in the thalamus. It is the relay that goes up to the sensory strip. it is the relay in the two important sensory pathways: the pain and temperature pathway (spinothalamic) and the vibration and proprioception (dorsal column pathway).

Question 9

Describe anatomy of subcortical white matter, corpus collosum and internal capsule in coronal section of the brain

Answer 9

There is subcortical white matter and the corpus collosum in the middle linking the two hemispheres. The internal capsule goes down to the middle of the brain with the thalamus medial and basal ganglia lateral.

Question 10

What is the internal capsule? Describe its function.

Answer 10

The internal capsule contains ascending projections to the cerebral cortex and descending projections that come from the cerebral cortex.

• The sensory pathways also go through the internal capsule going up so the pain and temperature pathway as well as the vibration and proprioception pathway can be seen going up. Ascending pathways must first go up to the thalamus and be relayed up.
• The motor fibres start in the motor cortex and go down through the internal capsule and then get tho the spinal cord by going through the brain stem. The motor fibres do not need to be relayued by the thalamus,

Question 11

Where in the brain is the internal capsule found?

Answer 11

It is between the basal ganglia and thalamus

Question 12

Describe structure of the brain stem

Answer 12

The midbrain is at the top, the pons is in the middle and the medulla at the bottom which eventually becomes the spinal cord.

Question 13

Describe structure of the midbrain

Answer 13

Two big roman pillars called cerebral peduncles. Between them is the interpeduncular fossa.

Question 14

Describe structure of the midbrain

Answer 14

Two big roman pillars called cerebral peduncles. Between them is the interpeduncular fossa. At the posterior we have the cerebral aquaduct going down and everything dorsal to that is the tectum (meaning roof) and anterior are the peduncles. The main thing in the peduncles is the substantia nigra, the nerve cells here project up to the basal ganglia for movement control. The crus cerebri is where the motor tract descends.

Question 15

What is the course of the motor cortex to spinal cord?

Answer 15

Motor cortex –> Internal capsule –> Crus cerebri –> Spinal Cord

Question 16

What do the dorsal columns do?

Answer 16

The dorsal columns of the spinal cord carry vibration and proprioception, so if you put a tuning fork on someones foot the info will be taken up the back of the spinal cord by these dorsal columns.
The dorsal columns continue up to the medulla and terminate at the dorsal column nuclei (posterior brain stem)

Question 17

What are the two sensory pathways?

Answer 17

1. Dorsal Column Pathway

2. Spinothalamic Tract

Question 18

Describe the dorsal column pathway

Answer 18

This is the vibration and proprioception pathway. It is tested clinically by a tuning fork and it tests discriminative touch and proprioception. The tuning fork stimulates low threshold mechanoreceptors. The neurones in this pathway have a large diameter that are typically myelinated with a fast conduction velocity.

Question 19

Describe the spinothalamic tract

Answer 19

This is the pathway that goes up the spine to thalamus. It is the pain (nocicetors) and temperature (thermoreceptor) pathway. This pathway has skinnier axons that are less myelinated (a-delta and c-fibres) hence impulses travel slower (because local spinal reflex would act before pain set in). The best clinical test for this is a pinprick.

Question 20

Describe a three neurone chain

Answer 20

Both dorsal column pathway and spinothalamic tract are three neurone pathways. A peripheral sensory nerves have their cell bodies in the dorsal root ganglia of the spinal cord. There is the middle neurone. The last step is the thalamus because this is the gateway to the cortex.
- When referring to these neurones we say first order neurone (lowest down), then second order neurone and then third order neurone.

Question 21

Describe how the dorsal column pathway is a three neurone chain

Answer 21

1. First order neurone has its cell body in dorsal root ganglion. This is a massive axon (for example, one end going down to little toe and other side travelling up to medulla).
2. Second order neurone has its cell body on the same side as dorsal root ganglion but its axon crosses the midline and then goes to thalamus.
3. Third order neurone has its cell body in the thalamus and projects via the internal capsule to the sensory strip of the parietal lobe.

Question 22

Describe how the spinothalamic tract is a three neurone chain

Answer 22

1. First order neurone comes in and synapses immerdiately with the second order neurone. This is all happening up the spinal cord, pain fibres coming in and immediately synapsing.
2. The second order neurone are in the spinal cord and cross the midline and go up the thalamus. This occurs all the way up the spinal cord.
3. The third order neurone has its cell body in the thalamus and projects via the internal capsule to the sensory strip of the parietal lobe.

Question 23

What is the main difference between the structure of the dorsal column pathway and the spinothalamic tract?

Answer 23

In dorsal column pathway, all the fibres cross over at once at the medulla (great sensory decussation) at the second order neurone while in the spinothalamic tract every incoming neurone synapses at the point over so there is crossing over at every level of the spinal cord.

Question 24

What is gracile fasiculus and cuneate fasiculus, Sensory decussion and medial lemniscus?

Answer 24

There are two dorsal column nuclei.
- The gracile fasiculus carries first order vibration neurones to the vibration nucleus, this is for the legs. It carries this to the gracile nucleus.
- Laterally we have the cuneate fasiculus which is the vibration pathway for the arms, it comes in at the cervical level where the brachial plexus is. It carries this to the cuneate nucleus.
These are all first order neurones carrying vibration information up to the medulla where the neurones synapse with the second order neurones.
- It is in the cuneate and gracile nuclei that the cell bodies of the second order neurones sit.
- Sensory decussation is where the second order neurones cross over to the medial lemniscus (this is like a twisted ribbon going up the pons to the thalamus.
- It then goes to the VP nucleus (cell bodies of third order neurones).

Question 25

Do vibration and pain signals travel on same side?

Answer 25

Vibration signals go up the same side, so vibration on right foot, the signals will go up the right-hand side of the spinal cord.
But if you put a pin in the foot, it will go up the opposite side in the spinothalamic tract. Axons are crossing at every single spinal level in the spinothalamic tract.