Lecture 12: Descending Pathways

Question 1

What is the role of Upper Motor Neurones and Lower Motor Neurones?

Answer 1

Upper motor neurones:

- Originate in the cerebrum and the subcortical structures
- Influence lower motor neurone activity (found in the anterior grey horn of the spinal cord - these then supply the specific skeletal muscles - or cranial nerves)
- Modify local reflex activities 
- Superimpose more complex patters of movements 
- Supply the muscles of the head and neck to the cranial nerve nucleus to synapse with a LMN that will send its axon via a cranial nerve 

Lower motor neurones:
- Originate from the brainstem (to supply the head and neck) or the ventral grey horn of the spinal cord (to supply the body)
Peripheral nerves to motor end plates/neuromuscular junction

Question 2

What is the internal capsule connected to? How is it organised?

Answer 2

Crus cerebri (cerebral peduncles/basis pedunculi).

It is somatopically organised. Motor nerves to the face pass through the genu of the internal capsule. The posterior limb of the internal capsule contains motor fibres organised from anterior to posterior: arm, trunk and legs.

The crus cerebri is also somatopically organised with the face more medial, then the arms, trunk and legs.

Question 3

What is the result of an internal capsule lesion? How is it at risk?

Answer 3

The middle cerebral artery gives rise to deep striate arteries to supply deep cortical structures such as the basal ganglia an the internal capsule. A haemorrhage in the lenticular striate (deep perforating arteries) puts the internal capsule in danger. The arteries are at risk in the case of hypertension. Many important sensory and motor fibres travel in this area.

Question 4

Describe the path of the lateral corticospinal tract.

Answer 4

• The UMN in the precentral gyrus sends an axon via the posterior limb of the internal capsule through the cerebral peduncles.
  
  • These fibres continue to travel through the ventral pons and into the pyramids of the medulla. At the decussation of the pyramids the majority of the fibres cross the middle (85%) and join the lateral corticospinal tract.
  • The UMN join the laterals corticospinal tract. At a specific spinal level it makes contact of a LMN at the ventral grey horn. It can then exit and supply the periphery. We have contralateral innervation.

The lateral corticospinal tract which carries contralateral information, supplies the limbs musculature.

Question 5

Describe the path of the anterior corticospinal tract.

Answer 5

• The UMN in the precentral gyrus sends an axon via the posterior limb of the internal capsule through the cerebral peduncles.
  
  • These fibres continue to travel through the ventral pons and into the pyramids of the medulla.
  • The remaining 15% that do not cross over at the decussation of pyramids descend ipsilaterally through to the spinal cord - some fibres join the anterior corticospinal tract. Some here cross the midline and contact at LMN for the muscles they will innervate.
  • They will innervate LMN at both sides - bilateral.

The anterior corticospinal tract is reserved for bilateral innervation. It is mainly axial muscles e.g. postural musculature, thoracic an abnormal flexors/extensors, pectoral girdle, pelvic girdle etc. These muscles do not work in isolation.

Question 6

What are the signs/symptoms of a LMN lesion?

Answer 6

• Lead to flaccid paralysis of the muscles involved
  - We see diminished (hyporeflexia ) or absent (areflexia) tendon reflexes at the level of the lesion
  - Muscles wasting
  - Muscles weakness
  - Hypotonia
  - Fasciculation (spontaneous muscle twitches) and fibrillations (rapid spontaneous muscle contractions)

Question 7

What are the signs/symptoms of a UMN lesion?

Answer 7

• Typically manifest initially as flaccid paralysis of opposite limbs and loss of tendon reflexes
  - After several days to a week motor function recovers but we see hypertonia
  - Long term we see increase, brisk (hyperreflexia) spinal reflex below, spastic paralysis of the involved muscles and loss of fine motor control, permanent inability to carry out fine movements of the hands and feet
  - Often we see other pathways appear to take over most corticospinal functions and so function returns - this movement however would not be as fine tuned
  - Axial muscle groups are spared - these are supplied bilaterally. An upper motor neurone on one side does matter as much as the other side will take over and innervate the muscles.

Question 8

What is the Babinski test? What does a positive result indicate?

Answer 8

Babinski sign - when we stroke the sole of the foot of a patient the toes should plantar flex. In an UMN lesion, one of the abnormal reflexes is the toes fan outwards. This is a pathological reflex.

Question 9

Which LMN have unilateral supply in the corticonuclear tract?

Answer 9

CN V (lower part) and CN XII

Question 10

Describe the facial nerve motor supply via the corticonuclear tract.

Answer 10

• At the level of the upper face at the precentral gyrus, the UMN sends it axon via the genu of the internal capsule. It then makes contact with the LMN at the facial nerve nuclei to meet the LMN.
  - The nerves to the upper facial muscles then sends bilateral innervation to the upper face
  - The upper face region of the left precentral gyrus for example supplies both the right and the left

     The Lower face region of the right precentral gyrus sends a UMN through the genu of the internal capsule to supply the contralateral facial nucleus. This then sends an axons to send a contralateral lower face. These fibres travel using CN VII.

Question 11

What is the difference in signs/symptoms in an infection in the facial canal (Bell’s palsy) or a TIA at the level of the internal capsule?

Answer 11

Bell’s palsy - Injury to the facial canal. This leads to paralysis of the upper and facial nerve as there is injury to the motor nerves from the ipsilateral side and the contralateral side supplying using it’s bilateral motor supply.

Internal capsule injury - This leads to paralysis of the lower face but the upper face muscles are still intact due to they bilateral supply. The contralateral side to the lesion supplies to both its ipsilateral side and contralateral side.

Question 12

What will result due to a lesion of the left hypoglossal nerve?

Answer 12

Tongue deviate to the left - the supply to the muscles on the right reduced. This means the muscles on the left (genioglossus ) will take control and deviate the tongue to the left.

Question 13

What will result due to a lesion of the left hypoglossal nerve?

Answer 13

Tongue deviate to the left - the supply to the muscles on the right reduced. This means the muscles on the left (genioglossus ) will take control and deviate the Tonge to the left.

https://www.youtube.com/watch?v=QJmhJHtKug8

Question 14

What is the result of a lesion in the fibres coming from the right side of the cortex to supply the tongue?

Answer 14

Deviate to the left - contralateral to the lesion

https://www.youtube.com/watch?v=QJmhJHtKug8

Question 15

Describe the extrapyramidal pathways.

Answer 15

1. Reticulospinal - starts in the reticular formation to the spinal cord
   
   2. Vestibulospinal - starts in the vestibular nuclei to the spinal cord
   3. Rubrospinal - red nucleus in the midbrain to the spinal cord
   4. Tectospinal - starts in the tectum of the brainstem to the spinal cord

Question 16

Describe the extrapyramidal pathways.

Answer 16

1. Reticulospinal - starts in the reticular formation to the spinal cord. It is important for fine tuning voluntary movement and is also important for muscles in breathing and consciousness.
   
   2. Vestibulospinal - starts in the vestibular nuclei to the spinal cord. It controls muscles important for posture (antigravity muscles).
   3. Rubrospinal - red nucleus in the midbrain to the spinal cord. It is important in controlling muscle tone.
   4. Tectospinal - starts in the tectum of the brainstem to the spinal cord. The tectospinal tract is responsible for controlling the movement of the head in response to auditory and visual stimuli.