(4.1+2) Descending Tract

Question 1

Name and describe the function of the two types of lower motor neurone.

Answer 1

• Alpha motor neurone -> extrafusal muscle fibres -> contraction -> shorten fibre length -> increase motor tone
• Gamma motor neurone -> intrafusal muscle fibres -> contraction to keep muscle spindle taut allowing alpha neurone to act on & expose central sensory nerve fibre endings to increase sensitivity

Question 2

Briefly describe how motor tone is maintained and adjusted.

Answer 2

• Tonic minimal stimuli by Alpha motor neurone
• Feedback from proprioception
• Adjustment of sensitivity by Gamma motor neurone

Question 3

When are muscle tone normally exhibited?

Answer 3

• Newborns yet develop

- REM sleep inhibits muscle tone

Question 4

What type of reflex is a patella tap reflex? Briefly describe its mechanism

Answer 4

Monosynaptic myotic reflex

• Muscle spindle proprioceptors detect stretch by shortened fibre length
• Group 1a Afferent fibres synapse directly with Alpha motor neurone (without interneurones)
• Alpha motor neurone causes contraction of the homonymous muscle

Question 5

Where does upper and lower motor neurone synapse?

Answer 5

Spinal ventral horn, at lamina IX

Question 6

Briefly describe an inverse myotic reflex.

Answer 6

• Golgi Tendon Organ detects tension
• Group 1b Afferent fibres transmit stimulus to dorsal horn
• Activation of interneurones -> inhibits Alpha motor neurone -> prevent muscle contraction

Question 7

Some diseases create polysynaptic reflex, how does it affect patient’s response?

Answer 7

• increased inhibitions from interneurones

- failure to evoke response

Question 8

Where does the Rubrospinal tract arise from? What response does it cause?

Answer 8

• Red nucleus

- Flexor reflex

Question 9

What response does Retoculospinal tract cause?

Answer 9

Extensor reflex

Question 10

What response does the Vestibulospinal tract cause?

Answer 10

Postural and balance

Question 11

Where does the Tectospinal tract arise and what response does it produce?

Answer 11

• Superior colliculus in midbrain

- Head, eyes and upper body response to auditory or visual stimuli

Question 12

Describe the course of Lateral Soinothalamic tract.

Answer 12

Cortex -> Internal capsule -> Thalamus -> Medullary pyramid -> decussate -> Spinal ventral horn

Question 13

Describe the course of Lateral Spinothalamic tract.

Answer 13

Cortex -> Internal capsule -> Thalamus -> Medullary pyramid -> Spinal level -> decussate -> Spinal ventral horn

Question 14

Which tract is responsible for cranial nerve motor movements? Describe its course.

Answer 14

• Corticospinal tract

Cortex -> Internal Capsule -> Thalamus -> Medullary pyramid -> Brainstem -> decussate -> Cranial N nucleus

Question 15

Compare and contrast the muscle tone, muscle power and reflex between UMN and LMN lesion.

Answer 15

UMN lesion

• Spastic paralysis (upper limbs flexed, lower limbs extended)
• Muscle weakness due to loss of voluntary control
• Hypereflexia

LMN

• Flaccid paralysis and muscle weakness
• Atonia
• Areflexia

Question 16

What test that you do on a patient’s foot can you do to recognise a UMN lesion?

Answer 16

Babinski sign:
- running sole of foot by a blunt instrument causes extended toes & dorsiflexed foot (replace the normal response of flexed toes)

Question 17

What is Pronator drift? What does it suggest?

Answer 17

Sign of UMN lesion:

- Eyes closed, arms outstretched, affected arm tend to drift downwards

Question 18

Which of UMN or LMN lesion may cause involuntary contractions? What is this sign called?

Answer 18

• UMN lesion

- Myoclonus

Question 19

Where is the lesion in Huntington’s disease? It produces a sign that causes a series of involuntary movement, what is it called?

Answer 19

• Disrupted synapse between Striatum and Subthalamic Nucleus

- Chorea

Question 20

What is the underlying cause of fasciculation?

Answer 20

LMN lesion -> nAChR becomes hyperexcitible to any ACh like chemicals

Question 21

What disease selectively destroy alpha motor neurone?

Answer 21

Polio myelitis -> destroy alpha soma -> unable to regenerate

Question 22

Compare and contrast the effects on bladder function between UMN and LMN lesions. At which spinal level can each be damaged?

Answer 22

UMN lesion: lesion above T12
- Loss of Hypogastric N -> loss of SNS -> leakage & urge incontinence

LMN lesion: lesion above S2-S4
- Loss of Pelvic N -> loss of PNS -> micturition inability hence overflow incontinence

Question 23

What would be the consequence of a alpha motor neurone lesion on the appearance of muscle it supplies?

Answer 23

• Begins with fasciculation

- Progress to muscle wasting & atrophy

Question 24

What’s a motor unit?

Answer 24

An alpha motorneurone + the muscle fibres it innervates