(4.1+2) Descending Tract Flashcards
Name and describe the function of the two types of lower motor neurone.
- 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
Briefly describe how motor tone is maintained and adjusted.
- Tonic minimal stimuli by Alpha motor neurone
- Feedback from proprioception
- Adjustment of sensitivity by Gamma motor neurone
When are muscle tone normally exhibited?
- Newborns yet develop
- REM sleep inhibits muscle tone
What type of reflex is a patella tap reflex? Briefly describe its mechanism
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
Where does upper and lower motor neurone synapse?
Spinal ventral horn, at lamina IX
Briefly describe an inverse myotic reflex.
- Golgi Tendon Organ detects tension
- Group 1b Afferent fibres transmit stimulus to dorsal horn
- Activation of interneurones -> inhibits Alpha motor neurone -> prevent muscle contraction
Some diseases create polysynaptic reflex, how does it affect patient’s response?
- increased inhibitions from interneurones
- failure to evoke response
Where does the Rubrospinal tract arise from? What response does it cause?
- Red nucleus
- Flexor reflex
What response does Retoculospinal tract cause?
Extensor reflex
What response does the Vestibulospinal tract cause?
Postural and balance
Where does the Tectospinal tract arise and what response does it produce?
- Superior colliculus in midbrain
- Head, eyes and upper body response to auditory or visual stimuli
Describe the course of Lateral Soinothalamic tract.
Cortex -> Internal capsule -> Thalamus -> Medullary pyramid -> decussate -> Spinal ventral horn
Describe the course of Lateral Spinothalamic tract.
Cortex -> Internal capsule -> Thalamus -> Medullary pyramid -> Spinal level -> decussate -> Spinal ventral horn
Which tract is responsible for cranial nerve motor movements? Describe its course.
- Corticospinal tract
Cortex -> Internal Capsule -> Thalamus -> Medullary pyramid -> Brainstem -> decussate -> Cranial N nucleus
Compare and contrast the muscle tone, muscle power and reflex between UMN and LMN lesion.
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
What test that you do on a patient’s foot can you do to recognise a UMN lesion?
Babinski sign:
- running sole of foot by a blunt instrument causes extended toes & dorsiflexed foot (replace the normal response of flexed toes)
What is Pronator drift? What does it suggest?
Sign of UMN lesion:
- Eyes closed, arms outstretched, affected arm tend to drift downwards
Which of UMN or LMN lesion may cause involuntary contractions? What is this sign called?
- UMN lesion
- Myoclonus
Where is the lesion in Huntington’s disease? It produces a sign that causes a series of involuntary movement, what is it called?
- Disrupted synapse between Striatum and Subthalamic Nucleus
- Chorea
What is the underlying cause of fasciculation?
LMN lesion -> nAChR becomes hyperexcitible to any ACh like chemicals
What disease selectively destroy alpha motor neurone?
Polio myelitis -> destroy alpha soma -> unable to regenerate
Compare and contrast the effects on bladder function between UMN and LMN lesions. At which spinal level can each be damaged?
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
What would be the consequence of a alpha motor neurone lesion on the appearance of muscle it supplies?
- Begins with fasciculation
- Progress to muscle wasting & atrophy
What’s a motor unit?
An alpha motorneurone + the muscle fibres it innervates
