Motor Neurones

Question 1

What are motor neurones

Answer 1

Nerve cell that provides efferent response of CNS

Question 2

How do you classify motor neurones

Answer 2

Upper motor neurones

Lower motor neurones

Question 3

Where are the cell bodies of UMN located

Answer 3

In the brain:
Cerebral cortex - pyramidal motor neurones/cortical efferents

Sub-cortical areas - extra-pyramidal motor neurones/brainstem and bulbar efferents

Question 4

Where are cell bodies of LMN located

Answer 4

Spinal cord: ventral horn lamina IX

Brainstem: cranial nerve nuclei

Question 5

What are the key properties of LMNs

Answer 5

Have cell body in CNS - so CNS lesions can present with LMN signs
Project into peripheral nervous system
Spinal LMNs innervate skeletal muscle (link UMN w muscle)
Cranial nerve LMNs innervate eyes, face, tongue
Receive descending inhibition from cortex by UMNs

Question 6

What are the signs of LMN lesion

Answer 6

Paralysis 
Weakness
Wasting
Hyporeflexia
Hypotonia 
Fasciculations
Fibrillations 
Negative Babinskis sign

Question 7

What is fasciculation

Answer 7

Rapid Uncoordinated contraction of group of muscle fibres, that is visible
Caused by increased expression of nAChR to compensate for lack of innervation

Question 8

What is fibrillation

Answer 8

Rapid uncoordinated contraction of individual muscle fibre, not visible

Question 9

Why do you get hypotonia with LMN lesion

Answer 9

LMNs send tonic stimulatory signal to muscle

Question 10

Why do you get hyporeflexia in LMN lesions

Answer 10

LMNs are the efferent fibre of spinal reflexes

Question 11

What is a spinal reflex

What are the components of spinal reflex arc

Answer 11

Involuntary and automatic reaction to stimulus that does not require the brain 
Receptor (in skin, muscle, tendon) 
Afferent fibre (to posterior horn)
Integration centre (lamina IX) 
Efferent fibre (LMN, anterior horn to effector) 
Effector (skeletal muscle)

Question 12

What is a stretch reflex

Answer 12

Monosynaptic reflex of muscle contraction in response to stretch

Question 13

List the steps of patellar reflex

Answer 13

1. Muscle spindle detects stretch in muscle fibre (stretch receptors that detect change in length of muscle)
2. Signal transmitted down afferent fibres to L3 L4 spinal roots
3. Afferent fibres synapse with efferent motor neurones in spinal cord
4. Motor neurone causes contraction of quadriceps muscle - knee extension
5. Afferent fibres descend in spinal cord to level of antagonist muscle innervation. Stimulate inhibitory interneurones to inhibit motor neurone of antagonist muscle. This inhibits tonic stimulation of muscle and facilitates knee extension.

Question 14

What is withdrawal reflex

List steps of the plantar reflex

Answer 14

Spinal reflex to withdraw part of body from noxious stimuli

1. Nociceptor of S1 dermatome stimulated
2. Afferent fibres enter nerve root L5 S1 and synapse with motor neurones
3. Motor neurones of L5 S1 nerve roots elicit plantar flexion response
4. There is descending pyramidal control of reflex arc to inhibit extensor response. Loss of descending inhibition in UMNL causes extensor/dorsiflexion - +ve Babinski sign

Question 15

What are the key properties of UMNs

Answer 15

Cell body in CNS: cortex, brainstem
Located entirely within CNS: UMNL always indicates CNS pathology
Synapse with LMNs in spinal cord: descending tracts are all UMNs
Net Inhibitory effect on LMNs: majority synapse via inhibitory interneurones

Question 16

What are the signs of UMN lesion

Answer 16

Weakness
Hypertonia
Hyperreflexia 
Spasticity
Clasp knife reflex 
\+ve Babinski sign

Question 17

Why do you get hypertonia in UMNL

Answer 17

Loss of descending tonic inhibition of LMN, continuous contraction

Question 18

What is spasticity

Answer 18

Combination of skeletal muscle paralysis, hyperreflexia, hypertonia
Muscles are fixed in position by the more powerful muscle of antagonist pair : UL flexed LL extended

Question 19

Why do you get hyperreflexia in UMNL

Answer 19

Loss of descending inhibition of LMNs, LMNs become hyperexcitable in spinal reflex

Question 20

What is clasp knife response

Answer 20

Sudden decrease in resistance when attempting to flex a joint
Mediated by Golgi tendon organs: stretch receptors only stimulated by extreme stretch
Sign of UMNL: muscles are hypertonic so initial resistance to flexion. Sudden drop in resistance with activation of GTO and stimulation of inhibitory interneurones that inhibit LMN, causing relaxation.