Disorders of the MotorU

Question 1

What is a motor unit

Answer 1

A alpha motor neurone (lower motor neurone) and all the extrafusal muscle fibres it innervates

Question 2

What is denervation?

What is re-innervation?

Answer 2

Denervation is the state where a muscle has lost its nerve supply

Re-Innervation is the regrowth of the axon of a lower motor neurone to resupply and re-innervate the muscle

Question 3

What is polio myelitis?

Answer 3

Infection caused by the polio virus

Is a communicable disease

Leads to infection of the Soma (cell bodies) in the ventral horn

Question 4

Why does polio lead to damage?

Answer 4

Infection of cell bodies in ventral horn
death of motor neurone cell body
denervation of muscle
paralysis 
loss of supply to respiratory muscles need assisted breathing

Question 5

Where are the cell bodies of lower motoneurons located

Answer 5

In the ventral horn of the spinal cord

Question 6

What is motor neurone disease?

Answer 6

unclear as to whether it is an upper motor neurone or lower motor neurone disease

Leads to spontaneous apoptosis and death of motor neurones

Question 7

How can are car crash lead to motor neurone injury

Answer 7

severing of the spinal cord or extreme compression due to a car crash injury can lead to death of motor neurones loss of muscle innervation and paralysis

Question 8

How can the axons of lower motor neurones be damaged?

Answer 8

complete transduction of the axon
crushing car crash injuries
stabbings

or Demyelination of axons
result in temporary faulty function of the axons and motor neurones e.g Gullian Barre syndrome

Question 9

How do nerve gases and pesticides affect the NMJ?

Answer 9

orgnaophosphates act by blocking AChE in the NMJ ie acting as a permanent inhibitor

prevents ACH release continuous nervous transmission and stimulation leading to prolonged muscle contraction, salvation tearing and deification if it autonomic neurones affected too eventually leads to asphyxiation and death

Question 10

What is Myasthenia Gravis

Answer 10

Autoimmune condition that result in destruction of nAChR’s on the post synaptic membrane

Question 11

Disease of the skeletal msucles

Answer 11

Lead to paralysis of the motor system

Duchenne Muscular Dystrophy
defects in muscle proteins lead to progressive muscle weakness and eventual muscle death
leads to death of individual eventually

Question 12

What are the characteristics lower motor neurone damage signs

Answer 12

Muscle weakness
loss of muscle tone
loss of stretch reflex
muscle atrophy wasting 
fasciculations uncontrolled contractions

Question 13

What is crocodile tears syndrome? What does this condition illustrate?

Answer 13

Where the regrowth of axon that used to supply the salivary glands innervates the lacrimal gland instead meaning when there is a stimulus such as taste smell or even sight of food the person begins to cry due to simulation of lacrimal gland

shows that even though regeneration may occur the re-innervation may be incorrect and go to the wrong effector

Question 14

What is the most common cause of botulims?

Answer 14

eating incorrectly preserved homemade food provides ideal conditions for the culture of the Botulin bacteria

Question 15

How does the toxin work and cause botulism?

Answer 15

Enters lower motor neurones that use ACh
favours highly active neurones
It is endocytosed by the presynaptic axon and one inside begins to cleave SNARE proteins involved in the release of ACh
this leads to reduced and eventually no postsynaptic action potentials and thus muscle paralysis

Can be fatal as can stop the muscles of respiration

Question 16

Why is diplopia a common one of the first symptoms of botulism

Answer 16

Extraoccular muscles of the eye are incredibly active toxin favours and enters better into highly active neurones

Question 17

What is the botulin toxin used for?

What happens if the toxin enters systemic circulation

Answer 17

Botox
Injected into the face at very high concentrations in localised areas leading to paralysis of muscles in the face and therefore reduction in wrinkles. This needs to be repeated regularly

Is rapidly metabolised if it enters systemic circulation so shouldn’t go onto effect other NMJ
Must be repeated as the toxin is broken down and metabolised over time

Question 18

What is suxamethonium?

Answer 18

A drug used to cause muscle paralysis and relaxation

Question 19

how does suxamethonium work?

What type of drug is it ?

Answer 19

It is a Agonist of the nACh receptor on the sarcolemma

Binds to nAChR on sarcolemma causing opening
leads to depolarisation and generation of an EPP and subsequent generation of an AP and initial muscle contraction

However unlike ACH it is not rapidly hydrolysed so leads to nACh channels to remain open this lead to the Em being constantly above threshold and the cell being unable to repolarise and thus the cell cannot fire any more AP’s
This leads to fasciculation’s and eventual muscle relation

Question 20

Why may repeated does of suxamethonium cause long term paralysis?

Answer 20

Acts in two phase, phase 1 where there is prolonged depolarisation resulting in fasiculations and then relaxation

And phase 2 that leads to nAChR desensitisation where they are no longer sensitive to ACh

Question 21

How can an overdose of suxamethonium be treated?

Answer 21

Add an antagonist of the nAChR to try and shut as many as possible preventing opening by suxamethonium