Wk4 Disorders Of Motor Unit

Question 1

What is the nerve muscle unit?

Answer 1

• It is the simplest formal arrangement between any 2 excitable tissues of the body
• Without the nerve-muscle unit, there would be no animal kingdom
• This system therefore deserves to be studied in its own right
• A large part of neurological practice specialises on clinical manifestations of disorders of the nerve-muscle unit

Question 2

What is innervation?

Answer 2

The normal state of nerve supply to a muscle or other target

The a-motorneurones innervante skeletal muscle

Lower motorneurones innervante skeletal muscle

Question 3

What is denervation?

Answer 3

Depriving the muscle of its nerve supply

Question 4

What can connection damage to nerve do?

Answer 4

Could lead to muscle death as muscle requires neural innervation and trop hic factors - keep muscle alive by determining biochemical factors

Question 5

Restoration of nerve supply to a muscle after nenervation

Answer 5

• Re-growth of a nerve to re-supply a muscle or other target is known as ‘re- innervation’
• Re-innervation of the original effector organ is often not always successful
• Most nerves often re-innervate an effector organ that is different from its original target
• This leads to emergence of all sorts of unexpected results (or syndromes)
– Crocodile tears syndrome is an interesting example

Question 6

Polio myelitis

Answer 6

• It is a communicable infection that targets cell bodies of lower motoneurones of the body
• Also known as infantile paralysis
• Caused by infection by the polio virus
• Leads to toxic infection (death) of Cell Bodies of Neurones of the ventral horn
• Immunization programmes are effective at preventing this disease
• Any motoneurones of the spinal cord are susceptible to this virus
• Death of motoneurones leads to denervation hence paralysis of muscles they supply
• It is very debilitating
• Wiped out by immunization in developed countries
• It is now largely a disease of the developing world
• Global air-travel means that it can still be acquired when holidaying in the disease’s hotspots after immunity has lapsed (usually after 10 years after innoculation)

Question 7

2 types of motorneurone disease

Answer 7

• There are two variants to it
1) The disease simultaneously kills both
– upper motoneurones –&
– Lower motoneurones
• This variant is known as progressive supranuclear palsy (American terminology)
2) The disease targets only the lower motoneurone

Question 8

Motorneurone disease

Answer 8

• Characterised by spontaneous genetically programmed death of motoneurones of the body
• Programmed cell death is known as apoptosis
• There are many variants to this disease
• One variant was renamed as Lou Gherig’s Disease after a famous baseball player succumbed to it
• Another variant is known as amyotrophic lateral sclerosis

Question 9

What are motorneurones susceptible to MD?

Answer 9

• Professor Stephen Hawking suffered from motoneurone disease
• His ability to communicate by coded eye movements is because some motoneurones of the body do not undergo apoptosis under MD
• These are as follows:
– Motoneurones supplying extraocular muscles
– Motoneurones supplying the anal sphincter
• Neural root values S2-S4

Question 10

Direct implication of The Ventral Horn of The Spinal Cord or Cranial Nerve Motor Nuclei

Answer 10

• Infection of the ventral horn of the grey matter of the cord would lead to death of motoneurones
– This is rare but possible
• Crushing injuries to the spinal cord that will include damage to the grey matter

Question 11

Implications of Disorders of The Motoneurone’s Axon & LMN Signs

Answer 11

Loss or damage to the motoneurone’s axon does not necessarily lead to death of the motoneurone
• However, it leads to the removal innervation to the muscle
• This is known as denervation

The closer the damage to the cell body, the more likely for neurone death

Question 12

Complete Transaction of Axons of A Motor Nerve

Answer 12

• Exemplified by
– crushing injuries to limbs
– Stabbing injuries
• Usually accidental
• There is often inevitable involvement of nerves supplying the affected part of the body
• This results in denervation of muscles involved

Question 13

Demyelination of Axons of Motoneurones Guillain-Barre Syndrome or Peripheral Neuropathies

Answer 13

• G-B Syndrome is acquired as a complication following a viral infection such as a common cold
• Diabetic neuropathy leads to demylination of axons of both sensory and motor axons
• Clinical signs reverse when the condition is cured or goes into remission
• Clinical motor signs can return with a relapse

Question 14

Implications of Disorders of The Neuromuscular Junction& LMN Signs

Answer 14

• The neuromuscular junction is a necessary element to the integrity of the motor unit
• There is a class of diseases that target the NMJ in its own right

Question 15

What is the NMJ?

Answer 15

• It is the tissue interface between the:
– Motoneurone 
– Muscle
• It is a synaptic interface
• It is a special synapse
• It is also called the end-plate

Question 16

NMJ 2.0

Answer 16

• The motoneurone
– Forms the pre-synaptic membrane
– Releases acetylcholine as the
neurotransmitter
• The muscle
– Forms the post-synaptic membrane
– Expresses nicotinic receptors that convert
chemical messages carried by Ach
• The synaptic cleft
– Expresses the enzyme acetylcholine esterase that destroys any Ach that is not bound to the post-synaptic membrane

Question 17

Botulinum toxin and NMJ

Answer 17

• Unskilled domestic preservation of cooked food can lead to inadvertent culturing of the botulinum toxin
• Badly preserved tinned foods are commonest cause of botulism
• Ingestion of food laced with the toxin would lead to paralysis of muscles of the body
• This is often an emergency as it can lead to death
• The botulinum toxin acts to deplete the presynatic terminal of the NMJ of its neurotransmitter
• Cosmetic use of “Botox” is based on understanding the actions of the botulinum toxin on the NMJ

Question 18

Nerve gases and NMJ

Answer 18

• Nerve agents are a class of phosphorus- containing organic chemicals (organophosphates)
• Organophosphates are used in formulation of common pesticides
• Widely used during the first and second world wars
• Still used as weapons of mass destruction
• Nerve Gases act by blocking acetylcholinesterase from breaking down Ach in the NMJ

Question 19

Myasthenia gravis and NMJ

Answer 19

• Myasthenia gravis is a rare genetic disease of the neuromuscular junction
• Nicotinic receptors of the post- synaptic membrane of the NMJ are selectively destroyed by the person’s immune system
• It affects any neuromuscular junction of the body in affected individuals
• It brings about flaccid weakness of the affected muscles
• It can cause respiratory failure if muscle of respiration are affected

Question 20

Disorders of skeletal muscles and LMN signs

Answer 20

• The skeletal muscle is part of the motor unit

* Any diseases or disorders of muscle would inevitably lead to lower motoneurone signs of myogenic origin

Question 21

Duchenne Muscular Dystrophy

Answer 21

• Genetic
• Progressive muscle weakness
• Defects in muscle proteins
• Death of muscle tissue
• Eventual death of the affected individual

Question 22

Lower motorneurone signs

Answer 22

• Lesions of lower motoneurones lead to characteristic impairments of movements
• There are a series of emergent clinical presenting signs known as lower motoneurone signs
• Lower motoneurone signs are very different from upper motoneurones signs
• We are expected to be able to spot lower motoneurone signs
• Eventually, we will be expected to explain the basis of lower motoneurone signs