Disorders of the motor unit

Question 1

The motor unit

Answer 1

The smallest functional unit of the motor system, comprised of an a-motoneurone and all the extrafusal muscle fibres it supplies. One motoneurone may supply between 10 (extra-ocular muscles) and 1000 (quadriceps) muscle fibres.

Question 2

a-motoneurone

Answer 2

A large, multipolar, efferent neurone of the SNS, supplying extrafusal skeletal muscle fibres. Also known as a lower motoneurone.

Question 3

Skeletal muscle

Answer 3

The effector organ of the SNS, responsible for voluntary movement.

Question 4

Neuromuscular junction

Answer 4

The site of communication between a neurone and a muscle fibre.

Question 5

Innervation

Answer 5

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

Question 6

Denervation

Answer 6

The deprivation or loss of nerve supply to a muscle.

Question 7

Re-innervation

Answer 7

Regrowth of a damaged nerve to re-supply the muscle, which is not always successful as a different target is often re-innervated.

Question 8

Polio myelitis

Answer 8

A communicable infection caused by the polio virus, that targets the cell body (soma) of the LMN. Death of the neurones of the ventral horn leads to denervation and paralysis of the muscles they supply. Immunisation programmes have been effective at preventing polio.

Question 9

Motoneurone disease

Answer 9

A disease causing the spontaneous, genetically programmed cell death (apoptosis) of motoneurones. Some variants target both upper and lower motoneurones, while others target LMNs only. Some motoneurones do not undergo apoptosis, namely those supplying the extraocular muscles and anal sphincter.

Question 10

Damage to the soma

Answer 10

Damage to the cell bodies contained within the ventral horn (grey matter) of the spinal cord caused by infection/trauma would lead to motoneurone death.

Question 11

Damage to the axon

Answer 11

Loss or damage to the axon leads to denervation, but not necessarily motoneurone death. The more distal the damage from the soma, the lower the risk of cell death and the more likely the chance of renervation.

Question 12

Disorders of the axon

Answer 12

Causes of axon damage are usually accidental (ie. crushing/stabbing injuries). Disorders may result from demyelination of axons, disrupting normal function (ie. Guillan-Barre syndrome, peripheral neuropathies).

Question 13

Botulinum toxin

Answer 13

Prevents the release of the neurotransmitter acetylcholine from the presynaptic terminal at the NMJ. It is found in badly preserved foods, where ingestion could lead to paralysis and death, but is also used for cosmetic and therapeutic purposes.

Question 14

Nerve gases

Answer 14

A class of phosphorus-containing organic compounds (organophosphates), which act by blocking acetylcholinesterase from breaking down ACh in the NMJ. Used in common pesticides and weapons of mass destruction.

Question 15

Myasthenia gravis

Answer 15

A rare genetic disease of the NMJ, in which nicotinic receptors of the post-synaptic membrane are selectively destroyed by an autoimmune response. It can target any NMJ, bringing about flaccid weakness of affected muscles. Symptoms include eyelid drooping (ptosis) and respiratory failure.

Question 16

Duchenne muscular dystrophy

Answer 16

A genetic defect in muscle proteins, leading to progressive muscle weakness, death of muscle tissue and eventual death of the individual.