Week 4 Flashcards
(47 cards)
What does a motor unit comprise of
A a-motoneuron
All extrafusal skeletal muscle fibres it supplies
What is the a-motoneurone
A specific subset of neurones of the nervous system through which all commands from the brain are relayed to skeletal muscles
What is a neuromuscular junction
The site of communication between the motoneurone and muscle
Theres 2 variants to the classification of motoneurone
Upper and lower motoneurones
Arranged strictly in series, damage to either of these leads to motoneurone signs
Where are upper motoneurones found
Reside in the brain in the cerebral cortex and travel down to the brain stem or spinal cord
Where are lower motoneurons’ cell bodies found
In the ventral horn of spinal cord grey matter or cranial nerve motor nuclei in the brain stem
What is meant by innervation
The term used to describe the normal state of nerve supply to a muscle of other target
Act of stimulating
Denervation
Depriving muscle of its nerve supply
Re-innervation
Re-growth of the nerve to re-supply the muscle
Re-innervation of the original effector organ is not always successful, most nerves reinnervate an effector organ that is different from its original target
This leads to emergence of all sorts of unexpected results
What leads to death of motoneurones
Loss or damage to the motoneurones cell body
Theres a class of diseases that target cell bodies of motoneurones
This would result in the denervation of muscles
What is poliomyelitis
Communicable infection that targets cell bodies of lower motoneurones of body
Also known as infantile paralysis
Caused by infection by polio virus
Leads to toxic infection of cell bodies of neurones of ventral horn
Any motoneurones of spinal cord are susceptible to virus
What does death of motoneurones lead to
Denervation hence paralysis of muscles they supply
Toxic infection of cell bodies of neurones of ventral horn can lead to death of dorsal horn too
What is motoneuron disease
Rare condition affecting brain and nerves
2 variants
Disease simultaneously kills both lower and upper motoneurones. Known as progressive supranuclear palsy
The disease targets only lower motoneurone
What is programmed cell death known as
Apoptosis
Which motoneurones dont undergo apoptosis under MD
Motoneurones supplying extraocular muscles
Motoneurones supplying anal sphincter
Implications of disorders of motoneurones axon and LMN signs
Loss or damage to motoneurones axon doesn’t necessarily lead to death of motoneurone
However it leads to the removal innervation to muscle- known as denervation
Complete transaction of axons of motor nerve
Exemplified by crushing injuries to limbs, stabbing injuries
Usually accidental
Often inevitable involvement of nerves supplying affected part of body
Results in denervation of muscles involved
What is Guillain-Barré syndrome
G-B syndrome is acquired as a complication following a viral infection such as common cold
Rare and serious condition mainly affects feet, hands, limbs causing numbness, weakness and pain
Demyelination of axons of motoneurones
Diabetic neuropathy leads to demyelination of axons of both sensory and motor axons
Clinical signs reverse when condition is cured or goes into remission
Clinical motor signs can return with relapse
Vision loss, muscle weakness, muscle stiffness and spasms, loss of coordination, changes in bladder and bowel function, walking problems
What is peripheral neuropathy
Type of nerve damage that can cause pain, weakness, numbness
Nerves located outside Brian and spinal cord
Implications of disorders of neuromuscular junction and LMN signs
Neuromuscular junction is necessary element to integrity of motor unit
Theres a class of diseases that target NMJ in its own right
LMN damage include weakness, muscle atrophy (wasting), and fasciculations (muscle twitching), decreased reflexes
What is the NMJ
It’s the tissue interface between motoneurone and muscle
It’s synaptic interface
Special synapse
Also called end plate
How does NMJ work
Motoneuron forms presynaptic membrane, releases acetylcholine as neurotransmitter
The muscle forms post synaptic membrane, expresses Nicotinic receptors that convert chemical messages carried by Ach
The synaptic cleft expresses enzyme acetylcholine esterase that destroys any Ach that isn’t bound to postsynaptic membrane
What does botulinum toxin do
Acts to deplete the presynaptic terminal of NMJ of its neurotransmitter
Effect of botulinum toxin is dry mouth and difficulty swallowing
