Disorders of the Motor Unit Flashcards
what does a motor unit comprise of?
The α- Motoneurone- A specific subset of neurones of the NS through which all commands from the brain are relayed to skeletal muscles
Skeletal Muscle- The only effector organ through which the brain expresses itself to the outside world
The Neuromuscular Junction- The site of communication between the motoneurone and muscle
what are the 2 classifications of motor neurones?
Upper Motoneurones
Lower Motoneurones
These are arranged strictly in-series
what happens when either of the 2 classifications of motor neurones are damaged?
Damage to any aspect of the Lower Motoneurone will lead to:
– “Lower Motoneurone Signs”
Damage to any aspect of the Upper Motoneurone will lead to:
– “Upper Motoneurone Signs”
define innervation
what does the alpha motor neurone innervate?
nerve supply to a muscle or other target.
alpha-motoneurone (lower motor neurones) innervate skeletal muscle
what are motor neurone diseases?
a class of disease that target cell bodies of motoneurons
Loss or damage of the motor neuron cell body will lead to death of the neuron
so muscle is left without innervation
it becomes denervated => neural dysfunction
what is denervation of muscle and what does it lead to?
Denervation- loss of nerve supply
denervation of muscle can result in the death of the muscle as the neurone provides the muscle with trophic factors which keeps it alive.
what is re-innervation?
When a nerve dies there may be a regrowth of that nerve (or a new nerve) to re-supply the muscle
is re-innervation always successful? why?
no
most nerves often lose their way and re-innervate an effector organ that is DIFFERENT from its original target.
This leads to emergence of lots of unexpected results – e.g. crocodile tears syndrome (wrong reestablishment of innervation with tear ducts leads to crying when eating)
what does death or dysufunction of a muscle give rise to?
how is this problematic when being identified?
Death or dysfunction of the muscles gives rise to neurological signs similar to lower motor neurone signs.
Lower motor neurone disorders will present different symptoms to upper motor neurone disorders.
This is problematic to trainees because there are 2 sources of dysfunction that are possible when a set of LMN disorder symptoms are present.
what is polio myelitis?
Also known as infantile paralysis
It is a communicable infection caused when an individual is infected with the polio virus
Leeds to toxic infection of cell bodies of the lower motor neurones which are located in the the ventral horn.
Any motor neurones of the spinal cord are susceptible to this virus
what does toxic infection lead to in polio myelitis?
This usually causes the death of the cell bodies.
Because they are post mitotic they can not be replaced so very serious.
Death of motor neurones leads to denervation hence paralysis of muscles they supply
in the 1990s, what was used to treat polio myelitis?
Until early 1990 Iron lungs were used to keep people suffering from polio alive as they lost control of their respiratory muscles.
what is given to prevent polio myelitis?
Immunisation programmes are effective at preventing this disease. Boosters given every 10 years. However still very apparent in developing world
compare the two images
Healthy (left) vs damaged (right) (atrophy of ventral horn – bottom left) spinal cord
how common is polio myelitis now?
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)