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)
what is a motor neurone disease characterised by?
Characterised by apoptosis (genetically programmed death) of motor neurones of the body
what are the 3 variants of a motor neurone disease?
- The disease simultaneously kills both upper motoneurone and lower motoneurons.
- The disease targets only the lower motor neurone.
- The disease only targets upper motor neurones
give examples of diseases that affect the upper motor neurone ONLY
Progressive supranuclear palsy
Lou Gherig’s disease (also known as ALS)
ALS - amyotrophic (without muscle support) lateral sclerosis
what is another cause of MND?
damage to axon – more forgiving than damage to cell bodies bc axons can regrow
HOWEVER– only possible if damage is distal – if it is closer to cell body damage to axon can result in death of whole motor neurone leading to denervation of muscle.
are all motor neurones susceptible to MND?
No, some motor neurones of the body do not undergo apoptosis under motor neurone disease
what motor neurones are not susceptible to MND?
These are as follow:
Motoneurone that supply extraocular muscles (this is why Stephen Hawkings could communicate with his eyes)
Motor neurones supplying the anal sphincter – neural root value s2-s4
explain the case of Stephen Hawking as an example of motor neurones not being susceptible to MND
Professor Stephen Hawking suffered from motoneurone disease
it affected the motorneurone that supplied the extraocular muscles
His ability to communicate by coded eye movements is because some motoneurones of the body do not undergo apoptosis under MD
how does trauma to the NS tissue cause motor neurone death?
Infection of the ventral horn of the grey matter of the cord would lead to death of motoneurones (rare)
Crushing injuries to the spinal cord damage the grey matter
how can complete transection (tear) of axons of a motor nerve be caused by?
crushing injuries to limbs
stabbing injuries – severe axons
usually accidental
If the axon that is severed supplies a muscle it will result in denervation of muscle.
what disorders lead to demyelination of the motor & sensory neurones?
Guillain-Barre Syndrome
Peripheral Neuropathies
what is Guillain-Barre syndrome and what does it lead to?
is an acquired complication of a viral infection e.g. common cold
leads to demyelination of axons on both motor and sensory neurones
what does diabetic neuropathy lead to?
leads to demyelination of axons on both motor and sensory neurones.
what happens when curing disorders that lead to demyelination?
Clinical signs reverse when the condition is cured or goes into remission
but can return in a relapse.
where is the NMJ in the motor unit?
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
what are the classes of diseases that target the NMJ?
Botulinum Toxin
Nerve gases
Myasthenia Gravis
what can lead to botulinum toxin?
Unskilled preservation of cooked food can lead to accidental 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
how does the botulinum toxin damage the NMJ?
The botulinum toxin inhibits exocytosis of ACh by the presynaptic neurone at NMJ.
It can also bind to NACh receptor and remains bound so receptors become unavailable.
Cosmetic use of “Botox” – paralysis of facial muscles to prevent wrinkles on skin.
what are nerve gases?
Widely used during the first and second world wars – nerve agents e.g. chlorine gas
Still used as weapons of mass destruction
Nerve agents are a class of phosphorus-containing organic chemicals (organophosphates)
Organophosphates are used in formulation of common pesticides – they interrupt the NMJ in insects.
how do nerve gases damage the NMJ?
It acts by blocking acetylcholinesterase from breaking down Ach in the our NMJ => muscle weakness
Also affects autonomic aspects of the nervous system
This can encourage glands to secret a lot of a hormone than usual.
what is Myasthenia gravis?
Myasthenia gravis is a rare genetic disease of the neuromuscular junction
It affects any neuromuscular junction of the body in affected individuals
how does Myasthenia graves damage the NMJ?
Nicotinic receptors of the post-synaptic membrane of the NMJ are selectively destroyed by the person’s immune system
It brings about flaccid weakness of the affected muscles – e.g. weakening of eyelid
It can cause respiratory failure if muscles of respiration are affected
what does disorder of the skeletal muscle lead to?
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
what is an example of a disorder that affects the skeletal muscle?
Duchenne Muscular Dystrophy
Genetic- causes defects in muscle proteins
Results in progressive muscle weakness and eventual death of muscle tissue and then eventual death of the affected individual
what are lower motor neurone signs?
Lesions of lower motoneurones (damage to cell body, axon, muscles or NMJ) lead to characteristic impairments of movements – known as LMN signs
Lower motoneurone signs are very different from upper motoneurone signs
