Neuromuscular Junction + Spinal Chord Flashcards
What is EPSP?
It is an exitatory post synaptic potential
–> potential that makes membrane less negative (lower threshold for fiering)
What is IPSP?
It is an inhibitory post synaptic potential
–> potential that makes membrane more negative (higher threshold for fiering)
How do you call process of graded effects that occur at a post-synaptical membrane that determine the response?
Summation
What is an alpha-motorneuron?
The lower motor neuron of the brainstem and spinal chord
What/where does an alpha-motor neuron innervate?
What is the consequence?
They innervate the extrafusal (outside a muscle fibre) skeletal muscle
–> cause muscle contraction
What is the motor neuron pool?
It contains all motor neurons that innervate one single, specific muslce
How are the alpha motor neuron pools organised in the ventral horn of a spinal chord?
Where are extensor and flexor motor neuron pools located in the ventral horn of the spinal chord?
Flexors are located more dorsally
Extensors are located more ventrally
What is a motor unit?
The unit of a single motor neuron and all muslce fibres it innervates
What is the smallest functional unit to produce force?
It is the motor unit
How many muscle fibres does an average motor neuron innervate in a motor unit?
About 600
Name the three types of muscle units
- S (Type I), slow
- FR (Type IIA), Fast, Fatigue resistant
- FF (Type IIB), Fast, Fatiguable
What are the propoerties of a Type I Motor unit?
It has slow conduction due to
- low diameter in cell body, axon
- small dendritic trees
–> slow twith, low tension but fatigue resistant
What are the properties of a Type IIA muscle unit?
FR –> Fast, Fatigue resistant
- larger cell body diameter, axonal diameter
- larger dendritic trees
–> Fast twitch, moderate tension, resistance to fatigue
What are the propoerties of a Type IIB motor unit?
FF –> Fast, fatigueable
- Larger diameter, larger dentritic trees
–> Fast twitch, high force, high fatigue
What are the two mechanisms by which force of muscle contraction it regulated?
- Recruitement
- targeted selection of motor neurons needed
- Rate coding
- the rate of AP fiering –> the higher the rate, the higher the force
Explain the concept of muscle unit recruitment
Done via the “size principle”
- smaller units are requruited first (normalls T1 units)
- As more force is required, more units are recruited
–> Allows fine tuning for low-force movements (e.g. writing)
Explain the concept of Rate coding in muscle force regulation
As the firing rate increases, the force produced by the unit increases
–> higher fiering rate= higher force
Which changes do muslce fibres undergo during training?
There is a shift from TIIB to TIIA –> Fatiguable to Fatigue resistant
What are the associated changes in muscle fibres with ageing?
Ageing is associated with both: loss of TI+II but more commonly TII
–> leading to slower muscle contraciton
The corticospinal tracts in the spinal chord controll voluntary movement. What is collective name of tractrs that modulate/controll involuntary movements ?
They are collectively known as extrapyramidal tracts e.g.
- Rubrospinal tracts
- Vestibulospinal tract etc. for balance reflex, maintaining posture etc.
What is the function of the extrapyramidal tracts?
Modulate involuntary movement via:
Automatic movements in responst to posture, balance changes, painful stimmuli etc.
What is a reflex?
An automatic response to an external stimulus without involvement of conciousness (no brain involved)
An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.
What are the componenets of a reflex?
What is a monosynaptic reflex?
A simple relex response with a single synapse between afferent and efferent pathways
–> Simple stretch responses: e.g. knee jerk etc.
What is a polysynaptic reflex?
A reflex that involved two or more synapses in the spinal chord, involves more muscles at different levels
–> Complex reflexes e.g. withdraw from sharp objects–> also involves balance, weight change onto other leg to maintain balance etc.
What is supraspinal control of reflexes?
Reflexes can be controlled by higher centres of the CNS
- Normally inhibitory Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex
Through which ways can supraspinal reflex control be achieved?
Higher centres influence reflexes by:
- Activating alpha motor neurons
- Activating inhibitory interneurons
- Activating propriospinal neurons
- Activating gamma motor neurons
- Activating terminals of afferent fibres
What is intrafusal innervation of muscle fibres?
Innervation within the muslce cells consisting of
- Gamma motor neurons
- sensory axons
What are the characteristics and function of gamma motor neurons?
They innervate the muscle intrafusally
–> respond intracellularly to muscle movement
Name examples of centres that are involved in higher controll of reflexes
- Cortex – corticospinal (fine control of limb movements, body adjustments)
- Red nucleus – rubrospinal (automatic movements of arm in response to posture/balance changes)
- Vestibular nuclei – vestibulospinal (altering posture to maintain balance)
- Tectum – tectospinal (head movements in response to visual information).
What is the difference between an intrafusal and extrafusal muscle fibre?
Intrafusal = skeletal muscle fibres that serve as sensory organs/proprioceptors which detect the amount and rate of change of the length of a muscle
Extrafusal = standard skeletal muscle fibres which are innervated by alpha motor neurones
What are signs and symptoms for a upper motor neuron lesion?
- HYPERREFLEXIA
- CLONUS
- BABINSKI’S SIGN
Explain the structure of a NMJ
It is just a specialised Synapse
- AP arrives
- Ca2+ influx
- Vesicle exocitosis
- Release of Ach
- Binds to recetors on Motor end plate
- Cause Ion channel opening an Ca2+ influx
- AP in mucle fibres
What are neurotrophic factors?
Are a type of growth factor
- Prevent neuronal death
- Promote growth of neurons after injury
What are the effects of neurotrophic factors on a motorunit?
They affect the function of the muscle fibres (by changing the innerveating nerve)
What are pyramidal tracts?
Connection of the motor cortex with the periphery (spinal chord) –>
- corticospinal tracts (lateral and anterior)
- Fine, precise movements
What are the extra-pyramidal tracts?
Connecting different parts of the CNS (includieg Cortex but also Cerebellum etc.) to periphery
- Mainly involved in gross, movement of proyimal muscles
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What is the exact order in which different motor fibres are recruited?
Slow, fast fatigue resistant, fast fatigable
