Flashcards in Motor: NMJ Deck (33):
Describe activation of the NMJ
ACh is released when SNARE proteins interact with the cell membrane.
Ca2+ influx causes ACh release
ACh binds to receptors on motor end plate.
Na+ influx triggers AP in muscle fibre
What is a miniature end plate potential?
At rest, ACh is released and transported across the synaptic cleft constantly at a very low rate.
What are alpha motor neurons/anterior horn cells?
Lower motor neurons of the brainstem and spinal cord that directly innervates a muscle via its axon.
If they innervate the face, they reside in the cranial nerve nuclei of the brainstem.
If they innervate the extrafusal muscle fibres of the limbs, they reside in the spinal cord.
What are the neurones which innervate the intrafusal muscle fibres and what does this mean?
Gamma motor neurones
Intrafusal skeletal muscle fibres are specialised sensory organs (proprioception), detect changes in amount and rate of change in length of muscle.
What is an extrafusal muscle fibre?
Generate tension by contracting, allow for skeletal muscle movement.
What is the somatotropin organisation of a-motor neurones in the spinal cord?
Located in the ventral horn
Flexors: posterior of ventral horn
Extensors: anterior of ventral horn
Distal: lateral of ventral horn
Proximal: medial of ventral horn
How many motor neurones and muscle fibres do human have on average?
420, 000 motor neurones
250 million muscle fibres
Around each motor neuron supplies 600 muscle fibres
What is a large innervation ratio and give an example?
Many muscle fibres are innervated by one motor neurone e.g. quadriceps
What are the three different motor unit types and describe their appearance
Slow; low tension, fatigue resistant. Small diameter, small dendritic tree, tin axons
Fast, fatigue resistant (Type 2a); Fast twitch, moderate tension. Larger diameter cell bodies, axons and dendritic trees.
Fast, fatiguable (Type 2b); Fast twitch, high tension, high fatigue. Same appearance as Type 2a.
How is muscle force regulated by the brain?
Recruitment and rate coding (frequency of AP)
In what order are the muscle fibres recruited?
Smaller units are recruited first (slow twitch), as force increases, more units are recruited (Type 2a, 2b).
5th motor neurone is first to be recruited and de-recruited.
What is the relationship between rate coding and recruitment?
At a low frequency firing rate, slow units are constantly firing. As the firing rate increases, the force produced by the unit increases.
What is the start off firing rate?
What are neurotrophic factors?
Provided by lower motor neurones to the muscle fibres; they are growth factors which prevent neuronal death. Necessary to stop nerve cell atrophy even with intact blood supply.
What change in muscle fibre type would you expect to se after training?
Type 2b to 2a
What change in muscle fibre type would you expect to se after a spinal cord injury?
Type 1 to 2
Which muscle fibre type is ageing associated with?
Loss of type 2 fibres
What is the somatotopic organisation of the lateral corticospinal tract?
Sacral is most lateral
What is a reflex?
An involuntary coordinated patten of muscle contractions from peripheral stimuli to nerve centre and outward to an effector without reaching the level of consciousness
What is the importance of testing reflexes?
Ability to voluntarily contract a muscle shows nothing is wrong with the motor neurones.
Absence of reflex when a stimulus is given indicates a sensory loss
How can you infer whether there is a monosynaptic or disynaptic connection?
The excitatory post-synaptic potential will be twice as long if it is disynaptic (more than one synapse between the afferent and efferent). Usually is 0.7ms
What is occurring in a reflex?
There is an inhibitory signal to the antagonist and excitatory signal to the agonist.
What is the Hoffman reflex?
You stimulate the nerve at the back of the knee (stimulus is of same duration and amplitude);
M wave is the direct motor response and is seen first
H wave is the second response having passed through the sensory nerve.
What is the difference between the H and M waves?
The H wave can be stimulated at a lower stimulus intensity. It will decrease in size and can disappear because they activate neurones which travel in opposite direction and can cancel out if they meet.
The M wave will increase in size but may plateau as it will have activated all the motor neurons and end plates.
What is the flexion withdrawal/crossed extensor reflex?
A type of polysnaptic reflex; stimulus is noxious, contract all muscles of the same side e.g. ipsilateral leg flexion, contralateral leg extension (crossed extensor)
What is the stretch reflex?
Monosynaptic reflex; sensory receptor (muscle spindles) cause sensory neuron excitation, at the integrating centre it causes activation of the motor neuron, the muscle contracts and the inhibitory interneurones cause the antagonistic muscle to relax.
What is meant by the supra spinal control of reflexes?
There is some descending inhibition of reflexes from the higher centres of CNS.
Inhibitory control dominates in normal conditions.
Decerebration shows there is excitatory control from supra spinal areas; leads to rigidity, spasticity, hyperreflexia
What is the Jendrassik Manoeuvre?
It is whereby the patellar-tendon reflex will be 2-3 times greater if a patient clenches their teeth at the same time.
What is the gamma reflex loop?
Whereby the gamma motor neuron fires and contracts the intrafusal muscle fibre which sends sensory info and activates the alpha motor neurone. This contracts the muscle to return it to the unstretched position which maintains its sensitivity.
By what mechanisms do the higher centres influence reflexes?
Activate alpha motor neurones (increased sensitivity)
Activate inhibitory interneurones
Activate proriospinal neruones
Activate gamma motor neurones (increased firing to contract the muscle spindle)
Activate terminals of afferent fibres
Which pathways and parts of the higher centres are involved in reflexes?
Corticospinal tract (fine control of limbs)
Rubrospinal (automatic movements in response to posture/balance change)
What is hyperreflexia?
Loss of descending inhibition e.g. in a stroke
Babinski's sign (plantar extension)