Neuro 7: Neuromuscular + spinal cord Flashcards
What are synapses ?
- allows contact from:
- -> neurons to muscle
- -> neurone to neurone
how can the memb potential of post synaptic neurone can be altered in 2 direction by inputs
a) can be made less -ve
- -> i.e brought closer to threshold for firing
- -> excitatory post synaptic potential (EPSP)
OR
b) can be made more negative
- -> i.e brought further away from threshold
- -> inhibitory post synaptic potentials (IPSP)
What is a neuromuscular junction.
- NMJ = specialized synapse btw motor neurone + motor end plate on muscle fibre cell memb
How is NMJ activated?
- AP arrives at NMJ
- Ca2+ influx causes Ach release
- Ach binds to receptors on motor end plate
- ion channel opens
- Na+ influx causes AP in muscle fibre
What does the degree of summation determine?
- determines how readily a neuron can reach threshold to produce an AP
What are Miniature End plate potentials?
- when at rest
individual vesicles release Ach at a low rate –> causing mEPP
What is an alpha motor neuron?
alpha motor neuron = lower motor neurons of the brainstem + spinal cord
–> they innervate the extrafusal muscle fibres of the skeletal muscles
–> activation causes muscle contraction
What is meant by intrafusal?
intrafusal : skeletal muscle fibres that acts as sensory organs that detects amount + rate of change in length of a muscle
- houses the sensory organs
What is meant by extrafusal ?
extrafusal: standard skeletal muscle fibre that are innervated by alpha motor neurones
- and they generate tension by contracting
- -> allowing skeletal muscle movement
- contains sensory organs = for reflex responses
What is a motor neurone pool?
motor neurone pool= collection of lower motor neurones that innervate a single muscle
describe the arrangement of alpha motor neurones.
- they are found in the anterior/ ventral horn of grey matter
- there are flexors + extensors
What is the difference between flexors and extensors
flexors = flexes the muscles –> allows you to curl up into a ball
extensors = allows you to be as tall + long as possible.
Define a motor unit:
motor unit - a single motor neurone together with all the muscle fibres that it innervates.
–> it is the smallest functional unit with which to produce force
note: stimulation of 1 motor unit –> causes contraction of all the muscle fibres in that unit.
-
What are the 3 types of motor unit?
- slow (S, type I)
- Fast, fatigue resistant (FR, Type 2a)
- Fast, fatiguable (FF, type 2b)
postural muscles = mainly _____ muscles
postural muscles = mainly slow muscles
Distinguish between the 3 different types of motor unit
SLOW, TYPE 1
- cell bodies = smallest diameter
- has small dendritic trees
- thinnest axon
- slowest conduction velocity
FR, TYPE 2A
- cell bodies = larger diameter
- has larger dendritic trees
- has thicker axons
- has faster conduction velocity
FF, TYPE 2B
- cell bodies = larger diameter
- has larger dendritic trees
- has thicker axons
- has faster conduction velocity
the 3 diff motor unit types = classified by:
the 3 diff motor unit types = classified by:
- amount of tension generated
- speed of contraction
- fatiguability of the motor unit
What are the 2 mechanisms by which the brain regulates the force a single muscle can produce?
- recruitment
2. rate coding
fatigue resistant muscles produce more/less force than slow fibres
and force = produced more quickly/slowly
fatigue resistant muscles produce MORE force than slow fibres + force = produced more QUICKLY
explain the difference between recruitment and rate coding
RECRUITMENT
= recruiting more motor units
- as muscles increase in force, more units are recruited
- smaller units are recruited first
RATE CODING
= changing the freq at which you send AP down the nerves
-
at ____ levels of force, the bottom motor unit = firing all the time
at LOW levels of force, the bottom motor unit = firing all the time
what is the order of recruitment?
slow –> fast fatigue resistant –> fast fatiguable
what are neurotrophic factors?
- neurotrophic factors = host of factors/ type of growth factor produced within the nerve
- -> transported through nerve to maintain nerve integrity + function
What do neurotrophic factors do?
- prevent neuronal death
- promote growth of neurons after injury
note: motor unit and characteristics are dependent on the nerve which innervates them –> if you cut a e.g slow nerve and switch it onto a different muscle type –> the muscle takes on the characteristics of the nerve and the muscle itself also becomes slow
-
most commonly type ___ may change to ____
following training
most commonly type 2B may change to 2A
following training
When might Type 1 change to change 2
- when there is severe deconditioning / spinal cord injury
-
Aging = associated with loss of what type of muscle fibre?
-loss of both type 1 + 2
but has preferential loss of type 2 fibres
–> so when you’re old there is a larger proportion of type 1 fibres (which are slower obv)
Why do aged muscle generally have slower contraction times?
- when you age –> there is preferential loss of type 2 fibres
- so there is a large proportion of type 1 fibre
–> so has slower contraction times
What is sarcopenia?
- loss of muscle
what does pyramidal tract largely control?
voluntary movements
define reflex
reflex = an automatic/ often inborn response to a stimulus
- that involves a nerve impulse passing inward from a receptor to a nerve centre
- -> then outward to an effector
- without reaching level of consciousness
the magnitude + timing of a coordinated muscle contraction/relaxation = determined by what 2 factors?
- intensity of the stimulus
2 onset of the stimulus
Why are reflexes important??
- important in maintaining upright posture
- and to reduce damage to parts of the body
what 3 things do you need for a reflex?
- an afferent signal
- some kind of relay neurone
- a motor neurone
describe how a reflex test is conducted
- muscle = stretched
- -> and amount of force the muscle produces in reflex action is recorded
What does a reflex test tell us?
- reflex testing can help determine whether there has been a sensory or a motor loss
- if you can voluntarily contract muscle –> motor neurone = normal
- if hit the tendon + nothing happens –> sensory loss
describe the difference between monosynaptic and disynaptic reflex
(MONOSYNAPTIC) - excitatory
stimulate extensor afferent –> you get response in extensor efferent –> causing contraction
(DISYNAPTIC) - inhibitory
if you excite flexor afferent nerve - there is an interneurone which inhibits the excitation of the extensor efferent
note:
time difference to generate an AP –> reflects difference in no. of synapses between monosynaptic(usually only 1) and disynaptic (usually more than 1)
How would a monosynaptic (stretch) reflex work
- when you strike the patellar tendon
- -> it makes the quadriceps stretch
- which sends an afferent signal
- this excites the efferents that goes to the quadriceps
- and inhibits the efferent to the hamstrings.
What is a key feature of the hoffman reflex?
- stimulus = identical each time a reflex is tested
(same duration + amplitude) - so you know any change in reflex size = not due to the input
what does the hoffman reflex test?
- it tests the integrity of the reflex pathways .
Describe what would happen if you stimulate the nerve at the back of the knee:
- you will observe 2 twitches
1. M wave: - direct motor response
- goes from motor neurone to muscle –> causing contraction
- H wave:
- caused by AP in sensory neurone
- which goes back to the spinal cord + excites the motor neurone
Why are sensory nerves more amendable to electrical stimuli?
- because they are larger
- so you can get a response from sensory nerve (H wave) at a lower stimulus intensity than the M wave
reflex responses
comprises of flexion withdrawal and crossed extensor.
explain how these work in response to a painful stimuli
e.g you step on pin multiple muscles on one limb contracts
Polysynaptic reflexes
- flexion withdrawal –> lifting leg up + make other leg straight
- crossed extensor –> allows you to stand in upright position even with one leg
What is the Jendrassik maneuver?
when testing the knee jerk, reflex on someone –> and you ask them to clench your teeth,
reflex will be 2-3x greater.
note: there is control from the brain
- pull hind leg of cat cat was decerebrated - pull hind leg of cat again - bigger response in the leg - because you take away the inhibitory control (that is usually dominant at rest)
CONCEPT IS SIMILAR FOR PATIENT WITH HYPERREFLEXIA
-
In supra spinal control of reflexes, what are the 5 ways in which higher centers influence reflexes?
- activating alpha motor neurons
- activating inhibitory interneurons
- activating propriospinal neurons
- activating gamma motor neurons
- activating terminal of afferent fibres
What is a gamma reflex loop?
- -> spindle of muscle is shortened
- -> to maintain its sensitivity
What is hyper reflexia?
- loss of descending inhibition
- due to e.g a stroke
What is a clonus?
- muscular spasm involving repeated / rhythmic contractions
- has hypertone in muscles
What is the babinski’s sign?
- if you stroke bottom of foot
- -> you will see plantar extension
- -> so toes fan out
What is hypo reflexia?
- below normal / absent reflexes
–> usually associated with lower motor neurone lesions –> because signal don’t go out / come out of spinal cords
what is the order of recruitment?
fast fatiguable –> fast fatigue resistant –> slow
whats does the vestibulospinal tract do?
- it regulates posture to maintain balance and facilitates mainly alpha motor neurones of the postural, anti-gravity (extensor) muscles
When you tap the patellar ligament - Why is there a difference in the time taken for these signals to reach the relevant muscles? (quadricep + hamstring)
- Signal to quadriceps = monosynaptic
- Signal to hamstring = disynaptic (polysnaptic)
- So signal to quadriceps = slightly faster
