Neuromuscular and spinal cord Flashcards
How can the membrane potential of the post synaptic neurone be affected?
It can be made less negative (closer to the threshold for firing)- an excitatory post synaptic potential (EPSP)
It can be made more negative (hyperpolarised)- inhibitory post-synaptic potential (IPSP)
What is a neuromuscular junction?
Specialised synapse between the motor neurone and the motor end plate on the muscle fibre cell membrane
What leads to the release of acetylcholine from the presynaptic cell?
Snare proteins interacting with the membrane and calcium influx
At rest in a muscle fibre, what is happening to the membrane potential?
There are small changes in membrane potential, these aren’t action potentials, they are miniature end plate potentials (mEPPs) it’s just the result of vesicles dumping their contents into the synaptic cleft
What are ventral horn cells/anterior horn cells/lower motor neurones more commonly known as?
Alpha motor neurones
What do alpha motor neurones innervate?
Extrafusal muscle fibres of the skeletal muscle
What are intrafusal muscle fibres?
Skeletal muscle fibres that serve as specialised sensory organs (proprioceptors) that detect the amount and rate of change in length of a muscle
What are extrafusal muscle fibres?
Standard skeletal muscle fibres that are innervated by alpha motor neurones and generate tension by contracting thereby allowing for skeletal muscle contraction
What are spindles?
Coiled, spring-like sensory receptors in the muscle that when stretched, feedback to the CNS and allows an excitatory reflex to be generated which is what you want when your patella tendon is hit by a joint hammer
What is a motor neurone pool?
Collection of lower motor neurones that innervate a single muscle
Where are alpha motor neurones found?
Anterior/ventral horn of grey matter
What do flexors and extensors allow?
Flexors allow you to curl up into a ball and extensors allow you to stand as tall as possible
What is the relation between alpha motor neurones and muscle fibres?
One alpha motor neurone can innervate several muscle fibres but one muscle fibre can only be innervated by one alpha motor neurone
When is there an exception to the relation between alpha motor neurones and muscle fibres?
Under pathological conditions, like when a nerve has been cut, the axon can sprout and begin to innervate muscle fibres that are already innervated by motor neurones
What is a motor unit?
A single motor neurone together with all the muscle fibres that it innervates. It is the smallest functional unit with which to make a force
What is the eyes muscle innervation ratio (number of muscle fibres innervated by a single motor neurone) like and why?
It is low as the eye has to be very finely controlled, if it was high, all of the muscles would contract simultaneous which isn’t what you want
What is the quadriceps muscle innervation ratio like and why?
It is high because you want all of them to contract simultaneously to generate power
What are the three types of motor unit?
Slow
Fast, non-fatiguable
Fast fatiguable
What are the characteristics of slow muscles?
They don’t produce much force but can work for a long time
What sort of muscles are mainly slow ones?
Postural
What are the two mechanisms by which the brain regulates the force that a single muscle can produce?
Recruitment- Recruiting more motor units (smaller/slower units are recruited first)
Rate coding- Changing the frequency with which you send action potentials down nerves
What are neurotrophic factors?
Factors that are produced within the nerve and are transported throughout the nerve to maintain the nerves integrity and function. They are a type of growth factor and they prevent death
What is the difference between CNS neurones and PNS neurones after damage
CNS don’t regenerate whereas PNS do- in the CNS there are millions of axons whereas PNS there are only few thousand so the consequences of rewiring incorrectly isn’t worth it
What happens if you switch a fast alpha motor neurone to a slow muscle and vice versa?
The slow muscle becomes fast and the fast muscle becomes slow so there is something else leading to this response
It is not easy to switch from one form of motor unit to another, but what type of switch is the most common?
Fast fatiguable to fast non-fatiguable with training
What happens to muscle fibres with ageing and what is it called?
Ageing is associated with loss of both types of muscle fibres but mainly type 2 (fast), this results in a large amount of type 1 in old people which is why aged muscle generally has slower contraction times- this is called sarcopenia
What is the corticospinal/pyramidal pathway mainly responsible for?
Voluntary movement
What is a reflex?
Automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (as a muscle or gland) without reaching the level of consciousness
How do reflexes differ from voluntary movements?
Reflexes can’t be stopped once they’ve been initiated
What are reflexes important for?
Maintaining body posture and preventing damage
When are reflex tests used?
If there is suspicion of damage to CNS or PNS
What do you need for a reflex?
An afferent signal, some kind of relay neurone and a motor neurone
How can reflex testing be used to determine whether there is sensory or motor nerve damage?
If you can voluntarily contract the muscle then there’s probably nothing wronf with motor. If you then hit the tendon and nothing happens, this shows a sensory loss
What do you measure when working out how many synapses there are?
You measure the volley (action potential) as a stimulus is set up in one of the two nerves
Give an example of the monosynaptic (stretch) reflex and describe what happens?
Striking the patellar tendon makes the quadriceps stretch. This sends an afferent signal that excites the efferents to the quadriceps and inhibits the efferents to the hamstrings
Why can’t you rely on knee jerk reflex alone with a tendon hammer?
Reflex depends very much on which part of tendon is hit and how hard
What did Hoffman come up to bypass problem with tendon hammer?
He came up with a way in which the stimulus can be identical everytime the reflex is tested in tems of duration and amplitude so change in reflex size isn’t due to input
What is the Hoffman reflex?
He reasoned that he could bypass physical stretch of muscle- if he delivered electrical stimulus to nerve carrying sensory and motor fibres then it would carry impulse along sensory fibre to spinal cord and via reflex arc back to muscle
What do you see when carrying out the Hoffman reflex at the back of the knee?
You will see two twitches:
M wave- Direct motor response- going from stimulated motor neurone directly to muscle
H wave- Short time later, another response in EMG and another twitch- caused by action potential in sensory neurone going back to spinal cord and exciting motor neurone
How are sensory nerves more amenable to electrical stimuli and what does this mean?
They are larger so you can get a response from sensory nerve (H wave) at lower intensity than M wave
What are flexor withdrawal polysynaptic reflexes?
Reflexes that go up and down spinal cord to innervate groups of muscle on same side
What are crossed extensor polysynaptic reflexes?
Reflexes that cross spinal cord to the other side such that other limbs do something to keep us upright
What is the Jendrassik manouvre?
If you’re testing the knee-jerk reflex on someone and ask them to clench their teeth, the reflex you get when you tap the patellar tendon will be 2 or 3 times greater
What is the Jendrassik manouvre an example of?
Descending control- very large inhibitory control over reflexes
What happens if you remove descending inhibitory control?
You will get very brisk reflexes and spasticity in muscles e.g. stroke arm spasticity
Give an example of upper motor neurone lesions?
Stroke
What do strokes lead to in terms of reflexes?
They lead to loss of descending inhibition of reflexes so you get hyper-reflexia
What is clonus?
Muscular spasm involving repeated, often rhythmic, contractions
What is hypo-reflexia commonly associated with?
Lower motor neurone lesions
