Neuromuscular and Spinal Cord Flashcards
What is the difference between EPSP (excitatory post-synaptic potential) and IPSP (inhibitory post-synaptic potential) in terms of membrane potential?
EPSP – makes the membrane potential less negative (bringing it closerto the threshold potential)
IPSP – makes the membrane potential more negative (hyperpolarisation)
You get graded effects – whether the neurone fires or not is dependent on the summation of inputs
Which proteins are involved in the release of acetylcholine at synapses?
SNARE proteins
What trigger acetylcholine release?
Calcium influx
If you record the post-synaptic membrane potential at any one time, you will see some small changes in membrane potential. What are these caused by?
Miniature end plate potentials
They are caused by the constant dumping of acetylcholine into the synapse
What is the difference between intrafusal and extrafusal muscle fibres?
Intrafusal – these are skeletal muscle fibres that serve as sensory organs (proprioceptors- muscle spindles) that detect the amount and rate of change of length of a muscle
Extrafusal – standard skeletal muscle fibres that are innervated by alpha motor neurones and generate tension by contracting, thereby allowing for skeletal muscle movement
What are alpha motor neurones? State some other names given to alpha motor neurones.
Anterior horn cells, ventral horn cells or lower motor neurones
They are motor neurones that innervate the extrafusal fibres of skeletal muscle
What is the name given to the sensory receptors in muscle that feedback to the CNS and allow an excitatory reflex to be generated?
Spindles (stretch receptors)
What is a motor neurone pool?
Collection of lower motor neurones that innervate a single muscle
Describe the arrangement of alpha motor neurones within the ventral horn.
Dorsal – flexors
Ventral – extensors
Medial – proximal
Lateral - distal
What is an important rule to remember regarding the connections between alpha motor neurones and muscle fibres?
One motor neurone can innervate several muscle fibres
But every muscle fibre can only be innervated by one motor neurone
Under what conditions can this rule be broken?
Under pathological conditions (e.g. severed nerve), the axonal regeneration can result in the innervation of muscle fibres that are already innervated
Define motor unit.
A single motor neurone together with all the muscle fibres that it innervates - it is the smallest functional unit that can generate force.
Describe and explain the difference in innervation ratio across different muscles in the body using examples.
Innervation ratio= how many muscle fibres a single neuron innervate in a motor unit.
Muscles that require very fine control (e.g. extrinsic eye muscles) havea low innervation ratio (few fibres innervated by a single neurone)
Muscle that are required to generate a lot of power have a high innervation ratio because when the motor unit fires, it will cause the contraction of a large mass of muscle fibres thus generating power (e.g. quadriceps)
What are the 3 types of motor unit?
Slow (Type 1)
Fast fatigue-resistant (Type 2A)
Fast fatiguable (Type 2B)
slow muscle fibres contract slowly with little force but can stay contracted for long time
Fast fibres are more easily fatigued (cannot contract for as long) and contract more powerfully
Note these are all subtypes skeletal muscle motor units
Describe the structural and functional differences of the neurons innervating the slow and fast twitch muscle fibres.
- cell body size
- size of dendritic trees
- axon thickness
- conduction velocity
Slow motor units (type 1) are innervated by neurons that are: Smallest diameter cell bodies Small dendritic trees Thinnest axons Slowest conduction velocity Fast motor units (types 2a and 2b) neurons that are: Larger diameter cell bodies Large dendritic trees Thicker axons Faster conduction velocity
What are 2 methods by which the brain regulates the force that a single muscle can produce?
Recruitment – recruiting more motor units for the muscle contraction
Rate Coding – increasing the frequency of action potentials travelling down the nerves to the muscle fibres
What principle governs recruitment?
Size principle
Smaller units are recruited first, which are generally slow fibres
Describe the order of recruitment of motor units with increasing force generation.
Slow –> Fast Fatigue-Resistant (2a)–> Fast Fatiguable (2b)
Slow muscle fibres tend to be in smaller motor units. Fast muscle fibres tend to be in larger motor units
What are neurotrophic factors?
Factors produced within the nerves and are transported throughout the nerve to maintain the nerves integrity and function.
They are a type of growth factor that prevents neuronal death and promotes the growth of neurones after injury.
What happens to a slow fibre when a fast nerve is transplanted onto it and what does this show?
It becomes faster
This shows that the function of the muscle fibre is INFLUENCED by the type of nerve that innervates it.
The action potentials can’t be the only thing being delivered to the muscles by the nerves, NEUROTROPHIC factors of the nerves may influence the muscle characteristics
How easy is it to switch from one motor unit type to another?
Type 2B to Type 2A can happen with training
There is usually no way of changing from type 2 to type 1 or vice versa except in the case of severe deconditioning e.g. zero gravity or spinal injury. Without gravity, theres no weight and hence astronauts dont have to use muscles to do anything. This may cause deconditioning/muscle wasting
How does muscle composition change with ageing?
Ageing is associated with a loss of type 1 and type 2 fibres with preferential loss of type 2 fibres
This means that a large proportion of muscle fibres in aged muscle are type 1
This loss of muscle is called sarcopenia
What tract is responsible for voluntary movements?
Pyramidal/Corticospinal tract
What is the role of extrapyramidal tracts?
It is responsible for automatic movements in response to stimuli (these are movements that your body makes without you being aware of it) and also in regulating pyramidal tracts
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 outwards to an effector (a muscle or gland) without reaching the level of consciousness
What are the components of a reflex arc?
Afferent signal
Relay neurone
Motor neurone
What are the two signals that are generated when the patellarligament is tapped?
There is an excitatory signal going to the quadriceps
There is also an inhibitory signal going to the hamstrings (antagonist)
Why is there a difference in the time taken for these signals to reach the relevant muscles?
The signal going to the quadriceps only has one synapse (monosynaptic reflex) whereas the signal to the hamstrings goes via an inhibitory interneurone so there are two synapses.
This means that the signal to the quadriceps arrives slightly faster than the signal to the hamstrings.
NB he said the monosynaptic reflex strictly only involves the quadricep contraction, the hamstring inhibition is an additional thing that is not part of the reflex hence you call the reflex monosynaptic
How did Hoffman plan on standardising the reflex test?
He considered bypassing the stretch that is caused by the tendon hammer on the patellar ligament and directly stimulating the nerve, which has sensory and motor fibres.
This would mean that the stimulus is identical every time (in duration and amplitude) and the magnitude of the reflex elicited will not be due to variations in input (how hard you tap the patellar tendon/where you tap it)
What are the two twitches that are seen when you stimulate the nerve behind the knee?
M wave – twitch due to the direct conduction of the impulse down the motor neurone to the muscle fibre
H wave – due to the action potential passing down the sensory neurone back to the spinal cord and then coming out via a motor neurone to stimulate the muscle
Why do sensory nerves show a response at lower stimulus intensity than motor nerves?
They are more susceptible to electrical stimuli because they’re larger
Example of a polysynaptic reflexes?
Flexion withdrawal/Crossed extensor reflex
Describe the supraspinal control of reflexes.
There is a large descending control over reflexes that only becomes noticeable when these descending controls are removed.
E.g they may exert inhibitory action on reflexes. If u remove the cerebral cortex and then stimulate the reflex, the reflex would be much greater
What is the Jendrassik manoeuvre?
Tap someone’s patellar tendon with a tendon hammer whilst they are clenching their teeth. The response elicited is 2-3 times greater.
If you decerebrate an animal (but keep them alive) and test their reflexes, what would you expect to observe?
Hyperreflexia
Increased muscle tone
this is due to loss of descending inhibitory signals
What is the gamma reflex loop?
It shortens the spindles in muscle to maintain its sensitivity
There is also facilitation from higher centres, which increase the sensitivity of the motor neurone to afferent input
What signs are seen with upper motor neurone lesions?
Hyperreflexia (loss of descending inhibitory control)
Clonus - involuntary rhythmic muscular contractions (loss of descending inhibitory control)
Babinski’s Sign
Hyporeflexia is often associated with upper or lower motor neuron lesions
lower motor neuron lesions (because neurons involved in the reflex itself are mostly lower motor neurons)
