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) 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 is the name given to the sensory receptors in muscle that feedback to the CNS and allow an excitatory reflex to be generated?
Spindles
What is a motor neurone pool?
Collection of lower motor neurones that innervate a single muscle
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 the rule muscle fibres can only be innervated by one motor neurone 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.
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 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
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 fast
This shows that the function of the muscle fibre is very much determined 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.
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 ages 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)
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 patellar ligament 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 in patellar reflex?
The signal going to the quadriceps only has one synapse (monosynaptic) 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.
Why do sensory nerves show a response at lower stimulus intensity than motor nerves?
They are more amenable to electrical stimuli because they’re larger
What names are given to the 2 polysynaptic reflexes?
Flexion withdrawal
Crossed extensor
Describe the supraspinal control of reflexes.
There is a large descending control over reflexes that only becomes noticeable when these descending controls are removed.
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
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
Clonus
Babinski’s Sign
What determines how readily a neuron can reach its action potential
Summation of all the ISPSs and ESPSs
What are alpha motor neurones
Lower motor neurones that innervate extrafusal muscle fibres
Where are they found
Either originating from brainstem (trigeminal nucleus) or ventral horn
What is the motor neuron pool
Where all alpha motor neuron innervating a single muscle are found
Somatotopic mapping of ventral horn
Distal pools more lateral
Proximal more medial
Functional mapping of ventral horn
Extensors more ventral
Flexors more dorsal
What is a motor unit
The motor neuron and the muscle fibres it innervates
What is smallest functional unit we talk about to give a force
Motor unit
3 types of motor unit
Slow
Fast fatigue resistant
Fast fatiguable
Order at which motor units are brought in
Slow then fast resistant then fast fatiguing
Differences between slow and fast motor units
Small have lower axon and cell body diameters
Smaller dendritic trees
Slow conduction
Alternative names for types of motor units
Slow can be type 1 or S
Fast resistant can be type 2a or FR
Fast fatiguing can be type 2b or FF
How to classify motor units
Force produced
Speed of contraction
Fatiguing
Importance of cross innervation
Possible for a fast twitch muscle and slow muscle to be cross innervated where motor neurone supplying them changes resulting in a change in fibre characteristics
Plasticity of a motor unit
Common for fibre types to change
Change in fibre types brought about from training
Type 2 b to 2A
Cause of change in T1 to T2
Severe deconditioning or spinal chord injury
Change brought about from space flights
T1 to T2
Example of polysynaptic withdrawal
Flexion withdrawal
What makes flexion withdrawal polysynaptic
Certain muscles have numerous motor units which also come at different spinal levels, these have to also have to receive the transmission. There is also issue with giving appropriate message to other leg as you don’t want that leg to flex as well. You need to stimulate the extensor in the other leg to maintain balance
Jendrassik manouever
Clenching teeth and making fists can potentials pterodactyl patellar reflex
What is the supraspinal effect on stretch reflexes
Higher centres of CNS exert excitatory and inhibitory regulation on reflexes
Proof of supraspinal control on reflexes
Decerebration revealed excitatory control as can leave muscles tonically stretched
How can higher centres influence reflexes
Activating 5 neurones: Alpha motor Inhibitory interneurones Propriospinal Gamma motor Terminals of afferent fibres
Propriospinal neurones
Give information about where we are in space
4 higher centres and their pathways in reflex control
Cortex- corticospinal
Red nucleus- rubrospinal
Vestibular nucleus- vestibulospinal
Tectum- tectospinal
Tectospinal pathway
Head movements in response to visual info
Gamma loop reflex
If knee is extended and goes slack the. The spindle is shortened to maintain sensitivity
Why does hyper-reflexia occur
Loss of descending inhibition
Causes of hyporeflexia
Normally lower motor neurone problem
