Neuromuscular and Spinal Cord Flashcards
What is the difference between EPSP and IPSP in terms of membrane potential? What do these result in?
EPSP: makes the membrane potential less negative (bringing it closer to the threshold potential)
IPSP: makes the membrane potential more negative (hyperpolarisation- further away from threshold for firing)
Graded effects- activity is a result of 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
Caused by constant low rate release of ACh into the synapse
What is the difference between intrafusal and extrafusal skeletal muscle fibres?
Intrafusal: serve as sensory organs (proprioceptors) that detect the amount + rate of change of length of a muscle
Extrafusal: innervated by alpha motor neurones + 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
Motor neurones that innervate the extrafusal fibres of skeletal muscle
Activation causes muscle contraction
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
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?
1 motor neurone can innervate several muscle fibres
But every muscle fibre can only be innervated by 1 motor neurone
When is a muscle fibre innervated by >1 motor neurone?
Under pathological conditions (e.g. severed nerve): 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 (the smallest functional unit that can generate force.)
Stimulation of 1 motor unit causes contraction of all muscle fibres in that unit
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) have a 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)
Describe the structural and functional differences between slow and fast twitch muscle fibres.
Slow: Smallest diameter cell bodies, Small dendritic trees, Thinnest axons, Slowest conduction velocity
Fast: Larger diameter cell bodies, Large dendritic cells, 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 AP’s travelling down the nerves to the muscle fibres
What principle governs recruitment?
Size principle
Smaller units are recruited first (generally slow twitch units)
Describe the order of recruitment of motor units with increasing force generation.
Slow –> Fast Fatigue-Resistant –> Fast Fatiguable
What are neurotrophic factors?
= a type of growth factor that prevent neuronal death + promote 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
Function of the muscle fibre is determined by the type of nerve that innervates it.
How easy is it to switch from one motor unit type to another?
Type 2B to Type 2A occurs with muscle training
Type 2 to type 1 occurs in severe deconditioning e.g. zero gravity or spinal injury
How does muscle composition change with ageing?
Ageing is associated with a loss of type 1 + type 2 fibres with preferential loss of type 2 fibres
Results in a larger proportion of type 1 fibres in aged muscle (slower contraction times)
What tract is responsible for voluntary movements?
Pyramidal/Corticospinal tract
What is the role of extrapyramidal tracts?
Automatic movements in response to stimuli
What is a reflex?
Automatic + often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre + then outwards to an effector (muscle or gland) without reaching the level of consciousness.
What are the 2 signals that are generated when the patellar ligament is tapped?
Excitatory signal to quadriceps= contraction
Inhibitory signal to hamstrings (antagonist)= relaxation
Why is there a difference in the time taken for signals to reach the quadriceps and hamstrings in the patellar reflex?
Signal going to quadriceps only has 1 synapse (monosynaptic) whereas the signal to the hamstrings goes via an inhibitory interneurone so there are 2 synapses.
Thus, signal to quadriceps arrives slightly faster than signal to 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 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.
Response elicited is 2-3 X greater.
If you decerebrate an animal and test their reflexes, what would you expect to observe?
Revelation of excitatory control from supraspinal areas
Rigidity + spasticity result from brain damage giving over active or tonic stretch reflex
What are gamma motor neurones reactive to?
Activity of muscle itself
control contraction of muscle fibers located within the muscle spindle based on afferent input
What signs are seen with upper motor neurone lesions?
Hyperreflexia (loss of descending inhibition)
Clonus (tonic involuntary behaviour of foot)
Babinski’s Sign (great toe will flex, small toes fan out)
What is the neuromuscular junction?
A specialised synapse between the motor neurone + motor end plate (muscle fibre cell membrane)
What does recruitment allow?
Fine control (e.g. when writing) under which low force levels are required
In rate coding, when does summation occur?
When units fire at frequency too fast to allow muscle to relax between arriving AP’s
What is the reticulospinal tract involved in?
Coordinating automated movements of locomotion + posture e.g. to painful stimuli
What is the rubrospinal tract involved in?
Automatic movements of arm in response to posture/ balance changes
What is the vestibulospinal tract involved in?
Regulating posture to maintain balance + facilitates mainly alpha motor neurones of the postural, extensor muscles
What determines magnitude and timing of a reflex?
Intensity + onset of stimulus
How do reflexes differ from voluntary movement?
Once they are released, they can’t be stopped
Describe what is seen in a flexion withdrawal and crossed extensor reflex
Pain sensation
Sensory neurone excited
Activation of interneurones in several spinal cord segments
Stimulation of flexor muscle to allow withdrawal from pain
Stimulation of extensor muscle in other leg to contract to maintain balance
What is the Gamma reflex loop?
Feedback loop in nervous system that regulates level of tension in muscles
Shortens the spindles in muscle to maintain its sensitivity
What is hypo-reflexia?
Below normal or absent reflexes
Mostly associated with lower motor neurone disease
