Neuromuscular and Spinal Cord Flashcards
What inputs can alter the membrane potential of a neuronal body?
- Excitatory post synaptic potential (EPSPs) - bring the potential closer t the threshold for firing
- Inhibitory post synaptic potential (IPSPs) - move the potential further away from. The threshold for firing
* EPSPs and IPSPs can also summary and the degree to which this occurs determines how readily a neuron will reach its threshold potential
What happens at the NMJ?
AP -> Ca2+ influx -> ACh release-> ACh binds to motor end-plate receptors -> ion channels open -> Na+ influx -> AP in muscle fibre
At. Rest individual vesicles release ACh at a very slow rate causing miniature end-plate potentials (mEPPs)
What are alpha motor neurone?
Lower motor neuronsof the brain stem and spinal cord
Innervate (extrafusal) muscle fibres of th skeletal muscle
*extrafusal is muscles that actually do the contracting
Activation causes muscle contraction
Motor neuron pool =all alpha neurones innervating one muscle fibre
**almost somatotropin arrangement of neurons in spinal cord - flexor axons tend to be located in posterior portion of ventral horn
What is the motor unit?
Single motor neurone together with the muscle fibre it innervates
Each muscle fibre only innervated by one distinct neuron
Competition for innervation of muscle fibres usually happens at birth
Motor unit is smallest functional unit that creates force
What are the different types of motor units?
Slow (S), type I - smallest soma, small dendritic trees, thinnest axons (slow conduction velocity)
Fast, fatigue resistant (FR), type IIA - larger Oma, larger dendritic tree, thicker axon (fast conduction)
Fats, fatigable (FF) type IIB - larger soma, larger dendritic tree, thicker axon (fast conduction)
*types not discretely located - spread out
What is the innervation ratio?
Number of muscle fibres innervated by a motor neuron
Higher number = more precise movement
How are the 3 types of muscle or units classified?
3 types are classified by amount of tension generated, speed and conduction and fatigability of motor unit
What mechanisms are used to regulate force produces?
- Recruitment - governered by ”size principle”
- smaller units ar recruited first (generally slow twitch)
- as more force is required, more units are required
- this allows for fine control when low force is required - Rate coding - rate of firing
- slow units fire at a slower frequency
- as the firing rate increases,force by the unit increases
- summation occurs when units fire at a rate too fast to allow the muscle to relax in between contractions
What are neurotrophic factors?
Form of growth factors - prevent neuronal cell death and promote growth of neurons after injury
What are the characteristics of motor units dependent on?
Nerve that innervated them
What can change the properties of muscle types?
training - type IIB -> IIA
deconditioning of spinal cord (microgravity) - type I -> type II
ageing - loss of type I and II (preferential loss of the II so larger proportion of type I)
How are the motor tracts organised in the spinal cord?
**Diagram from lecture
Pyramidal tracts 1a nd 1b are the lateral and anterior corticospinal tracts - responsible for majority of motor function
Some semblance of somatotropin representation in 1a - sacral, lumba, thoracic and cervical
What is a reflex?
An autonomic and often inborn response to a stimulus that involves a nerve passing inward from receptor to a nerve and then outward to an effector without reaching the level of consciousness
*once initiated, cannot be stopped
Reflex arc :
Sensory receptor -> sensory neuron -> integration in inter neuron -> motor neuron -> effector muscle
How can you see how many synapses there are in a reflex arc?
Difference win time from afferent volley to intracellular efferent recording gives experimenters an idea on the number of synapses in the reflex arc pathway - in increments of about 0.7ms
*momosynaptic = 0.7ms disynaptic = 1.6 ms
**inhibitory tends to be polysynaptic
What is the monosynaptic stretch reflex?
when patella ligament is hit
sensory stretch receptors activated -> integrating centre -> both inhibitory neuron and an excitatory neuron to two different muscle groups (one muscle stretched and one relaxed)
What is the Hoffman (H-) reflex?
done electronically ( not mechanically with patella hammer) -> differences in response due to person and not human error
initial M- wave is direct response of muscle to stretch with no synapses (or very few) - M- wave activates higher stimulus than H- wave
H- reflex is response of muscle a few ms later as response has travelled through a few synapses (multiple fibres
- H- wave activates at lower stimulus strength but is slower to fire
- H- wave decreases at higher stimulus because at higher stimuli, motor cells get activated that fire against normal flow and cancel out AP
What is the flexion withdrawal response?
withdrawal of one leg in reflex action and (crossed) extensor of the other leg in same reflex arc to ensure you continue to balance
Describe the supraspinal control of reflexes
e.g Jendrassik manoeuvre - clench teeth while knee jerk
higher centres of CNS exert inhibitory an excitatory regulation on stretch reflex whilst in normal conditions - dominant inhibitory control - decerebration/ reduced inhibition from CNS leads to hyperreflexia and spasticity
higher centres can influence reflexes by activating :
- alpha motor neurons
- inhibitory interneurons
- propriospinal neurons
- gamma motor neurons
- terminals of afferent fibres
higher centres and pathways involved are:
- cortext : corticospinal - fine control
- red nucleus: rubrospinal - autonomic movements of arm in response to posture/ balance changes
- vestibular nucleus : vestibulospinal - altering posture to maintain balance
- tectum: tectospinal - head movements in response to visual information
What is the gamma reflex loop?
ensures the muscle never remains slack and so can remain sensitive to stretch movements
gamma neurons shorten the spindle to maintain sensitivity
gamma and alpha motor neurons are found synonymously in the nervous system (they are found together and fire together when needs be)
What happens to reflexes in stroke patients?
loss of descending inhibition -> hyperreflexia
clonus due to continuous stretch and contraction until voluntary stopping
Babinski’s sign is another classic stroke sign
What is hyporeflexia commonly associated with?
lower motor neuron disease
