Neuromuscular and spinal cord control of movements Flashcards
What is the function of the synapse
Allows for contact from neurone to muscle or from neurone to neurone
Describe synapses
Structure is similar throughout the nervous system
May be simple or complex
Contact ratio ranges from 1:1 (muscle) to 10^3:1 (CNS)
synaptic cleft = 10-50nm
What are the two directions in which a membrane potential of the post synaptic neurone can be altered
-ve = further away from threshold for firing (inhibitory post synaptic potentials)
+ve = brought closer to threshold for firing (excitatory post synaptic potential)
What is summation
Graded effects of synaptic transmission
Occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
What does the degree of summation determine
how readily a neurone can reach threshold to produce an AP
How is the neuromuscular junction activated
- AP arrives at the pre-synaptic terminal causing opening of VGCa2+ channels and Ca2+ moves in
- Ca2+ causes the ACh vesicles to bind to the pre-synaptic membrane
- Ach is released into the synaptic cleft
- ACh binds to the receptors on the nicotinc ACh receptor (nAChR) and opening it
- Na+ moves into the sarcolemma and causes an AP here often leading to muscle contraction
What are mature end-plate potentials
At rest, individual vesicles release ACh at a very low rate
Describe the alpha motor neurone
Lower motor neurones of the brainstem and spinal cord
Innervates the extrafusal muscle fibres of the skeletal muscles
Activation causes contraction
What are the alpha motor neurons made up of and where is it
Cell bodies in grey matter of the spinal cord/brainstem
Cranial and spinal nerves included
In the spinal cord and brainstem
What is a motor unit
Single motor neurone together with all the muscle fibres that it innervates
Stimulation of one motor unit causes contraction of all the muscle fibres in that unit
What are the 3 types of motor unit
Slow (S, type I)
Fast, fatigue resistant (FR, Type IIA)
Fast, fatiguable (FF, type IIB)
Describe slow motor units
Smallest diameter cell bodies
Small dendritic trees
Thinnest axons
Slowest conduction velocity
Describe fast, fatigue resistant motor units
Larger diameter cell bodies
Larger dendritic trees
Thicker axons
Faster conduction velocity
Describe fast, fatiguable motor units
Larger diameter cell bodies
Larger dendritic trees
Thicker axons
Faster conduction velocity
What is recruitment
The acquisition of more motor units as more force is required
Allows fine control (e.g. writing), under which low force levels are required
(basically spatial summation)
In what order does recruitment occur in
Governed by the size principle - smaller motor units are recruited first (generally slow twitch)
S to FR to FF
What is Rate Coding
The increase in firing rate to generate more force by the motor unit
As firing rate increases, force produced increases
(basically temporal summation)
Explain how rate coding is associated with summation
Summation/tetany occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
Describe the plasticity of motor units/muscle fibres
Fibre types can change properties under conditions
What are the changes in motor units during training
FF(IIB) to FR(IIA)
What are the changes in motor units in ageing
Loss of Type I and II, but more II S type (I) mainly
Where is the vestibulospinal tract and what is it involved in
Ventral
regulates posture to maintain balance, and facilitates mainly α motoneurones of the postural, anti-gravity (extensor) muscles
Where is the anterior corticospinal tract and what is it involved in
Dorsal to vestibulospinal tract
Control voluntary movements
Where is the rubrospinal tract and what is it involved in
Lateral
Automatic movements of arm in response to posture/balance changes
Where is the reticulospinal tract and what is it involved in
Dorsal to the anterior corticospinal tract
Coordinate automated movements of locomotion and posture (e.g. to painful stimuli)
What are the components of a reflex arc
Sensory receptor Sensory neuron Integrating center Motor neuron Effector
What are the types of spinal reflexes
Stretch reflex
Flexion reflex
Crossed extensor reflex
Describe the general reflex pathway
- Receptors sense the stimuli
- Sensory neurones stimulated
- Synapse with interneurons in posterior horn
- Synapse with motor neurones in anterior horn
- Effector activated
Describe the stretch reflex pathway
Monosynaptic
Same as the normal reflex but there is an inhibitory signal to the antagonistic muscle causing relaxation
What is the Hoffman reflex
If you stimulate a muscle fibre then it will be stimulated by:
The direct stimulation to the motor neurone and cause contraction (SHORT ROUTE) also creating an M wave
The sensory neurone going back through the spinal cord to the motor neurone and causing contraction (LONG ROUTE) also creating an H wave
Hence you end up with 2 contractions of the muscle
Describe the flexion reflex
Polysynaptic (as there is an interneurone stimulated in the spinal cord)
Several spinal segments are stimulated from one sensory neurone
Causes contraction of the flexor muscles and therefore withdraws the arm/leg
Describe the crossed extensor reflex
Polysynaptic
The sensory neurone synapses with several interneurons including those from the other side and so you get flexion of the stimulated side and extension of the non-stimulated side (in this case)
So there is excitatory stimulation to both flexor and extensor muscles on both sides
What are muscle spindles
Specialised receptors within the skeletal muscles to detect changes in length and proprioception
avoid damage/overuse/pain
What do gamma neurones do
They keep the muscle spindles sensitive by causing intrafusal muscle contraction to keep up with the changing muscle fibre length caused by alpha motor neurons
What is the gamma reflex loop
If the knee is extended and the muscles fo slack, the spindle is shortened to maintain its sensitivity
Give an example fo supra spinal control of reflexes
1 .If your bicep muscle is at normal length (N) then if a weight is suddenly dropped on your hand it will move to a stretched length(S)
- When it moves to S then the spindles within the muscle fib§res are stretched and cause the a reflex to occur and contract your bicep (as there are sensory neurons wrapped around the spindle)
- So your extrafusal muscle fibres have shortened but your spindles are still stretched
- Supraspinal areas control the gamma motor neurones, causing stimulation of these to contract the spindle back to the correct now contracted length and keep the muscle sensitive to any other changes (Like if you dropped the weight suddenly)
Where does UMN originate and terminate
originate in cerebral cortex and terminate in the spinal cord/brainstem
Where does LMB ornate and terminate
LMN originate in the spinal cord/brainstem and terminate at the effector muscle
What causes hyperreflexia
loss of the supraspinal reflex control when the UMNs are damaged and so the reflexes are present and very active as there is no suppression
What causes hyporeflexia
LMNs are not functioning to effectively contract the muscles in the reflex loop
What occurs in an upper motor neurone lesion
Hypertonia Hyperreflexia No atrophy Clonus prescient Lesion from motor cortex through to anterior horn
What occurs in a lower motor neurone lesion
Hypotonia
Hyporeflexia
Severe atrophy
Fasciculations and fibrillations
