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.
Excitatory post synaptic potentials and inhibitory post synaptic potentials can summate
The degree of summation will determine how readily a neurone can reach threshold to produce an AP
Describe the neuromuscular junction
Specialised synapse between the motor neurone and the motor end plate, the muscle fibre cell membrane
How is the neuromuscular junction activated§
- AP arrives
- Calcium influx
- ACh release
- ACh binds to receptor on motor end plate
- Ion channels open to allow sodium to influx -> AP
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 extramural muscle fibres of the skeletal muscles
Activation causes contraction
What is the motor neurone pool
Contains all alpha motor neurons innervating a single muscle
What is a motor unit
Single motor neurone together with all the muscle fibres that it innervates
Smallest functional unit to produce force
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 are the 3 motor unit types classified according to
Amount of tension generated
Speed of contraction
Fatiguability
What is recruitment
Mechanism by which the brain regulates the force a single muscle can produce
Governed by the size principle - smaller motor units are recruited first (generally slow twitch)
More force required -> more units required
Allows fine control (e.g. writing), under which low force levels are required
What is rate coding
Mechanism by which the brain regulates the force a single muscle can produce
Motor units can fire at a range of frequencies
Slow units = lower frequency
As firing rate increases, force produced increases
Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
What are neurotrophic factors
Type of growth factor
Prevents neuronal death
Promotes growth of neurones after injury
What are the effects of neurotrophic factors
Motor unit and fibre characteristics are dependent on the nerve which innervates them.
If a fast twitch muscle and a slow muscle are cross innervated, the soleus becomes fast and the FDL becomes slow.
The motor neurone has some effect on the properties of the muscle fibres which it.
Describe the plasticity of motor units/muscle fibres
Fibre types can change properties under conditions
Training = IIB -> IIA
Severe deconditioning/ spinal cord injury = =I -> II
Microgravity in spaceflight = slow -> fast
Ageing = loss of type I slower contraction
What is the vestibulospinal tract involved in
Ventral
regulates posture to maintain balance, and facilitates mainly α motoneurones of the postural, anti-gravity (extensor) muscles
What are the Sacral and anterior corticospinal tracts involved in
Control voluntary movements
What is the role of the reticulospinal tract
Coordinates automated movements of locomotion and posture e.g. to painful stimuli
What is the role of the rubrospinal tract
Automatic movements of arm in response to posture/balance changes
What is a reflex
An automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (as a muscle or gland) without reaching the level of consciousness.
What are the components of a reflex arc
Sensory receptor Sensory neuron Integrating center Motor neuron Effector
Describe the monosynaptic (stretch) reflex
- Stretching stimulates the sensory receptor (muscle spindle)
- Sensory neuron is excited
- Sensory neuron activates motor neurones in the integrating centre (spinal cord)
- Motor neuron excited
- Effector contracts and relives the stretching
- Inhibitory neurone activated
- Motor neurone to antagonistic muscles inhibited
- Antagonistic muscles relax
What is the Hoffman reflex
-
Describe a polysynaptic reflex
Flexion withdrawal
- Stepping on a tack stimulates the sensory receptor
- Excited sensory neurone
- Sensory neurones activates interneurones in several spinal cord segments, ascending and descending
- Motor neurones are excited
- Effector contracts
Describe the supra spinal control of reflexes
Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex
Inhibitory control dominates in normal conditions
Decerebration reveals the excitatory control from supra spinal nerves
How may rigidity and spasticity occur via supra spinal reflex control
Brain damage giving over-active or tonic stretch reflex.
What are the mechanisms by which higher centres influence reflexes
Activating:
- Alpha neurones
- Inhibitory interneurones
- Propriospinal neurons
- Gamma motor neurons
- Terminals of afferent fibres
Which higher centres and pathways are involved in supra spinal control of reflexes
Cortex – corticospinal (fine control of limb movements, body adjustments)
Red nucleus – rubrospinal (automatic movements of arm in response to posture/balance changes)
Vestibular nuclei – vestibulospinal (altering posture to maintain balance)
Tectum – tectospinal (head movements in response to visual information).
What is the gamma reflex loop
If the knee is extended and the muscles fo slack, the spindle is shortened to maintain its sensitivity
What is hyper-reflexia
Below normal or absent reflexes
Loss of descending inhibition
Mostly associated with lower motor neuron diseases
May be due to stroke
Describe the structure of the alpha motor neuron
Extensors are more ventral, flexors dorsal
Grey matter is more proximal
