Neuromuscular and spinal cord Flashcards
How may the membrane potential of the postsynaptic neurone be altered by inputs?
It can be made less negative, i.e. be brought closer to threshold for firing- this is an excitatory postsynaptic potential (EPSP).
It can be made more negative, i.e. be brought further away from threshold for firing- this is an inhibitory postsynaptic potential (IPSP).
What is an excitatory postsynaptic potential (EPSP)?
Membrane potential of the postsynaptic neurone can be made less negative, i.e. be brought closer to threshold for firing.
What is an inhibitory postsynaptic potential (IPSP)?
Membrane potential of the postsynaptic neurone can be made more negative, i.e. be brought further away from threshold for firing.
What does the degree of summation of EPSPs and IPSPs determine?
How readily a neuron can reach threshold to produce an action potential.
What is a neuromuscular junction?
A specialised synapse between the lower motor neuron and the motor end plate (highly folded muscle fibre membrane).
How is a neuromuscular junction activated?
When an action potential arrives at the NMJ, Ca2+ influx causes ACh release from vesicles.
ACh binds to receptors on the motor end plate.
Ion channel opens- Na+ influx causes action potential in muscle fibre.
What is a miniature end plate potential (mEPP)?
At rest, individual vesicles release ACh at a very low rate.
What is an alpha motor neurone?
Lower motor neurones of the brainstem and spinal cord.
Innervate the extrafusal muscle fibres of the skeletal muscles.
Their activation causes muscle contraction.
Motor neurone pool contains all alpha motor neurones innervating a single muscle.
Where are alpha motor neurones found?
Ventral horn of spinal cord.
What is a motor unit?
A single motor neurone and all of the muscle fibres it innervates.
The smallest functional unit that causes contraction/ produces force.
Stimulation of one motor unit causes contraction of all the muscle fibres in that unit.
Approximately how many motor neurones do humans have?
420,000
Approximately how many skeletal muscle fibres do humans have?
250 million
On average how many muscle fibres does each motor neurone supply?
About 600
What are the different types of motor unit?
Slow (S, type I).
Fast, fatigue resistant (FR, type IIA).
Fast, fatiguable (FF, type IIB).
What are the features of a slow (S, type I) motor unit?
Smallest diameter cell bodies. Small dendritic trees. Thinnest axons. Slowest conduction velocity. Slow twitch. Low tension. Fatigue resistant.
What are the features of a fast, fatigue resistant (FR, type IIA) motor unit?
Larger diameter cell bodies. Larger dendritic trees. Thicker axons. Faster conduction velocity. Fast twitch. Moderate tension. Fatigue resistant.
What are the features of a fast, fatiguable (FF, type IIB) motor unit?
Larger diameter cell bodies. Larger dendritic trees. Thicker axons. Faster conduction velocity. Fast twitch. High tension. High fatigue.
How are the 3 different motor unit types classified?
Amount of tension generated.
Speed of contraction.
Fatiguability of the motor unit.
What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?
Recruitment.
Rate coding.
What is recruitment?
Motor units are not randomly recruited- there is an order.
Governed by the ‘size principle’.
Smaller units are recruited first (generally the slow twitch units).
As more force is required, more units are recruited.
This allows fine control (e.g. when writing), under which low force levels are required.
What is rate coding?
A motor unit can fire at a range of frequencies.
Slow units fire at a lower frequency.
As the firing rate increases, the force produced by the unit increases.
Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
What are neurotrophic factors?
Growth factors.
Prevent neuronal death.
Promote growth of neurons 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, slow becomes fast and fast becomes slow.
The motor neuron has some effect on the properties of the muscle fibres it innervates.
Discuss the plasticity of motor units/ muscle fibres.
Fibre types can change properties under many different conditions.
Type IIB to IIA most common following training.
Type I to II possible in cases of severe reconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types.
Ageing associated with loss of type I and II fibres but also preferential loss of type II. This results in a larger proportion of type I fibres in aged muscles (slower contraction times).
