The Lower Motorneurone Flashcards
What is a motorneurone?
It is a somatic efferent that supplies skeletal muscles to:
- Bring about displacement of limbs (i.e. movements)
- Set muscle tone
What are the two basic divisions of type of motorneurone?
Upper motorneurones
(Motorneurones that eventually supply skeletal muscles indirectly)
Lower motorneurones
(Motorneurones that supply skeletal muscles directly
Describe the variety of UMNs.
All descending tracts are UMN
All interneurons are UMNs
All upper motorneurones exert their actions via lower motorneurones
What is a lower motorneurone?
It is a somatic motor efferent with a cell body in either:
- Lamina IX of the spinal cord (i.e. spinal motor nucleus)
- Cranial nerve motor nucleus
Its axon supplies skeletal muscles of the body directly
What are some lower motorneurone signs?
Flaccid muscle weakness Hypotonia or atonia Hyporeflexia or areflexia Denervation muscle atrophy Fasciculation (acute phase) Muscle wasting
What is the difference between a lower motorneurone and an alpha-motorneurone?
Alpha-motorneurones are a subdivision of lower motorneurones
What is a motor unit?
A motor unit consists of an alpha-motorneurone plus all the muscle fibres it supplies
What types of lower motorneurone are there?
Alpha-motorneurones
Beta-motorneurones
Gamma-motorneurones
Define a reflex.
It is an involuntary, unlearned, repeatable, automatic reaction to a specific stimulus that does not require the brain to be intact
(The neuronal pathway describing a reflex is known as a reflex arc)
What are the components to a reflex arc?
5 components:
- A receptor (or transducer)
- An afferent fibre
- An integration centre
- An efferent fibre
- An effector
What are the different types of muscle stretch reflexes?
Monosynaptic stretch reflex
(Only has one synapse, i.e. one afferent fibre then one lower motorneurone)
Disynaptic stretch reflex
(Has two synapses, i.e. interneuron)
Oligosynaptic stretch reflex
(Has multiple synapses)
What is the relevance of different types of muscle stretch reflexes?
The more synapses ad intermediate neurones, then the more inhibition that can be on that reflex, and so it can make it harder to activate that reflex.
Why do we have muscle stretch reflexes?
To hold a bowl of soup
To keep a constant tone in our muscles
So if you have your arm outstretched, then gravity would make it fall, this would lengthen the stretch fibres I the muscle which would then send a reflex to increase motor tone and stop your arm from falling.
How does the muscle stretch reflex work?
When a muscle relaxes it effectively lengthens
This stretches muscle length fibres
They send an action potential down three routes:
- The brain via dorsal columns
- The cerebellum via spinocerebellar tracts
- Directly to spinal motorneurones
This keeps the body aware of muscle position and keeps tone.
What is muscle tone?
In a normal awake neurological status, lower motorneurones tonically supply muscles with background electrical impulses
These lead to background minimal contraction of the muscle
This gives the muscle a small amount of force
Also known as muscle tone
Muscle tone allows us to maintain body posture and hold our heads upright
