Lower Motor Neurones Flashcards
In which parts of the nervous system would you find the cell body and the axon of a lower motor neurone?
- Cell body in the CNS
- Axon found in the PNS
Where are upper motor neurones found?
Upper motor neurones completely contained within the CNS
UMN connect to LMN directly or via interneurones
What are the defining characteristics of a lower motor neurone?
- Cell body found within brain/ spinal cord
- Axon projects into the PNS
- Neurone innervates skeletal/ smooth muscle
- Neurone can be both somatic and autonomic
What are the 3 main functions of lower motor neurones?
1) Maintaining posture
2) Voluntary posture
3) Visceral motor function (glandular secretion + smooth muscle)
What is meant by muscle tone?
ACh is continuously released at the neuromuscular junction causing miniature depolarisation of the muscle end plate.
What happens to this tone if there are problems with LMNs?
- Inhibits normal muscle tone
- Atropy occurs (wasting or thinning of muscle mass)
What is the hierarchal relationship between upper and lower motor neurones?
- UMNs are the managers. They receive input from motor planning areas of the forebrain and when activated they can either activate or inhibit LMNs
- LMNs are the workers, they receive input from the UMN and also receive sensory input from muscles.
What are muscle spindles?
Muscle spindles are the sensory organs in muscles.
What do the intrafusal fibres in muscle spindles detect?
They detect muscle stretch
What runs off of muscle spindles?
There is a sensory neurone that runs from the muscle spindle and synapses on a lower motor neurone
What is the name of the reflex that helps to control posture?
Myostatic reflex
What type of reflex is the myostatic reflex?
Monosynaptic reflec (no interneurone)
What happens when muscle spindles detect stretch?
Muscle spindle detect stretch, info is sent via a sensory neurone to the spinal canal (sensory neurone enters the spinal cord via the dorsal root), sensory neurone synapses with LMN in the spinal cord, signal sent to the muscle along the LMN (LMN leaves the spinal cord via ventral horn).
What are gamma motor neurones?
The motor neurones that innervate intrafusal muscle fibres within the species
What do gamma motor neurones do when a muscle contracts?
What is the benefit of this?
The gamma motor neurones stimulate the intrafusal fibres to shorten too.
This means spindle fibres stay taut and can still sense stretching.
What type of motor neurone innervates the ordinary extrafusal muscle fibres?
Alpha α motor neurones
What is the correct term to describe the simultaneous action of α and γ motor neurones?
Co-activation of α and γ motor neurones, specifically lower MNs.
What are the sensory organs of tendons?
Golgi tendon organs
What other structures help to detect tension in a tendon?
Sensory nerve endings between the collagen fibres
What happens when the golgi tendon organs detect too much tension?
- Signal sent via a sensory neurone to an interneurone in the spinal cord
- This is an inhibitory motor neurone
- Inhibitory signal then sent to the tendon via a lower motor neurone
Tendon reflexes are reverse myostatic reflexes, what is their function?
To prevent excessive force through the tissue and provide monitoring to the CNS
How does the patellar reflex work?
- Contact to the patellar ligament
- Stretches the quadricep
- Muscle spindle in the quadriceps detect stretch
- A sensory neurone synapses with an α motor neurone
- The reflex then causes contraction of extensor to restore length
How is flexion of the leg inhibited during the patellar reflex?
There is a sensory neurone that synapses with an inhibitory interneurone in the spinal cord.
The interneurone then synapses with a lower motor neurone that sends an inhibitory signal to the flexor muscle
How do you know if a reflex involves the use of an interneurone or not?
When a neural pathway leads back to the same muscle, interneurone NOT seen
- Different muscle, interneurone is seen.
What does ipsilateral mean?
On the same side of the body
What does contralateral mean?
On the other side of the body
What is the cross extensor reflex?
A reflex used to help maintain posture in which the interneurone uses both contralateral and ipsilateral fibres
During the cross extensor reflex, what happens on the ipsilateral side?
What about on the contralateral side?
On the ipsilateral side:
- Extensor muscles relax
- Flexor muscles contract
On the contralateral side:
- Extensor muscles contract
- Flexor muscles relax
What is polyradiculoneuritis?
In inflammatory disease affecting ventral spinal nerve roots. Causes severe limb weakness.
What is myasthenia gravis?
An auto-immune disease where antibodies are more against the ACh receptors at the neuromuscular junction. Can be treated with AChE inhibitors.
