4.1 - The Lower Motoneurone Flashcards
Define the function of a motoneurone
Supplies skeletal muscle to set muscle tone and bring about movement.
Motoneurones can be categorised as upper or lower motoneurones. Where is the cell bodies of each located?
Upper - Cell bodies in brain and synapse within the CNS
Lower - cell bodies in spinal cord (lamina IX) or in cranial nerve motor nuclei
How can upper motoneurones be further categorised? Where are their cell bodies located?
Cortical efferents and brainstem efferents
Cortical - cell bodies in cerebral cortex
Brainstem - cell bodies in the sub cortical brain areas
In what part of the spinal cord can descending tracts be found?
Anterior or lateral funiculus
How do motoneurones ensure there are no uncontrolled limb movements?
when not in use lower motoneurones are always under inhibition from the upper motoneurones and the brain.
Cortex removes this inhibition when muscle movement is desired.
What are the clinical signs of lower motoneurone lesions?
Muscle weakness, hypotonia or atonia, hypo or areflexia, atrophy, fasciculations
What is the difference between a LMN and an alpha-motoneurone?
usually synonymous term.
Define a motor unit
an alpha-motoneuron and all the muscle fibres it supplies
Define a reflex / reflex arc
involuntary, unlearned, repeatable, automatic reaction to a specific stimulus that does not require the brain to be intact.
What are the 5 components necessary for a reflex arc?
- a receptor e.g. muscle spindle
- an afferent fibre e.g. muscle spindle afferent
- An integration centre e.g. lamina IX of spinal cord
- An efferent fibre e.g. alpha motoneurone
- An effector e.g. muscle
What are the 2 types of LMNs?
alpha - innervates skeletal muscle fibres.
Gamma - innervates infrasual spindles found in muscle spindles which gives info on muscle length
Describe how a reflex arc occurs
tap tendon of skeletal muscle which produces vibration in the belly of the muscle and stretches the muscle spindle which produces an AP and is sent to the sensory neuron which synapses with an alpha-motoneurone to trigger contraction of muscle
How is motor neurone generated?
Tonic contraction of LMNs. Muscle fibres contract randomly to produce sufficient tone but prevent fatiguing of muscles
Explain the ‘size principle’ with regards to increasing muscle tone
when tone needs to be increased the LMNs recruit fibres according to their size, with the smallest motor units contracting first and the largest contracting last
Why is motor tone suppressed in the new born?
to allow easier passage out of the vagina.
