Muscle Receptors And Spinal Reflexes Flashcards
Where are the cell bodies of lower motor neurones located?
Spinal cord (lamina IX) Cranial nerve nuclei eg facial nucleus
What is the effect of most upper motor neurones? Inhibitory or excitatory?
Inhibitory Only one (normally inhibitory) can predominate over the lower motor neurones at one time
What controls the lower motor neurones?
The upper motor neurones, normally have an inhibitory effect
What allows limb movements to occur?
When the cortex gives permission for movements to occur by removing inhibition on a segment of the spinal cord - results in voluntary movement
What allows there to be motor tone in a motionless but conscious individual?
They have a large input from the descending inhibitory neurones
But the lower motor neurones still have sufficient output to cause motor tone
During deep sleep, which are the only muscles not to be paralysed by descending inhibition?
Breathing muscles
Extra-ocular muscles
What does addition of extra interneurones (to produce disynaptic, trisynaptic etc. reflexes) allow?
Greater control to the finer movements of the body
They are also inhibited by the descending tracts
What is a spinal reflex?
An involuntary, unlearned, automatic reaction to a specific stimulus that does not require the brain
What are the five components of a reflex arc?
A receptor eg muscle stimulus
An afferent fibre eg muscle spindle afferent
An integration centre eg lamina IX of the spinal cord
An efferents fibre eg α-motor neurone
An effector eg muscle
What are the two types of lower motor neurones and what do they innervate?
α motorneurones: extrafusal fibres (normal skeletal muscle fibres)
γ motorneurones: intrafusal fibres (found in muscle spindles) - causes contraction of the poles of the intrafusal fibre, stretching the central zone and activating the peripheral process of the afferent neurone
Differences in structure between α and γ motorneurones?
α:
- myelinated
- large cell bodies
- α conduction velocity
γ
- small cell bodies
- γ conduction velocity
Where are muscle spindles found?
In skeletal muscles
More numerous in muscles that control fine movements
What does each muscle spindle consist of?
A connective tissue capsule in which there are 8-10 intrafusal fibres - known as a fusical
Efferents innervation provided to polar ends of intrafusal fibres innervated by gamma motorneurones
Therefore, muscle spindles detect changes in length of muscle
Function of muscle spindles?
Detect muscle length
Function of Golgi tendons?
Detect tension in tendons - activated by contraction or stretching of the muscle
Why do gamma neurones need to innervate muscle spindles?
To keep them taut
Allows firing of alpha neurones to continue without descending inhibition allowing muscle tone when at rest
What is the structure of the Golgi tendon organ?
A large unmyelinated fibre that enters a connective tissue capsule and subdivided into many unmyelinated receptor endings that intermingle and encircle collagenous fascicles
What is motor tone produced by?
Tonic contraction of lower motor neurones to their muscle fibres
What prevents muscle fibres from becoming fatigued in muscle tone?
Fibres contract randomly to produce sufficient tone
What happens when tone in a muscle needs to be increased?
Get an orderly recruitment pattern caused by size principle
- smaller motor units are recruited first
- larger ones recruited last
What is the reflex when providing muscle tone?
Feedback from muscle spindle afferents
Causes reflex contraction of the muscle that that spindle innervates
Allows for muscle tone and an ability to judge passive displacements
Do babies have muscle tone?
No - if it does not develop, is a sign of brain injury
What is the reflex seen in the knee jerk response?
Myotatic/stretch reflex
What happens in the stretch reflex?
Afferent receptor: muscle spindle
Fast-conducting, large, myelinated axons (group 1a afferents) provide the afferent response
Muscle is stretched, action potentials are produced by the muscle spindle due to deformation
They synapse directly with α-motorneurones in the spinal cord
α-motorneurones innervate the extrafusal fibres causing contraction of the homonymous muscle
