Lecture 11 - Neuromuscular Reflexes Flashcards
What are reflexes
Rapid, automatic responses generated by the circuitry of the spinal cord
What do reflexes coordinate
The action of muscle groups at the spinal level
When skeletal muscle is pulled, it pulls back and this is known as
The myotatic reflex
What is change in length/rate of muscle registered by
A sensory organ known as the muscle spindle
What type of arc is the myotatic reflex
A monosynaptic reflex arc
The stretch of a muscle spindle causes the activation of
Ia afferents
When the Ia afferents are stimulated this causes
The excitatory synaptic transmission in the spinal cord which activates the alpha motor neurone
When alpha motor neurones are stimulated they cause
The contraction of the homonymous muscle
What does the muscle spindle consist of
A fibrous capsule, intrafusal fibres, sensory afferents that innervate intrafusal fibres, and gamma motor neurone efferents that innervate the intrafusal fibres
What do intrafusal fibres consist of
A non-elastic equatorial region and contractile polar ends
What is the non-contractile equatorial region of the intrafusal fibre innervated by
Ia sensory neurones
What type of input does the contractile polar ends receive
Efferent input from gamma motor neurones
Where are the cell bodies from gamma motor neurones found
In the ventral horn of the spinal cord
What does stimulation of gamma motor neurones cause
The contraction of the muscle spindle
During voluntary movements, what alpha and gamma motor neurones are active
Both alpha and gamma motor neurones are co-activated
Why are alpha and gamma motor neurones both activated
So that the intrafusal fibres contract in parallel with the extrafusal fibres
What maintains the sensitivity of the muscle spindle
Alpha and gamma motor neurones being co-activated and the intrafusal and extrafusal fibres contracting in parallel
What are the two groups of intrafusal fibres
Nuclear bag fibres and chain fibres
Two types of nuclear bag fibres
Bag 1/dynamic and bag 2/static
Characteristics of bag 1 fibres
Very sensitive to the rate of change of muscle length and are innervated by dynamic gamma motor neurones
Characteristics of bag 2
Sensitive to the absolute length of the muscle and are innervated by static gamma motor neurones
Characteristics of chain fibres
Sensitive to the absolute length of the muscle and are innervated by static gamma motor neurones
What are the two types of afferent fibres that innervate intrafusal fibres
Ia afferents and II fibres
What do Ia afferents form
A primary annulospial nerve ending
What do II fibres form
A flowerspray ending on all intrafusal fibres except the bag 1 dynamic type
What do both types of afferent fibres respond to
Stretch
What are Ia fibres more sensitive to
The rate of change of the intrafusal fibre
What are II fibres more sensitive to
The absolute length of the intrafusal fibres
Stimulation of the static gamma fibres causes
The static response to increase
Which motor neurones are active in activities in which muscle length changes slowly and predictably
Only static gamma motor neurones
Which motor neurones are active when muscle length changes rapidly and unpredictably
Dynamic gamma motor neurones
Where are the Golgi tendon organs located
The junction of the muscle and tendon
What do the Golgi tendon organs monitor
The changes in muscle tension
What are Golgi tendon organs in series with
Extrafusal fibres
What innervates the Golgi tendon organs
Ib sensory afferents
Characteristics of Ib afferents
Myelinated and slightly slower conducting than Iafibres
Where do group Ib afferents synapse
Upon inhibitory interneurones
What do the inhibitory interneurones synapse upon
The alpha motor neurones
What forms the basis of the inverse myotatic reflex
Ib afferents entering the spinal cord and synapse upon inhibitory interneurones, which in turn synapse upon the alpha motor neurones of the homonymous muscle
What is the polysnaptic pathway
The way in which an inhibitory interneurone is interposed between the Ib afferents and alpha motor neurones
What does the inverse myotatic reflex protect
The muscle from overloading
As muscle tension increases, what occurs
Inhibition of alpha motor neurones slows its contraction
When muscle tension falls, what occurs
The inhibition of the alpha motor neurone is reduced and muscle contraction increases
Where are proprioceptive axons present
In connective tissue of joints
What do proprioceptive axons respond to
Changes in angle, direction and velocity of movement of a joint
What do the proprioceptive axons prevent
Excessive flexion and extension
What are proprioceptive axons made up of
A mixture of rapidly adapting (RA) and slow adapting (SA) units that either have a high threshold (HT) or low threshold (LT) for activation
Where are free nerve endings found
In the capsule and connective tissue
What type of proprioceptive axons are found within free nerve endings
High threshold and slow adapting receptors
What is the role of receptors in free nerve endings
They have a protective role
Paciniform endings are found where
In the periosteum
What type of proprioceptive axons are found within the paciniform endings
Low threshold and slow adapting receptors
What are the proprioceptive axon receptors in the paciniform
The are accelerated detectors
Where are ruffini endings found
Mainly in joint capsules
What type of proprioceptor axons are found in the ruffini endings
Low threshold and slow adapting
What do the proprioceptor axons within the ruffini endings do
They monitor the static position and speed of movements
Where do spinal interneurones receive input from
Primary sensory organs (Ia and Ib fibres), descending axons from the brain, collaterals of the lower motor neurones and other interneurones
What are the two types of interneurone input
Excitatory or inhibitory
What do the inhibitory interneurones mediate
The inverse myotatic reflex and reciprocal inhibition between extensor and flexor muscles
What do the interneurones integrate
Incoming information to generate an output
What are the two types of interneurones
Excitatory and inhibitory
What does the myotatic reflex cause
The homonymous extensor muscle to contract but the antagonist flexor muscle must simultaneously relax
What is reciprocal inhibition important in
The inhibition of movement by the motor cortex
What initiates the myotatic reflex
Voluntary contraction of an extensor muscle will stretch an antagonist flexor
What do the excitatory interneurones mediate
The flexor reflex and crossed extensor reflex
What happens in the flexor reflex
A noxious stimulus causes the limbs to flex by contraction of flexor muscles via excitatory interneurones and the relaxation of extensor muscles via excitatory and inhibitory interneurones
What happens in the crossed extensor reflex
A noxious stimulus causes the limb to extend by contraction of the extensor muscles via excitatory interneurones and relaxation of the flexor muscle via excitatory and inhibitory interneurones
What does the crossed extensor reflex enhance
Postural support during withdrawal of the foot from a painful stimulus
