Spinal Reflexes Flashcards
What are the subdivisions of intrafusal fibres?
Nuclear bag fibres = bag 1 (dynamic), bag 2 (static)
Chain fibres
What are the features of bag 1 fibres?
Very sensitive to rate of change of muscle length
Innervated by dynamic gamma motor neurons
What are the features of bag 2 fibres?
More sensitive to absolute length of muscle
Innervated by static gamma motor neurons
What are some features of chain fibres?
Sensitive to absolute length of muscle
Innervated by static gamma motor neurons
What are the two types of afferent fibre that innervate the intrafusal fibres?
Ia (Aalpha) fibres and II (Abeta) fibres
How are Ia afferent fibres organised?
Form primary annulospinal nerve ending winding around the centre of all intrafusal fibres
How are II afferent fibres organised?
Form flowerspray endings on all intrafusal fibres except bag 1 type = more slowly conducting
Do both types of afferent fibres that innervate intrafusal fibres respond to stretch the same way?
No = Ia fibres are more sensitive to rate of change of stretch, whereas II fibres are more sensitive to absolute length of intrafusal fibres
What does the Ia afferent fibre respond to?
Rate of change of muscle length and absolute length of the muscle
What does stimulation of the static gamma motor neuron cause?
Increase of steady state
What does stimulation of the dynamic gamma motor neuron cause?
Enhances the dynamic response of the Ia afferent fibre to stretch
What are the only kind of activities that cause static gamma motor neurons to be active?
Activities in which muscle length changes slowly and predictably
What behaviours cause dynamic gamma motor neurons to be active?
Behaviours in which muscle length changes rapidly and unpredictably
What do dynamic and static gamma motor neurons form?
A fusimotor system
Where are golgi tendon organs located?
At the junction of muscle and tendon = in series with extrafusal fibres
What is the function of golgi tendon organs?
Monitor changes in muscle tension = act to regulate muscle tension to protect from overload and keep within optimal range
What are golgi tendon organs innervated by?
Groub Ib sensory afferents = myelinated, slightly slower conducting than Ia fibres
Where do group Ib afferents synpase once they enter the spinal cord?
Upon inhibitory interneurons which in turn synapse upon alpha motor neuorns of the homonymous muscle = forms basis of reverse myotatic reflex
What does the reverse myotatic reflex involve?
Polysynaptic pathway = inhibitory neuron is interposed between the Ib afferent and alpha motor neuron
What is the reverse myotatic reflex important in?
Execution of fine motor acts
Where are proprioceptive axons located?
In the connective tissue of joints
What do proprioceptive axons respond to?
Changes in angle, direction and velocity of movement of a joint = also prevent excessive flexion or extension
What are proprioceptive axons a mix of?
Fast adapting and slowly adapting units that have either high threshold or low threshold for activation
What are examples of endings of proprioceptive axons?
Free nerve endings, golgi type endings, Paciniform endings, Ruffini endings
What are some features of free nerve endings of proprioceptive axons?
Found in capsule and connective tissue
Most numerous
High threshold and slowly adapting
Nocicepetive function
What are some features of golgi type endings of proprioceptive axons?
Found only in ligaments
High threshold and slowly adapting
Protective role
What are some features of Paciniform endings on proprioceptive axons?
Found in periosteum near articular attachments and fibrous part of joint capsule
Low threshold and slowly adapting
Acceleration detectors
What are some features of Ruffini endings on proprioceptive axons?
Found mainly in joint capsule
Low threshold and slowly adapting
Static position and speed of movements
Where does proprioceptive info arise from?
Muscle spindles, golgi tendon organs and joint receptors
Where do spinal interneurons receive input from?
Primary sensory axons, descending axons from the brain, collaterals of LMN and other interneurons
What types of input can spinal interneurons receive?
Excitatory or inhibitory
What is the function of spinal interneurons?
Integrate incoming info to generate an output
What is the function of inhibitory interneurons?
Mediate inverse myotatic response and reciprocal inhibition between extensor and flexor muscles
What is the myotatic reflex an example of?
Reciprocal inhibition
What occurs in the myotatic reflex?
Causes homonymous extensor muscle to contract but also causes simultaneous relaxation of antagonist flexor muscle
What is the innervation of the myotatic reflex?
Ia afferent from muscle spindle extensor makes excitatory monosynaptic contact with alpha motor neuron innervating the homonymous muscle
How does the Ia afferent fibre inhibit the alpha motor neuron that supplies the flexor muscle in the myotatic reflex?
Via polysynaptic pathways involving interneurons
What is reciprocal inhibition important in?
Initiation of movement by the motor cortex
What do excitatory interneurons mediate?
The flexor reflex and crossed extensor reflex
What is the flexor reflex?
Noxious stimulus causes limb to flex by contraction of flexor muscles via exicitatory interneurons, and relaxation of extensor muscles via excitatory and inhibitory interneurons
What is the crossed extensor reflex?
Noxious stimulus causes limb to extend by contraction of extensor muscles via excitatory interneurons, and relaxation of flexor muscles via excitatory and inhibitory interneurons
What does the crossed extensor reflex enhance?
Postural support during withdrawl of a body part from painful stimulus
