Spinal Reflexes Flashcards
What is the motor cortex responsible for?
Planning, initiating and directing voluntary movements
What are the brainstem centres responsible for?
Rhythmic, stereotyped movements and postural control
What is the function of the basal ganglia?
Initiation of intended movement and suppression of unwanted movement
What is the function of the cerebellum?
Coordination of ongoing movement
Define the term “motor nuclei”.
Clusters of motor neurons that innervate the same muscle
- hundreds of motor neurons together
- also called motor pools
Which regions of the spinal cord are dedicated to mediating motor function ?
Ventral horn - laminae VII-IX
Which part of the ventral horn innervates axial musculature?
Motor neurons located most medically in ventral horn
Which part of the ventral horn innervates proximal musculature?
Motor neurons which lie lateral to the those that innervate the axial musculature
- distal parts of extremities are innervated by those which are most lateral in the ventral horn
What are the 2 types of lower motor neurons found in the ventral horn?
- Large - alpha motor neurons
- Smaller - gamma motor neurons
Define the term “motor unit”.
A single motor neuron and all of the muscle fibres it innervates
What are the different types for motor units available?
- Slow
- small units
- important for activities that require sustained muscular contraction (example: maintaining upright posture) - Fast fatigable
- larger motor neurons which innervate larger muscle fibres that generate more force, however these fibres have few mitochondria, therefore easily fatigued
- needed for brief exertions that require large forces (running and jumping) - Fast fatigue resistant
- intermediate size and not as fast as FF motor units
- they generate more force than slow motor units
Describe the phenomenon of motor unit recruitment. Also detail which units gets recruited first.
Makes use of the size principle - motor units are recruited into action during muscle contraction according to their size and other electrophysiological characteristics
- gradual increase in muscle tension results in recruitment of motor units in a fixed order
Recruitment order: slow (standing), fast fatigue resistant (run/walking) and then fast fatigable (gallop and jump)
- slow have the lowest threshold
Define what a reflex is.
It is a rapid, automatic and predictable response to specific stimuli
- a specific reflex produces the same motor response each time
List the 4 levels of classification of reflexes.
- By development
- By type of motor response
- By complexity of neural circuit
- By site of information processing
Discuss the types of reflexes which are dependent on development.
Innate
- basic neural reflexes formed before birth
- genetically programmed
- examples: withdrawal, chewing and visual tracking
Acquired
- rapid, automatic learned motor patterns
- repetition enhances them
- Examples: breaking a car in emergency
Discuss the types of reflexes which are dependent on the nature of resulting motor response.
Somatic
- control skeletal muscle contractions
- superficial reflexes (stimulate skin and mucous membranes)
- stretch or deep tendon reflexes
Visceral/autonomic
- control other effectors (smooth muscle, cardiac muscle or glands)
Discuss the types of reflexes which are dependent on the complexity of the neural circuit.
Monosynaptic
- single synapse (simplest reflex arc)
- sensory neuron synapses directly with motor neuron
- fast response
Polysynaptic
- at leats one Interneuron (most common)
- slower response (delay increase with number of synapses involved)
Discuss the types of reflexes which are dependent on the site of information processing
Spinal
Cranial
What is the purpose of the stretch/myotatic reflex and what structure mediated this function?
It monitors and maintains the length of a muscle
- mediated by muscle spindle
What are the different types of fibres which make up the muscle spindle?
Extramural and intrafusal fibres
Intrafusal fibres have 2 classes - nuclear bag fibres and the nuclear chain fibres
Which afferent and efferent neurons supply the stretch/myotatic reflex?
Afferent:
- supplied by 2 types - group Ia and II axons
- group Ia: specific to the nuclear bag fibres
- group II: specific to the nuclear chain fibres
Efferent:
- gamma motor neuron
Outline the pathway of a stretch/myotatic reflex in the case a hammer being struck on the patella.
Hammer tap stretches tendon which in turn stretches the sensory receptors in leg extensor muscle
Sensory neuron synapses with and excites motor neuron in the spinal cord
Sensory neuron also excites spinal interneuron
Interneuron synapse inhibits motor neuron to flexor muscles
Motor neuron conducts action potential to synapses on extensor muscle fibres causing contraction
Flexor muscle relaxes because the activity of its motors neurons have been inhibited
What the function of the Golgi tendon organ reflex and what structure mediates this function?
It monitors and maintains the force of contraction of a muscle
- mediated by the GTO which is located in the tendon
What is the sensory afferent fibres which supplies the Golgi tendon organ reflex? And what stimulus does it respond to?
Supplied by group Ib fibres (sensory afferents)
Sensory afferent is woven into collagen fibres and respond to stretch in these fibres that is generated when a muscle contracts
Outline the pathway of the Golgi tendon organ reflex.
When muscle contracts, the force acts directly on the tendon, leading to an increase in tension of the collagen fibrils in tendon organ - subsequent compression of intertwined sensory nerve endings. Activation of cationic mechanosensitive ion channels in nerve endings of GTO results in a generator potential that triggers generation of APs that propagate along group Ib axon to spinal cord
Ib axons from GTO contact GABAergic inhibitory local circuit neurons in the spinal cord that synapse with alpha motor neurons that innervate the same muscle
Negative feedback system that regulates muscle tension
- it deceases activation of muscle when large forces are generated, therefore protects muscle
- counteracts small changes in muscles tension by increasing or decreasing inhibition of motor neurons
What is the function of the flexor/crossed extensor reflex and which structures mediates this?
It facilitates withdrawal from noxious stimulus
- mediated by A delta nociceptors which carry fast pain signals
Outline the pathway of the flexor/crossed extensor reflex in the case of stepping on a tack with right leg.
- Stepping on a tack stimulates nociceptors
- Sensory neuron generates APs which are propagated through the spinal cord segments
- In the spinal cord (integrating centre), the sensory neuron activates interneurons over several spinal cord segments
- Interneuons activate motor neurons
- Flexor muscles in the thigh (effectors) contract, producing withdrawal of the right leg
Extensor muscles in the thigh contract, proving support from the left leg
The patellar reflex is commonly used in neurological examinations. What is usually tested?
The integrity of limb, excitability of motor neurons and skeletal muscles
The patellar reflex is commonly used in neurological examinations. What does it mean if there decreased activity and what could be the causes of this.
Abnormality of the reflex arc caused by:
- damage to the afferent nerve fibres (occurs in peripheral neuropathy from HIV or diabetes)
- damage to spinal cord grey matter
- damage to lower motor neuron
The patellar reflex is commonly used in neurological examinations. What does it mean if it is hyperactive?
Corticospinal tract lesions
Why is alpha gamma coactivation important with regards to the stretch/myotatic reflex?
To maintain spindle sensitivity
- Allows spindles to function as all lengths during movement and postural adjustments
