Spinal Reflexes

Question 1

What is the motor cortex responsible for?

Answer 1

Planning, initiating and directing voluntary movements

Question 2

What are the brainstem centres responsible for?

Answer 2

Rhythmic, stereotyped movements and postural control

Question 3

What is the function of the basal ganglia?

Answer 3

Initiation of intended movement and suppression of unwanted movement

Question 4

What is the function of the cerebellum?

Answer 4

Coordination of ongoing movement

Question 5

Define the term “motor nuclei”.

Answer 5

Clusters of motor neurons that innervate the same muscle
- hundreds of motor neurons together
- also called motor pools

Question 6

Which regions of the spinal cord are dedicated to mediating motor function ?

Answer 6

Ventral horn - laminae VII-IX

Question 7

Which part of the ventral horn innervates axial musculature?

Answer 7

Motor neurons located most medically in ventral horn

Question 8

Which part of the ventral horn innervates proximal musculature?

Answer 8

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

Question 9

What are the 2 types of lower motor neurons found in the ventral horn?

Answer 9

1. Large - alpha motor neurons
2. Smaller - gamma motor neurons

Question 10

Define the term “motor unit”.

Answer 10

A single motor neuron and all of the muscle fibres it innervates

Question 11

What are the different types for motor units available?

Answer 11

1. Slow
   - small units
   - important for activities that require sustained muscular contraction (example: maintaining upright posture)
2. 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)
3. Fast fatigue resistant
   - intermediate size and not as fast as FF motor units
   - they generate more force than slow motor units

Question 12

Describe the phenomenon of motor unit recruitment. Also detail which units gets recruited first.

Answer 12

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

Question 13

Define what a reflex is.

Answer 13

It is a rapid, automatic and predictable response to specific stimuli
- a specific reflex produces the same motor response each time

Question 14

List the 4 levels of classification of reflexes.

Answer 14

1. By development
2. By type of motor response
3. By complexity of neural circuit
4. By site of information processing

Question 15

Discuss the types of reflexes which are dependent on development.

Answer 15

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

Question 16

Discuss the types of reflexes which are dependent on the nature of resulting motor response.

Answer 16

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)

Question 17

Discuss the types of reflexes which are dependent on the complexity of the neural circuit.

Answer 17

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)

Question 18

Discuss the types of reflexes which are dependent on the site of information processing

Answer 18

Spinal

Cranial

Question 19

What is the purpose of the stretch/myotatic reflex and what structure mediated this function?

Answer 19

It monitors and maintains the length of a muscle
- mediated by muscle spindle

Question 20

What are the different types of fibres which make up the muscle spindle?

Answer 20

Extramural and intrafusal fibres

Intrafusal fibres have 2 classes - nuclear bag fibres and the nuclear chain fibres

Question 21

Which afferent and efferent neurons supply the stretch/myotatic reflex?

Answer 21

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

Question 22

Outline the pathway of a stretch/myotatic reflex in the case a hammer being struck on the patella.

Answer 22

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

Question 23

What the function of the Golgi tendon organ reflex and what structure mediates this function?

Answer 23

It monitors and maintains the force of contraction of a muscle
- mediated by the GTO which is located in the tendon

Question 24

What is the sensory afferent fibres which supplies the Golgi tendon organ reflex? And what stimulus does it respond to?

Answer 24

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

Question 25

Outline the pathway of the Golgi tendon organ reflex.

Answer 25

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

Question 26

What is the function of the flexor/crossed extensor reflex and which structures mediates this?

Answer 26

It facilitates withdrawal from noxious stimulus
- mediated by A delta nociceptors which carry fast pain signals

Question 27

Outline the pathway of the flexor/crossed extensor reflex in the case of stepping on a tack with right leg.

Answer 27

1. Stepping on a tack stimulates nociceptors
2. Sensory neuron generates APs which are propagated through the spinal cord segments
3. In the spinal cord (integrating centre), the sensory neuron activates interneurons over several spinal cord segments
4. Interneuons activate motor neurons
5. 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

Question 28

The patellar reflex is commonly used in neurological examinations. What is usually tested?

Answer 28

The integrity of limb, excitability of motor neurons and skeletal muscles

Question 29

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.

Answer 29

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

Question 30

The patellar reflex is commonly used in neurological examinations. What does it mean if it is hyperactive?

Answer 30

Corticospinal tract lesions

Question 31

Why is alpha gamma coactivation important with regards to the stretch/myotatic reflex?

Answer 31

To maintain spindle sensitivity
- Allows spindles to function as all lengths during movement and postural adjustments