Week 2 - Reflexes

Question 1

What is a reflex?

Answer 1

Automatic, involuntary actions that occur in response to a stimulus.

→ Typically predictable in nature + fast - the response occurs before the impulse reaches the brain
→ All research circuits share common structural characteristics

Question 2

What is the reflex arc?

Answer 2

The name given to the neural pathway that controls a reflex response.

Question 3

What are the stages of the reflex arc?

Answer 3

→Stimulus is detected by the specialised receptor
→Receptor converts energy detected into an electrical signal
→Signal carried by afferent (sensory) pathway towards CNS
→ In CNS, integration occurs: some synaptic connection before signal gets conveyed by efferent (motor) pathway
→Signal back out of CNS in order to elicit an effector response

Question 4

How are reflexes classified by development?

Answer 4

Innate reflexes - genetically determined
Acquired reflex - learned

Question 5

How are reflexes classified by response?

Answer 5

Somatic reflexes - control skeletal muscle contractions, include superficial and stretch reflexes

Visceral (autonomic) reflexes - control actions of smooth and cardiac muscles + glands

Question 6

How are reflexes classified by complexity of circuit?

Answer 6

Monosynaptic - one synapse

Polysynaptic - multiple synapses

Question 7

How are reflexes classified by processing site?

Answer 7

(where integration takes place)

Spinal reflexes - processing in the spinal cord
Cranial reflexes - processing in the brain

Question 8

What is an example of a monosynaptic spinal reflex?
(somatic-propriocepive)

Answer 8

The stretch reflex
(the “myotactic reflex”)

Question 9

What is the stimulus for the stretch reflex?

Answer 9

Muscle stretch detected by the “muscle spindle” receptors

Question 10

What is the response of the stretch reflex?

Answer 10

Muscle contraction (occurs within 1-2 milliseconds)

Question 11

What is the reflex arc for the stretch reflex?

Answer 11

𝑹𝒆𝒄𝒆𝒑𝒕𝒐𝒓
→Muscle spindle

𝘼𝙛𝙛𝙚𝙧𝙚𝙣𝙩 𝙥𝙖𝙩𝙝𝙬𝙖𝙮
→ Ia and group II fibres

𝙄𝙣𝙩𝙚𝙜𝙧𝙖𝙩𝙞𝙤𝙣
→ Synapse in the spinal cord (monosynaptic)

𝙀𝙛𝙛𝙚𝙧𝙚𝙣𝙩 𝙥𝙖𝙩𝙝𝙬𝙖𝙮
→ Alpha Motor Neurone (aMN)

𝙀𝙛𝙛𝙚𝙘𝙩𝙤𝙧
→ Skeletal muscle

Question 12

What is the structure and function of a muscle spindle?

Answer 12

The muscle spindle contains intrafusial muscle fibres (non-contractile).
→ consists of connective tissue capsule that is fixed at each end to adjacent muscle fibres
→ ranges in length from 4-10mm

𝑫𝒆𝒕𝒆𝒄𝒕𝒊𝒏𝒈
- change in muscle length
- rate of change in length (velocity)

Question 13

How does the muscle spindle contribute to the stretch reflex?

Answer 13

Receptor - gets stretched when muscle stretches and sends signal to Ia and II fibres

Question 14

What are the 2 types of intrafusial muscle fibres?

Answer 14

1 - nuclear bag fibre
2 - nuclear chain fibre

Question 15

What is the sensory afference ?

Answer 15

Wraps around intrafusal muscle fibres so when muscle stretched, intrafusal muscle fibres also stretched + they deform in mechanical shape, stimulating afferent nerves to take signal back towards the spinal cord.

Question 16

Types of afferent pathway = type Ia and type II - what are they?

Answer 16

Type Ia = Annulospinal afferent - Encoding info about the dynamic/actively changing state of the receptor (speed/velocity, position of muscle during any type of movement).

Type II = Flower spray afferents - Slowly adapting receptors that send continuous information about the overall muscle position/length of muscle back to SC.

Question 17

Stages of the stretch reflex at the elbow?

Answer 17

1. Biceps tendon tapped w/reflex hammer - stretches extrafusial muscle fibres (contractile)
2. Leads to stretch of muscle spindle - stimulates afferent nerves
3. Afferent nerves send info. about velocity of movement + muscle length to spinal cord
4. Sensory info enters dorsal root - single monosynaptic connection in the ventral horn
5. Afferent nerve stimulates efferent nerve (alpha motor neurone) - produces action potential back to same muscle
6. Produces reflex contraction

Question 18

What is reciprocal innervation?

Answer 18

When a muscle is stretches the sensory afferents increase their firing rate leading to contraction of the same muscle, the synergist and 𝙧𝙚𝙡𝙖𝙭𝙖𝙩𝙞𝙤𝙣 𝙤𝙛 𝙩𝙝𝙚 𝙖𝙣𝙩𝙖𝙜𝙤𝙣𝙞𝙨𝙩.

Interneuron switches signal from excitatory to inhibitory.

Needs to inhibit antagonist otherwise muscles are working against each other.

Question 19

How is a brisk reflex explained?

Answer 19

(The brain modulates the sensitivity of the circuit via descending control. It can turn up or dial down the amount of inhibition).

Losing the connection/modulation between the brain and reflex means the muscle spindle is more sensitive to stretch thus showing a brisk reflex.

Question 20

What is the flexor withdrawal reflex?

Answer 20

This reflex protects against tissue necrosis from contact with noxious stimuli such as pain or heat - it is important in avoiding significant dangers.

Question 21

What is the crossed extensor reflex?

Answer 21

This is a contralateral reflex that allows the body to compensate on one side for a stimulus on the other e.g. when one foot steps on a nail, the crossed extensor reflex shifts the body’s weight onto the other foot, protecting and withdrawing the foot on the nail.

Works together with the flexor withdrawl reflex.

Question 22

How does the brain modulate refexes?

Answer 22

Stretch reflex is modulated (e.g. if a hazard such as ice is identified the brain will upregulate muscle tone).

The brain can upregulate (greater response) and downregulate (downregulation) muscle tone which will then affect the extent of the reflex response.

Gamma motor control (inhibitory) from brain comes down spinal cord and innervates muscle spindle directly and increases/decreases (regulates amount of inhibition) the sensitivity of the muscle spindle.