Spinal Reflexes

Question 1

What is the stretch reflex

Answer 1

Simplest reflex and found in all muscles. Uses info from muscle spindles which monitor muscle length

Question 2

Give an example of a stretch reflex

Answer 2

knee jerk reflex

Question 3

How does the stretch reflex work summary

Answer 3

• Muscle stretch - stimulates muscle spindles
• Causes reflex muscle contraction - muscle shortens to previous length (e.g. heavy object stopped fromdropping on outstretched arms)

stretch activates 1a afferent sensory nerves in muscle spindle –> inc number of APs in 1a affernets projecting through dorsal horn into the spinal cord

Question 4

How do spindle sensory afferents divide?

Answer 4

3 types of connections (2 in spinal cord, 1 in brain)

Question 5

Give 3 connections made by spindle sensory afferents

Answer 5

1. many directly contact a-monorneurones in the stretched muscle: causing rapid contraction of the agonist muscle - is monosynaptic reflex with 1 synapse and no interneurons involved
2. Since muusles use agonist and antagonists to move joints, sensory fibbres from stretched spindle also connect indirectly with antagonist muscles 9when agonist conntracts, antagonist relaxes). Happens because spindle afferents connect with and activate inhibitory interneurones which decrease activation of a-motorneurones to the antagonist muscle - which then relaxes - process called RECRIPROCAL INHIBITION
3. Spindle afferent firing also travels up the dorsal columns to thalamus and somatosensory cortex - to tell brain aout length of muscles

Question 6

describe nerual pathway of stretch reflex

Answer 6

see diagram/sheet

Question 7

afferent neuron

Answer 7

sensory neuron

Question 8

Other names for the inverse stretch reflex

Answer 8

Golgi-tendon organ or clasp-knide reflex

Question 9

Explain the inverse stretch/Golgi-tendon organ reflex

what causes it, what happens and what this causes

Answer 9

• Caused by 1b afferent nerves from the Golgi Tendon Organs (GTO) which monitor muscle tension
• Muscle contracts and shortens, pulls the tendon and sensory 1b afferent nerves from the GTOs, inc firing of AP
• This causes:
  1. Activation of inhibitory interneurones to the agonist muscle and a dec in contraction strength
  2. Activation of exitory interneurones to the antagoist muscles
  3. Info about muscle tension ascends in the dorsal columns to the somatosensory cortex

Question 10

Why is the GTO called the “clasp-knife reflex” and what is it’s function

Answer 10

greatly inc tension leads to collapse of resistance - protective mechanism to prevent muscle/tendon damage

Question 11

briefly describe process of GTO reflex

Answer 11

Pull hard on tendon, GTO afferents inc firing but synaps with inhibitory interneurons so reduce motoneurone firing - muscle inhibited and relaxes rapidly.

Question 12

What is the GTO reflex and what does it prevent

Answer 12

Reflex is polysnaptic and protective and avoids muscle contacting so hard tendons are ripped from bone

Question 13

Flexor-withdrawal reflex

Answer 13

uses info from pain receptors (nociceptors) in skin/muscles/joints and withdraw part of the body away from stimulus and towards the body (flex affected part)

Question 14

What is the flexor/withdrawal reflex

Answer 14

Polysynaptic and protective

Question 15

What do inc Ap in nociceptor nerves cause in the flexor/withdrawal reflex

5 steps

Answer 15

1. Inc activity in flexor muscle via number of excitatory interneurons
2. At same time, via number of extiroy/inhibitory interneurones, the antagonistic extensors are inhibited
3. Excitatory interneurons cross spinal cord and excite the contralateral extensors
4. other interneurones cross the spinal cord, synapse with inhibitory neurons and they inhibit the contralateral flexors - helps maintain an upright posture by extending the limb opposite the flexed one (shifts body weight)
5. Sensory info ascends to the brain in the contralateral spinothalamic tract

Question 16

What is the flexor reflex also associated with

Answer 16

crossed extensor reflex

Question 17

What does flexor reflex cause

Answer 17

Ipsilateral flexion in response to pain (on same side of body)

??? as also have crossed extensor reflex

Question 18

briefly explain the circuitry of the flexor reflex

Answer 18

• Small diameter A# nociceptive fibres triccering pain enter cord
• branch and activate interneurons in several spinal segments above entry point
• activate alpha motorneurons controlling all the flexor muscles of the affected limb

Question 19

Why is the flexor and crossed extensor reflex slower than the stretch reflex

Answer 19

Several interneurons in pathway of flexor/crossed extensor, each with a small synaptic delay. Nociiceptive sensory receptors have a small diameter compared to muscle spindle afferents (stretch reflex), meaning they conduct slower

Question 20

Summarise what the flexor/crossed extensor reflex actually looks like

Answer 20

Leg flexes from pain, other one extends to maintain balance

Question 21

What else does our brain do to these inbuilt spinal reflexes

Answer 21

Uses them as building blocks and adds voluntary control from the thalamus and cortex to reshape them into finely controlled movements

Question 22

What can we do to reflexes

Answer 22

Override them consciously

Question 23

How does the GTO reflex work

Answer 23

1. Hold heavy load
2. Neuron from GTO fires
3. Motor neuron inhibites
4. Muscle relaxes
5. load is released

if was child then could override

Question 24

4 types of reflex

Answer 24

1. Stretch reflex
2. Golgi tendon reflex
3. Withdrawal reflex
4. Crossed extensor reflex

Question 25

how can we override the GTO reflex

Answer 25

descending voluntary exitation of alpha motorneurons overriides the inhibiton from the GTO s and maintains muscle contraction - preventing GTO reflex

Question 26

Explain the synapses that each alpha motorneuron recieves and where they come from

Answer 26

Thalamus and cortex, causing exitory/inhibitory postsynaptic potentials (EPSP/IPSP). Some make direct contact but most act through interneurons.

Each alpha motorneurone must intergrate these signals, net effect at cell body is summed (exitation minus inhibition). Membrane potential changes on distant dendrites will havbe less of an effec than those from nearer the cell body

Question 27

How can we override the stretch reflex?

Answer 27

Strong descending inhibition in patient hyperpolarizes alpha motoneurones and the stretch reflex is hard to evoke
High activity in upper motor neurones spreads to and depolarises lwer level motorneurones and this overcomes the descending inhibition

Question 28

What does the absence of the stretch reflex not necessarily show

Answer 28

evidence of nerve damage

Question 29

Can we override the withdrawal reflex

Answer 29

Yes, if for ex, dropping a hot object would cause more harm (we can override it)

Question 30

What is the clinical relevane of reflexes

Answer 30

From reflexes can work out spinal level of problem

Question 31

In a stretch reflex, what is spindle input

Answer 31

highly localised and affects only alpha motoneurones at 1 or 2 spinal segments

Question 32

What happens to pain fibre input in the withdrawle reflex

Answer 32

Diffues, and spreads over several spinal segments. Powerful pain causes greater segmental spread, more muscles recreuited and response is larger

For ex, if finger touch potato, small responese, but if hand touch then withdraw whole arm = big response

this is facilitation

Question 33

What does facilitation do?

Answer 33

Enhance the effectiveness of sensory inputs

Question 34

Explain the process of facilitation

Answer 34

If have A.B.C as neuronal inputs, each casuing AP in 4 motorneurons.

Input A will cause slight depolarisation of B, but not to threshold: said to be in subliminal zone of input A.

if have input in A and C then overlap big enough to fire AP in B also. This activates 12 motorneurones (not just 4 or 8). This is FACILITATION

Question 35

Jendrassik manoeuvre

Answer 35

a medical maneuver wherein the patient clenches the teeth, flexes both sets of fingers into a hook-like form, and interlocks those sets of fingers together. The tendon below the patient’s knee is then hit with a reflex hammer to elicit the patellar reflex. - should stop knee reflex

Question 36

role of interneurons in reflexes

Answer 36

coordinate the motor response necessary to address a stimulus. Additionally, these neurons are responsible for communicating with the brain for cognition.

Question 37

Ipsilateral

Answer 37

same side (e.g. side of body stood on pin like right leg)

Question 38

Contralateral

Answer 38

Opposite side of body (e.g. right leg flexes after standing on pin so contralateral response in left leg by extending)

Question 39

Role of higher centres of reflex

Not sure about answer???

Answer 39

brain overriding reflex (e.g. inhibiting or facilitating alpha motorneurons to intiiate desired response

Question 40

Function of stretch reflex

Answer 40

To relieve stretch on your tendons/muscles