Reflexes

Question 1

the stretch reflex is found in

Answer 1

ALL muscles.

Best example is the patellar tendon or knee-jerk reflex.

Question 2

A Stretch reflex =

Answer 2

follows a sharp tap to an inelastic tendon.

force is transmitted to the muscle fibres – they are more elastic than tendons and so are more able to stretch.

Question 3

stretch activates the sensory nerves in the muscle spindle which

Answer 3

increases thee number of action potentials in afferent nerves projecting through the dorsal horn into the spinal cord.

Question 4

spindle sensory afferents divide to make 3 types of connections

Answer 4

MONOSYNAPTIC REFLEX
RECIPROCAL INHIBITION.
Inverse stretch reflex

Question 5

MONOSYNAPTIC REFLEX

Answer 5

no interneurones involved – it is the only such connection known.
- many directly activate the alpha-motoneurone pool to the muscle which was stretched: causes rapid contraction of the AGONIST muscle.

Question 6

The stretch reflex is a

Answer 6

classic negative feedback loop.
Muscle stretch - stimulates muscle spindles
Causes reflex muscle contraction – muscle shortens to previous length

Question 7

RECIPROCAL INHIBITION.

Answer 7

spindle afferents connect with and activate inhibitory interneurones which decrease activation of alpha-motoneurones to the antagonist muscle - which then relaxes.

• muscles use agonists and antagonists to move joints - sensory fibres from the stretched spindle also connect indirectly with and influence the antagonist muscles.
  So, when the AGONIST muscle contracts,
  the ANTAGONIST muscle relaxes (stretches).

Question 8

Spindle afferent firing also travels

Answer 8

up the dorsal columns to the somatosensory cortex - to tell the brain about length of muscles

Question 9

Inverse stretch reflex

Answer 9

(clasp-knife or Golgi-tendon organ reflex)
causes:
- Activation of inhibitory interneurones to the agonist muscle and a decrease in contraction strength.
- Activation of excitatory interneurones to antagonist muscles.
- Again, information about muscle tension ascends in the dorsal columns to the somatosensory cortex.

Question 10

clasp-knife reflex

Answer 10

• because greatly increasing tension in tendon leads to a collapse of resistance
• like a spring-loaded knife opening up
• it is a protective mechanism to prevent muscle damage

Question 11

Golgi-tendon organ reflex : inverse stretch reflex : clasp-knife reflex

Answer 11

agonist muscle is inhibited – and relaxes rapidly
antagonist muscle is activated – and contracts

this reflex is polysynaptic (disynaptic) and protective

it prevents muscles contracting so hard that the tendon insertion is torn away from the bone

Question 12

Flexor (Withdrawal) reflex - with crossed extension.

Answer 12

use information from pain receptors (nociceptors) in skin, muscles and joints.

• they are polysynaptic and protective
• they withdraw part of the body away from the painful stimulus and in towards the body - so flex the affected part.

Question 13

Increased sensory APs from pain receptors cause

Answer 13

• increased activity in the flexor muscles of the affected part via a number of excitatory interneurones.
• At the same time, via a number of excitatory and inhibitory interneurones, the antagonistic extensors are inhibited

Question 14

FLEXOR REFLEX

Answer 14

ipsilateral flexion in response to pain

Question 15

Circuitry of flexor withdrawal reflex

Answer 15

• Small diameter Aδ nociceptive fibres triggering pain enter cord
• they branch a lot and activate interneurons in several spinal segments above entry point
• which activate α motoneurones controlling all the flexor muscles of the affected limb

Question 16

contralateral limb extends

Answer 16

and prevents you from falling over say when you step on a pin so you can stay balanced.

Question 17

Sensory information ascends to the brain in the

Answer 17

contralateral spinothalamic tract.

• This basic circuitry is similar to that for walking
• hardwired at spinal cord level.

Question 18

Several excitatory interneurones which cross

Answer 18

the spinal cord excite the contralateral extensors

At the same time, via several interneurones, there is inhibition of the contralateral flexors

Question 19

the flexor and crossed extensor reflex is

Answer 19

far slower than the stretch reflex.
this is because:
- there are several interneurons in the pathway - each with a small synaptic delay
- nociceptive sensory fibres have a smaller diameter than muscle spindle afferents and so conduct more slowly

Question 20

reflexes are “hard-wired” building blocks integrated into the voluntary control of movement and they interact with other reflexes: but

Answer 20

can be over-ridden consciously.
example of this is load- if something is precious you are going to hold on to it until you can safetly put it down somewhere - eg a baby

Question 21

can the GTO reflex be overridden?

Answer 21

The GTO reflex can be over-ridden by voluntary input from the CNS

Question 22

if excess load is placed n muscle

Answer 22

golgi tendon reflex is activated (motor neuron inhibited)causing relaxation and thus protecting muscle.

Question 23

when holding heavy loads descending voluntary excitation of

Answer 23

alpha motoneurones can override the inhibition from the GTOs and maintain contraction.

Question 24

stretch reflex can be overriidden when

Answer 24

strong descending inhibition hyperpolarises alpha motorneurons and the stretch reflex cannot be evoked.

Question 25

activation of gamma motor neurons depends on

Answer 25

descending pathways.

Question 26

high gamma-motoneurone activation of muscle spindles, causes

Answer 26

muscles to become extremely resistant to stretch and muscle is “spastic”.

• Many neurological disorders are associated with altered gamma-motoneurone discharge to spindles

Question 27

reflexes allow

Answer 27

spinal level localisation of a problem

Question 28

in stretch reflex, spindle input is

Answer 28

highly localised and affects only alpha motoneurones at one or two spinal segments

Question 29

in withdrawal reflex, pain fibre input is

Answer 29

diffuse and spreads through several spinal segments - the more powerful the pain stimulus, the greater the spinal spread and the larger the response.

Question 30

Facilitation

Answer 30

increases the effects of sensory inputs

• finger on a hot plate - just withdraw hand
• whole hand in contact with hot surface, whole arm will be withdrawn and extension of the contralateral arm too.

Question 31

Facilitation occurs between

Answer 31

• similar inputs eg. pain fibre inputs

- diverse inputs eg. burnt hand on a hotplate and biceps stretch reflex

Question 32

Facilitation explained

Answer 32

A alone = 4 motoneurones activated
C alone = 4 motoneurones activated

A + C =12 motoneurones activated
(B is in the middle)

When A and C are active together there is sufficient excitation to bring neurones in B to threshold