Reflexes

Question 1

what is the simplest reflex of all? and in which muscle is it found

Answer 1

the stretch reflex

it’s found in all muscles

Question 2

What is the best example of the stretch reflex?

Answer 2

Patella tendon or knee-jerk reflex

Question 3

where does the info about there being a stretch in the muscle come from?

Answer 3

the muscle spindles which are the sensory receptors found in muscle fibres

Question 4

how does the knee-jerk reflex work?

Answer 4

sharp tap to the tendon below the knee causes a stretch reflex
this force is transmitted to the muscle fibres (quadriceps) and these stretch slightly
this stretch activates the sensory nerves in the spindles and increases the number of A.P fired in afferent nerves to the spinal cord

Question 5

what 3 connections can the sensory afferent make once they reach the spinal cord?

Answer 5

1. directly activate the alpha motoneurone pool in the muscle fibre
2. connect indirectly with and influence the antagonist muscles
3. afferent info that comes in can ascend in dorsal column to the somatosensory cortex in brain and give info about muscle length

Question 6

what does directly activating the alpha motoneurone pool do?

Answer 6

efferent motor info goes straight and synapse on muscle without interneurones being involved- this results in rapid contraction of agonist muscle.
it’s a monosynaptic reflex and this is the only one of its kind

Question 7

connecting indirectly with and influencing the antagonist muscles

Answer 7

need both antagonist and agonist muscles to move joints
spindle afferents connect with and activate inhibitory interneurones which decrease the activation of alpha motoneurones to the antagonist which then relaxes it (stretches it) this is called RECIPROCAL INHIBITION

Question 8

What are alpha motoneurones?

Answer 8

motor neurones of the brain stem or spinal cord.
they innervate extrafusal muscle fibres (skeletal muscle fibres that are innervated by alpha motoneurones)
they initiate contraction directly

Question 9

what type of classic mechanism is the stretch reflex?

Answer 9

negative feedback loop as the after the muscle stretches it stimulates muscle spindles and the reflex muscle contracts and then shortens back to previous length

Question 10

what is the golgi tendon organ?

Answer 10

a proprioceptive sensory receptor (receives stimuli from within the body) is an organ that senses changes in muscle tension

Question 11

what happens to the GTO when a muscle contracts and shortens?

Answer 11

it pulls on the tendon and the 1b sensory nerves from GTO increase firing of action potentials which causes 3 things
like when you get given a heavy bag and drop it- agonist relaxes, antagonist contracts- arm drops

Question 12

what 3 things are caused by the GTO firing A.Ps

Answer 12

1. activation of inhibitory interneurones to the agonist muscle
2. activation of excitatory interneurones to antagonist muscles
3. ascends dorsal column to somatosensory cortex

Question 13

what do the GTO (sensory receptors) respond to?

Answer 13

massive increases in tension
prevents the muscle from being torn away from the bone by relieving tension (collapse of resistance)
- this is a protective measure and is polysynaptic
think arm wrestle and giving up

Question 14

Flexor reflex what is it?

Answer 14

they withdraw part of the body away from the painful stimulus and IN TOWARDS the body- so flex the affected part

info comes from pain receptors (nociceptors) in skin, muscles and joints
they are polysynaptic and protective

Question 15

how do flexor reflexes work?

Answer 15

an increase in sensory APs from the pain receptors to the spinal cord which causes
an increase in 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 (they relax)

Question 16

what type of flexion is the flexor reflex

Answer 16

ipsilateral flexion

so sensory info comes from same side of the body that the motor efferent info goes to

Question 17

how does the flexor withdrawal reflex affect whole limbs

Answer 17

once afferent info enters spinal cord they branch a lot and so activate interneurons in several spinal segments hence they can activate alpha motoneurones controlling all the flexor muscles of the affected limb

Question 18

what has to happen if you withdraw a limb from something in order to prevent falling over?

Answer 18

you need the contralateral limb to extend (ie other leg or arm) to keep balance

Question 19

how is the contralateral limb extended to keep balance and maintain an upright posture

Answer 19

several excitatory interneurones which cross the spinal cord excite the contralateral extensors SWITCH SIDES
at the same time, via several interneurones there is inhibtion of the contralateral flexors

Question 20

in terms of the flexor withdrawl reflex how is sensory info sent to the brain

Answer 20

it ascends to the brain in the contralateral spinothalamic tract

Question 21

why do nociceptive sensory fibres conduct more slowly than muscle spindle afferents?

Answer 21

they have smaller diameters so the flexor and the crossed extensor reflex is far slower than the stretch reflex

Question 22

Can the GTO reflex be over-ridden?

Answer 22

YES

if given a heavy load and it was books you would drop it but if it was a child or baby you wouldn’t!!

Question 23

can the stretch reflex be over-ridden?

Answer 23

yes
strong descending inhibition (messages from the brain inhibiting the reflex) hyperpolarises alpha motoneurones and the stretch reflex can not be evoked

Question 24

what does high gamma motoneurone activation of muscle spindles cause?

Answer 24

it causes muscles to become extremely resistant to stretch and muscle is
spastic- continuously contracted

Question 25

why is the stretch reflex important clinically?

Answer 25

important in assessing integrity of whole spinal cord circuit

Question 26

contralateral

Answer 26

opposite sides of body

Question 27

how many spinal segments does the withdrawal reflex spread through?

Answer 27

several

Question 28

how many spinal segments does the stretch reflex spread through?

Answer 28

only 1 or 2, very localised

Question 29

facilitation

Answer 29

larger response due to greater spinal spread and pain stimulus
brings other neurons nearby to threshold due to significant excitation