Reflexes

Question 1

Describe the stretch reflex

Answer 1

stimulates muscle spindles, causes reflex muscle contraction - muscle shortens to previous length

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

stretch activates 1a afferent sensory nerves in muscle spindle - in creases number of AP’s in 1a afferent projecting through dorsal horn into the spinal cord

Question 2

Describe the reflex mediated by Golgi tendon organs

Answer 2

Inverse stretch reflex

caused by 1b afferent nerves from Golgi tendon organs - which monitor muscle tension

muscle contracts and shortens - pulls on tendon and sensory 1b afferent nerves from GTO, increasing firing of AP’s

Question 3

Describe the causes of the Inverse stretch reflex

Answer 3

1. Activation of inhibitory interneurons to the agonist muscle and a decrease in contraction strength
2. Activation of excitatory interneuron to antagonist muscles
3. Information about muscle tension ascends in the dorsal columns to the somatosensory cortex

reflex is polysynaptic and protective

Question 4

Explain the role of interneurons

Answer 4

they connect spinal motor and sensory neurons

As well as transferring signals between sensory and motor neurons, interneurons can also communicate with each other, forming circuits of various complexity

They are multipolar, just like motor neurons

Question 5

Describe ipsilateral and contralateral reflexes

Answer 5

ipsilateral - ‘same side’ eg. 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

contralateral - ‘opposite side’ eg. 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

Question 6

Describe the flexor - withdrawal reflex

Answer 6

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

withdraw part of the body away from the pain stimulus and in towards the body - so flex affected part

increased AP’s in nociceptor nerves cause 5 things to happen:

1. increase activity in the flexor muscles of the affected part via a number of excitatory interneurons
2. antagonist extensors are inhibited
3. excitatory interneurons cross the spinal cord and excite the contralateral extensors
4. other interneurons cross the spinal cord and synapse with inhibitory neurons and they inhibit the contralateral flexors
5. sensory information ascends to the brain in the contralateral spinothalamic tract

Question 7

Explain the role of facilitation

Answer 7

enhances the effectiveness of sensory inputs

occurs between the same stimuli, pain fibres and between different stimuli

Question 8

Explain the role of inhibition of the GTO reflex

Answer 8

many a motorneurons are from thalamus and cortex and cause excitatory and inhibitory postsynaptic potentials (EPSP’s &IPSP’s)

some make direct contact, bust most act through interneurons

each a motorneuron has to integrate these signals - their net effect at the cell body is summed (total excitation minus total inhibition)

membrane potential changes on distant dendrites have less effect than those nearer cell body

descending voluntary excitation of a a motorneuron overrides the inhibition from the GTO and maintains muscle contraction - preventing GTO reflex

Question 9

Describe the different spindle sensory afferent nerves

Answer 9

(two in the spinal cord, one in the brains)

1. monosynaptic reflex - many directly contact a motorneurons in stretches muscle (one synapse, no interneurons involved), causes rapid contraction of agonist muscle
2. Reciprocal inhibition - uses agonist and antagonist to move joints, sensory fibres from the stretched spindle also connect indirectly with the antagonist muscle, when agonist contracts antagonist relaxes
3. spindle afferent firing also travel up the dorsal columns to thalamus and somatosensory cortex - to tell the brain about length of muscle