Reflexes

Question 1

What is the crossed-extension reflex?

Answer 1

Enhance postural support during withdrawal of a foot (weight bering foot) from painful stimulus

Question 2

Flexion-withdrawal uses what type of reflex pathway?

Answer 2

Polysynaptic reflex pathway

Question 3

What type of inhibition is seen in flexion withdrawal?

Answer 3

Reciprocal inhibition: sensory signal both excites motor neurones that innervate flexor muscles of stimulated limb and inhibits motor neurones that innervate extensor muscles

Question 4

Explain why a positive Babinski sign occurs

Answer 4

The corticospinal tract usually inhibits the flexion withdrawal reaction with non-painful stimuli at cutaneous stimulation but this does not happen in those with demyelination in corticospinal tract

Question 5

How does the crossed-extension reflex demonstrate reciprocal inhibition?

Answer 5

Contraction of extensor muscles in contralateral limb counteracts increase load caused by lifting stimulated leg

Question 6

How is reciprocal inhibition mediated in crossed extension reflex?

Answer 6

Mediated by axon collaterals which cross the midline of the cord and excite the alpha motor neurones of contralateral limb extensor muscles -> can be modulated according to posture

Question 7

What do we use the Hoffman reflex for?

Answer 7

To examine the monosynaptic connections from 1a sensory fibres to spinal motor neurones in humans

Question 8

How do we elicit a Hoffman reflex?

Answer 8

Electrically stimulating 1a fibres in a peripheral nerve and recording the reflex response in homonymous muscle.

Question 9

Where is the Hoffman reflex seen?

Answer 9

-> readily measured in soles muscle -> response depends on stimulus strength on 1a fibres from soles and its synergists

Question 10

Describe the Golgi Tendon Organ reflex

Answer 10

Skeletal muscle contraction causes the antagonist muscle to simultaneously lengthen and relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex. In locomotion Ib activity is depressed due to modification by descending signals to allow excitation

Question 11

What detects change in length for stretch reflex?

Answer 11

Muscle spindle

Question 12

Describe the stretch reflex

Answer 12

A stretch stimulates muscle spindles in intrafusal fibres -> afferent fibres carry impulses to CNS-> make monosynaptic excitatory contact with a.m.n that innervate extrafusal muscle fibres -> contract

Question 13

What controls the sensitivity of stretch receptors?

Answer 13

Alpha - extrafusal (regulated by descending pathways, Gamma - polar, contractile elements of intrafusal

Question 14

What happens at activation of stretch receptors? What kind of loop does this cause?

Answer 14

At activation increased tension to sensory endings which lowers threshold of stretch receptors -> lowers threshold of stretch receptors to externally applied stretch which increases sensitivity of stretch reflex -> Gamma Reflex Loop

Question 15

Clasp Knife Reaction

Answer 15

When 1b afferent fibres enter spinal grey matter and inhibit alpha m.n. via interneurone -> abrupt relaxation after initial contraction on rapid passive stretch of spastic muscle

Question 16

Skeletofusiomotor innervation

Answer 16

When collaterals from alpha motor neurones innervate intrafusal fibres

Question 17

How is the αMN influenced?

Answer 17

1. Via 1a inhibitory IN
2. Via 1b autogenic inhibition
3. Descending signals: brainstem and CST projections.
4. Presynaptic, synaptic and direct effects
5. Overflow of voluntary command e.g. Jendrassik manoeuvre