Control Lecture 14: Spinal reflexes

Question 1

Which reflex uses muscle spindles are its receptor? For this reflex give the sensory afferent, change in stimulus and proposed function.

Answer 1

Myotatic Reflex

(a) Sensory afferent - 1a
(b) Sensory Stimulus is a change in muscle stretch
(c) Function: Causes muscle contraction in response to stretching. Functions in antigravity, posture and movement.

Question 2

Which reflex uses 1b sensory afferents? For this reflex give the receptor, change in stimulus and proposed function.

Answer 2

(a) Inverse Myotatic Reflex
(b) Golgi Tendon
(c) Sensory Stimulus is a change in tension
(d) Proposed function is tension feedback and overload protection

Question 3

Which reflex acts on noxious stimulation? For this reflex give the receptor, sensory afferent and proposed function.

Answer 3

(a) Flexor Withdrawal
(b) Alpha-Delta, C afferents, Group III/IV Afferent
(c) Sensory Stimulus is noxious stimulation
(d) Proposed function is to avoid damage through withdrawal and maintained balance on limb withdrawal

Question 4

What is a monosynaptic reflex?

Answer 4

A reflex with one synapse. A polysynpatic reflex is one with 2 or more synapses; this is slower but more complex.

Question 5

Describe the neural connections in the patellar reflex?

Answer 5

Patellar reflex uses the Myotatic Stretch Reflex.

1. The Quadriceps tendon is tapped. This change in stretch is detected by sensed by muscle spindles.
2. The afferent fibres travel to the spinal cord - they are called 1a afferent. 
3. Afferent fibres synapse directly with LMN in the ventral horn of the spinal cord. They initiate contraction of the quadricep muscle (it is a monosynaptic reflex). We then get extension of the quadriceps - contraction of the agonist homonymous muscle i.e. the muscle form which afferent arose.

This tests the integrity of the LMN and afferent fibres. All of this is happening ipsilaterally.

Question 6

Describe the myotonic antagonist response.

Answer 6

Reciprocal inhibition of antagonist muscle during the myotatic reflex. Using the example of when the quadriceps contracts, we need the hamstrings to relax.

	1. 1a afferent take information from the receptor and come to synapse with a LMN in the ventral horn which increases the contraction of the quadriceps. There is direct excitation to the agonist muscle. 
	2. A copy of the information being sent towards the spinal cord, sends information to an inhibitor interneuron. 
	3. We want to decrease the activity of the alpha motor neurone going to the hamstring (the flexor muscle - the antagonist).

Question 7

Describe the reflex that occurs with increased tension on the quadriceps.

Answer 7

The Inverse myotonic reflex.

It senses tension in the muscle via the Golgi Tendon Organ. It is more of a protective reflex. It kicks in when the tension in the muscle is too high so we do not rip of the fibres.

	1. The receptors sense increased tension. Since the fibres came from the Golgi tendon organ, the afferent fibres is 1b afferent fibres. 
	2. There is increased firing in 1b afferent
	3. Indirect inhibition via an inhibitory interneuron (disynaptic link) of the alpha motor neuron. 
	4. Decreased alpha motor neuron activity to homonymous muscle. 
	5. Relaxation/inhibition of the homonymous muscle. 
	6. Simultaneously, there is indirect excitation of motor neurons innervating antagonist muscle groups

Question 8

Describe the reflex that occurs when you step on a lego.

Answer 8

The Flexor Withdrawal Reflex.

	1. Nociceptors initiate firing in Alpha-Delta and C afferents. They come to the dorsal horn of the spinal cord.
	2. There is polysynaptic activation of ipsilateral flexors so that we can withdraw from the stimulus - increased excitation in hamstrings.
	3. Polysynaptic inhibition of ipsilateral extensors - decrease activity of the quadriceps Another copy of the information crosses the midline - they come and communicate with the other side to keep balance. There is activation of the quadriceps and inhibition to the flexors. We decrease the activity of the contralateral muscles.