Spinal Reflexes

Question 1

What is the purpose of reflexes?

Answer 1

To maintain homeostasis (i.e balance and posture) and to protect (i.e touching hot kettle)

Question 2

What is the purpose of a spinal reflex and what are its five components?

Answer 2

Automatic, subconscious responses to changes in the external environment that don’t require the brain!

1. Receptor which detects the stimulus
2. A sensory neuron which carries the afferent impulse and enters the spinal cord dorsally
3. The integration centre which processes the impulse from sensory to motor neuron
4. The motor efferent neuron which exits the spinal cord ventrally and conducts the output to the periphery
5. The effector which causes muscle contraction

Question 3

What is the difference between monosynaptic and polysynaptic? Name an example of each

Answer 3

The integration centre can be

• Monosynaptic: one synapse between sensory-motor neuron, i.e muscle spindles -> alpha-MN
• Polysynaptic: multiple synapses which may involve an interneuron, i.e tendon and withdrawal reflex

Question 4

Describe four ways that reflexes can be classified

Answer 4

1. By development: innate reflexes (developed genetically or developmentally) or acquired reflexes
2. By response: somatic reflexes (controls skeletal muscle contractions and includes superficial and stretch reflexes) or visceral (autonomic) reflexes which controls the actions of smooth + cardiac muscles and glands
3. By complexity of circuit: monosynaptic or polysynaptic
4. By processing site: spinal reflexes process in the spinal cord and cranial reflexes process in the brain

Question 5

Describe the patella reflex

Answer 5

1. Stretch to patella tendon
2. Sensed by muscle spindle as a stretch to the muscle
3. 1a afferent neuron to the spinal cord
4. Synapse with ventral horn
5. alpha-motor neuron is efferent
6. Quad contracts and knee extends

Question 6

Which reciprocal process is also involved in the patella reflex?

Answer 6

An inhibitory interneuron is used to relax the antagonistic hamstring

Question 7

Name five important deep tendon reflexes and the nerve roots they test

Answer 7

1. Supinator: C5,6
2. Biceps: C5,6
3. Triceps: C7,8
4. Quad (patella): L3 and L4
5. Ankle/Achilles: S1

Question 8

How are reflexes recorded?

Answer 8

On a scale of 0-4

Or with +s and - (+++ is hyperactive, ++ is normal, + is diminished, - is absent)

Question 9

Describe the stretch (myotatic reflex) - postural reflex, what is it important for?

Answer 9

Resists the stretching of muscle and maintains its length, important for posture, balance and holding heavy objects

1. A muscle’s spindle afferents will detect its stretch and fire to inform the CNS
2. Muscle spindle afferents also make monosynaptic and oligosynaptic contacts with alpha-motor neurons - so the continual firing of muscle spindle afferents results in a reflex contraction of the muscles in which it resides

Question 10

What does the myotatic reflex stimulate and inhibit?

Answer 10

Stimulates LMN supplying synergist (agonist) to stimulate a contraction
Inhibits LMN supplying antagonist

Question 11

Define three major differences between the stretch and golgi tendon reflex

Answer 11

Their responses:

• Stretch: responds to stretching within the muscles
• Golgi tendon: responds to over contraction of the muscle

Sensed and afferent:

• Stretch: sensed by muscle spindles and uses 1a afferent sensory fibres
• Golgi tendon: sensed by golgi tendon organ and uses 1b afferent sensory fibres

Causes:

• Stretch: contraction of synergistic muscles and reciprocal relaxation of antagonist muscle
• Golgi tendon: relaxes the agonist and contracts the antagonist

Question 12

What is proprioception and which reflex is it important for?

Answer 12

Proprioception is perception or awareness of positioning and movement of the body

The stretch reflex is important for proprioception

Question 13

Describe the pathway of the withdrawal reflex

Answer 13

1. Painful stimulus stimulates nociceptors
2. Afferent sensory neuron synapses with multiple (excitatory and inhibitory) interneurons; this affects different segments of the spinal cord and activates ascending pathways for pain and postural adjustment
3. Interneurons synapse with efferent neurons

Question 14

What are the tonic neck and grasp neonatal reflexes? When do they disappear and where are their origins?

Answer 14

Tonic neck: A fencing posture when supine

Grasp: A finger in a baby’s palm stimulates hand, elbow and shoulder flexion

Both disappear after 4-6 months and originate in the brainstem and vestibular nuclei

Question 15

Define the term “reflex movement”

Answer 15

An unlearned, automatic displacement of a limb in response to a specific stimulus applied to a part of the body

Question 16

What is ‘reinforcement’?

Answer 16

This is used when a reflex appears to be absent, ask the patient to interlock fingers and pull apart, or clench their teeth

Question 17

List three superficial reflexes and their associated nerve roots

Answer 17

Cremasteric; L1-2
Abdominal: T8-12
Plantar: S1-2