Spinal Reflexes

Question 1

What is a reflex?

Answer 1

A rapid, involuntary, yet stereotyped and co-ordinated response to a sensory stimulus

Question 2

What do spinal reflexes require?

Answer 2

Stimulation

Question 3

The stretch (myotactic) reflex

Answer 3

• Knee jerk isn’t a response to pain in knee
• The tap stretches the thigh extensor muscle and associated tendon sets in motion to correct the stretching
• Maintains body posture

Question 4

The myotactic reflex steps

Answer 4

1. Sensory fibres sense muscle stretch and send signals to spinal cord
2. Direct connection to motor neuron fires action potential which contracts the biceps muscle
3. Simultaneously, a distinct connection to an inhibitory interneuron inhibits the firing of motor neurons connected to the triceps, thus relaxing the antagonistic muscle, this is known as reciprocal inhibition

Question 5

What are muscle spindles innervated by?

Answer 5

Ia sensory fibres

Question 6

What does the Golgi Tendon Organ do?

Answer 6

Detects muscle tension due to muscle contraction not stretch

Question 7

Golgi Tendon Organ steps

Answer 7

• GTO sensory (Ib) afferents activated
• Leads to activation of inhibitory interneurons which inhibit alpha motor neurons that innervate the same muscle
• Negative feedback circuit that regulates muscle tension and protects the muscle from damage

Question 8

Flexor (withdrawal) reflex

Answer 8

• Quick contraction of flexor muscles to withdraw a limb from an injurious stimulus
• Activates nociceptive sensory receptors

Question 9

Polysynaptic reflex

Answer 9

One or more interneurons connect afferent (sensory) and efferent (motor) signals

Question 10

Parallel after-discharge circuit

Answer 10

• Time taken to cross each synapse is the same
• Stimulus initiated by a point will take different times to reach output neuron if there are many pathways with different numbers of synapses to cross
• Results in initial signal being sustained over extended period of time

Question 11

Because rapid withdrawal of limb may lead to imbalance, ____ reflexes often include a ______ element.

Answer 11

• Flexor

- Contralateral

Question 12

Crossed extensor reflex

Answer 12

• Provides postural support during limb removal

- Due to contralateral motor neurons to extensor being excited

Question 13

If the spinal cord connections to the brain are severed what happens to the legs?

Answer 13

Basic alternation of activity, and the co-ordination between legs, continues

Question 14

Central pattern generators

Answer 14

Presence of local circuits that can generate the pattern of alternating flexion and extension

Question 15

Monosynaptic neuron

Answer 15

When a reflex arc consists of only two neurons in an animal (one sensory neuron, and one motor neuron)

Question 16

Examples of monosynaptic reflexes

Answer 16

• Patellar reflex

- Achilles reflex

Question 17

Example of polysynaptic reflex

Answer 17

Withdrawal reflex