5.5.7: Reflex actions

Question 1

What are reflex actions?

Answer 1

Responses to changes in the environment that do not involve any processing in the brain to coordinate the movement.

Question 2

Why is the nervous pathway as short as possible in a reflex action?

Answer 2

-So that the reflex is rapid.

Question 3

What do most reflex pathways consist of?

Answer 3

-Just three neurones:

Sensory neurone→relay neurone→motor neurone

Question 4

What is the brain’s role in a reflex action?

Answer 4

-The brain may be informed that the reflex has happened, but is not involved in coordinating the response.

Question 5

What is the survival value of a reflex action?

Answer 5

-A reflex may be used to get out of danger, to avoid damage to part of the body, or it may be used to maintain balance.

Question 6

What are two examples of reflex actions?

Answer 6

• Blinking
• Knee jerk reflex

Question 7

What does the blinking reflex cause?

Answer 7

Temporary closure of the eyelids to protect the eyes from damage.

Question 8

The nervous pathway for the blinking reflex passes through part of the brain. What type of reflex is this?

Answer 8

• A cranial reflex.
• However, the pathway is a direct pathway that does not involve any thought processes in the higher parts of the brain.

Question 9

Why is the blinking reflex a reflex arc?

Answer 9

The receptor and effector are in the same place.

Question 10

Blinking may be stimulated by sudden changes in the environment such as:

Answer 10

• A foreign object touching the eye (the corneal reflex)
• Sudden bright light (optical reflex)
• Loud sounds
• Sudden movements close to the eye.

Question 11

Describe the steps in the corneal reflex.

Answer 11

1. The reflex is mediated by a sensory neurone from the cornea, which enters the pons.
2. A synapse connects the sensory neurone to a relay neurone, which passes the ap to the motor neurone.
3. The motor neurone passes back out of the brain to the facial muscles, causing the eyelid to blink.

Question 12

Why is the corneal reflex very rapid (0.1 seconds)?

Answer 12

It is a short and very direct pathway.

Question 13

How do the sensory neurones involved in the corneal reflex allow the reflex to be overridden by conscious control?

Answer 13

1. sensory neurone also passes the ap to myelinated neurones in the pons.
2. These carry the ap to the sensory region in the cerebral cortex,
3. informs the higher centres of the brain that the stimulus has occurred.
4. The higher parts of the brain (cerebral cortex) can send inhibitory signals to the motor centre in the pons.
5. myelinated neurones carrying impulses to and from the cerebral cortex transmit aps faster than the non-myelinated relay neurones in the pons.
6. Therefore, the inhibitory aps can prevent the formation of an ap in the motor neurone.

Question 14

What is the benefit of the optical reflex?

Answer 14

-This protects the light-sensitive cells of the retina from damage.

Question 15

How does the opical reflex happen?

Answer 15

• The stiulus is detected by the retina and the reflex is mediated by the optical centre in the cerebral cortex.
  
  • It is a little slower than the corneal reflex.

Question 16

What type of reflex is a knee jerk reflex?

Answer 16

A spinal reflex.

-The nervous pathway passes through the spinal cord rather than through the brain.

Question 17

What is the knee-jerk reflex involved in?

Answer 17

coordinated movement and balance.

Question 18

What happens when the muscles at the front of the thigh (the quadriceps) contract?

Answer 18

The leg straightens.

Question 19

What are the muscles at the front of the thighs called?

Answer 19

The quadriceps.

Question 20

How are the quadriceps attached to the lower leg bones at the front of the knee?

Answer 20

Via the patella tendon.

Question 21

When is a knee-jerk reaction caused? (what are the steps?)

Answer 21

• Quadriceps are stretched unexpectedly.
• Muscle spindles (stretch receptors) detect the increase in the length of the muscle.
• Nerve impulse pass along sensory neurones to motor neurones in the spinal cord.
• Motor neurones carry nerve impulse to effectors (quadricep muscles)
• Causes them to contract, leg moves forwards.

Question 22

Why is the knee jerk reflex unusual?

Answer 22

It only consists of two neurones:

Sensory neurone→motor neurone

Therefore, there is one less synapse is involved and the response is quicker.

Question 23

Why can the brain not inhibit the knee-jerk reflex?

Answer 23

• Higher parts of the brain are informed that the reflex is occuring.
• However, because there is no relay neurone, the brain cannot inhibit the reflex.
• In the absence of a relay neurone, the motor neurone is stimulated directly by the sensory neurone and there is insufficient delay to enable inhibition.

Question 24

What does Inhibition rely on?

Answer 24

Inhibition relies on rapid myelinated neurones carrying the inhibitory ap to the synapse before the motor neurone is stimulated.

Question 25

Why do doctors test your reflexes by tapping below the knee cap?

Answer 25

-It causes an immediate response that cannot be inhibited.

Question 26

Why is the reflex contraction inhibited when walking pr running even though the knee must bend and will stimulate muscle spindles?

Answer 26

* A **complex pattern** of **nervous impulses** coming from the **cerebellum** is able to **inhibit** the **reflex** contractions. * As **action potentials** are sent to the **muscles** behind the thigh (**hamstring**), stimulating it to **contract**, **inhibitory** aps are sent to the synapse in the reflex arc to **prevent** the reflex contraction of the opposing muscle.