5.5.7- reflex actions

What is a reflex action?

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

Nervous pathway

Sensory neurone, relay neurone, motor neurone

Why is the reflex pathway short ?

So the reflex is rapid

What is the point of reflex actions?

Survival value- a reflex can be used to get out of danger, avoid damage to part of the body, or it may be used to maintain balance

Blinking reflex

Causes temporary closure of the eyelids to protect the eyes from damage

What type of reflex is the blinking reflex?

Cranial reflex- it passes through part of the brain, but is a direct pathway that does not involve processes in the higher part of the brain

Reflex arc

Receptor and effector in the same place

Stimulation of blinking

• A foreign object touching the eye (corneal reflex)

• Sudden bright light (optical reflex)

• Loud sounds

• Sudden movements close to the eye

What is the corneal reflex mediated by?

A sensory neurone from the cornea, which enters the pons. A synapse connects the sensory neurone to a relay neurone, which passes the action potential to the motor neurone.

Where does the motor neurone pass?

Back out of the brain to the facial muscles, causing the eyelids to blink.

How long does the corneal reflex take?

0.1 seconds. The corneal reflex causes both eyes to blink, even if only one cornea is affected.

Where does the sensory neurone involved in the corneal reflex pass?

The action potential to myelinated neurones in the pons

Where do the myelinated neurones carry the action potential to?

The sensory region in the cerebral cortex to inform the higher centres of the brain that the stimulus has occurred. This allows the reflex to be overridden by conscious control

What can the higher parts of the brain do?

Send inhibitory signals to the motor centre in the pons

What do the myelinated neurones carrying impulses to and from the cerebral cortex do?

Transmit action potentials much more rapidly than the non-myelinated relay neurone in the pons. The inhibitory action potentials can prevent the formation of an action potential in the motor neurone.

Optical reflex

Protects the light-sensitive cells of the retina from damage. The stimulus is detected by the retina and the reflex is mediated by the optical centre in the cerebral cortex. Slower than the corneal reflex.

What type of reflex is the knee-jerk reflex?

Spinal reflex- the nervous pathway passes through the spinal cord rather than through the brain

What is the knee jerk reflex involved in?

Coordinated movement and balance.

Process of the knee jerk reflex

• Muscle at the front of the thigh (quadriceps) contracts to straighten the leg

• This muscle is attached to the lower leg via the patella tendon that connects the patella to the lower leg bones at the front of the knee

• When the muscles at the front of the thigh are stretched, specialised stretch receptors called muscle spindles detect the increase in length of the muscle

• If this stretching is unexpected, a reflex action causes contraction of the same muscle

What is this reflex part of?

The mechanism that enables us to balance on two legs. Contraction of the muscles straightens the knee or brings the body back above the legs.

Why is the knee jerk reflex unusual?

Only has two neurones- sensory and motor

What happens as a result of there being no relay neurone?

The brain cannot inhibit the reflex. Inhibition relies on rapid myelinated neurones carrying the inhibitory action potentials to the synapse before the motor neurone is stimulated. 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.

What happens when we are walking or running?

The knee must bend and will stimulate the muscle spindles. The 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 action potentials are sent to the synapse in the reflex arc to prevent the reflex contraction of the opposing muscle.