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REFLEX ACTIONS.

• Learning Objectives; • To know that reflex actions are automatic

and give rapid response to stimuli.• Understand why a reflex arc pathway is

followed.• Explain how reflexes serve to protect us.

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What types of reflexes do you know?

Common reflexes

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Tasks; (use pages 48 and 49 of AQA Biology )1. Write a sentence saying why reflex actions are

important.2. Explain how a reflex action works.3. Explain how nerves are “joined together”and how

the impulse passes along these joins.4. Draw and label a reflex arc, explaining each stage

of the nerve impulse.5. Complete the SUMMARY QUESTIONS IN FULL.6. EXTENSION= answer exam Q 1 + 2 page 61.

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Reflexes are fast, automatic protective biological control systems that link a stimulus to a response.

Many reflexes such as sneezing and focusing your eyes occur naturally, but other reflexes can be learned, i.e. conditioned responses.

Reflex reactions happen without you having to think about them – they are involuntary. This is because the central nervous system (CNS) sends electrical signals to the muscles before the brain can pick up the message.

What are reflexes?

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What do neurones look like?

The axons of most neurones are wrapped in an insulating lipid layer called the myelin sheath. Why is this important?

Thin projections called dendrites extend from the cell body and connect with other neurones, allowing electrical impulses to pass from one to the other.

Neurones are elongated cells consisting of a cell body and long, thin axon.

myelin sheath

axon

cell body

dendrites

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What are sensory neurones?

Sensory neurones transmit messages from sense receptors, such as the eye or nose, to the brain or spinal cord.

cell body

nerve impulse from sense organ

nerve impulse to CNS

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What are motor neurones?

Motor neurones transmit messages from the brain and spinal cord to the muscles and glands.

cell body

muscle

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A reflex arc

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What is a synapse?

A synapse is a junction between two neurones across which electrical signals pass. The human body contains up to 500 trillion synapses.

presynaptic cell

postsynaptic cell

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The release of neurotransmitters

When a nerve impulse arrives at the end of one neurone it triggers the release of neurotransmitter molecules from synaptic vesicles.

synaptic vesicle

neurotransmitter molecules

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Continuing the impulse

The neurotransmitters diffuse across the synaptic cleft and bind with receptors on the next neurone, triggering another impulse.

nerve impulse

receptor

synaptic cleft

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The sequence of a reflex arc