The Nervous System and Reflexes

Responding to the environment

Organisms must detect and respond to changes (stimuli). In humans this is done by the nervous system (fast, electrical) and the endocrine system (slower, hormonal). This note focuses on the nervous system and reflexes.

The nervous system

The nervous system has two main parts:

• Central nervous system (CNS) – the brain and spinal cord; it coordinates responses.
• Peripheral nervous system – the nerves connecting the CNS to the rest of the body.

Information travels as electrical impulses along neurones (nerve cells).

Receptors and effectors

• Receptors – cells that detect a stimulus (e.g. light receptors in the eye, temperature receptors in the skin).
• Effectors – muscles (which contract) or glands (which secrete hormones) that carry out the response.

The pathway of a response

stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response

• Sensory neurone – carries impulses from receptors to the CNS.
• Relay neurone – found in the CNS, passes impulses on.
• Motor neurone – carries impulses from the CNS to effectors.

Synapses

Where two neurones meet there is a tiny gap called a synapse. The electrical impulse can't cross the gap directly, so:

1. The impulse triggers the release of chemicals (neurotransmitters).

2. These diffuse across the gap.

3. They trigger a new impulse in the next neurone.

Reflexes

A reflex action is a rapid, automatic response that does not involve conscious thought — it protects the body (e.g. pulling your hand off something hot, blinking). Because it skips the conscious brain, it is very fast.

The pathway is called a reflex arc:

stimulus → receptor → sensory neurone → relay neurone (spinal cord) → motor neurone → effector → response

The impulse goes through the spinal cord, not the conscious brain, which is why reflexes are so quick.

Worked example

Put in order the parts of a reflex arc when you touch a sharp pin.

1. Receptor in the skin detects the pain (stimulus).

2. Sensory neurone → spinal cord.

3. Relay neurone passes it on.

4. Motor neurone → muscle (effector).

5. Muscle contracts to pull the hand away (response). ✓

Common mistakes

• Mixing up sensory (to CNS) and motor (from CNS) neurones.
• Saying impulses "jump" the synapse — they cross as chemicals that diffuse.
• Thinking a reflex involves the conscious brain — it doesn't (that's why it's fast).

Exam tips

• Learn the response pathway in order — a very common fill-in-the-blanks question.
• Explain reflexes as fast, automatic and protective, bypassing the conscious brain.
• Describe synaptic transmission as chemicals diffusing across the gap.

Key facts to remember

• CNS = brain + spinal cord; impulses travel along neurones (sensory → relay → motor).
• Pathway: stimulus → receptor → sensory → CNS → motor → effector (muscle/gland) → response.
• Reflexes are rapid, automatic, protective and go through the spinal cord; synapses are crossed by diffusing chemicals.