How does the body send fast messages, and how does a reflex protect us automatically?
Describe the parts of the human nervous system and explain the reflex arc as a fast, automatic response
A scaffolded answer to the N(A)-Level Biology outcome on the nervous system. The central and peripheral nervous systems, the three types of neurone, and the reflex arc that protects the body automatically.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
This outcome wants you to describe the parts of the human nervous system, name the three types of neurone, and explain how a reflex arc works as a fast, automatic response. You should be able to put the steps of a reflex in the right order, from stimulus to response, and explain why a reflex is so quick and why that is useful. The marks reward the correct sequence and the idea that a reflex bypasses thinking to protect the body.
The answer
The parts of the nervous system
The nervous system carries fast electrical messages around the body to coordinate what it does. It has two main parts:
- The central nervous system (CNS): the brain and the spinal cord. This is the control centre that processes information and decides on responses.
- The peripheral nerves: the nerves that carry messages between the CNS and the rest of the body.
The three types of neurone
A neurone is a nerve cell that carries an electrical message called a nerve impulse. There are three kinds:
- Sensory neurones carry messages from the receptors (sense organs) to the CNS.
- Relay neurones are found in the CNS and pass messages from one neurone to another.
- Motor neurones carry messages from the CNS to the effectors (muscles or glands) that carry out the response.
The reflex arc
A reflex is a fast, automatic response that protects the body, such as pulling your hand off something hot. It does not need the brain to think about it. The message travels along a pathway called the reflex arc, in this order:
- Stimulus (the hot object).
- Receptor in the skin detects it.
- Sensory neurone carries the message to the spinal cord.
- Relay neurone in the spinal cord passes it on.
- Motor neurone carries the message to the muscle.
- Effector (the arm muscle) contracts.
- Response (the hand is pulled away).
Why reflexes are fast and useful
A reflex is fast because the message takes a short route through the spinal cord and does not have to travel up to the brain and wait for a decision. Being automatic and quick means reflexes protect the body from harm before any damage is done.
Examples in context
Example 1. Why your eye blinks when something flies near it. The blink is a reflex. A receptor detects the object, the message races through the reflex arc, and the eyelid muscle closes the eye, all before you have time to think. It protects the delicate eye from being hit, a perfect example of a fast, automatic reflex.
Example 2. Why a doctor taps below your knee. Tapping the tendon below the kneecap triggers the knee-jerk reflex, making the lower leg kick out automatically. Doctors use it to check that the reflex arc through the spinal cord is working. It shows reflexes happening without any thinking.
Try this
Q1. Name the two parts of the central nervous system. [1 mark]
- Cue. The brain and the spinal cord.
Q2. State the job of a motor neurone. [1 mark]
- Cue. It carries the nerve message from the CNS to an effector (a muscle or gland).
Q3. Explain why a reflex action is faster than a thought-out movement. [2 marks]
- Cue. The message takes a short, automatic route through the spinal cord and does not have to travel to the brain and wait for a decision.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksDescribe the path of the nerve message in a reflex arc when a person touches a hot object and pulls their hand away. Name the parts the message passes through in order.Show worked answer →
The hot object is the stimulus. A receptor in the skin detects the heat and sends an electrical message (a nerve impulse) along a sensory neurone to the spinal cord. In the spinal cord the message passes to a relay neurone, which passes it to a motor neurone. The motor neurone carries the message to a muscle in the arm (the effector), which contracts and pulls the hand away (the response).
What markers reward: the correct order (stimulus, receptor, sensory neurone, spinal cord/relay neurone, motor neurone, effector/muscle, response) and the idea that the message is electrical. Saying the brain decides to move the hand is wrong here; a reflex does not need the brain.
Original4 marksExplain why a reflex action is fast, and why being fast is useful.Show worked answer →
A reflex is fast because the nerve message takes a short route, usually through the spinal cord, and does not have to travel to the brain and wait for a decision. It is automatic, so no thinking is involved, which saves time.
Being fast is useful because reflexes protect the body from harm. For example, pulling your hand away from something hot quickly reduces the damage to the skin. A slower, thought-out response could cause more injury.
What markers reward: the message taking a short, automatic route (through the spinal cord, not the brain) so it is quick, and the link to protecting the body from harm. Saying reflexes are fast because nerves are fast, with no mention of the short route, scores only partly.
Related dot points
- Define homeostasis and explain how blood glucose is controlled by insulin and the role of negative feedback
A scaffolded answer to the N(A)-Level Biology outcome on homeostasis. What homeostasis is, how insulin lowers blood glucose, the idea of negative feedback, and what happens in diabetes.
- Describe the structure of the human eye and explain how it responds to changes in light intensity
A scaffolded answer to the N(A)-Level Biology outcome on the human eye. The main parts and their jobs, how an image is formed on the retina, and the reflex that changes the size of the pupil in bright and dim light.
- Describe how cells are specialised for their functions and explain how cells are organised into tissues, organs, organ systems and organisms
A scaffolded answer to the N(A)-Level Biology outcome on specialised cells and levels of organisation. Examples such as the red blood cell, root hair cell and nerve cell, and the ladder from cell to tissue to organ to organism.
- Describe the structure of the human heart and the circulatory system, and explain how blood is pumped around the body in a double circulation
A scaffolded answer to the N(A)-Level Biology outcome on the human circulatory system. The structure of the heart, the three types of blood vessel, and how a double circulation carries blood to the lungs and the body.
- Describe the parts of the human digestive system, the process of physical and chemical digestion, and the absorption of digested food
A scaffolded answer to the N(A)-Level Biology outcome on human digestion. The main parts of the digestive system, physical and chemical digestion by enzymes, and how digested food is absorbed in the small intestine.