Reflex Arcs & Nervous System Overview
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of the nervous system from GCSE. You can test your knowledge on this below.
What is the function of the nervous system?
To detect and respond to changes in the environment (stimuli).
What is a reflex action?
A rapid, automatic response to a stimulus that protects body tissues from damage.
What are the main parts of a reflex arc?
Receptor → Sensory neurone → Relay neurone → Motor neurone → Effector
Topic Explainer Video
Check out this @JoeDoesBiology video that explains reflex arcs or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Overview of the Nervous System
The Nervous System is divided into:
1. Central Nervous System (CNS):
- Brain and spinal cord containing relay neurones.
- Coordinates the response to sensory information.
2. Peripheral Nervous System (PNS):
- All neurones that connect the CNS to the rest of the body.
- Subdivided into:
-
- Sensory Neurones: carry impulses from receptors to the CNS.
- Motor Neurones: carry impulses from the CNS to effectors.
Functional Divisions of the PNS
1. Somatic Nervous System
- Controls voluntary responses (e.g. moving muscles).
2. Autonomic Nervous System (ANS)
- Controls involuntary responses (e.g. heart rate, digestion).
- Further divided into:
- Sympathetic (fight or flight).
- Parasympathetic (rest and digest).
Reflex Arcs
Reflexes are:
- Rapid.
- Automatic.
- Protective.
Basic Sequence:
- Stimulus (e.g. increase in temperature).
- Receptor detects stimulus (e.g. thermoreceptors in the skin) and converts it into an electrical impulse.
- Sensory neurone carries electrical impulse to spinal cord in the CNS.
- Relay neurone in the CNS, processes and relays the electrical impulse.
- Motor neurone carries electrical impulse to effector.
- Effector (muscle/gland) produces a response (e.g. muscle contracts to pull the hand away).
Important: Reflexes bypass the brain for a faster response, although the brain is usually informed afterward.
Importance of Reflex Arcs
- Protect body tissues from damage (e.g. withdrawing from hot objects to prevent burns).
- Enable organisms to respond quickly to danger.
- Do not require conscious thought - faster than voluntary actions and automatic.
- They are innate – they do not need to be learnt.
- They enable homeostatic control.
Key Terms
- Neurone: A specialised cell that transmits electrical impulses.
- CNS: Central nervous system (brain + spinal cord).
- Reflex arc: The pathway followed by nerve impulses during a reflex action.
- Sensory neurone: Carries impulse from receptor to CNS.
- Relay neurone: Found in the CNS, connects sensory and motor neurones.
- Motor neurone: Carries impulse from CNS to effector.
- Effector: Muscle or gland that brings about a response.
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Exam Tip
In 6-mark reflex questions, always mention all parts of the pathway in order. (stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response).
Don’t forget: relay neurone is found only in the spinal cord or brain (CNS).
No answer provided.
1. This nerve pathway may be regarded as a simple reflex arc. Use the diagram to explain why. (1 mark)
2. Suggest 2 advantages of simple reflex actions. (2 marks)
1. Only 3 neurones / nerve cells are involved in a reflex arc.
2. Rapid.
Protect against damage to body tissues.
Do not have to be learnt.
Enable homeostatic control.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!