- Define and explain The Reflex Arc in your own words
- Use key terms such as reflex accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on The Reflex Arc: map the five stages of a spinal reflex and explain why speed matters.
Map the five stages of a spinal reflex and explain why speed matters.
Key ideas
The reflex arc
A spinal reflex has five stages: receptor detects the stimulus, sensory neurone carries the impulse to the spinal cord, a relay neurone passes it across, a motor neurone carries the response out, and the effector (muscle) responds. Bypassing the brain makes the response faster, which protects the body — you snatch your hand from a hot surface before you consciously feel pain.
Neurones and synapses
A neurone carries an electrical impulse along its axon, insulated by a myelin sheath that speeds transmission. At the synapse, the impulse triggers release of neurotransmitter, which diffuses across the gap and binds to receptors on the next neurone, starting a new impulse. Because transmission is one-way and chemical, synapses introduce a tiny delay but allow impulses to be directed and integrated.
Key term — reflex: A rapid, automatic response to a stimulus that bypasses conscious thought, coordinated through the spinal cord or brain stem.
State the five stages of a spinal reflex arc in order.
Receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector (muscle).
Answer: Receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector (muscle).
- Claiming reflexes involve the brain thinking Spinal reflexes are coordinated by relay neurones in the spinal cord; the brain is informed afterwards, which is why you feel the pain after you have already moved.
- Saying impulses jump the synapse electrically The impulse travels electrically along the axon but crosses the synapse chemically, by diffusion of neurotransmitter. The gap cannot be jumped electrically.
Practice
The impulse travels only to the spinal cord via short relay neurones and does not travel up to the brain for conscious processing, so the pathway is shorter and the response quicker.
Nervous responses are very fast and short-lived, travelling as electrical impulses along neurones to specific targets. Hormonal responses are slower and longer-lasting, travelling in the blood to widespread targets.
When blood glucose falls, the pancreas releases glucagon, which stimulates the liver to convert stored glycogen back into glucose and release it into the blood.
The arriving impulse triggers release of neurotransmitter into the synaptic gap; it diffuses across and binds to receptors on the next neurone, triggering a new impulse there.
Quick check
Which of these best defines "reflex"?
Why is type 1 diabetes treated with insulin injections?
- The Reflex Arc: map the five stages of a spinal reflex and explain why speed matters.
- The reflex arc: A spinal reflex has five stages: receptor detects the stimulus, sensory neurone carries the impulse to the spinal cord, a relay neurone passes it across, a motor neurone carries the response out, and the effector (muscle) responds.
- synapse: The junction between two neurones, where the impulse is transmitted by diffusion of neurotransmitter across a tiny gap.
- Watch out for: claiming reflexes involve the brain thinking