Your body is coordinated by electrical impulses that race along neurones to and from the brain and spinal cord. A reflex arc wires a receptor straight to an effector so you pull your hand from a hot plate before you even feel the pain. At each junction between neurones, a synapse, chemicals carry the message across a tiny gap. The detail of synapses is Extended content.
Electrical impulses travel along neurones. The mammalian nervous system is the central nervous system (brain and spinal cord) plus the peripheral nervous system (all the nerves outside them); together they coordinate and regulate body functions. A reflex action automatically and rapidly links a stimulus to the response of an effector (a muscle or gland) through a reflex arc: receptor, sensory neurone, relay neurone, motor neurone, effector. A synapse is a junction between two neurones.
Fig. 1: a simple reflex arc; the relay neurone in the spinal cord links the sensory and motor neurones
Section 01
Brain, spinal cord and nerves.
Switch between the whole system, the central nervous system and the peripheral nervous system, then pick the types of neurone. Watch the yellow impulses travel to and from the brain and spinal cord.
Impulses: information travels along neurones as electrical impulses.
Central nervous system (CNS): the brain and the spinal cord. It receives impulses from receptors, processes the information and sends impulses to effectors.
Peripheral nervous system (PNS): the nerves outside the brain and spinal cord, linking the CNS with every part of the body.
Role: the nervous system coordinates and regulates body functions, so that the right effectors respond at the right time.
Neurone
Carries impulses
How to spot it in a diagram
Sensory
from a receptor to the CNS
long fibre from the receptor end; cell body off to the side of the fibre
Relay
between neurones, inside the CNS
short, with many branches, wholly inside the brain or spinal cord
Motor
from the CNS to an effector
cell body at one end with many dendrites; long axon to a muscle or gland
Examiner trap
A nerve and a neurone are not the same thing. A neurone is a single nerve cell; a nerve is a bundle of many neurone fibres.
Section 02
The reflex arc.
Press Touch the hot plate and follow the impulse. The panel shows which part of the arc the impulse is in and the time since the touch: the whole response takes only about a tenth of a second.
Receptor: heat receptors in the skin of the fingertip detect the stimulus and start an impulse.
Sensory neurone: carries the impulse to the spinal cord.
Relay neurone: in the spinal cord, passes the impulse to a motor neurone.
Motor neurone: carries the impulse to the effector.
Effector: the biceps muscle contracts and the hand is pulled away. Effectors are muscles or glands.
Reflex action: an automatic and rapid way of integrating and coordinating a stimulus with the response of an effector. You do not decide to do it, which saves time and protects the body.
Examiner trap
In a spinal reflex the response does not wait for the brain. Impulses do also travel up to the brain, which is why you feel the pain a moment after the hand has moved.
Stage 1 · Learn
Check what the sim just showed you
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In the nervous system tab, which structures make up the central nervous system?
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Which type of neurone is found only inside the CNS?
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In the reflex arc tab, what is the effector?
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In the reflex arc, what is the correct order of the impulse after the receptor?
Section 03
Across the synapse.
Extended. A synapse is a junction between two neurones. The impulse cannot jump the gap, so chemicals carry the message across. Send an impulse, then try sending one backwards.
Structure: the end of the first neurone contains vesicles filled with neurotransmitter molecules; a narrow synaptic gap separates it from the next neurone, whose membrane carries receptor proteins.
1. An impulse arrives and stimulates the release of neurotransmitter molecules from the vesicles into the synaptic gap.
2. The neurotransmitter molecules diffuse across the gap.
3. They bind with receptor proteins on the next neurone.
4. An impulse is then stimulated in the next neurone.
One direction only: only the first neurone has vesicles and only the next neurone has receptors, so synapses ensure impulses travel in one direction only.
Examiner trap
Neurotransmitter crosses the gap by diffusion, not as an electrical impulse and not by active transport. The impulse itself never crosses the gap.
Stage 2 · Exam
Exam-style questions
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Which description of a reflex action is correct?
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Which list contains only effectors?
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Extended: what happens straight after neurotransmitter molecules are released into the synaptic gap?
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Extended: why can impulses cross a synapse in one direction only?
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