IGCSE Biology / Coordination
The three types of neurone, and the synapse
Edexcel International GCSE Biology (4BI1), chapter 6: Coordination, specification point 2.88, 2.89. Free lesson.
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Neurones carry impulses around the nervous system, and synapses pass signals between them. Knowing what each neurone connects and how a synapse works helps you explain the direction of nerve signals.
What the parts of a neurone do
The three types of neurone have the same main parts, although their shapes differ. In a motor neurone, the cell body is at one end, in the spinal cord. Its axon carries an impulse away from the cell body, and the myelin sheath insulates the axon so the impulse travels faster. Dendrites receive impulses from other neurones. The cell body contains the nucleus. The axon terminals end close to the effector, such as a muscle.
What each type of neurone connects
Each type is named by what it connects to what. The central nervous system (CNS) is the brain and spinal cord. Sensory neurones carry impulses towards it, while motor neurones carry impulses away from it. Sensory neurone: connects a receptor to the CNS. Its cell body sits in a bulge partway along the neurone. Relay neurone: lies within the CNS and connects a sensory neurone to a motor neurone. It is short and has no long axon. Motor neurone: connects the CNS to an effector. Its cell body is at the CNS end, and its axon can be long. Sensory and motor neurones can be very long.…
How a signal crosses a synapse
A synapse is the gap between neurones. An electrical impulse cannot cross the gap directly. Instead, the signal changes from electrical to chemical, then back to electrical in the next neurone.
Why synapses work in one direction
The two sides of a synapse have different roles. Only the first side releases neurotransmitter, and only the next side has receptors for it. This means the signal can pass in one direction only. Remember the order: sensory in, relay between, motor out. Myelin insulates the axon and speeds up the impulse.