Figure 16 shows the number of neurones in the brain of different animals - Edexcel - GCSE Biology - Question 10 - 2019 - Paper 1

Myelination is essential for motor neurones to ensure rapid conduction of action potentials. The myelin sheath insulates the axon and allows the electrical impulses to jump between nodes of Ranvier, significantly increasing the speed of signal transmission. In contrast, whilst some neurones in the brain are also myelinated, many are unmyelinated, allowing for slower impulses, which is often adequate for processing complex information where speed is less critical. Additionally, the dense networks of connections in the brain require a more meditative propagation of signals, which can be accommodated by unmyelinated neurones.

In this step, the axon of the sensory neurone in Figure 17 should be identified and labeled clearly, often indicated as the long extension leading away from the neurone's cell body.

Sensory neurones are responsible for transmitting sensory information from sensory receptors to the central nervous system (CNS). They convert external stimuli (like light, sound, and touch) into electrical impulses that the brain can interpret, allowing for the perception of the environment.

At synapses, impulses are transmitted via chemical signals. When an action potential reaches the axon terminal of a presynaptic neuron, it triggers the release of neurotransmitters from synaptic vesicles into the synaptic cleft. These neurotransmitters bind to receptor sites on the postsynaptic neurone, leading to the opening of ion channels and the generation of a new action potential in the receiving neurone. This process ensures that signals can continue propagating between neurones, facilitating communication throughout the nervous system.