Neurones and the Synapse

Three types of neurones

• A sensory neurone carries nerve impulses from a receptor to the central nervous system; it has a long dendron bringing information in and a short axon.
• A relay neurone (or interneurone) is found entirely within the brain or spinal cord, where it connects a sensory neurone to a motor neurone.
• A motor neurone carries nerve impulses from the central nervous system to an effector, such as a muscle or gland; it has short dendrites and a long axon.

Structure of a neurone

A typical neurone has a cell body containing the nucleus, short branching dendrites that receive impulses from other neurones, and a long axon that carries the impulse away toward the next neurone or an effector. Many axons are wrapped in a myelin sheath, a fatty insulating layer formed by Schwann cells, which speeds up the impulse and insulates the axon from surrounding cells.

Gaps in this sheath, called nodes of Ranvier, allow the impulse to jump from node to node, transmitting it even faster.

The synapse: crossing the gap

Because neurotransmitter is released only from the presynaptic side and received only on the postsynaptic side, a synapse ensures that a nerve impulse travels in one direction only.

1. A nerve impulse arrives at the end of the axon of the first (presynaptic) neurone.
2. This triggers the release of a chemical called a neurotransmitter into the tiny gap between the two neurones, the synaptic cleft.
3. The neurotransmitter diffuses across the synaptic cleft.
4. It binds to specific receptors on the membrane of the second (postsynaptic) neurone.
5. This triggers a new electrical impulse in the postsynaptic neurone, and the neurotransmitter is then broken down or removed so the signal does not continue indefinitely.

How it is examined

Exam questions on this standard often show a labelled diagram of a neurone and ask you to name its parts, give you a description or diagram to identify which type of neurone it is, or ask you to sequence the events at a synapse. A common question asks why impulses at a synapse can only travel in one direction, which should be answered in terms of where the neurotransmitter is released and received.

Worked exam-style question

Question. A doctor tests a patient's reflexes and finds that nerve impulses travel unusually slowly along one of the patient's leg nerves. Further investigation shows that the myelin sheath around the axons of this nerve has been damaged in several places.

(a) State the normal function of the myelin sheath and of the nodes of Ranvier found along it. (b) Explain, using the idea of the nodes of Ranvier, why a damaged myelin sheath would slow down the nerve impulse.

(c) Name the type of neurone most directly responsible for carrying the impulse that produces the patient's leg movement. (d) State one structural feature of that type of neurone that suits its normal function.

Model answer. (a) The myelin sheath insulates the axon and speeds up the transmission of the nerve impulse; the nodes of Ranvier are gaps in the sheath that allow the impulse to jump from node to node, transmitting it faster. (b) With the myelin sheath damaged, the impulse can no longer jump efficiently between exposed nodes, so it must instead travel along more of the axon membrane directly, which is slower than jumping between nodes.

(c) The motor neurone, since it carries impulses from the central nervous system to the effector muscles that produce the leg movement. (d) A motor neurone has a long axon to carry the impulse over the distance from the spinal cord to the leg muscles, and short dendrites to receive input from a relay neurone.

Practice question

Try this. A student is asked why a nerve impulse arriving at a synapse cannot travel backwards, from the second (postsynaptic) neurone to the first (presynaptic) neurone. Explain the answer in terms of where the neurotransmitter is released and where it is received.

Exam tip

Key terms

These glossary terms connect directly to this standard:

• Neuron, a nerve cell adapted to carry electrical impulses; sensory, relay and motor neurones are its three main types.
• Synapse, the junction between two neurones, where a nerve impulse is passed on chemically using a neurotransmitter.
• Stimulus, a change in the environment first detected by a receptor, which a sensory neurone then carries as an impulse.
• Reflex action, a fast, involuntary response built from a chain of neurones connected by synapses.

Source:SRC-DSKP-EN

Frequently asked questions

More for Coordination and Response in Humans