International A Level Biology / The nervous system and neurones
Sensory, relay and motor neurones revision notes
Edexcel International A Level Biology (YBI11), chapter 23: The nervous system and neurones, specification point 8.1. Free lesson.
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Sensory, relay and motor neurones have different structures because they carry impulses along different routes. Knowing each neurone’s structure and function helps you explain how impulses travel from a receptor to an effector.
How to label a motor neurone
A motor neurone carries impulses from the CNS (brain and spinal cord) to an effector, such as a muscle or gland. Its cell body is at one end, and its long axon can be over a metre long. Cell body: contains the nucleus and much rough endoplasmic reticulum, which makes proteins. Dendrites carry impulses towards the cell body. The axon carries impulses away from it. Schwann cells wrap around the axon. Their layers of membrane form the myelin sheath, an electrical insulator. Gaps between Schwann cells are nodes of Ranvier. At the far end, axon terminals form…
How Schwann cells make the myelin sheath
Each Schwann cell covers a short length of axon. It wraps its cell surface membrane around the axon many times, adding layers rich in lipid. These layers form the myelin sheath, while the cell’s nucleus remains on the outside. The axon runs through the middle of each Schwann cell. Schwann cells are separate cells, not part of the neurone. Neighbouring Schwann cells do not quite meet, leaving a node of Ranvier between them. Myelin insulates the axon, so ions can cross the membrane only at the nodes. This makes impulses travel much faster.
How the three neurones differ
Each type is named for its function, or job. In a route through the nervous system, the sensory neurone carries impulses in to the CNS, the relay neurone passes them between neurones, and the motor neurone carries them out to an effector. Sensory neurone: carries impulses from a receptor to the CNS. It has a long dendron, a short axon, and a cell body outside the CNS, partway along the fibre. Sensory neurones run in nerves leading to the CNS. Relay neurone: passes impulses from sensory to motor neurones. It has short dendrites and a short axon, and stays wholly inside the brain or…
Answer structure and function questions
A structure answer describes what a neurone looks like. A function answer gives the job it does, so check which one the question asks for. For example, “It has a cell body partway along a long dendron” describes the structure of a sensory neurone. Its function is: “It carries impulses from a receptor to the CNS.”
Compare sensory and motor neurones
For a comparison question, give clear differences in both cell body position and fibre length. These features distinguish the two structures. A sensory neurone’s cell body is outside the CNS, partway along the fibre. A motor neurone’s cell body is inside the CNS, at one end. A sensory neurone has a long dendron and short axon. A motor neurone has many short dendrites and a long axon.