Neurones and the reflex arcEdexcel International A Level Biology: Revision notes
Section 1
Three types of neurone
A neurone is a nerve cell specialised to carry electrical impulses. There are three types, each linked to one stage of the pathway.
- A sensory neurone carries impulses from a receptor to the central nervous system (CNS). Its cell body lies to one side of the axon, in the dorsal root ganglion, so one long fibre carries the impulse straight to the spinal cord.
- A relay neurone (intermediate neurone) lies wholly within the CNS and connects sensory neurones to motor neurones.
- A motor neurone carries impulses from the CNS to an effector. Its cell body is at one end, in the grey matter of the spinal cord, with many short dendrites to receive impulses and a long axon to the effector.
Direction matters: sensory neurones carry impulses towards the CNS, motor neurones carry them away from it.
Section 2
Schwann cells and myelination
Schwann cells wrap themselves around the axons of many neurones. As they wrap round many times, they build up layers of membrane, forming the myelin sheath.
Myelin contains a lot of lipid, so it is an electrical insulator: ions cannot cross the membrane where myelin is present. There are small gaps between neighbouring Schwann cells, called nodes of Ranvier. Myelinated neurones conduct impulses faster than unmyelinated neurones.
Schwann cells do not carry impulses and are not neurones. They support and insulate the axon.
Section 3
The spinal cord: grey and white matter
A transverse section of the spinal cord shows two regions:
- The grey matter is the central, H-shaped region. It contains the cell bodies of relay and motor neurones and the synapses between neurones.
- The white matter surrounds the grey matter. It consists mainly of myelinated axons running up and down the cord to and from the brain. The myelin gives it its pale colour.
Sensory neurones enter the cord through the dorsal root, and motor neurones leave through the ventral root.
Section 4
The spinal reflex arc
A reflex arc is the pathway that links a stimulus to an involuntary response:
stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response
The sensory neurone synapses with a relay neurone in the grey matter, which synapses with a motor neurone. The effector is a muscle, which contracts, or a gland, which secretes. For example, in the withdrawal reflex the leg muscle contracts to move the foot away from a sharp object.
Reflexes are rapid (few synapses, and the pathway is short because the brain is not involved first) and involuntary. They protect the body from harm. The brain is still informed, as impulses also travel up the white matter, so you feel pain after you have withdrawn.
Reflex speed: few neurones, few synapses, no need to go to the brain for a decision.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Neurones and the reflex arc
- A person touches a hot saucepan handle and pulls their hand away before they feel any pain. The withdrawal is a spinal reflex.Explain why the withdrawal reflex is faster than a conscious, voluntary response.2 marks
- A neurobiologist examines a transverse section through a human spinal cord and a longitudinal section through a motor neurone with its myelin sheath.Describe how Schwann cells form a myelin sheath and explain why the sheath insulates the axon.2 marks
- A child steps on a sharp drawing pin and lifts her foot straight away. A student describes the pathway. A receptor in the skin detects the pin. A neurone carries the impulse into the spinal cord through the dorsal root, and the cell body of this neurone lies in a swelling outside the cord. In the spinal cord the impulse passes across a synapse to a short neurone, then across a second synapse to a neurone that leaves the cord through the ventral root and reaches a muscle in the leg.Describe the pathway of the impulse in this reflex, from the receptor to the response, naming the types of neurone involved.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).