What you need to know
- 8.10The peripheral nervous systemAll the nerves outside the brain and spinal cord. is the part outside the brain and spinal cord.
Do not turn sympathetic and parasympathetic into a long extra topic. Use them only to place the pupil and other involuntary motor paths. Brain-region functions are the next lesson.
By the end, you should be able to…
- Separate sensory and motor divisions of the peripheral nervous system.
- Place involuntary motor paths as autonomic.
- Keep cranial and spinal nerves outside the central organs.
Build your understanding
Nerves are the peripheral wiring
Cranial nerves leave the brain. Spinal nerves leave the cord. Both are peripheral even though they plug into the centre. A sensory pathway brings impulses from receptors. A motor pathway carries impulses to effectors. Most spinal nerves contain both kinds of fibre. “A nerve” is a bundle. It is not one neurone.
The cell body of a sensory neuroneA neurone that carries impulses from a receptor to the central nervous system. in a dorsal root ganglion is peripheral. The cell body of a motor neuroneA neurone that carries impulses from the central nervous system to an effector, such as a muscle. that supplies a skeletal muscle is inside the cord, but its axon in the spinal nerve is peripheral. Use that split if a question asks where a named part sits.
Somatic and autonomic motor paths
Somatic motor neurones supply skeletal muscle, including the voluntary withdrawal reflex and movements you decide to make. Autonomic motor paths supply glands, cardiac muscle and smooth muscle. You do not choose each heart beat or each change in pupil size.
The autonomic paths are taught as sympathetic and parasympathetic, with opposite effects on many organs. Pupil dilation is the sympathetic-side example from 8A 6. Constriction is the parasympathetic-side example. heart rateThe number of heartbeats per minute. rises with sympathetic activity and falls with parasympathetic activity. Learn the pattern as examples of peripheral motor control. Do not memorise a full organ table unless a question gives the organ.
Still part of one system
Peripheral does not mean independent. A sensory nerve is useless for a reflex if the cord synapse is destroyed, and a motor nerve is useless if the effector is. The specification sentence is the division of the mammalian nervous system into central and peripheral parts.
Receptors in the eye, the skin and the ear are the starts of peripheral sensory neurones. The brain regions that interpret them are central. Do not call the retina part of the spinal cord.
Make your reasoning visible
Bundle versus cell
A nerve contains many axons. A neurone is one cell.
Involuntary still uses neurones
Autonomic does not mean hormonal. The pupil still has a nerve pathway.
Avoid this misconception
The claim: The peripheral nervous systemAll the nerves outside the brain and spinal cord. is only the autonomic system, and a nerve is a single axon.
The correction: Peripheral includes sensory fibres and both somatic and autonomic motor fibres, in cranial and spinal nerves. A nerve is a bundle of axons.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Which organs are not part of the peripheral nervous system?
Show suggested answer
The brain and the spinal cord. They are central. Nerves outside them are peripheral.
Practice question
Retrieval and explanation · suggested answer
What is the difference between a sensory and a motor pathway?
Show suggested answer
Sensory pathways carry impulses from receptors toward the CNS. Motor pathways carry impulses from the CNS to effectors.
Practice question
Retrieval and explanation · suggested answer
Why is pupil dilation a peripheral motor event even though you do not decide to do it?
Show suggested answer
The iris muscles are effectors supplied by autonomic motor neurones outside the central organs. Involuntary does not mean the pathway is hormonal or inside the brain only.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A cut destroys a spinal nerve but not the cord. The muscle in that body wall no longer contracts, and touch on that strip of skin is not felt. The other side is normal. Explain using sensory and motor fibres. Why does this not show that the central nervous system has been removed?
Show a hint
One nerve can contain both fibre types. The cord is still there.
Explore the reasoning
That spinal nerve carried both sensory axons from the skin and motor axons to the muscle. Cutting it stops both on that side only. The brain and cord are intact, so the central nervous system has not been removed. A reflex that needed those axons will fail on that side even though the grey matter is still there.
Recall this lesson
Lesson recall questions
Question 1 of 5: List the two divisions of the mammalian nervous system named in this specification point.
CNS and PNS
Practice question
Retrieval and explanation · suggested answer
List the two divisions of the mammalian nervous system named in this specification point.
Lesson: 8B 1: The Central Nervous System
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A scan can show the centre the nerves plug into
8B · The Peripheral Nervous SystemNIBIB’s MRI description includes the spinal cord as a soft tissue that can be pictured without ionising radiation. A spinal nerve leaving that cord is the peripheral part.…Specification 8.10Read & use the scienceClose article
Why this matters
NIBIB’s MRI description includes the spinal cord as a soft tissue that can be pictured without ionising radiation. A spinal nerve leaving that cord is the peripheral part. The page does not define somatic and autonomic paths. Use it to keep “what we can picture” separate from “how the pathways are classified”. 8.10: peripheral nerves connect receptors and effectors to the brain and spinal cord. A scan or growth response is indirect evidence: it cannot alone identify every signalling step or prove the proposed cause.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.10 Central and peripheral nervous systems.
Use the science
Why is “the MRI is the peripheral nervous system” wrong?
Explore a possible response
An MRI is a picture. The peripheral nervous system is the nerves and ganglia outside the brain and cord. The cord itself, even on a clear scan, is central.
Explore the source
- NIBIB: magnetic resonance imagingUS National Institute of Biomedical Imaging and Bioengineering
A peripheral nerve test asks where the fault lies
8B · The Peripheral Nervous SystemNINDS describes peripheral neuropathy as damage to nerves outside the brain and spinal cord. Clinicians may use nerve-conduction studies to estimate how strongly and quickly signals pass along…Specification 8.10Read & use the scienceClose article
Why this matters
NINDS describes peripheral neuropathy as damage to nerves outside the brain and spinal cord. Clinicians may use nerve-conduction studies to estimate how strongly and quickly signals pass along some peripheral nerves. This shows why the 8.10 distinction between central and peripheral systems matters beyond a labelled diagram: a problem in an arm nerve is not the same location as one in the spinal cord or brain. A slow test result still needs interpretation; it cannot identify every cause from conduction speed alone. Sensory and motor fibres may be affected differently, so symptoms and measurements should be compared.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.10 Central and peripheral nervous systems.
Use the science
Why would normal brain imaging not rule out a problem in a peripheral sensory nerve?
Explore a possible response
The nerve lies outside the brain and spinal cord. A brain image does not directly test conduction along that peripheral pathway.
Explore the source
- Peripheral neuropathyUS National Institute of Neurological Disorders and Stroke