Whether the forehead wrinkles decides whether you are looking at a stroke or a nerve — and that is anatomy, not neurology.
Neuroanatomy has a reputation for being the least learnable part of the course. It becomes learnable when each fact is attached to a sign you will actually see.
This chapter covers the three areas the examination concentrates on: the cranial nerves that produce recognisable deficits, the ventricular system and what happens when it blocks, and why a stroke in one place looks different from a stroke in another.
The cranial nerves worth knowing cold
Twelve cranial nerves, of which a handful account for nearly all clinical questions.
III — oculomotor. Supplies most of the eye movements and the eyelid, and carries the fibres that constrict the pupil. A complete palsy gives an eye that is down and out, with drooping of the lid.
The clinically decisive detail is the pupil. The pupil-constricting fibres run on the outside of the nerve, so a compressing lesion — an aneurysm, or a brain being pushed against the nerve by rising pressure — reaches them first and produces a dilated pupil. A third nerve palsy with a dilated pupil is a surgical emergency until proven otherwise, while one that spares the pupil more often reflects small-vessel disease. That distinction is anatomy doing clinical work.
V — trigeminal. Sensation to the face, and the muscles of chewing. It is the afferent limb of the corneal reflex, which is why a trigeminal lesion abolishes the reflex on that side even though the blink itself is a facial nerve function.
VII — facial. The muscles of facial expression — dealt with below, because it deserves its own section.
VIII — vestibulocochlear. Hearing and balance. Unilateral sensorineural hearing loss with imbalance is the combination that needs explaining rather than reassurance, as the ENT chapter sets out.
IX and X — glossopharyngeal and vagus. Palate, swallow and voice. Their failure produces an unsafe swallow, which is why swallowing is assessed before anything is given by mouth after a stroke — the aspiration that follows a missed assessment causes pneumonia and death.
The forehead rule
This is the single highest-yield fact in the chapter, and it comes directly from how the facial nerve nucleus is wired.
The part of the nucleus supplying the forehead receives input from both hemispheres. The part supplying the lower face receives input from the opposite hemisphere only.
Everything follows:
A stroke damages one hemisphere's input. The lower face is weak because it had only that one supply — but the forehead still works, because the other hemisphere is still supplying it. Forehead spared.
A lesion of the nerve itself, after everything has joined together — Bell's palsy, a lesion in the ear, a parotid tumour — knocks out the whole side. Forehead involved.
💡 And one management point that is routinely forgotten in the excitement of making the diagnosis: check whether the eye closes. A facial palsy that prevents complete eye closure leaves the cornea exposed, and an exposed cornea ulcerates and can perforate. That patient needs eye protection — lubrication and taping at night — arranged the same day, whatever the cause of the palsy turns out to be.
The ventricles and hydrocephalus
Cerebrospinal fluid is produced inside the ventricles, flows through a series of progressively narrower channels, escapes around the outside of the brain and spinal cord, and is absorbed there.
Production continues regardless of whether flow is obstructed. That single fact is why any block raises pressure: fluid keeps arriving behind an obstruction that will not let it past.
Two patterns follow:
Obstruction within the system — a tumour, or a channel that is congenitally narrow. The ventricles above the block enlarge; those below do not.
Failure of absorption — typically after meningitis or a subarachnoid bleed, where inflammation or blood scars the absorbing surfaces. The whole system enlarges.
The paediatric difference matters clinically: an infant's skull can still expand, because the sutures have not fused. So instead of pressure rising steeply, the head grows — which is precisely why head circumference is measured and plotted at every check. In an older child or adult the skull is fixed, so the same process produces headache, vomiting and reduced consciousness instead.
Stroke: the deficit tells you the vessel
The body is represented along the surface of the motor cortex in an orderly strip — conventionally described as a homunculus — with the leg at the top and over the midline, and the face and hand at the bottom, near the side.
Arteries supply different parts of that strip, so the pattern of weakness identifies the vessel:
Middle cerebral artery supplies the lateral surface, where face and arm are represented. So the deficit is face and arm worse than leg, with speech disturbance when the dominant hemisphere is involved.
Anterior cerebral artery supplies the medial surface, where the leg sits. So the deficit is leg worse than arm.
Notice that you are not memorising a table — you are reading a map. Given the homunculus and which surface each artery reaches, the pattern is derivable.
Why a tiny lesion can paralyse a whole side
Fibres leaving the cortex spread across its whole surface, then converge into a narrow band — the internal capsule — before continuing down. Everything that was spread over a wide area is now packed into a small one.
So a small infarct in the internal capsule damages fibres for the face, arm and leg together, producing a dense weakness of the entire opposite side. And because the lesion is below the cortex, there is typically no speech disturbance and no visual field loss — which is what distinguishes it clinically from a large cortical stroke that would produce the same weakness plus those extra deficits.
The general principle is worth carrying beyond this example: the damage a lesion does depends on how tightly packed the fibres are, not on how big the lesion is. That is why a very small brainstem stroke can be devastating — the brainstem contains motor fibres, sensory fibres, cranial nerve nuclei and the pathways controlling consciousness and breathing, all within a few centimetres.
Two things from embryology that explain clinical findings
The neural tube closes early — often before a pregnancy is recognised. Failure of that closure produces spina bifida and anencephaly. This is the anatomical reason folic acid must be taken before conception rather than started at the booking visit, a point the prescribing and antenatal chapters make from the other direction.
The fetal circulation has shunts that must close after birth. Before birth the lungs are not used, so blood bypasses them through openings between the atria and between the pulmonary artery and the aorta. At birth those should close. When one does not, or when a lesion depends on it staying open, the newborn presents in the way the paediatric cardiology chapter describes — well at first, then collapsing as the duct closes over the first days.
Putting it together
A dilated pupil with a third nerve palsy suggests compression, because those fibres run on the outside of the nerve.
Forehead spared means stroke; forehead involved means the nerve — and always check the eye closes.
CSF production continues whatever blocks the flow, so a block raises pressure; in a baby the head grows instead.
Face and arm means middle cerebral; leg means anterior cerebral, read off the homunculus rather than memorised.
A tiny internal capsule lesion paralyses a whole side, because packing rather than size determines damage.
The neural tube closes before most women know they are pregnant, which is why folate must precede conception.
No CSF volumes or pressures, coma scores or vessel calibres appear here: those belong with the clinical chapters that own the procedures. Use your local protocol.
Syllabus points
The cranial nerve signs that are actually tested
Dilated pupil in third nerve palsy means compression
The forehead rule — stroke vs facial nerve
Check the eye closes; the cornea is at risk
CSF flow, obstruction and absorption failure
Why a baby's head grows instead
Homunculus and stroke territory
Internal capsule; packing rather than size
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