2.6.1 General brain anatomy, cerebral lobes, limbic system, cranial nerves and spinal cord

Orientation and basic terms

• The neuraxis bends at the midbrain. In the forebrain, dorsal means superior and ventral means inferior. In the spinal cord, dorsal means posterior and ventral means anterior.

• Grey matter holds cell bodies, dendrites and synapses, and white matter holds the axons that connect them.

• A cluster of cell bodies is a nucleus inside the central nervous system and a ganglion outside it.

• Gyri are the raised ridges of the cortical surface, and sulci are the grooves between them.

Divisions of the brain

The brain is divided into forebrain, midbrain and hindbrain. The forebrain is the cerebrum and the diencephalon.

Diencephalon

The diencephalon surrounds the third ventricle.

• The thalamus is the largest part of the diencephalon. It relays sensory, motor and limbic information to the cortex.

• The lateral geniculate nucleus is a thalamic relay in the visual pathway, between the optic tract and the optic radiation.

• The hypothalamus lies below the thalamus and controls endocrine and autonomic function.

• The mammillary bodies are paired swellings on the underside of the posterior hypothalamus. They receive the fornix and project to the anterior thalamus.

• The subthalamus lies inferolaterally and connects with the basal ganglia.

• The pineal gland is an unpaired epithalamic structure that often calcifies. It, rather than the suprachiasmatic clock, makes melatonin in darkness.

• Parinaud's syndrome is upgaze palsy with light-near dissociation. A pineal mass above the tectum causes it by compressing the dorsal midbrain.

The hippocampus is limbic cortex of the temporal lobe. It is not part of the diencephalon.

Brainstem

The brainstem lies below the forebrain and contains long tracts, cranial nerve nuclei and reticular systems.

• Opioids cause nausea through the area postrema. Antipsychotics are antiemetic because they block dopamine D2 receptors there.

Cerebellum

The cerebellum lies behind the brainstem.

• The vermis is the midline strip that joins the two cerebellar hemispheres.

• Folia are the fine transverse gyri of the cerebellar surface. The arbor vitae is the branching white matter core seen on a sagittal cut.

• The flocculonodular lobe (vestibulocerebellum) works with the vestibular system for balance and posture.

• The dentate nucleus is the largest deep cerebellar nucleus and is buried in the white matter of each hemisphere. Its output leaves through the superior cerebellar peduncle to the red nucleus and thalamus.

Midline partitions

• The septum pellucidum is a thin midline sheet beneath the corpus callosum. It divides the frontal horns of the two lateral ventricles.

• The falx cerebri is a fold of dura in the longitudinal fissure between the cerebral hemispheres. It is distinct from the falx cerebelli, the tentorium cerebelli and the diaphragma sellae.

Lobes and sulci

• The central sulcus (fissure of Rolando) separates the frontal lobe in front from the parietal lobe behind.

• The lateral sulcus is the same as the Sylvian fissure. It separates the frontal and parietal lobes above from the temporal lobe below, and both Broca's and Wernicke's areas border it.

• The insula is true cortex: a lobe buried in the lateral sulcus beneath the opercula, not a subcortical nucleus. It processes taste, visceral sensation and interoception.

Brodmann put the human prefrontal region at roughly 29 to 30% of the cortical surface.

Cortical structure

• Korbinian Brodmann divided the whole cerebral cortex into numbered cytoarchitectonic areas, based on cell type and layering.

• Pyramidal cells are the main cortical projection neurones and make up roughly 70 to 80% of neocortical neurones.

• Betz cells are giant pyramidal neurones in layer V of the primary motor cortex (area 4). They are the largest neurones in the nervous system.

• Layer IV (the internal granular layer) consists mainly of stellate cells and is the main target of thalamocortical afferents.

Primary cortical areas

Tap to enlarge

• A unilateral motor cortex lesion causes contralateral upper motor neurone weakness with raised tone, because corticospinal fibres cross at the medullary pyramids.

Parietal gyri

• The superior parietal lobule combines touch, proprioception and vision into a representation of the body and space.

• The supramarginal gyrus (area 40) handles phonological processing and lies next to the angular gyrus.

• The angular gyrus (area 39) sits in the inferior parietal lobule. It caps the superior temporal sulcus near the parieto-occipital junction.

• The dominant angular gyrus supports reading, writing, naming, word comprehension and the retrieval of arithmetic facts. It links visual and auditory word forms to meaning.

Tap to enlarge

Temporal and occipital gyri

• The superior temporal gyrus holds the primary auditory cortex and, on the left, Wernicke's area.

• Reduced left superior temporal gyrus volume is among the most replicated structural findings in schizophrenia. It goes with more severe hallucinations and thought disorder.

• The parahippocampal gyrus is marked by the uncus and the entorhinal cortex. Its anterior part feeds neocortical input to the hippocampus through the perforant path, and the gyrus helps memory and the recognition of scenes and places.

• The uncus is the hooked anterior end of the parahippocampal gyrus, lying over the amygdala. Olfactory auras arise here, because the content of an aura shows where a seizure starts; temporal seizures may also produce déjà vu.

• The fusiform gyrus is the lateral occipitotemporal gyrus, bounded by the occipitotemporal and collateral sulci. Its face area serves face recognition, and its left-sided part handles written words.

• The lingual gyrus lies below the calcarine sulcus and above the collateral sulcus, and supports visual memory and imagery. Solms showed that medial occipitotemporal damage here abolishes the visual content of dreams.

Hemispheric dominance and asymmetry

• Language dominance is left-sided in about 99% of right-handers and about two-thirds of left-handers. About one-third of left-handers therefore have right-sided or mixed dominance.

• Language follows the dominant hemisphere, not the left side as such. A right-sided Wada result puts Wernicke's area on the right, and handedness alone cannot establish dominance.

• Memory loss after a one-sided lesion is material-specific. Removing the right hippocampus impairs memory for faces, designs and routes.

Planum temporale

• The planum temporale lies on the upper surface of the superior temporal gyrus, behind Heschl's gyrus, and forms part of Wernicke's area.

• Leftward asymmetry: in most healthy right-handed brains, the planum temporale and Heschl's gyrus are larger on the left. The planum temporale shows the clearest structural asymmetry.

• In many people with developmental dyslexia, this leftward asymmetry is reduced or absent. In schizophrenia, it is reduced or reversed.

Language areas and aphasias

• Broca's area lies in the posterior inferior frontal gyrus of the dominant hemisphere. It covers the pars opercularis and triangularis, which are areas 44 and 45.

• Wernicke's area lies in the posterior superior temporal gyrus of the dominant temporal lobe, not the frontal lobe. It corresponds to area 22, whereas primary auditory cortex is area 41.

• Wernicke's area is the main cortical site for language comprehension.

• The arcuate fasciculus connects the posterior temporal and inferior frontal language areas.

• Arterial supply: both language areas lie in middle cerebral artery territory, which is part of the anterior circulation. Carotid-territory ischaemia therefore produces aphasia.

Tap to enlarge

Fluency is what separates Broca's aphasia from Wernicke's aphasia.

• The transcortical aphasias (extrasylvian or watershed aphasias) spare the perisylvian circuit of Broca's area, arcuate fasciculus and Wernicke's area. Repetition therefore survives.

• Nominal (anomic) aphasia is a pure naming deficit. It localises to the dominant angular gyrus.

Parietal syndromes

• Ideomotor apraxia and Gerstmann signs come from the dominant side, unlike neglect and dressing apraxia.

• Acalculia follows a dominant angular gyrus lesion because that gyrus retrieves arithmetic facts. Do not confuse the angular gyrus with the adjacent supramarginal gyrus.

Visual recognition and visual fields

• Pure alexia is a disconnection syndrome. The right hemianopia and splenial damage keep written words from reaching the left language areas.

• Meyer's loop is the part of the optic radiation that carries the upper opposite visual field through the temporal lobe.

• The calcarine cortex receives the opposite half field from both eyes, whichever hemisphere is dominant.

Tap to enlarge

Frontal lobe

The prefrontal regions lie in front of the motor cortex.

• The dorsolateral prefrontal cortex lies on the lateral frontal convexity, in middle cerebral artery territory.

• Orbitofrontal damage often follows head injury that bruises the floor of the anterior cranial fossa. Formal tests and language stay normal.

• Personality does not lateralise to one hemisphere.

• Akinetic mutism leaves the patient awake and tracking with the eyes, but silent and still. It follows bilateral anterior cerebral artery infarction and comes with leg-predominant weakness.

Basal ganglia, cerebellum and thalamus

• The basal ganglia select and release a movement and suppress its rivals, working through cortico-striato-thalamic loops. This is why Parkinson's disease slows the initiation of movement.

• The nucleus accumbens is the ventral striatum, where the head of the caudate and the putamen join ventrally. It therefore belongs to the basal ganglia.

• The nucleus accumbens receives mesolimbic dopamine from the ventral tegmental area. A dopamine rise there signals reward and reinforcement, which is how most drugs of misuse act.

• Subthalamic nucleus infarction causes contralateral hemiballismus, with large, flinging proximal movements. It does this by releasing the thalamus and motor cortex from indirect pathway inhibition.

• The cerebellum compares the intended movement with feedback from the limb and smooths the movement.

• Cerebellar damage gives ipsilateral ataxia, intention tremor, past-pointing, hypotonia and a pendular knee jerk.

• A lesion of the ventral posterior thalamus, the thalamic sensory relay, causes contralateral loss of every sensory modality over the face, arm and leg. Two-point discrimination and stereognosis are lost secondarily.

The limbic system

• The limbic system includes the hippocampal and parahippocampal regions, the amygdala, the cingulate gyrus, the hypothalamus, and the anterior and mediodorsal thalamic nuclei.

• The fornix, cingulum, mammillothalamic tract and stria terminalis connect these structures.

• The cingulate gyrus arches above the corpus callosum.

• The hippocampus lies in the medial temporal lobe, in the floor of the temporal horn of the lateral ventricle. It lays down new declarative memories.

• Place cells (described by O'Keefe) give the hippocampus a map of space.

• The amygdala lies deep in the anterior medial temporal lobe, beneath the uncus and in front of the hippocampus. It belongs to the cerebrum (telencephalon) and drives the hypothalamus through the stria terminalis.

• Learned fear: in classical conditioning, the amygdala forms and expresses learned fear through its lateral and central nuclei. The hippocampus records the episode and its context.

• Kluver-Bucy syndrome follows bilateral amygdala damage, for example after herpes simplex encephalitis. It causes placidity, loss of fear, hyperorality and hypersexuality.

• Olfaction is the only sense whose input reaches primary cortex without a thalamic synapse. It travels in the lateral olfactory tract to the piriform cortex.

Tap to enlarge

Diencephalic amnesia

• Wernicke's encephalopathy: thiamine deficiency damages the mammillary bodies. In Korsakoff's syndrome, the mammillary bodies look atrophic on magnetic resonance imaging (MRI).

• The mediodorsal thalamic nucleus carries the lesion most closely tied to the persistent amnesia of Korsakoff syndrome, rather than the hippocampus.

Hypothalamus

• Ventromedial lesions or tumours cause hyperphagia and obesity. Destroying the lateral hypothalamic area causes food refusal.

• Dopamine D2 blockade removes the arcuate brake on prolactin.

Arterial supply

Tap to enlarge

• The hippocampal formation is supplied chiefly by hippocampal branches of the posterior cerebral artery, with a smaller contribution from the anterior choroidal artery.

Brainstem vascular syndromes

• Locked-in syndrome follows a ventral pontine infarct from basilar occlusion. The infarct cuts the corticospinal and corticobulbar fibres, leaving quadriplegia and anarthria.

• In locked-in syndrome, awareness stays intact and vertical eye movements usually remain. This is because the dorsal arousal system and the midbrain vertical gaze centres are spared.

• Lateral medullary (Wallenberg) syndrome follows occlusion of the posterior inferior cerebellar artery or its parent vertebral artery.

• Its features are bulbar palsy, ipsilateral Horner syndrome, crossed loss of pain and temperature (ipsilateral face, contralateral body), ipsilateral ataxia and vertigo. Power is spared.

• Subclavian steal arises from left subclavian stenosis proximal to the origin of the vertebral artery. Using the arm reverses vertebral flow, causing vertigo and diplopia.

• Subclavian steal also gives a weak, delayed pulse and an inter-arm systolic pressure gap above 20 mmHg.

Tap to enlarge

Cerebrospinal fluid

Flow

The choroid plexus secretes cerebrospinal fluid (CSF), which flows in this order:

1. From the lateral ventricles through the interventricular foramina (of Monro) to the third ventricle.

2. Through the cerebral aqueduct (of Sylvius) to the fourth ventricle.

3. Out of the fourth ventricle through the single midline median aperture (of Magendie) to the cisterna magna, and through the paired lateral foramina (of Luschka), into the subarachnoid space.

4. Through the arachnoid granulations into the venous sinuses.

Tap to enlarge

• A colloid cyst at the front of the third ventricle can block the foramen of Monro. It causes positional headache and sudden collapse.

• The cerebral aqueduct runs through the midbrain beneath the tectum. Aqueduct stenosis is a common cause of non-communicating hydrocephalus in infancy.

• Arachnoid granulations are outpouchings of arachnoid through the dura into the dural venous sinuses, chiefly the superior sagittal sinus. Through them, CSF crosses one way into venous blood.

• Non-communicating hydrocephalus follows a block within the ventricular pathway. Fluid collects upstream of the block, so the pattern of ventricular enlargement shows where the block is.

• Communicating hydrocephalus follows failure of the arachnoid granulations, for example after subarachnoid blood.

Composition

Cranial nerves

Tap to enlarge

Skull foramina and cranial fossae

• The petrous ridge marks the border between the middle and posterior cranial fossae.

• The cribriform plate of the ethmoid bone lies in the anterior cranial fossa.

• The optic canal passes through the lesser wing of the sphenoid. Both the optic canal and the foramen ovale, which lies in the greater wing of the sphenoid, open into the middle fossa.

• The jugular foramen and the foramen magnum open into the posterior fossa.

• The foramen magnum lies in the occipital bone and transmits the medulla, the spinal cord and the vertebral arteries.

• The foramen spinosum carries the middle meningeal artery, which is linked with extradural haematoma. It is distinct from the adjacent foramen ovale.

• The facial nerve leaves the skull through the stylomastoid foramen, which lies between the styloid and mastoid processes.

• The trigeminal nerve is the largest cranial nerve. Its mandibular division is the only division with motor fibres.

Tap to enlarge

Reflexes

Each cranial nerve reflex has a separate afferent limb and efferent limb, often in different nerves.

• The vestibulo-ocular reflex uses nerves III, IV and VI as its motor limb, because it moves the eyes in every plane.

Eye movement palsies and pupils

Tap to enlarge

• Argyll Robertson pupil is light-near dissociation: the pupil still constricts to a near target but loses direct and consensual constriction to light. It comes from a lesion in the midbrain light reflex arc.

Lower cranial nerves

• A lower motor neurone vagus lesion weakens the palate, pharynx and larynx, causing hoarseness and dysphagia. The vagal motor fibres arise from the nucleus ambiguus; the glossopharyngeal nerve does not supply this motor function.

• Jugular foramen lesions affect nerves IX to XI, causing dysphagia, hoarseness, a reduced gag and a weak shoulder shrug.

• Accessory nerve injury weakens the shoulder shrug by weakening the trapezius.

• A lower motor neurone hypoglossal lesion makes the protruded tongue deviate towards the side of the lesion, with wasting and fasciculation on that side. The intact genioglossus pushes the tip of the tongue over to the weak side.

• An upper motor neurone hypoglossal lesion makes the tongue deviate away from the side of the lesion.

Tap to enlarge

Auditory pathway

• The auditory pathway begins where the hair cells of the organ of Corti synapse on afferent fibres of the vestibulocochlear nerve, which carry the signal to the cochlear nuclei.

• The inferior colliculus feeds the medial geniculate nucleus, which relays from the auditory brainstem to the primary auditory cortex.

Spinal cord

• Spinal segments are cervical, thoracic, lumbar, sacral and coccygeal, each with paired roots. Dorsal roots carry sensory input and hold the sensory ganglia, and ventral roots carry motor output.

• The grey matter forms dorsal sensory horns and ventral motor horns. Lateral horns in the thoracic and upper lumbar cord contain preganglionic sympathetic neurones.

Tap to enlarge

• The fasciculus gracilis carries the leg and lower trunk below about T6. The fasciculus cuneatus carries the arm and upper trunk.

• Corticospinal fibres run from the motor cortex through the internal capsule, cerebral peduncle and pons to the medullary pyramids.

• Lesion level: a corticospinal lesion above the pyramidal crossing causes contralateral weakness. A lateral corticospinal tract lesion in the cord causes ipsilateral weakness below the injury.

• Cord hemisection causes ipsilateral weakness and loss of vibration and proprioception. Pain and temperature are lost on the opposite side, starting slightly lower.