Brain

Encephalon

Also known as: encephalon

The ≈1.3 kg organ within the skull containing ≈86 billion neurons, made up of the cerebrum, diencephalon, brainstem and cerebellum; it generates perception, movement, cognition and consciousness and regulates homeostasis.

Show on body map Brain — Lateral View Brain — Midsagittal Section

Overview

Mass≈1.2–1.4 kg (♂ > ♀, unrelated to intelligence)

Neurons≈86 billion

Blood flow≈750 mL/min (15% of cardiac output)

O₂ consumption≈20% of resting body total

Glucose use≈120 g/day

Volume of CSF around and within it≈150 mL

The adult brain weighs ≈1.2–1.4 kg (≈2% of body mass) yet receives ≈15% of the cardiac output and uses ≈20% of resting oxygen. It has four major parts:
• the cerebrum: two cerebral hemispheres separated by the longitudinal fissure and joined by the corpus callosum. It is responsible for higher functions and holds the primary motor and sensory cortex.
• the diencephalon: thalamus, hypothalamus, epithalamus and subthalamus, around the third ventricle. It relays information to the cortex and controls the autonomic and endocrine systems.
• the brainstem: midbrain, pons and medulla. It carries the long tracts and holds the nuclei of cranial nerves III–XII and the vital centres.
• the cerebellum: in the posterior cranial fossa, for coordination and motor learning.

The brain floats in cerebrospinal fluid, which fills the four ventricles and the subarachnoid space. It is wrapped by the three meninges, and dural folds (the falx cerebri and tentorium cerebelli) divide the cranial cavity into compartments. Its arterial supply comes from two internal carotid arteries (anterior circulation) and two vertebral arteries joining as the basilar artery (posterior circulation). These are linked at the base of the brain by the circle of Willis. Venous blood drains through superficial and deep cerebral veins into the dural venous sinuses and then the internal jugular veins.

Because the skull is rigid, any expanding lesion (haematoma, tumour, oedema) raises intracranial pressure and can push brain tissue across the dural partitions. These herniation syndromes are neurological emergencies.

Location: Cranial cavity: cerebrum in the anterior and middle cranial fossae and above the tentorium; brainstem and cerebellum in the posterior cranial fossa.

Parts & subdivisions

Cerebrum
Two hemispheres (≈83% of brain mass) with cortex, white matter, basal nuclei and limbic structures.
Diencephalon
Thalamus, hypothalamus, epithalamus (pineal gland) and subthalamus.
Brainstem
Midbrain, pons and medulla oblongata.
Cerebellum
≈10% of brain mass but ≈80% of its neurons.
Ventricular system
Two lateral ventricles, third ventricle, cerebral aqueduct, fourth ventricle.
Embryological divisions
Forebrain (prosencephalon: telencephalon + diencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon: metencephalon — pons and cerebellum; myelencephalon — medulla).

Anatomy

Arterial supply: Internal carotid arteries → anterior and middle cerebral arteries (anterior circulation); vertebral arteries → basilar artery → posterior cerebral arteries, plus cerebellar and pontine branches (posterior circulation). The two are linked by the anterior and posterior communicating arteries (circle of Willis).
Venous drainage: Superficial cerebral veins → superior sagittal and other dural sinuses; deep veins (internal cerebral veins → great cerebral vein of Galen) → straight sinus → confluence → transverse → sigmoid sinuses → internal jugular veins.
Development: From the cranial neural tube: three primary vesicles (week 4) → five secondary vesicles (week 5). The cephalic, cervical and pontine flexures fold the tube. Brain weight at birth is ≈350–400 g (≈25% of adult) and triples by about age 2–3, mainly through growth of dendrites, synapses and myelin rather than new neurons.

Function

Perception and integration of all sensory modalities
Planning, initiation and coordination of movement
Language, memory, emotion, reasoning and personality
Control of autonomic, endocrine and homeostatic functions
Regulation of consciousness and sleep–wake cycles
Origin of cranial nerves III–XII (brainstem) and of I and II (forebrain outgrowths)

Physiology: Cerebral blood flow is ≈750 mL/min (≈50 mL/100 g/min). Grey matter receives roughly four times more than white matter. Flow is autoregulated over a wide range of arterial pressure and rises steeply with PaCO₂. The brain relies almost entirely on glucose (≈120 g/day) and oxidative metabolism; in prolonged starvation it shifts partly to ketone bodies.

Clinical significance

Brain herniation syndromes

• Subfalcine (cingulate) herniation: the cingulate gyrus shifts under the falx and may compress the ACA, causing leg weakness.
• Uncal (transtentorial) herniation: the medial temporal lobe is forced through the tentorial notch. It compresses CN III (ipsilateral fixed dilated pupil, then "down and out" eye), the cerebral peduncle (hemiparesis, sometimes ipsilateral from the contralateral peduncle being pushed against the tentorium: the Kernohan notch phenomenon) and the PCA (occipital infarction).
• Central herniation: downward displacement of the diencephalon and brainstem; it may tear the paramedian basilar perforators (Duret haemorrhages).
• Tonsillar herniation ("coning"): the cerebellar tonsils are forced through the foramen magnum, compressing the medulla and causing respiratory arrest. This is why LP is contraindicated with a posterior-fossa mass or signs of raised ICP.

Traumatic brain injury and the Glasgow Coma Scale

Severity is graded by the GCS (3–15), which scores eye opening (1–4), verbal response (1–5) and motor response (1–6). A score of ≤8 indicates severe injury and usually the need for airway protection. Diffuse axonal injury from rotational acceleration shears long tracts (corpus callosum, brainstem) and can cause coma with a near-normal CT.

Brain death

Brain death is irreversible loss of all brain function, including the brainstem. It is confirmed when there are no brainstem reflexes (pupillary, corneal, oculocephalic, oculovestibular, gag and cough) and no respiratory drive on apnoea testing despite a raised PaCO₂, in the absence of confounders such as sedatives, hypothermia or metabolic derangement.

Test yourself

Which cranial nerve is the only one to emerge from the dorsal surface of the brainstem?

Oculomotor (III)
Hypoglossal (XII)
Abducens (VI)
Trochlear (IV)

Answer: Trochlear (IV)

The trochlear nerve emerges just below the inferior colliculi, decussates, and supplies the contralateral superior oblique. It is also the thinnest cranial nerve and has the longest intracranial course.