Brain · Grey Matter

The brain (the encephalon) is everything of the central nervous system that sits inside the skull: the cerebrum with its two hemispheres, the cerebellum behind and below it, and the brainstem that continues into the spinal cord.

Brain. The brain (the encephalon) is everything of the central nervous system that sits inside the skull: the cerebrum with its two hemispheres, the cerebellum behind and below it, and the brainstem that continues into the spinal cord.

An adult brain weighs about 1.2 to 1.4 kg, roughly 2 % of body weight, with an average volume near 1,260 cm³ in men and 1,130 cm³ in women and a wide normal range around those means. The two hemispheres are joined across the midline by the corpus callosum, the largest bundle of fibres between them. Its tissue comes in two kinds that look different to the eye: grey matter, where the cell bodies and synapses sit, and white matter, the cabling of myelinated axons between them.

The divisions follow development. The forebrain gives the cerebral hemispheres (the cerebral cortex, the basal ganglia, the hippocampus and amygdala) and the diencephalon (thalamus and hypothalamus); the midbrain, pons and medulla form the brainstem; the cerebellum grows from the hindbrain beside the pons.

Mass and neurons are distributed very unevenly. The cerebellum is about a tenth of the mass and holds about 80 % of the roughly 86 billion neurons; the cerebral cortex is most of the mass and holds about 16 billion.

Every function sits in a network rather than a single place. The lobes and structures named on these cards are useful addresses, and each card says which other regions the function needs.

A city map is a fair picture of the atlas.

Districts have characters (the port, the market, the government quarter), but nothing happens in one district alone, and the roads between them matter as much as the buildings.

Questions: Is there a simple rule of thumb for where a function lives in the brain? A rough one works: senses arrive at the primary areas (vision at the back, touch behind the central sulcus, hearing on the upper temporal lobe, taste and the body's state in the insula), and movement leaves from in front of the central sulcus, refined by the cerebellum and selected by the basal ganglia. Rules and control sit in the prefrontal cortex, meaning in the temporal lobe, space and number in the parietal lobe, emotion in the amygdala, insula, cingulate and ventromedial prefrontal cortex, and vital functions in the brainstem, hypothalamus and thalamus. Every real task recruits several of these at once with overlap, so the rule says where to look first and never where a function is confined. Which part of the brain holds most of its neurons? The cerebellum does, although it is only about a tenth of the brain's mass: counts by Azevedo, Herculano-Houzel and colleagues found about 69 billion of the brain's roughly 86 billion neurons there, about 80 %. Most of them are granule cells, the smallest neurons in the brain, packed in the cerebellar cortex at enormous density. The cerebral cortex, which makes up most of the mass, holds about 16 billion neurons, its bulk going into large cells, long connections and white matter.