SIMPLY: THE BRAIN

COMPLEX CORTEX

The cerebral cortex is the largest part of the human brain. Its multiple folds allow more cortical tissue to fit inside the skull. The cortex carries out the brain's most complex information processing, and its massive expansion in humans is elemental to the sophistication of human intelligence. Divisible into over 50 regions, each with a unique internal organization and different connections to other brain regions, the cortex is conventionally divided into four main lobes. Each lobe has characteristic primary functions.

GENERATING RESPONSES

The limbic system is a collection of structures that lie in the brain's interior. It is elemental to generating and controlling our emotional lives and for forming memories. The powerful interplay between smells, memories, and feelings arises here. The limbic structures are highly connected to the cortex and the brain's reward centres and control our motivational states and goal-directed behaviours. To help do this, the limbic system also has a large influence on the body's hormonal system.

SURVIVAL MACHINE

The brain must monitor a person's most basic needs: Am I hungry or thirsty? Am I full? Am I too hot or too cold? Is there enough oxygen in my blood? Do I need the toilet? Once these basic physiological needs are recognized, the brain instigates a behavioural response to seek food, water, or shelter. It also induces unconscious physiological responses, such as directing the body to breathe deeper, to sweat, or to shiver. These coordinated responses act to avert bodily harm.

BRAIN CELLS

Neurons are the cells in the brain that relay and process information. There are around 86 billion in a human brain. Neurons have a cell body from which wiry processes extend to form connections with other neurons. On one side, neurons have short dendrites, which collect incoming signals. On the other, a neuron has a very long axon, along which outgoing signals travel.

MAKING CONNECTIONS

The junction between two neurons is called a synapse. Neurons are seperated by a synaptic cleft: a gap 10,000 times thinner than a human hair. An electrical impulse in the first (presynaptic) neuron causes the release of chemical messengers, known as neurotransmitters, which cross the synaptic cleft to the second (postsynaptic) neuron. They attach to receptors and change the electrical activity of that neuron, thus passing the message on.

PASSING MESSAGES

Information travels around the brain via electrical pathways formed by axons. Each neuron has one of these long tendrills extending from it. Changes in the voltage across the surface of a neuron's dendrites and cell body generate brief electrical spikes - called action potentials - that move down the axon. Then, when action potentials reach the axon's outgoing synapses, they cause the release of chemical neurotransmitters. These diffuse to the receiving neuron, binding to receptors that change the voltage across that neuron's dendrites. A neuron's combined synaptic inputs make it more or less likely, in turn, to fire action potentials.

Some cerebellum neurons receive as many as 200,000 inputs on their extensively branched dendrites.

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