The Brain

Lore & Background

Invertebrate brains arise from paired segmental ganglia of the ventral nerve cord, while vertebrate brains develop axially from the midline dorsal nerve cord as a vesicular enlargement at the rostral end of the neural tube. All vertebrate brains can be embryonically divided into three parts: the forebrain, midbrain, and hindbrain. The spinal cord, which coordinates somatic functions below the head, is a caudal extension of the myelencephalon. Together, the brain and spinal cord constitute the central nervous system in all vertebrates.

Reader's Guide

The brain exerts centralized control over a body's other organs by generating patterns of muscle activity and driving the secretion of hormones. This allows rapid and coordinated responses to changes in the environment. Some basic responses such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior requires the information-integrating capabilities of a centralized brain.

Did You Know?

• In humans, the cerebral cortex contains approximately 14–16 billion neurons, and the cerebellum contains an estimated 55–70 billion neurons.
• Each neuron is connected by synapses to several thousand other neurons, typically communicating via dendrites and axons.
• The human brain has been estimated to contain approximately 100 trillion synapses.
• Activity-dependent modification of synapses is widely believed to be the brain's primary mechanism for learning and memory.