Scientists divide the nervous system into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS, which includes the brain and spinal cord, receives, processes, interprets, and stores information and sends out messages destined for muscles, glands, and organs.

The peripheral nervous system consists of the somatic nervous system, which permits sensation and voluntary actions, and the autonomic nervous system, which regulates blood vessels, glands, and internal (visceral) organs. The autonomic system usually functions without conscious control. The autonomic nervous system is further divided into the sympathetic nervous system, which mobilizes the body for action, and the parasympathetic nervous system, which conserves energy.

Neurons are the basic units of the nervous system. They are held in place by glial cells, which nourish, insulate, protect, and repair neurons.

Each neuron consists of dendrites, a cell body, and an axon. In the peripheral nervous system, axons (and sometimes dendrites) are collected together in bundles called nerves. Many axons are insulated by a myelin sheath that speeds up the conduction of neural impulses and prevents signals in adjacent cells from interfering with one another.

Research has disproven two old assumptions: that neurons in the human central nervous system cannot be induced to regenerate and that no new neurons form after early infancy. Embryonic stem cells are pluripotent, meaning that they can generate many different kinds of cells in the body. These stem cells in various organs, including in brain areas associated with learning and memory, continue to divide and mature throughout adulthood, giving rise to new neurons. A stimulating environment seems to enhance this process of neurogenesis.

Communication between two neurons occurs at the synapse. When a wave of electrical voltage (action potential) reaches the end of a transmitting axon, neurotransmitter molecules are released into the synaptic cleft. When these molecules bind to receptor sites on the receiving neuron, that neuron becomes either more or less likely to fire.

Neurotransmitters play a critical role in mood, memory, and psychological well-being. Serotonin, dopamine, acetylcholine, and norepinephrine systems travel different paths through the brain; GABA and glutamate are distributed through the entire brain. Hormones, produced mainly by the endocrine glands, affect and are affected by the nervous system. Neuroscientists are especially interested in melatonin, which promotes sleep and helps regulate bodily rhythms; oxytocin and vasopressin, which play a role in attachment and trust; adrenal hormones such as epinephrine and norepinephrine, which are involved in emotions and stress; and the sex hormones, which are involved in the physical changes of puberty, the menstrual cycle (estrogens and progesterone), sexual arousal (testosterone), and some nonreproductive functions, including mental functioning.

In the lower part of the brain, in the brain stem, the medulla controls automatic functions such as heartbeat and breathing, and the pons is involved in sleeping, waking, and dreaming. The reticular activating system (RAS) screens incoming information and is responsible for alertness. The cerebellum contributes to balance and muscle coordination, and plays a role in cognitive and emotional learning.

The thalamus directs sensory messages to appropriate higher centers in the brain. Smell is the only sense that bypasses the thalamus, with specialized cells located instead in the olfactory bulb.

The hypothalamus is involved in emotion and in drives associated with survival. It also controls the operations of the autonomic nervous system and sends out chemicals that tell the pituitary gland when to “talk” to other endocrine glands.

The amygdala is responsible for evaluating sensory information and quickly determining its importance, and thus your initial decision to approach or withdraw from a person or situation. It is also involved in forming and retrieving emotional memories.

The hippocampus moderates the reticular activating system. It has also been called the “gateway to memory” because it plays a critical role in the formation of long-term memories for facts and events and other aspects of memory.

Much of the brain's circuitry is packed into the cerebrum, which is divided into two hemispheres (connected by the corpus callosum) and is covered by thin layers of cells known collectively as the cerebral cortex.

The occipital, parietal, temporal, and frontal lobes of the cortex have specialized (but partially overlapping) functions. The association cortex appears to be responsible for higher mental processes. The frontal lobes, particularly areas in the prefrontal cortex, are involved in social judgment, making and carrying out plans, and decision making.

Studies of patients who have had split-brain surgery (a severing of the corpus callosum) show that the two cerebral hemispheres have somewhat different talents. In most people, language is processed mainly in the left hemisphere, which generally is specialized for logical, symbolic, and sequential tasks. The right hemisphere is associated with visual–spatial tasks, facial recognition, and the creation and appreciation of art and music.

In most mental activities the two hemispheres cooperate as partners, with each making a valuable contribution. The brain is more like an interactive federation than a house divided.

The brain's circuits are not fixed and immutable but are continually changing in response to information, challenges, and changes in the environment, a phenomenon known as plasticity. In some people who have been blind from an early age, brain regions usually devoted to vision are activated by sound—a dramatic example of plasticity.

Brain scans and other techniques have revealed many male–female differences in brain anatomy and function. Controversy exists, however, about what such differences mean in real life. Some of the brain research has focused on behavioral or cognitive differences that are small and insignificant. Some findings have been widely accepted but then have failed to replicate. Some male–female brain differences may be related to differences in behavior or performance, whereas others may not. Finally, sex differences in experience could affect brain organization and function rather than the other way around.