All behavior ultimately arises from the activity of the nervous system, whose basic working unit is the neuron. A neuron has three main parts: dendrites, which receive signals from other neurons; the cell body, or soma, which processes information; and the axon, which transmits signals onward. Many axons are wrapped in a myelin sheath, a fatty insulating layer that speeds up neural transmission; damage to this myelin, as occurs in multiple sclerosis, slows signal conduction. When a neuron fires, it produces an action potential, a brief electrical charge that travels down the axon. The action potential follows an "all-or-none" principle: it either fires completely or not at all. At the end of the axon, the signal crosses a synapse, the tiny gap between one neuron's axon terminal and the next neuron's dendrite, by releasing chemical messengers called neurotransmitters.
Different neurotransmitters have distinctive roles in the brain. Dopamine is involved in reward, motivation, pleasure, and movement; imbalances are linked to Parkinson's disease on one end and to schizophrenia on the other. Serotonin regulates mood, sleep, appetite, and body temperature, and low levels are associated with depression and anxiety disorders. GABA serves as the main inhibitory neurotransmitter, reducing neural activity and helping to calm anxiety, while glutamate acts as the main excitatory neurotransmitter, supporting learning and memory, though excess glutamate can cause excitotoxicity and neural damage. Acetylcholine plays a key role in muscle contraction, attention, and memory, and the degeneration of acetylcholine-producing neurons is a hallmark of Alzheimer's disease. Endorphins, by contrast, function as natural painkillers that reduce the perception of pain and can produce feelings of euphoria, sometimes called "runner's high."
The neurons of the body are organized into the central nervous system, consisting of the brain and spinal cord, which processes and integrates information from the entire body, and the peripheral nervous system, made up of all nerves outside the brain and spinal cord. The peripheral system includes the somatic nervous system, controlling voluntary actions, and the autonomic nervous system, controlling involuntary functions such as heart rate, digestion, and breathing. The autonomic system is in turn split into the sympathetic branch, which activates the fight-or-flight response by increasing heart rate, dilating pupils, and releasing adrenaline during emergencies, and the parasympathetic branch, which promotes rest-and-digest functions, slowing heart rate and stimulating digestion once a threat has passed.
Within the brain itself, the cerebral cortex is the outer layer responsible for higher-order functions such as thinking, planning, language, and voluntary movement. It is divided into four lobes: the frontal lobe, which handles executive functions, planning, decision-making, personality, and voluntary movement; the parietal lobe, which processes sensory information; the temporal lobe, which manages hearing and language; and the occipital lobe, located at the back of the brain, which is responsible for visual processing. Deeper structures form the limbic system, which is involved in emotion, motivation, and memory. Within this system, the amygdala processes emotions, especially fear and aggression, and is critical for the fight-or-flight response. The hippocampus is essential for forming new long-term memories and for spatial navigation; damage to it, as famously seen in patient H.M., results in an inability to form new declarative memories. The hypothalamus regulates homeostasis, including body temperature, hunger, thirst, and sleep, and links the nervous system to the endocrine system via the pituitary gland. The thalamus acts as the brain's relay station, routing incoming sensory information to the appropriate cortical areas, except for smell. Finally, the cerebellum, located at the back of the brain, coordinates balance, movement, and motor learning, helping to fine-tune voluntary actions for smooth execution.