Everything psychological is simultaneously biological. The links between biology and behavior are a key part of the biopsychosocial approach.

Brain plasticity allows us to adapt to new environments; our brain is a work in progress.

Biological psychologists study the links between biological (genetic, neural, hormonal) and psychological processes. In cognitive neuroscience, people from many fields join forces to study the brain activity linked with cognition.

Neurons (nerve cells) are the basic building blocks of the nervous system.

A neuron has dendrites (extensions of the cell body) that receive messages and an axon that sends messages to other neurons or to muscles and glands. Some axons are encased in a myelin sheath, which enables faster communication. Glial cells provide myelin, and they support, nourish, and protect neurons; they may also play a role in learning, thinking, and memory.

An action potential is a nerve impulse—a brief electrical charge that travels down an axon.

Neurons transmit information in a chemistry-to-electricity process, sending action potentials down their axons. They receive incoming excitatory or inhibitory signals through their dendrites and cell body.

Neurons fire in an all-or-none response when combined incoming signals are strong enough to pass a minimum threshold. A brief refractory period follows.

The response triggers a release of chemical messengers (neurotransmitters) across the tiny gap (synapse) separating a sending neuron from a receiving cell.

Specific neurotransmitters, such as serotonin and dopamine, travel designated pathways in the brain. Neurotransmitters affect particular behaviors and emotions, such as hunger, movement, and arousal.

Endorphins are natural opiates released in response to pain and intense exercise.

Drugs and other chemicals affect brain chemistry at synapses.

The nervous system’s two major divisions are the central nervous system (CNS)—the brain and spinal cord—and the peripheral nervous system (PNS)—the sensory and motor neurons connecting the CNS to the rest of the body.

Interneurons communicate within the brain and spinal cord and between motor neurons and sensory neurons.

In the PNS, the somatic nervous system controls voluntary movements of the skeletal system. The autonomic nervous system (ANS) controls the involuntary muscles and the glands. The subdivisions of the ANS are the sympathetic nervous system (which arouses) and the parasympathetic nervous system (which calms).

The endocrine system is the body’s slower information system. Its glands secrete hormones into the bloodstream, which influence brain and behavior.

In times of stress or danger, the autonomic nervous system (ANS) activates the adrenal glands’ fight-or-flight response.

The pituitary (the endocrine system’s master gland) triggers other glands, including sex glands, to release hormones, which then affect the brain and behavior. This complex feedback system reveals the interplay between the nervous and endocrine systems.

To study the brain, researchers consider the effects of brain damage.

They also use MRI scans to reveal brain structures.

Researchers use EEG recordings and PET and fMRI (functional MRI) scans to reveal brain activity.

The brainstem, the oldest part of the brain, controls automatic survival functions.

The medulla controls heartbeat and breathing. Just above the medulla, the pons helps coordinate movements and control sleep.

The thalamus, sitting at the top of the brainstem, acts as the brain’s sensory control center.

The reticular formation controls arousal.

The cerebellum, attached to the rear of the brainstem, helps process sensory input, coordinate muscle movement, and enable nonverbal learning and memory.

The limbic system is linked to emotions, drives, and memory, and its neural centers include the amygdala, hypothalamus, and hippocampus:

The cerebral cortex has two hemispheres, and each hemisphere has four lobes:

The motor cortex (at the rear of the frontal lobes) controls voluntary muscle movement.

The somatosensory cortex (at the front of the parietal lobes) registers and processes body touch and movement sensations.

The cerebral cortex is mostly association areas, which are involved primarily in higher-level functions, such as learning, remembering, thinking, and speaking. Higher-level functions require the coordination of many brain areas.

Association areas, vast in humans, interpret, integrate, and act on sensory information and link it with stored memories. More intelligent animals have larger association areas.

Evidence from brain damage shows that the neurons in association areas are busy with higher mental functions, so a bullet would not land in an “unused” area.

The brain’s plasticity allows it to modify itself after some types of damage, especially early in life.

The brain often attempts self-repair by reorganizing existing tissue. Neurogenesis, less common, is the formation of new neurons.

The corpus callosum (a large band of nerve fibers) normally connects the two brain hemispheres. If surgically severed (for example, to treat severe epilepsy), a split brain results.

Split-brain research shows that in most people, the hemispheres are specialized, though they work together in a normal brain. The left hemisphere usually specializes in verbal processing. The right hemisphere usually specializes in visual perception and the recognition of emotion.

Consciousness is our awareness of ourselves and our environment. We process information at an explicit, conscious level (sequential processing of whatever requires focused attention) and at an implicit, unconscious level (parallel processing of routine business).

We selectively attend to, and process, a very limited portion of incoming information, blocking out much and often shifting the spotlight of our attention from one thing to another.

Focused intently on one task, we often display inattentional blindness to other events and change blindness (a form of inattentional blindness) to changes around us.

The circadian rhythm is our internal biological clock; it regulates our daily cycles of alertness and sleepiness.

Nightly sleep cycles every 90 minutes through recurring stages:

Age, genetic, and social-cultural factors affect sleep patterns.

Psychologists suggest five possible reasons why sleep evolved:

Sleep loss causes fatigue and irritability, and impairs concentration, productivity, and memory consolidation. It can also lead to depression, obesity, joint pain, a suppressed immune system, and slowed performance (with greater vulnerability to accidents).

The major sleep disorders are insomnia (recurring wakefulness); narcolepsy (sudden, uncontrollable sleepiness, sometimes lapsing directly into REM sleep); sleep apnea (the repeated stopping of breathing while asleep); and sleepwalking, sleeptalking, and night terrors.

Our dreams often include ordinary events and everyday experiences, but with a vivid, emotional, and often bizarre flavor. Most dreams are bad dreams—of personal failures, dangers, or misfortunes.

There are five major views of the function of dreams:

Most sleep theorists agree that REM sleep and its associated dreams serve an important function, as shown by the REM rebound that occurs following REM deprivation in humans and other species.