Every thought, feeling and behavior runs on biology: genes and environment, a nervous system built from neurons, a brain with specialized parts, a daily sleep cycle, and senses that turn the world into signals. Most exam questions here give you a scenario and ask you to name the structure, chemical or process at work.
Heredity ("nature") and environment ("nurture") interact to shape behavior. The evolutionary perspective asks how natural selection favored behaviors that helped survival and reproduction. Twin, family and adoption studies separate the effects of genes and environment.
The central nervous system is the brain and spinal cord. The peripheral nervous system connects it to the body: the somatic system handles voluntary movement, and the autonomic system handles involuntary processes, split into the arousing sympathetic and calming parasympathetic divisions.
Neurons transmit information; glial cells support them. A neuron fires when stimulation reaches threshold, and the action potential is all-or-nothing. Neurotransmitters are excitatory or inhibitory; hormones act similarly from outside the nervous system. Drugs work as agonists, antagonists or reuptake inhibitors.
The brainstem runs basic life functions, the cerebellum coordinates movement and balance, and the cerebral cortex is divided into two hemispheres and four lobes. The limbic system handles memory and emotion. Language sits mostly in the left hemisphere (Broca: production; Wernicke: comprehension). The brain can rewire itself (plasticity), and is studied with EEG, fMRI, case studies and lesioning.
Sleep follows a roughly 24-hour circadian rhythm. A night cycles through NREM stages 1–3 and REM sleep; deep NREM gets shorter and REM gets longer as the night goes on. REM is "paradoxical": brain waves look awake but the body is most relaxed, and it is when most dreaming happens. Sleep is thought to consolidate memories and restore the body.
Sensation detects stimuli and transduces them into neural signals. The absolute threshold is the weakest stimulus detected 50% of the time; Weber's law says the just-noticeable difference is a constant proportion. Vision runs on rods and cones and two color theories; hearing on pitch and loudness; and the chemical, touch, pain, vestibular and kinesthetic senses each have their own structures and theories.