Psychology · Foundations
The nervous system The body's rapid communication network: the system of neurons, nerves, tracts, and associated tissues that coordinates the body's activity in response to signals from inside and outside the body. Full entry → is the body's rapid communication network: it senses what is happening inside and around you and coordinates your responses. It has two major divisions — the central nervous system (the brain and spinal cord) and the peripheral nervous system, which connects the center to the rest of the body. Signals travel both ways: sensory messages in, motor commands out. The brain's three headline parts — cerebrum The largest part of the brain, associated with higher-level processes such as thought, language, and memory. Full entry →, cerebellum A brain region below the cerebrum that controls balance and smooth, coordinated movement. Full entry →, and brainstem The part of the brain that connects the cerebrum with the spinal cord and hosts basic life processes such as breathing. Full entry → — handle thinking, coordination, and the basics of staying alive, and the whole system rewires itself with experience.
Every thought, feeling, memory, and movement you will study in psychology arrives through this one network. When a drug lifts someone's mood, when practice turns a clumsy skill into an automatic one, when a head injury changes a personality, the action happens in the nervous system. Knowing its basic map — two divisions, three headline brain parts, two directions of traffic, one capacity to rewire — gives you a frame for every later topic, from sensation to sleep to psychological disorders. It also keeps you honest: the mind runs on this hardware, so understanding the hardware is not optional.
Psychology's account of the nervous system starts with a working definition. The APA Dictionary of Psychology describes the nervous system as the system of neurons, nerves, tracts, and associated tissues that, together with the endocrine system, coordinates the activities of the organism in response to signals received from the internal and external environments. In plainer terms for this lesson: the nervous system is the body's rapid communication network — it picks up signals from inside and outside the body and coordinates the response. Rapid matters: the endocrine system also coordinates the body, but with slower chemical messages carried in the bloodstream; nervous-system signals are electrical and near-instant.
The standard account organizes the network into two major divisions, and OpenStax's Psychology 2e states them directly: the central nervous system (CNS) The division of the nervous system made up of the brain and spinal cord; it integrates incoming sensory messages and outgoing motor messages. Full entry → is made up of the brain and spinal cord, and the peripheral nervous system (PNS) The division of the nervous system that lies outside the brain and spinal cord and connects the central nervous system to the rest of the body. Full entry → connects the CNS to the rest of the body. Everything outside the skull and spine — every nerve reaching into your face, arms, legs, and organs — belongs to the peripheral division. The APA Dictionary defines the CNS as the complex of neurons, axons, and supporting tissue that constitutes the brain and spinal cord, coordinating and integrating incoming sensory and outgoing motor messages. The network is built from cells, and one kind earns a single line here: neurons, the basic cellular unit of the nervous system, are nerve cells that carry signals from place to place — OpenStax describes them as interconnected processors that handle the nervous system's work. How neurons fire and how they talk through neurotransmitters belong to their own topics.
The brain sits at the center of the network, and this lesson names its three major parts without going deep — the anatomy lives in a sibling topic. The cerebrum is the largest part of the brain, and OpenStax ties it to the higher-level processes that define human mental life: consciousness, thought, emotion, reasoning, language, and memory. The cerebellum (Latin for little brain) sits below it; the APA Dictionary says it modulates muscular contractions to produce smooth, accurately timed movements, and OpenStax adds that it receives messages from muscles, tendons, joints, and the ear to control balance, coordination, movement, and motor skills. The brainstem connects the cerebrum with the spinal cord, and OpenStax places the basic processes of life — breathing, digestion — plus automatic functions like heart rate under its medulla and pons. Three one-liners to keep: cerebrum, thinking and higher mental processes; cerebellum, balance and smooth, coordinated movement; brainstem, basic life-support like breathing. Where these parts sit and what their sub-structures do is the job of the brain-structures topic.
The nervous system is not a one-way street. The peripheral division carries messages in both directions, and the APA Dictionary spells out the arrangement: afferent fibers bring messages from the sense organs to the CNS, while efferent fibers transmit messages from the CNS to the muscles and glands. OpenStax puts it in plainer words: sensory nerves bring messages in; motor nerves send messages out, and messages travel to and from the brain. An original example: Ana lines up for a 100-meter race. When the starter's gun fires, her ears turn the sound into a signal that sensory nerves carry inward — through her peripheral nerves and up her spinal cord to her brain. Her brain recognizes the sound, and in the same instant motor commands travel outward, down the spinal cord and out through motor nerves to the muscles of her legs and arms, pushing her out of the blocks. One event, two directions of traffic, coordinated by the central nervous system in a fraction of a second.
A common picture of the brain is a fixed machine: set at birth, aging like hardware. The evidence says otherwise. The APA Dictionary defines neural plasticity — also called neuroplasticity The ability of the nervous system to change in response to experience or environmental stimulation; also called neural plasticity. Full entry → — as the nervous system's ability to reshape itself with experience. OpenStax describes the same capacity: neuroplasticity refers to how the nervous system can change and adapt, and it can occur through personal experiences, developmental processes, or in response to damage. OpenStax is careful about the time course: the nervous system is most plastic when we are very young, yet it remains capable of remarkable change later in life. An original example: Nadia takes a job at a coffee shop and spends her first two weeks hunting for the right buttons on the drink machine. By the third week her hands move to the correct settings without her eyes looking. Nothing about the machine changed; her nervous system did. Repeated experience reshaped the pathways behind that routine — that is plasticity in action.
Psychology's reality check on the nervous system is this: the brain is the hardware your mind runs on. The biopsychology perspective, which OpenStax summarizes as exploring how biology, especially the nervous system, influences behavior, is built on that fact. When memory fails, when mood shifts, when personality changes after injury, the underlying machinery is nervous-system tissue. The mind does not float free of the body. But the honest framing cuts both ways. Psychology also studies the mind and behavior at their own level — OpenStax's working definition of the field is the science of the mind and behavior — and the hardware alone does not explain everything about experience. The brain's basic map tells you where mental life runs, not what a thought means or why a person chooses as they do. The hardware framing is the floor, not the ceiling.
Eli explains
Explain it like I’m 10
Picture the nervous system as the body's messaging service. Specialized cells called neurons are the couriers; the peripheral nervous system is the delivery routes running to every corner of the body; the central nervous system is the sorting office where the brain and spinal cord read the incoming messages and send replies. Messages flow in from your senses, get processed at the center, and flow back out as commands to your muscles. Three big departments handle the brain's work: the cerebrum for thinking, the cerebellum for smooth, coordinated movement, and the brainstem for the automatic basics like breathing. And the system is not fixed: practice rewires the routes, which is why skills that start clumsy end up automatic.
Think of the nervous system as a city's communication grid. The central nervous system is the network's core — the buildings where information arrives and decisions are made. The peripheral nervous system is the miles of lines running out to every neighborhood: sensors feeding reports in, commands traveling back out. When a signal arrives from a sense organ, it travels along the lines to the core; the core responds and sends instructions back along the lines to the muscles. The whole city stays coordinated because the lines run in both directions and the core never stops listening.
A messaging grid has one headquarters, but the brain is really many specialized regions working at once, not a single sorting office. Delivery routes also stay the same once built, while the nervous system constantly rewires itself — that is the whole point of neuroplasticity. And a communication grid processes one message at a time, while the nervous system coordinates thousands of signals simultaneously, every second, without you noticing.
Ana lines up for a 100-meter race. When the starter's gun fires, her ears convert the sound into a signal that sensory nerves carry inward through her peripheral nervous system, up her spinal cord, and into her brain. Her brain recognizes the starting signal and, in the same instant, sends motor commands outward: down the spinal cord and out through motor nerves to the muscles of her legs and arms, driving her out of the blocks. Notice the two-way traffic: one event, messages rushing in, then messages rushing out. Then add plasticity: Ana has practiced starts hundreds of times, so that command pathway is deeply worn. Her first start of the season felt slow because the pathway had grown rusty; each practice session made the response faster and smoother — experience rewiring the route. One race start shows both the two-way traffic and the rewiring in a single motion.
The nervous system is the body's rapid communication network — two divisions, three headline brain parts, signals flowing both ways — and its ability to rewire with experience is why practice, recovery, and change are possible at all.
3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.
The nervous system is basically the brain.
The brain is one organ inside the system. The nervous system includes the central division (brain plus spinal cord) and the peripheral division, whose nerves reach every part of the body — most of the network lives outside the skull.
Messages only travel one way, from the body to the brain.
Traffic runs both ways: sensory (afferent) messages carry information in from the sense organs, and motor (efferent) messages carry commands out from the central nervous system to muscles and glands.
The adult brain is fixed and cannot change.
Plasticity is strongest in childhood, but the nervous system remains capable of change throughout life, in response to experience and even to injury.
Neuroplasticity means the brain can fully repair any damage.
Plasticity is change and adaptation, not unlimited regeneration. The nervous system can reorganize and compensate after injury, but lost functions are not simply rebuilt.
Each of the three named brain parts works alone.
The cerebrum, cerebellum, and brainstem are described one at a time for clarity, but they operate as one coordinated system — a single action like catching a ball engages all three.
The CNS (brain and spinal cord) is the integrating core; the PNS is the network of nerves outside it that carries messages between the core and the rest of the body.
Sensory (afferent) nerves carry information inward from the sense organs to the CNS; motor (efferent) nerves carry commands outward from the CNS to muscles and glands.
The cerebrum is the largest part of the brain and handles higher-level processes such as thought and language; the cerebellum is smaller and handles balance and smooth, coordinated movement.
The nervous system is most changeable early in life, but it remains capable of meaningful change later — practice and experience keep reshaping it.
EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.
Researched 2026-08-22
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