Every time you pick up a cup, read a sentence, or feel the floor under your feet, signals are traveling along a network that reaches nearly every part of your body. So what is the nervous system, exactly? It is that network: an organ system built from specialized cells, organized into a central hub and a branching set of connections that run out to your skin, muscles, and organs. Once you see the map, the vocabulary that comes with it gets a lot easier to read.

In Google Trends data for the US over the past 7 days, related searches for the seed term "nervous system" were led by "the nervous system" with a top score of 100, followed by "what is nervous system" (46), "what is the nervous system" (39), "central nervous system" (28), "what is a nervous system" (27), "peripheral nervous system" (14), "somatic nervous system" (11), "nerve" (10), "neurons" (9), and "what is central nervous system" (9). These top scores are relative interest, scaled so the most popular term in the set equals 100; they are not search counts. None of these terms was listed as rising. The pattern is clear enough: most people are asking the basic definition first, then looking for the names of the parts.

What is the nervous system? Two parts, one network

Anatomy textbooks start with one simple split. OpenStax Anatomy and Physiology 2e (section 12.1, "Basic Structure and Function of the Nervous System") puts it in a single line: the central nervous system is the brain and spinal cord, and the peripheral nervous system is everything else.

That split is about location, not about two separate systems working on their own. The central part sits protected inside bone. The peripheral part is the wiring that connects that center to the rest of you. Signals move in both directions across the boundary, so it helps to think of one network with two regions.

Cleveland Clinic's overview page on the nervous system describes it as the body's command center, and names neurons, or nerve cells, as the basis of the whole system. Keep that idea in mind: every part of the map below is built from neurons and the cells that support them.

The central nervous system: brain and spinal cord

If you have typed "what is central nervous system" into a search bar, here is the short answer. The central nervous system, or CNS, is made of two structures: the brain and the spinal cord. Cleveland Clinic states it the same way: your brain and spinal cord make up your CNS.

Both structures are enclosed in bone. Per OpenStax section 12.1, the brain is contained within the cranial cavity of the skull, and the spinal cord sits within the vertebral cavity of the vertebral column. That bony housing is one of the easiest ways to remember where the CNS ends.

OpenStax also points out a naming detail that trips up a lot of readers. A bundle of axons inside the CNS is called a tract. The same kind of bundle outside the CNS is called a nerve. So, strictly speaking, nerves belong to the peripheral side of the map.

The peripheral nervous system: nerves and ganglia

The peripheral nervous system, or PNS, is defined by what it is not. OpenStax section 13.4, "The Peripheral Nervous System," describes the PNS as everything that is not the CNS. In practice, that means two kinds of structures: nerves and ganglia.

Nerves. These are the cables of the PNS. OpenStax section 13.4 sorts them into two groups by where they connect. There are twelve cranial nerves, designated CN I through CN XII, and there are 31 spinal nerves, each named for the level of the spinal cord where it emerges. Spinal nerves are numbered from top to bottom.

Ganglia. OpenStax section 12.1 defines a ganglion as a cluster of neuron cell bodies in the PNS. The plural is ganglia. Where a nerve is a bundle of axons, a ganglion is a gathering point for the cell bodies those axons come from.

Somatic vs. autonomic: two divisions by job

The central and peripheral split is anatomical, a map of where things are. Textbooks also divide the nervous system by function, a map of what things do. Those two maps overlap: each functional division has parts in both the CNS and the PNS.

The somatic nervous system. OpenStax section 12.1 says the somatic nervous system is responsible for conscious perception and voluntary motor responses, and its motor output goes to skeletal muscle, the muscle that moves the skeleton. Cleveland Clinic phrases it as the division that guides your voluntary movements. OpenStax adds a nuance: some somatic responses, such as reflexes, happen without a conscious decision. If you want to see how skeletal muscle differs from the other two muscle types, our explainer on skeletal, smooth, and cardiac muscle lays them side by side.

The autonomic nervous system. The other division handles the work you never have to think about. OpenStax describes it as responsible for involuntary control of the body, with motor output to smooth muscle, cardiac muscle, and glandular tissue. It has its own subdivisions and vocabulary, which we cover in a separate piece: the autonomic nervous system, explained.

What is a nerve? A wrapped bundle of axons

A nerve looks like a single cord, but it is really a cable made of many smaller cables. OpenStax section 13.4 describes three layers of connective tissue, from the outside in:

• Epineurium: the fibrous connective tissue that surrounds the outer surface of the whole nerve.
• Perineurium: the sheath that wraps each fascicle, a smaller bundle of axons inside the nerve.
• Endoneurium: the loose connective tissue that surrounds each individual axon.

The root "neur" in all three names means nerve, and the prefixes tell you the position: epi (upon, the outer layer), peri (around, each fascicle), and endo (within, each single axon). Learn those three prefixes and the cross section of a nerve becomes easy to label.

What are neurons? The cells the network is built from

Neurons are the signaling cells of the nervous system. OpenStax section 12.2, "Nervous Tissue," names their main parts:

• Cell body (soma): the main part of the neuron.
• Dendrites: usually highly branched extensions that receive information from other neurons at contact points called synapses.
• Axon: the fiber extension of the neuron. OpenStax notes that neurons are usually described as having one, and only one, axon. Axons bundled together are what form a nerve.

Neurons do not work alone. OpenStax section 12.2 describes glial cells as the supporting cells of nervous tissue and lists six types: four found in the CNS and two found in the PNS. Some glial cells produce myelin, an insulating wrap around many axons. In the CNS, one oligodendrocyte can provide myelin for several axon segments; in the PNS, a Schwann cell wraps a portion of just one axon.

Put the pieces together and the map closes neatly. Axons from many neurons are bundled into a nerve. Cell bodies cluster in ganglia. Nerves connect to the brain and spinal cord, and the brain and spinal cord are made of neurons and glia too.

Our take

We think the nervous system looks harder than it is. Most of the intimidating terms are just labels on a map with two regions (central and peripheral) and two jobs (somatic and autonomic). Once you know which region and which job a word belongs to, you can place it.

We also like how much of the vocabulary is built from a few roots. Neur means nerve. Epi, peri, and endo tell you outer, around, and within. Cranial points to the skull, and spinal points to the vertebral column. Read anatomy that way and a textbook diagram stops feeling like a foreign language.

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FAQ

What is a nervous system?

A nervous system is the network of specialized cells that carries signals through the body. Cleveland Clinic describes it as the body's command center and names neurons, or nerve cells, as its basis.

What is the central nervous system?

The central nervous system is the brain and spinal cord. Per OpenStax Anatomy and Physiology 2e (section 12.1), the brain sits within the cranial cavity of the skull and the spinal cord within the vertebral cavity of the vertebral column.

What is the difference between the central and peripheral nervous system?

Location. OpenStax defines the central nervous system as the brain and spinal cord and the peripheral nervous system as everything else, which means the nerves and ganglia outside the skull and vertebral column. OpenStax section 13.4 lists 12 cranial nerves and 31 spinal nerves on the peripheral side.

What are neurons?

Neurons are the signaling cells of nervous tissue. OpenStax section 12.2 describes a cell body (soma), usually highly branched dendrites that receive information at synapses, and typically one axon.