Nervous System Organization: CNS and PNS
The nervous system is the body’s rapid communication network, and it is organized into the central nervous system (CNS), the brain and spinal cord, and the peripheral nervous system (PNS), the nerves that connect the CNS to the rest of the body. In brief: the CNS processes information and decides on responses, while the PNS carries signals to the CNS (sensory pathway) and from the CNS (motor pathway). The adult human brain has roughly 86 billion neurons and weighs about 1.3 to 1.4 kilograms.
The nervous system lets organisms detect changes in their environment and respond within fractions of a second. This guide explains how the system is divided, the main parts of the brain and their functions, the structure and role of the spinal cord, the somatic and autonomic divisions of the PNS, a worked example that traces a signal and the common mistakes.
How is the nervous system organized?
The nervous system is built from nervous tissue, which contains neurons (nerve cells) and supporting glial cells. Its three overall functions are to collect information through sensory receptors, to integrate it in the brain and spinal cord, and to produce a response through effectors, which are muscles and glands.
The two main divisions are:
- Central nervous system (CNS): the brain and spinal cord. It is protected by bone (skull and vertebrae), three membranes called meninges and a cushion of cerebrospinal fluid.
- Peripheral nervous system (PNS): all the nerves outside the CNS, including 12 pairs of cranial nerves from the brain and 31 pairs of spinal nerves from the spinal cord.
What are the parts of the brain?
The brain has several regions that work together.
- Cerebrum: the largest part, divided into left and right hemispheres joined by a bundle of fibers (the corpus callosum). Its outer layer, the cerebral cortex, is folded and about 2 to 4 millimeters thick. It controls conscious thought, memory, language, voluntary movement and the interpretation of senses. Its lobes include the frontal (planning, movement), parietal (touch, spatial awareness), temporal (hearing, memory) and occipital (vision) lobes.
- Cerebellum: at the back of the head, below the cerebrum. It coordinates movement, balance and posture.
- Brain stem: consists of the midbrain, pons and medulla oblongata. The medulla controls automatic functions such as heart rate, breathing and blood pressure. The brain stem also connects the brain to the spinal cord.
- Hypothalamus: a small region that regulates body temperature, hunger, thirst and sleep cycles. It controls the pituitary gland, which links the nervous and endocrine systems.
- Thalamus: relays sensory signals to the cortex.
Different regions are specialized, but most functions involve several areas communicating. Damage to a region, for example by a stroke, can affect the associated function, such as speech or movement on one side of the body. The left hemisphere controls the right side of the body, and the right hemisphere controls the left side.
What does the spinal cord do?
The spinal cord is a cylinder of nervous tissue about 45 centimeters long in adults and about as thick as a finger. It runs inside the vertebral column from the brain stem to the lower back. It has two functions: it carries signals up to the brain and down from it, and it controls simple, fast reflexes that do not involve the brain, as in withdrawing a hand from a hot object.
In cross-section, the inner part is gray matter, which contains the cell bodies of neurons and looks like a butterfly, and the outer part is white matter, which contains bundles of axons covered with fatty myelin. Sensory nerve fibers enter through the back (dorsal) root, and motor fibers leave through the front (ventral) root.
What does the peripheral nervous system do?
The PNS has two functional divisions.
Somatic nervous system
It controls voluntary movements of skeletal muscles and carries sensory information from the skin, muscles and joints. It is what you use to decide to pick up a pencil.
Autonomic nervous system
It controls involuntary functions of internal organs, such as heart rate, digestion and the size of pupils. It has two parts that often have opposite effects:
- Sympathetic: the “fight or flight” response. It increases heart rate and breathing, widens the pupils and diverts blood to muscles, preparing the body for action.
- Parasympathetic: the “rest and digest” response. It slows the heart, stimulates digestion and conserves energy.
Divisions of the nervous system
| Division | Parts | Main function | Control |
|---|---|---|---|
| CNS | Brain, spinal cord | Processing and decisions | Both |
| PNS: somatic | Nerves to skeletal muscle and sense organs | Voluntary movement, sensation | Voluntary |
| PNS: sympathetic | Nerves to organs | Prepares for action | Involuntary |
| PNS: parasympathetic | Nerves to organs | Rest and digestion | Involuntary |
Worked example: tracing a voluntary movement
You see a ball flying toward you and catch it. Describe the route of the signals.
- Receptors: light-sensitive cells in the retina detect the ball. Sensory neurons in the optic nerve carry the signal to the brain.
- Processing: the occipital lobe interprets the image, and areas in the parietal and frontal lobes judge position and speed and decide to catch the ball.
- Command: the motor cortex in the frontal lobe sends signals down the spinal cord through motor neurons.
- Coordination: the cerebellum helps to fine-tune the timing and balance.
- Effectors: motor neurons of the somatic nervous system carry signals to the muscles of the arm and hand, which contract to catch the ball.
- Autonomic side effect: the sympathetic system speeds up your heart rate.
Nerve signals travel at speeds up to about 120 meters per second in large, myelinated fibers. Suppose the signal must travel about 1.2 meters from the brain to the hand: 1.2 ÷ 120 = 0.01 s. The whole process, including processing time, typically takes about 0.2 seconds.
How was the nervous system studied?
Ancient Greek writers disagreed about whether the heart or the brain was the seat of thought. Galen showed by experiments on animals that nerves carry commands from the brain. Santiago Ramón y Cajal’s drawings in the late 1800s established that the nervous system consists of separate neurons. In the twentieth century, imaging methods such as EEG, CT, MRI and functional MRI allowed researchers to study the living brain.
Which mistakes are common?
Mistake 1: Saying that the spinal cord is part of the PNS
The spinal cord is part of the CNS. Spinal nerves that branch off it belong to the PNS.
Mistake 2: Using nerve and neuron as synonyms
A neuron is a single cell. A nerve is a bundle of many axons wrapped in connective tissue.
Mistake 3: Believing that we use only 10 percent of the brain
This is a myth. Imaging shows that all regions of the brain are active at different times.
Mistake 4: Thinking that the cerebellum controls thinking
It coordinates movement and balance. Thinking is mainly a function of the cerebral cortex.