Nervous system

In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. It helps animals detect changes in their environment and respond to them, working together with the endocrine system to keep everything running smoothly. Nervous tissue first appeared in simple wormlike organisms around 550 to 600 million years ago.

In vertebrates, the nervous system has two main parts: the central nervous system, made up of the brain and spinal cord, and the peripheral nervous system, which includes nerves that connect the brain and spinal cord to the rest of the body. These nerves carry messages both to and from the central system. The peripheral system also has two subsystems: one that controls voluntary movements and another that manages automatic body functions like digestion and heart rate.

The nervous system is made of special cells called neurons that can send electrical and chemical signals quickly through long fibers known as axons. These signals allow animals to sense the world around them and react accordingly. Other supporting cells, called glial cells, help keep the neurons healthy and functioning well. From simple worms to large African elephants, nervous systems vary greatly in size and complexity, but they are essential for almost all animals except a few very simple ones.

Structure

The nervous system is made up of nerves, which are bundles of fibers called axons that come from the brain and spinal cord. These nerves spread out to every part of the body. People have known about nerves for a very long time, but we only learned about their inner workings with microscopes.

The nervous system has two main types of cells: neurons and glial cells. Neurons are special cells that talk to each other using tiny connections called synapses. They help turn things like light and sound into signals the body can understand. Glial cells support and protect neurons, helping them work better.

In animals with backbones, like humans, the nervous system has two main parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves outside the brain and spinal cord). These parts work together to help the body react to what it senses.

Function

The nervous system sends signals from one part of the body to another, helping it respond to changes around it. It works with the endocrine system to coordinate actions and reactions. Nerve signals travel much faster and more specifically than hormonal signals, allowing quick and precise responses.

The nervous system controls the body by sensing the environment, processing that information, and creating responses. This helps animals develop advanced abilities like vision, social interactions, and coordination. In humans, it supports complex features such as language, abstract thinking, and culture.

Neurons and synapses

Neurons send signals through long arms called axons. These signals are electrochemical waves known as action potentials. Neurons connect at points called synapses, where they release chemicals called neurotransmitters. These chemicals cross a tiny space and bind to receptors on the next neuron, causing it to react. This process happens very quickly and can be either stimulating or calming for the receiving neuron.

There are many types of synapses and neurotransmitters, allowing for a wide range of effects. Some synapses help form memories by changing strength over time. Networks of connected neurons can process information in many ways, helping animals detect features, generate patterns, and control actions.

Neural circuits and systems

Neurons can work together in circuits to perform various functions. Simple circuits, called reflexes, help the body react quickly to dangers, like pulling a hand away from something hot. More complex circuits involve many parts of the brain and allow for detailed processing of signals, such as seeing an object and deciding to look at it.

The nervous system can also create rhythms and patterns on its own, without needing outside signals. For example, it controls the sleep-wake cycle through internal clocks. These patterns help coordinate many body functions over different timescales.

Mirror neurons

Main article: Mirror neuron

Mirror neurons are special neurons that activate both when an animal acts and when it observes the same action done by another. They have been found in primates and may help with understanding others' actions and learning new skills by imitation. Scientists are still studying exactly how these neurons work and what roles they play in behavior and cognition.

Development

Main articles: Development of the nervous system and Development of the nervous system in humans

The nervous system begins to form very early in an animal's development. In vertebrates, it starts with a thin strip of cells along the back called the neural plate. This strip folds and closes to form a tube, which becomes the central nervous system (CNS), while other parts become the peripheral nervous system (PNS).

Scientists have discovered that special signals from other cells tell these early cells to become part of the nervous system. These signals help create the building blocks of nerves, called neurons, which are important for sending messages throughout the body. Studies in both insects and vertebrates show that similar processes help build nervous systems across many different animals.

Pathology

Main article: Neurology

See also: Psychiatry

The nervous system has special protections to keep it safe. The brain and spinal cord are surrounded by tough layers called meningeal membranes and are enclosed in the bones of the skull and vertebral column. There is also a barrier called the blood–brain barrier that stops many harmful chemicals from reaching the brain.

Nerves can sometimes be hurt, which might cause pain or loss of feeling. This can happen if they get pressed or damaged, like in carpal tunnel syndrome. Some problems with nerves are caused by other illnesses, such as diabetes or shingles. Doctors use many methods to help treat nervous system problems.