Neurons are cells that transport information. Like most cells, neurons have a cell body containing a nucleus.
Extending from the cell body are special parts called dendrites. Also extending from the cell body is a long, thin projection called an axon. At the end of the axon are axon terminals. Bundles of axons in the body are called nerves.
Dendrites pick up incoming signals and deliver them to the cell body. A neuron has many dendrites, so information can enter a neuron from many places at once.
Axons send signals out from the cell body. A neuron has one axon, but that axon may branch into many axon terminals. This allows information to be sent from one neuron to many places at once.
The brain is a highly organized network of billions of cells protected by the skull.
Voluntary actions, the things we choose to do, are directed by the brain. This includes activities such as, walking, jumping, and reading. The brain also directs many involuntary actions. For instance, the brain controls blinking, heartbeat, and digestion.
The spinal cord is a thin cord of neurons that is only about 1 inch in diameter protected by a series of bony disks called the vertebral column.
The spinal cord has two major functions:
The central nervous system is composed of the brain and the spinal cord:
Neural signaling is the function of the nervous system. Each neuron receives information through its dendrites from other neurons or from the environment. Neurons carry this information through their cell body and axon to their axon terminals. The information is delivered to the dendrites of the next neuron in the pathway or to the body.
There are three major types of neurons in the nervous system: sensory neurons, motor neurons, and interneurons.
Sensory neurons receive and carry information from the outside environment or from inside the body.
Axons of sensory neurons then carry this information to other neurons located in the brain or spinal cord.
Motor neurons cause actions. Motor neurons receive information from the axon terminals of sensory neurons or other neurons. The axons of motor neurons are often located in nerves together with axons of sensory neurons.
The axon terminals of motor neurons are located in muscles. The information delivered to muscle causes the muscle to contract.
Interneurons carry information within the brain and spinal cord. Interneurons are neurons that are not motor neurons or sensory neurons.
The dendrites of interneurons receive signals from the axon terminals of sensory neurons or other interneurons.
The axon terminals of interneurons deliver information to other neurons.
Sometimes the body must respond instantly to a signal from the environment.
If your hand touches a hot stove, you will pull your hand away without thinking about it. Such quick, automatic responses are called reflex actions.
Information flows more quickly through short pathways than long ones. We can respond more quickly when information does not have to go all the way to the brain. The neurons of reflex pathways can function without instructions from the brain. Information flows from the body to the spinal cord. The information then flows back out to the body, and the body protects itself. Although the brain is not involved in the reflex, it is informed about what is going on, so learning can occur.
In the simplest reflex pathways, information flows from the body to a sensory neuron that connects to a motor neuron in the spinal cord. Information then flows back to the body.
Voluntary actions, such as talking, eating, or walking, involve making a choice. To make a choice, we use the brain.
Voluntary pathways require that information collected from sensory neurons go to the brain. Interneurons carry information within the brain and spinal cord.
Information that activates a voluntary pathway can generate many different responses. For instance, if someone stands near a chair, he or she can choose either to sit or to remain standing. If sitting were a reflex, people might sit down any time they are near a chair.