The brain, nerves, and sense organs make up the body’s nervous system. The nervous system carries information around the body in the form of electrical signals. The various parts of the system are in constant communication and continually active. Messages stream into the brain from the senses every split second.
At the same time, the brain coordinates the exact tension of hundreds of muscles around the body, from the tiny muscles that move the eyes to the large muscles needed for running.
The brain and spinal cord make up the central nervous system, which serves as the nervous system’s control center. In very simple animals, brains do little more than coordinate basic reflexes, a task carried out in humans by the spinal cord. The human brain, however, has evolved into something far more complex, able to generate an inner world and a conscious self that can reflect, plan, and make decisions.
Filling the space inside the skull, the human brain is dominated by a large, folded structure called the cerebrum. Far larger in humans than in other animals, the cerebrum is responsible for conscious thought, planning, social judgment, and language.
Beneath the cerebrum is the limbic system, which generates emotions, and the cerebellum, which coordinates movement.
Some of the responses made by our nervous system are under voluntary control, while others are involuntary - they happen without our choice. The voluntary division is called the somatic nervous system, while the involuntary division is called the autonomic nervous system.
Somatic Nervous System
This branch of the nervous system controls parts of the body that you control at will, such as the muscles you use to a book or kick a ball.
Autonomic Nervous System
This part of the nervous system controls organs without you having to think. It makes your heart speed up and slow down, pushes food through your intestines, and controls how wide the pupils in your eyes are.
The cells that make up the nervous system are called nerve cells, or neurons. Neurons transmit electric signals along long fibers called axons, which are sheathed by a fatty material called myelin to help the signal travel faster. There are three main types of neurons.
1. Unipolar Neurons: These neurons typically carry incoming sensory signals. The cell body has a single extension.
2. Bipolar Neurons: These neurons are found in the eyes and in muscles. The cell body of a bipolar neuron has two extensions.
3. Multipolar Neurons: Found in the brain, these are the most common neurons. The cell body has multiple extensions.