Nervous System

Functions of the Nervous System
The nervous system collects information about the body’s internal and external environment, processes that information, and responds to it. These functions are accomplished by the peripheral nervous system and the central nervous system. The peripheral nervous system, which consists of nerves and supporting cells, collects information about the body’s external andinternal environment. The central nervous system, which consists of the brain and spinal cord, processes that information and creates a response that is delivered to the appropriate part of the body through the peripheral nervous system.
Neurons
What is the function of neurons?
The messages carried by the nervous system are electrical signals called impulses. Nervous system impulses aretransmitted by cells called neurons.
Types of Neurons
Neurons can be classified into three types according to the direction in which an impulse travels. Sensory neurons carry impulses from the sense organs, such as the eyes and ears, to the spinal cord and brain. Motor neurons carry impulses from the brain and the spinal cord to muscles and glands. Interneurons do the high-level work. They processinformation from sensory neurons and then send commands to other interneurons or motor neurons.
Structure of Neurons
Although neurons come in many shapes and sizes, they all have certain features in common. The largest part of a typical neuron is its cell body, which contains the nucleus and much of the cytoplasm.
Spreading out from the cell body are short, branched extensions calleddendrites. Dendrites receive impulses from other neurons and carry impulses to the cell body. The long fiber that carries impulses away from the cell body is the axon. An axon ends in a series of small swellings called axon terminals. Neurons may have dozens of dendrites, but usually they have only one axon. In most animals, axons and dendrites of different neurons are clustered into bundles of fiberscalled nerves.
In some neurons, the axon is surrounded by an insulating membrane known as the myelin (my uh lin) sheath. The myelin sheath that surrounds a single, long axon has many gaps, called nodes, where the axon membrane is exposed. As an impulse moves along the axon, it jumps from one node to the next.
The Nerve Impulse
An impulse begins when a neuron is stimulated by another neuron or bythe environment. Once it begins, the impulse travels quickly down the axon away from the cell body toward the axon terminals.
This reversal of charges, from more negatively charged to more positively charged, is called a nerveimpulse, or an action potential.
Threshold Not all stimuli are capable of starting an impulse. The minimum level of a stimulus that is required to cause an impulse in aneuron is called its threshold. Any stimulus that is weaker than the threshold will not produce an impulse
The point at which a neuron transfers an impulse to another cell is called synapse A space, called the synaptic cleft, separates the axon terminal from the adjacent cell.
The axon terminal at a synapse contains tiny vesicles fi lled with neurotransmitters.
Neurotransmitters are chemicalsthat transmit an impulse across a synapse to another cell. When an impulse arrives at the synapse, neurotransmitters are released from the axon, diffuse across the synaptic cleft, and bind to receptors on the membrane of the receiving cell.
The Brain and Spinal Cord
The control point of the central nervous system is the brain. Each of the major areas of the brain—the cerebrum, cerebellum, andbrain stem—are responsible for processing and relaying information.
Most of the neurons that enter and leave the brain do so in a large cluster of neurons and other cells known as the spinal cord. The spinal cord is the main communication link between the brain and the rest of the body. The spinal cord is a bit like a major telephone line, carrying thousands of signals at once between the...