Nervous System

The nervous system is a network of nerve cells that allows an animal to collect, process, and respond to information. As an internal communications system, the nervous system enables an animal to react and adjust to changes in its environment. Almost all animals have some type of nervous system, but the human nervous system allows us to speak, solve problems, and have creative ideas—activities that make humans different from all other animals.

Animals have a nervous system but plants do not. Plants react and respond to changes in their environment by slowly altering their growth patterns using different hormones. When a plant inclines itself toward a light source or drops its leaves, it is doing so on command from a naturally occurring chemical called a hormone that it produces as a response to something outside itself. Such a slow system of internal communication could only be practical for an organism that can make its own food and does not move. For animals, however, their very existence and reproductive ability often rests on being able to react immediately to something in their environment. Often they are either trying to catch something to eat or they are trying to escape being caught. Movement is, therefore, essential to animals, and their nervous systems must always allow them to act appropriately, efficiently, and most important of all, rapidly.

EVOLUTIONARY DEVELOPMENT OF THE NERVOUS SYSTEM

Even the simplest multicellular organism has to constantly gather and analyze information about its environment if it is to maintain its inner balance (known as homeostasis) and survive in a constantly changing habitat. The single-celled amoeba does not have a real nervous system, but it still responds appropriately to stimuli like light or food. However, more complex organisms need to do more than simply react to stimuli and therefore need a more complex communications system. Probably the simplest nervous system is the one used by the class of animals called Scyphozoa (phylum Cnidaria), better known as jellyfish, hydra, and sea anemones. These animals use a system described as a nerve net that directly connects the receiving cell to the cell that does the responding. Flatworms are more complex than jellyfish and concentrate both their receiving and sending sensors in their forward end (like a head). This means that their front part is the first to meet the stimuli, and this permits them to react more rapidly. Flatworms also have bilateral symmetry (a body that is basically the same on both halves). Bilateral symmetry includes a typical vertebrate system with pairing of nerves down either side of a central column.

Continuing up the ladder past the jellyfish and the flatworm, the beginnings of the vertebrate (an animal with a backbone) model can be seen inside an earthworm or a grasshopper. Besides having bilateral symmetry, they also demonstrate what is called segmentation. They have identical segments (each of which has a pair of nerves) linked together and connected to a central organ that could be called a primitive brain. Once this model was established and seemed to work well, it eventually evolved into the sophisticated nervous system that is basic to humans and all other animals with backbones. The backbone or the vertebral column evolved into its present form because it proved, first of all, to be an ideal up-and-down framework to support the body and make it both strong and flexible. Its hollow center was eventually expanded and modified so that it could hold a spinal cord connected to a brain. Finally, the skull that held the brain then slowly developed a range of sensory equipment (ears, eyes, nose) that gathered information about the outside world and fed it into the brain.

THE CENTRAL NERVOUS SYSTEMS

Today, the nervous system of all vertebrates can be divided into two main parts: the central nervous system and the peripheral nervous system.

SANTIAGO RAMON Y CAJAL

Spanish histologist (a person specializing in the study of tissues and organs) Santiago Ramon y Cajal (1852–1934) laid the foundations of modern neurology, which is the scientific study of the nervous system. He established that the nervous system is made up of independent units of nerve cells called neurons, and also made important discoveries relating to the transmission of nerve impulses and the cellular structures of the brain.

Santiago Ramon y Cajal was born in the remote village of Petilla de Aragon, Spain. His father was a self-trained country doctor who wanted his son to have a real medical education. Therefore, the family moved to the university city of Zaragoza, and young Santiago studied medicine there. Santiago was a rebellious youth who was more interested in drawing than studying, and his father finally forced him to work for a barber and a shoemaker as a way of disciplining him and making him appreciate school. This apparently worked, and he earned his medical degree from the University of Zaragoza in 1873. He then joined Spain's army medical service and served as army surgeon in Cuba for a year. However, while in Cuba he caught malaria and returned to Spain to recuperate. Going back to school, he earned his doctorate in medicine in 1879 and began a teaching career. Preferring to do research and to teach rather than practice medicine, he turned to the field of anatomy, which had always been his favorite subject. Anatomy studies the structure of living things and figures out how the different parts of an organism are shaped and how they fit together. His favorite branch of anatomy was histology.

Working with an old, abandoned microscope he found at the University of Zaragoza, Ramon y Cajal eventually turned toward the study of the most complex tissues in the body, which are those of the nervous system. These can only be studied under the microscope if they are stained (dyed), and Ramon y Cajal was able to improve upon some of the stains in use at this time. When he began his research on the nervous system, little or nothing was known about what might be called the path that a nervous impulse takes. Most scientists thought it traveled over something like a connected grid, or network of wires, but no one really knew for sure since they had never been able to examine nervous tissue closely. Using his own improved stain techniques on brain tissue, Ramon y Cajal was able, by 1889, to demonstrate that the nervous system was by far much more complex than anyone had imagined. He then went on to show that the neuron, or nerve cell, was the basic unit of the nervous system, and that it was very different from the other ordinary cells of the body. Ramon y Cajal then offered a controversial "neuron theory" that few accepted. He stated that the nervous system consists of a network of separate nerve fibers whose ends never actually "touch" or are not actually connected with the surrounding nerve cells. Today it is known that neurons are indeed not "hard-wired" together but instead have a synapse or space between them, across which the electrical charge or nervous impulse "jumps." He also stated that nerve impulses travel in only one direction, and that the brain's neurons had different structural patterns in different areas, suggesting that one part might have a different job than another. We now call this "localization," meaning that a certain part of the brain controls memory and another intelligence. Ramon y Cajal also conducted important research on the tissues of the inner ear and eye. Overall, modern neurology really began with his work, since he established the correct role played by the neuron and the nervous impulse.

The central nervous system consists of the brain and spinal cord and acts as network central, or the "main switchboard" for the entire system. The cord itself lies within and is protected by a central canal that is made up of stacks of bony vertebrae. Like an electrical cable, it has pathways that lead to and from the brain, and its nerve cells are lined up in a columnar way. Inside the backbone, the spinal column is bathed in cerebrospinal fluid that circulates around both it and the brain and acts as a liquid shock absorber. At the top of the column sits the brain—the system's real control center. It is actually a continuation of the spinal cord, which extends upwards and expands into segments called ventricles and aqueducts. The brain itself has three parts: the hind-brain, the midbrain, and the forebrain. The hindbrain occupies the rear of the skull and connects with the cord. It contains the medulla oblongata and the cerebellum. The medulla controls the body's involuntary processes like breathing and the heartbeat. The cerebellum coordinates the body's many muscles and allows it to move properly. The midbrain is sort of a coordinating center for information collected by sight and hearing, and it also relays information to higher centers of the brain. The forebrain contains the higher brain centers such as the cerebrum. The cerebrum is the part of the brain involved in voluntary actions and with functions related to memory, learning, and sensing things. In humans, the cerebrum is larger than the hindbrain and midbrain put together, and is the place that governs thinking, reasoning, and the use of complex language.

THE PERIPHERAL NERVOUS SYSTEM

The peripheral nervous system can be described as a branching network of nerves that carry signals to and from the central nervous system. This system runs throughout the body of a vertebrate and has two different types of nerves: afferent nerves (or sensory neurons) bring input into the central nervous system, while efferent nerves (or motor neurons) take output away and communicate it to the muscles and glands. Some nerves in the peripheral nervous system can be controlled voluntarily, such as those that allow us to walk or run, but those that are not under our control (like the production of saliva or our beating heart) are considered a division of the peripheral nervous system called the autonomic nervous system. This system regulates functions like digestion that we cannot control. These nerves keep the body running smoothly by automatically adjusting its many systems.

NEURONS

The entire system works as it does because it is based on the neuron or nerve cell, the fundamental unit of communication in all nervous systems. Neurons never act alone. Rather, they transmit impulses to one another in the form of electrical signals and link together the entire nervous system. In many ways, neurons are the body's electrical highway. The neuron is a specialized cell and consists of three parts: the soma, the dendrites, and the axons. The soma is the cell body with its cytoplasm and nucleus. Dendrites and axons are hairlike arms or branches that extend from the body and channel information in one direction. Dendrites form the input part of the system and carry information toward the cell body. Axons are for output and therefore carry information away from the cell body. A typical neuron has several dendrites and one axon. Finally, neurons pass impulses to one another in a one-way direction across a space called a synapse. When a nerve impulse reaches the end of an axon and arrives at the synapse, a transmitter substance is released from the axon into the synapse. This chemical neurotransmitter goes across and binds to a receptor in the adjoining dendrite and triggers an impulse in it. The brain of the average adult contains about 1,000,000,000 neurons. Since the functioning of the entire system is dependent on the precise and proper functioning of the different types of neurons, anything that interrupts or disturbs that synaptic function can cause a problem with the organism. Today, scientists know of many genetic and infectious diseases that can severely interfere with this function. Many drugs have also been developed that can positively or negatively affect it.

[See alsoBrain; Muscular System ]