Ch. 3 S. 1 The Nervous System Obj: Explain how messages are transmitted by neurons, and describe the functions of the spinal cord and the peripheral nervous system.

The Nervous System The nervous system regulates our internal functions. It is also involved in how we react to the external world. Even learning and memory are made possible by the nervous system. When we learn a new behavior or acquire new information, the nervous system registers that experience and changes to accommodate its storage.

Two main parts: the central nervous system and the peripheral nervous system. Central nervous system – consists of the brain and the spinal cord. Peripheral nervous system – made up of nerve cells that send messages between the central nervous system and all the parts of the body.

Neurons Nerve cells, called neurons, run through our entire bodies and communicate with each other. Neurons send and receive messages from other structures in the body, such as muscles and glands. These messages can affect events ranging from the sensation of a pinprick to the first steps of a child, from the writing of a poem to the memory of a past event. Each of us has more than 100 billion neurons, most of which are found in the brain.

Components of a Neuron Neurons are somewhat like trees in structure. Parts of neurons resemble the branches, trunk, and roots of a tree. And, as in forests, many nerve cells lie alongside one another like a thicket of trees. Unlike tress, however, neurons can also lie end to end. Their “roots” are intertwined with the “branches” of neurons that lie below.

Every neuron consists of a cell body, dendrites, and an axon. The cell body produces energy that fuels the activity of the cell. Branching out from the cell body are think fibers called dendrites. The dendrites receive information from other neurons and pass the message through the cell body. While the dendrites carry information to the cell body, the axon carries messages away. A neuron has many dendrites but usually only one axon.

Axons vary greatly in length. Some are just a tiny fraction of an inch, while others stretch to several feet. Because of the length of their axons, some neurons in your legs are several feet long. Many axons are covered with myelin, a white fatty substance that insulates and protects the axon. This myelin sheath, or casing, also helps to speed up the transmission of the message. At the end of the axon, smaller fibers branch out. These fibers are called axon terminals.

The Communication Process Messages are sent from the axon terminals of one neuron to the dendrites of other neurons. In order for a message to be sent from one neuron to another neuron, it must cross the synapse. The synapse is a junction between the axon terminals of one neuron and the dendrites of another neuron. Messages travel in only one direction. Thus, messages are received by the dendrites and travel through the cell body and the axon to the axon terminals. From there, they cross synapses to the dendrites of other neurons. New synapses can develop between neurons that were not previously connected, as when we learn something new.

The information that is sent and the place to which it goes depend on a number of factors. These factors include the locations of the neuron in the body and the events that produced the message. Sensory neurons are nerve cells that carry information received by the senses to the central nervous system. Motor neurons, on the other hand, are nerve cells that carry information from the central nervous system to the muscles and the glands and influence their functioning.

Occasionally, something happens to disrupt the message-sending process. For example, a hard blow to the head from a car accident or a sports injury can cause a concussion-an injury in which the soft tissue of the brain hits against the skull. Sometimes, the person is affected for only a few seconds. Other times, the person may experience effects for a much longer time.

Neurotransmitters: Chemical Messengers Neurons send messages across synapses through the release of neurotransmitters. Neurotransmitters are chemicals that are stored in sacs in the axon terminals. A neuron fires, or sends its message, by releasing neurotransmitters-much like droplets of water shooting out of a spray bottle. Neurons can fire hundreds of times every second.

There are several types of neurotransmitters. Each has its own structure and fits into a receptor site on the next neuron, similar to the way in which a key fits into a lock. The message is converted into an electrical impulse that travels the length of the neuron. The message is then transmitted to the next neuron by neurotransmitters. The process continues until the message arrives at its destination. This whole process takes only a fraction of a second.

Neurotransmitters are involved in everything people do. Whenever a person waves a hand, yawns, or thinks about a friend, neurotransmitters are involved. Some diseases and psychological disorders may also be caused by the presence of too much or too little of various neurotransmitters. Researchers have identified dozens of neurotransmitters and their functions. Examples: noradrenaline-prepares the body for action; seratonin-emotional arousal and sleep; dopamine-motor behavior, but a deficiency plays a role in Parkinson’s and an excess may contribute to schizophrenia.

The Central Nervous System Consists of the neurons of the spinal cord and the brain. The spinal cord extends from the brain down the back. It is a column of nerves about as thick as a thumb, and it is protected by the bones of the spine. It transmits messages between the brain and the muscles and the glands throughout the body.

The spinal cord is also involved in spinal reflexes. A spinal reflex is a simple, automatic response to something. For example, if a person touches a hot stove, a message goes immediately from his or her hand to the spinal cord. A message to remove the hand is then sent back to motor neurons in the hand. The removal of the hand is a spinal reflex. (The person may also register pain in his or her brain. But the pain is not what causes the reflex. In fact, the pain may not even be felt until after the hand has been removed.)

The Peripheral Nervous System The peripheral nervous system lies outside the central nervous system and is responsible for transmitting messages between the central nervous system and all parts of the body. The two main divisions of the peripheral nervous system are the somatic nervous system and the autonomic nervous system.

The Somatic Nervous System The somatic nervous system transmits sensory messages to the central nervous system. It is activated by touch, pain, changes in temperature, and changes in body position. The somatic nervous system enables us to experience the sensations of hot and cold and to feel pain and pressure. For example, we can feel the softness a cat’s fur. The somatic system also alerts us that parts of the body have moved or changed positions. It sends messages to the muscles and the glands and helps us maintain posture and balance.

The Autonomic Nervous System The word autonomic means “occurring involuntarily,” or automatically. The autonomic nervous system regulates the body’s vital functions, such as heartbeat, breathing, digestion, and blood pressure. We generally do not have to think about these activities-they occur automatically and are essential for keeping us alive.

Psychologists are interested in the autonomic system because of its involvement in the experience of emotion. The response of the autonomic nervous system is particularly important when a person experiences something stressful in the environment. This system has two divisions: the sympathetic and the parasympathetic nervous system. These systems generally have opposing functions.

The sympathetic system is activated when a person is going into action. This is the “fight or flight” response. It does this by suppressing digestion, increasing the heart and respiration rates, and elevating the blood pressure. The parasympathetic system restores the body’s reserves of energy after an action has occurred. Heart rate and blood pressure are normalized, breathing is slowed, and digestion returns to normal.