p CHAPTER 4 How Do Neurons Convey Information? Electricity and Neurons How Neurons Integrate What Is Electricity? Information Early Clues to Electrical Activity in the Nervous Excitatory and Inhibitory Postsynaptic Potentials System Focus on Disorders: Myasthenia Gravis Modern Tools for Measuring a Neuron’s Summation of Inputs Electrical Activity The Axon Hillock How the Movement of Ions Creates Electrical Charges Into the Nervous System and Back Out The Electrical Activity How Sensory Stimuli Produce Action Potentials of a Membrane How Nerve Impulses Produce Movement The Resting Potential Focus on Disorders: Lou Gehrig’s Disease Graded Potentials The Action Potential Using Electrical Activity The Nerve Impulse to Study Brain Function Saltatory Conduction and Myelin Sheaths Single-Cell Recordings EEG Recordings Focus on Disorders: Epilepsy Event-Related Potentials Mason Morfit / FPG International / PictureQuest Micrograph: Dr. David Scott/Phototake 112 I p igure 4-1 is perhaps the most reproduced drawing body (not shown here) causes the head to turn toward the in behavioral neuroscience. Taken from René painful stimulus and the hands to rub the injured toe. F Descartes’s book titled Treatise on Man, it illus- Descartes’s theory was inaccurate, as discussed in trates the first serious attempt to explain how information Chapter 1. Even at the time that his book appeared, this travels through the nervous system. Descartes proposed theory did not receive much support. It was clear from the that the carrier of information is cerebrospinal fluid flow- examination of nerves that they were not tubes, and the ing through nerve tubes. When the fire in Figure 4-1 burns idea that muscles fill with fluid as they contract proved to the man’s toe, it stretches the skin, which tugs on a nerve be equally wrong. If an arm muscle is contracted when tube leading to the brain. In response to the tug, a valve in the arm is held in a tub of water, the water level in the tub a ventricle of the brain opens and cerebrospinal fluid does not rise, as it should if the mass of the muscle were flows down the tube and fills the leg muscles, causing increasing owing to an influx of fluid. them to contract and pull the toe back from the fire. The Still, Descartes’s theory was remarkable for its time flow of fluid through other tubes to other muscles of the because it considered the three basic processes that un- derlie a behavioral response: 1. Detecting a sensory stimulus and sending a message to the brain 2. Deciding, by using the brain, what response should be made 3. Sending a response from the brain to command mus- cles to move Descartes was trying to explain the very same things that we want to explain today. If it is not stretched skin tug- ging on a nerve tube that initiates the message, the mes- sage must still be initiated in some other way. If it is not the opening of valves to initiate the flow of cerebrospinal fluid to convey the information, the flow of information must Figure 4-1 still be sent by some other means. If it is not the filling of In Descartes’s concept of how the nervous system conveys muscles with fluid that produces movements, some other information, heat from a flame causes skin on the foot to stretch, mechanism must still cause muscles to contract. What all and this stretching pulls a nerve tube going to the brain. The pull opens a valve in the brain’s ventricle. The fluid in the ventricle these other mechanisms are is the subject of this chapter. flows through the nerve tube to fill the muscles of the leg, causing We will examine how information gets from the environ- the foot to withdraw. Tubes to other muscles (not shown) cause ment to neurons, how neurons conduct the information the eyes and head to turn to look at the burn and cause the hand and body to bend to protect the foot. throughout the nervous system, and how neurons ulti- From Descartes, 1664. mately activate muscles to produce movement. I 113 p 114 I CHAPTER 4 ELECTRICITY AND NEURONS The first hints about how the nervous system conveys its messages came in the second half of the eighteenth century with the discovery of electricity. By following the clue that electricity was in some way implicated in neural messages, scientists eventually provided an accurate answer to the three questions to be examined in this chapter. What Is Electricity? Link to an introductory review Electricity is a flow of electrons from a body that contains a higher charge (more of electricity at the Web site at electrons) to a body that contains a lower charge (fewer electrons). The body with the www.worthpublishers.com/kolb/ higher electrical charge is called the negative pole, because electrons are negatively chapter4. charged and this body has more of them. The body with the lower electrical charge is called the positive pole. Electricity is measured in volts, which describe the difference Negative pole: in electrical potential between the two poles. The term potential is used here because more electrons the electrons on the negative pole have the potential to flow to the positive pole. The Positive pole: fewer electrons negatively charged electrons are attracted to the positive pole because opposite Current: charges are attracted to each other. A flow of electrons is called a current. If you look flow of electrons from at a battery, you will see that one of its poles is marked “Ϫ” for negative and the other negative to positive pole Electrical potential: “ϩ” for positive. These two poles are separated by an insulator, a substance through difference in electrical charge which electrons cannot flow. Therefore, a current of electrons flows from the negative (measured in volts) between Ϫ ϩ negative and positive poles ( ) to the positive ( ) pole only if the two poles are connected by a conducting medium, such as a wire. If a wire from each pole is brought into contact with tissue, the current will flow from the wire connected to the negative pole into the tissue and then from the tissue into the wire connected to the positive pole. Such wires are called electrodes. Electrons can accumulate on many substances, including ourselves, which is why you sometimes get a shock from touching a metal object after walking on a carpet. From the carpet, you accumulate relatively loose electrons, which give you a greater negative charge than that of objects around you. In short, you become a negative pole. Electrons normally leave your body as you walk around, because the earth acts as a positive pole. If you are wearing rubber-soled shoes, however, you retain an electrical potential because the soles of the shoes act as an insulator. If you then touch a metal object, such as a water fountain, electrons that are equally distributed on your body suddenly rush through the contact area of your fingertips. In fact, if you watch your fingertips just before they touch the water fountain, you will see a small lightning bolt as the electrons are transferred. These electrons leaving your fingertips give you the shock. Combing your hair is another way to accumulate electrons. If you then hold a piece of paper near the comb, the paper will bend in the comb’s direction. The nega- tive charges on the comb have pushed the negative charges on the front side of the pa- per to the back side of the paper, leaving the front side of the paper positively charged. Because unlike charges attract, the paper bends toward the comb. Early Clues to Electrical Activity in the Nervous System In 1731, Stephen Gray performed a similar experiment. He rubbed a rod with a piece of cloth to accumulate electrons on the rod. Then he touched the charged rod to the feet of a boy suspended on a rope and brought a metal foil to the boy’s nose. The foil bent on approaching the boy’s nose, being attracted to it, and, as the foil and nose p HOW DO NEURONS CONVEY INFORMATION? I 115 touched, electricity passed from the rod, through the boy, to the foil. Yet the boy was Electrical stimulation. The flow of elec- completely unaware that the electricity had passed through his body. Gray speculated trical current from the tip of an electrode that electricity might be the messenger in the nervous system. Although this conclu- through brain tissue that results in changes sion was not precisely correct, two other lines of evidence suggested that electrical ac- in the electrical activity of the tissue. tivity was somehow implicated in the nervous system’s flow of information. One of these lines of evidence consisted of the results of electrical-stimulation studies, the other of the results of electrical-recording studies. ELECTRICAL-STIMULATION STUDIES Visit the CD and find the area on Electrical-stimulation studies began in the eighteenth century when an Italian scien- electrical stimulation in the module on tist, Luigi Galvani, observed that frogs’ legs hanging on a wire in a market twitched Research Methods. You’ll see a model of during a lightning storm. He surmised that sparks of electricity from the storm were an electrical stimulator and a video clip activating the muscles. Investigating this possibility, he found that, if an electrical cur- of the self-stimulation of a rat. rent is applied to a dissected nerve, the muscle to which the nerve is connected con- tracts. Galvani concluded that the electricity flowed along the nerve to the muscle. He was wrong in this conclusion, but his experiment was pointing scientists in the right Figure 4-2 direction.