Biological Psychology 3.pdf - 3D4Medical.com/Getty Images...

This preview shows page 1 out of 124 pages.

Unformatted text preview: 3D4Medical.com/Getty Images 03007_02_ch2_p026-047.indd 26 11/5/08 Nerve Cells and Nerve Impulses CHAPTER OUTLINE MODULE 2.1 The Cells of the Nervous System Anatomy of Neurons and Glia The Blood-Brain Barrier The Nourishment of Vertebrate Neurons In Closing: Neurons MODULE 2.2 The Nerve Impulse The Resting Potential of the Neuron The Action Potential Propagation of the Action Potential The Myelin Sheath and Saltatory Conduction Local Neurons In Closing: Neural Messages Exploration and Study 2 MAIN IDEAS 1. The nervous system is composed of two kinds of cells: neurons and glia. Only the neurons transmit impulses from one location to another. 2. The larger neurons have branches, known as axons and dendrites, which can change their branching pattern as a function of experience, age, and chemical influences. 3. Many molecules in the bloodstream that can enter other body organs cannot enter the brain. 4. The action potential, an all-or-none change in the electrical potential across the membrane of a neuron, is caused by the sudden flow of sodium ions into the neuron and is followed by a flow of potassium ions out o the neuron. 5. Local neurons are small and do not have axons or action potentials. Instead, they convey information to nearby neurons by graded potentials. A nervous system, composed of many individual cells, is in some regards like a human society composed of man people: Each individual maintains an identity, and yet th whole can accomplish far more than any of the individual could alone. We begin our study of the nervous system by ex amining single cells; later, we examine how cells act together. Advice: Parts of this chapter and the next assume that you understand basic chemical concepts. If you need to refresh your memory, read Appendix A. OPPOSITE: An electron micrograph of neurons, magnified tens of thousands of times. The color is added artificially. For objects this small, it is impossible to focus light to obtain an image. It is possible to focus an electron beam, but electrons do not show color. 2 03007_02_ch2_p026-047.indd 27 11/5/08 MODULE 2.1 The Cells of the Nervous System Y our nervous system controls everything you do, ranging from walking to changes in heart rate and breathing to the most complex kinds of problem solving. To understand how the nervous system works, we have to start with its microscopic units—the cells. Anatomy of Neurons and Glia The nervous system consists of two kinds of cells: neurons and glia. Neurons receive information and transmit it to other cells. Glia serve many functions that are difficult to summaCerebral cortex and associated areas:12 to 15 billion neurons Cerebellum: 70 billion neurons Spinal cord: 1 billion neurons Figure 2.1 Estimated numbers of neurons in humans Because of the small size of many neurons and the variation in cell density from one spot to another, obtaining an accurate count is difficult. (Source: R. W. Williams & Herrup, 1988) rize, and we shall defer that discussion until later in the chapter. According to one estimate, the adult human brain contains approximately 100 billion neurons (R. W. Williams & Herrup, 1988) (Figure 2.1). An accurate count would be more difficult than it is worth, and the actual number of neurons varies from person to person. The idea that the brain is composed of individual cells is now so well established that we take it for granted. However, the idea was in doubt until the early 1900s. Until then, the best microscopic views revealed little detail about the organization of the brain. Observers noted long, thin fibers between one neuron’s cell body and another, but they could not see whether each fiber merged into the next cell or stopped before it (Albright, Jessell, Kandel, & Posner, 2001). Then, in the late 1800s, Santiago Ramón y Cajal used newly developed staining techniques to show that a small gap separates the tips of one neuron’s fibers from the surface of the next neuron. The brain, like the rest of the body, consists of individual cells. APPLICATIONS AND EXTENSIONS Santiago Ramón y Cajal, a Pioneer of Neuroscience Two scientists are widely recognized as the main founders of neuroscience: Charles Sherrington, whom we shall discuss in Chapter 3, and the Spanish investigator Santiago Ramón y Cajal (1852–1934). Cajal’s early career did not progress altogether smoothly. At one point, he was imprisoned in a solitary cell, limited to one meal a day, and taken out daily for public floggings—at the age of 10—for the crime of not paying attention during his Latin class (Cajal, 1901– 1917/1937). (And you thought your teachers were strict!) Cajal wanted to become an artist, but his father insisted that he study medicine as a safer way to make 28 03007_02_ch2_p026-047.indd 28 11/5/08 Bettmann/CORBIS 2.1 The Cells of the Nervous System Santiago Ramón y Cajal (1852–1934) How many interesting facts fail to be converted into fertile discoveries because their first observers regard them as natural and ordinary things! . . . It is strange to see how the populace, which nourishes its imagination with tales of witches or saints, mysterious events and extraordinary occurrences, disdains the world around it as commonplace, monotonous and prosaic, without suspecting that at bottom it is all secret, mystery, and marvel. a living. He managed to combine the two fields, becoming an outstanding anatomical researcher and illustrator. His detailed drawings of the nervous system are still considered definitive today. Before the late 1800s, microscopy could reveal few details about the nervous system. Then the Italian investigator Camillo Golgi found a way to stain nerve cells with silver salts. This method, which completely stained some cells without affecting others at all, enabled researchers to examine the structure of a single (nuclear envelope) (nucleolus) Nucleus (membrane-enclosed region containing DNA; hereditary control) Plasma membrane (control of material exchanges, mediation of cellenvironment interactions) 2 cell. Cajal used Golgi’s methods but applied them to infant brains, in which the cells are smaller and therefore easier to examine on a single slide. Cajal’s research demonstrated that nerve cells remain separate instead of merging into one another. Philosophically, we can see the appeal of the old idea that neurons merge. We describe our experience as undivided, not the sum of separate parts, so it seems that all the cells in the brain should be joined together as one unit. How the separate cells combine their influences is a complex and still mysterious process. The Structures of an Animal Cell Figure 2.2 illustrates a neuron from the cerebellum of a mous (magnified enormously, of course). Except for their distinctiv shapes, neurons have much in common with the rest of th body’s cells. The surface of a cell is its membrane (or plasma mem brane), a structure that separates the inside of the cell from th outside environment. It is composed of two layers of fat mol ecules that are free to flow around one another, as illustrated in Figure 2.3. Most chemicals cannot cross the membrane, bu (ribosomes) Endoplasmic reticulum (isolation, modification, transport of proteins and other substances) Mitochondrion (aerobic energy metabolism) Figure 2.2 An electron micrograph of parts of a neuron from the cerebellum of a mouse The nucleus, membrane, and other structures are characteristic of most animal cells. The plasma membrane is the border of the neuron. Magnification approximately x 20,000. (Source: Micrograph courtesy of Dennis M. D. Landis) 03007_02_ch2_p026-047.indd 29 11/5/08 30 Chapter 2 Nerve Cells and Nerve Impulses Phospholipid molecules specific protein channels in the membrane permit a controlled flow of water, oxygen, sodium, potassium, calcium, chloride, and other important chemicals. Except for mammalian red blood cells, all animal cells have a nucleus, the structure that contains the chromosomes. A mitochondrion (pl.: mitochondria) is the structure that performs metabolic activities, providing the energy that the cell requires for all other activities. Mitochondria require fuel and oxygen to function. Ribosomes are the sites at which the cell synthesizes new protein molecules. Proteins provide building materials for the cell and facilitate various chemical reactions. Some ribosomes float freely within the cell. Others are attached to the endoplasmic reticulum, a network of thin tubes that transport newly synthesized proteins to other locations. Protein molecules The Structure of a Neuron Figure 2.3 The membrane of a neuron Courtesy of Bob Jacobs, Colorado College Embedded in the membrane are protein channels that permit certain ions to cross through the membrane at a controlled rate. Figure 2.4 Neurons, stained to appear dark Note the small fuzzy-looking spines on the dendrites. Figure 2.5 The components of a vertebrate motor neuron The cell body of a motor neuron is located in the spinal cord. The various parts are not drawn to scale; in particular, a real axon is much longer in proportion to the soma. Dendrite Nucleus Myelin sheath Axon Presynaptic terminals Axon hillock Soma 03007_02_ch2_p026-047.indd 30 Neurons are distinguished from other cells by their shape (Figure 2.4). The larger neurons have these components: dendrites, a soma (cell body), an axon, and presynaptic terminals. (The tiniest neurons lack axons, and some lack well-defined dendrites.) Contrast the motor neuron in Figure 2.5 and the sensory neuron in Figure 2.6. A motor neuron has its soma in the spinal cord. It receives excitation from other neurons through its dendrites and conducts impulses along its axon to a muscle. A sensory neuron is specialized at one end to be highly sensitive to a particular type of stimulation, such as light, sound, or touch. The sensory neuron shown in Figure 2.6 is a neuron conducting touch information from the skin to the spinal cord. Tiny branches lead directly from the receptors into the axon, and the cell’s soma is located on a little stalk off the main trunk. Dendrites are branching fibers that get narrower near their ends. (The term dendrite comes from a Greek root word meaning “tree”; a dendrite is shaped like a tree.) The dendrite’s surface is lined with specialized synaptic receptors, at which the dendrite receives information from other neurons. (Chapter 3 concerns synapses.) The greater the surface area of a dendrite, the more information it can receive. Some dendrites branch widely and therefore have a large surface area. Some also contain dendritic spines, the short outgrowths that increase the surface area available for synapses (Figure 2.7). The shape of dendrites varies enormously from one neuron to another and can even vary from one time to another for a given neuron. The shape of the dendrite has much to do with how the dendrite combines different kinds of input (Häusser, Spruston, & Stuart, 2000). Dendritic spines Muscle fiber 11/5/08 2.1 The Cells of the Nervous System 3 Cross-section of skin Sensory endings Axon Soma Nucleus Skin surface Figure 2.6 A vertebrate sensory neuron Note that the soma is located on a stalk off the main trunk of the axon. (As in Figure 2.5, the various structures are not drawn to scale.) The cell body, or soma (Greek for “body”; pl.: somata), contains the nucleus, ribosomes, mitochondria, and other structures found in most cells. Much of the metabolic work of the neuron occurs here. Cell bodies of neurons range in diameter from 0.005 mm to 0.1 mm in mammals and up to a full millimeter in certain invertebrates. Like the dendrites, the cell body is covered with synapses on its surface in many neurons. The axon is a thin fiber of constant diameter, in most cases longer than the dendrites. (The term axon comes from a Greek 03007_02_ch2_p026-047.indd 31 STOP & CHECK 1. What are the widely branching structures of a neuron called And what is the long thin structure that carries information to another cell called? ANSWER The widely branching structures of a neuron are called ndrites, and the long thin structure that carries informan to another cell is called an axon. Text not available due to copyright restrictions word meaning “axis.”) The axon is the information sender o the neuron, conveying an impulse toward other neurons or an organ or muscle. Many vertebrate axons are covered with an insulating material called a myelin sheath with interruption known as nodes of Ranvier (RAHN-vee-ay). Invertebrat axons do not have myelin sheaths. An axon has many branches each of which swells at its tip, forming a presynaptic terminal also known as an end bulb or bouton (French for “button”). Thi is the point from which the axon releases chemicals that cros through the junction between one neuron and the next. A neuron can have any number of dendrites, but no mor than one axon, which may have branches. Axons can range to meter or more in length, as in the case of axons from your spina cord to your feet. In most cases, branches of the axon depar from its trunk far from the cell body, near the terminals. Other terms associated with neurons are afferent, effer ent, and intrinsic. An afferent axon brings information into structure; an efferent axon carries information away from structure. Every sensory neuron is an afferent to the rest of th nervous system, and every motor neuron is an efferent from th nervous system. Within the nervous system, a given neuron i an efferent from one structure and an afferent to another. (You can remember that efferent starts with e as in exit; afferent start with a as in admission.) For example, an axon that is efferen from the thalamus may be afferent to the cerebral cortex (Figur 2.8). If a cell’s dendrites and axon are entirely contained within a single structure, the cell is an interneuron or intrinsic neuron of that structure. For example, an intrinsic neuron of the thala mus has its axon and all its dendrites within the thalamus. 11/5/08 32 Chapter 2 Nerve Cells and Nerve Impulses The function of a neuron relates to its shape (Figure 2.9). For example, the widely branching dendrites of the Purkinje cell of the cerebellum (Figure 2.9a) enable it to receive input from a huge number of axons. By contrast, certain cells in the retina (Figure 2.9d) have only short branches on their dendrites and therefore pool input from only a few sources. B Afferent (to B) A Efferent (from A) Glia Figure 2.8 Cell structures and axons It all depends on the point of view. An axon from A to B is an efferent axon from A and an afferent axon to B, just as a train from Washington to New York is exiting Washington and approaching New York. Variations Among Neurons Neurons vary enormously in size, shape, and function. The shape of a given neuron determines its connections with other neurons and thereby determines its contribution to the nervous system. Neurons with wider branching connect with more neurons. Glia (or neuroglia), the other major components of the nervous system, do not transmit information over long distances as neurons do, although they do exchange chemicals with adjacent neurons. In some cases, that exchange causes neurons to oscillate in their activity (Nadkarni & Jung, 2003). The term glia, derived from a Greek word meaning “glue,” reflects early investigators’ idea that glia were like glue that held the neurons together (Somjen, 1988). Although that concept is obsolete, the term remains. Glia are smaller but also more numerous than neurons. Overall, they occupy about the same volume (Figure 2.10). The brain has several types of glia with different functions (Haydon, 2001). The star-shaped astrocytes wrap around the presynaptic terminals of a group of functionally related axons, Apical dendrite Dendrites Basilar dendrites Axon (a) Axon (c) 10 m (b) (d) (e) Figure 2.9 The diverse shapes of neurons (a) Purkinje cell, a cell type found only in the cerebellum; (b) sensory neurons from skin to spinal cord; (c) pyramidal cell of the motor area of the cerebral cortex; (d) bipolar cell of retina of the eye; (e) Kenyon cell, from a honeybee. (Part e, from R. G. Coss, Brain Research, October 1982. Reprinted by permission of R. G. Coss.) 03007_02_ch2_p026-047.indd 32 11/5/08 2.1 The Cells of the Nervous System 3 Axon Schwann cell Astrocyte Capillary (small blood vessel) Schwann cell Astrocyte Radial glia Oligodendrocyte Myelin sheath Axon Migrating neuron Microglia Microglia Figure 2.10 Shapes of some glia cells Oligodendrocytes produce myelin sheaths that insulate certain vertebrate axons in the central nervous system; Schwann cells have a similar function in the periphery. The oligodendrocyte is shown here forming a segment of myelin sheath for two axons; in fact, each oligodendrocyte forms such segments for 30 to 50 axons. Astrocytes pass chemicals back and forth between neurons and blood and among neighboring neurons. Microglia proliferate in areas of brain damage and remove toxic materials. Radial glia (not shown here) guide the migration of neurons during embryological development. Glia have other functions as well. as shown in Figure 2.11. By taking up chemicals released by axons and then releasing them back to axons, an astrocyte helps synchronize the activity of the axons, enabling them to send messages in waves (Angulo, Kozlov, Charpak, & Audinat, 2004; Antanitus, 1998). Astrocytes also remove waste material created when neurons die and control the amount of blood flow to each brain area (Mulligan & MacVicar, 2004). An additional function is that during periods of heightened activity in some brain area, astrocytes dilate the blood vessels to bring more nutrients into that area (Filosa et al., 2006; Takano et al., 2006). Furthermore, astrocytes release chemicals that modify the activity of neighboring neurons (Perea & Araque, 2007). Clearly, astrocytes do more than just support neurons. They are an important contributor to information processing. Microglia, very small cells, also remove waste material as well as viruses, fungi, and other microorganisms. In effect, they function like part of the immune system (Davalos et al., 2005). Oligodendrocytes (OL-i-go-DEN-druh-sites) in the brain and spinal cord and Schwann cells in the periphery are specialized types of glia that build the myelin sheaths that surround and insulate certain vertebrate axons. Radial glia guide the migration of neurons and their axons and dendrites during embryonic development. When embryological development finishes, most radial glia differentiate into neurons, 03007_02_ch2_p026-047.indd 33 Neuron Astrocyte Synapse enveloped by astrocyte Figure 2.11 How an astrocyte synchronizes associated axons Branches of the astrocyte (in the center) surround the presynaptic terminals of related axons. If a few of them are active at once, the astrocyte absorbs some of the chemicals they release. It then temporarily inhibits all the axons to which it is connected. When the inhibition ceases, all of the axons are primed to respond again in synchrony. (Source: Based on Antanitus, 1998) 11/5/08 Chapter 2 Nerve Cells and Nerve Impulses and a smaller number differentiate into astrocytes and oligodendrocytes (Pinto & Götz, 2007). STOP & CHECK STOP & CHECK 4. Identify one major advantage and one disadvantage of having a blood-brain barrier. 4. The blood-brain barrier keeps out viruses (an advantage) and also most nutrients (a disadvantage). 34 ANSWER 2. Identify the four major structures that compose a neuron. 3. Which kind of glia cell wraps around the synaptic terminals of axons? How the Blood-Brain Barrier Works 2. Dendrites, soma (cell body), axon, and presynaptic terminal. 3. Astrocytes. ANSWERS The Blood-Brain Barrier The blood-brain barrier (Figure 2.12) depends on the arrangement of endothelial cells that form the walls of the capillaries (Bundgaard, 1986; Rapoport & Robinson, 1986). Outside the brain, such cells are separated by small gaps, but in the brain, they are joined so tightly that virtually nothing passes between them. Althou...
View Full Document

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture