You shake my nerves and you rattle my brain Jerry Lee Lewi
You shake my nerves and you rattle my brain. Jerry Lee Lewis 022_069_CogNeu_4e_Ch02.indd 22 7/17/13 9:28 AM
Structure and chapter Function of the 2 Nervous System ONE DAY IN 1963,neuroscientist Jose Delgado coolly stood in a bullring in Cordoba,Spain, facing a charging bull.He did not sport the Spanish matador's typical gear of toreador pants, jacket,and sword,however.No theoretical scientist he,Delgado stepped into came he OUTLINE The Structure of Neurons cidly loo eemingly,this Neuronal Signaling Synaptic Transmission The Role of Glial Cells the The Bigger Picture or d b Overview of Nervous System ch to tre Structure of ne A Guided Tour of the Brain and hrain fun ion.he de d his device mplants ever to be used.Exceedingly contro at the time,his devices were The Cerebral Cortex the forerunners of the now common inracranial devices used for stimulating the brain to treat disorders like Parkinson's discase,chronic pain,and other maladies Development of the Nervous System ables running to and from our brains.He also understood that inside our brains,neurons form an intricate wiring pattern:An electrical signal initiated at one location could travel to another location to trigger a muscle to contract or initiate a behavior,such as aggres- sion,to arise or cease.Delgado was banking on the hope that he had figured out the cor neurons use dge is the on whi neurona signaling are built or us,it i of the n s what this chapter In many of the fo we at wha 23
23 chapter 2 OUTLINE The Structure of Neurons Neuronal Signaling Synaptic Transmission The Role of Glial Cells The Bigger Picture Overview of Nervous System Structure A Guided Tour of the Brain The Cerebral Cortex Development of the Nervous System ONE DAY IN 1963, neuroscientist Jose Delgado coolly stood in a bullring in Cordoba, Spain, facing a charging bull. He did not sport the Spanish matador’s ty pical gear of toreador pants, jacket, and sword, however. No theoretical scientist he, Delgado stepped into the ring in slacks and a pullover sweater while holding a small device in his hand (and a cape, for good eff ect). He was about to see if it worked. As the bull came charging toward him, Delgado stood his ground, trigger fi nger itchy on the device’s butt on. And then he calmly pushed it. Th e bull slammed on the brakes and skidded to a stop, standing a few fee t before the scientist (Figure 2.1). Th e bull placidly looked at the smiling Delgado. See mingly, this was no ordinary bull; but yet it was. One odd thing about this bull, however, gave Delgado his confi dence: An electric stimulator had bee n surgically implanted in its caudate nucleus. Th e device in Delgado’s hand was a transmitt er he had built to activate the stimulator. By stimulating the bull’s caudate nucleus, Delgado had turned off its aggression. Years before, Delgado had bee n horrifi ed by the increasingly popular fr ontal lobotomy surgical procedure that destroyed brain tissue and function. He was interested in fi nding a more conservative approach to treating mental disorders through electrical stimulation. Using his knowledge of the electrical nature of neurons, neuroanatomy, and brain function, he designed his devices, the fi rst neural implants ever to be used. Excee dingly controversial at the time, his devices were the forerunners of the now common intracranial devices used for stimulating the brain to treat disorders like Parkinson’s disease, chronic pain, and other maladies. Delgado understood that our nervous system uses electrochemical energy for communication and that nerves can be thought of as glorifi ed electrical cables running to and fr om our brains. He also understood that inside our brains, neurons form an intricate wiring patt ern: An electrical signal initiated at one location could travel to another location to trigger a muscle to contract or initiate a behavior, such as aggression, to arise or cease. Delgado was banking on the hope that he had fi gured out the correct circuit involved in aggressive behavior. Delgado’s device was built with the knowledge that neurons use electrochemical signals to communicate. Th is knowledge is the foundation on which all theories of neuronal signaling are built. Th us, for us, it is important to understand the basic physiology of neurons and the anatomy of the nervous system, which is what this chapter discusses. In many of the following chapters, we will look at what Structure and Function of the Nervous System 022_069_CogNeu_4e_Ch02.indd 23 8/2/13 9:06 AM
24 CHAPTER 2 Structure and Function of the Nervous System together into circuits that form the brain and extend out to The Structure of Neurons The nervous system is composed of two main classe of cells:neurp s and glial cells.Neu s are the basic signalingunits that transmit information throughout the yous system.as ramon y caial and others of his time deduced neurons take in information make a"decision' about it following some relatively simple rules,and then, by changes in their activity levels,pass it along to other FIGURE 2.1 Jose Delgado halting a charging bull by remote contro neurons.Neurons vary in their form,location,and inter- connectivity within the nervous system (Figure 2.2),and results from the activity within and among specific circuits these variations are closely related to their functions. (Le.,perception,cognition,emotion,action). Glial cells are nonneural cells that serve various func Since all theories of how the brain enables the mind ions in the nervous system,some of which are only now must uimately mesh with the actual nuts and bolts of the eing elucidated. nervous system,we ne underst of it egin v and an overvi within a neuror I1 A en,we turn to t es the GURE 2.2 Ma embryonic rat (fluorescent micrograph)
24 | CHAPTER 2 Structure and Function of the Nervous System together into circuits that form the brain and extend out to form the entire nervous system. We survey the anatomy and functions of the brain and the nervous system. Finally, we look at the development of the nervous system— prenatally, in the years following birth, and in adults. The Structure of Neurons Th e nervous system is composed of tw o main classes of cells: neurons and glial cells. Neurons are the basic signaling units that transmit information throughout the nervous system. As Ramón y Cajal and others of his time deduced, neurons take in information, make a “ decision” about it following some relatively simple rules, and then, by changes in their activity levels, pass it along to other neurons. Neurons vary in their form, location, and interconnectivity within the nervous system (Figure 2.2), and these variations are closely related to their functions. Glial cells are nonneural cells that serve various functions in the nervous system, some of which are only now being elucidated. Th ese include providing structural support and electrical insulation to neurons, and modulating neuronal activity . We begin with a look at neuronal structure and function, and then we return to glial cells. Th e standard cellular components found in almost all eukaryotic cells are found in neurons as well. A cell membrane encases the cell body (in neurons, it is sometimes called the results fr om the activity within and among specifi c circuits (i.e., perception, cognition, emotion, action). Since all theories of how the brain enables the mind must ultimately mesh with the actual nuts and bolts of the nervous system, we nee d to understand the basics of its organizational structure, function, and modes of communication. In this chapter, we begin with the anatomy of the neuron and an overview of how information is transferred both within a neuron, and fr om one neuron to the next. Th en, we turn to the bigger picture. Our neurons are strung FIGURE 2.1 Jose Delgado halting a charging bull by remote control. FIGURE 2.2 Mammalian neurons show enormous anatomical variety. (Clockwise from upper left) Neuron from the vestibular area of the brain—glial cells are the thin white structures (confocal light micrograph); Hippocampal neuron (fl uorescent micrograph); Mouse neuron and spinal cord ganglia (transmission electron micrograph); Multipolar neuron cell body from human cerebral cortex (scanning electron micrograph); Neuron from the brain; Nerve culture from dorsal root ganglia of an embryonic rat (fl uorescent micrograph). 022_069_CogNeu_4e_Ch02.indd 24 7/17/13 9:28 AM
The Structure of Neurons 25 ous with that in the cell body. soma:Greek for "body").which contains the metabolic Neurons.unlike other cells.possess uniau cytological machinery that maintains the neuron:a nucleus.endoplas- features and physiological properties that enable them to mic reticulum.a cytoskeleton.mitochondria.Golgi appara transmit and process information rapidly.The two predomi- nant cellular components unique to neurons are the dendrites and axon.Dendrites are branching extensions of the neuron intracellular fluid that is made up of a combination of ions. that receive inputs from other neurons.They take many var- predominantly ions of potassium,sodium,chloride,and cal- ied and complex forms,depending on the type and location of the neuron.The arborizations may look like the branches alty extracellular f a in the that is widerin one dir n the er.(a) rtex showing a Purkinje cell.(b)Co
The Structure of Neurons | 25 Neurons, unlike other cells, possess unique cytological features and physiological properties that enable them to transmit and process information rapidly. Th e tw o predominant cellular components unique to neurons are the dendrites and axon. Dendrites are branching extensions of the neuron that receive inputs fr om other neurons. Th ey take many varied and complex forms, depending on the ty pe and location of the neuron. Th e arborizations may look like the branches and tw igs of an old oak tree , as see n in the complex dendritic structures of the cerebellar Purkinje cells ( Figure 2.4), soma ; Gree k for “body”), which contains the metabolic machinery that maintains the neuron: a nucleus, endoplasmic reticulum, a cytoskeleton, mitochondria, Golgi apparatus, and other common intracellular organelles (Figure 2.3). Th ese structures are suspended in cytoplasm, the salty intracellular fl uid that is made up of a combination of ions, predominantly ions of potassium, sodium, chloride, and calcium, as well as molecules such as proteins. Th e neuron, like any other cell, sits in a bath of salty extracellular fl uid, which is also made up of a mixture of the same ty pes of ions. FIGURE 2.4 Soma and dendritic tree of a Purkinje cell from the cerebellum. The Purkinje cells are arrayed in rows in the cerebellum. Each one has a large dendritic tree that is wider in one direction than the other. (a) Sagittal section through a cerebellar cortex showing a Purkinje cell. (b) Confocal micrograph of a Purkinje cell from mouse cerebellum. The cell is visualized using fl ourescence methods. a b Dendrites Axon FIGURE 2.3 Idealized mammalian neuron. A neuron is composed of three main parts: a cell body, dendrites, and an axon. The cell body contains the cellular machinery for the production of proteins and other cellular macromolecules. Like other cells, the neuron contains a nucleus, endoplasmic reticulum, ribosomes, mitochondria, Golgi apparatus, and other intracellular organelles (inset). The dendrites and axon are extensions of the cell membrane and contain cytoplasm continuous with that in the cell body. Mitochondria Axon Axon terminals Cell body Dendrites Golgi apparatus Endoplasmic reticulum Ribosomes Nucleus 022_069_CogNeu_4e_Ch02.indd 25 7/17/13 9:28 AM
26 CHAPTER 2 Structure and Function of the Nervous System FIGURE 2.5 Spinal motorn d their or they may be much simpler.such as the dendrites in spi neuron transmits the signal to other neurons or other nal motor neurons(Figure 2.5).Many dendrites also have cell types.Transmission occurs at the synapse,a spe specialized processes called spines,little knobs attached cialized structure where two neurons come into close by small necks to the surface of the dendrites,where the contact so that chemical or electrical signals can be dendrites receive inputs from other neurons(Figure 2.6). passed from one cell to the next.Some axons branch The axon is a single process that extends from the to form axon collaterals that can transmit signals cell body.This structure represents the output side of to more than one cell (Figure 2.7).Many axons are he axon terminals.where the engt e axons,there are evenly space gaps in the hese gaps nly refe the g mist I lis-A t he who first de em. gnal TAKE-HOME MESSAGES Neurons and glial cells make up the nervous system. Neurons ar the cells that trans consist of a cell soma (hody)axon and dendrites Neurons communicate with other neurons and cells at specialized structures called synapses,where IGURE 2.6 De 6emeneuonesrneasenascaneconmee dendrites (green)and the spines (redy
26 | CHAPTER 2 Structure and Function of the Nervous System FIGURE 2.6 Dendritic spines on cultured rat hippocampal neurons. Neuron has been triple stained to reveal the cell body (blue), dendrites (green), and the spines (red). FIGURE 2.5 Spinal motor neuron. (a) Neurons located in the ventral horn of the spinal cord send their axons out the ventral root to make synapses on muscle fi bers. (b) A spinal cord motor neuron stained with cresyl echt violet stain. a b Dendrites Axon or they may be much simpler, such as the dendrites in spinal motor neurons (Figure 2.5). Many dendrites also have specialized processes called spines , litt le knobs att ached by small necks to the surface of the dendrites, where the dendrites receive inputs fr om other neurons (Figure 2.6). Th e axon is a single process that extends fr om the cell body. Th is structure represents the output side of the neuron. Electrical signals travel along the length of the axon to its end, the axon terminals, where the neuron transmits the signal to other neurons or other cell ty pes. Transmission occurs at the synapse, a specialized structure where tw o neurons come into close contact so that chemical or electrical signals can be passed fr om one cell to the next. Some axons branch to form axon collaterals that can transmit signals to more than one cell (Figure 2.7). Many axons are wrapped in layers of a fatty substance called myelin . Along the length of the axons, there are evenly spaced gaps in the myelin. Th ese gaps are commonly referred to as the nodes of Ranvier (see Figure 2.11), named after the French histologist and anatomist Louis-Antoine Ranvier, who fi rst described them. Later, when we look at how signals move down an axon, we will explore the role of myelin and the nodes of Ranvier in accelerating signal transmission. TAKE-HOME MESSAGES ■ Neurons and glial cells make up the nervous system. ■ Neurons are the cells that transmit information throughout the nervous system. Most neurons consist of a cell soma (body), axon, and dendrites. ■ Neurons communicate with other neurons and cells at specialized structures called synapses, where chemical and electrical signals can be conveyed between neurons. 022_069_CogNeu_4e_Ch02.indd 26 7/17/13 9:28 AM