This is just one of the solutions for you to be successful. As understood, feat does not recommend that you have fabulous points. The text is organized around a series of key experiments to illustrate how scientific progress is made and helps. Principles of Neurobiology is accompanied by a rich package of online student and instructor resources including animations, journal club suggestions, figures in PowerPoint, and a Question Bank for adopting instructors.
In , Matthias Schleiden and Teodor Schwann formally proposed the cell theory: all living organisms are composed of cells as their basic units. Te cell theory was widely accepted in almost every discipline of biology by the second half of the nineteenth century, except among researchers studying the nervous system.
Although cell bodies had been observed in nervous tissues, many histologists of that era believed that nerve cells were linked together by their elaborate processes to form a giant net, or reticulum, of nerves. Proponents of this reticular theory believed that the reticulum as a whole, rather than its individual cells, constituted the unit of the nervous system. Among the histologists who supported the reticular theory of the nervous system was Camillo Golgi, who made many important contributions to science, including the discovery of the Golgi apparatus, an intracellular organelle responsible for processing proteins in the secretory pathway Figure When a piece of neural tissue is soaked in a solution of silver nitrate and potassium dichromate in the dark for several weeks, black precipitates microcrystals of silver chromate stochastically form in a small fraction of nerve cells, rendering these cells visible against an unstained background.
Importantly, once black precipitates form within a cell, an autocatalytic reaction occurs such that the entire cell, including most or all of the elaborate extensions, can be visualized in its native tissue Figure Golgi stain thus enabled visualization of the entire morphology of individual neurons for the frst time.
Despite inventing this key method for neuronal visualization, however, Golgi remained a believer in the reticular theory Box Instead, neurons intimately contact each other, with communication between distinct neurons occurring at these contact sites Box Te term synapse was later coined by Charles How is the nervous system organized? An individual Purkinje cell in the mouse cerebellum is stained black by the formation of silver chromate precipitate, allowing visualization of its complex dendritic tree.
The axon, which is not included in this image, projects downward from the cell body indicated by an asterisk. The inset shows a higher magnifcation of a dendritic segment, highlighting protruding structures called dendritic spines. Tey shared the Nobel Prize for Physiology or Medicine, the frst to be awarded for fndings in the nervous system.
However, their debates on how nerve cells constitute the nervous system— via a reticular network or as individual neurons communicating with each other through synaptic contacts—continued during their Nobel lectures Figure A,B. For example, utilizing the brainbow method Section Was he not a careful observer?
After all, he made many great discoveries, including those describing the Golgi apparatus. On the one hand, there is his method, which has generated a prodigious number of observations that have been enthusiastically confrmed.
But on the other, there are his interpretations, which have been questioned and rejected. Golgi was trained in a scientifc environment in which this reticular theory was the dominant interpretation of nervous system organization and so tried to ft his observations into existing theory.
The dendritic, cell body, and axonal layers are indicated on the left. Note that axons below the cell bodies have defnitive endings. C Hippocampal granule cells labeled by the brainbow technique, which allows the spectral separation of individual neurons expressing different mixtures of cyan, yellow, and red fuorescent proteins. Not only cell bodies but also some dendrites above and axon terminals below can be resolved by different colors.
A, after Golgi C [] Nobel Lecture. He believed that it was their axons, which formed an inseparable giant net as he viewed them Figure A, bottom , that performed all the special functions of the nervous system. Tis story teaches an important lesson: scientists need to be observant, but they also need to be as objective and unbiased as possible when interpreting their own observations. For example, during development, neurons begin with only cell bodies.
Axons then grow out from the cell bodies toward their fnal destinations. Tis was demonstrated by observing axon growth in vitro via experiments made possible by tissue culture techniques, which were initially developed for the purpose of visualizing neuronal process growth Figure Axons are led by a structure called the growth cone, which changes its shape dynamically as axons extend.
We will learn more about the function of the growth cone in axon guidance in Chapter 5. Te fnal pieces of evidence that neuronal processes are not fused with each other came from observations made possible by the development of electron microscopy, a technique allowing visualization of structures at nanometer nm resolution.
Conventional light microscopy, which scientists since Hooke have used to observe biological samples, cannot resolve structures less than nm apart because of the physical properties of light.
Te use of electron microscopy to examine chemical synapses so named because communication between cells is mediated by release of chemicals called neurotransmitters revealed that the synaptic cleft, a 20— nm gap, separates a neuron from its target, which can be another neuron or a muscle cell Figure A. Synaptic partners are not symmetric: presynaptic terminals of neurons contain small synaptic vesicles flled How is the nervous system organized?
Postsynaptic target cells have postsynaptic specializations also called postsynaptic densities enriched in neurotransmitter receptors on their plasma membrane surfaces. Chemical synapses are the predominant type of synapse allowing neurons to communicate with each other and with muscle cells.
We will study them in greater detail in Chapter 3. Neurons can also communicate with each other by electrical synapses mediated by gap junctions Figure B. Here, each partner neuron contributes protein subunits to form gap junction channels that directly link the cytoplasms of two adjacent neurons, allowing ions and small molecules to travel between them.
Tese gap junctions come closest to what the reticular theory would imagine as a fusion between diferent neurons. However, macromolecules cannot pass between gap junctions, and the neurons remain distinct cells with highly regulated communication. Te existence of gap junctions, therefore, does not violate the premise that individual neurons are the building blocks of the nervous system.
Te dendritic morphologies and axonal projection patterns of specifc types of neurons are characteristic and are often used for classifcation. For example, the most frequently encountered type of neuron in the mammalian cerebral cortex and hippocampus, the pyramidal neuron, has a pyramid-shaped cell body with an apical dendrite and several basal dendrites that branch extensively Figure A.
Much of the dendritic tree sprouts dendritic spines Figure inset , which contain postsynaptic specializations in close contact with presynaptic terminals of partner neurons. Another widely encountered neuronal type, basket cells Figure B , wrap their axon terminals around the cell bodies of pyramidal cells in the cerebral cortex or Purkinje cells Figure in the cerebellum.
A Electron micrograph of a chemical synapse between the presynaptic terminal of a motor neuron and the postsynaptic specialization of its target muscle cell. A synaptic cleft separates the two cells. The arrow points to a synaptic vesicle. B Electron micrograph of an electrical synapse gap junction between two dendrites of mouse cerebral cortical neurons. Two opposing pairs of arrows mark the border of the electrical synapse.
Asterisks indicate mitochondria in both micrographs. A, courtesy of Jack McMahan. Harris, SynapseWeb. Figure The fr st time-lapse depiction of a growing axon. Frog embryonic spinal cord tissue was cultured in vitro. Growth of an individual axon was sketched with the aid of a camera lucida at the time indicated on the left hour. The stationary blood vessel oval provided a landmark for the growing tips of the axon, called growth cones, which undergo dynamic changes in shape, including both extensions and retractions.
A distance scale is at the bottom of the fgure. A A pyramidal cell from rabbit cerebral cortex. A typical pyramidal cell has an apical dendrite blue that gives off branches as it ascends, several basal dendrites blue that emerge from the cell body, and an axon red that branches locally and projects to distant targets. B A basket cell from mouse cerebellum. C A motor neuron from cat spinal cord. Its bushy dendrites blue receive input within the spinal cord, and its axon red projects outside the spinal cord to muscle, while also leaving behind local branches.
D A mammalian sensory neuron from a dorsal root ganglion. A single process from the cell body bifurcates into a peripheral axon dashed to indicate the long distance with terminal endings in the skin equivalent of dendrites for collecting sensory information and a central axon that projects into the spinal cord. E A motor neuron from the fruit fy ventral nerve cord equivalent to the vertebrate spinal cord. Most invertebrate central neurons are unipolar: a single process extends out of the cell body, giving rise to dendritic branches blue and an axon red.
In all panels, asterisks denote axon initiation segments; as will be discussed in Section 1. Te spinal cord motor neuron extends bushy dendrites within the spinal cord Figure C and projects its axon out of the spinal cord and into muscle. Located in the dorsal root ganglion just outside the spinal cord, a sensory neuron of the somatosensory system which processes bodily sensation extends a single process that bifurcates, forming a peripheral axon that gives rise to branched terminal endings and a central axon that projects into the spinal cord Figure D.
Most vertebrate neurons have both dendrites and an axon leaving the cell body, and hence are called multipolar or bipolar if there is only a single dendrite ; somatosensory neurons are pseudounipolar because, although there is just one process leaving the cell body, it gives rise to both peripheral and central branches. What is the direction of information fow within individual neurons?
Terefore, every neuron has 1 a receptive component, the cell body and dendrites; 2 a transmission component, the axon; and 3 an efector component, the axon terminals. With few exceptions the somatosensory neuron being one , this important principle has been validated by numerous observations and experiments since it was proposed a century ago and has been used extensively to deduce the direction of information fow in the vertebrate CNS. We will study the cell biological basis of neuronal polarization in Chapter 2.
How did observing the morphologies of individual neurons lead to the discovery of this rule? By examining diferent neurons along the visual pathway Figure , for example, one can see that at each connection, dendrites are at the receiving end, facing the external world, while axons are oriented so as to deliver such information to more central targets, sometimes at a great distance from the cell body where the axon originates.
Tis applies to neurons in other sensory systems as well. Conversely, in motor systems, information must generally fow from the CNS to the periphery. Te morphology of the motor neuron indeed supports the notion that its bushy dendrites receive input within the spinal cord, and its long axon, projecting to muscle, provides output Figure C. Neuronal processes in invertebrates can also be defned as dendrites and axons according to their functions, with dendrites positioned to receive information and axons to send it.
However, the morphological diferentiation of most invertebrate axons and dendrites, especially in the CNS, is not as clear-cut as it is for vertebrate neurons.
Most often, invertebrate neurons are unipolar, extending a single process giving rise to both dendritic and axonal branches Figure E. Dendritic branches are often, but not always, closer to the cell body. In many cases, the same branches can both receive and send information; this occurs in some vertebrate neurons as well, as we will learn in Chapters 4 and 6.
We now know that the nervous system uses electrical signals to propagate information. It was known by the beginning of the twentieth century that electrical signals were spread in neurons via transient changes in membrane potential, the electrical potential diference across the neuronal membrane.
As we will learn in more detail in Chapter 2, neurons at the resting state are more negatively charged inside the cells compared to outside the cells. When neurons are excited, their membrane potentials change transiently, creating nerve impulses that propagate along their axons.
But how is information relayed through nerve impulses? Quantitative studies of how sensory stimuli of diferent magnitudes induce nerve impulses provided important clues. Studies of muscle contraction in response to electrical stimulation of motor nerves suggested that an elementary nerve impulse underlies diferent stimulus strengths.
An all-or-none conduction principle became evident when amplifers for electrical signals built in the s made it possible to record nerve impulses from single axon fbers in response to sensory stimulation. Edgar Adrian and co-workers systematically measured nerve impulses from somatosensory neurons Figure D that convey information about touch, pressure, and pain to the spinal cord.
Tey found that individual nerve impulses were of a uniform size and shape, whether they were elicited by weak or strong sensory stimuli; stronger stimuli increased the frequency of such impulses but not the properties of each impulse Figure Tese experiments led to two important concepts in modern neuroscience.
Te frst concept is the presence of an elementary unit of nerve impulses that axons use to convey information across long distances; we now call this elementary unit an action potential. In Chapter 2, we will study in greater detail the molecular basis of action potentials, including why they exhibit the all-or-none property. Te second concept is that neurons use the frequency of action potentials to convey the intensity of signals. Whereas the frequency of action potentials is the most widely used means to convey signal intensity throughout the nervous system, the timing of action potentials can also convey important information.
In addition to action potentials, another important form of communication within neurons are graded potentials—membrane potentials that vary continuously 13 photoreceptor cells bipolar cell retinal ganglion cell Figure 1—16 Neurons and information fow in the ver tebrate retina. Visual information is collected by photoreceptor cells in the retina, communicated to the bipolar cell, and then to the retinal ganglion cell, which projects a longdistance axon into the brain.
Note that for both the bipolar cell and the retinal ganglion cell, information is received by their dendrites blue and sent via their axons red. The photoreceptor processes can also be divided into a dendrite equivalent that detects light blue and an axon that sends output to the bipolar cell. Arrows indicate the direction of information fow.
We will learn more about these cells and connections in Chapter 4. A Experimental setup for applying a specifed amount of pressure to the toe of a cat, while recording nerve impulses action potentials from an associated sensory nerve. The x axis shows the time scale in units of seconds s. A typical neuron in the mammalian CNS receives thousands of inputs at dendritic spines blue distributed along its dendritic tree.
Inputs are collected in the form of synaptic potentials, which travel toward the cell body blue arrows and are integrated at the axon initial segment red to produce action potentials. Action potentials propagate to axon terminals red arrows and trigger neurotransmitter release, thus conveying information to postsynaptic partner neurons. One type of graded potential, called synaptic potentials, is produced at postsynaptic sites in response to neurotransmitter release from presynaptic partners.
Unlike action potentials, the sizes of graded potentials vary depending on the strength of the input stimuli and the sensitivity of postsynaptic or sensory neurons to those stimuli. Some neurons, including most neurons in the vertebrate retina, do not fre action potentials at all.
Tese non-spiking neurons use graded potentials to transmit information, even in their axons. Synaptic potentials are usually produced at dendritic spines, along the dendrite tree, and at the soma of a neuron. A typical mammalian neuron contains thousands of postsynaptic sites on its dendritic tree, allowing it to collect input from many individual presynaptic partners Figure As we will learn later, there are two kinds of inputs: excitatory inputs facilitate action potential production in the postsynaptic neuron, whereas inhibitory inputs impede action potential production.
In most neurons, the purpose of these synaptic potentials is to determine whether, when, and how frequently the neuron should fre action potentials so that information can propagate along its axon to its own postsynaptic target neurons. Te site of action potential initiation is typically the axon initial segment or the axon hillock adjacent to the soma Figure A—C.
Tus, synaptic potentials generated in dendrites must travel through the soma to the axon initial segment to contribute to action potential generation.
Te rule of action potential initiation near the soma has notable exceptions. For example, in the sensory neuron in Figure D, action potentials are initiated at the junction between terminal endings and the peripheral axon of the sensory neuron such that sensory information can be transmitted by the peripheral and central axon to the spinal cord across a long distance.
In invertebrate neurons, which are mostly unipolar, action potential initiation likely occurs at the junction between the dendritic and axonal compartments Figure E.
How is information transmitted between neurons? At electrical synapses, membrane potential changes are directly transmitted from one neuron to the next by ion fow across gap junctions Figure B. At chemical synapses, the arrival of action potentials or graded potentials in non-spiking neurons at presynaptic terminals triggers neurotransmitter release.
Neurotransmitters difuse across the synaptic cleft and bind to their receptors on postsynaptic neurons to produce synaptic potentials Figure ; Figure A. Te process of neurotransmitter release from the presynaptic neuron and neurotransmitter reception by the postsynaptic neuron is collectively referred to as synaptic transmission.
Tus, whereas intraneuronal communication is achieved by membrane potential changes in the form of graded potentials and action potentials, interneuronal communication at chemical synapses relies on neurotransmitter release and reception.
We will 15 How is the nervous system organized? Te simplest circuits in vertebrates, those that mediate the spinal refexes, comprise as few as two interconnected neurons: a sensory neuron that receives external stimuli and a motor neuron that controls muscle contraction. Many fundamental neurobiological principles have been derived from studying these simple circuits. Te underlying circuit mechanism for this knee-jerk refex has been identifed: sensory neurons embed their endings in specialized apparatus called muscle spindles in an extensor muscle whose contraction extends the knee joint.
Tese sensory neurons detect stretching of the muscle spindles caused by the physical impact of the hammer and convert this stimulus into electrical signals—namely, receptor potentials—at the sensory endings. Next, the peripheral and central axons of the sensory neurons propagate these electrical signals to the spinal cord as action potentials. Tere, central axon terminals of the sensory neurons release neurotransmitters directly onto the dendrites of their partner motor neurons.
Tese motor neurons extend their own axons outward from the spinal cord and terminate in the same extensor muscle in which the sensory neurons embed their endings. Sensory axons are also called aferents, referring to axons projecting from peripheral tissues to the CNS, whereas motor axons are called eferents, referring to axons that project from the CNS to peripheral targets.
Both the sensory and motor neurons in this circuit are excitatory neurons. When excitatory neurons are activated—that is, when they fre action potentials and release neurotransmitters—they make their postsynaptic target cells more likely to fre action potentials. Terefore, mechanical stimulation activates sensory neurons.
Tis in turn activates the postsynaptic motor neurons. Neurotransmitter release at motor axon terminals leads to contraction of the extensor muscle. Te knee-jerk refex involves coordination of more than one muscle.
Te fexor muscle, which is antagonistic to the extensor muscle, must not contract at the same time in order for the knee-jerk refex to occur. As we will learn in Chapter 8, contraction of extensor muscles increases the angle of a joint, while contraction of fexor muscles decreases the angle of a joint.
Terefore, the sensory axons must inhibit contraction of the corresponding fexor muscle in addition to causing contraction of the extensor muscle. Tis inhibition is mediated by inhibitory interneurons in the spinal cord, a second type of postsynaptic neuron targeted by the sensory axons. Note that neurobiologists use the term interneuron in two diferent contexts. In a broad context, all neurons that are not sensory or motor neurons are interneurons.
But in most contexts, the term interneuron refers to neurons that confne their axons within a specifc region, in contrast to projection neurons, whose axons link diferent regions of the nervous system. Te spinal inhibitory interneurons ft both criteria. In this refex circuit, activation of sensory neurons causes excitation of these inhibitory interneurons, which in turn inhibit the motor neurons innervating the fexor muscle.
Tis inhibition makes it more difcult for the fexor motor neurons to fre action potentials, causing the fexor muscle to relax. Tus, coordinated contraction of the extensor muscle and relaxation of the fexor muscle brings the lower leg forward.
First analyzed in studies of spinal refexes by Charles Sherrington in the s, the role of inhibition is crucial in coordinating neuronal function throughout the nervous system. In summary, the knee-jerk refex involves one of the simplest neural circuits: coordinated excitation and inhibition is executed by monosynaptic connections between sensory neurons and motor neurons and disynaptic connections between sensory neurons and a diferent group of motor neurons via inhibitory interneuron intermediates.
A simple neural circuit is responsible for the involuntary jerk that results when the front of the knee is hit with a hammer. In this simplifed scheme, a single neuron represents a population of neurons performing the same function. The sensory neuron extends its peripheral axon to the muscle spindle of the extensor muscle and its central axon to the spinal cord.
In the spinal cord, the sensory neuron has two postsynaptic targets: the green motor neuron that innervates the extensor muscle, and the red inhibitory interneuron that synapses with the yellow motor neuron innervating the fexor muscle. When the knee is hit, mechanical force activates the sensory neuron, resulting in excitation of the extensor motor neuron, which causes contraction of the extensor muscle following solid arrows.
At the same time, sensory neuron activation causes inhibition of the fexor motor neuron, which relaxes the fexor muscle dashed arrow. The spinal cord is drawn as a cross section. The gray matter at the center contains cell bodies, dendrites, and the synaptic connections of spinal cord neurons; the white matter at the periphery consists of axons of projection neurons.
The sensory neuron cell body is located in a dorsal root ganglion adjacent to the spinal cord. Most neural circuits are much more complex than the spinal cord refex circuit.
Box discusses commonly used circuit motifs we will encounter in this Box Common neural circuit motifs Te simplest circuit consists of two synaptically connected neurons, such as the sensory neuron—extensor motor neuron circuit in the knee-jerk refex.
In circuits containing more than two neurons, individual neurons can receive input from and send output to more than one partner. Further complexity arises when some neurons in a circuit are excitatory and others are inhibitory. Te nervous system employs many circuit motifs—common confgurations of neural circuits that allow the connection patterns of individual neurons to execute specifc functions.
Here, we introduce the most common circuit motifs Figure Convergent excitation Figure A refers to a circuit motif wherein several neurons synapse onto the same postsynaptic neuron. Conversely, divergent excitation Figure B refers to a motif wherein a single neuron synapses onto multiple postsynaptic targets via branched axons axonal branches are also called collaterals.
Convergent and divergent connections allow individual neurons to integrate input from multiple presynaptic neurons and to send output to multiple postsynaptic targets, respectively.
Serially connected excitatory neurons constitute a feedforward excitation motif Figure C for propagating information across multiple brain regions, as in the relay of somatosensory stimuli to the primary somatosensory cortex Figure When a postsynaptic neuron synapses onto its own presynaptic partner, this motif is called feedback excitation Figure D. Neurons that transmit parallel streams of information can also excite each other, forming a recurrent lateral excitation motif Figure E.
When excitatory and inhibitory neurons interact in the same circuit, as is most often the case, many interesting circuit motifs with diverse functionalities can be constructed. Te names of motifs involving inhibitory neurons usually emphasize the nature of the inhibition. In feedforward inhibition Figure F , an excitatory neuron synapses onto both an excitatory neuron and an inhibitory neuron, and the inhibitory neuron further synapses onto the excitatory postsynaptic neuron.
In feedback inhibition Figure G , the postsynaptic excitatory neuron synapses onto an inhibitory neuron, which synapses back onto the postsynaptic excitatory neuron. In both cases, inhibition can control the duration and magnitude of the excitation of the target neuron.
In recurrent cross inhibition Figure H , two parallel excitatory pathways crossinhibit each other via inhibitory neuron intermediates; the inhibition of the fexor motor neuron in the knee-jerk refex discussed in Section 1. In lateral inhibition Figure I , an inhibitory neuron receives excitatory input from one or several parallel streams of excitatory neurons and sends inhibitory output to many postsynaptic targets of these excitatory neurons.
Lateral inhibition is widely used in processing sensory information, as we will study in greater detail in Chapters 4 and 6. Finally, when an inhibitory neuron synapses onto another inhibitory neuron, the excitation of the frst inhibitory neuron reduces the inhibitory output of the second inhibitory neuron, causing disinhibition of the fnal target neuron Figure J. Te circuit motifs discussed here are often used in combinations, giving rise to many diferent ways of processing information.
In Chapter 3, we will encounter another group of neurons, the modulatory neurons, which can act on both excitatory and inhibitory neurons to up- or downregulate their excitability or synaptic transmission, adding further richness to the information processing functions of neural circuits. In Chapter 14, we will examine circuit architecture from theoretical and computational perspectives.
We will see that excitatory and inhibitory neurons can be connected in specifc ways to produce logic gates for computation; these logic gates are also the bases of all operations in modern computers Section A convergent excitation B divergent excitation F feedforward inhibition G feedback inhibition C D feedback excitation feedforward excitation H recurrent cross inhibition I lateral inhibition E recurrent lateral excitation J disinhibition Figure Common circuit motifs.
In all panels, the general information fow is from left to right. Green, excitatory neuron; red, inhibitory neuron; gray, any neuron. A—E Circuit motifs consisting of only excitatory neurons. F—J Circuit motifs that include inhibitory neurons.
See text for more details. For recurrent inhibition H and lateral inhibition I , only the feedforward modes are depicted; the feedback modes of these motifs may also be used not shown , in which case the inhibitory neuron s receive input s from postsynaptic excitatory neurons, as in Panel G.
For example, a subject becomes aware that a hammer has hit her knee because sensory neurons also send axonal branches that ascend along the spinal cord. After passing through relay neurons in the brainstem and thalamus, sensory information eventually reaches the primary somatosensory cortex, the part of the cerebral cortex that frst receives somatosensory input from the body Figure Cortical processing of such sensory input generates the perception that her knee has been hit.
Such information also propagates to other cortical areas, including the primary motor cortex. Te primary motor cortex sends descending output directly and indirectly to spinal cord motor neurons to control muscle contraction Figure , in case we want to move our leg voluntarily in contrast to the knee-jerk refex, which is involuntary. We will study these sensory and motor pathways in greater detail in Chapters 6 and 8, but in general we know far less about the underlying mechanisms of these ascending, cortical, and descending circuits than we do about the spinal cord refex circuit.
Elucidating the principles of information processing in complex neural circuits that mediate sensory perception and motor action is one of the most exciting and challenging goals of modern neuroscience. However, throughout prior centuries, philosophers argued about whether brain functions underlie mind, let alone whether specifc brain regions are responsible for specifc mental activities.
Even in the early twentieth century, a prevalent view was that any specifc mental function is carried out by neurons across many areas of the cerebral cortex. Franz Joseph Gall developed a discipline called phrenology in the early nineteenth century. Gall supposed that all behavior emanates from the brain, with specifc brain regions controlling specifc functions.
Te centers for each mental function, he reasoned, grow with use, creating bumps and ridges on the skull. Based on this reasoning, Gall and his followers attempted to map human mental function to specifc parts of the cortex, correlating the size and shape of the bumps 17 Figure Sensory and motor pathways between the spinal cord and cerebral cortex.
Some sensory neurons, in addition to participating in the spinal cord refex circuit, send an ascending branch that connects with relay neurons in the brainstem, which deliver information to neurons in the primary somatosensory cortex via intermediate neurons in the thalamus.
Through intercortical connections, information is delivered to neurons in the primary motor cortex, which send descending output directly and indirectly to spinal cord motor neurons for voluntary control of muscles. Shown here are the most direct routes for these ascending and descending pathways.
The spinal cord is represented in cross section. The brain is shown from a sagittal view not at the same scale as the spinal cord. According to phrenology, the brain is divided into individual areas specialized for defned mental functions.
The size of each area is modifed by use. For example, a cautious person would have an enlarged area corresponding to cautiousness. Brain lesions provided the frst instances of scientifc evidence that specialized regions of the human cerebral cortex perform specifc functions. Each hemisphere of the cerebral cortex is divided into four lobes, the frontal, parietal, temporal, and occipital lobes, based on the major folds called fssures separating the lobes Figure A.
In the s, Paul Broca discovered lesions in a specifc area of the human left frontal lobe Figure B in patients who could not speak. A Major fssures divide each cerebral cortex hemisphere into frontal, parietal, temporal, and occipital lobes. Observation of similar lesions in language-defcient patients led Broca to propose that the area is essential for language production. In the twentieth century, two important techniques—brain stimulation and brain imaging—confrmed and extended fndings from lesion studies, revealing in greater detail specifc brain regions that perform distinct functions.
Brain stimulation is a standard procedure for mapping specifc brain regions to guide brain surgeries, such as severing axonal pathways to treat intractable epilepsy. Epilepsy is a medical condition characterized by recurrent seizures—strong surges of abnormal electrical activity that afect part or all of the brain; Box Tese brain stimulation studies have identifed additional areas involved in language production.
One of the most remarkable methods developed in the late twentieth century is the noninvasive functional brain imaging of healthy human subjects as they perform specifc tasks.
Te most widely used technique is functional magnetic resonance imaging fMRI , which monitors signals originating from changes in blood fow that results from local neuronal activity. By allowing researchers to observe whole-brain activity without bias while subjects perform specifc tasks, fMRI has revolutionized our understanding of brain regions implicated in specifc functions. Because fMRI ofers higher spatial resolution than do lesion studies, it has enabled researchers to ask more specifc questions.
For example, do bilingual speakers use the same cortical areas for their native and second languages? Te answer depends on the cortical area in question and the age at which an individual acquires the second language. Experimental studies of mammalian model organisms, which share this gross organization Figure B , complement our understanding.
We will study the organization and function of many nervous system regions in detail in subsequent chapters. An important organizational principle worth emphasizing now is that the nervous system uses maps to represent information.
We have already seen this phenomenon in our earlier discussion of the auditory and visual maps that barn owls use to target their prey. Two striking examples of maps in the human brain are the motor homunculus and the sensory homunculus Figure For example, stimulation of cortical neurons in specifc parts of the primary motor cortex elicits movement of specifc body parts on the contralateral side.
Movement of the left side of the body is controlled by the right side of the brain and vice versa. Systematic studies revealed a cortical topographic map corresponding to movement of specifc body parts: nearby neurons in the motor homunculus control the 19 A native English second French B native 1 Turkish native 2 English common C native English second French common Figure Representations of native and second languages as revealed by functional magnetic resonance imaging fMRI.
The detection of blood-fow signals associated with brain activity by fMRI provides a means for imaging the brain loci where native and second languages are processed.
In the brain scans on the left side of the fgure, green rectangles highlight language-processing areas in the left hemisphere; the highlighted areas are magnifed on the right. This book was written to introduce students to the major issues, ex perimental strategies, and current knowledge base in cellular, molecular, and developmental neuroscience.
The concept for the book arose from a section of an introductory neuroscience course given to first-year medical students at the University of Virginia School of Medicine.
The text pre sumes a basic, but not detailed, understanding of nervous system orga nization and function, and a background in biology. While some of the specific information presented undoubtedly will be outdated rapidly, the "gestalt" of this emerging field of inquiry as presented here should help the beginning stu dent organize new information. To understand how such a complex system functions requires the construction and analysis of computational models at many different levels.
This book provides a step-by-step account of how to model the neuron and neural circuitry to understand the nervous system at all levels, from ion channels to networks.
Starting with a simple model of the neuron as an electrical circuit, gradually more details are added to include the effects of neuronal morphology, synapses, ion channels and intracellular signalling. The principle of abstraction is explained through chapters on simplifying models, and how simplified models can be used in networks. This theme is continued in a final chapter on modelling the development of the nervous system.
Requiring an elementary background in neuroscience and some high school mathematics, this textbook is an ideal basis for a course on computational neuroscience. Modern neurobiology is an open science, with no restrictions placed on either methodological approaches or conceptual orientation. New developments continue to occur at an explosive pace. As an introductory textbook, this work aims to clearly and simply present the principles of neurobiology considered by the author to be most important, based on numerous examples.
It has been written specifically for beginners, with a carefully considered balance of concepts and principles, details and illustrations. It is intended to guide students through the subject without overwhelming them with the huge influx of information that has developed in the field.
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