Again, the electrical —30 signal spreads along the bipolar cell, but in this case, it suppresses the release —40 of transmitter. Excitatory and inhibitory synaptic potentials in ganglion cells —50 are shown in Figure 1.

In neurons throughout the nervous system, combined excitatory and —60 inhibitory inputs determine whether or not the threshold for the initiation Light on of an action potential will be reached.

For example, a ganglion cell, as men- tioned, receives both excitatory and inhibitory inputs. If excitation is sufficient 0 0.

In motor cells in the spinal cord, to use a different example, B Inhibitory synaptic potential excitatory and inhibitory influences from different fibers determine whether Membrane potential mV. A motor cell of this sort receives 10, or more —55 incoming fibers Figure 1. Intracellular recordings from a ganglion cell —70 showing excitatory and inhibitory synaptic potentials.

A A ganglion cell is depolarized by Light on continuous release of excitatory transmitter during retinal illumination. If the depolarizing synaptic potential is large enough, threshold is crossed and action potentials are initiated in the ganglion cell.

B Illumination of a different group of photoreceptors causes inhibition. Hy- 0 0. After Time s Baylor and Fettiplace, A B Astrocytic processes Oligodendrocyte Dendrites.

Cell body. Giant boutons. A Approximately 10, presynaptic axons converge to form endings that are distributed over the surface of a motor neuron in the spinal cord. The drawing is based on a reconstruction made from electron micro- graphs. B This drawing shows the divergence of the axon of a single horizontal cell that branch- es extensively to supply many postsynaptic target cells. A from Poritsky, ; B after Fisher and Boycott, An individual Purkinje cell in the cerebellum receives more than , inputs.

Electrical Transmission Although synaptic transmission between most neurons involves the release of transmitter molecules, the membranes of many cells in the retina and in the rest of the nervous sys- tem are instead linked by specialized junctions.

At such synapses electrical transmission occurs. The pre- and postsynaptic membranes are closely apposed and linked by channels that connect the intracellular fluids of the two cells. This close connection allows local electrical potentials and even action potentials to spread directly from cell to cell without a chemical transmitter and without delay.

Metabolites and dyes can also spread from cell to cell. One important example in the retina is provided by the horizontal cells, which are electrically coupled in this way. By virtue of this property, graded depolarizing or hyperpo- larizing potentials can spread from one horizontal cell to the next, with marked effect on the processing of visual information in the retina.

Electrical synapses are found throughout the CNS of vertebrates and invertebrates. They also connect non-neuronal cells of other tissues in the body.

Modulation of Synaptic Efficacy Chemically mediated synaptic transmission shows great plasticity. Dramatic changes oc- cur in the amount of transmitter that is released by a signal—such as an action potential or a local potential—that invades a presynaptic terminal.

The photoreceptors in the retina provide an example: The amount of the transmitter glutamate that is released by a rod or a cone in response to a standard light stimulus can be increased or decreased by feedback to the terminal from horizontal cells. The horizontal cells themselves are influenced by other photoreceptors.

This feedback loop plays a critical role in the way the eye adapts to different levels of illumination. Other mechanisms that influence transmitter release depend on the history of impulse activity. During and after a train of impulses in a neuron, the amount of transmitter it releases can increase or decrease dramatically, depending on the frequency and duration of the activity. Modulation of efficacy can also be postsynaptic in origin. Long- and short- term plasticity are the focus of intense contemporary research.

Integrative Mechanisms 13 Sherrington, C. Reprint, Yale create their own new messages with new meanings. Sherrington 14 Kuffler, S. Once again, retinal ganglion cells provide an excellent example of integration. Kuffler14 was the first to show that a ganglion cell responds best to a small light spot or dark spot that falls on a few receptors in a particular region of the retina. Previously investigators had used flashes of bright light in an attempt to achieve maximal stimulation of the retina.

Massive stimulation of this type applied to the retina, the ear, and other sensory systems gives little information about how information is processed with fine discrimination under normal conditions. Kuffler showed that a small region of illumination gives rise to a brisk discharge of action potentials Figure 1.

A larger spot shone over the same part of the retina is far less effective: this is because an additional group of receptors arranged circumferentially around the first set also responds to the change in illumination. The action of these pho- toreceptors on bipolar cells gives rise to inhibition of ganglion cell firing Figure 1. Summation of the excitatory effect of a small central spot and the inhibitory effect from the surrounding region causes the ganglion cell to be relatively insensitive to diffuse light Figure 1.

In a second major category of ganglion cells, the optimal visual stimulus consists of a small dark spot surrounded by light. The meaning of the signal in a ganglion cell has thereby become more complex than information simply about light or dark. Instead, the action potentials report the presence of a contrasting pattern of light in a particular region of the visual field. This occurs be- cause each ganglion cell is influenced, albeit indirectly, not by one photoreceptor but by many.

For any given ganglion cell, the specific connections through bipolar, horizontal,. Pattern of Illumination of Ganglion cell Ganglion cell illumination photoreceptors response of retina. Extracellular recordings made from a single ganglion cell in the retina of a lightly anesthetized cat, while patterns of light were presented to the eye see Figure 1. A A small spot of light presented to a centrally located group of photore- ceptors gives rise to excitation and a brisk discharge of action potentials.

B Light presented as a ring, or annulus, to illuminate a circumferential group of photoreceptors gives rise to inhibition of the ganglion cell, which prevents the cell from firing. Removal of the inhibition at the end of illumi- nation is equivalent to excitation, which gives rise to a burst of action potentials.

C Illumination of both groups of receptors causes integration of excitation and inhibition and a weak discharge of action potentials. After Kuffler, B, Biol. On Intelligence. Times Books, New York. Complexity of the Information Conveyed by Action Potentials At a distance of only three synaptic relays beyond the retina, even more sophisticated in- formation about the visual world is provided by the action potentials in cortical nerve cells.

Hubel and Wiesel15 showed that cortical neurons do not respond simply to light or dark on the retina. Instead, their activation depends on the pattern of retinal illumination.

Specific, distinctive patterns are the required and most effective stimuli for different types of corti- cal cells. For example, one type of cortical cell in the visual pathway responds selectively to a bar of light with a specific orientation vertical, oblique, or horizontal , moving in a particular direction in a particular part of the visual field vertical in the case of the cell illustrated in Figure 1.

The firing of these cells is not influenced by diffuse light or by a bar of an inappropriate orientation or one moving in the wrong direction. Hence, its action potentials provide precise information about the visual stimulus to higher centers in the brain. This increase in the meaning attributed to a stereotyped action potential is explained by the precise connections of lower-order cells to the cortical cell and the way in which the cortical cell integrates incoming signals by summation of localized graded potentials.

Complex integration occurs in other sensory systems. Thus, the position and direction of a mechanical stimulus, moving along a fingertip, act as selective stimuli for particular cells in the region of the cerebral cortex that are concerned with tactile stimuli. Mice, which use their whiskers much as we use our fingertips, have cells in the cortex that not only can distinguish the direction of motion but the roughness or smoothness of a surface.

Two important conclusions about signaling in the nervous system are: 1 nerve cells act as the building blocks for perception and 2 the abstract significance of the message can be extremely complex, depending on the number of inputs a neuron receives. It turns A out that the progressive integration of information derived from lower order units can lead to the generation of highly complex and specific stimulus requirements for higher order central neurons.

For example, later it will be shown see Chapter 23 that specific cells exist Light in visual association areas that respond selectively to a face. In addition, the temporal group- ing and patterning of impulses can provide information about the quality of the stimulus. Reverse Traffic of Signals from Higher to Lower Centers Implicit in the discussion of information transfer in this chapter is the concept of linear progression from receptors to perception or from motor commands to movement.

It will C be shown however that extensive signaling also occurs in the opposite direction: from the brain towards sensory receptors as well as from each higher center back towards the lower centers from which it received its inputs. In a few cases, the significance of descending information is understood, but in general, the functional role remains to be discovered. Extracellular recordings from a neuron in the cerebral cortex of a lightly anesthetized cat.

Action potentials in this cell indicate that a bar of light that is almost vertical shines on one particular part of the visual field. The small draw- ings to the left of the graphs show how visual stimuli such as bars or edges with different orienta- E tions and positions are presented to the eye. A The cortical cell fires a burst of action potentials when the light stimulus consists of a vertical bar of light in one particular part of the visual field.

B—E Bars with different orientations or diffuse light fail to evoke action potentials. The cortical cell integrates information arriving by way of relays from a large number of photoreceptors, some of which corresponding to those illuminated by the vertical bar give rise to excitation on the cortical 0 1 2 3 cell, the others giving rise to inhibition.

After Hubel and Wiesel, Higher Functions of the Brain In spite of the alarming flow of new treatises that appear day by day on consciousness, learning, and memory, only rudimentary information is available at present about the way in which the brain creates a complete image of the outside world, with its forms, colors, depth, and motion, or about the way in which it composes and executes complex, integrated movements of the body.

Indeed, this open frontier is one of the most appealing aspects of research on the nervous system. We do not know how the tennis player runs to hit the ball in exactly the right place on the racket, so as to drive it to the far corner of the court, or how the coordinated finger and arm movements required for playing the violin are initiated or executed—let alone how we think and feel.

New approaches that shed light on mechanisms for the individual steps involved in higher functions often have their origin in psychophysical experiments. For example, tests on normal human subjects made with precise quantitative stimuli have shown that under suitable conditions, a person can detect the arrival of single quanta of light on photorecep- tors of the eye.

By carefully designed behavioral experiments made on rats and mice, one can produce symptoms of stress and anxiety resembling those seen in patients. As a further step, it then becomes possible to assess which brain structures and mechanisms play a part in such disorders of higher functions. In addition to the insights that these experiments provide about our emotions and our minds, they are essential for the development of new drugs that can mitigate the suffering of patients.

Moreover, information flows back from the clinic toward basic research. Clinical observations, particularly on patients with discrete circumscribed lesions, provide unparalleled insights into mechanisms of perception, movement, and speech. Cellular and Molecular Biology of Neurons Like other types of cells, neurons possess the cellular machinery for metabolic activity for synthesizing intracellular and membrane proteins, and for distributing them to precise locations in the cell.

Each type of neuron synthesizes, stores, and releases its characteristic transmitter s. The receptors for specific transmitters are located at well-defined sites on the postsynaptic cell under the presynaptic terminals.

In addition, other membrane proteins, known as pumps and transporters, maintain the constancy of the internal and external milieu of the cell. The presynaptic terminals of optic nerve fibers of ganglion cells like those of photoreceptors and bipolar cells, and indeed like all presynaptic nerve terminals contain in their membranes specific channels through which calcium ions can flow. Cal- cium entry triggers the release of transmitters and can activate intracellular cascades of enzymes and regulate numerous other cellular processes.

A major specialization in the cell biology of neurons, compared to other types of cells, arises from the presence of the axon. Axons do not have adequate machinery for synthesiz- ing all the proteins they need. Hence, essential molecules are carried to the nerve terminals by a process known as axonal transport, often over long distances. Molecules required for maintenance of structure and function, as well as for the appropriate membrane channels, travel from the cell body in this way; similarly, molecules taken up at the ending are carried back to the cell body.

Neurons are different from most other cells in that, with few exceptions, they cannot divide after differentiation. As a result, in an adult human being, neurons in the central nervous system that have been destroyed usually cannot be replaced.

Signals for Development of the Nervous System The high degree of organization in a structure such as the retina poses a fascinating prob- lem. Whereas a computer requires a brain to wire it, the brain must establish and tune its own connections. What seems so puzzling is how the proper assembly of the parts endows the brain with its extraordinary properties.

In the mature retina, each cell type is situated in the correct layer—or even sublayer— and makes the correct connections with the appropriate targets.

This arrangement is a prerequisite for function. A gene known as eye- less controls development of the eye in the fruit fly. After deletion of this gene, eyes fail to appear.

Overexpression leads to the development of ectopic eyes that are morphologically normal. A This scanning electron micrograph shows ectopic eyes on the antenna right arrow and on the wing left arrow. B Here, the wing eye is shown at higher magnifica- tion. A gene with strikingly similar se- quence homology in the mouse can be inserted into the fly genome, and it also leads to the formation of ectopic eyes.

After Halder, Callaerts, and Gehring, ; micrographs kindly provided by W. The axons must find their way over long distances through the optic nerve to end in the appropriate layer of the next relay station. Similar processes must occur for the various divisions of the nervous system so that complex structures required for function are formed. Study of the mechanisms by which highly complex structures, such as the retina, are formed presents a key problem in modern neurobiology.

An understanding of how intricate wiring diagrams are established in development often provides clues about function and about the genesis of functional disorders. In other words, if you know how an electrical circuit has been wired, you may be able to understand what the components are doing and, consequently, you may be able to repair it.

Certain specific molecules are essential for dif- ferentiation, outgrowth of axons, pathfinding, synapse formation, and survival of neurons. Such molecules are now being identified at an ever-increasing rate, and their mechanisms of action are being studied.

Interestingly, molecular signals that give rise to the outgrowth of axons and formation of connections can be regulated by electrical signals. Activity plays a role in determining the pattern of connections. Genetic approaches have made it possible to identify genes that control the differentia- tion of entire organs, such as the eye as a whole.

Gehring17 and his colleagues have studied the expression of a gene in the fruit fly Drosophila , known as eyeless, that controls the development of the eyes.

After deletion of this gene in the germline, eyes fail to develop in the progeny for generation after generation. Homologous genes in mice and humans known as small eye and aniridia, respectively share extensive sequence identity and have similar developmental functions. If the fly eyeless gene or the mammalian homologue of the gene is introduced and overexpressed in the fly, it develops multiple ectopic eyes over its antennae, wings, and legs Figure 1.

The gene can therefore orchestrate the forma- tion of an entire eye, in a mouse or a fly, even though the eyes themselves have completely different structures and properties.

Regeneration of the Nervous System after Injury Not only does the nervous system wire itself when it is developing, but it can also restore certain connections after injury again something your computer cannot do!

For ex- ample, axons in an arm can grow back after the nerve has been injured so that function can be restored; the hand can once again be moved, and sensation returns. Similarly, in a frog, fish, or an invertebrate like the leech, lesions in the central nervous system are fol- lowed by axon regeneration and functional recovery.

After the optic nerve of a frog or a fish has been cut, fibers grow back to the brain and the animal can see again. However, in the adult mammalian CNS regeneration does not occur. The molecular signals that cause 17 Halder, G. Suggested Reading All the experiments and concepts described in this introductory chapter are treated in more detail and fully referenced in later chapters.

The following sources represent key reviews that show how essential concepts of neurobiology have developed over the years. Currently in print and of great interest: Hawkins, J. Hubel, D. Visual Perception. Oxford University Press, Oxford. Harder to buy new but still fascinating: Adrian, E. The Physical Background of Perception. Helmholtz, H. Southhall ed. Dover, New York. Hodgkin, A. The Conduction of the Nervous Impulse. Liverpool University Press, Liver- pool, England. Katz, B.

Nerve, Muscle, and Synapse. McGraw-Hill, New York. Histology of the Nervous System, 2 vols. Translated by Neely Swanson and Larry Swanson. Sherrington, C. The Integrative Action of the Nervous System. They are sent by ganglion. N cell axons to a relay, the lateral geniculate nucleus LGN , and then to higher centers that produce our perception of scenes with objects and background, movement, shade, and color. Signaling at each level is best analyzed in terms of the receptive fields of neurons.

A recep- tive field in the visual system is defined as the area of the retinal surface or corresponding region of the visual field that, upon illumination, enhances or inhibits the activity of a neuron.

A useful strategy for analyzing the visual system is to define the optimal pattern of illumination and the receptive field for each neuron. The receptive fields of most retinal ganglion cells and neurons in the lateral geniculate nucleus consist of small circular areas on the retina. The cells respond to contrast rather than diffuse il- lumination.

Geniculate axons project to form a new map of the visual field in primary visual cortex. The receptive fields of neurons in the primary visual cortex for the most part consist of lines, bars, or edges with a particular orientation. Cortical neurons give no response to diffuse illumination.

The optimal stimulus for a simple cell is an oriented edge or bar, which may be light or dark, with a defined width, shining on a precise place in the retina.

Complex cells also respond to oriented bars but their discharges are evoked over a wider area than the simple cells. End inhibition, which is a decrease in the response of a neuron as the length of an image increases, gives rise to more elaborate stimulus requirements, such as a corner or a line that stops. Most cortical cells respond to appropriate illumination of both eyes.

Receptive fields of simple cells result from convergence of a number of geniculate afferents with adjoining field centers. The response properties of complex cells depend on inputs from simple and other cortical cells.

Cortical neurons detect only the edges of white or black patterns on a background with inverse contrast. The overall levels of illumination are measured by specialized retinal ganglion cells that project to areas other than the visual cortex.

The progression of receptive field properties from retina to complex cells suggests that inputs from one level are combined to produce more abstract requirements at the next. Information also flows in the opposite direction, for example from cortex to LGN, from one cortical layer to another and back.

Throughout the visual pathways, the emphasis is on contrast, color, movement, depth, and boundaries, rather than on light detection. This distinction enables the nervous system to fo- cus on what is important to the animal and to jettison irrelevant information in the visual fields. This chapter describes the functional properties of neurons at successive stages in the vi- sual pathways. We deal first with the output of the eye; second, with the next relay station, the lateral geniculate nucleus; and then with the primary visual cortex, the initial receiv- ing center for visual information.

Our aim is to show how neuronal activity is related to higher functions, such as visual perception, using as background knowledge only the basic information provided in Chapter 1. Chapter 3 shows in greater detail how structure and function are intimately related at every level see also Chapter Experiments performed in recent years have produced an overwhelming body of work on psychophysics, color vision, dark adaptation, retinal pigments, transduction, transmit- ters, and the organization of the retina see Chapter Each of these topics can form the basis of a self-contained monograph see the Suggested Reading section at the end of the chapter.

The same applies to comparative aspects of the visual system in invertebrates, lower vertebrates, and mammals. Since a comprehensive account is not possible within the scope of this book, we have selected experiments that provide a continuous thread, extending from the properties of cells in the retina to mechanisms that underlie perception. Pathways in the Visual System The initial step in visual processing is the formation on each retina of a sharp image of the outside world.

Essential for clear vision are: 1 correct focus of the image by ad- justment of the curvature of the lens accommodation , 2 regulation of light entering the eye by the diameter of the pupil, and 3 convergence of the two eyes to ensure that matching images fall on corresponding points of both retinas. Our vision depends criti- cally on the region of the visual field that is being analyzed Figure 2. We can read. Lens Pupillary muscles Ciliary muscle.

Bipolar, horizontal, amacrine, and ganglion cells. Ganglion cell axons Fovea Fovea Blind spot. Optic nerve Rods and cones. Densely packed Optic nerve cones. Cross section through the eye. Light must pass through the lens and layers of cells in order to reach the rod and cone photoreceptors. The fovea is a specialized area, containing only densely packed, slender cones.

It is used for fine discrimination. In the fovea, the superficial layers of cells are spread apart and this feature permits light to have more direct access to the photoreceptors than elsewhere in the retina.

The point at which the optic nerve exits the eye has no photoreceptors and constitutes a blind spot. Signaling in the Visual System The R right side of each retina, shown in green, Lens projects to the right lateral geniculate nucleus. Thus, the right visual cortex receives information exclusively from the Right eye Optic chiasm left half of the visual field. Visual Visual fields cortex. Left eye Lateral geniculate nucleus L. This loss of acuity arises from the way in which visual information is processed; it is not the result of blurred images or optical distortion outside the central region.

The pathways from the eye to the cerebral cortex are illustrated in Figure Lateral 2. The optic nerve fibers that arise from ganglion cells in the retina end on layers of cells in a relay station of the thalamus, which as mentioned is called the lateral geniculate nucleus geniculate means bent like a knee. In each of the six principal layers of this structure Figure 2.

Geniculate axons in turn project through the optic radiation Dorsal to the cerebral cortex. The six layers of the visual cortex and the arrangements P6 of maps are dealt with in Chapter 3. For present purposes, it is sufficient to state that in the monkey, the optic radiation ends on a folded plate of cells P5 about 2 mm thick see Figure 2. This region of the brain is known as the primary visual cortex, or visual area 1, also called V1 , which lies posteriorly P4 in the occipital lobe.

Adjacent regions of cortex are also concerned with vision. From the primary visual cortex, the progression through the brain becomes P3 ever more complex, with no end point in sight. Medial Figure 2. The right side of each retina projects to the right cerebral M1 hemisphere. Because of optical reversal by the lens, the right side of each retina receives the image of the visual world on the left side of the head.

Each cerebral hemisphere, therefore, sees the opposite side of the outside world. Accordingly, people with damage to the right cerebral hemisphere caused by K trauma or disease become blind in the left visual field, and vice versa.

Other pathways that branch off to the midbrain are not described here. Nucleus LGN has six major layers designated parvo- cellular, or P 3, 4, 5, 6 and magnocellular, or M 1, 2 , separated by the koniocellular K layers. In the monkey, Convergence and Divergence of Connections each layer is supplied by only one eye and contains cells with specialized response properties.

Red signifies By examining the cellular anatomy of the various structures in the visual input from the contralateral eye and blue from the ipsi- pathway, one can exclude the possibility that information is handed on un- lateral eye.

After Hendry and Calkins, The neurons Light converge and diverge extensively at every stage; that is, each cell receives many inputs and makes connections with a number of other cells see Chapter 1. Just as a To optic nerve ganglion cell is supplied indirectly from numerous rods and cones, so a neuron in the LGN receives its input from many ganglion cells and it in turn supplies Ganglion cell many cortical neurons.

Hence, as impulses travel to the cortex and within the cortex itself there occurs a fun- Inner neling and, simultaneously, a dispersal of information. Moreover, except at the level of the ganglion cells, information is simultaneously Bipolar cell flowing in the opposite direction, for example from cortex down to lateral geniculate nucleus. Outer Receptive Fields of Ganglion and plexiform layer Geniculate Cells Concept of Receptive Fields Cone Diffuse flashes of light are of little or no use for assess- ing function in the visual system.

Instead, the technique of illuminating selected areas of the retina led to the Rod concept of the receptive field. The concept has provided a key for understanding the significance of the signals, not only in the retina, but at successive stages in the FIGURE 2.

As previously mentioned in Chapter 1, the term illustrate rod and cone pathways to ganglion cells see Chapter After Dowl- receptive field was coined originally by Sherrington in ing and Boycott, ; Daw, Jensen, and Brunken, By definition, illumination outside a receptive field produces no effect on firing.

The area itself can be subdivided into distinct regions, some of which increase activity and others of which suppress it. The Output of the Retina Many years before the electrical responses of photoreceptors or bipolar cells in the retina could be measured, important information was obtained by recording from ganglion cells.

Thus, the first analysis of signaling in the retina was made at the output stage, the end result of synaptic interactions. It was a simplification and shortcut to go straight to the output. As discussed in Chapter 1 see Figure 1. Stephen Kuffler first defined the organization of the receptive field in the cat visual system. Hubel has succinctly 1 Conley, M. What is especially interesting to me is the unexpectedness of the results, as reflected J.

Eye, Brain and Vision. The principal novelty in the study of the visual system was the use of discrete, circum- Scientific American Library, New York. When one then shines pat- terns of light onto the screen or displays On-center cell Off-center cell computer-generated images, these will responses responses be well focused on the retinal surface Light see Figure 1. Central spot Such procedures had been foreshad- of light owed by pioneering work on the eye of a simple invertebrate, the horseshoe crab Limulus,3 and on the retina of the frog.

Rabbit ganglion cells have spot elaborate receptive fields that respond 0 0. Baylor, On-center cells respond best to a spot personal communication. Illumination indicated by Annular illumination the red bar above records of the sur- Ganglion and Geniculate Cell rounding area with a spot or a ring of light Receptive Field Organization reduces or suppresses the discharges and causes responses when the light is turned When one records from a particular cell off.

Illumination of the entire receptive field in the visual system, the first task is to find elicits weak discharges because center and the location of its receptive field. Off-center cells slow the visual system show discharges at rest down or stop signaling when the central Diffuse illumination even in the absence of illumination.

Ap- area of their field is illuminated and acceler- propriate stimuli do not necessarily initi- ate when the light is turned off. Light shone onto the surround of an off-center receptive ate activity but may modulate the resting field causes excitation of the neuron. After discharge, causing either an increase or a Kuffler, Figure 2. For the on-center receptive field in Figure 2.

When the inhibitory annular 7 Barlow, H. An off-center field has 8 Barlow, H. For either cell, the On-center field Off-center field. Central illumination Light. The concentric recep- tive fields of cells in the LGN resemble those of ganglion cells in the retina, consisting of on-center and off-center types. The responses illustrated are from an on-center cell in the cat LGN. The red bar above each record indicates illumination.

Furthermore, the same spot of light can have opposite effects, depending on the exact position of the stimulus within the receptive field. In one area, a small spot of light excites the cell for the duration of illumination, while simply shifting the spot by 1 mm or less across the retinal surface gives rise to inhibition.

Again, as in the retina, two basic receptive field types predominate, on-center and off-center geniculate cells. The receptive fields of both types are roughly concentric. While ganglion and geniculate cells have very similar receptive field organization, they are not identical. For example, descending connections from layer 6 of the visual cortex project to geniculate neurons to modulate their firing; there is, however, no comparable descending input to ganglion cells.

In addition there are subtle differences in receptive field properties, such as even greater failure of geniculate cells to respond to diffuse illumination. It is a general problem that the precise part played by thalamic structures including the LGN in transferring information to the cortex is still not fully understood12,13 see Chapter Sizes of Receptive Fields 13 Guillery, R. Brain Res. Neighboring cells in the visual system collect information from very similar, but not 14 Borghuis, B.

Throughout the visual system, neurons processing related information are clustered together. In sensory systems this means that the central neurons dealing with a particular area of the surface can communicate with each other over short distances. This appears to be an economical arrangement, as it minimizes the need for long lines of communication and simplifies formation of connections see Chapters 3 and Since neighboring regions of the retina make connections with neighboring geniculate cells, the receptive fields of adjacent neurons overlap over most of their area.

This extensive representation of the fovea reflects the high density of foveal receptors necessary for high-acuity vision. Moreover, the size of the receptive field of a ganglion, geniculate, or cortical cell depends on its location in the retina or visual field.

The receptive fields of cells situated in the central areas of the retina have much smaller centers than those at the periphery; receptive fields are smallest in the fovea, where the acuity of vision is highest. Note that receptive fields can be described either as dimensions on the retina or as degrees of arc subtended by the stimulus. There are similar gradations of receptive field size and spatial dimension in the somatosensory system.

A higher-order sensory neuron in the brain, responding to a fine touch applied to the skin of the fingertip, has a much smaller receptive field than that of a neuron whose field is on the skin of the upper arm see Chapter To discern the form of an object, we use our fingertips and fovea, not the less discriminating regions with poorer resolution. Classification of Ganglion and Geniculate Cells Superimposed on the general scheme of on- or off-center receptive fields, ganglion cells in the monkey retina can be grouped into two main categories denoted as M and P.

The criteria are both anatomical and physiological. Genetic single-neuron tracing from the olfactory bulb to higher brain centers in zebrafish. Neuron-glia synapses in the brain. Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising.

If you continue browsing the site, you agree to the use of cookies on this website. See our User Agreement and Privacy Policy. See our Privacy Policy and User Agreement for details. Published on Jun 5,. We use cookies to give you the best possible experience. By using our website you agree to our use of cookies. Dispatched from the UK in 2 business days When will my order arrive? John G. Home Contact us Help Free delivery worldwide. Free delivery worldwide. It presents a broad outline of neural development principles as exemplified by key experiments and observations from past and recent times.

The text is organized along a development pathway from the induction of the neural primordium to the emergence of behavior. It covers all the major topics including the patterning and growth of the nervous system, neuronal determination, axonal navigation and targeting, synapse formation and plasticity, and neuronal survival and death. This new text reflects the complete modernization of the field achieved through the use of model organisms and the intensive application of molecular and genetic approaches.

The original, artist-rendered drawings from the First Edition have all been redone and colorized to so that the entire text is in full color. This new edition is an excellent textbook for undergraduate and graduate level students in courses such as Neuroscience, Medicine, Psychology, Biochemistry, Pharmacology, and Developmental Biology. Updates information including all the new developments made in the field since the first edition Now in full color throughout, with the original, artist-rendered drawings from the first edition completely redone, revised, colorized, and updated.

Biophysics of Computation: Information Processing in Single Neurons challenges this notion, using richly detailed experimental and theoretical findings from cellular biophysics to explain the repertoire of computational functions available to single neurons. The author shows how individual nerve cells can multiply, integrate, or delay synaptic inputs and how information can be encoded in the voltage across the membrane, in the intracellular calcium concentration, or in the timing of individual spikes.

Key topics covered include the linear cable equation; cable theory as applied to passive dendritic trees and dendritic spines; chemical and electrical synapses and how to treat them from a computational point of view; nonlinear interactions of synaptic input in passive and active dendritic trees; the Hodgkin-Huxley model of action potential generation and propagation; phase space analysis; linking stochastic ionic channels to membrane-dependent currents; calcium and potassium currents and their role in information processing; the role of diffusion, buffering and binding of calcium, and other messenger systems in information processing and storage; short- and long-term models of synaptic plasticity; simplified models of single cells; stochastic aspects of neuronal firing; the nature of the neuronal code; and unconventional models of sub-cellular computation.

Biophysics of Computation: Information Processing in Single Neurons serves as an ideal text for advanced undergraduate and graduate courses in cellular biophysics, computational neuroscience, and neural networks, and will appeal to students and professionals in neuroscience, electrical and computer engineering, and physics.

It serves as a comprehensive introduction for medical students, physician assistants, and nurse practitioners, and is also a handy reference and refresher for residents and practitioners. Lists, tables, and clear illustrations throughout expedite review, while the engaging Secrets Series format makes the text both enjoyable and readable. New lead editors, Drs.