Opportunities in Biology (1989) / Chapter Skim
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6. The Nervous System and Behavior
6. The Nervous System and Behavior
Pages 175-223
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From page 175... ...
The real magnitude of this challenge can perhaps be best judged by considering the structural and functional complexity of the human brain and the bewildering complexity of human behavior. The human brain is thought to be composed of about a hundred billion (10~)
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From page 176... ...
Some cells were found to have only a single process, others just two processes, and still others-including the overwhelming majority of neurons in the brains of vertebrates-have several, often scores, of processes. In most cases we can recognize a single process, the axon, that serves to conduct information-usually in the form of all-or-none signals known as action potentials or nerve impulses to other cells.
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From page 177... ...
The fact that most neurons have to survive and continue to function effectively throughout the life of the organism for 70 or more years in the case of neurons in the human brain is one of their most impressive characteristics. Recent developments in molecular and cell biology are beginning to influence the study of these phenomena, and there is every reason to be confident that they will soon be as well understood as the mechanisms involved in impulse conduction and synaptic transmission.
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From page 178... ...
That it has been possible to produce machines that can duplicate some aspects of these higher brain functions has suggested that developments in computer science and especially artificial intelligence may inform our understanding of how the brain functions in much the same way as molecular and cellular neuroscience have been informed by concurrent developments in molecular and cell biology. But this remains to be seen, and for the present the single greatest challenge to neuroscience is to elucidate how the brain works.
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From page 179... ...
Clinical neurologists led the way by analyzing brain function resulting from localized or more general brain pathologies. Much of what we know about the organization of the human brain has come from studies of this kind, and when patients with specific brain lesions (such as an interruption of the corpus callosum that unites the two cerebral hemispheres, or localized damage to the speech areas or to those concerned with memory processing)
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From page 180... ...
, event-related potential recordings from the scalp with computerized averaging techniques, and magnetoencephalography. The full impact of these technologies has yet to be felt, but it is already clear that they will enable us to study many aspects of brain function in the intact human brain that hitherto could be analyzed only in the brains of experimental animals.
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From page 181... ...
These action potentials propagate along the length of the sensory neurons, past the cell bodies, and to the axons, which extend into the spinal cord. At the ends of the axons the action potentials cause a chemical transmitter to be released.
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From page 182... ...
NERVE CELL COMMUNICATION Nerve Cells Communicate by Electrical awl Chemical Signals To understand how neurons and synapses work, we need to understand how a nerve impulse or action potential in a presynaptic neuron causes the release of a chemical neurotransmitter at the synapses formed by its axon. But fast we must focus on the ionic currents that produce the action potential in the presynaptic neuron and on the way these currents interact with the structures in the terminal parts of the axon to bring about the release of the transmitter.
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From page 183... ...
But at present, the conceptual gap between the primary structure of the channel proteins and their function is too large to allow us to make this prediction with any degree of confidence. Synaptic Transmission: The Nerve-Muscle Junction as a Prototypical Example The nerve impulse is essentially a form of electrical signaling, with the wave of ionic currents sweeping down the surface of the axon at speeds in the range of 1 to 100 meters per second.
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From page 184... ...
Because of its ease of access and because it was the first site at which chemical transmission was identified, the neuromuscular junction between the motor axon terminals and muscle cells-has been the most intensively studied synapse and illustrates the major features of synaptic transmission. When an action potential invades a motor nerve terminal, it releases the transmitter ACh after an irreducible delay of about 0.5 to 1.0 milliseconds.
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From page 185... ...
The function of calcium in this context can best be understood if we shift our attention from the receptors on the postsynaptic cell the muscle to the presynaptic terminals of the motor neuron. The chemical transmitter is released from the axon terminals, not as isolated molecules, but in packets containing about 5,000 molecules of ACh.
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From page 186... ...
Yet another distinction between peptides and more conventional transmitters is the manner in which they are synthesized. Most conventional transmitters are synthesized by enzymes located in nerve terminals; peptides, on the other hand, are synthesized as parts of larger polyproteins on polynbosomes in the cell body.
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From page 187... ...
Since some of the processes may be extremely long (the axons of motor cells in the spinal cord that innervate muscles in the human foot extend a meter or more) , and since all the genetic and most of the biosynthetic machinery of the cell is confined to the cell body, an elaborate transport system has been evolved to support and maintain neuronal processes.
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From page 188... ...
Slow Axonal Transport Several Components of Slow Axonal Transport Have Been Defined, and It Is Likely That They Will Be Understood in Terms of Molecular and Physical Mechanisms Witiun the Next 5 Years Slow axonal transport moves materials at a rate of between 0.5 and 5 mm/day and consists primarily of cytoskeletal proteins transported from the cell body to the end of neural processes. In mature neurons, slow axonal transport maintains the nerve cell processes, whereas in developing axons the rate of process extension corresponds fairly closely to the rate of slow axonal transport.
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From page 189... ...
Rapid Axonal Transport Recent Studies of Rapid Axonal Transport Exemplify How Current Technologies Can Be Used to Understand the Cell Biology of Neurons A broad spectrum of materials are transported along neuronal processes with velocities in the range of 20 to 500 mm/day. Included in these materials are several synaptic components including some of the synaptic vesicle membrane proteins and a number of transmitter-synthesizing enzymes.
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From page 190... ...
Against these advantages is the fact that development in most invertebrates seems to be rigidly programmed and shows little of the plasticity so characteristic of vertebrate development. Needless to say, among vertebrates, the greatest interest centers on understanding the development of the human brain, but as yet we have hardly progressed beyond the descriptive level.
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From page 191... ...
was rudimentary. The availability of modern molecular genetic techniques should make it possible in the near future to identify the factors involved in neural induction and to isolate and characterize them.
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From page 192... ...
. The Overwhelming Majority of Nerve Cells Must Undergo at Least One Major Phase of Migration in Order to Reach Their Definitive Locations The withdrawal of neurons from the cell cycle appears to be the trigger for their outward migration from the ventricular or subventricular zones.
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From page 193... ...
Antibodies to N-CAM selectively prevent nerve cell aggregation in vitro and also perturb normal neurite fasciculation. The widespread distribution of both molecules at key stages in the development of the brain and spinal cord suggests that they may each play a critical role in several of the morphogenetic events being considered here.
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From page 194... ...
According to this hypothesis, each small group of neighboring neurons requires a distinctive cytochemical label that is also expressed on the surfaces of the growing axons; the presence of matching or complementary labels on the target cells enables the axons to recognize and form synapses with their appropriate partners. A good deal of experimental evidence is consonant with this hypothesis (and none yet contradicts it)
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From page 195... ...
Nerve Cell Death During Neural Development Allows for the Fine-Tuning of the Nervous System It has been known for more than 80 years that some nerve cells die during normal neural development, but it was not until the late 1940s that the full significance of such cell deaths came to be appreciated. We now know that, in almost every part of the nervous system, neurons are initially overproduced; at some later period, between 15 and 85 percent of the initial population degenerate.
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From page 196... ...
In several instances we now recognize what are referred to as critical periods during which the relevant neural systems seem to be particularly susceptible to external environmental influences. The final form of the brain and its functional capacities are shaped by the interplay of intrinsic (genetic and epigenetic)
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From page 197... ...
For example, if the upper and lower eyelids of one eye are sutured closed throughout this critical period and then the eye is reopened, the animal subsequently behaves as if it were blind in the deprived eye. When the visual cortex is explored neurophysiologically, the cells that would normally be activated by stimulating that eye are found to be either silent or dominated largely by inputs from the nondeprived eye.
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From page 198... ...
What changes occur in the brain when behavior is modified as a result of experience? In a prescient lecture in 1894, Ramon y Cajal proposed that learning might produce prolonged changes in the effectiveness of the synaptic connections between nerve cells and that these changes could serve as the mechanism for memory.
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From page 199... ...
Short-term sensitization has been analyzed in detail in two invertebrate reflex systems, in which presynaptic facilitation is initiated by modulatory transmitters acting through two second-messenger systems: cyclic AMP-dependent protein phosphorylation and calcium ion concentration. The presynaptic facilitation involves a change in the potassium channel that broadens the action potential and increases calcium influx.
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From page 200... ...
With the tools of modern molecular biology, it should now be possible to identify the genes and proteins necessary for long-term memory in simple animals and to use these genes and proteins as probes to explore tong-term memory in higher forms. We Are Beginning to Understand the Mechanisms Underlying Various Simple Forms of Learning and the Short-Term Memory to Which They Give Rise One of He major findings to emerge from this research is that short-term memory involves second-messenger systems similar to those used for other
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From page 201... ...
Between the photoreceptors and the output stage of the retina are several classes of interneurons whose functional properties and chemical transmitters are well known because of intracellular labeling methods and immunocytochemistry. The actual output of the retina is carried by distinct classes of retinal ganglion cells that transmit qualitatively different Apes of information (for example, "on"
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From page 202... ...
The axons of the visual ganglion cells form the optic nerve and conduct visual information from the eye to a number of subcortical visual centers in the brain, some of which are concerned with various visual reflexes, the control of eye movements, and the regulation of longer-term light-induced hormonal changes, including those responsible for circadian and other behavioral rhythms. Of greatest interest for visual perception is the relay of information to the primary visual cortex through the lateral geniculate nucleus, in which the different types of ganglion cell axons are spatially segregated.
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From page 203... ...
Positron emission tomography can be used to map functionally distinct regions of the human visual cortex, which will play an important role in linking human psychophysical studies to animal models. Neural recordings with multiple electrode arrays offer the prospect of understanding how neural ensembles process information whose representation at the level of single cells is not explicit.
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From page 204... ...
The Studies of the Control of Eye Movements and Visual Attention Have Borrowed the Attentional Paradigms of Experimental Psychology and Applied Them to Experiments with Animals These studies have revealed the existence of complex mechanisms involved in the spatial distribution of attention; companion neurophysiological studies have contributed to our understanding of the neural mechanisms involved. As indicated earlier, the development of the awake, behaving monkey for central nervous system recording has been, and will continue to be, of the utmost value in studies of the oculomotor system and of the higher functions of the visual nervous system more generally.
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From page 205... ...
Computational Approaches to Visual Function Although computer vision has developed without reference to biological visual systems, it has become increasingly apparent that the types of algorithms used in machine vision are likely to throw light on how the brain processes visual information. Although most of the effort in computer vision has so far been devoted to the applied problem of programming computers to deal sensibly with image data, a growing number of workers are turning their attention to biological visual systems and are trying to model different aspects of visual behavior (and the underlying neural mechanisms)
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From page 206... ...
Research in motor control now focuses on intact, awake animals and, where possible, on human beings. Current approaches to the study of motor control derive from techniques for recording from single nerve cells in awake animals performing skilled motor tasks.
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From page 207... ...
Especially striking has been the finding that the premolar areas of the cerebral cortex become active when subjects mentally rehearse or plan complex motor actions; other areas become active only when the movement is performed. These results are now guiding studies of the activity of single neurons in experimental animals performing similar tasks.
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From page 208... ...
Artificial intelligence has also influenced studies of the role of the vestibular system, that part of the motor system which orients the head, eyes, and the body and helps to maintain balance. While current attempts to apply models derived from control theory, robotics, or artificial intelligence are unlikely to be the final word in motor control, they are now making an important contribution by sensitizing neuroscientists to the importance of generating testable models of motor control that make explicit performance predictions.
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From page 209... ...
Changes in brain actwity~ behave been examined radiologically And the: ~dbfi ciOnGies: of dopamine shown quantitatively in :~living patients. ~ Studies of the ;~a~ct~n~of single nerve Reilly A animals havens begun to elucidate how disor dered nerve ~cel I movement produces the~1rozen.
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From page 210... ...
can be applied. As Our Ability to Explore the Human Brain Has Changed, So Has Our Definition of What Cognition Is and Our Understanding of How It Can Be Studied Experimentally Computational models of cognition arising from efforts to develop artificial intelligence can provide an analytical basis for neural studies of mental processes.
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From page 211... ...
This prevents us from taking full advantage of the opportunity to combine findings based on the more spatially specific neural imaging techniques with those derived from the more accurate, temporally precise recordings of eventrelated activity. Nor do we know how the activity of individual cells or of such cellular configurations as cortical columns relates to the computations described by computational models.
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From page 212... ...
Studies in humans suggest that the temporal lobe and closely associated structures of the limbic system including, in particular, the hippocampus, may be critically involved in the acquisition of declarative memory. The structures are not themselves thought to be sites for memory storage, but are somehow involved in the process by which memories are placed into storage or are retrieved and read out from storage.
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From page 213... ...
is important for selection when the animal attends by making eye movements, whereas thalamic (pulvinar) and cortical (parietal lobe)
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From page 214... ...
Neural imaging techniques have revolutionized the practice of clinical neurology. In the near future, our understanding of the neural mechanisms underlying selective attention and language should assist the neurosurgeon in the delicate task of avoiding the most critical areas when performing needed surgery.
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From page 215... ...
A knowledge of the morphological features and physiological mechanisms underlying behavioral patterns is crucial to our understanding of the evolutionary constraints on behavioral~cological adaptations. Without an appreciation of the behavioral mechanisms involved in such key phenomena as competition, parent-offspring relationships, communication, and interspecific interaction within ecological communities, an adequate and precise description of ecological organization is not possible.
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From page 216... ...
Interesting evolutionary parallels can be drawn between kin recognition systems and the immune system. In most organisms studied to date, kin recognition labels seem to be chemical probably complex blends of specific chemical compounds that are ultimately genetically determined.
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From page 217... ...
Here, too, recognition systems play a crucial role. Conflict and competition appear to constitute a major force in structuring ecologial communities, but little is known about the behavioral mechanisms underlying competition.
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From page 218... ...
It has recently been argued that the social system itself, by communicators processes, can develop the properties of problem solving; it can develop what amounts to a cognitive system that encompasses but also exceeds the cognitive capabilities of the individual components. It has even been suggested that we compare the coordinating mechanism active in such superorganisms with the interactive neuronal processes that endow central nervous systems with their acknowledged cognitive capacities.
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From page 219... ...
to remove the offending neurof~bromas that can disfigure the entire body, occlude the auditory canal, and extend into the brain and spinal cord to pose an immediate life threat. Neurofibromatosis is an autosomal dominant disorder, which means that each child of a parent with neurof~bromatosis has a 50 percent chance of inheriting the disease.
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From page 220... ...
Molecular Pharmacology, Modern Imaging, and the Diagnosis and Treatment of Psychiatric Disorders Diagnosis in psychiatry is less precise than in neurology or in the rest of medicine because most psychiatric diseases cannot as yet be localized to specific regions of the brain, much less to particular proteins in specific nerve cells. Thus, the diagnosis of psychiatric disorders must rely primarily upon clinical symptoms.
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From page 221... ...
That patients with chronic schizophrenia have enlarged cerebral ventricles relative to those with the acute forms of the disorder, along with differential drug responses, has prompted the definition of two subtypes of the illness, type I and type II schizophrenia. Type I manifests positive symptoms, a good response to neuroleptic drugs, and no enlargement of the cerebral ventricles, whereas type II schizophrenia is characterized by negative symptoms, a poor drug response, and enlarged ventricles.
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From page 222... ...
Perhaps the most exciting possibility is the potential identif~cation of the molecular genetic abnormality associated with schizophrenia, as we have discussed in relation to neurological disease. In families with an extremely high incidence of schizophrenia, one might be able to search for specific genetic markers differentiating schizophrenics from the general population by using strategies that have been successful in conditions such as Huntington's disease.
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From page 223... ...
It is honed that molecular Genetic techniques can be aDDlied to the diagnosis and -- -rip -A of ¢~ treatment of affective illness, with the ultimate view of identifying specific molecular aberrations that reflect the cause of the illness.
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