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The Nervous System
Pages 92-108

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From page 92... ... Increasing evidence indicates that many hormones effect a change in the function of some portions of the genome, as we have already noted for cyclic adenylate. Insulin, thyroxin, sex steroids, and the plant kinins have all been shown to elicit a burst of RNA synthesis, presumably of messages previously repressed. Read the entire page →
From page 93... ... Adult structures - ~ Adult ~ eland FIGURE 25 Endocrine relations in insects. JH, juvenile hormone; PGH, ecdysone. Read the entire page →
From page 94... ... But this explains why the cerebral cortex requires so many cells. It is not that the actions mediated by individual cells are trivial; rather, it is because the cortex receives such an enormous amount and variety of information from the sensory apparatus at the body surface (eyes, ears, nose, skin) Read the entire page →
From page 95... ... The narrow cylindrical axon varies from a fraction of a millimeter to more than a meter long and conducts the action potential generated in the cell body to the terminal contacts in a repetitive code of all-or-none impulses. SIGNALING IN NEURONS THE TRANSFER OF INFORMATION The complex organization of the nervous system is based upon a precise, selective interconnection of neurons, many of which are present and operative at birth. Read the entire page →
From page 96... ... (B ) Several axons form synapses on the cell body and base of dendrites; motor neurons are densely covered with such synapses. Read the entire page →
From page 97... ... Surface polarity Stimulus applied FRONTIERS OF BIOLOGY 97 Action Resting potential potential pi �-i At- row Temporal sequence of sodium and potassium fluxes N,a + PROPAGATING NEURON new \ 4. ./1 Recovery of resting potential + + + ~ Na k=G Impulse propagation FIGURE 27 Monophasic nerve action potential. Read the entire page →
From page 98... ... During the subsequent period, while the original sodium and potassium concentrations are being restored, the nerve cannot fire. INITIATION OF IMPULSE ACTIVITY AT SENSE ORGANS Entering the central nervous system is a vast network of nervous fibers whose peripheral endings constitute the sensory apparatus, distributed in sheets such as those of the retina of the eye, the cochlea of the ear, or the skin. Read the entire page →
From page 99... ... In the neurons of the brain, each presynaptic cell either excites, i.e., lowers the voltage difference between inside and outside, and leads to initiation of an impulse, or inhibits, i.e., increases the resting potential, thereby preventing or depressing the ability of the receiving cell to discharge. On most central interneurons, as is the case for other central neurons, there are both inhibitory and excitatory synapses; the balance of their influences determines whether a neuron will or will not generate an action potential and whether its frequency of discharge will increase or decrease. Read the entire page →
From page 100... ... For example, a ganglion of only nine interconnected cells is resp~nsible for the rhythmic beat of the heart of crustaceans. More complex motor reflexes are controlled by segmental ganglia that may contain but 500 cells; an excellent beginning has been made in tracing the connection of the individual nerve cells in such an apparatus. Read the entire page →
From page 101... ... The afferent input is distributed widely among various regions of the central nervous system, and a great amount of sensory trans ~ . _ formation occurs at the earliest points in the sensory pathways as a result of lateral interaction between adjacent elements. Read the entire page →
From page 102... ... By either.firing or not firing they then determine whether efferent impulses will then flow along axons that lead to the musculature. LARGER BRAINS Early studies of the visual systems of larger brains made it evident that the retina projects in an orderly fashion upon the visual cortex region of the central nervous system. Read the entire page →
From page 103... ... FRONTIERS OF BIOLOGY field occur at various levels between the retina and its final representation, as can be observed with microelectrodes implanted at various levels along the optic tracts. The first neural elements in the mammalian visual system whose receptive fields have been successfully studied are the ganglion cells of the retina, each of which is receiving information from a considerable ~ r ~ number ot rod or cone cells in the retina proper, as shown in Figure 28. Read the entire page →
From page 105... ... INTERCALATED SYSTEMS: HOMEOSTATIC REGULATION Between the major sensory and motor systems there exists the great mass of the central nervous system, composed of subsystems organized for the regulation of intrinsic brain functions and for the control of the function of other organs. These are particularly voluminous and diverse in the forebrain, where they are represented by the large association areas of the cerebral cortex, the limbic system, a heterogeneous array of large neural structures in the medial and basal walls of the cerebral hemispheres and the corpus striatum. Read the entire page →
From page 107... ... Yet there is no general diminution in the overall activity of the sleeping brain no change in cerebral blood flow or oxygen consumption, no apparent overall decrease in the activity of cortical neurons. Two important discoveries underlie modern concepts of the mechanisms involved. Read the entire page →
From page 108... ... An alternative approach, which appears to warrant aggressive continued study, rests on the observation that spinal fluid taken from a goat that has been kept awake for several days contains a material of low molecular weight that induces deep natural sleep in other animals. If confirmed, this observation offers enormous potential, not only for understanding of the mechanisms involved in sleep, but also for the development of the ideal sedative or anesthetic. Read the entire page →

From page 92...

... Increasing evidence indicates that many hormones effect a change in the function of some portions of the genome, as we have already noted for cyclic adenylate. Insulin, thyroxin, sex steroids, and the plant kinins have all been shown to elicit a burst of RNA synthesis, presumably of messages previously repressed.

The Nervous System Pages 92-108

The Nervous System
Pages 92-108