Neuro-Case-1-Reviewer-2022 PDF

Introduction: This is a three-week module integrating anatomy, physiology and biochemistry related to the NERVOUS SYSTEM. This module is designed for maximum student participation in the teaching-learning process while at the same time ensuring comprehensive and deep understanding of cardio-...

Introduction: This is a three-week module integrating anatomy, physiology and biochemistry related to the NERVOUS SYSTEM. This module is designed for maximum student participation in the teaching-learning process while at the same time ensuring comprehensive and deep understanding of cardio-respiratory anatomy and functions that are part of the foundations for their future roles as providers of health care, researchers and teachers. General Objective: The students should be able to: 1. Discuss the case given, by being able to identify the pertinent data and generate learning issues from the cases. 2. Discuss the structural organization and physiological functions of the body in terms of the nervous system. 3. Discuss the embryology, anatomy, histology and biochemistry of the nervous system. Specific Objectives: At the end of this case the students should be able to understand the physiology and anatomy of the peripheral nervous and the autonomic nervous system. They should be able to know its differences and functions in the human body. Case 2: Puting Van Handout 1: Laura, an 18yo/female while walking along the side of the Ligao Poblacion Park, a white van suddenly comes near her and open the door. An unknown man suddenly grab her left hand, to her shock she quickly punch the man right into his nose and she was able to run away. Guide Questions for 1st Handout: 1. What is the nervous system? What are its 2 divisions? 2. What is the integrative function of the nervous system? 3. What is the basic unit of the nervous system? Discuss its parts. 4. What are Schwann cells and satellite cells of ganglia? 5. Describe the nerve organization in the PNS. 6. What is the difference of myelinated from unmyelinated fibers? 7. What are ganglia? Describe sensory and autonomic ganglia. 8. Discuss the physiological anatomy of the synapse. 9. Discuss the action potential in the presynaptic and postsynaptic terminals. 10. What are the chemical substances that function as synaptic transmitters? 11. What are the different types of sensory preceptors? 12. Discuss the transduction of sensory stimuli into nerve impulses. Handout No. 2 She run straight to the barangay hall, while seated inside the office the barangay chairman, her heart was still pounding and her knees are trembling. She can’t believe that she was able to run that fast and punched the unknown man. At the moment she just froze into his seat and felt so nervous. Guide Questions for 2nd Handout: 13. What is the autonomic nervous system? 14. Describe the general organization of the ANS. 15. Describe the physiological anatomy of the sympathetic and parasympathetic nervous system. 16. What are the cholinergic and adrenergic fibers? 17. Discuss the mechanism of transmitter secretion and removal of postganglionic endings? 18. What are the two principal types of acetylcholine receptors? 19. What are the two major adrenergic receptors? 20. List the excitatory and inhibitory actions of sympathetic and parasympathetic stimulation. 21. What is the function of the adrenal medullae? 22. Describe the sympathetic and parasympathetic tone. 23. What do you call the reaction of Laura during the incident causing her to run and punch the culprit? TUTORS GUIDE 1. What is the nervous system? What are its 2 divisions? The human nervous system, by far the most complex system in the body, is formed by a network of many billion nerve cells (neurons), all assisted by many more supporting cells called glial cells. Each neuron has hundreds of interconnections with other neurons, forming a very complex system for processing information and generating responses. Nerve tissue is distributed throughout the body as an integrated communications network. Anatomically, the general organization of the nervous system has two major divisions: Central nervous system (CNS), consisting of the brain and spinal cord Peripheral nervous system (PNS), composed of the cranial, spinal, and peripheral nerves conducting impulses to and from the CNS (sensory and motor nerves, respectively) and ganglia that are small groups of nerve cells outside the CNS. Cells in both central and peripheral nerve tissue are of two kinds: nerve cells, or neurons, which usually show numerous long processes; and various glial cells (Gr. glia, glue), which have short processes, support and protect neurons, and participate in many neural activities, neural nutrition, and defense of cells in the CNS. Neurons respond to environmental changes (stimuli) by altering the ionic gradient that exists across their plasma membranes. All cells maintain such a gradient, also called an electrical potential, but cells that can rapidly change this potential in response to stimuli (e.g., neurons, muscle cells, some gland cells) are said to be excitable or irritable. Neurons react promptly to stimuli with a reversal of the ionic gradient (membrane depolarization) that generally spreads from the place that received the stimulus and is propagated across the neuron’s entire plasma membrane. This propagation, called the action potential, the depolarization wave, or the nerve impulse, is capable of traveling long distances along neuronal processes, transmitting such signals to other neurons, muscles, and glands. By collecting, analyzing, and integrating information in such signals, the nervous system continuously stabilizes the intrinsic conditions of the body (e.g., blood pressure, O2 and CO2 content, pH, blood glucose levels, and hormone levels) within normal ranges and maintains behavioral patterns (e.g., feeding, reproduction, defense, interaction with other living creatures). A. Central Nervous System Begins to form in the third week of embryonic development as the neural plate. The neural plate becomes the neural tube, which gives rise to the brain and spinal cord. B. Peripheral Nervous Sytem Consists of spinal, cranial and visceral nerves and spinal, cranial and autonomic ganglia.  It is derived from 3 sources: 1. Neural Crest Cells: gives rise to peripheral ganglia, Schwann cells and afferent nerve fibers 2. Neural Tube: gives rise to all preganglionic autonomic fibers and all fibers that innervate skeletal muscles 3. Mesoderm: gives rise to the dura mater and to connective tissue investments of peripheral nerve fibers (endoneurium, perineurium and epineurium). 2. What is the integrative function of the nervous system? PROCESSING OF INFORMATION—“INTEGRATIVE” FUNCTION OF THE NERVOUS SYSTEM One of the most important functions of the nervous system is to process incoming information in such a way that appropriate mental and motor responses will occur. More than 99 percent of all sensory information is discarded by the brain as irrelevant and unimportant. For instance, one is ordinarily unaware of the parts of the body that are in contact with clothing, as well as of the seat pressure when sitting. Likewise, attention is drawn only to an occasional object in one’s field of vision, and even the perpetual noise of our surroundings is usually relegated to the subconscious. However, when important sensory information excites the mind, it is immediately channeled into proper integrative and motor regions of the brain to cause desired responses. This channeling and processing of information is called the integrative function of the nervous system. Thus, if a person places a hand on a hot stove, the desired instantaneous response is to lift the hand. Other associated responses follow, such as moving the entire body away from the stove and perhaps even shouting with pain. 3. What is the basic unit of the nervous system? Discuss its parts. The functional unit in both the CNS and PNS is the neuron or nerve cell. Some neuronal components have special names, such as “neurolemma” for the cell membrane. Most neurons consist of three main parts (Figure 9–3): The cell body, or perikaryon, which contains the nucleus and most of the cell’s organelles and serves as the synthetic or trophic center for the entire neuron. The dendrites, which are the numerous elongated processes extending from the perikaryon and specialized to receive stimuli from other neurons at unique sites called synapses. The axon (Gr. axon, axis), which is a single long process ending at synapses specialized to generate and conduct nerve impulses to other cells (nerve, muscle, and gland cells). Axons may also receive information from other neurons, information that mainly modifies the transmission of action potentials to those neurons. Neurons and their processes are extremely variable in size and shape. Cell bodies can be very large, measuring up to 150 μm in diameter. Other neurons, such as the cerebellar granule cells, are among the body’s smallest cells. Neurons can be classified according to the number of processes extending from the cell body (Figure 9–4): Multipolar neurons, which have one axon and two or more dendrites Bipolar neurons, with one dendrite and one axon Unipolar or pseudounipolar neurons, which have a single process that bifurcates close to the perikaryon, with the longer branch extending to a peripheral ending and the other toward the CNS. Anaxonic neurons, with many dendrites but no true axon, do not produce action potentials, but regulate electrical  changes of adjacent neurons. Most neurons are multipolar. Bipolar neurons are found in the retina, olfactory mucosa, and the (inner ear) cochlear and vestibular ganglia, where they serve the senses of sight, smell, and balance, respectively. Pseudounipolar neurons are found in the spinal ganglia (the sensory ganglia found with the spinal nerves) and in most cranial ganglia. Because the fine processes emerging from perikarya are seldom seen in sections of nervous tissue, it is difficult to classify neurons structurally by microscopic inspection. Nervous components can also be subdivided functionally. Sensory neurons are afferent and receive stimuli from the receptors throughout the body. Motor neurons are efferent, sending impulses to effector organs such as muscle fibers and glands. Somatic motor nerves are under voluntary control and typically innervate most skeletal muscle; autonomic motor nerves control the “involuntary” activities of glands, cardiac muscle, and most smooth muscle. Interneurons establish relationships among other neurons, forming complex functional networks or circuits (as in the CNS and retina). Interneurons are generally multipolar or anaxonic and are estimated to include 99% of the neurons in the human CNS. In the CNS most neuronal perikarya occur in the gray matter, with axons concentrated in the white matter. These terms refer to the general appearance of unstained CNS tissue caused in part by the different densities of nerve cell bodies. In the PNS cell bodies are found in ganglia and in some sensory regions, such as the olfactory mucosa, and axons are bundled in nerves. 4. What are Schwann cells and satellite cells of ganglia? Schwann cells (named for 19th century German histologist Theodor Schwann), sometimes called neurolemmocytes, are found only in the PNS and differentiate from precursors in the neural crest. Schwann cells have trophic interactions with axons and importantly allow for their myelination, like the oligodendrocytes of the CNS. As discussed with peripheral nerves, one Schwann cell forms myelin around a segment of one axon, in contrast to the ability of oligodendrocytes to branch and ensheath parts of more than one axon. Also derived from the embryonic neural crest, small satellite cells form an intimate covering layer over the large neuronal cell bodies in the ganglia of the PNS (Figures 9–9f). Satellite cells exert a trophic or supportive effect on these neurons, insulating, nourishing, and regulating their microenvironments. Fig. 9e shows how a series of Schwann cells covers the full length of an axon. Satellite Cells of Ganglia Also derived from the embryonic neural crest, small satellite cells form an intimate covering layer over the large neuronal cell bodies in the ganglia of the PNS. Satellite cells exert a trophic or supportive effect on these neurons, insulating, nourishing, and regulating their microenvironments. 5. Describe the nerve organization in the PNS. The main components of the peripheral nervous system (PNS) are the nerves, ganglia, and nerve endings. Nerves are bundles of nerve fibers (axons) surrounded by Schwann cells and layers of connective tissue. Nerve Fibers Nerve fibers are analogous to tracts in the CNS, containing axons enclosed within sheaths of glial cells specialized to facilitate axonal function. In peripheral nerve fibers, axons are sheathed by Schwann cells or neurolemmocytes). The sheath may or may not form myelin around the axons, depending on their diameter. Nerve Organization In the PNS nerve fibers are grouped into bundles to form nerves. Except for very thin nerves containing only unmyelinated fibers, nerves have a whitish, glistening appearance because of their myelin and collagen content. Axons and Schwann cells are enclosed within layers of connective tissue. Immediately around the external laminae of the Schwann cells is a thin layer called the endoneurium, consisting of reticular fibers, scattered fibroblasts, and capillaries. Groups of axons with Schwann cells and endoneurium are bundled together as fascicles by a sleeve of perineurium, containing flat fibrocytes with their edges sealed together by tight junctions. From two to six layers of these unique connective tissue cells regulate diffusion into the fascicle and make up the blood-nerve barrier that helps maintain the fibers’ microenvironment. Externally, peripheral nerves have a dense, irregular fibrous coat called the epineurium, which extends deeply to fill the space between fascicles. Very small nerves consist of one fascicle. Small nerves can be found in sections of many organs and often show a winding disposition in connective tissue. Peripheral nerves establish communication between centers in the CNS and the sense organs and effectors (muscles, glands, etc). They generally contain both afferent and efferent fibers. Afferent fibers carry information from internal body regions and the environment to the CNS. Efferent fibers carry impulses from the CNS to effector organs commanded by these centers. Nerves possessing only sensory fibers are called sensory nerves; those composed only of fibers carrying impulses to the effectors are called motor nerves. Most nerves have both sensory and motor fibers and are called mixed nerves, usually also with both myelinated and unmyelinated axons. 6. What is the difference of myelinated from unmyelinated fibers? Myelinated Fibers As axons of large diameter grow in the PNS, they are engulfed along their length by a series of differentiating neurolemmocytes and become myelinated nerve fibers. The plasma membrane of each covering Schwann cell fuses with itself around the axon, and the fused membrane (or mesaxon) becomes wrapped around the axon as the glial cell body moves circumferentially around the axon many times (Figure 9–21). The multiple layers of Schwann cell membrane unite as a thick myelin sheath. Composed mainly of lipid bilayers and membrane proteins, myelin is a large lipoprotein complex that, like cell membranes, is partly removed by standard histologic procedures. Unlike oligodendrocytes of the CNS, a Schwann cell forms myelin around only a portion of one axon.  With high-magnification TEM, the myelin sheath appears as a thick electron-dense axonal covering in which the concentric membrane layers may be visible. The prominent electron-dense layers visible ultrastructurally in the sheath, the major dense lines, represent the fused, protein-rich cytoplasmic surfaces of the Schwann cell membrane. Along the myelin sheath, these surfaces periodically separate slightly to allow transient movement of cytoplasm for membrane maintenance; at these myelin clefts (or Schmidt- Lanterman clefts) the major dense lines temporarily disappear. Faintly seen ultrastructurally in the light staining layers are the intraperiod lines that represent the apposed outer bilayers of the Schwann cell membrane. Membranes of Schwann cells have a higher proportion of lipids than do other cell membranes, and the myelin sheath serves to insulate axons and maintain a constant ionic micro-environment most suitable for action potentials. Between adjacent Schwann cells on an axon the myelin sheath shows small nodes of Ranvier, where the axon is only partially covered by interdigitating Schwann cell processes. At these nodes the axolemma is exposed to ions in the + interstitial fluid and has a much higher concentration of voltage-gated Na channels, which renew the action potential and produce saltatory conduction (L. saltare, to jump) of nerve impulses, their rapid movement from node to node. The length of axon ensheathed by one Schwann cell, the internodal segment, varies directly with axonal diameter and ranges from 300 to 1500 μm. Unmyelinated Fibers Unlike the CNS where many short axons are not myelinated at all but run free among the other neuronal and glial processes, the smallest-diameter axons of peripheral nerves are still enveloped within simple folds of Schwann cells (Fig. 9.25). In these unmyelinated fibers the glial cell does not form the multiple wrapping of a myelin sheath. In unmyelinated fibers, each Schwann cell can enclose portions of many axons with small diameters. Without the thick myelin sheath, nodes of Ranvier are not seen along unmyelinated nerve fibers. Moreover, these small-diameter axons have evenly distributed voltage-gated ion channels; their impulse conduction is not saltatory and is much slower than that of myelinated axons. 7. What are ganglia? Describe sensory and autonomic ganglia. Ganglia are typically ovoid structures containing neuronal cell bodies and their surrounding glial satellite cells supported by delicate connective tissue and surrounded by a denser capsule. Because they serve as relay stations to transmit nerve impulses, at least one nerve enters and another exits from each ganglion. The direction of the nerve impulse determines whether the ganglion will be a sensory or an autonomic ganglion. Sensory Ganglia Sensory ganglia receive afferent impulses that go to the CNS. Sensory ganglia are associated with both cranial nerves (cranial ganglia) and the dorsal roots of the spinal nerves (spinal ganglia). The large neuronal cell bodies of ganglia (Figure 9–29) are associated with thin, sheetlike extensions of small glial satellite cells. Sensory ganglia are supported by a distinct connective tissue capsule and an internal framework continuous with the connective tissue layers of the nerves. The neurons of these ganglia are pseudounipolar and relay information from the ganglion’s nerve endings to the gray matter of the spinal cord via synapses with local neurons. Autonomic Ganglia Autonomic (Gr. autos, self + nomos, law) nerves effect the activity of smooth muscle, the secretion of some glands, heart rate, and many other involuntary activities by which the body maintains a constant internal environment (homeostasis). Autonomic ganglia are small bulbous dilations in autonomic nerves, usually with multipolar neurons. Some are located within certain organs, especially in the walls of the digestive tract, where they constitute the intramural ganglia. The capsules of these ganglia may be poorly defined among the local connective tissue. A layer of satellite cells also envelops the neurons of autonomic ganglia (Figure 9–29), although these may also be inconspicuous in intramural ganglia. Autonomic nerves use two-neuron circuits. The first neuron of the chain, with the preganglionic fiber, is located in the CNS. Its axon forms a synapse with postganglionic fibers of the second multipolar neuron in the chain located in a peripheral ganglion system. The chemical mediator present in the synaptic vesicles of all preganglionic axons is acetylcholine. As indicated earlier autonomic nerves make up the autonomic nervous system. This has two parts: the sympathetic and the parasympathetic divisions. Neuronal cell bodies of preganglionic sympathetic nerves are located in the thoracic and lumbar segments of the spinal cord and those of the parasympathetic division are in the medulla and midbrain and in the sacral portion of the spinal cord. Sympathetic second neurons are located in small ganglia along the vertebral column, while second neurons of the parasympathetic series are found in very small ganglia always located near or within the effector organs, for example in the walls of the stomach and intestines. Parasympathetic ganglia may lack distinct capsules altogether, perikarya and associated satellite cells simply forming a loosely organized plexus within the surrounding connective tissue. 8. Discuss the physiological anatomy of the synapse. Figure 46-6 shows a typical anterior motor neuron in the anterior horn of the spinal cord. It is composed of three major parts: the soma, which is the main body of the neuron; a single axon, which extends from the soma into a peripheral nerve that leaves the spinal cord; and the dendrites, which are great numbers of branching projections of the soma that extend as much as 1 millimeter into the surrounding areas of the cord. As many as 10,000 to 200,000 minute synaptic knobs called presynaptic terminals lie on the surfaces of the dendrites and soma of the motor neuron, with about 80 to 95 percent of them on the dendrites and only 5 to 20 percent on the soma. These presynaptic terminals are the ends of nerve fibrils that originate from many other neurons. Many of these presynaptic terminals are excitatory—that is, they secrete a neurotransmitter that excites the postsynaptic neuron. However, other presynaptic terminals are inhibitory— that is, they secrete a neurotransmitter that inhibits the postsynaptic neuron. Neurons in other parts of the cord and brain differ from the anterior motor neuron in (1) the size of the cell body; (2) the length, size, and number of dendrites, ranging in length from almost zero to many centimeters; (3) the length and size of the axon; and (4) the number of presynaptic terminals, which may range from only a few to as many as 200,000. These differences make neurons in different parts of the nervous system react differently to incoming synaptic signals and, therefore, perform many different functions. Presynaptic Terminals. Electron microscopic studies of the presynaptic terminals show that they have varied anatomical forms, but most of them resemble small round or oval knobs and, therefore, are sometimes called terminal knobs, boutons, end-feet, or synaptic knobs.  Figure 46-5A illustrates the basic structure of a chemical synapse, showing a single presynaptic terminal on the membrane surface of a postsynaptic neuron. The presynaptic terminal is separated from the postsynaptic neuronal soma by a synaptic cleft having a width usually of 200 to 300 angstroms. The terminal has two internal structures important to the excitatory or inhibitory function of the synapse: the transmitter vesicles and the mitochondria. The transmitter vesicles contain the neurotransmitter that, when released into the synaptic cleft, either excites or inhibits the postsynaptic neuron. It excites the postsynaptic neuron if the neuronal membrane contains excitatory receptors, and it inhibits the neuron if the membrane contains inhibitory receptors. The mitochondria provide adenosine triphosphate (ATP), which in turn supplies the energy for synthesizing new transmitter substance. When an action potential spreads over a presynaptic terminal, depolarization of its membrane causes a small number of vesicles to empty into the cleft. The released transmitter in turn causes an immediate change in permeability characteristics of the postsynaptic neuronal membrane, which leads to excitation or inhibition of the postsynaptic neuron, depending on the neuronal receptor characteristics. 9. Discuss the action potential in the presynaptic and postsynaptic terminals. Mechanism by Which an Action Potential Causes Transmitter Release from the Presynaptic Terminals—Role of Calcium Ions The membrane of the presynaptic terminal is called the presynaptic membrane. It contains large numbers of voltage-gated calcium channels. When an action potential depolarizes the presynaptic membrane, these calcium channels open and allow large numbers of calcium ions to flow into the terminal. The quantity of neurotransmitter that is then released from the terminal into the synaptic cleft is directly related to the number of calcium ions that enter. The precise mechanism by which the calcium ions cause this release is not known, but it is believed to be the following. When the calcium ions enter the presynaptic terminal, they bind with special protein molecules on the inside surface of the presynaptic membrane, called release sites. This binding in turn causes the release sites to open through the membrane, allowing a few transmitter vesicles to release their transmitter into the cleft after each single action potential. For the vesicles that store the neurotransmitter acetylcholine, between 2,000 and 10,000 molecules of acetylcholine are present in each vesicle, and there are enough vesicles in the presynaptic terminal to transmit from a few hundred to more than 10,000 action potentials. Action of the Transmitter Substance on the Postsynaptic Neuron—Function of “Receptor Proteins” The membrane of the postsynaptic neuron contains large numbers of receptor proteins, also shown in Figure 46- 5A. The molecules of these receptors have two important components: (1) a binding component that protrudes outward from the membrane into the synaptic cleft— here it binds the neurotransmitter coming from the presynaptic terminal— and (2) an intracellular component that passes all the way through the postsynaptic membrane to the interior of the postsynaptic neuron. Receptor activation controls the opening of ion channels in the postsynaptic cell in one of two ways: (1) by gating ion channels directly and allowing passage of specified types of ions through the membrane, or (2) by activating a “second messenger” that is not an ion channel but instead is a molecule that protrudes into the cell cytoplasm and activates one or more substances inside the postsynaptic neuron. These second messengers increase or decrease specific cellular functions. Neurotransmitter receptors that directly gate ion channels are often called ionotropic receptors, whereas those that act through second messenger systems are called metabotropic receptors. 10. What are the chemical substances that function as synaptic transmitters? More than 50 chemical substances have been proved or postulated to function as synaptic transmitters. Many of them are listed in Tables 46-1 and 46-2, which provide two groups of synaptic transmitters. One group comprises small- molecule, rapidly acting transmitters. The other is made up of a large number of neuropeptides of much larger molecular size that usually act much more slowly.  The small molecule, rapidly acting transmitters cause most acute responses of the nervous system, such as transmission of sensory signals to the brain and of motor signals back to the muscles. The neuropeptides, in contrast, usually cause more prolonged actions, such as long term changes in numbers of neuronal receptors, long term opening or closure of certain ion channels, and possibly even long term changes in numbers of synapses or sizes of synapses. 11. What are the different types of sensory preceptors? Table 47-1 lists and classifies five basic types of sensory receptors: (1) mechanoreceptors, which detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor; (2) thermoreceptors, which detect changes in temperature, with some receptors detecting cold and others warmth; (3) nociceptors (pain receptors), which detect physical or chemical damage occurring in the tissues; (4) electromagnetic receptors, which detect light on the retina of the eye; and (5) chemoreceptors, which detect taste in the mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the body fluids, carbon dioxide concentration, and other factors that make up the chemistry of the body. Let’s discuss the function of a few specific types of receptors, primarily peripheral mechanoreceptors, to illustrate some of the principles by which receptors operate. Other receptors are discussed in other chapters in relation to the sensory systems that they subserve. Figure 47-1 shows some of the types of mechanoreceptors found in the skin or in deep tissues of the body. DIFFERENTIAL SENSITIVITY OF RECEPTORS How do two types of sensory receptors detect different types of sensory stimuli? The answer is “by differential sensitivities.”That is, each type of receptor is highly sensitive to one type of stimulus for which it is designed and yet is almost nonresponsive to other types of sensory stimuli. Thus, the rods and cones of the eyes are highly responsive to light but are almost completely nonresponsive to normal ranges of heat, cold, pressure on the eyeballs, or chemical changes in the blood. The osmoreceptors of the supraoptic nuclei in the hypothalamus detect minute changes in the osmolality of the body fluids but have never been known to respond to sound. Finally, pain receptors in the skin are almost never stimulated by usual touch or pressure stimuli but do become highly active the moment tactile stimuli become severe enough to damage the tissues. Modality of Sensation—the “Labeled Line” Principle Each of the principal types of sensation that we can experience—pain, touch, sight, sound, and so forth—is called a modality of sensation. Yet, despite the fact that we experience these different modalities of sensation, nerve fibers transmit only impulses. Therefore, how do different nerve fibers transmit different modalities of sensation? The answer is that each nerve tract terminates at a specific point in the central nervous system, and the type of sensation felt when a nerve fiber is stimulated is determined by the point in the nervous system to which the fiber leads. For instance, if a pain fiber is stimulated, the person perceives pain regardless of what type of stimulus excites the fiber. The stimulus can be electricity, overheating of the fiber, crushing of the fiber, or stimulation of the pain nerve ending by damage to the tissue cells. In all these instances, the person perceives pain. Likewise, if a touch fiber is stimulated by electrical excitation of a touch receptor or in any other way, the person perceives touch because touch fibers lead to specific touch areas in the brain. Similarly, fibers from the retina of the eye terminate in the vision areas of the brain, fibers from the ear terminate in the auditory areas of the brain, and temperature fibers terminate in the temperature areas. This specificity of nerve fibers for transmitting only one modality of sensation is called the labeled line principle. 12. Discuss the transduction of sensory stimuli into nerve impulses. LOCAL ELECTRICAL CURRENTS AT NERVE ENDINGS—RECEPTOR POTENTIALS All sensory receptors have one feature in common. Whatever the type of stimulus that excites the receptor, its immediate effect is to change the membrane electrical potential of the receptor. This change in potential is called a receptor potential. Mechanisms of Receptor Potentials. Different receptors can be excited in one of several ways to cause receptor potentials: (1) by mechanical deformation of the receptor, which stretches the receptor membrane and opens ion channels; (2) by application of a chemical to the membrane, which also opens ion channels; (3) by change of the temperature of the membrane, which alters the permeability of the membrane; or (4) by the effects of electromagnetic radiation, such as light on a retinal visual receptor, which either directly or indirectly changes the receptor membrane characteristics and allows ions to flow through membrane channels.  These four means of exciting receptors correspond in general to the different types of known sensory receptors. In all instances, the basic cause of the change in membrane potential is a change in membrane permeability of the receptor, which allows ions to diffuse more or less readily through the membrane and thereby to change the transmembrane potential. Maximum Receptor Potential Amplitude. The maximum amplitude of most sensory receptor potentials is about 100 millivolts, but this level occurs only at an extremely high intensity of sensory stimulus. This is about the same maximum voltage recorded in action potentials and is also the change in voltage when the membrane becomes maximally permeable to sodium ions. Relation of the Receptor Potential to Action Potentials. When the receptor potential rises above the threshold for eliciting action potentials in the nerve fiber attached to the receptor, then action potentials occur, as illustrated in Figure 47- 2. Note also that the more the receptor potential rises above the threshold level, the greater becomes the action potential frequency. RECEPTOR POTENTIAL OF THE PACINIAN CORPUSCLE—AN EXAMPLE OF RECEPTOR FUNCTION Note in Figure 47-1 that the Pacinian corpuscle has a central nerve fiber extending through its core. Surrounding this central nerve fiber are multiple concentric capsule layers, and thus compression anywhere on the outside of the corpuscle will elongate, indent, or otherwise deform the central fiber. Figure 47-3 shows only the central fiber of the Pacinian corpuscle after all capsule layers but one, have been removed. The tip of the central fiber inside the capsule is unmyelinated, but the fiber does become myelinated (the blue sheath shown in the figure) shortly before leaving the corpuscle to enter a peripheral sensory nerve. Figure 47-3 also shows the mechanism by which a receptor potential is produced in the Pacinian corpuscle. Observe the small area of the terminal fiber that has been deformed by compression of the corpuscle, and note that ion channels have opened in the membrane, allowing positively charged sodium ions to diffuse to the interior of the fiber. This action creates increased positivity inside the fiber, which is the “receptor potential.” The receptor potential in turn induces a local circuit of current flow, shown by the arrows, that spreads along the nerve fiber. At the first node of Ranvier, which lies inside the capsule of the Pacinian corpuscle, the local current flow depolarizes the fiber membrane at this node, which then sets off typical action potentials that are transmitted along the nerve fiber toward the central nervous system. 13. What is the autonomic nervous system? The autonomic nervous system is the portion of the nervous system that controls most visceral functions of the body. This system helps to control arterial pressure, gastrointestinal motility, gastrointestinal secretion, urinary bladder emptying, sweating, body temperature, and many other activities. Some of these activities are controlled almost entirely and some only partially by the autonomic nervous system. One of the most striking characteristics of the autonomic nervous system is the rapidity and intensity with which it can change visceral functions. For instance, within 3 to 5 seconds it can increase the heart rate to twice normal, and within 10 to 15 seconds the arterial pressure can be doubled. At the other extreme, the arterial pressure can be decreased low enough within 10 to 15 seconds to cause fainting. Sweating can begin within seconds, and the urinary bladder may empty involuntarily, also within seconds. 14. Describe the general organization of the ANS. The autonomic nervous system is activated mainly by centers located in the spinal cord, brain stem, and hypo- thalamus. In addition, portions of the cerebral cortex, especially of the limbic cortex, can transmit signals to the lower centers and in this way can influence autonomic control. The autonomic nervous system also often operates through visceral reflexes. That is, subconscious sensory signals from visceral organs can enter the autonomic ganglia, the brain stem, or the hypothalamus and then return subconscious reflex responses directly back to the visceral organs to control their activities. The efferent autonomic signals are transmitted to the various organs of the body through two major subdivisions called the sympathetic nervous system and the parasympathetic nervous system, the characteristics and functions of which are described next 15. Describe the physiological anatomy of the sympathetic and parasympathetic nervous system. Physiological Anatomy of the Sympathetic Nervous System Figure 61-1 shows the general organization of the peripheral portions of the sympathetic nervous system. Shown specifically in the figure are (1) one of the two paravertebral sympathetic chains of ganglia that are interconnected with the spinal nerves on the side of the vertebral column, (2) prevertebral ganglia (the celiac, superior mesenteric, aortico-renal, inferior mesenteric, and hypogastric), and (3) nerves extending from the ganglia to the different internal organs. The sympathetic nerve fibers originate in the spinal cord along with spinal nerves between cord segments T1 and L2 and pass first into the sympathetic chain and then to the tissues and organs that are stimulated by the sympathetic nerves. Preganglionic and Postganglionic Sympathetic Neurons The sympathetic nerves are different from skeletal motor nerves in the following way: Each sympathetic pathway from the cord to the stimulated tissue is composed of two neurons, a preganglionic neuron and a postganglionic neuron, in contrast to only a single neuron in the skeletal motor pathway. The cell body of each preganglionic neuron lies in the intermediolateral horn of the spinal cord; its fiber passes through a ventral root of the cord into the corresponding spinal nerve, as shown in Figure 61-2. Immediately after the spinal nerve leaves the spinal canal, the preganglionic sympathetic fibers leave the spinal nerve and pass through a white ramus into one of the ganglia of the sympathetic chain. The fibers then can take one of the following three courses: (1) they can synapse with postganglionic sympathetic neurons in the ganglion that they enter; (2) they can pass upward or downward in the chain and synapse in one of the other ganglia of the chain; or (3) they can pass for variable distances through the chain and then through one of the sympathetic nerves radiating outward from the chain, finally synapsing in a peripheral sympathetic ganglion. The postganglionic sympathetic neuron thus originates either in one of the sympathetic chain ganglia or in one of the peripheral sympathetic ganglia. From either of these two sources, the postganglionic fibers then travel to their destinations in the various organs.  Sympathetic Nerve Fibers in the Skeletal Nerves. Some of the postganglionic fibers pass back from the sympathetic chain into the spinal nerves through gray rami at all levels of the cord, as shown in Figure 61- 2. These sympathetic fibers are all very small type C fibers, and they extend to all parts of the body by way of the skeletal nerves. They control the blood vessels, sweat glands, and piloerector muscles of the hairs. About 8 percent of the fibers in the average skeletal nerve are sympathetic fibers, a fact that indicates their great importance. Segmental Distribution of the Sympathetic Nerve Fibers. The sympathetic pathways that originate in the different segments of the spinal cord are not necessarily distributed to the same part of the body as the somatic spinal nerve fibers from the same segments. Instead, the sympathetic fibers from cord segment T1 generally pass (1) up the sympathetic chain to terminate in the head; (2) from T2 to terminate in the neck; (3) from T3, T4, T5, and T6 into the thorax; (4) from T7, T8, T9, T10, and T11 into the abdomen; and (5) from T12, L1, and L2 into the legs. This distribution is only approximate and overlaps greatly. The distribution of sympathetic nerves to each organ is determined partly by the locus in the embryo from which the organ originated. For instance, the heart receives many sympathetic nerve fibers from the neck portion of the sympathetic chain because the heart originated in the neck of the embryo before translocating into the thorax. Likewise, the abdominal organs receive most of their sympathetic innervation from the lower thoracic spinal cord segments because most of the primitive gut originated in this area. Special Nature of the Sympathetic Nerve Endings in the Adrenal Medullae. Preganglionic sympathetic nerve fibers pass, without synapsing, all the way from the intermediolateral horn cells of the spinal cord, through the sympathetic chains, then through the splanchnic nerves, and finally into the two adrenal medullae. There they end directly on modified neuronal cells that secrete epinephrine and norepinephrine into the blood stream. These secretory cells embryologically are derived from nervous tissue and are actually postganglionic neurons; indeed, they even have rudimentary nerve fibers, and it is the endings of these fibers that secrete the adrenal hormones epinephrine and norepinephrine. Physiological Anatomy of the Parasympathetic Nervous System The parasympathetic nervous system is shown in Figure 61-3, which demonstrates that parasympathetic fibers leave the central nervous system through cranial nerves III, VII, IX, and X; additional parasympathetic fibers leave the lowermost part of the spinal cord through the second and third sacral spinal nerves and occasionally the first and fourth sacral nerves. About 75 percent of all parasympathetic nerve fibers are in the vagus nerves (cranial nerve X), passing to the entire thoracic and abdominal regions of the body. Therefore, a physiologist speaking of the parasympathetic nervous system often thinks mainly of the two vagus nerves. The vagus nerves supply parasympathetic nerves to the heart, lungs, esophagus, stomach, entire small intestine, proximal half of the colon, liver, gallbladder, pancreas, kidneys, and upper portions of the ureters. Parasympathetic fibers in the third cranial nerve go to the pupillary sphincter and ciliary muscle of the eye. Fibers from the seventh cranial nerve pass to the lacrimal, nasal, and submandibular glands, and fibers from the ninth cranial nerve go to the parotid gland. The sacral parasympathetic fibers are in the pelvic nerves, which pass through the spinal nerve sacral plexus on each side of the cord at the S2 and S3 levels. These fibers then distribute to the descending colon, rectum, urinary bladder, and lower portions of the ureters. Also, this sacral group of parasympathetics supplies nerve signals to the external genitalia to cause erection.  Preganglionic and Postganglionic Parasympathetic Neurons. The parasympathetic system, like the sympathetic system, has both preganglionic and postganglionic neurons. However, except in the case of a few cranial parasympathetic nerves, the preganglionic fibers pass uninterrupted all the way to the organ that is to be controlled. The postganglionic neurons are located in the wall of the organ. The preganglionic fibers synapse with these neurons, and extremely short postganglionic fibers, a fraction of a millimeter to several centimeters in length, leave the neurons to innervate the tissues of the organ. This location of the parasympathetic postganglionic neurons in the visceral organ is quite different from the arrangement of the sympathetic ganglia because the cell bodies of the sympathetic postganglionic neurons are almost always located in the ganglia of the sympathetic chain or in various other discrete ganglia in the abdomen, rather than in the target organ. 16. What are the cholinergic and adrenergic fibers? The sympathetic and parasympathetic nerve fibers secrete mainly one or the other of two synaptic transmitter substances, acetylcholine or norepinephrine. The fibers that secrete acetylcholine are said to be cholinergic. Those that secrete norepinephrine are said to be adrenergic, a term derived from adrenalin, which is an alternate name for epinephrine. All preganglionic neurons are cholinergic in both the sympathetic and the parasympathetic nervous systems. Acetylcholine or acetylcholine-like substances, when applied to the ganglia, will excite both sympathetic and parasympathetic postganglionic neurons. Either all or almost all of the postganglionic neurons of the parasympathetic system are also cholinergic. Conversely, most of the postganglionic sympathetic neurons are adrenergic. However, the postganglionic sympathetic nerve fibers to the sweat glands and perhaps to a very few blood vessels are cholinergic. Thus, the terminal nerve endings of the parasympathetic system all or virtually all secrete acetylcholine. Almost all of the sympathetic nerve endings secrete norepinephrine, but a few secrete acetylcholine. These neurotransmitters in turn act on the different organs to cause respective parasympathetic or sympathetic effects. Therefore, acetylcholine is called a parasympathetic transmitter and norepinephrine is called a sympathetic transmitter. The molecular structures of acetylcholine and norepinephrine are the following: 17. Discuss the mechanism of transmitter secretion and removal of postganglionic endings? Secretion of Acetylcholine and Norepinephrine by Postganglionic Nerve Endings. A few of the postganglionic autonomic nerve endings, especially those of the parasympathetic nerves, are similar to but much smaller than those of the skeletal neuromuscular junction. However, many of the parasympathetic nerve fibers and almost all the sympathetic fibers merely touch the effector cells of the organs that they innervate as they pass by, or in some instances, they terminate in connective tissue located adjacent to the cells that are to be stimulated. Where these filaments touch or pass over or near the cells to be stimulated, they usually have bulbous enlargements called varicosities; it is in these varicosities that the transmitter vesicles of acetylcholine or norepinephrine are synthesized and stored. Also in the varicosities are large numbers of mitochondria that supply adenosine triphosphate, which is required to energize acetylcholine or norepinephrine synthesis. When an action potential spreads over the terminal fibers, the depolarization process increases the permeability of the fiber membrane to calcium ions, allowing these ions to diffuse into the nerve terminals or nerve varicosities. The calcium ions in turn cause the terminals or varicosities to empty their contents to the exterior. Thus, the transmitter substance is secreted. Synthesis of Acetylcholine, Its Destruction after Secretion, and Its Duration of Action. Acetylcholine is synthesized in the terminal endings and varicosities of the cholinergic nerve fibers, where it is stored in vesicles in highly concentrated form until it is released. The basic chemical reaction of this synthesis is the following: Choline acetyltransferase Acetyl-CoA + Choline ⎜⎜⎜⎜⎜⎜⎜→ Acetylcholine Once acetylcholine is secreted into a tissue by a cholinergic nerve ending, it persists in the tissue for a few seconds while it performs its nerve signal transmitter function. Then it is split into an acetate ion and choline, catalyzed by the enzyme acetylcholinesterase that is bound with collagen and glycosaminoglycans in the local connective tissue. This mechanism is the same as that for acetylcholine signal transmission and subsequent acetylcholine destruction that occurs at the neuromuscular junctions of skeletal nerve fibers. The choline that is formed is then transported back into the terminal nerve ending, where it is used again and again for synthesis of new acetylcholine. Synthesis of Norepinephrine, Its Removal, and Its Duration of Action. Synthesis of norepinephrine begins in the axoplasm of the terminal nerve endings of adrenergic nerve fibers but is completed inside the secretory vesicles. The basic steps are the following: Hydroxylation 1. Tyrosine ⎜⎜⎜⎜⎜→ Dopa Decarboxylation 2. Dopa ⎜⎜⎜⎜⎜⎜→ Dopamine 3. Transport of dopamine into the vesicles Hydroxylation 4. Dopamine ⎜⎜⎜⎜⎜→ Norepinephrine In the adrenal medulla, this reaction goes still one step further to transform about 80 percent of the norepinephrine into epinephrine, as follows: Methylation 5. Norepinephrine ⎜⎜⎜⎜⎜→ Epinephrine After secretion of norepinephrine by the terminal nerve endings, it is removed from the secretory site in three ways: (1) reuptake into the adrenergic nerve endings by an active transport process, accounting for removal of 50 to 80 percent of the secreted norepinephrine; (2) diffusion away from the nerve endings into the surrounding body fluids and then into the blood, accounting for removal of most of the remaining norepinephrine; and (3) destruction of small amounts by tissue enzymes (one of these enzymes is monoamine oxidase, which is found in the nerve endings, and another is catechol-O-methyl transferase, which is present diffusely in the tissues). Ordinarily, the norepinephrine secreted directly into a tissue remains active for only a few seconds, demonstrating that its reuptake and diffusion away from the tissue are rapid. However, the norepinephrine and epinephrine secreted into the blood by the adrenal medullae remain active until they diffuse into some tissue, where they can be destroyed by catechol-O-methyl transferase; this action occurs mainly in the liver. Therefore, when secreted into the blood, both norepinephrine and epinephrine remain active for 10 to 30 seconds, but their activity declines to extinction over 1 to several minutes. 18. What are the two principal types of acetylcholine receptors? Acetylcholine activates mainly two types of receptors, which are called muscarinic and nicotinic receptors. The reason for these names is that muscarine, a poison from toadstools, activates only muscarinic receptors and will not activate nicotinic receptors, whereas nicotine activates only nicotinic receptors. Acetylcholine activates both of them. Muscarinic receptors, which use G proteins as their signaling mechanism, are found on all effector cells that are stimulated by the postganglionic cholinergic neurons of either the parasympathetic nervous system or the sympathetic system. Nicotinic receptors are ligand-gated ion channels found in autonomic ganglia at the synapses between the preganglionic and postganglionic neurons of both the sympathetic and parasympathetic systems. (Nicotinic receptors are also present at many non-autonomic nerve endings—for instance, at the neuromuscular junctions in skeletal muscle).  An understanding of the two types of receptors is especially important because specific drugs are frequently used as medicine to stimulate or block one or the other of the two types of receptors. 19. What are the two major adrenergic receptors?  Two major classes of adrenergic receptors also exist; they are called alpha receptors and beta receptors. There are two major types of alpha receptors, alpha1 and alpha2, which are linked to different G proteins. The beta receptors are divided into beta1, beta2, and beta3 receptors because certain chemicals affect only certain beta receptors. The beta receptors also use G proteins for signaling. Norepinephrine and epinephrine, both of which are secreted into the blood by the adrenal medulla, have slightly different effects in exciting the alpha and beta receptors. Norepinephrine excites mainly alpha receptors but excites the beta receptors to a lesser extent as well. Epinephrine excites both types of receptors approximately equally. Therefore, the relative effects of norepinephrine and epinephrine on different effector organs are determined by the types of receptors in the organs. If they are all beta receptors, epinephrine will be the more effective excitant. Table 61-1 lists the distribution of alpha and beta receptors in some of the organs and systems controlled by the sympathetic nerves. Note that certain alpha functions are excitatory, whereas others are inhibitory. Likewise, certain beta functions are excitatory and others are inhibitory. Therefore, alpha and beta receptors are not necessarily associated with excitation or inhibition but simply with the affinity of the hormone for the receptors in the given effector organ. A synthetic hormone chemically similar to epinephrine and norepinephrine, isopropyl norepinephrine, has an extremely strong action on beta receptors but essentially no action on alpha receptors. 20. List the excitatory and inhibitory actions of sympathetic and parasympathetic stimulation. Table 61-2 lists the effects on different visceral functions of the body caused by stimulating either the parasympathetic nerves or the sympathetic nerves. Note again that sympathetic stimulation causes excitatory effects in some organs but inhibitory effects in others. Likewise, parasympathetic stimulation causes excitation in some organs but inhibition in others. Also, when sympathetic stimulation excites a particular organ, parasympathetic stimulation sometimes inhibits it, demonstrating that the two systems occasionally act reciprocally to each other. However, most organs are dominantly controlled by one or the other of the two systems. There is no generalization one can use to explain whether sympathetic or parasympathetic stimulation will cause excitation or inhibition of a particular organ. Therefore, to understand sympathetic and parasympathetic function, one must learn all the separate functions of these two nervous systems on each organ, as listed in Table 61-2. 21. What is the function of the adrenal medullae? Stimulation of the sympathetic nerves to the adrenal medullae causes large quantities of epinephrine and norepinephrine to be released into the circulating blood, and these two hormones in turn are carried in the blood to all tissues of the body. On average, about 80 percent of the secretion is epinephrine and 20 percent is norepinephrine, although the relative proportions can change considerably under different physiological conditions. The circulating epinephrine and norepinephrine have almost the same effects on the different organs as the effects caused by direct sympathetic stimulation, except that the effects last 5 to 10 times as long because both of these hormones are removed from the blood slowly over a period of 2 to 4 minutes. The circulating norepinephrine causes constriction of most of the blood vessels of the body; it also causes increased activity of the heart, inhibition of the gastrointestinal tract, dilation of the pupils of the eyes, and so forth. Epinephrine causes almost the same effects as those caused by norepinephrine, but the effects differ in the following respects: First, epinephrine, because of its greater effect in stimulating the beta receptors, has a greater effect on cardiac stimulation than does norepinephrine. Second, epinephrine causes only weak constriction of the blood vessels in the muscles, in comparison with much stronger constriction caused by norepinephrine. Because the muscle vessels represent a major segment of the vessels of the body, this difference is of special importance because norepinephrine greatly increases the total peripheral resistance and elevates arterial pressure, whereas epinephrine raises the arterial pressure to a lesser extent but increases the cardiac output more. A third difference between the actions of epinephrine and norepinephrine relates to their effects on tissue metabolism. Epinephrine has 5 to 10 times as great a metabolic effect as norepinephrine. Indeed, the epinephrine secreted by the adrenal medullae can increase the metabolic rate of the whole body often to as much as 100 percent above normal, in this way increasing the activity and excitability of the body. It also increases the rates of other metabolic activities, such as glycogenolysis in the liver and muscle and glucose release into the blood. In summary, stimulation of the adrenal medullae causes release of the hormones epinephrine and norepinephrine, which together have almost the same effects throughout the body as direct sympathetic stimulation, except that the effects are prolonged, lasting 2 to 4 minutes after the stimulation is over. Value of the Adrenal Medullae to the Function of the Sympathetic Nervous System. Epinephrine and norepinephrine are almost always released by the adrenal medullae at the same time that the different organs are stimulated directly by generalized sympathetic activation. Therefore, the organs are actually stimulated in two ways: directly by the sympathetic nerves and indirectly by the adrenal medullary hormones. The two means of stimulation support each other and either can, in most instances, substitute for the other. For instance, destruction of the direct sympathetic pathways to the different body organs does not abrogate sympathetic excitation of the organs because norepinephrine and epinephrine are still released into the circulating blood and indirectly cause stimulation. Likewise, loss of the two adrenal medullae usually has little effect on the operation of the sympathetic nervous system because the direct pathways can still perform almost all the necessary duties. Thus, the dual mechanism of sympathetic stimulation provides a safety factor, with one mechanism substituting for the other if it is missing. Another important value of the adrenal medullae is the capability of epinephrine and norepinephrine to stimulate structures of the body that are not innervated by direct sympathetic fibers. For instance, the metabolic rate of almost every cell of the body is increased by these hormones, especially by epinephrine, even though only a small proportion of all the cells in the body are innervated directly by sympathetic fibers. 22. Describe the sympathetic and parasympathetic tone. Normally, the sympathetic and parasympathetic systems are continually active, and the basal rates of activity are known, respectively, as sympathetic tone and parasympathetic tone. The value of tone is that it allows a single nervous system to both increase and decrease the activity of a stimulated organ. For instance, sympathetic tone normally keeps almost all the systemic arterioles constricted to about one-half their maximum diameter. By increasing the degree of sympathetic stimulation above normal, these vessels can be constricted even more; conversely, by decreasing the stimulation below normal, the arterioles can be dilated. If it were not for the continual background sympathetic tone, the sympathetic system could cause only vasoconstriction, never vasodilation. Another interesting example of tone is the background “tone” of the parasympathetics in the gastrointestinal tract. Surgical removal of the parasympathetic supply to most of the gut by cutting the vagus nerves can cause serious and prolonged gastric and intestinal “atony” with resulting blockage of much of the normal gastrointestinal propulsion and consequent serious constipation, thus demonstrating that parasympathetic tone to the gut is normally very much required. This tone can be decreased by the brain, thereby inhibiting gastrointestinal motility, or it can be increased, thereby promoting increased gastrointestinal activity. Tone Caused by Basal Secretion of Epinephrine and Norepinephrine by the Adrenal Medullae. The normal resting rate of secretion by the adrenal medullae is about 0.2 μg/kg/min of epinephrine and about 0.05 μg/kg/min of norepinephrine. These quantities are considerable—indeed, enough to maintain the blood pressure almost normal even if all direct sympathetic pathways to the cardiovascular system are removed. Therefore, it is obvious that much of the overall tone of the sympathetic nervous system results from basal secretion of epinephrine and norepinephrine in addition to the tone resulting from direct sympathetic stimulation. Effect of Loss of Sympathetic or Parasympathetic Tone after Denervation. Immediately after a sympathetic or parasympathetic nerve is cut, the innervated organ loses its sympathetic or parasympathetic tone. In many blood vessels, for instance, cutting the sympathetic nerves results in substantial vasodilation within 5 to 30 seconds. However, over minutes, hours, days, or weeks, intrinsic tone in the smooth muscle of the vessels increases—that is, increased tone caused by increased smooth muscle contractile force that is not the result of sympathetic stimulation but of chemical adaptations in the smooth muscle fibers themselves. This intrinsic tone eventually restores almost normal vasoconstriction. Essentially the same effects occur in most other effector organs whenever sympathetic or parasympathetic tone is lost. That is, intrinsic compensation soon develops to return the function of the organ almost to its normal basal level. However, in the parasympathetic system, the compensation sometimes requires many months. For instance, loss of parasympathetic tone to the heart after cardiac vagotomy increases the heart rate to 160 beats/min in a dog, and this rate will still be partially elevated 6 months later. 23. What do you call the reaction of Laura during the incident causing her to run and punch the culprit? “ALARM” OR “STRESS” RESPONSE OF THE SYMPATHETIC NERVOUS SYSTEM When large portions of the sympathetic nervous system discharge at the same time—that is, a mass discharge— this action increases the ability of the body to perform vigorous muscle activity in many ways, as summarized in the following list: 1. Increased arterial pressure 2. Increased blood flow to active muscles concurrent with decreased blood flow to organs such as the gastrointestinal tract and the kidneys that are not needed for rapid motor activity 3. Increased rates of cellular metabolism throughout the body 4. Increased blood glucose concentration 5. Increased glycolysis in the liver and in muscle 6. Increased muscle strength 7. Increased mental activity 8. Increased rate of blood coagulation The sum of these effects permits a person to perform far more strenuous physical activity than would otherwise be possible. Because either mental or physical stress can excite the sympathetic system, it is frequently said that the purpose of the sympathetic system is to provide extra activation of the body in states of stress, which is called the sympathetic stress response. The sympathetic system is especially strongly activated in many emotional states. For instance, in the state of rage, which is elicited to a great extent by stimulating the hypothalamus, signals are transmitted downward through the reticular formation of the brain stem and into the spinal cord to cause massive sympathetic discharge; most aforementioned sympathetic events ensue immediately. This is called the sympathetic alarm reaction. It is also called the fight-or-flight reaction because an animal in this state decides almost instantly whether to stand and fight or to run. In either event, the sympathetic alarm reaction makes the animal’s subsequent activities vigorous.