ORGANIZATION-OF-THE-NERVOUS-SYSTEM_240814_222534.pdf

ORGANIZATION OF THE NERVOUS SYSTEM General Design of the Nervous System tactile receptors on the surface of the body, or other kinds of receptors. Central Nervous System Neuron: The Basic Functional Unit...

ORGANIZATION OF THE NERVOUS SYSTEM General Design of the Nervous System tactile receptors on the surface of the body, or other kinds of receptors. Central Nervous System Neuron: The Basic Functional Unit This sensory experience can either cause immediate reaction from the Central nervous system contains brain, or memory of the experience more than 100 billion neurons can be stored in the brain for Incoming signals enter this neuron minutes, weeks, or years and through synapses located mostly on determine bodily reactions at some the neuronal dendrites, but also on future date. the cell body. Conversely, the output This information enters the central signal travels by way of a single nervous system through peripheral axon leaving the neuron. nerves and is conducted immediately This axon has many separate to multiple sensory areas in: branches to other parts of the (1) the spinal cord at all levels nervous system or peripheral body. (2) the reticular substance of the A special feature of most synapses is medulla, pons, and mesencephalon that the signal normally passes only of the brain in the forward direction (from the (3) the cerebellum axon of a preceding neuron to (4) the thalamus dendrites on cell membranes of (5) areas of the cerebral cortex subsequent neurons) This forces the signal to travel in required directions for performing specific nervous functions. Motor Part of the Nervous System- Effectors The most important eventual role of the nervous system is to control the various bodily activities. This is achieved by controlling: Sensory Part of the Nervous System- (1) contraction of appropriate Sensory Receptors skeletal muscles throughout the body Most activities of the nervous system are (2) contraction of smooth muscle in initiated by sensory experience exciting the internal organs sensory receptors, whether visual receptors (3) secretion of active chemical in the eyes, auditory receptors in the ears, substances by both exocrine and pg. 1  ORGANIZATION OF THE NERVOUS SYSTEM endocrine glands in many parts of Many, if not most, of what we call the body. subconscious activities of the body These activities are collectively are controlled in the lower areas of called motor functions of the nervous the brain-that is, in the medulla, system, and the muscles and glands pons, mesencephalon, hypothalamus, are called effectors because they are thalamus, cerebellum, and basal the actual anatomical structures that ganglia. For instance, subconscious perform the functions dictated by the control of arterial pressure and nerve signals respiration is achieved mainly in the medulla and pons.. MAJOR LEVELS OF CENTRAL Control of equilibrium is a combined NERVOUS SYSTEM FUNCTION Three function of the older portions of the major levels of the central nervous system cerebellum and the reticular have specific functional characteristics: substance of the medulla, pons, and (1) the spinal cord level mesencephalon. Feeding reflexes, such as salivation and licking of the (2) the lower brain or subcortical level lips in response to the taste of food, (3) the higher brain or cortical level are controlled by areas in the medulla, pons, mesencephalon, SPINAL CORD LEVEL amygdala, and hypothalamus.. We often think of the spinal cord as being In addition, many emotional patterns only a conduit for signals from the periphery such as anger, excitement, sexual of the bod- to the brain, or in the opposite response, reaction to pain, and direction from the brain back to the body. reaction to pleasure can still occur This supposition is far from the truth. Even after destruction of much of the after the spinal cord has been cut in the high cerebral cortex. neck region, many highly organized spinal HIGHER BRAIN OR CORTICAL cord functions still occur. LEVEL For instance, neuronal circuits in the cord After the preceding account of the many can cause: nervous system functions that occur at (1) walking movements the cord and lower brain levels, one may ask, what is left for the cerebral cortex to (2) reflexes that withdraw portions of the do? The answer to this question is body from painful objects complex, but it begins with the fact that (3) reflexes that stiffen the legs to support the cerebral cortex is an extremely large the body against gravity memory storehouse. (4) reflexes that control local blood vessels, The cortex never functions alone gastrointestinal movements, or urinary but always in association with excretion. lower centers of the nervous system. Without the cerebral LOWER BRAIN OR SUBCORTICAL LEVEL pg. 2  ORGANIZATION OF THE NERVOUS SYSTEM cortex, the functions of the lower brain centers are often imprecise. The vast storehouse of cortical information usually converts these functions to determinative and precise operations. Finally, the cerebral cortex is essential for most of our thought processes, but it cannot function by itself. In fact, it is the lower brain centers, not the cortex, that initiate wakefulness in the cerebral cortex, thus opening its bank of memories to the thinking machinery of the brain. Thus, each portion of the nervous system performs specific functions, but it is the cortex that opens a world of stored information for use by the mind 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 per cent 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 pg. 3  ORGANIZATION OF THE NERVOUS SYSTEM But, when important sensory information excites the mind, it is In addition, some postsynaptic immediately channeled into neurons respond with large proper integrative and motor numbers of output impulses, and regions of the brain to cause others respond with only a few. desired responses. This channeling and processing of Synapses perform a selective information is called the action, often blocking weak integrative function of the signals while allowing strong nervous system signals to pass, but at other times selecting and amplifying certain Thus, if a person places a hand weak signals, and often on a hot stove, the desired channeling these signals in many instantaneous response is to lift directions rather than only one the hand. And other associated direction responses follow, such as moving the entire body away from the SYNAPTIC FUNCTIONS OF stove, and perhaps even shouting NEURONS: with pain Synapse: junction pt. from one to the next :an advantageous site of Role of Synapses in Processing Information substance or the control of signal transmission The synapse is the junction point : described as "interneuronal from one neuron to the next. junction" : almost all synapses in the CNS It is important to point out that transmit signals through synapses determine the directions neurotransmitter substance that the nervous signals will chemical synapse spread through the nervous system. Some synapses transmit signals from one neuron to the next with ease, whereas others transmit signals only with difficulty. Also, facilitatory and inhibitory signals from other areas in the nervous system can control synaptic transmission, sometimes opening the synapses for transmission and at other times closing them. pg. 4  ORGANIZATION OF THE NERVOUS SYSTEM MECHANISM: Action Potential Causes Transmitter Release (Pre- Synaptic Knob) Pre-Synaptic Terminal : Depolarization of PST Na+ & Ca++ influx Ca++ bind with protein receptors (release sites) Local Transmitter Vesicles bind with Pre-S. memb. Exocytosis of Neurotransmitter into Synaptic cleft PARTS of a SYNAPSE: 1. Pre-synaptic Terminals (terminal knobs, buotons, end- feet, syn.knobs) - synaptic cleft: 200-300 Å width - synaptic vesicles with neurotransmitter - mitochondria: provide ATP - voltage-gated Ca++ channels 2. Post-synaptic neuron - receptor proteins: 2 components a. Binding components b. lonophore component 2 types: 1) Chemically activated ion channels Na+ channels - excitatory K+ channels Inhibitory Cl-channels 2) Enzymes : active internal metabolic system to : - increase in no. of receptors - decrease in no. of receptors modulators - 2nd messengers pg. 5  ORGANIZATION OF THE NERVOUS SYSTEM 3. Origin of RMP (neuronal soma) Na-K pump Extrusion of more Na+ to the outside than K+ to the interior (3:2) ▸ Interior of cell more negative since (-) charged ions inside soma cannot diffuse outward (protein ions, PO4 cpds). ELECTRICAL EVENTS: (large motor 4. Uniform distribution of potential inside neurons in spinal cord) soma 1 >Any change in potential in any part of A. RMP: neuronal soma intrasomal fld. almost exact equal change 1. (-) 65 mv. 2. Conc. diff. of IONS across neuronal in potential somal membrane: at all points inside the soma. a. Na+ ECF = 142 meq/L ICF 14 meq/L Na- >Very highly conductive electrolytic K pump- continually pumps OUT Na+ solution in the ICF of soma Very large Nernst potential (EMF) = +61 mv diameter of soma almost no resistance to allows Na+ to normally diffuse INWARD conduction of electrical current. B. EPSP (Excitatory Postsynaptic b. K+: ECF = 4.5 meq/L ICF = 120 meq/L Potential) K+ pump: pumps K+ INWARD Nernst 1. in voltage above the normal RMP. potential = -86 mv 2. Transmitter acts on memb. excitatory tends to move K+ OUTWARD receptor = memb. permeability to Na+ & K+. C. Cl: ECF = 107 meq/L ICF =8 meq/L 3. Mainly the Na+ influx neutralize Memb. is highly permeable to Cl- ions Neg. negativity inside the memb. e.g. from -65 voltage (-65 mv) inside soma repels Cl- to mV -45 mV EPSP = +20 Mv the C. Action Potential (A.P.) 1. Begins in axon hillock: 7x more conc. of voltage-gated Na+ channels than soma. OUTSIDE Nernst potential = -70 mV EPSP to elicit A.P. at axon hillock = Therefore: Cl- normally tend to diffuse +15 +20 mV INWARD at soma = +30mV pg. 6  ORGANIZATION OF THE NERVOUS SYSTEM 2. Travels peripherally along axon, soma & Occurs when activity is present in dendrites more than one synaptic knob at the same time Effect of summing simultaneous postsynaptic potentials by excitation of multiple pre- synaptic terminals on widely spaced areas of the membrane. TEMPORAL SUMMATION Occurs if repeated afferent C. Presynaptic Inhibition 1. Caused by stimuli cause new EPSP's that "presynaptic" synapses that lie on terminal would produce depolarization nerve fibrils before they themselves Involves single pre-synaptic terminate on the following neuron. terminal. ↳ ***NOTE: If the EPSP in both spatial & temporal summation Secrete a transmitter that depresses voltage is large enough to reach a of Action Potential at syn. memb. (partial depolarization) firing level, a full-pledged ACTION POTENTIAL is produced. Ca++ that enter the terminal Relation of State of Excitation of the Neuron to Rate of Firing "Excitatory State. Transmitter released by the terminal " The "excitatory state" of a neuron is defined as the summated degree of Excitation of neuron (post-syn.) excitatory drive to the neuron. If there is a higher degree of excitation than inhibition of the neuron at any given instant, then it is said that there is an excitatory state. Conversely, if there is more inhibition than excitation, then it is said that there is an A. Spatial inhibitory state. When the excitatory state of a neuron rises above the threshold for B. Temporal excitation, the neuron will fire repetitively SUMMATION OF EPSP & IPSP: as long as the excitatory state remains at that SPATIAL SUMMATION level pg. 7  ORGANIZATION OF THE NERVOUS SYSTEM Some Special Characteristics of Synaptic coffee, tea, and cocoa, respectively, Transmission Fatigue of Synaptic all increase neuronal excitability, Transmission presumably by reducing the When excitatory synapses are repetitively threshold for excitation of neurons. stimulated at a rapid rate, the number of Strychnine is one of the best known discharges by the postsynaptic neuron is at of all agents that increase excitability first very great, but the firing rate becomes of neurons. However, it does not do progressively less in succeeding this by reducing the threshold for milliseconds or seconds. This is called excitation of the neurons; instead, it fatigue of synaptic transmission inhibits the action of some normally The mechanism of fatigue is mainly inhibitory transmitter substances, exhaustion or partial exhaustion of the stores especially the inhibitory effect of of transmitter substance in the presynaptic glycine in the spinal cord. Therefore, terminals The excitatory terminals on many the effects of the excitatory neurons can store enough excitatory transmitters become overwhelming, transmitter to cause only about 10,000 action and the neurons become so excited potentials, and the transmitter can be that they go into rapidly repetitive exhausted in only a few seconds to a few discharge, resulting in severe tonic minutes of rapid stimulation muscle spasms. Part of the fatigue process probably results Most anesthetics increase the from two other factors as well: neuronal membrane threshold for 1) progressive inactivation of many of the excitation and thereby decrease postsynaptic membrane receptors synaptic transmission at many points 2) slow development of abnormal in the nervous system. Because many concentrations of ions inside the of the anesthetics are especially lipid postsynaptic neuronal cell soluble, it has been reasoned that Enter some of them might change the Effect of Hypoxia on Synaptic physical characteristics of the Transmission neuronal membranes, making them Neuronal excitability is also highly less responsive to excitatory agents. dependent on an adequate supply of oxygen. Effect of Acidosis or Alkalosis on Cessation of oxygen for only a few Synaptic Transmission seconds can cause complete inexcitability of Most neurons are highly responsive some neurons. This is observed when the to changes in pH of the surrounding brain's blood flow is temporarily interrupted, interstitial fluids. because within 3 to 7 seconds, the person -Normally, alkalosis greatly becomes unconscious. increases neuronal excitability. For Enter instance, a rise in arterial blood pH Effect of Drugs on Synaptic from the 7.4 norm to 7.8 to 8.0 often Transmission: causes cerebral epileptic seizures Caffeine, theophylline, and because of increased excitability of theobromine, which are found in some or all of the cerebral neurons. pg. 8  ORGANIZATION OF THE NERVOUS SYSTEM This can be demonstrated especially well by asking a person who is predisposed to epileptic seizures to overbreathe. The overbreathing blows off carbon dioxide and therefore elevates the pH of the blood momentarily, but even this short time can often precipitate an epileptic attack. -Conversely, acidosis greatly depresses neuronal activity; a fall in pH from 7.4 to below 7.0 usually causes a comatose state. For instance, in very severe diabetic or uremic acidosis, coma virtually always develops pg. 9