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Chapter 3

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Mamdouh Shoukri

Lesson 3 1) Communication in the nervous system a. Nervous tissue: the basic hardware: The cells in the nervous system fall into two major categories: glia and neurons i. Neurons are individual cells in the nervous system that receive, integrate, and transmit information. Majority communicates only with other neurons. Small minority receives signals from outside the nervous system or carry message from the nervous system to the muscles that move the body. 1. Soma, or cell body, contains the cell nucleus and much of the chemical machinery common to most cells. 2. Dendrites: are the parts of a neuron that are specialized to receive information. 3. Axon: a long, thin fibre that transmits signals away from the soma to other neurons or to muscles or glands. a. Many axons are wrapped in cells with a high concentration of a white, fatty substance called myelin. b. Myelin sheath is insulating material, derived from glial cells, that encases some axons. It can speed up the transmission of signals that move along axons. Example: multiple sclerosis. 4. Terminal buttons: small knobs that secrete chemicals called neurotransmitters. These chemicals serve as messengers that may activate neighboring neurons. 5. Synapses: a junction where information is transmitted from one neuron to another. 6. Summary: information is received at the dendrites, is passed through the soma and along the axon, and is transmitted to the dendrites of other cells at meeting point called synapse. But many exemptions. ii. Glia is cells found throughout the nervous system that provides various types of support for neurons. 1. Much smaller than neurons, outnumber neurons by about 10 to 1. 2. Over 50% of the brain’s volume. 3. Glial cells supply nourishment to neurons, help remove neurons’ waste products, and provide insulation around many axons. The myelin sheaths that encase some axons are derived from special types of glial cells. 4. Orchestrating the development of the nervous system in the human embryo. 5. Glia may also send and receive chemical signals; they may be implicated in diseases such as amyotrophic lateral sclerosis and Parkinson’s disease. 6. Memory formation: Alzheimer’s disease b. The neural impulse: using energy to send information i. The neuron at rest: a tiny battery 1. Neural impulse: a complex electrochemical reaction. The electrical discharge that travels along a nerve fiber; "they demonstrated the transmission of impulses from the cortex to the hypothalamus" 2. Ions: electrically charged atoms and molecules. 3. The cell membrane is semipermeable, permitting movement of some ions. Positively charged sodium and potassium ions and negatively charged chloride ions flow back and forth across the cell membrane, but they do not cross at the same rate. This difference in flow rates leads to a slightly higher concentration of negatively charged ions inside the cell. The resulting voltage means that the neuron at rest is a tiny battery, a store of potential energy. 4. The resting potential of a neuron is its stable, negative charge when the cell is inactive. ii. The action potential 1. Constant voltage of a neuron leads to the cell is quiet and no messages are being sent. 2. When the neuron is stimulated, channels in its cell membrane open, briefly allowing positively charged sodium ions to rush in. 3. Action potential: a very brief shift in a neuron’s electrical charge that travels along an axon. 4. Absolute refractory period: the minimum length of time after an action potential during which another action potential cannot begin. 5. Followed by a brief relative refractory period: the neuron can fire, but its threshold for firing is elevated, so more intense stimulation is required to initiate an action potential. iii. The all-or-none law 1. The neural impulse is an all-or-none proposition. 2. Neuron’s action potentials are all the same size. Weaker stimuli do not produce smaller action potentials. 3. Neurons can convey information about the strength of a stimulus. They do so by varying the rate at which they fire action potentials. Stronger stimulus will cause a cell to fire a more rapid volley of neural impulses than a weaker stimulus will. 4. Various neurons transmit neural impulses at different speeds. Thicker axons transmit more rapidly than thinner ones do. c. The synapse: where neurons meet (depend on chemical messengers) i. Sending signals: chemicals as couriers. 1. Two neurons don’t actually touch. They are separated by the synaptic cleft: a microscopic gap between the terminal button of one neuron and the cell membrane of another neuron. Signals have to cross this gap to permit neurons to communicate. 2. Presynaptic neuron: neuron that sends a signal across the gap. 3. Postsynaptic neuron: neuron that receives the signals. 4. The arrival of an action potential at an axon’s terminal buttons triggers the release of Neurotransmitters: chemicals that transmit information from one neuron to another. 5. Synaptic vesicles: store neurotransmitters chemicals in the terminal buttons. 6. The neurotransmitters are released when a vesicle fuses with the membrane of the presynaptic cell and its contents spill into the synaptic cleft. After their release, neurotransmitters diffuse across the synaptic cleft to the membrane of the receiving cell. There they may bind with special molecules in the postsynaptic cell membrane at various receptor sites: specifically tuned to recognize and respond to some neurotransmitters but not to others. ii. Receiving signals: postsynaptic potentials 1. When a neurotransmitter and a receptor molecule combine, cause Postsynaptic potential (PSP): a voltage change at a receptor site on a postsynaptic cell membrane. It does not follow the all-or-none law as action potentials do. They are graded. They vary in size and they increase or decrease the probability of a neural impulse in the receiving cell in proportion to the amount of voltage change. 2. Two types of messages can be sent from cell to cell: excitatory and inhibitory. Excitatory PSP is a positive voltage shift that increases the likelihood that the postsynaptic neuron will fire action potentials. Inhibitory PSP is a negative voltage shift that decreases the likelihood that the postsynaptic neuron will fire action potentials. They depend on which receptor sites are activated in the postsynaptic neuron. 3. The excitatory or inhibitory effects produced at a synapse last only a fraction of a second. Then neurotransmitters drift away from receptor sites or are inactivated by enzymes that metabolize (convert) them into inactive forms. Most are reabsorbed into the presynaptic neuron through reuptake: a process in which neurotransmitters are sponged up from the synaptic cleft by the presynaptic membrane. 4. Process: a. Synthesis and storage of neurotransmitter molecules in synaptic vesicles b. Release of neurotransmitter molecules into synaptic cleft. c. Binding of neurotransmitters at receptor sites on postsynaptic membrane. d. Inactivation by enzymes or removal drifting away of neurotransmitters. e. Reuptake of neurotransmitters sponged up by the presynaptic neuron. iii. Integrating signals: neural networks 1. A neuron must integrate signals arriving at many synapses before it “decides” whether to fire a neural impulse. Enough excitatory PSPs, action potential fires. 2. Our perceptions, thoughts and actions depend on patterns of neural activity in elaborative neural networks. These networks consist of interconnected neurons that frequently fire together or sequentially to perform certain functions. 3. Elimination of old synapses appears to play a larger role in the sculpting of neural networks than the creation of new synapses. The nervous system normally forms more synapses than needed and then gradually eliminates the less active synapses. 4. Synaptic pruning is a key process in the formation of the neural networks that are crucial to communication in the nervous system. 5. Donald Hebb: the organization of behavior. Cell assemblies. Hebbian learning rule. One neuron stimulating another neuron repeatedly produces changes in the synapse. d. Neurotransmitters and behavior i. Acetylcholine: 1. ACh has been found throughout the nervous system. It is the only transmitter between motor neurons and voluntary muscles. (Activates motor neurons controlling skeletal muscles.) 2. Contribute to attention, arousal, and memory. 3. Some Ach receptors stimulated by nicotine. When smoke, some of your Ach synapses will be stimulated by the nicotine that arrives in your brain. At these synapses, the nicotine acts like Ach itself. It binds to receptor sites for ACh, causing postsynaptic potentials. Nicotine is an ACh agonist: a chemical that mimics the action of a neurotransmitter. 4. Antagonist: a chemical that opposes the action of a neurotransmitter. Like curare. It temporarily blocks the action of the natural transmitter by occupying its captor sites, rendering them unusable. As a result, muscles are unable to move. ii. Monoamines: 1. Monoamine: a. Include three neurotransmitters: dopamine, norepinephrine, and serotonin. b. Abnormal levels of monoamines in the brain have been related to the development of certain psychological disorders. c. Temporary alterations at monoamine synapses also appear to account for the powerful effects of amphetamines and cocaine. 2. Dopamine (DA): (L-dopa: treat Parkinson) a. Contributes to control of voluntary movement, pleasurable emotions. b. Decreased levels associated with Parkinson’s disease. The reduction in dopamine synthesis occurs because of the deterioration of a structure located in the midbrain. c. Over activity at DA synapses associated with schizophrenia. d. Cocaine and amphetamines elevate activity at DA synapses. e. Dopamine hypothesis asserts that abnormalities in activity at dopamine synapses play a crucial role in the development of schizophrenia. 3. Norepinephrine (NE): a. Contributes to modulation of mood and arousal b. Cocaine and amphetamines elevate activity at NE synapses 4. Serotonin: a. Involved in regulation of sleep and wakefulness, eating, aggression. b. Abnormal levels may contribute to depression and obsessive-compulsive disorder. c. Prozac and similar antidepressant drugs affect serotonin circuits. d. Dysregulation in serotonin circuits has also been implicated as a factor in eating disorders, such as anorexia and bulimia and in obsessive-compulsive disorders. iii. GABA (gamma-aminobutyric acid) and Glutamate: 1. Serves as widely distributed inhibitory transmitter. 2. Valium and similar antianxiety drugs work at GABA synapses. 3. GABA (consist of amino acid) receptors are widely distributed in the brain and may be present at 40% of all synapses. GABA appears to be responsible for much of the inhibition in the central nervous system. (Only has inhibitory effect) It also contributes to the regulation of anxiety in humans and that it plays a central role in the expression of seizures. 4. Glutamate is another amino acid neurotransmitter that is widely distributed in the brain. It has both inhibitory and exhibitory effects. It is best known for its contribution to learning and memory. 5. Long-term potentiation (LTP): durable increases in excitability at synapses along a specific neural pathway. One of the basic building blocks of memory formation. iv. Endorphins 1. Morphine exerts its effects by binding to specialized receptors in the brain. 2. Endorphins: internally produced chemicals that resemble opiates in structure and effects. 3. Endogenous opioids also contribute to the modulation of eating behaviour and the body’s response to stress. 4. Contribute to pain relief and perhaps to some pleasurable emotions. 2) Looking inside the brain: research methods a. Electrical recordings i. The electroencephalograph (EEG): a device that monitors the electrical activity of the brain over time by means of recording electrodes attached to the surface of the scalp. ii. An EEG electrode sums and amplifies electric potentials occurring in many thousands of brain cells. iii. The resulting EEG recordings are translated into line tracings, commonly called brain waves. iv. The EEG is often used in the clinical diagnosis of brain damage and neurological disorders. v. In research applications, EEG can be used to identify patterns of brain activity that occur when participants engage in specific behaviors or experience specific emotions. vi. EEG is invaluable to researchers exploring the physiology of sleep. b. Lesioning i. Case study method. Most conduct with animal. ii. Limitations: subjects are not plentiful, and neuroscientists can’t control the location or severity of their subjects’ brain damage. Variations in the participant’s histories create a host of extraneous variable that make it difficult to isolate cause-and-effect relationships between rain damage and behaviour. iii. Lesioning involves destroying a piece of the brain. It is typically done by inserting an electrode into a brain structure and passing a high-frequency electric current through it to burn the tissue and disable the structure. c. Electrical stimulation of the brain i. Electrical stimulation of the brain (ESB) involves sending a weak electric current into a brain structure to stimulate (activate) it. ii. Most on animal, sometime on human who has brain surgery. iii. Wilder Penfield: Montreal neurological institute and hospital. Treatment of epilepsy. iv. Both techniques depend on the use of stereotaxic instruments that permit researchers to implant electrodes at precise locations in animals’ brains. d. Transcranial magnetic stimulation i. Transcranial magnetic stimulation (TMS) is a new technique that permits scientists to temporarily enhance or depress activity in a specific area of the brain. ii. In essence, this technology allows scientists to create “virtual lesions” in human subjects for short periods of time, using a painless, noninvasive method. iii. Moreover, this approach circumvents the host of uncontrolled variables that plague the study of natural lesions in humans who have experienced brain damage. iv. Limitation: it cannot be used to study areas deep within the brain. e. Brain-imaging procedures i. Computerized tomography (CT) scan: a computer-enhanced x-ray of brain structure. CT is the least expensive, and it has been widely used in research. It can portray only brain structure. ii. Position emission tomography (PET) scanning is proving especially valuable. It can examine brain function, mapping actual activity in the brain over time. It can provide a color-coded map indicating which areas of he brain become active when subjects clench their fist, sing, or contemplate the mysteries of the universe. Because PET scans monitor chemical processes, they can also be used to study the activity of specific neurotransmitters. iii. Magnetic resonance imaging (MRI) scan: uses magnetic fields, radio waves, and computerized enhancement to map out brain structure. Better images of brain structure than CT scans. 3-D of the brain, high resolution. iv. Functional magnetic resonance imaging (fMRI): new variation on MRI technology that monitors blood flow and oxygen consumption in the brain to identify areas of high activity. Like PET scans, it can map actual activity in the brain over time, but with vastly greater precision. 3) Organization of the nervous system a. The peripheral nervous system: The first and most important division separates the central nervous system (The brain and spinal cord) from the peripheral nervous system. Peripheral nervous system: made up of all those nerves that lie outside the brain and spinal cord. Nerves: bundles of neuron fibers (axons) that are routed together in the peripheral nervous system. i. The somatic nervous system 1. Somatic nervous system: made up of nerves that connect to voluntary skeletal muscles and to sensory receptors. 2. These functions require two kinds of nerve fibers: a. Afferent nerve fibers: axons that carry information inward to the central nervous system from the periphery of the body. b. Efferent nerve fibers are axons that carry information outward from the central nervous system to the periphery of the body. ii. The autonomic nervous system 1. Autonomic nervous system (ANS): made up of nerves that connect to the heart, blood vessels, smooth muscles, and glands. It controls automatic, involuntary, visceral functions that people don’t normally think about, like heart rate, digestion, and perspiration. 2. The autonomic nervous system can be subdivided into two branches: a. Sympathetic division: the branch of the autonomic nervous system that mobilizes the body’s resources for emergencies. It creates the fight-or-flight response. Activation of the sympathetic division slows digestive processes and drains blood from the periphery, lessening bleeding in the case of an injury. Key sympathetic nerves send signals to the adrenal glands, triggering the release of hormones that ready the body for exertion. b. Parasympathetic division: the branch of the autonomic nervous system that generally conserves bodily resources. It activates processes that allow the body to save and store energy. Slow heart rate, reduce blood pressure, and promote digestion. b. The central nervous system: consists of the brain and the spinal cord. Protected by enclosing sheaths called the meninges. The cerebrospinal fluid (CSF) nourishes the brain and provides a protective cushion for it. Ventricle: the how cavities in the brain that are filled with CSF. i. The spinal cord 1. The spinal cord connects the brain to the rest of the body through the peripheral nervous system. It is an extension of the brain. 2. It houses bundles of axons that carry the brains commands to peripheral nerves and that relay sensations from the periphery of the body to the brain. 3. Many forms of paralysis result from spinal cord damage, a fact that underscores the critical role the spinal cord plays in transmitting signals from the brain to the motor neurons that move the body’s muscles. ii. The brain 1. The hindbrain: include the cerebellum and two structures found in the lower part of the brainstem: the medulla and the pons. a. Cerebellum: relatively large and deeply folded structure located adjacent to the back surface of the brainstem. It is critical to the coordination of movement and to the sense of equilibrium, or physical balance. It plays a key role in organizing the sensory information that guides these movements. It is one of the structures first depressed by alcohol. Damage to the cerebellum disrupts fine motor skills. b. Medulla: which attaches to the spinal cord, is in charge of largely unconscious but vital functions, including circulating blood, breathing, maintain muscle tone, and regulating reflexes such as sneezing, coughing, and salivating. c. Pons: includes a bridge of fibers that connects the brainstem with the cerebellum. The pons also contains several clusters of cell bodies involved with sleep and arousal. 2. The midbrain: the segment of the brainstem that lies between the hindbrain and the forebrain. a. It contains an area that is concerned with integrating sensory processes, such as vision and hearing. b. An important system of dopamine-releasing neurons that projects into various higher brain centers originates in the midbrain. c. Reticular formation: running through both the hindbrain and the midbrain. d. It is best known for its role in the regulation of sleep and arousal. 3. The forebrain: the largest and most complex region of the brain, encompassing a variety of structures, including the thalamus, hypothalamus, limbic system and cerebrum. a. Cerebrum: the seat of complex thought. Responsible for sensing, thinking, learning, emotion, consciousness, and voluntary movement. It is divided into two halves called hemispheres. i. This fissure descends to a thick band
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