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Chapter 3

PSY100 Ch3

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University of Toronto St. George
Dan Dolderman

CHAPTER 3 Nervous system pg 88 Neurons differ from most other cells because they are excitable. Have 3 functions: reception, conduction, transmission. “nerve” refers to a bundle of axons carrying info between brain and body. Boundary of a neuron defined by its membrane, lipid bilayer. - regulates concentration of electrically charge molecules that are the basis of the neuron’s electrical activity Sensory neurons - afferent neurons o carry signals from body to brain Motor neurons - direct muscles to contract or relax, produce movement - efferent neurons o from brain to body Interneurons - communicate within local or short-distance circuits - integrate neural activity within a single area rather than transmitting info to brain or organs together, sensory and motor neurons control movement. Nerves that provide info from muscles are somatosensory, general term for sensations experienced from within the body. Neurons do NOT communicate randomly or arbitrarily. They SELECTIVELY communicate with other neurons to form circuits or neural networks. These develop through maturation and experience, forming PERMANENT alliances among groups of neurons. Multiple sclerosis - decay of myelin sheath surrounding axons - early symptoms: numbness of limbs and blurry vision - axons short-circuit - motor actions become jerky, lose ability to coordinate movements - movement, sensation, coordination impaired over time - axons are exposed and may themselves start to break down - sclerosis means hardening, scarring in brains of patients - both genetic and environmental causes - genetic: more common in identical twins than nonidentical. More common among Caucasians than Asians. - Environment: more common in cold climates than warm climates. o Connection between cold weather and MS is unclear but growing up in a cold climate seems more relevant than moving to one as a young adult, moving from cold to warm after childhood does not reduce risk o Some say autoimmune disorder, body views myelin sheath as intruder  May have its origin in a slow-acting infection contracted early in childhood - no cure Neurons don’t touch, separated by synaptic cleft. - site of chemical communication - inside terminal buttons are vesicles containing neurotransmitters - generic word for chemical substances that carry signals across synaptic cleft - bind to receptors on postsynaptic neuron - causes ion channels to open - changes membrane potential at that location o if neurotransmitter depolarizes membrane, it is excitatory higher probability receiving neuron will fire o if neurotransmitter hyperpolarizes membrane, it is inhibitory, less likely to fire Receptors are SPECIALIZED protein molecules. - lock-key - but drugs and toxins can mimic neurotransmitters and bind with their receptors as if real thing - addictive drugs like heroin and cocaine have their effects because are structurally similar to natural neurotransmitters and receptors can’t differentiate them Terminating synaptic transmission 1) reuptake a. most common b. neurotransmitter molecules are taken back into presynaptic terminal buttons c. cycle of reuptake and release repeats continuously d. action potential prompts release and then take it back for recycling 2) enzyme deactivation a. enzyme destroys neurotransmitter in synaptic cleft b. different enzymes break down different neurotransmitters 3) autoreception a. bind with their own receptors b. autoreceptors monitor how much neurotransmitter has been released into synapse c. when excess detected, autoreceptors signal the neuron to stop releasing the neurotransmitter All three methods regulate activity of neurotransmitters in synaptic cleft. Unless one occurs, NT continues to bind and blocks new signals from getting through Drugs and toxins can - alter how the NT is synthesized - raise or lower the amount of NT released from terminal buttons - change the way the NT is deactivated in synaptic cleft by blocking reuptake or preventing enzyme deactivation drugs that enhance action of NT are agonists; drugs that inhibit action are antagonists. NTs are not a property of the chemicals themselves, but rather a function of the receptors to which they bind! Same NT can be excitatory or inhibitory or produce radically different effects based on the properties of the receptor. Four categories: acetylcholine, monoamines, amino acids, and peptides. Pg 98 table! RULE: RECEPTOR CONTROLS THE PROCESS! Acetylcholine  motor control over muscles  binds with receptors on muscle cells, relax or contract  Ach excites skeletal muscles and inhibits heart muscles  Learning, memory, sleeping, dreaming o ACh antagonists cause temporary amnesia o Alzheimer’s disease associated with diminished ACh functioning o Toxics that mimic ACh can bind to receptors and cause temporary paralysis  E.g. curare, herbal poison used by South American Indians. Competes with ACh, inhibits mechanisms that produce muscle movement. Paralyzes limbs and lungs, asphyxiating them  Botulism, form of food poisoning. Inhibits release of ACh. Leads to trouble with breathing, difficulty chewing, and often death. • Used in cosmetic surgery is smaller doses o Paralyze in eyebrow region o Wear off every 2 to 4 months - nicotine o excites ACh receptors o has several cognitive effects o increase attention, improve problem solving, and facilitate memory  only evidence suggests for current smokers  nonsmokers perform more poorly on cognitive tasks or former smokers  over time suffer deficits in cognitive performance UNLESS ingest nicotine (why hard to give up) o nicotine patch have vivid dreams because activation of ACh neurons has affect on state of sleep associated with dreams Monoamines  synthesized from single amino acids  regulate states of arousal and affect (feelings) and to motivate behavior  MAO is enzyme that interrupts activity of all monoamines FOUR types: epinephrine, norepinephrine, serotonin, dopamine Epinephrine - found mostly in body, small amounts in brain - initially called adrenaline, basis for “adrenaline rush” Norepinephrine - arousal, alertness - esp imp for vigilance, heightened sensitivity to what’s going on around you - useful for fine-tuning clarity of attention - inhibits responsiveness to weak synaptic inputs and strengthens or maintains responsive to strong synaptic inputs Serotonin - many different behaviors - esp emotional states, impulse control, dreaming - LOW levels: sad and anxious, food cravings, aggression  drugs block reuptake of serotonin treat depression, OCD, eating disorders, obesity - LSD structure similar  enters brain and binds with serotonin receptors • if those receptors involved in dreaming, causes hallucinations Dopamine - many brain functions, esp motivation and motor control - reward - eating when hungry, drinking when thirsty, sex when aroused  activate dopamine, pleasure - motor control: guiding behavior towards objects and experiences that get reward - certain drugs are dopamine agonists. Cause pleasure. - Cocaine blocks reuptake of dopamine, allows it to have longer-lasting effect on postsynaptic receptors, heightened arousal, euphoria - Controlling voluntary muscle movements o Dopamine depletion in Parkinson’s disease  Muscular rigidity, tremors, difficulty initiating voluntary action  Dopamine producing neurons slowly die off  Later stages: cognitive and mood disturbances, death  Injection of L-DOPA chief building blocks of dopamine help surviving neurons produce MORE dopamine • Only temporary recovery • When too many neurons die, diminishes effect - transplanting fetal tissue into brains so new fetal cells will produce dopamine
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