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Chapter 3

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Psychology 1000

Chapter 3: Biological Foundations of Behavior The Neural Bases of Behavior - Neurons: basic building blocks of thee nervous system; these never cells are linked together in circuits o Cell Body (soma): contains biochemical structures needed to keep the neuron alive; carries genetic information that determines how the cell develops and functions o Dendrites: emerge from cell body; act like antennas that collect messages from neighboring neurons and send them to the cell body o Axon: conduct electrical impulses away from the cell body to other neurons, muscles, or glands; axon branches out at its end to form a number of axon terminals - Neurons can vary in size and shape (more than 200 different types of neurons) - Neurons receive, process and send messages  Glial Cells: surround neurons and hold them in place; also manufacture nutrient chemicals that neurons need, form myelin sheath, and absorb toxins and waste materials  Blood-Brain Barrier: prevents many substances from the entering the brain Nerve Conduction: An Electrochemical Process - Neurons generate electricity and release chemicals - Neurons are surrounded by a salty liquid environment rich in positive sodium ions (Na+) - Inside the neuron, there are some positively charged potassium ions (K+), but overall there are more negative ions; thus, the inner membrane carries a negative charge - Resting potential of about -70 millivolts (mV) - This resting state is said to be polarized - The action potential: sudden reversal in the neuron’s membrane voltage, during which the membrane voltage momentarily moves from -70mV to +40mV - This shift from negative to positive is called depolarization - Graded Potentials: small changes that occur as a result of axons stimulating the dendrites or the cell body o Proportional to the amount of incoming stimulation o If the graded potentials are not very strong, the neuron will be partially depolarized, but not enough to generate an action potential o If the graded potentials are strong enough to reach the threshold, the neuron discharges with an action potential o Action potential obeys the all-or-none law (either occurs with maximum intensity or does not occur at all) - Steps of action potential  Initially, graded potentials change the membrane charge by opening ion channels for sodium ions  Sodium ions enter the cell and partially depolarize the membrane  If the membrane charge reaches about -55 mV, the threshold is reached (neuron fires)  The influx of Na+ causes the cell to be more positively charged than the outside of the cell (complete depolarization)  State of complete depolarization is what constitutes the action potential  Sodium ion channels close and potassium ion channels open (to restore polarity)  By the time the membrane is reploarized, action potential has started a chain reaction “wave” that flows along the membrane as succeeding sodium channels open and the process repeats itself  Immediately after an impulse passes any give point along the axon, there occurs a refractory period, a time period during which the membrane is not excitable and cannot discharge another action potential - How does the nervous system tell the difference between a dim light and a bright light? o Strong stimulus may increase the rate of firing of one neuron o Strong stimulus may increase the number of neurons o Either way, information is provided concerning the nature of the stimulus The Myelin Sheath - Axons that transmit information throughout the brain and spinal cord are covered by myelin sheath - Derived from glial cells - Myelin sheath is interrupted at regular intervals by the nodes of Ranvier - In myelinated axons, the action potential can skip from node to node (accounts for high conduction speed) - Multiple Sclerosis (MS) is an autoimmune disease in which the body’s immune system attacks the myelin sheath; damage to the myelin sheath disrupts the delicate timing of nerve impulses, resulting in jerky and uncoordinated movements How Neurons Communicate: Synaptic Transmission - Neurons are individual cells, therefore, they do not make physical contact with each other - As a result, neurons communicate at a synapse - Tiny gap between the axon terminal of one neuron and the dendrite of the next neuron is called the synaptic cleft - Neurotransmitters: chemical substances that carry messages across the synapse to either excite other neurons or inhibit their firing; involve five steps o Synthesis: chemical molecules are formed inside the neuron o Storage: the chemical molecules are stored in chambers called synaptic vesicles within the axon terminals o Release: when an action potential reaches the axon terminal, the vesicles move to the surface of the axon and are released into the fluid-filled space between the axon of the sending (presynaptic) neuron and membrane of the receiving (postsynaptic) neuron o Binding: the molecules cross the synaptic space and bind to receptor sites (these sites have a specially shaped surface that fits a specific transmitter molecule)  Binding can excite the postsynaptic membrane by stimulating inflow of sodium ions (excitatory transmitters)  Binding can also inhibit the postsynaptic membrane by stimulating outflow of potassium ions and hyperpolarizing the membrane (inhibitory transmitters)  Balance between excitatory and inhibitory processes must be maintained if the nervous system is to function properly o Deactivation: neurotransmitters continue to either excite or inhibit the neuron until they are deactivated  Some transmitters are broken down by other chemicals located in the synaptic space  Some transmitters are reabsorbed into the presynaptic axon terminal - Many drugs target the transmitter’s receptor, binding to the receptor in place of transmitters o Either mimic the naturally occurring transmitter (e.g. opiates) or, o Bind to the receptor and have no effect other than denying the transmitter access to its receptor (e.g. caffeine) - A drug’s psychological effects are determined not by its actions at the synapse, but by which specific chemical transmitter it targets Specialized Transmitter Systems - Glutamate: excitatory; involved in learning and memory - GABA: inhibitory; important for motor control and for the control of anxiety - Acetylcholine (ACh): excitatory; involved in memory and in muscle activity; underproduction of this transmitter will result in Alzheimer’s disease - Dopamine: can be inhibitory or excitatory; involved in voluntary movement, emotional arousal, learning, motivation, and experiencing pleasure; underproduction of dopamine leads to Parkinson’s diseases; overproduction of dopamine leads to Schizophrenia - Serotonin: inhibitory; involved in mood, sleep, eating, and arousal, and may be an important transmitter underlying pleasure and pain; underproduction of serotonin leads to depression, sleeping and eating disorders - Endorphins: excitatory; reduce pain and increase feelings of well-being - Neuromodulators: have a more widespread and generalized influence on synaptic transmission; play important roles in functions such as eating, sleep, and stress The Nervous System - Body’s master control centre - Sensory Neurons: carry input messages from the sense organs to the spinal cord and brain - Motor Neurons: transmit output impulses from the brain and spinal cord to the body’s muscles and organs - Interneurons: perform connective or associative functions; link the input and output functions; outnumber sensory and motor neurons - Nervous system is broken down into two major divisions o Central Nervous System (CNS): consists of brain and spinal cord o Peripheral Nervous System (PNS): composed of all the neurons that connect the CNS with the muscles, glands, and sensory receptors  Somatic Nervous System: voluntary muscles activation; sense and respond to environment  Sensory Neurons: transmit messages from the eyes, ears and other sensory receptors; axons of sensory neurons group together to form sensory nerves  Motor Neurons: send messages from the brain and spinal cord to the muscles that control our voluntary movements; axons combine to form
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