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Chapter 3

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Western University
Psychology 1000

Psychology – Chapter 3: Biological Foundations of Behaviour The Neural Bases of Behaviour: Neurons: Specialized nerve cells that are the basic building blocks of the nervous system; have been sculpted by nature to perform their function of receiving, processing, and sending messages Neurons have three main parts: 1. Cell Body (or soma): - Contains the biochemical structures needed to keep the neuron alive - Its nucleus carries the genetic information that determines how it develops and functions 2. Dendrites: - Branchlike fibres emerging from the cell body - Special receiving units like antennas that collect messages from neighbouring neurons and send them on to the cell body - Incoming information is there combined and processed 3. Axon: - One single axon extends from the side of the cell body - Conducts electrical impulses away from the bell body to other neurons, muscles, or glands - Branches out at its end to form many axon terminals - Connect to dendritic branches from many other neurons to mass messages Glial Cells: Support neurons in their functions  Surround neurons and hold them in place  Manufacture nutrient chemicals that neurons need  Form the myelin sheath around axons  Absorb toxins and waste materials that might damage neurons The Blood-Brain Barrier: prevents many substances, including a wide range of toxins, from entering the brain. The walls of the blood vessels in the brain have smaller gaps than elsewhere in the body and are covered by a special type of glial cell that keep foreign substances from the brain Nerve Conduction: Neurons` own chemical substances are a source of energy. They are surrounded by salty liquid. Its high concentration of sodium has a positive electrical charge The inside of the neuron has some pos charged potassium ions, it contains many more that are negative, so the inside of the neuron is electrically negative in relation to the outside and produces an electrical resting potential of about -70 mV. When at this resting state, neuron is polarized The Action Potential: or a nerve impulse, a sudden reversal in the neuron`s membrane voltage momentarily moves from -70 mV to +40 mV. Shift from negative to positive voltage is called depolarization 7 Steps of Action Potential: 1. Resting state of -70 mV 2. Depolarization to -55 mV (threshold of excitation, must reach -55 for reaction) 3. At threshold, voltage-gated Na+ channels open and Na+ enters cell 4. Membrane potential increases to +40 mV, K+ channels open, Na+ channels close 5. K+ leaves cell 6. Hyperpolarization of cell (returns to below -70mV) 7. Resting membrane potential restored (-70mV) Graded Potentials: small shifts that occur in the cell membrane’s electrical potential when the dendrites or the cell body of a neuron are stimulated by axons from other neurons The Myelin Sheath: - A fatty insulating substance on the axon of some neurons that increases the speed of neural transmission; derived from glial cells during development - Nodes of Ranvier: interruptions in the myelin sheath at regular intervals where the myelin is either extremely thin or absent - Myelin Sheath allow for electrical conduction to skip from node to node and increase conduction speeds - Multiple Sclerosis: Occurs when a person’s own immune system attacks the myelin sheath; disrupts the precise timing of nerve impulses, resulting in jerky uncoordinated movements How Neurons Communicate: Synaptic Transmission - Nervous system operates as a giant communications network; action requires transmission of nerve impulses between neurons - Neurons do not make physical contact with each other but communicate at a synapse: a functional but not physical connection between a neuron and its target - Synapse occurs by neurons releasing chemicals that carry messages to the next neuron in the circuit - Gap between the axon terminal of one neuron and the dendrite of the next is called the synaptic cleft Neurotransmitters: Neurons produce neurotransmitters: chemical substances that carry messages across the synapse to either excite other neurons or inhibit their firing. From axon terminals of presynaptic neuron to dendrites of postsynaptic neuron has five steps: 1. Synthesis: chemical molecules are formed inside the neuron 2. Storage: molecules are stored in chambers called synaptic vesicles within the axon terminals 3. Release: molecules move to the surface of the axon terminal and are released into the space between the pre and postsynaptic neurons when an action potential comes down to the axon 4. Binding: molecules cross the synaptic space and bind to receptor sites: large protein molecules embedded in the receiving neuron’s cell membrane 5. Deactivation: can be deactivated by other chemicals found in the synaptic space, or by the mechanism reuptake: transmitter molecules are reabsorbed into the presynaptic axon terminal Two categories of neurotransmitters: Excitatory: o Binds to the receptor site and produces a depolarizing (excitatory) reaction by stimulating the inflow of positively charged ions (Na+) o The stimulation may exceed the action potential threshold and cause the postsynaptic neuron to fire an action potential Inhibitory: o Hyperpolarizes the postsynaptic membrane by stimulating ion channels that allow K+ ions to flow out, or negatively charges ions to flow in to the neuron o Makes the membrane potential even more negative, making it harder to reach action potential For the nervous system to function properly, a perfect balance between excitatory and inhibitory processes must be maintained. Neurotransmitters: Neurotransmitter Major Function Associated Disorders Glutamate (Glutamic Acid) Excitatory; control of behaviours Learning and memory GABA Inhibitory; motor behaviour, Destruction of GABA producing anxiety neurons – Huntington’s Acetylcholine (ACh) Excitatory; memory, motor, Undersupply – memory loss in learning, sleep Alzheimer’s Norepinephrine Excitatory & Inhibitory; arousal, Undersupply – depression attention, eating Serotonin Inhibitory (mostly); sleep, Undersupply – depression, emotion, thermoregulation, sleeping, and eating disorders eating Dopamine Inhibitory & Excitatory; voluntary Undersupply – Parkinson’s and movement, emotional arousal, depression motivation Oversupply – Schizophrenia Drug Effects: o Increase or decrease amount of neurotransmitters o Processes that terminate transmitter action o Stimulates or block receptor sites - Caffeine: Blocks adenosine receptor sites—prevents sleep - Nicotine: Stimulates receptor molecules, duplicating effect of ACh - Cocaine: Stimulates the release of dopamine and prevents reuptake - Curare: Blocks receptor sites for ACh, if blocked, heart stops and you die - Black Widow Venom: Stimulates release of ACh, heart goes faster until it blows up - Botulism Toxin: Blocks release of ACh, just makes you sick Endorphins: reduce pain and increase feeling of well-being; family of neurotransmitters Neuromodulators: circulate through the brain and either increase or decrease (modulate) the sensitivity of neurons to their specific transmitters The Nervous System: The body’s master control centre The system’s input, output, and integration functions are carried out by three types of neurons: 1. Sensory Neurons: carry input messages from the sense organs to the spinal cord and brain 2. Motor Neurons: transmit output impulses from the brain and spinal cord to the body’s muscles and organs 3. Interneurons: perform connective or associative functions within the nervous system Nervous system is divided into two major subsystems: Central Nervous System (CNS): consists of all the neurons in the brain and spinal cord Peripheral Nervous System (PNS): composed of all the neurons that connect the CNS with the muscles, glands, and sensory receptors Nervous System Peripheral Nervous System Central Nervous System Somatic Division Autonomic Division Brain Spinal Cord Sympathetic Parasympathetic The Peripheral Nervous System: contains all neural structures that lie outside the brain and spinal cord o Allows us to sense what is going on inside and outside our bodies and to respond with muscles and glands o Divided in to somatic nervous system and autonomic nervous system Somatic Nervous System: sensory and motor neurons that control out voluntary movements o sensory neurons: transmit messages from the eyes, ears and other sensory receptors o motor neurons: send messages from the brain and spinal cord to the muscles that control our voluntary movements o sensory neurons group together and motor neurons group together in to strands o these form sensory nerves and motor nerves (also called tracts) o allows you to sense and respond to your environment Autonomic Nervous System: controls glands and involuntary muscles that form the heart, blood vessels, and the lining of the stomach and intestines o concerned with involuntary functions: respiration, circulation, digestion o also involved in motivation, emotional behaviour, and stress responses o consists of the sympathetic nervous system and the parasympathetic nervous system Sympathetic Nervous System: has an activation function and acts as a total unit; speeds up systems when stressed Parasympathetic Nervous System: affects one or two organs at a ti
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