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Chapter 3

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Psychology 1000
Mark Holden

Chapter3: BiologicalFoundationsof Behaviour Neural Bases of Behaviour Neurons:  Human brain weighs only 3lbs, but is critical to our experiences and behaviour  Neurons: o ‘nerve cells’ that are arranged in a network o 100 billion neurons in the brain at the time of birth o Most neurons when 4-5 months in the womb (200 billion neurons at the time) o 50-100 thousand neurons per second are being grown when you are a fetus at around 4 months o Send and receive electrical signals  Glial cells: o ‘nurse cells’ that ‘take care’ of the neurons o 10x as many glial cells as neurons  3 types of neurons: o Sensory Neurons  Detect stimuli and send signals to the brain o Inter-neurons (association neurons)  Receive messages from other neurons, interpret, and pass along the response  Far and away the most common neuron o Motor Neurons  Carry signals from brain (or spinal cord) to muscles and organs  3 Main Components of Neurons: o Dendrites (receiving)  Branchlike fibres emerging from cell body  Receive messages from other neurons (receives messages from up to 1000+ other neurons)  Passes message on to the cell body o Cell Body (soma) (Processing)  Contains nucleus, DNA  determines how the cell develops ad functions  Contains necessary cellular structures  keeps the neuron alive  Combines and processes all incoming signals  can also receive signals directly o Axon (sending)  On the other side of the cell body  Sends out signals (electrical impulses) to other neurons/muscles (sends signals up to 50,000 other neurons)  Glial Cells: o Sometimes called ‘nurse’ or ‘glue’ cells o Way more glial cells than neurons in the brain (10x) o Surround neurons and ‘take care’ of them  Keep them in place (hold them)  Manufacture needed nutrients (feed them)  Absorb toxins and waste (protect them)  During development, they help convey the neurons the right place (guide them) o Help form the myelin sheath around the axon of some neurons  Myelin sheath:  Fatty layer of insulation around a neuron’s axon  Makes sending signals much faster  Sheath is like the beads or sausage links  gaps between the sausages are the Nodes of Ranvier  Not all neurons have a myelin sheath o Also help protect the entire brain from toxins  Blood-brain Barrier:  Specialized barrier that prevents many toxins from entering the brain  Blood vessels in brain are tightly packed (smaller gaps), and the glial cells are there to also cover them  Together, these prevent most toxins from entering the brain Electrical Activity of Neurons  Neurons do 2 important things: 1) Generate electricity that creates nerve impulses. 2) Release chemicals that allow them to communicate with other neurons and with muscles and glands.  How nerve impulses occur: o At rest, the neuron has an electrical resting potential due to the distribution of positively and negatively charged chemicals (ions) inside and outside the neuron o When stimulated, a flow of ion in and out through the cell membrane reverses the electrical charge of the resting potential, producing an action potential, or nerve impulse o The original distribution of ions is restored, and neuron is again at rest  Neuron’s Cell Membranes: o Separates the inside of the cell from the outside (like our skin) o This cell membrane has channels in it  These channels can be open or closed  The channels are specific for certain types of ions only (sodium and potassium) o The cell membrane also contains a special structure called a sodium-potassium pump Resting Potential:  Refers to when a neuron has built up a charge and is ready to fire  The inside of the neuron is relatively more negatively charged than the outside of the neuron  Sodium-potassium pumps actively transport potassium and sodium ions across the cell membrane  Pushes 3 sodium ions out and pulls in only 2 potassium ions  Concentration of sodium ions outside is 10x of inside  Means less positive ions inside than outside, therefore inside is relatively more negative  Outside: lots of Sodium (+), some Cl (-)  Inside: lots of anions (-), some potassium (K+)  Can say that the neuron is ‘polarized’  Difference is mostly due to high concentration of sodium on one side, and A- on the other side  Difference is -70mV (millivolts)  Sodium ‘wants’ to get inside the cell; potassium ‘wants’ to get out of the cell  Cannot do this because ion channels are closed Action Potential:  Sudden, wave-like depolarization of the membrane  Difference suddenly reverses to +40mV  Discovered by Hodgkin and Huxley (Nobel Prize 1963)  How to get Action Potential: 1. Action potential begins when sodium channels start to open  Sodium rushes into the cell (positive sodium is attracted by relatively negative interior)  Also wants to balance the concentration gradient (10x as much Na)  Neuron becomes depolarized (-70mV  +40mV in less than a millisecond)  Potassium channels still closed 2. Once one sodium channel opens, it causes the one next to it to open  Sodium channels are voltage gated (think of a locked gate that is barred from the inside)  This is why it is a ‘wave-live depolarization’ 3. As the next sodium (Na+) channel is opening, 2 things happen:  The first Na+ channel shuts again  The K+ channels open  K+ rushes out of the cell  Positive potassium ion is repelled by the now positive interior (wants to also balance concentration gradient) 4. At this point, Na+ is inside the cell, K+ is on the outside  This repolarizes the cell at about -70mV  Na+ and K+ on the wrong sides  Sodium-Potassium pumps go to work  because pump pushes 3 Na+ out and brings in only 2 K+, the outside becomes extra positive relative to the inside  hyperpolarization  Eventually, the standard resting potential levels are reached  During hyper-polarization, neuron cannot fire again  Refractory period: the period of time after a neuron has fired during which it cannot fire again (2ms)  All-or-None: o Action potentials always occur at the same maximal intensity (or they do not occur at all) o Threshold Potential:  To trigger an action potential, the amount of sodium entering must be enough to raise the difference to -50mV  If threshold is met, we get a full action potential (max intensity)  If threshold is not met, then the neuron doesn’t fire  A sub-threshold change in voltage is called a graded potential  Several graded potentials can add up to reach threshold and cause action potential  Myelin Sheath o Fatty layer of insulation wrapped around the axon of some (not all) neurons o Speeds up conduction (electrical impulses) o In unmyelinated axons, the next channel is nearby o In myelinated axons, the next channel is all the way at the next node of ranvier  Signal jumps from node to node, making electrical conduction much faster o Myelin is not completely formed until well after birth (partially explains some of the gains of development) o Multiple sclerosis = immune system attacks myelin How Neurons Communicate: Synaptic Transmission  Neurons do not actually touch each other  Synapse (connection between neuron and its target): Synaptic cleft  a tiny space between the axon terminals of one neuron and the dendrites of the next neuron Neurotransmitters:  Chemical substances that help neurons ‘communicate’ with other neurons  5 steps of Chemical Communication: o Synthesis  Neurons make the neurotransmitter o Storage  Stored in the synaptic vesicles (bubbles with magic pixie dust) o Release  When the action potential reaches the axon terminals of the ‘pre-synaptic neuron’, it causes the vesicles to go to the ends and ‘pop’  Neurotransmitter is released into the synaptic cleft o Binding  The molecules of neurotransmitter attach to special receptors on the ‘post- synaptic neuron’  Each receptor will only fit together with a specific type of neurotransmitter o Deactivation  Receptors and neurotransmitters are ‘pulled apart’ to stop the signal Excitation and Inhibition  Excitation: o Some neurotransmitters make the post-synaptic neuron more likely to fire  excitatory neurotransmitter o Binding with the receptor site causes some sodium to enter the post-synaptic neuron o If enough enters to reach the threshold, the neuron will fire  Inhibition: o Some neurotransmitters make the post-synaptic neuron less likely to fire o Binding with the receptor site causes either the potassium to leave, or chloride to enter the post-synaptic neuron o Makes the inside even more negative (hyperpolarized), now it’s even harder to reach the threshold of -50Mv o Inhibitory neurons helps fine-tune brain activity and behaviour  Some neurotransmitters can be either excitatory OR inhibitory. In these cases, depends on specific receptors of the post-synaptic neuron  Neuron receives a combination of excitatory/inhibitory inputs, whether the neuron fires depends on additive effects of the inputs  Deactivation: o 2 parts to deactivating a signal  Enzymes will break down the neurotransmitter, even as it’s bound to the receptor site  Reuptake: the pre-synaptic neuron re-absorbs the neurotransmitter that is left in the synaptic cleft  Different areas have more or less of certain types of receptor, which makes these areas more or less sensitive to specific neurotransmitters. This helps prevent ‘crosstalk’ from other areas Drugs:  Agonist: o Increases the activity of a neurotransmitter o Can enhance neuron’s ability to synthesize, store, or release more neurotransmitter o Can stop or slow deactivation  Antagonist o Decreases the activity of a neurotransmitter o Can reduce a neurons ability to synthesize, store, or release more neurotransmitter o Can prevent neurotransmitter from binding  Neuromodulators o Increase or decrease sensitivity of neurons to whatever neurotransmitters they receive o Much more broad general influence on the brain o Glial cells will modulate the amount of neuromodulators released  Alcohol: o both agonist and antagonist effects o agonist for GABA  alcohol lowers overall neural activity o antagonist for Glutamate  alcohol diminishes clear thinking  Caffeine o Antagonist for Adenosine (Adenosine inhibits neurotransmitters) o Causes increased activity because it inhibits a inhibitor  Nicotine o Agonist for Acetylcholine (ACh)  can fit in the same receptor sites o Agonist for Dopamine  dopamine is involved in pleasure, may explain addictiveness  Amphetamines o Agonist for Dopamine  results in the high feeling o Agonist for Norepinephrine (drug related to adrenaline)  increases release and prevents reuptake  Cocaine o Same as amphetamines, except it only blocks reuptake  Date Rape Drugs o Agonist for GABA  GABA is an inhibitory neurotransmitter widespread in the brain Specific Neurotransmitters: (6)  Glutamate o Excitatory o Found throughout the brain  important for learning and memory o Too much glutamate  seizures  GABA o Inhibitory o Found throughout the brain  especially important for motor control, lower anxiety o Too much GABA  basically, think drunkenness  Acetylcholine (ACh): o Excitatory o Involved in muscle movement and memory o Too little ACh  Alzheimer’s Disease, paralysis o Too much ACh  convulsions, muscle contractions (black widow spider venom)  Dopamine: o Excitatory or Inhibitory o Involved in reward, pleasure, voluntary motor control, control of thought processes o Too little  Parkinson’s Disease, Depression o Too much  Schizophrenia, hallucinations  Serotonin: o Inhibitory o Involved in mood, eating, sleep, sex o Too little  depression  Endorphins: o Inhibitory o Involved in inhibiting pain (binds to same receptors as opium) o Too little  hyper-sensitivity o Too much  insensitivity to pain The Nervous System  2 Major divisions: 1. Central Nervous System (CNS)  All neurons within the brain and spinal cord 2. Peripheral Nervous System (PNS)  All neurons through the rest of the body Peripheral Nervous System  2 major divisions of the PNS: 1. Somatic Nervous system:  Involved in voluntary movements  Sensory neurons  axons of these become sensory nerves  Motor neurons  axons of these become motor nerves 2. Autonomic Nervous System:  Controls glands and smooth (involuntary) muscles (i.e. heart, intestines)  Involuntary body functions (breathing, digestion, etc.)  Also involved in the body’s response to stress, motivation, emotion Autonomic Nervous System:  2 divisions of the autonomic nervous system: o Sympathetic Nervous System  Prepares the body for stress (activation)  Increased heart rate, increased breathing rate, more blood to muscles, pupil dilation, slows digestion  Called the Fight-Flight Response  tends to act as one big unit (i.e. heart rate and breathing rate tend to both increase) o Parasympathetic Nervous System  Slows it all down, back to normal levels  Tends to act more specifically, slowing one or two organs at a time (breathing might return to normal quickly, heart rate might take longer) o The sympathetic and parasympathetic systems work together to maintain homeostasis  Homeostasis: balanced internal state Central Nervous System  2 major components of the CNS: 1. Spinal Cord:  Connects most neurons in the PNS with the CNS  Inside the vertebrae  Grey (H-shaped) area are neuron cell bodies and their connections  White surrounding area are myelinated axons connecting to other levels of the spinal cord and brain  Spinal Reflexes: reflexes that are triggered at the spinal cord, without involving the br
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