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Chapter 3

Chapter 3 summary

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Psychology 1000

Chapter 3 Biological Foundations of Behaviour Chapter 3: Biological Foundations of Behaviour The Neural Bases of Behaviour Neurons - Basic building blocks of the nervous system - Nerve cells are linked together in circuits - A cell body, dendrites, and an axon - The cell body (soma), contains biochemical structures needed to keep the neuron alive - Its nucleus carries genetic information - Emerging from the cell body are dendrites (branch like fibres) - Antennas that collect messages from neighbouring neurons send them on to the cell body - Incoming information is combined and processed - Extending from one side of the cell body is a single axon - AXON conducts electrical impulses away from the cell body to other neurons, muscles, or glands - Axon branches out to form axon terminals which connect with dendrite branches from numerous neurons - Glial cells: o Surround the neurons and hold them in place o Manufacture nutrient chemicals that neurons need o From myelin sheath around some axons, and absorb toxins and waste materials that might damage neurons o Protect the brain from toxin - BLOOD-BRAIN BARIER prevents many substances (including a wide range of toxins) from entering the brain - Walls of the blood vessels within the brain contain smaller gaps o Covered by specialized type of glial cell - Smaller gaps and glial cells keep foreign substances from gaining access to the brain 1 Chapter 3 Biological Foundations of Behaviour The Electrical Activity of Neurons - Neurons: o Generate electricity that creates nerve impulses o Release chemicals that allow them to communicate with other neurons and with muscles and glands - Three Steps: 1. At rest, the neuron has an electrical resting potential due to distribution of positively and negatively charged chemicals (ions) inside and outside the neuron 2. When stimulated flow of ions in and out through the cell membranes reverses the electrical charge of the resting potential o Produces an action potential or nerve impulse 3. Original distribution of ions is restored, and neuron is again at rest - Neurons are surrounded by body fluids o Separated from this liquid by protective membrane o Cell membrane is a bit like a selective sieve, allowing certain substances to pass through ion channels into the cell while refusing or limiting passage to other substances - An ion channel is a passageway or channel in the membrane that can open to allow ions to pass through - Salty fluid outside the neuron are positively charge sodium ions (NA+) and negatively charged chlorine ions (Cl-) - Inside neurons are large negatively charged protein molecules (anions or A-) and positively charged potassium ions (K+) - Interior of the cell negative compared to the outside - Internal difference is the neurons resting potential o State of polarization Action Potential - Sudden reversal in the neurons membrane voltage o Membrane voltage momentarily moves from -70millivolts to +40millivolts o Negative to positive voltage called depolarization - Resting state, the neurons sodium and potassium channels are closed - Concentration of Na ions is 10 times higher outside - Neuron is stimulated so nearby sodium channels open up - Positively charged sodium ions flood into the axon o Creates a state of depolarization - The interior now becomes positive - Reflex action to restore the resting potential the cell closes its sodium channels o Positively charged potassium ions flow out their channels, restoring the negative resting potential - The excess sodium ions flow out to the neuron, and the escaped potassium ions are recovered - After an impulse passes a point along the axon 2 Chapter 3 Biological Foundations of Behaviour o Recovery period as K+ ions flow out of the interior - Absolute refractory period is when the membrane is not excitable and cannot generate another action potential ALL-OR-NONE LAW action potentials occur at a uniform and maximum intensity, or they dont occur at all GRADED POTENTIAL - Changes in the negative resting potential that dont reach the -50mV action potential threshold Myelin Sheath - Fatty, whitish insulin layer derived from glial cells during development - Interrupted at regular intervals by the nodes of Ranvier o At these parts myelin is either thin or absent - Unmyelinated axons, the action potential travels down the axon length like a burning fuse - Myelinated axons, electrical conduction can skip from node to node - Multiple sclerosis: o Persons own immune system attacks the myelin sheath o Disrupts the delicate timing of nerve impulses o Results in jerky, uncoordinated movements How Neurons Communicate: Synaptic Transmission SYNAPSE functional connection between a neuron and its target 3 Chapter 3 Biological Foundations of Behaviour - Neurons released chemicals that carried the message from one neuron to the next cell in the circuit SYNAPTIC CLEFT tiny gap or space between the axon terminal of one neuron and the dendrite of the next neuron Neurotransmitters - Chemical substances that carry messages across the synapse to either excite other neurons or inhibit their firing - Five Steps: 1. Synthesis o Chemical molecules are formed inside the neuron 2. Storage o Then molecules are stored in chambers (synaptic vesicles) within the axon terminal 3. Release o Action potential comes down the axon terminal o Molecules are released into the fluid-filled space between the axon of the sending neuron to the membrane of the receiving neuron 4. Binding o Molecules cross the synaptic space and bind to the receptor site o Receptor sites have specially shaped surface that fits a specific transmitter molecule 5. Deactivation Excitation, Inhibition, and Deactivation - Reaction will depolarize the postsynaptic cell membrane - Stimulates the inflow of sodium - Excitatory transmitters are neurotransmitters that create depolarization - If the stimulation exceeds the action potential can cause the postsynaptic neuron to fire an action potential - The chemical reaction can hyperpolarize the postsynaptic membrane by stimulating ion channels o Positively charged potassium ions to flow out of the neuron o Negatively charged ions (chloride) flow into the membrane - Membrane potential is now even more negative - Every neuron is constantly bombarded with excitatory and inhibitory neurotransmitters from other neurons o Determines whether the cell fires an action potential - Balance between excitatory and inhibitory processes must be maintained if the nervous system is to function properly - A neurotransmitter molecule binds to its receptor o Continues to activate/inhibit the neuron until its shut off/deactivated - Transmitter molecules are deactivated by other chemicals located in the synaptic space - Deactivation mechanism is reuptake 4
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