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Chapter 3

Chapter 3 Notes

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Psychology 1000
Stephen Biggs

Chapter 3 Biological Foundations of Behaviour nd September 22 /2011 The Neural Bases of Behaviour Structure of the Nervous System Biopsychology The study of the biological bases of thoughts, feelings, and behaviours Neural Impulses: The Beginning Hodgkin and Huxley in the 1950s were able to study the neurons in Giant Squid without microscopes They noted a saline solution inside and outside of neurons This saline solution contained ions They noticed that neurons were different at rest than when activated Neurons Specialized cells Brain contains 100 billion neurons at birth Less able to reproduce than other cells Easier for young people than old people to reproduce neurons Designed to receive, process, and send messages Resting potential of -70 millivolts This means that at rest, the neurons have a negative charge inside of them compared to the outside Contains: Cell body (soma) Contains the biochemical structure needed to keep the neuron alive Nucleus carries the genetic information that determines how the cell develops and functions Combines and processes information sent from the dendrites Dendrites Emerge from the cell body Receiving units Collect messages from neighbouring neurons and send them to the cell body Can receive input from 1000 or more neighbouring neurons Axon Extends from one side of the cell body Conducts electrical impulses away from the cell body to other neurons, muscles, glands Each axon can connect to dendrite branches from numerous neurons It is possible for a single neuron to pass messages to as many as 50 000 other neurons Branches out to form axon terminals At the end of axons there are terminal buttons synapse the postsynaptic membrane on the dendrite of another neuron (this is the path that information follows as it is being transferred from one neuron to another) Glial Cells Neural Glue Surround neurons and hold them in place Provide support and structure for neurons, the brain, and the nervous system They are constantly replacing themselves Manufacture nutrient chemicals that neurons need Form the myelin sheath around some axons Absorb toxins and waste materials that might damage neurons The blood-brain barrier prevents many substances and toxins from entering the brain The walls of the blood vessels within the brain contain very small gaps, and are also covered by specialized types of glial cells Smaller gaps + glial cells keep foreign substances out of the brain During prenatal brain development, glial cells guide newly divided neurons to their targeted place in the brain Outnumber neurons 10 to 1 (There are approximately 1 trillion) Clear waste, provide structural support, and help to supply nutrients to the important neurons Nerve Conduction: An Electrochemical Process Neurons generate electricity and release chemicals Neurons own chemical substances are a source of energy Neurons are surrounded by a salty, positively charged, liquid environment On the interior, Neurons carry a negative charge (in relation to the outside) Resting potential is -70 mV In this resting state, the neuron is said to be polarized The Action Potential Stimulation causes the cell membrane to open briefly and allow positively charged sodium ions to flow in, and then the shift in electrical charge travels along the neuron Sudden reversal of a neurons membrane voltage from -70 millivolts to +40 millivolts Depolarization describes this shift from negative to positive voltage Graded potentials are small shifts that occur in the cells membranes electrical potential and are proportional to the amount of incoming stimulation Occurs when the dendrites on the cell body receive stimulation from other neurons They are cumulative If the graded potentials are not very strong, the neuron will be partially depolarized but not enough to generate an action potential If the graded potentials are large enough to reach the action potential threshold, then an action potential will occur Over time the graded potentials can accumulate and eventually cause an action potential Action potentials obey the all-or-none law which means that either it occurs or it doesnt Camera analogy once you press down the button it doesnt matter how hard you press it, it will always have the same brightness of flash and also the flash wont happen if you dont push the button down all the way (graded potentials) Graded potentials act on tiny protein structures in the cell membrane called ion channels Each ion channel allows specific ions to cross the cell membrane to enter or leave the cell Positively charged ions (like sodium) can enter through ion channels and make the neuron less negative than it was (partial depolarization Action potential threshold is around -55 millivolts Resting potential is -70 millivolts When a neuron is at resting potential, positively charged sodium ions are kept in salty fluid outside of the cell in a 10:1 concentration When a cell reaches the threshold for generating an action potential, the sodium ion channels open and positively charged sodium ions flow into the neuron, attracted by the negative electrical force inside of the cell which makes the neuron more positively charged than the outside of the cell These sodium ion channels do not stay open for very long, they close in less than 1/1000 of a second To restore resting polarity, potassium channels open and positively charged potassium ions leave the cell which allows the cells negative resting potential to be restored The whole process only takes a few milliseconds at any point on the membrane, but the action potential has started a wave that flows along the membrane and repeats the process After the action potential, the sodium ions are pumped back out and the potassium ions are pumped back in PAGE 89 FOR DIAGRAMS A refractory period occurs after an impulse passes along the axon During this time the membrane is not excitable and it cannot discharge another action potential th It lasts 1/1000 of a second This limits the rate at which action potential can be triggered in a neuron The limit is approximately 300 nerve impulses per second The rate of firing of an individual neuron, and the number of neurons that fire act in response to high-intensity stimulation The Myelin Sheath The myelin sheath is a tube-like, fatty, whitish insulation layer derived from glial cells that covers many axons This allows for much faster transfer of information The nodes of Ranvier are areas where the myelin sheath is very thin, or even absent Unmyelinated axons: The action potential travels down the axon length like a burning fuse Grey matter Myelinated axons: Electrical conduction can skip from node to node, and this accounts for high conduction speeds of over 300km/h White matter Mostly higher animals have myelin sheaths Also, in many nerve fibers the myelin sheath is not formed until sometime after birth Damage in myelin coating Grey matter has no myelin sheath on outside and therefore has a much slower rate of transfer
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