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Chapter 3

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Psychology 1000
Derek Quinlan

Chapter 3  Mean: Add all the numbers up and divide by how many are present.  Median: rank the numbers from lowest to highest and find the middle number  Mode: number that shows up most frequently  Chapter 3:  Neurons: are the basic building blocks of the nervous system  Emerging from the cell body are branchlike fibres called “dendrites”: these specialized receiving units are like antennas that collect messages from neighbouring neurons and send them on to the cell body  Extending from one side of the cell body is a single “axon”: which conducts electrical impulses away from the cell body to other neurons, muscles, or glands. The axon branches out at its end to form a number of axon terminals-as many as several hundred in some cases  Blood-brain barrier: prevents many substances, including a wide range of toxins, from entering the brain  Neurons do two important things: 1) they generate electricity that creates nerve impulses. 2) they also release chemicals that allow them to communicate with other neurons and with muscles and glands  Nerve Impulses occur: 1) 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 2) when stimulated, a slow of ions in and out through the cell membrane reserves the electrical charge of the resting potential, producing an action potential, or nerve impulse 3) the original distribution of ions is restored, and the neuron is again at rest  Action Potential: is a sudden reversal in the neuron’s membrane voltage, during which the membrane voltage momentarily moves from -70 millivolts (inside) to +40 millivolts  This shift from negative to positive voltage is called “Depolarization”: what happens in the neuron to cause the action potential? Hodgins and Huxley found that the key mechanism is the action of sodium and potassium ion channels in the cell membrane.  The internal difference of around 70 millivolts is called the neurons “resting potential”  During this “Absolute Refractory Period”: 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 do not occur at all  The negative potential inside the axon has to be changed from -70monillivolts to about -50millivolts (the action potential threshold) by the influx of sodium ions into the axon before the action potential will be triggered. Changes in the negative resting potential that do not reach the -50millivolts action potential threshold are called “graded” potentials”. Under certain circumstances, graded potentials caused by several neurons can add up to trigger an action potential in the postsynaptic neuron  For a neuron to function properly, sodium and potassium ions must enter and leave the membrane at just the right rate  Myelin Sheath: a fatty, whitish insulation layer derived from glial cells during development. The myelin sheath is interrupted at regular intervals by the “nodes of Ranvier”: where the myelin is either extremely thin or absent  In unmyelinated axons: the action potential travels down the axon length like a burning fuse.  In myelinated axons: electrical conduction can skip from node to node, and these “great leaps” from one gap to another account for high conduction speeds of more than 300 kilometres per hour  The myelin sheath: is commonly found in the nervous system of higher animals. People with multiple sclerosis can be seen with damage to the myelin sheath  Synapse: a functional (but not physical) connection between a neuron and its target. This idea was controversial: How could a neuron influence the functioning of the heart, or a skeletal muscle, or another neuron within the brain if these cells did not actually touch? What carried the message from one neuron to the next? The controversy persisted until the 1920s, when Otto Loewi, in a series of simple but elegant experiments, demonstrated that neurons released chemicals, and it was these chemicals that carried the message from one neuron to the next cell in a circuit  A tiny gap or space called “Synaptic Cleft”: 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.  Process of chemical communication has Five steps: synthesis, storage, release, binding, and deactivation  Synthesis: the chemical molecules are formed inside the neuron. The molecules are then stored in chambers called “Synaptic vesicles” within the axon terminals. When an action potential comes down the axon, these vesicles move to the surface of the axon terminal and the molecules are released into the fluid-filled space between the axon of the sending (presynaptic) neuron and the membrane of the receiving (postsynaptic) neuron. The molecules cross the synaptic space and bind to “receptor sties” –large protein molecules embedded in the receiving neuron’s cell membrane. Receptor sites look like Lilly pads   Deactivation mechanism is called “Reuptake”: in which the transmitter molecules are reabsorbed into the pre-synaptic axon terminal.  Perhaps the best understood neurotransmitter is “Acetylcholine (ACh)”: which is involved in memory and muscle activity. Underproduction of ACh is though to be an important factor in Alzheimer’s disease, a degenerative brain disorder involving profound memory impairment that afflicts between 5-10% of all people over 65 years old.  Dopamine: mediates a wide range of functions, including motivation, reward, and feelings of pressure; voluntary motor control; and control of though processes.  *depression involves abnormal sensitivity to* Serotonin: a neurotransmitter that influences mood, eating, sleep, and sexual behaviour  Endorphins: reduce pain and increase feelings of well-being  Neuromodulators: have a more widespread and generalized influence on synaptic transmission.  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: which far outnumber sensory and motor neurons, perform connective or associative functions within the nervous system.  Central Nervous system: consisting of all the neurons in the brain and spinal cord  Peripheral Nervous system: composed of all the neurons that connects the central nervous system with the muscles, glands, and sensory receptors.  Somatic nervous system: consists of the sensory neurons that are specialized to transmit messages from the eyes, ears, and other sensory receptors, brain and spinal cord to the muscles that control our voluntary m
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