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Chapter 3

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PSYO 1021
Sean Barrett

Chapter 3: Biological Foundations of Behavior 1. Name the three main parts of the neuron and describe their functions. Each neuron has three main parts: a cell body, dendrites and an axon. The cell body (soma) contains the biochemical structures needed to keep the neuron alive, and its nucleus carries the genetic information that determines how the cell develops and functions. Emerging from the cell body are branchlike fibers called dendrites. These specialized receiving units are like antennas that collect messages from neighboring neurons and send them on to the cell body. There the incoming information is combined and processed. The many branches of the dendrites can receive input from 1000 or more neighboring neurons. The surface of the cell body also has receptor areas that can be directly stimulated by other neurons. 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. Each axon may connect with dendritic branches from numerous neurons, making it possible for a single neuron to pass messages as many as 50,000 other neurons. 2. Which structural characteristics permit the many possible interconnections among neurons? The structure of the dendrites and axons make it possible to have trillions of interconnections in the brain. 3. How do glial cells differ from neurons? What three functions do they have in the nervous system? Neurons are supported in their functions by glial cells (from the Greek word glue). Glial cells surround neurons and hold them in place. Glial cells also manufacture nutrient chemicals that neurons need, form the myelin sheath around some axons, and absorb toxins and waste materials that might damage neurons. 4. What causes the negative resting potential of neurons? When is a neuron said to be in a state of polarization? In the salty fluid outside the neurons are positively charged sodium ions (Na+) and negatively charged chloride ions (CL-). Inside the neuron are large negatively charged protein molecules (anions or A-) and positively charged potassium ions (K+). The high concentration of sodium ions in the outside the cell, together with the negatively charged protein ions inside, results in an uneven distribution of positive and negative compared to the outside. This internal difference (of around -70 millivolts) is called the neuron’s resting potential. At rest the neuron is said to be in a state of polarization. 5. What chemical changes cause the process of depolarization that created graded and action potentials? How do these potentials differ? An 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. In a resting state, the neuron’s sodium and potassium channels are closed and the concentration of Na+ ions is 10 times higher outside the neuron than inside it. But when a neuron is stimulated sufficiently, nearby sodium channels open up. Attracted by the negative protein ions inside, positively charged sodium ions flood into the axon creating a state of depolarization. In an instant the interior now becomes positive in relation to the outside, creating the action potential. 6. What is the nature and importance of the myelin sheath? Which disorder results from inadequate myelinization? Many axons that transmit information throughout the brain and spinal cord are covered by tube-like myelin sheath, a fatty, whitish insulation layer derived from glial cells during development. The tragic effects of damage to the myelin coating can be seen in people who suffer form multiple sclerosis. This progressive disease occurs when the person’s own immune system attacks the myelin sheath. Damage to the myelin sheath disrupts the delicate timing of nerve impulses, resulting in jerky uncoordinated movements and in the final stages paralysis. 7. How do neurotransmitters achieve the processes of excitation and inhibition of postsynaptic neurons? The binding of a transmitter molecule to the receptor site produces a chemical reaction that can have one of two effects on the postsynaptic neuron. In some cases, the reaction will depolarize (excite) the postsynaptic cell membrane by stimulating the inflow of sodium or other positively charged ions. Neurotransmitters that create depolarization are called excitatory transmitters. 8. Describe two methods by which neurotransmitter molecules are deactivated at the synapse. 9. Describe the role of (a) acetylcholine, (b) dopamine, (c) serotonin, and (d) endorphins in psychological functions. 10. What are the three major types of neurons? What are the their functions? The three major types of neurons are the sensory neurons, motor neurons and interneurons. Sensory neurons carry input messages form 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. 11. Differentiate between the central nervous system and the peripheral nervous system. What are the two divisions of the peripheral nervous system? The nervous system can be broken down into several interrelated subsystems. The two major divisions are the central nervous system, consisting of all the neurons in the brain and spinal cord, and the peripheral nervous system, composed of all the neurons that connect the central nervous system with the muscles, glands, and sensory receptors. 12. Describe the two divisions of the autonomic nervous system, as well as their roles in maintaining homeostasis. The sympathetic nervous system has an activation or arousal function, and it tends to act as a total unit. The parasympathetic nervous system slows down body processes and maintains or returns you to a state of rest. While the sympathetic system speeds up the heart rate, the parasympathetic system slows it down. By working together to maintain equilibrium in our internal organs, the two divisions can maintain homeostasis, a delicately balance or constant internal state. 13. How do spinal reflexes occur? Spinal reflexes are simple stimulus-response sequences that can be triggered at the level of the spinal cord without any involvement of the brain. For example, if you touch something hot, sensory receptors in your skin trigger nerve impulses in sensory nerves that flash into your spinal cord and synapse inside with interneurons. The interneurons then excite motor neurons that send impulses to your hand so that it pulls away from the hot object. Other interneurons simultaneously carry the “Hot!” message up the spinal cord to your brain, but it is a good thing that you don’t have to wait for the brain to tell you what to do in such emergencies. Getting message to and from the brain takes slightly longer so the spinal cord reflex system significantly reduces reaction time, and in this case potential tissue damage. 14. Describe four methods used to study brain-behavior relations. Neuropsychological tests are used to measure verbal and non-verbal behaviors that are known to be affected by particular types of brain damage. Destruction and stimulation techniques, where researchers can produce brain damage under carefully controlled conditions in which specific nervous tissue is destroyed with electricity, with co
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