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Chapter 3

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Western University
Psychology 1000

Psychology 1000 Chapter 3 (Biological Foundations of Behaviour) History  Descartes reflex arc, stimuli transferred from periphery to brain and reflected back; o How?  “Animal spirits” o Importance of pineal gland  Place of divine intervention  Swammerdam‟s Frog Experiment  Bell (1800s) o Types of nerves  Afferent fibres (sensory)  Efferent fibres (to the effectors, motor)  Bell & Muller o Specific nerve energy o Speed of impulse  9000 ft/min to 57 billion ft/sec o Hemlholtz  50-100 m/s  Reaction time o Maskelyne and Kinnebrook The Neural Bases of Behaviour  The brain is a grapefruit sized mass of tissue, weighing about three pounds.  The nervous system is built upon a cell called the neuron. They link within the system like how electronics are linked within a computer. The body contains about 100 billion neurons. o The neuron contains three main parts; the cell body (soma), dendrites and an axon.  The soma contains the structures that are required to keep the neuron alive as well as genetic information, determining how the cell develops and functions  Dendrites receive information (in the form of neurotransmitters) from other neurons and send them into the soma.  Dendrites can receive information from up to 1000 or more neighbouring neurons.  An axon conducts electrical information in the form of impulses to other neurons, muscles, glands, etc. An axon branches out to terminals, varying in the amount of neurons on axon terminal may touch. o Neurons can vary in size and shape; more than 200 different types of neurons have been seen.  Bipolar  Found in visual system  Unipolar  Multipolar o Neurons can be classified by function:  Sensory neurons are in Afferent fibres.  Motor neurons are in Efferent fibres.  Inter Neuron act as relay stations.  Act as relay stations. o Neurons are supported by glial cells that support the neuron in its structure as well as function; manufacturing chemicals that neurons need, forming the myelin sheath, absorb toxins and waste that may damage the neurons.  Glial cells outnumber neurons by a ratio of about 10:1.  Glial cells protect brain from toxins; current theory suggests the blood-brain barrier is made from glial cells. Not only that, research has indicated that the glial cells are somehow involved with communication among neurons.  Neurons generate electricity and send out chemicals to other neurons as well as glands and muscles. Nerve activation: o Neurons have a resting potential due to the balance of positive and negative ions within and + outside the neuron (Na ions are situated more on the outside of the neuron). o Na ions flow into the neuron, causing an imbalance within the neuron. + + + o After, the Na gates close and the K gates open, causing K to rush outside the cell, causing the negative overall charge to get restored. o The action potential is repeated for the length of the neuron. o After, during the absolute refractory period, the membrane cannot generate another impulse for a certain length of time, places an upper limit on the rate at which nerve impulses occur; about 300 impulses a second. o However, the action-potential is either all or nothing; the resting potential has to reach from - 70mV to -50 mV (the action potential threshold). Changes in the resting potential that do not reach -50 mV are called graded potentials.  Intensity is directly proportional to frequency of firing.  Axons that transmit information are covered by the myelin sheath, a white, fatty insulation that covers the neuron. The myelin sheath „ends‟ at certain points called nodes of Ranvier, where the myelin is extremely thin or absent. Within the unmyelinated axons, the axons are considerably slower than the myelinated axons; up to 300 km/h. The myelinated sections „jump‟ from node to node, therefore increasing the speed. o Myelin is more commonly found in the nervous systems of „higher‟ animals. o Myelin sheaths for many nerve fibres are not completely formed until sometime after birth. o People with ALS and other nervous system disorders, occurs due to the fact that the myelin sheath is being destroyed or wears out. How Neurons Communicate: Synaptic Transmission  Chemical communication involves five steps: o Synthesis: chemical neurons being formed within the neuron. o Storage: chemical molecules are stored within synaptic vesicles within the axon terminals.  Each neurotransmitter is stored within one axon; i.e. dopamine and serotonin would be „housed‟ in separate axon terminals. o Release: when the action potential comes down the axon, then the vesicles moved to the axon terminal. After, the molecules are released from the sending (presynaptic) to the receiving (postsynaptic) neuron. o Binding: the molecules cross the synaptic cleft and then attach the molecules to receptor sites.  Each receptor site conforms to one neurotransmitter only.  Binding a neurotransmitter to a receptor can produce one of two effects: o Depolarization: the reaction may excite the postsynaptic cleft cells by stimulating the inflow of sodium or other molecules.  The depolarization may exceed the action-potential threshold and cause another firing of the neuron.  Neurons that depolarize are called excitatory transmitters.  They cause an EPSP (excitatory postsynaptic potential) o Hyperpolarization: the reaction may cause an influx of negative ions to flow into the neuron or cause positive ions to flow out.  They inhibit the firing of the neuron by making the action-potential threshold harder to reach (-70 mV unhyperpolarized to -72 mV hyperpolarized).  Neurons that hyperpolarize are called inhibitory transmitters.  They cause an IPSP (inhibitory postsynaptic potential). o A balance of hyperpolarization and depolarization must be maintained if the nervous system is to work properly.  Seizures are caused by large amounts of neurons firing in a runabout fashion. o Once a neurotransmitter binds with a receptor, it continues to „do its thing‟ until deactivated.  Some are deactivated by chemicals within the synaptic cleft.  Others are reabsorbed into the presynaptic cleft and reused.  Reabsorption is called reuptake.  When the receptor is clear, it returns to resting state and waits until next stimulation. o Transmitter molecules are in many different shapes.  Only 100-150 are known/suspected to be transmitters, however many more are present.  Each substance has a different effect on a neuron.  The brain only recognizes certain chemicals, so communication between „systems‟ does not occur.  Each neurotransmitter (NT) has a specialized function.  GABA (gamma-aminobutryic acid) is vital in motor control and anxiety control.  Glutamate has roles involved with memory and learning. However, too much of glutamate can cause seizures since it has an excitatory effect on the brain (mainly within the cerebral cortex)  The best understood NT is acetylcholine (ACh), involved in memory and muscle activity. o Underproduction of ACh is supposedly an important factor in Alzheimer‟s disease. o ACh not being activated can cause muscular distress; Botulism causes paralysis in muscle activity, it being fatal if the respiratory muscles are affected. On the other hand, if too much ACh is present, it can cause violent muscle contraction and even death (Black widow spider bite) o Curare, a plant, blocks receptor sites for ACh, causing paralysis. o Nicotine duplicates the effect of ACh and stimulates the receptor sites at the same time.  Dopamine (DA) is a NT with a wide variety of functions; from feelings of pleasure, motor control and control of thoughts.
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