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PSYC Week 7.docx

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PSYC 100
Meredith Chivers

Week 7 In Chapter 4, we explore the biological basis of the stuff from which ‘you’ are made – the brain, and the nerve cells that comprise it. Biology of Behaviour  Neuroscientists: attempt to determine the relationship between behaviours, including thoughts and feelings, and activity in the brain and hormonal systems.  Questions that a neuroscientist might ask include: 1. What does the brain do? The brain is an organ, like the heart or lungs. Hippocrates, the ancient Greek physician, decided that the brain was the most powerful organ because it controlled all the other organs. He was correct according to most psychologists—ultimately, our nervous system (of which the brain is a part) controls all behaviour and mental processes. Therefore, it is important to psychologists to understand the brain's structure and functions. 
 2. What is the nervous system? The nervous system can be divided into two different parts  Peripheral nervous system (PNS) o The PNS includes cranial nerves and spinal nerves, which transmit sensory information from the rest of the body to the CNS, and transmit motor and other commands in the other direction, from the CNS to muscles, glands and internal organs.  Central nervous system (CNS). Figure 4.2 in your text (p. 91) 
 o The CNS includes the brain and spinal cord. The Peripheral Nervous System (PNS)  The PNS is divided into skeletal and autonomic portions.  The skeletal portion controls o Various muscles (e.g. your leg muscles) o Relaying motor commands from the brain  The autonomic portion exerts o Automatic control of the “involuntary” muscles (e.g., heart and diaphragm) and many internal organs (e.g., the viscera).  The autonomic nervous system (ANS) (page 120-121 of your textbook.) o ANS consists of nerves that control the function of the glands and internal organs o It is especially important for physiological responses to emotional situations, such as sweating, crying, salvation, and sexual arousal. o It is subdivided into the sympathetic division, which promotes a ‘fight or flight’ response, involves high arousal/alertness, mobilizes the body for rapid energy expenditure, and inhibits digestion. o The parasympathetic division promotes a ‘rest and digest’ response, enhancing the internal processes related to the digestion of food. Neuron: refers to an individual neural (or nerve) cell. Nerve: always refers to a bundle of fibers. Each fiber in the bundle is a long process (an axon) extending from a neuron’s cell body. Nerves transmit information within the PNS, and between the PNS and CNS. Nerves that are entirely within the brain and spinal cord (i.e., the CNS) are called tracts. The Central Nervous System (CNS)
  The CNS is composed of two broad classes of cells; neurons, and glia.  The glia or glial cells are supporting cells.  There are different kinds of glia and they serve supportive and protective functions – helping the neurons to do their work.  Glia serves a supportive role, they o Supply oxygen and nutrients to neurons o Remove dead cells and germs o Serve as protective insulation around the axons of neurons. o Communication in the brain What are Neurons?  Neurons are specialized cells capable of transmitting information  Complex circuits, made up of thousands of neurons sending and receiving signals, are the functional basis of all psychological activity.  Although we can describe the action of single neurons, the complexity of human thought and behaviour is the result of billions of neurons, each connected with tens of thousands of other neurons.  Neurons aren’t connected randomly – they selectively communicate with other neurons to form circuits or networks  Neurons are specialized to transmit information in a number of ways  One way that they are specialized to transmit information comes from their shape. (Figure 4.5 on p. 93) presents a schematic picture of a "prototypical" neuron, which has several distinct features including: o Cell body – contains structures that maintain cell health and metabolism. o Nucleus – contains genes (DNA). It is where the proteins that "run" the cell 
get made. o Dendrites - look similar to trees, and are responsible for receiving 
information. o Axon - transmits information (analogous to a telephone wire). o Axon Terminals / Synapse – conveys information to the next neuron. 
In terms of transmitting information, then, the dendrites receive information - whether from another neuron or from external stimulation - the cell body "analyzes" it, and the axon transmits it. The information is passed on to the next neuron (or a muscle) at the axon terminal. The axon terminal is not directly connected to the next neuron - rather, there is a small gap between a 
sending neuron and the receiving structure - this is called the “synapse”. An example of a real neuron is shown in Figure 4.7 at the top of page 93. 
 Three Neuron Types and the Spinal Reflex.  There are three general types of neurons: 1. Sensory neurons—Detect information from the physical world, and pass that information along to the brain (e.g. light receptor neurons in the eye or touch receptor neurons in the skin). o Sensory neurons are often called afferent neurons because they send signals FROM the body TO the brain. 
 2. Motor neurons – direct muscles to relax or contract, producing movement. o They are efferent neurons, sending information FROM the brain TO the body. 3. Interneurons – These are neurons that communicate within short or long distance circuits. o They are between sensory/motor neurons, and work to integrate and communicate information o Basically, an interneuron is any neuron that is not a sensory or motor neuron  These 3 types of neuron are involved in a very primitive, but functional, behaviour known as the spinal reflex.  The spinal reflex is controlled entirely at the level of the spinal cord; the brain is not directly involved in producing a spinal reflex, there are many types  When you cross your leg and your doctor hits you on the knee, he or she is testing a spinal reflex. It is the reflexive withdrawal from pain.  Example: o Let's say the hand (to which the skin and muscle, shown, belong) is exposed to a flame. The sensory neuron’s dendrites in the skin would send a signal up the sensory neuron to the spinal cord. The sensory signal would then be sent to the interneuron, which in turn would signal the motor neuron. This signal would then travel to the muscle and activate it to pull the hand away from the flame.  The brain isn’t conscious of the pain we know this by observing that we remove our hand from the painful stimulus even before we are consciously aware of it.  The brain is bypassed because it would slow down the system too much. The brain eventually does become aware of the pain. Presumably this is so we can learn to avoid the behaviour (touching a flame) that produced the pain. Neurons as Telephone Wires (limits to an analogy)  One can think of neurons and nerves as being like telephone wires - they both relay information.  Telephone wires use electrical impulses to send information and neurons operate in a similar (but not exactly the same) manner  Information transmitted by neurons travels, at best, at about 100 meters per second Membranes and Potentials  Recall that that the neuron is a highly specialized cell, "specialness" is said to come from its shape  The neuron's most important specialty is its ability to maintain and change its electrical charge  Let's start with a neuron that is resting o Physiologists use a device with needle electrodes to measure the electrical state differences between the outside and inside of a resting neuron. o They found that the INSIDE of a neuron is NEGATIVELY CHARGED relative to the OUTSIDE, the difference being about -70 mV (milliVolts). o This charge is like the charge of a battery, only much smaller. The Neuron’s Resting Potential and the Semipermeable Membrane  How does a neuron maintain its charge?  The membrane of the neuron has a special property: it is semipermeable. It allows some chemicals to cross (enter and leave the neuron) and it keeps others out. Ion Channels  Cell membranes have ion channels in them.  These are tiny holes through the membrane that allow certain ions to pass in and out; see Fig 4.8 (p. 95) and Fig. 4.13 (p. 98) that depict ion channels.  The cell membrane provides each kind of ion with its own channel.  Ion channels are like gates: they can be open, or closed.  When they are open, ions can pass; when they are closed, ions cannot pass. Ion Channels and the Resting Potential *  Remember that the resting potential of a neuron is - 70 mV.  This potential is determined by the relative amount of positive and negative ions floating inside and outside the neuron.  Whether ions can get into a cell depends in part on whether the ion channels are open or closed.  For a neuron at rest, most of the potassium (K+) channels are open and a fair number of K+ ions can pass back and forth across the membrane.  In contrast, most of the sodium (Na+) channels are closed. A mechanism actively pumps any sodium that leaks into the cell back out resulting in a lot of sodium (Na+) being outside of the neuron at rest.  Ions with similar charge tend to be repelled from one another. Because there is a lot of positively charged sodium outside the cell, the positive charged potassium tends to get pushed into the cell (through the open potassium gates). So, the inside of the cell has a lot of potassium.  Another thing contributing to this process is the presence of negatively charged proteins found inside the neuron.  These proteins are too large to pass through the membrane.  The net result is that, if you add together all the charges inside and outside the cell, the inside is more negatively charged than the outside by 70 mV-> This is how the resting potential is created. The Action Potential  For an animation of the action potential, step by step, see the movie at: http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php In the section on Neural Signalling: Conduction  The neuron maintains a negative electrical charge.  When the neuron becomes excited (e.g. from stimulation to its dendrites) the ion channel gates change their state.  Changing ion concentrations on both sides of the membrane very briefly causes the membrane to change its electrical potential from -70 mV to +40 mV. This brief reversal of charge is called an action potential.  It is propagated down an axon like a wave  The action potential is the effect of multiple successive brief depolarizations all along the cell membrane. The action potential is the way that neurons transmit signals from their cell body to the end of their axon. How the Action Potential Occurs  If there is enough stimulation, then, the neuron becomes sufficiently positively charged (e.g., its electrical potential moves from -70 to about -55 mV and it achieves the threshold of activation)  When the neuron depolarizes and hits -55 mV, all the sodium gates open up suddenly.  Remember that the outside of the cell has a lot of sodium, while the inside has very little.  Chemicals in solution tend to like to be as diffuse as possible and will flow down a "concentration gradient" (i.e., they will move from more concentrated areas to less concentrated areas)  Opposites attract—the positively charged sodium is attracted to the negatively charged proteins inside the neuron  So, once the sodium channels open up, the sodium rushes into the cell.  This rush of sodium causes the charge of the inside of the neuron to become positive relative to the outside (it depolarizes).  It goes all the way up to +40 mV.  This sudden change in the potential of the membrane is the action potential  After about half a millisecond, the potassium (K+) starts to move out of the cell and the Na+ gates close. As a consequence, the potential begins to go back toward the resting level (it repolarizes). Then active pumps called ion transporters work to pump the Na+ out, and the K+ back in.  It takes a while for the resting concentrations to be achieved inside and outside the cell  Refractory period is a period of time, when the neuron becomes slightly more negative inside than it is during the resting potential (-80 mV), lasts a few milliseconds, during which the neuron is much less sensitive to stimulation. Axonal Propagation  The action potential only occurs on a small part of 7 the neuronal membrane  It starts at a point just at the base of the axon  When an action potential occurs at this first point of the axon, the change in the membrane potential causes the sodium channels in the neighbouring region of the axon to open up which generates an action potential in this region as well  Thus, the action potential flows down the axon. It is sort of like the "wave" you create by flicking a skipping rope  Because each section enters a refractory period at the end of the action potential, this keeps the action potential from "bouncing" back in the direction it came  During the refractory period, that section of the axon is less likely to be stimulated enough to generate an action potential o Therefore the action potential can only keep going in one direction – down the axon. The Nature of Action Potentials  Action potentials are the method by which a neuron sends information from one end of its axon to the other  Important points about the nature of the action potential: o The action potential is an "all or none" event. If a neuron is sufficiently depolarized it will generate an action potential. If it does not reach the threshold of activation (sufficiently depolarized) the impulse will decay rapidly and will not reach the end of the axon (see the ‘failed initiations’ in the Figure, above). Some have likened the action potential to the firing of a gun. Both are "all or none" events. As a result, the action potential is sometimes referred to as the "firing" of the neuron. o The action potential is a biologically active process. It involves opening and closing channels and the movement of ions across the membrane. Because it is a biological process, it is relatively slow. o The action potential can be sped up by myelin. Remember the myelin sheath on the axon (Fig. 4.5 p. 93) it is a fatty covering, made up of a kind of glial cell called a Schwann cell. The Schwann cells insulate the neuron, allowing the electrical impulse of the action potential to travel very rapidly. But the Schwann cells aren’t very big; each one can cover about 100 micrometers of an axon, so when axons are long, many Schwann cells, sitting adjacent to each other along the axon like a string of sausages, are required. The action potential is renewed by depolarization at the exposed membrane at each little gap between successive Schwann cells (between two sausages) – allowing it to “leap” to the next little gap. These little gaps are called nodes of Ranvier. So the charge seems to jump from node to node, rather than having to travel the whole axon. This is called saltatory conduction and lets the charge travel much faster than it would down an unmyelinated axon. o Multiple Sclerosis is an autoimmune disorder in which the afflicted person's immune system begins to destroy his or her myelin. The result is a degradation of neural transmission, producing the symptoms associated with MS, including slurring of speech, loss of vision, loss of motor control, and cognitive impairments. How Neurons Talk to One Another—The Synapse.  So, a neuron transmits information by sending electrical impulses known as action potentials down the axon. Recall the spinal reflex.  This system consisted of three different neurons  The question is: How do these neurons communicate with one another? o It turns out that these neurons are not connected directly. o There is a small space between the axon terminal buttons of one neuron and the dendrites of the next neuron. This space is called the synapse. (Figure 4.6 p. 94) o The action potential is like an electrical impulse—it flows down to the end of axon. o However, this electrical impulse does not jump across the synapse; this would require an extraordinary amount of electrical energy to accomplish. o See the ‘Synapses’ animation on this site: http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php  When the action potential reaches the end of the axon, it causes the release of special chemicals into the synapse. These chemicals are called neurotransmitters - they transmit a signal from one neuron to the next.  Neurotransmitters are found at the end of the axon in an area referred to as
the axon terminal where they are held in small packets known as vesicles  When the action potential reaches the axon terminal, it causes the vesicles to move to the membrane of the axon terminal, fuse with it and release the neurotransmitter into the synaptic cleft (the space between the two neurons)  The neurotransmitter travels across the synapse to the next neuron's dendrite (Fig. 4.12, p. 97). See the ‘“classic” chemical neurotransmission’ animation on this site: http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php  When the neurotransmitter, having crossed the synaptic cleft, arrives at the dendrite of the next neuron, it attaches to a structure known as a neurotransmitter receptor (Fig4.13)  The receptor is often a chemical gated channel these channels open under special circumstances  In this case, they open when they are stimulated by the appropriate neurotransmitter  If a sufficient quantity of neurotransmitter attaches (i.e., if the depolarization caused by the incoming neurotransmitter is sufficient that the threshold of activation is crossed), then the postsynaptic neuron (the neuron that receives the neurotransmitter) will generate an action potential  So, neurons can transmit information within themselves through action potentials and transmit information between themselves (across synapse) through the release of neurotransmitters. Getting Rid of Neurotransmitters. • The neurotransmitter sends the message from one neuron to the next. • When it has finished its job, it must be eliminated; otherwise the message would continue to be sent • This is accomplished in two general ways. o One is synaptic reuptake where the neurotransmitter is taken back into the axon terminal of the cell that released the neurotransmitter (the presynaptic cell), so it can be used again; Figure 4.14. o The second is by enzymatic breakdown, in which the neurotransmitter is disabled by an enzyme. It is converted into a chemical that has no effect on the binding site. Neurotransmitters and Action Potentials
 • Recall that action potentials are all-or-none events. The role of a neurotransmitter in evoking an action potential is not “all or none”. • We know that we must exceed the threshold of activation before an action potential occurs. • In order for an action potential to occur, the postsynaptic cell must be sufficiently stimulated • Two general factors influence whether or not an action potential will occur: o The number of neurons that are connected to dendrites of postsynaptic neuron. Most neurons receive inputs from hundreds of other neurons. If sufficient numbers of neurons release enough neurotransmitters on the surface of the cell, then an action potential will occur. Example: the neurotransmitter excites the postsynaptic cell (by opening sodium channels). o There are also neurotransmitters that inhibit the postsynaptic cell (by opening chloride and potassium channels). These neurotransmitters actually make the postsynaptic cell more negatively charged than it normally would be (hyperpolarized). The likelihood of an action potential depends on sum of all excitatory and inhibitory potentials on the neuron at a given time. Neurotransmitters and Behaviour  Many behaviours are linked to specific types of neurotransmitters  We can usually tell that a behaviour is linked to a neurotransmitter system, when a drug known to affect a neurotransmitter also affects a behaviour. (pp. 99-105.)  Neurotransmitters can be classified into three ‘families’ based on their chemical structure  These families are: o the amines o the amino acids o the peptides.  The neurotransmitter members of each of these 3 ‘families’ have similar properties and functions. http://thebrain.mcgill.ca/flash/i/i_01/i_01_m/i_01_m_ana/i_01_m_ana.html – 2 Amines The amines include dopamine, epinephrine and norepinephrine, serotonin, and acetylcholine. 1. Many researchers believe that dopamine is the primary neurotransmitter that communicates which activities might be rewarding. Drinking when thirsty, eating when hungry, or having sex when aroused all lead to activation of dopamine receptors and are therefore experienced as pleasurable. Dopamine is also activated by stress, and has been implicated in motor control and planning, guiding behavior towards objects and events that will lead to reward. People with Parkinson’s disease, which is characterized by muscle tremor, rigidity and difficulty initiating voluntary action, have dopamine depletion in their midbrain. Parkinson’s disease is treated by drugs that increase the level of dopamine in the brain. Drugs that reduce dopamine activity also reduce schizophrenic symptoms. The dopamine hypothesis of schizophrenia (page 577) 2. Epinephrine and norepinephrine. Epinephrine is mostly found in the body, although small amounts are present in brain. It is also called adrenaline, a burst of energy caused by the release of epinephrine in the body. A shot of epinephrine is used to treat people who are having difficulty breathing. Norepinephrine (noradrenaline) is involved in arousal and vigilance, heightened sensitivity to what is going on around you. 3. The neurotransmitter serotonin is another monoamine. Low levels of serotonin are associated with sad and anxious moods. Antidepressant drugs called "serotonin-selective reuptake inhibitors", like Prozac, increase the activity of serotonin by allowing it to stay in the synapse for a longer period of time. 4. Acetylcholine is responsible for motor control at the junction between nerves and muscles. It is also involved in learning and memory, and in sleep and dreaming. Drugs that prevent acetylcholine from working can cause temporary memory impairment, and Alzheimer’s disease, which is characterized by severe memory loss, is associated with diminished acetylcholine functioning. Amino acids: glutamate and GABA. 1. Glutamate acts as the primary excitatory neurotransmitter in the brain. It is fast acting; glutamate receptors aid learning and memory by strengthening synaptic connections. 2. GABA (short for gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in the brain. Without GABA, synaptic excitation might get out of control and spread through the brain. It is thought that an abnormality in GABA function might be related to epileptic seizures. Benzodiazepines (like Valium) and alcohol (ethyl alcohol) work to assist GABA in binding with one of its receptors, and are known to have relaxing effects. Benzodiazepines are used to treat anxiety disorders. Peptides  Peptides are a large family of neurotransmitters. Some peptides modulate emotions, and others are involved in the perception of pain, while others regulate responses to stress.  One important group of neurotransmitter peptides is the opioids, so called because they bind to the same post-synaptic receptors as opium and its derivatives morphine and heroin.  Opium has been cultivated for more than 5000 years, and has been used to treat pain since at least the 1400s. One of the active ingredients in opium is a plant alkaloid, morphine, named for the Greek god of dreams (Morpheus). Morphine is still one of the most widely used treatments for pain, despite its addictive potential.  The opioid peptides were discovered in the 1970s when researchers went looking for brain chemicals that mimicked the action of morphine. It was hoped that such chemicals would also act as analgesics (drugs that diminish the perception of pain) and that a greater understanding of them would shed light on the mechanisms of drug addiction.  These peptides were originally called endorphins but this term is now reserved for one specific class of opioid peptides (the other classes are enkephalins and dynorphins).  Morphine and heroin mimic the action of opioid peptides by binding to brain opioid receptors. This produces profound analgesia (pain reduction), drowsiness and nausea. Indeed, one of the biggest problems with drugs like morphine is that many people feel too sick to continue taking the drug, even if they are in pain. Morphine also produces a general state of well-being or euphoria which may reduce the subjective experience of pain – people still feel it, but they don’t experience it as aversive.  Opioid peptides (particularly the endorphins) act the same way, permitting an animal to carry on with important behaviours like eating, moving, and mating even when they are in pain, which has survival value. Because endorphins are released under stressful or painful states, they might explain the ‘runners’ high’ that habitual runners can experience.  One theory is that the body produces opioid peptides to cope with anticipated pain; if that pain doesn’t occur, the result is pleasure. Neurotransmitters and Drugs  Any drug that affects behaviour does so by changing the nature of neurotransmitter activity at the synapse, generally in one of three ways. o The drug can block or enhance neurotransmitter synthesis in the pre- synaptic neuron o The drug can block or enhance neurotransmitter release from the pre- synaptic neuron o Block or enhance binding at the post-synaptic receptors 
  Drugs that diminish the
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