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Chapter 4

Chapter 4- Biology of Behaviour.doc

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Steve Joordens

Chapter 4- Biology of Behaviour The Brain and Its Components • Pierre Flourens said that different parts of the nervous system were responsible for different functions • Donald Hebb considered how individual nerve cells are organized into larger units and that they were structurally organized and could generate the higher processes of the brain (Ex: memory, thought, and decision) Modules- nerve cells of the brain that are organized into units • They are connected to other neural circuits, receiving info from some of them, processing this info, and sending the results to other modules Structure of the Nervous System • The brain has 3 major functions: o Controlling behaviour o Processing and retaining the info we receive from the environment o Regulating the body’s physiological processes • Needs to receive info from the body’s sense receptors, and must be connected with the muscles and glands of the body if it is to affect behaviour and physiological processes Central nervous system- the brain and the spinal cord Spinal cord- a long, thin collection of nerve cells attached to the base of the brain and running the length of the spinal column • Contains circuits of nerve cells that control some simple reflexes (Ex: automatically moving away from a hot object) Nerve- a bundle of fibres that transmits info between the central nervous system and the body’s sense organs, muscles, and glands • The central nervous system communicates with the rest of the body through nerves • Bundles of 1000s of individual fibres, all wrapped in a tough, protective membrane • Nerve fibres transmit messages through the nerve, from a sense organ to the brain or from the brain to the muscle or gland, that make up the white matter and other axon tracts Peripheral nervous system- the cranial and spinal nerves • That part of the nervous system peripheral to the brain and spinal cord 1 • The nerves, which are attached to the spinal cord and to the base of the brain • Sensory information (info about what is happening in the environment or within the body) is conveyed from sensory organs to the brain and spinal cord • Consists of the nerves that connect the central nervous system with sense organs, muscles, and glands • Nerves carry both incoming and outgoing info • The sense organs detect changes in the environment and send signals though the nerves to the muscles (causing behaviour) and the glands (producing adjustments in internal physiological processes) Cranial nerve- a bundle of nerve fibres attached to the base of the brain • Convey sensory info from the head and carries messages to muscles and glands Spinal nerves- a bundle of nerve fibres attached to the spinal cord • Conveys sensory info from the body and carries messages to muscles and glands • The cranial nerves and spinal nerves carry info away from the central nervous system • The human brain has 3 major functions: o The brain stem- the “stem” of the brain, including the medulla, pons, and mid-brain  Primarily control of physiological functions and automatic behaviours o Cerebral hemispheres- the largest part of the brain  Covered by the cerebral cortex and containing parts of the brain that evolved most recently o Cerebellum- a pair of hemispheres resembling the cerebral hemispheres but much smaller and lying beneath and in back of them  Controls posture and movements • The brain is protected in the skull, and the spinal cord runs through the middle of the column of hollow bones (vertebrae) Vertebra- one of the bones that protect the spinal cord and create the vertebral column Meninges- the 3-layered set of membranes that enclose the brain and spinal cord Cerebrospinal fluid (CSF) – the liquid in which the brain and spinal cord float • Provides a shock-absorbing cushion 2 • This liquid fills the space between two of the meninges, providing a liquid cushion surrounding the brain and spinal cord and protecting them from being busied by the bones that protect them • The brain and spinal cord doesn’t come into contact with the bones of the skull and vertebrae Blood- brain barrier- a barrier between the blood and the brain produced by the cells in the walls of the brain’s capillaries • Prevents some substances from passing from the blood into the brain • Make it less likely that toxic chemicals found in what we eat or drink can find their way into the brain, where they might do damage to neurons Cerebral cortex- the outer layer of the cerebral hemispheres of the brain • Approximately 3 mm thick • It is very wrinkled because it is full of bulges called gyri (gyrus) and large grooves called fissures, which expand the amount of surface area of the cortex and greatly increase the number of nerve cells it can contain • The thin layer of grey matter on the outside of the brain Grey matter- the portions of the central nervous system that are abundant in cell bodies of neurons rather than axons • Colour appears grey relative to white matter • It is in the cerebral cortex that perceptions take place, memories are stored, and plans are formulated and executed White matter- the portions of the central nervous system that are abundant in axons rather than cell bodies of neurons • The colour derives from the presence of the axons’ myelin sheaths • The nerve cells in the cerebral cortex are connected to other parts of the brain by a layer of nerve fibres because of the shiny white appearance of the substance that costs and insulates them Cells of the Nervous System Neurons/nerve cells- consists of a cell body with dendrites and an axon whose branches end in terminal buttons that synapse with muscle fibres, gland cells, or other neurons • Bring sensory information to the brain, store memories, reach decisions, and control the activity of the muscles • Can receive information from other neurons or cells in sense organs, process this info, and communicate the processed information to other neurons or to cells in muscles, glands, or internal organs 3 Glial cell- a cell of the central nervous system that provides support for neurons and supplies them with some essential chemicals • Neurons are assisted in their task by this cell • Some types of glial cells form long fibres that guide developing neurons from their place of birth to their final resting place • Others manufacture chemicals that neurons need to perform their tasks and absorb chemicals that might impair neurons’ functioning • Some form protective insulating sheaths around nerve fibres and serve has the brain’s immune system, protecting it from invading micro- organisms that might affect it Dendrites- a tree-like part of a neuron on which other neurons form synapses • Receive messages from other neurons • Transmit the info they receive down their “trucks” of the cell body Dendritic spines- a small bud-like protuberance on the surface of a neuron’s dendrite • Appear on neurons in the brain Soma- a cell body • Largest part of a neuron • Contains the mechanisms that control the metabolism and maintenance of the cell • Also receives messages from other neurons Axon- a long, thin part of a neuron attached to the soma • Divides into a few or many branches, ending in terminal buttons • Carries messages (action potentials), which consists of brief changes in the electrical charge of the axon, away from the soma toward the cells, with which the neuron communicates Terminal buttons- the rounded swelling at the end of the axon of a neuron • They rest against dendrites, dendritc spines, the soma, or the axon of another neuron NOTE: Nerve cell is made up of: dendrites, soma, axon, and terminal buttons Neurotransmitter/ transmitter substance- a chemical released by the terminal buttons that causes the postsynaptic neuron to be excited or inhibited • Affects the activity of the other cells with which the neuron communicates and the message is conveyed chemically from one neuron to another 4 • Drugs can alter a person’s behaviour by affecting the chemical transmission of messages between cells Myelin sheaths- the insulating material that encases most large axons • Myelin (protein and fat) us produced by glial cells that wrap parts of themselves around segments of the axon, leaving small bare patches of the axon between them • The main function of myelin is to insulate axons from one another and to prevent the scrambling of messages The Excitable Axon: The Action Potential • The message carried by the axon (the action potential) involves an electrical current, but it does not travel down the axon the way that electricity travels through a wire Action potential- a brief electrochemical event that is carried by an axon from the soma of the neuron to its terminal button • Causes the release of a neurotransmitter • Caused by the opening of some ion channels in the membrane at the end of the axon nearest the soma, that permits positively charged sodium ions to enter, which produces another reversal at that point • A brief reversal of the membrane’s electrical charge, where the ion channels close and another set of ion channels opens, letting positively charged potassium ions out of the axon, which restores the normal electrical charge Ion- a positively or negatively charged particle • It is produced when many substances dissolve in water • Axonal membrane contains special submicroscopic proteins that serve as ion channels or ion transporters Ion channels- a special protein molecule located in the membrane of a cell • Can open and close to controls the entry or exit of particular ions • The membrane of the axon contains 2 types of ion channels: o Sodium channels o Potassium channels Ion transporters- a special protein molecule located in the membrane of a cell • Actively transport ions into or out of the cell using the energy resources of the cell • Pumps sodium ions out of the axon and pump potassium ions back in, restoring the normal balance 5 • When the axon is in resting state, the outside of the membrane is positively charged (inside is negatively charged) because the fluid inside the axon contains more negatively charged ions and fewer positively charged ions, and since the axon’s ion channels are closed, ions cannot move into or out of the axon All-or-none law- the principle that once an action potential is triggered in an axon, it is propagated, without getting smaller, to the end of the axon Sensory neuron- a neuron that detects changes in the external or internal environment and sends information about these changes to the central nervous system Motor neuron- a neuron whose terminal buttons form synapses with muscle fibres • When an action potential travels down its axon, the associated muscle fibres will twitch Synapses Synapse- the junction between the terminal button of one neuron and the membrane of a muscle fibre, a gland, or another neuron Synaptic transmission- info is passed from one cell to another Presynaptic neuron- a neuron whose terminal buttons form synapses with and excites or inhibits another neuron that sends the message (sends messages) Postsynaptic neuron- a neuron with which the terminal buttons of another neuron form synapses and that is excited or inhibited by that neuron (receive messages) • When the axon of a motor neuron fires, all of the muscle fibres with which it forms synapses will contract with a brief twitch because a muscles is controlled by a large number of motor neurons, each of which forms synapses with different groups of muscles fibres • 2 types of synapses: o Excitatory synapses- when the axon fires, the terminal buttons release a neurotransmitter that excites the postsynaptic neurons with which they form synapses o Inhibitory synapses- when they are activated, the lower the likelihood that the axons of the postsynaptic neurons will fire Synaptic cleft- a fluid-filled space between the presynaptic and postsynaptic membranes 6 • The terminal button releases a neurotransmitter into this space that causes reactions in the postsynaptic neuron that either excite or inhibit it by special submicroscopic protein molecules embedded in the postsynaptic membrane Neurotransmitter receptors- a special protein molecule located in the membrane of the postsynaptic neuron that responds to molecules of the neurotransmitter • After their release from a terminal button, molecules of a neurotransmitter find their way to the receptor molecules, attach to them, and activate them, where they produce excitatory or inhibitory effects on the postsynaptic neuron by opening ion channels Reuptake- the process by which a terminal button retrieves the molecules of a neurotransmitter that it has just released • Terminates the effect of the neurotransmitter on the receptors of the postsynaptic neuron • After the signal is send, the neurotransmitters return to the sending neuron Drugs and Behaviour • Some substances can affect people’s moods in ways that they want to experience all the time • Drugs can use for good or bad Effects of Drugs on Synaptic Transmissions • Drugs can affect and alter our thoughts, perceptions, emotions, and behaviour by affecting the activity of neurons in the brain Stimulating or Inhibiting the Release of Neurotransmitters • Some drugs can stimulate or prevent certain terminal buttons to releasing their neurotransmitter when the axon fires Stimulating or Blocking Postsynaptic Receptors • Some drugs mimic the effects of particular neurotransmitters by directly stimulating particular kinds of receptors, which can work like a master key, turning the receptors on even when the neurotransmitters is not present • Some drugs bind with receptors and do not simulate them, which blocks receptors, making them inaccessible to the neurotransmitter and inhibiting synaptic transmission Inhibiting Reuptake 7 • Some drugs inhibit the process of reuptake so that molecules of the neurotransmitter continue to stimulate the postsynaptic receptors for a long time, which increases the effect of the neurotransmitter Neurotransmitters, Their Actions, and Drugs That Affect Them • In the brain, most synaptic communication is accomplished by 2 neurotransmitters: o Glutamate- the most important excitatory neurotransmitter in the brain and spinal cord o Gamma-amino butyric acid (GABA) - the most important inhibitory neurotransmitter in the brain  Glycine- found in the lower brain stem and the spinal cord Glutamate • The most important excitatory neurotransmitter in the brain and major excitatory neurotransmitter in the spinal cord • All sensory organs transmit info to he brain through axons whose terminals release glutamate • NMDA receptor is responsible for many of the changes in synaptic connections that are responsible for learning GABA • Some drugs depress behaviour, causing relaxation, sedations, and even loss of consciousness Barbiturates- a drug that causes sedation • One of several derivatives of barbituric acid Antianxiety drugs- a “tranquilizer” which reduces anxiety Benzodiazepine- a class of drug having anxiolytic (“tranquilizing”) effects • Effects various parts of the brain including the regions that is involved in fear and anxiety Acetylcholine Acetylcholine (ACh) – a neurotransmitter found in the brain, spinal cord, and parts of the peripheral nervous system • Responsible for muscular contraction • Primary neurotransmitter secreted by the axons of motor neurons • Axons and terminal buttons of ACh neurons are distributed widely throughout the brain o Activates the brain mechanisms responsible for REM sleep (the phase of sleep during which most dreaming occurs) o Activating neurons in cerebral cortex and facilitating learning (perpetual learning) 8 o Controls the functions of another part of the brain involved in learning: the hippocampus • 2 drugs that affect the release of ACh: o Botulinum toxin: a drug that prevents the release of ACh by terminal buttons  Produced by a bacterium that can grow in improperly canned food  Very dilute solution and extremely strong poison  Usually referred to as Botox, to stop muscular contractions that are causing wrinkles o Black widow spider venom- a drug that stimulates the release of ACh by terminal buttons  Can be fatal to infants or to frail, elderly people  Venom is less toxic than botulinum toxin • After being released by the terminal buttons, ACh is deactivated by an enzyme, called acetylcholinesterase (AChE), that is present in the postsynaptic membrane o Neostigmine- a drug that enhances the effects of acetylcholine by blocking the enzyme that destroys it • Drugs that affects acetylcholine receptors: o Nicotine- a drug that blinds with and stimulates acetylcholine receptors, mimicking the effects of this neurotransmitter  Highly addictive drug  Plays a role in the reinforcement (reward) mechanisms of the brain o Curare- a drug that binds with and blocks acetylcholine receptors, preventing the neurotransmitter from existing its effects  Can cause paralysis Monoamines Monoamines- a category of neurotransmitters that include dopamine, norepinephrine, and serotonin • Serve to modulate the function of widespread regions of the brain, increasing or decreasing the activities of particular brain functions Dopamine (DA) – a monoamine neurotransmitter involved in control of brain mechanisms of movement and reinforcement • Including movement, attention, learning, and the reinforcing effects of drugs that people tend to abuse Parkinson’s disease- a neurological disorder characterized by tremors, rigidity of the limbs, poor balance, and difficulty in imitating movements • Caused by degeneration of a system of dopamine- secreting neurons 9 • Are given a drug called -LOPA, which is taken up by the DA neurons that still survive and it converted to dopamine, and these neurons release more dopamine that improves the patients’ symptoms Schizophrenia- a serious psychological disorder whose symptoms include hallucinations, delusions, and disruption of normal, logical thought processes • Drugs such as Thorazine (chlorpromazine) and Clozaril (clozapine) relive the symptoms of the disorder, by blocking particular types of dopamine receptors • Several drugs inhibit the reuptake of dopamine, serving ti prolong and strengthen its effects, such as amphetamine and cocaine Norepinephrine (NE) – a monoamine neurotransmitter involved in alertness and vigilance and control of REM sleep Serotonin- a monoamine neurotransmitter involved in the regulation of mood • In the control of eating, sleep and arousal and in the regulation of pain • A deficiency in the release of this in the cerebral cortex is linked with alcoholism and anti-social behaviour • Drugs such as Protaz (fluoxetine) inhibit the reuptake of serotonin and strengthen and prolong its effects, and used to treat depression, anxiety disorder, and obsessive-compulsive disorder Lysergic acid diethylamide (LSD) – a hallucinogenic drug that blocks a category of serotonin receptors • Produces distortions of visual perceptions that some people find awesome and fascinating but that simply frighten other people Peptides Neuromodulators- a substance secreted in the brain that modulates the activity of neurons that contain the appropriate receptors • As these chemicals diffuse through the brain, they can activate or inhibit circuits of neurons that control a variety of functions and they can modulate particular categories of behaviour • Most neuromodulators are peptides • Neurons send messages to other neurons, which effect many sites in the brain simultaneously leading to many different behaviour al effects Peptides- a category of neurotransmitters and neuromodulators that consist of 2 or more amino acids • Linked by peptide bonds • Endogenous opioid- a neuromodulator whose action is mimicked by a natural or synthetic opiate, such as opium, morphine, and heroin 10 o Reduce pain because they have direct effects on the brain o Simulate special opioid receptors located on neurons in several parts of the brain o Effects include decreased sensitivity to pain and a tendency to persist in ongo
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