Chapter THREE.docx

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University of Toronto Scarborough
Steve Joordens

Chapter THREE NEURONS: THE ORIGIN OF BEHAVIOUR - There are approximately 100 billion cells in our brain that perform a variety of tasks that allow us to function as a human being. - Humans have thoughts, feelings, and behaviours that are often accompanied by visible signals. - All of the thoughts, feelings, and behaviours spring from cells in the brain that take in information and produce some kind of output trillions of times a day. o These cells are neurons, cells in the nervous system that communicate with one another to perform information-processing tasks. DISCOVERY OF HOW NEURONS FUNCTION - During the 1800s, scientists began to turn their attention from studying the mechanics of limbs, lungs, and livers to studying the harder-to-observe workings of the brain. - In the late 1880s, a Spanish physician named Santiago Ramon y Cajal learned about a new technique for staining neurons in the brain. o The stain highlighted the appearance of entire cells, revealing that they came in different shapes and sizes. o During an 1887 visit to Madrid a colleague showed Cajal some samples stained with the new technique, and his imagination was immediately captured by the idea that it could provide important new insights into the nature and structure of the nervous system. o Using this technique, Cajal was the first to see that each neuron was composed of a body with many threads extending outward toward other neurons o Surprisingly, he also saw that the threads of each neuron did not actually touch other neurons. o Cajal then arrived at the fundamental insight that neurons are the information-processing units of the brain and that even though he saw gaps between neurons, they had to communicate in some way. COMPONENTS OF THE NEURON - Cajal discovered that neurons are complex structures composed of three basic parts: o The CELL BODY THE DENDRITES THE AXON - Like cells in all organs of the body, neurons also have a cell body (also called the soma) the largest component of the neuron that coordinates the information-processing tasks and keeps the cell alive. o Functions such as protein synthesis, energy production, and metabolism take place here. o The cell body contains a nucleus; this structure houses chromosomes that contain our DNA, or the genetic blueprint of who we are. o The cell body is surrounded by a porous cell membrane that allows molecules to flow into and out of the cell. - Unlike the other cells in the body, neurons have two types of specialized extensions of the cell membrane that allow them to communicate: DENDRITES and AXONS - Dendrites receive information from other neurons and relay it to the cell body. o The term dendrite comes from the Greek word for “tree”; most neurons have many dendrites that look like tree branches. - The Axon transmits information to other neurons, muscles, or glands. o Each neuron has a single axon that sometimes can be very long, even stretching up to a meter from the base of the spinal cord down to the big toe. - In many neurons, the axon is covered by a myelin sheath an insulating layer of fatty material o This myelin sheath is composed of glial cells which are support cells found in the nervous system. o Although there are 100 billion neurons processing information in our brain, there are 10-50 times that many glial cells serving a variety of functions. o Some glial cells digest parts of dead neurons, others provide physical and nutritional support for neurons, and others form myelin to help the axon transmit information more efficiently. o An axon insulated with myelin can more efficiently transmit signals to the other neurons, organs, or muscles. o With the demyelinating diseases, such as multiple sclerosis the myelin sheath deteriorates, slowing the transmission of information from one neuron to another. This leads to a lot of problems, such as loss of feeling in the limbs, partial blindness, and difficulties in coordinated movement and cognition. - There is a small gap between the axon of one neuron and the dendrites or cell body of another. o This gap is part of the synapse: the junction or region between the axon of one neuron and the dendrites or cell body of another. o The transmission of information across the synapse is fundamental to communication between neurons, a process that allows us to think, feel, and behave. MAJOR TYPES OF NEURONS - There are three major types of neurons, each performing a distinct function: o Sensory neurons receive information from the external world and convey this information to the brain via the spinal cord.  They have specialized endings on their dendrites that receive signals for light, sound, touch, taste, and smell.  In our eyes, sensory neurons’ endings are sensitive to light o Motor neurons carry signals from the spinal cord to the muscles to produce movement  These neurons often have long axons that can stretch to muscles at our extremities o Interneurons connect sensory neurons, motor neurons, or other interneurons.  Some interneurons carry information from sensory neurons into the nervous system, others carry information from the nervous system to motor neurons, and still others perform a variety of information-processing functions within the nervous system.  Interneurons work together in small circuits to perform simple tasks, such as identifying the location of a sensory signal, and much more complicated ones, such as recognizing a familiar face. NEURONS SPECIALIZED BY LOCATION - Besides specialization for sensory, motor, or connective functions, neurons are also somewhat specialized depending on their location. o For instance Purkinje cells are a type of interneuron that carries information from the cerebellum to the rest of the brain and spinal cord.  These neurons have dense, elaborate dendrites that resemble bushes. o Pyramidal cells are found in the cerebral cortex, and have a triangular cell body and a single, long dendrite among many smaller dendrites o Bipolar cells are a type of sensory neuron found in the retinas of the eye, and have a single axon and a single dendrite. - The brain processes different types of information, so a substantial amount of specialization at the cellular level has evolved to handle these tasks. THE ELECTROCHEMICAL ACTIONS OF NEURONS: INFORMATION PROCESSING - The communication of information within and between neurons proceeds in two stages – conduction and transmission. o The first stage is the conduction of an electric signal over relatively long distances within neurons, from the dendrites, to the cell body, then throughout the axon. o The second stage is the transmission of chemical signals between neurons over the synapse. - Together these stages are what scientists generally refer to as the electrochemical action of neurons. ELECTRIC SIGNALING: CONDUCTING INFORMATION WITHIN A NEURON - The neuron`s cell membrane is porous – it allows small electrically charged molecules, called ions, to flow in and out of the cell. THE RESTING POTENTIAL: THE ORIGIN OF THE NEURON`S ELECTRICAL PROPERTIES - Neurons have a natural electric charge called the resting potential, which is the difference in electric charge between the inside and outside of a neuron`s cell membrane. o The resting potential is similar to the difference between the ``+`` and ``-`` poles of a battery, and just like a battery, resting potential creates the environment for a possible electrical impulse. o First discovered by biologists in the 1930s - The resting potential arises from the difference in concentrations of ions inside and outside the neuron`s cell membrane. o Ions can carry a positive (+) or a negative (-) charge. o In the resting state, there is a high concentration of a positively charged ion, potassium (K+), as well as negatively charged protein ions (A-), INSIDE the neuron’s cell membrane compared to outside it. o By contrast, there is a high concentration of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-) OUTSIDE the neuron’s cell membrane. o Since both the inside and the outside of the neuron contain one positively and one negatively charged ion, you might think that the positive and negative charges would simply cancel each other out, so that the resting potential is neither positive nor negative. WRONG! o The behavior of the K+ ion provides the key to understanding this seemingly odd state of affairs. o Raising the concentration of K+ in the fluid outside the neuron to match the concentration of K+ inside the neuron causes the resting potential to disappear. - The concentration of K+ inside and outside an axon is controlled by channels in the axon membrane that allow molecules to flow in and out of the neuron. - In the resting state, the channels that allow K+ molecules to flow freely across the cell membrane are open, while channels that allow the flow of Na+ and the other ions noted earlier are generally closed. - Because there is a naturally higher concentration of K+ molecules INSIDE the neuron, some K+ molecules move out of the neuron through the open channels, leaving the inside of the neurone with a charge of about -70 millivolts relative to the outside. THE ACTION POTENTIAL: SENDING SIGNALS ACROSS THE NEURON - Biologists working with the squid giant axon noticed that they could produce a signal by stimulating the axon with a brief electric shock, which resulted in the conduction of a large electric impulse down the length of the axon. o This electric impulse is called an action potential, which is an electric signal that is conducted along the length of a neuron’s axon to the synapse. - The action potential occurs only when the electric shock reaches a certain level, or threshold. - When the shock was below this threshold, the researchers recorded only tiny signals. - When the shock reached the threshold, a much larger signal, the action potential, was observed - Increases in the electric shock above the threshold did NOT increase the strength of the action potential. - The action potential always occurs with exactly the same characteristics and at the same magnitude regardless of whether the stimulus is at or above the threshold. - The action potential occurs when there is a change in the state of the axon’s membrane channels. - During the resting potential, only the K+ channels are open. However, when an electric charge is raised to the threshold value, the K+ channels briefly shut down, and other channels that allow the flow of a positively charged ion, Na+, are opened. - We know that Na+ is much more concentrated outside the axon than inside. - When the Na+ channels open, those positively charged ions flow inside, increasing the positive charge inside the axon relative to that outside. - This flow of Na+ into the axon pushes the action potential to its maximum value of +40 millivolts. - After the action potential reaches its maximum, the membrane channels return to their original state, and K+ flows out until the axon returns to its resting potential. o This leaves a lot of extra Na+ ions inside the axon and a lot of extra K+ ions outside the axon - During this period where the ions are imbalanced, the neuron CANNOT initiate another action potential, so it is said to be in a refractory period, the time following an action potential during which a new action potential cannot be initiated. - The imbalance in ions eventually is reversed by an active chemical “pump” in the cell membrane that moves Na+ outside the axon and moves K+ inside the axon o The pump does NOT operate during the action potential. - When an action potential is generated at the beginning of the axon, it spreads a short distance, which generates an action potential at a nearby location on the axon. That action potential also spreads, initiating an action potential at another nearby location thus transmitting the charge down the length of the axon - This simple mechanism ensures that the action potential travels the full length of the axon and that it achieves its full intensity at each step, regardless of the distance traveled. - The myelin sheath, which is made up of glial cells that coat and insulate the axon, facilitates the transmission of the action potential. - Myelin doesn’t cover the entire axon; rather, it clumps around the axon with little break points between clumps. o These breakpoints are called the nodes of Ranvier - When an electric current passes down the length of a myelinated axon, the charge seems to “jump” from node to node rather than having to traverse the entire axon. o This is called salutatory conduction, and it helps speed the flow of information down the axon. CHEMICAL SIGNALING: TRANSMISSION BETWEEN NEURONS - Axons usually end in terminal buttons, which are knoblike structures that branch out from an axon. o Terminal buttons are filled with tiny vesicles, or “bags”, that contain neurotransmitters, chemicals that transmit information across the synapse to a receiving neuron’s dendrites. - The dendrites of the receiving neuron contain receptors, parts of the cell membrane that receive neurotransmitters and either initiate or prevent a new electric signal - As K+ and Na+ flow across a cell membrane, they move the sending neuron, or presynaptic neuron, from a resting potential to an action potential. - The action potential travels down the length of the axon to the terminal buttons, where it stimulates the release of neurotransmitters from vesicles into the synapse. - These neurotransmitters float across the synapse and bind to receptor sites on a nearby dendrite of the receiving neuron, or postsynaptic neuron - A new electric potential is initiated in that neuron, and the process continues down that neuron’s axon to the next synapse and the next neuron. - This electrochemical action, called synaptic transmission, allows neurons to communicate with one another and underlies our thoughts, emotions, and behaviour. - What tells the dendrites which of the neurotransmitters flooding into the synapse to receive? o Neurons tend to form pathways in the brain that are characterized by specific types of neurotransmitters; one neurotransmitter might be prevalent in a different part of the brain o Neurotransmitters and receptor sites act like a lock-and-key system. o Only some neurotransmitters bind to specific receptor sites on a dendrite. - What happens to the neurotransmitters left in the synapse after the chemical message is relayed to the postsynaptic neuron? - Neurotransmitters leave the synapse through three process: o First, reuptake occurs when neurotransmitters are reabsorbed by the terminal buttons of the presynaptic neuron’s axon o Second, neurotransmitters can be destroyed by enzymes in the synapse in a process called enzyme deactivation; specific enzymes break down specific neurotransmitters. o Finally, neurotransmitters can bind to the receptor sites called autoreceptors on the presynaptic neurons.  They detect how much of a neurotransmitter has been released into a synapse and signal the neuron to stop releasing. TYPES AND FUNCTIONS OF NEUROTRANSMITTERS - Acetylcholine (Ach) – a neurotransmitter involved in a number of functions, including voluntary motor control, was one of the first neurotransmitters discovered. o it is found in neurons of the brain and in the synapses where axons connect to muscles and body organs, such as the heart o activates muscles to initiate motor behaviour, but it also contributes to the regulation of attention, learning, sleeping, dreaming, and memory. - Dopamine – is a neurotransmitter that regulates motor behaviour, motivation, pleasure, and emotional arousal. o Because of its role in basic motivated behaviours, such as seeking pleasure or associating actions with rewards, dopamine plays a role in drug addiction. - Glutamate – is a major excitatory neurotransmitter involved in information transmission throughout the brain. o It enhances the transmission of information o GABA (gamma-aminobutyric acid), is the primary inhibitory neurotransmitter in the brain. o Inhibitory neurotransmitters stop the firing of neurons, an activity that also contributes to the function of the organism - Norepinephrine – a neurotransmitter that influences mood and arousal, is particularly involved in states of vigilance, or a heightened awareness of dangers in the environment o Serotonin is involved in the regulation of sleep and wakefulness, eating, and aggressive behaviour. - Endorphins –are chemicals that act within the pain pathways and emotion centers of the brain. - Even a slight imbalance can dramatically affect behaviour. HOW DRUGS MIMIC NEUROTRANSMITTERS - Agonists are drugs that increase the action of a neurotransmitter - Antagonists are drugs that block the function of a neurotransmitter. - If, by binding to a receptor, a drug activates the neurotransmitter, it is an agonist - If it blocks the action of the neurotransmitter, it is an antagonist. o For instance ingesting t-dopa will elevate the amount of L-dopa in the brain and spur the surviving neurons to produce more dopamine. L-dopa acts as an agonist for dopamine. - Some unexpected evidence also highlights the central role of dopamine in regulating movement and motor performance. o For example the six people from San Francisco were admitted with certain symptoms and were diagnosed with Parkinson’s disease. But infact they were all heroin addicts. - Many other drugs, including some street drugs, alter the actions of neurotransmitters o Methamphetamine – affects pathways for dopamine, serotonin, and norepinephrine at the neuron’s synapses, making it difficult to interpret exactly how it works.  But the combination of its agonist and antagonist effects alters the functions of neurotransmitters that help us perceive and interpret visual images. o Amphetamine – is a popular drug that stimulates the release of norepinephrine and dopamine.  Both amphetamine and cocaine prevent the reuptake of norepinephrine and dopamine and prevention of their reuptake floods the synapse with those neurotransmitters, resulting in increased activation of their receptors.  Both of these are strong agonists, although the psychological effects of the two drugs differ somewhat because of subtle distinctions in where and how they act on the brain. - Norepinephrine and dopamine play a critical role in mood control, such that increases in either neurotransmitter result in euphoria, wakefulness, and a burst of energy. - However, norepinephrine also increases heart rate. - An overdose of amphetamine or cocaine can cause the heart to contract so rapidly that heartbeats do not last long enough to pump blood effectively, leading to fainting and sometimes to death. - It has also been found that the size of a brain structure known as the amygdala, which plays an important role in recognizing expressions of fear, is reduced in regular cocaine users. o Prozac – a drug commonly used to treat depression, is another example of a neurotransmitter agonist.  Prozac blocks the reuptake of the neurotransmitter serotonin, making it part of a category of drugs called selective serotonin reuptake inhibitors, or SSRIs.  Patients suffering from clinical depression typically have reduced levels of serotonin in their brains.  By blocking reuptake, more of the neurotransmitter remains in the synapse longer and produces greater activation of serotonin receptors.  Serotonin elevates mood, which can help relieve depression. - An antagonist with important medical implications is a drug called propranolol, one of a class of drugs called beta-blockers that obstruct a receptor site for norepinephrine in the heart. - Because norepinephrine cannot bind to these receptors, heart rate slows down, which is helpful for disorders in which the heart beats too fast or irregularly. - Beta-blockers are also prescribed to reduce the agitation, racing heart, and nervousness associated with stage fright. THE ORGANIZATION OF THE NERVOUS SYSTEM - The nervous system is an interacting network of neurons that conveys electrochemical information throughout the body. DIVISIONS OF THE NERVOUS SYSTEM - There are TWO major divisions of the nervous system: o The central nervous system (CNS) is composed of the brain and spinal cord.  The central nervous system receives sensory information from the external world, processes and coordinates this information, and sends commands to the skeletal and muscular systems for action.  At the top of the CNS rests the brain, which contains structures that support the most complex perceptual, motor, emotional, and cognitive functions of the nervous system.  The spinal cord branches down from the brain; nerves that process sensory information and relay commands to the body connect to the spinal cord. o The peripheral nervous system (PNS) – connects the central nervous system to the body’s organs and muscles. - The PNS is itself composed of two major subdivisions: o The somatic nervous system is a set of nerves that conveys information into
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