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Chapter 3

Chapter 3 - Neuroscience and Behaviour.odt

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Steve Joordens

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Chapter 3 – Neuroscience and Behaviour It was an unusual night, even for the late shift in the hospital emergency room. Seventeen-year- old David saw people who weren’t there and 75-year-old Betty saw, but didn’t recognize, her own husband. They discovered he was suffering from hallucinations–a side effect of abusing methamphetamine. David’s prolonged crystal meth habit had altered the normal functioning of some chemicals in his brain, distorting his perception of reality and “fooling” his brain into perceiving things that were not actually there. Doctors diagnosed Betty with a rare disorder called prosopagnosia, which is an inability to recognize familiar faces –a result of the brain damage caused by her stroke. The anticipation you have, the happiness you feel, and the speed of your feet are the result of information processing in your brain. In a way, all of your thoughts, feelings, and behaviors spring from cells in the brain that take in information and produce some kind of output trillions of times a day. These cells are neurons, cells in the nervous system that communicate with one another to perform information-processing tasks. Cajal was the first to see that each neuron was composed of a body with many threads extending outward toward other neurons. Surprisingly, he also saw that the threads of each neuron did not actually touch other neurons. Cajal discovered that neurons are complex structures composed of three basic parts: the cell body, the dendrites, and the axon. Like cells in all organs of the body, neurons 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. Functions such as protein synthesis, energy production, and metabolism take place here. The cell body contains a nucleus; this structure houses chromosomes that contain your DNA, or the genetic blueprint of who you are. The cell body is surrounded by a porous cell membrane that allows molecules to flow into and out of the cell. Unlike 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. The term dendrite comes from the Greek word for “tree”; indeed, most neurons have many dendrites that look like tree branches. The axon transmits information to other neurons, muscles, or glands. In many neurons, the axon is covered by a myelin sheath, an insulating layer of fatty material. The myelin sheath is composed of glial cells, which are support cells found in the nervous system.An axon insulated with myelin can more efficiently transmit signals to other neurons, organs, or muscles. In fact, with demyelinating diseases, such as multiple sclerosis, the myelin sheath deteriorates, slowing the transmission of information from one neuron to another. There’s a small gap between the axon of one neuron and the dendrites or cell body of another. 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. There are three major types of neurons, each performing a distinct function: sensory neurons, motor neurons, and interneurons. 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. 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. However, most of the nervous system is composed of the third type of neuron, interneurons, which 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. Besides specialization for sensory, motor, or connective functions, neurons are also somewhat specialized depending on their location. For example, 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. Pyramidal cells, found in the cerebral cortex, have a triangular cell body and a single, long dendrite among many smaller dendrites. Bipolar cells, a type of sensory neuron found in the retinas of the eye, have a single axon and a single dendrite. The communication of information within and between neurons proceeds in two stages— conduction and transmission. Together, these stages are what scientists generally refer to as the electrochemical action of neurons.As you’ll recall, the neuron’s cell membrane is porous: It allows small electrically charged molecules, called ions, to flow in and out of the cell. 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. 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. 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, which dissipated rapidly. When the shock reached the threshold, a much larger signal, the action potential, was observed. The action potential is all or none: Electric stimulation below the threshold fails to produce an action potential, whereas electric stimulation at or above the threshold always produces the action potential.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. 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. Myelin doesn’t cover the entire axon; rather, it clumps around the axon with little break points between clumps, looking kind of like sausage links. These breakpoints are called the nodes of Ranvier, after French pathologist Louis-Antoine Ranvier, who discovered them. 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. This process is called saltatory conduction, and it helps speed the flow of information down the axon. Axons usually end in terminal buttons, which are knoblike structures that branch out from an axon.Aterminal button is 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. 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.Anew 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 ultimately underlies your thoughts, emotions, and behavior. Neurotransmitters leave the synapse through three processes. First, re-uptake occurs when neurotransmitters are reabsorbed by the terminal buttons of the presynaptic neuron’s axon. Second, neurotransmitters can be destroyed by enzymes in the synapse in a process called enzyme deactivation; specific enzymes break down specific neurotransmitters. Finally, neurotransmitters can bind to the receptor sites called autoreceptors on the presynaptic neurons.Autoreceptors detect how much of a neurotransmitter has been released into a synapse and signal the neuron to stop releasing the neurotransmitter when an excess is present. - Acetylcholine (ACh), a neurotransmitter involved in a number of functions, including voluntary motor control, was one of the first neurotransmitters discovered.Acetylcholine is found in neurons of the brain and in the synapses where axons connect to muscles and body organs, such as the heart. Acetylcholine activates muscles to initiate motor behavior, but it also contributes to the regulation of attention, learning, sleeping, dreaming, and memory - Dopamine is a neurotransmitter that regulates motor behavior, motivation, pleasure, and emotional arousal. Because of its role in basic motivated behaviors, 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. This means that glutamate enhances the transmission of information. Too much glutamate can overstimulate the brain, causing seizures. GABA(gamma-aminobutyric acid), in contrast, is the primary inhibitory neurotransmitter in the brain. Inhibitory neurotransmitters stop the firing of neurons, an activity that also contributes to the function of the organism. Too little GABA, just like too much glutamate, can cause neurons to become overactive. - Norepinephrine, a neurotransmitter that influences mood and arousal, is particularly involved in states of vigilance, or a heightened awareness of dangers in the environment. Similarly, serotonin is involved in the regulation of sleep and wakefulness, eating, and aggressive behavior. Because both neurotransmitters affect mood and arousal, low levels of each have been implicated in mood disorders. - Endorphins are chemicals that act within the pain pathways and emotion centers of the brain. The term endorphin is a contraction of endogenous morphine, and that’s a pretty apt description. Morphine is a synthetic drug that has a calming and pleasurable effect; an endorphin is an internally produced substance that has similar properties, such as dulling the experience of pain and elevating moods. The drug LSD, for example, is structurally very similar to serotonin, so it binds very easily with serotonin receptors in the brain, producing similar effects on thoughts, feelings, or behavior. Many drugs that affect the nervous system operate by increasing, interfering with, or mimicking the manufacture or function of neurotransmitters. Agonists are drugs that increase the action of a neurotransmitter. Antagonists are drugs that block the function of a neurotransmitter. For example, a drug called L-dopa has been developed to treat Parkinson’s disease, a movement disorder characterized by tremors and difficulty initiating movement and caused by the loss of neurons that use the neurotransmitter dopamine. Dopamine is created in neurons by a modification of a common molecule called L-dopa. Ingesting L-dopa will elevate the amount of L-dopa in the brain and spur the surviving neurons to produce more dopamine. In other words, L-dopa acts as an agonist for dopamine. Many other drugs, including some street drugs, alter the actions of neurotransmitters. Let’s look at a few more examples. 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. Amphetamine is a popular drug that stimulates the release of norepinephrine and dopamine. In addition, both amphetamine and cocaine prevent the reuptake of nor-epinephrine and dopamine. The combination of increased release of norepinephrine and dopamine and prevention of their reuptake floods the synapse with those neurotransmitters, resulting in increased activation of their receptors. Nor-epinephrine 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. 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 Neurons are the building blocks that form nerves, or bundles of axons and the glial cells that support them. The nervous system is an interacting network of neurons that conveys electrochemical information throughout the body. There are two major divisions of the nervous system: the central nervous system and the peripheral nervous system. 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. The peripheral nervous system (PNS) connects the central nervous system to the body’s organs and muscles. The peripheral nervous system is itself composed of t
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