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Neurons: how are signals transmitted (Action Potentials)
Chapter 23 The Nervous System’s Functional Units Neurons: Structure Neurons: how are signals transmitted (Action Potentials) Neurotransmitters Nervous tissue is made up of neurons and neuroglial cells. The neuroglial cells are supporting cells and they may be regulators, but the neurons are the functional units of the NS
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Brain Research through Advancing Innovative Neurotechnologies initiative: $100 million project
On April 2nd, President Barack Obama announced the BRAIN project. “ As humans we can identify galaxies light years away, we can study particles smaller than an atom. But we still haven’t unlocked the mystery of the three poinds of matter that sits between our ears.” -President Obama On April 2nd, President Barack Obama announced that America’s government will back a project intended to unlock the mysteries of the human brain >100 billion neurons in the human brain alone!!
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Nobel Prize in Physiology or Medicine 2014 for discovering the inner GPS of the brain
John O'KeefePrize May-Britt Moser Edvard I. Moser The Nobel Prize in Physiology or Medicine 2014 was "for their discoveries of cells that constitute a positioning system in the brain".
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FUNCTIONS OF THE NERVOUS SYSTEM
The nervous system—present in almost all multicellular animals—has three primary functions. RECEIVE INPUT The nervous system collects information about the internal and external environment. PROCESS INFORMATION The nervous system interprets the incoming stimuli and determines a response. INITIATE RESPONSE The nervous system sends signals to muscles and glands in response to the internal and external environment.
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Nervous System Functions
Pathways of Information Flow Receive sensory input Integrate the input Respond to stimuli Somatic & Autonomic
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THE VERTEBRATE NERVOUS SYSTEM
In vertebrates, the nervous system is divided into the central nervous system and the peripheral nervous system. CENTRAL NERVOUS SYSTEM Composed of the neurons and other supporting cells that make up the brain and spinal cord Brain Spinal cord 31 spinal nerves and 123 cranial nerves PERIPHERAL NERVOUS SYSTEM Composed of neurons that detect stimuli and neurons that transmit signals to the muscles and glands Neurons (bundled together into nerves)
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NEURON STRUCTURE Neurons—individual cells that specialize in carrying electrical signals—are the building blocks of the nervous system. They are composed of three distinct elements. Stimulus DENDRITES Sense and respond to stimulation from outside the cell and send that information toward the cell body CELL BODY Contains the nucleus and other cellular machinery AXON Long tube-like projection that extends from the cell body and transmits signals to other cells GLIAL CELLS Support cells that protect, insulate, and nourish the neurons
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Neurons come in different shapes, but all have some common features
DIRECTION OF SIGNAL
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Axons Cell Body Dendrites
Myelinated = 330 mi/hr Unmyelinated ~10 mi/hr
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Nerve signals flow from dendrites to axons
One neuron: Two neurons: SYNAPSE
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Support cells: Glial cells Protect, insulate and nourish
Each bead-like structure that is strung along an axon is a single oligodendrocyte Microglia Microglia process harmful bacteria and act as the brain's immune cells.
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Astrocytes Astrocytes can release gliotransmitters
(like glutamate) by exocytosis to send signals to neighboring neuron End-feet" connect to blood vessels in the brain. And regulate local blood flow to neurons
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Myelin Sheaths Axons are insulated, by a fatty coating called the myelin sheath, preventing the action potential from weakening as it travels down the axon. The lack of myelin on an axon can be seen in babies when they first start trying to walk and their gross motor control isn’t very good.
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Neurons Come in Three Types
There are three types of neurons: sensory neurons, which collect information from the five senses; interneurons, which communicate messages from sensory neurons to motor neurons; and motor neurons, which initiate a reaction to the communicated stimuli.
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Three Types of Neurons 1 Sensory Neuron Interneuron 2 3 Motor Neuron
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B. dendrites; motor neurons C. axons; sensory neurons
You take a sip of coffee, and it’s too hot. The ___ of your __ receive this stimulus A. axons; motor neurons B. dendrites; motor neurons C. axons; sensory neurons D. dendrites; sensory neurons E. axons; interneurons Answer: 4
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Sensory neurons: from skin and joints are affected by syphilis
Interneurons: affected by Parkinson’s Motor neurons: affected by polio
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Drawing of neuronal circuit by Santiago Ramon y Cajal: 1901
Modern day photomicrographs
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Clarity: See through Brain Nature: April 10, 2013
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How a “reflex action” occurs
1 2 3 4 5
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Ouch!! What is letter G? A. Effector cells B. Interneuron
C. Motor neuron D. Sensory receptors E. Synapse Ouch!! Ans: A
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How do neurons send “signals”?
How does the nerve cell speak Through Action potentials and neurotransmitters
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Human Connectome Project
Map of the network of connections between neurons in the human brain:Connectomes Signals are transmitted from one brain region to another Connectome of roundworm C. elegans has been done (300 neurons and 7000 connections) TED talk: I am my connectome
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Resting Potential Action Potential
Neurons are excitable cells (their membrane potential can change rapidly) Membrane potential (action potential) depend on ions Resting Potential Action Potential -70 mV +30mV (depolarized)
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Using Giant Axon of Squid to figure out nerve impulses
Voltage gated channels Sodium gates Potassium gates Pottasium channels (non-gated ion channels) Sodium potassium pumps
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Dendrites receive external stimuli.
A neuron may have hundreds or even thousands of dendrites. Dendrites receive stimuli in one of two ways. Those on motor neurons and interneurons generally connect with and receive signals from other neurons. Sensory neuron dendrites, on the other hand, are modified to respond to a specific external stimulus such as a touch or sound, light, or a chemical.
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Axon Terminal Buttons Regardless of the species under observation, each neuron has one axon. This projection leaves the cell body and can extend several feet or more. At its end, the axon branches into several (or hundreds of) axon terminals, also called terminal buttons, which are knob-like ends of the axon, positioned very close to a muscle cell or gland or to the dendrites of another neuron. And in response to an action potential, these axon terminals release the contents of vesicles, small sacs of chemicals inside the axon terminal, into the space between the cells, potentially influencing adjacent cells (Figure 23-9).
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The action potential travels along a neuron
1 2 3 Axon Action potential Axon segment Na+ K+
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If these channels do not work, disorders can result.
Channelopathies (disorders of ion channels) Epilepsy: potassium channel mutations Muscle weakness Ciguatoxin: Shellfish: blocks sodium channel (numbness, muscle weakness) Neurotoxin by puffer fish paralyzes
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The action potential is an “all-or-nothing” spike
Action potential is triggered only when threshold potential is reached - Always the same size
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The presence of myelin allows an axon to ___
Produce more frequent action potentials Conduct impulses more rapidly Produce action potentials of larger amplitude Produce action potentials of longer duration Ans: B
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How are powerful stimuli sensed?
The intensity of the sensation depends on the number the number of neurons that fire. Action potentials become more frequent for a strong signal Figure 23-9 Heavy or light? We experience gradations of sensation depending on the number of neurons that fire.
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Pick the term that describes the membrane potential changes from -80mV to -70mV
Hyperpolarize Depolarize Repolarize Ans: B
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A junction between a neuron and another cell is called a synapse.
How do neurons pass information? A junction between a neuron and another cell is called a synapse. Signals are passed between neurons mostly by Chemical signals (neurotransmitters) in humans Action potentials travel WITHIN neurons. Neurotransmitters travel BETWEEN neurons (at synapses).
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Vesicles release neurotransmitters into the synapse
Action potential Presynaptic membrane Positively charged ions Vesicles Neurotransmitter molecules Synaptic cleft Postsynaptic membrane Receptor proteins Ion channels This causes a channel to open, allowing ions to flow into the cell, stimulating or inhibiting it. neurotransmitters are then released from the receptors, and either taken back in and recycled by the initial axon , or are broken down by enzymes 1 2 Neurotrans-mitters bind to receptor on the adjacent cell membrane 3 4 Vesicles release neurotransmitters into the synapse
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Neurotransmitters must be removed from the synapse
They are either destroyed by enzymes, or.. Re-absorbed by the neuron that released them Medications and recreations drugs can interfere with these processes
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At the Synapse, Several Things Occur…
The action potential in a pre synaptic neuron triggers calcium to enter the neuron. Calcium causes sacs called vesicles to release neurotransmitters into the synaptic cleft. 2. The neurotransmitter diffuses to nearby receptor sites of a post synaptic neuron. 3. The neurotransmitter attaches to postsynaptic receptors. 4. Gates open in the postsynaptic cell membrane.
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At the Synapse, Several Things Occur…
5. Open gates enable the signal to pass to the postsynaptic cell. 6. Neurotransmitter is released from the postsynaptic cell receptors. 7. Neurotransmitter is recycled by being taken back into the pre-synaptic neuron or broken down by enzymes.
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What is labeled as #4 ? Transporter Neurotransmitter Vesicles Receptor
5 1 What is labeled as #4 ? Transporter Neurotransmitter Vesicles Receptor 2 3 Ans: D Synaptic vesicle full of neurotransmitter Synaptic cleft Neurotransmitter receptor Calcium Channel Fused vesicle releasing neurotransmitter Neurotransmitter re-uptake pump 4 6
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Neurotransmitters Examples
Excitatory: increases membrane potential and increases chance for threshold to be reached Inhibitory: decreases membrane potential and decreases chance for threshold to be reached Examples 1. Acetylcholine: stimulates muscle contraction 2. Dopamine : feeling good 3. Serotonin: feeling good, Sleep, appetite and mood, pain 4. Endorphins: decrease pain 5. Glutamate: Excitatory NT 6. GABA: Inhibitory NT
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Acetylcholine Acetylcholine is the neurotransmitter released by motor neurons at the point where they synapse with muscle cells. When enough acetylcholine binds to a muscle cell, the muscle contracts. Deficiency: Alzheimers Autoimmune disease: destroy Ach receptors: Myasthenia gravis
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Curare is a poison that works by blocking the receptor sites where acetylcholine binds to muscle cells Botox Botulinum toxin—a protein produced by several species of bacteria—is the most toxic substance known: less than a microgram is lethal if injected into a person or inhaled. Also known as Botox, this toxin is an increasingly popular drug used in cosmetic procedures, with more than a billion dollars spent on it worldwide each year (Figure 23-14). What does this have to do with neurotransmitters? When injected into muscles, Botox blocks the fusion of acetylcholine-filled vesicles with the presynaptic neuron’s membrane, preventing release of acetylcholine into the synapse and thus paralyzing the muscle. A tiny amount of Botox can prevent a muscle from contracting for three or four months! Tiny injections of Botox can smooth lines in the forehead and can also reduce “crow’s-feet” wrinkles around the eyes. Because Botox essentially paralyzes muscles, however, it can lead to problems.
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Glutamate and GABA (work together)
Glutamate: excitatory NT and over half of all brain synapses release glutamate. It starts action potential or keeps it going. GABA: inhibitory NT. Released by 30-40% of brain synapses and it stops action potentials. Caffeine ____ glutamate and _____ GABA activity Caffeine increases glutamate and decreases GABA activity
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Serotonin generally is an inhibitory NT.
Dopamine controls movement and posture. Also chief happiness NT. Dopamine Deficiency causes Parkinsons. Loss of control of motor functions Serotonin generally is an inhibitory NT. It affects appetite, sleep, anxiety, pain, mood and produces feelings of contentment and satiation Problems: anxiety disorders, OCD, and depression
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Drugs can hijack pleasure pathways
Our nervous system can be tricked by chemicals Drugs—whether recreational or therapeutic, whether found in nature or made in the laboratory—can work by mimicking neurotransmitters The French poet Jacques Prevert tinkers with synaptic activity in his brain, in Paris, 1955. Drugs can literally trick our brains by mimicking the neurotransmitter action we have seen earlier in the chapter. Examples of Drugs Cocaine Morphine and Heroin Nicotine Prozac, Zoloft Ritalin
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Cocaine, Prozac, and Zoloft
When someone snorts a bit of cocaine, it crosses the blood-brain barrier and quickly reaches the brain’s pleasure centers. Once there, the cocaine binds to the sites on the presynaptic membrane where dopamine is normally reabsorbed from the synaptic cleft by the cells that originally released it. As long as these reuptake sites are blocked by the drug, dopamine released by that neuron remains in the synaptic cleft, repeatedly stimulating the postsynaptic cell and causing nonstop activity in the pleasure center. Antidepressants work by a mechanism almost identical to that of cocaine. In addition to dopamine, another neurotransmitter important in our brain’s pleasure centers is serotonin. The antidepressants Prozac and Zoloft block serotonin from being reabsorbed and recycled by the presynaptic cells that released it, prolonging its effect (see Figure 23-47). This is why these drugs are called selective serotonin reuptake inhibitors (SSRIs). The net result is that people taking these medications generally experience an elevated mood because of serotonin’s prolonged stay in the synaptic cleft.
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Morphine and Heroin Mimic endorphins and bind to their receptor sites.
Nicotine Mimics acetylcholine. Fooled by the nicotine binding to acetylcholine receptors, cells release adrenaline and other stimulating chemicals, including pleasure-causing dopamine. Rapid surges, then rapid depletions. The endorphins are our body’s natural painkillers. Produced by our brain, endorphins block pain messages arriving from throughout the body. Under a variety of situations of extreme stress—such as if an individual has just been seriously injured in a fight or is in mile 12 of a half-marathon—the body responds by releasing endorphins. Shortly after entering the bloodstream, nicotine begins mimicking one of the body’s most common and important neurotransmitters: acetylcholine. Fooled by the nicotine binding to acetylcholine receptors, cells release adrenaline and other stimulating chemicals, including pleasure-causing dopamine. Nicotine causes rapid surges, then rapid depletions, of these chemicals, leaving smokers happy for a short while but soon yearning for another cigarette
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Do human drug addictions and dependencies reflect differences in our genes?
The allele of the DRD4 gene found among one-third of the smokers in a study was the same one that appears to cause individuals to exhibit a variety of personality traits associated with risk-taking and novelty-seeking behavior.
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Alcohol interferes with many different neurotransmitters.
Alcohol fools at least four different receptor molecules. Blocks glutamate receptors Blocks dopamine reuptake Releases endorphins Increases efficiency of serotonin receptors Alcohol is a great neurotransmitter impersonator, fooling at least four different receptor molecules. Let’s look at some of its effects (Figure 23-51).
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Alcohol interferes with many different neurotransmitters.
Dopamine, serotonin, gaba, glutamate Alcohol ___ GABA activity and ___ Glutamate activity. Increases ; Increases Increases; Decreases Decreases; Increases Decreases; Decreases Figure Alcohol impersonates several neurotransmitters, with several consequences. Alcohol is a great NT impersonator fooling at least four different receptor molecules Ans:B
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