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Ted Petit (185)
Chapter 2

PSYB65 Chapter 2.docx

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Ted Petit

Chapter 2: Neuroanatomy  Cells of the Nervous System • Neurons and glia are the specialized cells of the nervous system o Glia provide support functions o Neurons react and respond to stimuli, basis of behaviour  Learn and store information about their external environment Neurons and Glia: Structure and Function Gross Anatomy of the Neuron • Neuron consist of three main components: o Dendrites – receive information from other neurons o Soma – or cell body, contains the genetic machinery and most of the metabolic machinery needed for common cellular functions o Axon – sends neural information to other neurons • Information is passed from the axon to the dendrite across a gap called synapse o Events occurring in the axon is presynaptic o Events happening in the dendrite is postsynaptic • Dendrites increase the surface area available for the reception of signals from the axons of other neurons o Sent in form of electrical charge or action potential o Covered with tiny spines  Can form synapses with other neurons • Axon thought of as an information sender o Neuron has only one axon  Axon can divide at its far end into many branches • Increasing the number of synapses it can form o Long thin fiber or wire that can pass its message along to many different cells simultaneously o Covered with insulation – myelin  Speed rate of information transfer and to ensure that the message gets to the end of the axon o End of axon is terminal button  Information is sent from the terminal button across the synapse to the dendrite Internal Anatomy of the Neuron • Plasma membrane – consists of a bilayer of continuous sheets of phospholipids that separate two fluid (H2O) environments – one inside the cell (cytoplasm) and the other outside the cell o Within membrane are proteins and channels that allow the passage of materials into and out of the neuron Structure and Function of Neurons • Unipolar neurons have only one process emanating from the cell body • Bipolar neurons have two processes • Multipolar have numerous processes extending from the cell body • Neurons with no axons or only very short axons are called interneurons o Integrate information within a structure rather than sending information between structures • Neurons can be classified by the type of signals that they process o Sensory neurons process information elicited from sensory-type stimuli o Interneurons make connections between cells o Afferent – bringing information to the CNS or structure o Efferent – sending information from the brain or away from a structure Glia • Perform an essential role in the functioning of the CNS • Perform support functions, different types of glia providing different types of support • Different types of glia: o Astrocytes  Largest glia  Fill space neuron neurons  Nutritive and metabolic functions for neurons  Regulate the chemical content of the extracellular space  Storage of neurotransmitters  Transmission of information in the nervous system o Oligodendrocytes  Make myelin  Wrap their processes around most axons in the brain and spinal cord • Made of myelin  Axons outside the brain and spinal cord are frequently myelinated • Myelin provided by Schwann cells o Only provide one segment of myelin to an axon o Oligodendrocytes contribute many segments to many axons o Microglia  Smallest of the glia  Phagocytes that remove debris from the nervous system  Debris can accumulate in the brain as a result of injury, disease, infection, or aging  Made outside of the brain and spinal cord by macrophages  Excessive activation of microglia has been implicated in neurodegenerative diseases • Eg. Multiple sclerosis andAlzheimer’s Communication within the Neuron: The Action Potential • Electrical events that underlie the transmission (or inhibition) of information rely on the balance of ions between the inside of the neuron (intracellur) and the outside of the neuron • Neuron at rest, electrical charge of -70 mV o Electrical charge on the inside of the neuron is 70 mV less than the charge on the outside o Initial state of the neuron is called resting potential + o At rest, extracellular fluid contains high concentrations of Na o Intracellular fluid contains high concentrations of K o Simple solutions, ions are distributed homogeneously that are found in equal amounts throughout the solution • In the brain, ions are concentrated in either the extracellular or intracellular fluid o Neurotransmitter diffuses across synapse  Open ion channels that allow rapid influx (inflow) of Na into the neuron and the rapid efflux of K from the neuron • When the change in the membrane potential moves from its resting state of -70 mV to +50 mV (depolarization), an action potential occurs o When action potential occurs, neurotransmitters are released from the terminal buttons o As the neuron becomes depolarized, K channels open, K ions rapidly leave the neuron +  Efflux of K triggers closing of the sodium channels • Neuron returns to its resting state (repolarization) + + • K channels take longer than necessary to close, some additional K leaks out o Temporary change in the membrane beyond -70 mV (hyperpolarization) • Several features of an action potential: o There are times when an action potential cannot be triggered  Inability to open sodium channels (absolute refractory period) o All or none  Once the neuron becomes sufficiently depolarized, sodium channels open and an action potential occurs o Myelination of axons  Small gaps in myelin (nodes of Ranvier) • Ion channels and sodium potassium pumps occur only at the nodes of Ranvier  Jumping of the action potential from one node of Ranvier to another is saltatory conduction • Occurs down the entire length of the axon Communication between Neurons: The Synapse • Most synapses are axodendritic o Consist of axons that form synapses with dendrtic spines • Dedrodendritic synapses o Dendrites forming synapses with other dendrites • Axoaxonic synapses o Axons forming synapses with other axons • Neurotransmitter release is triggered by arrival of an action potential at the terminal button of the axon 2+ o Action potential causes calcium channels to open, and Ca rushes into the neuron o Increase in concentration of Ca causes the neurotransmitter to be released into the synapse by exocytosis  Membrane of the vesicle fuses with the axonal membrane (at the active zone), results in an opening in the vesicle allowing the neurotransmitter to flow into the synapse o Postsynaptic effects occur when the neurotransmitter binds to a protein embedded in the postsynaptic membrane known as a receptor o Most part, receptors are specific  Only one type of neurotransmitter can bind to a given receptor o Two types of receptors located on the postsynaptic membrane:  Transmitter-gated ion channels  G-protein-coupled receptors  *Often dendrites will have a mixture of the two type of receptors located in their membrane • Frequently bind different neurotransmitters • Transmitter-gated ion channels/ionotropic receptors o Proteins that control an ion channel o When a neurotransmitter binds to a transmitter-gated ion channel, channel changes conformation (open/close) o Ionotropic receptors result in quick changes in ionic concentrations and often appear in situations in which a fast response is required o Functional consequence of receptor binding often depends on the ion that is controlled by the receptor o Excitory postsynaptic potential (EPSP)  When dendrite is depolarized by the release of a neurotransmitter from the presynaptic site o Inhibitory postsynaptic potential (IPSP)  Dendrite is hyperpolarized by the release of a neurotransmitter from the presynaptic site o Postsynaptic potentials are not actively propagated  Smaller the farther they travel, differ in the degree to which they depolarize or hyperpolarize the neuron • G-protein-coupled receptors/metabotrpic receptors o Produce slower, more diverse, and more sustained responses than transmitter-gated ion channel receptors do o Occur more frequently in the nervous system than do transmitter-gated ion channel receptors o Use multistep process to produce their responses  Begins with neurotransmitter binding to receptor • Subunit of G-protein breaks away and can either move along the inside of the membrane and bind to an ion channel or trigger the synthesis of other chemicals o Binding of G-protein receptors can result in IPSPs or EPSPs, or result in changes in gene expression o *G-protein receptors can have more diverse effects than inotropic receptors o Neurotransmitter recepetors on the presynaptic membrane  Autoreceptors • Primary function to regulate and monitor the amount of neurotransmitter in the synapse • Located on the presynaptic cell membrane and bind the neurotransmitter related by the presynaptic axon • Terminating the activity of neurotransmitters: o Reuptake and enzynmatic degradation  Reuptake is more common and involves the presynaptic neuron reabsorbing the neurotransmitter from the synapse and repacking it in vesicles to be used again  Enzymatic degradation is when a neurotransmitter is broken down into an inactive form by an enzyme present in the synapse • Inactive forms are absorbed into the presynaptic neuron to be resynthesized into the neurotransmitter Neurotransmitters • Divided into small and large molecule neurotransmitters o Either excitory or inhibitory • Small-molecule neurotransmitters tend to be related in a direct fashion, activating either ionotropic or metabotropic receptors that act directly on ion channels o Fast responses (excitatory/inhibitory) • Large-molecule neurotransmitters tend to be released diffusely, activating metabotrpic receptors, and produce either metabolic or genetic alterations within the neuron o Slower, longer-lasting responses Acetylcholine • First neurotransmitter to be identified • Neurons that release this are called cholinergic • Used by all motor neurons in the brain and spinal cord • Synthesized by enzymatic conversion from choline o Commonly found in vegetables and egg yolk • Ach is deactivated into choline and acetic acid by acetylchlinesterase (AChE) o Choline is reabsorbed presynpatically • Rate of degradation is very fast • Receptors forAch: o Muscarinic and nicotinic receptors  Most common is muscarinic • Commonly found throughout the rain and in cardic and smooth muscle  Nicotinic receptors are ionotropic and excitatory • Only occur in few locations within the brain Monoamines • Derived from single amino acid from tryptophan (indoleamines) and those derived from tyrosine (catecholamines) o Both readily available in diet  Eg. Meat and dairy products • Serotonin o Only indoleamine neurotransmitter and relatively rare in the nervous system involved in brain systems that regulate eating, sleep, and emotion behaviour o Removed from the synapse by reuptake into the presynaptic neuron o Drugs that affect rate at which 5-HT is reabsorbed are potent antidepressants o Almost all 5-HT are metabotropic • Three catecholaminergic neurotransmitters: o Dopamine o Norepinephrine o Epinephrine • Catecholamine-containing neurons are numerous in the nervous system o Regulate movement, mood, motivation, and attention • Dopaminergic neurons are located in the areas of the brain involved with movement and reward o All metabotropoic o Depletion of DAin the brain occurs in those with movement disorder Parkinson’s o Drugs that stimulate release of DA (eg. amphetamines) are very addictive • Adrenergic neurons located throughout the brain o Many in the locus coeruleus o Act as a neurotransmitter in the brain, and also as a hormone that is released by the adrenal glands o Play role in mediating the hormonal effects of catecholamines o Drugs used to treat acute symptoms of asthma affect adrenergic neurons Soluble Gases • Most recently discovered group of neurotransmitters • NO and CO rapidly synthesized within the nervous system and undergo very rapid degeneration • NO does not need a receptor to produce effects o Not sure whether CO is a neurotransmitter o NO released from the postsynaptic site and acts on presynaptic site o Viagra affects NO, NO plays important role in the brain’s ability to learn Amino Acids • Apartate, glutamate, glycine, gamma-aminobutyric acid (GABA) • Most part, receptors for the amino acids are all ionotropic o Involved in fast responses within nervous system  Learning, and memory • Most prevalent excitatory amino acid neurotransmitter is glutamate • Most prevalent inhibitory amino acid neurotransmitter is GABA Neuropeptides • Endorphins, substance P, cholycystoinin, and insulin • Endorphins play role in pain medication o Drugs act as endorphin receptors, providing potent relief from pain • Substance P is a neurotransmitter that plays a significant role in sensory transmission o Especially touch, temperature and pain
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