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Chapter 2

Chapter 2 - Neuroanatomy.doc

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Ted Petit

Chapter 2: Neuroanatomy Module 2.1: Cells of the Nervous System • Every living thing made up of cells. • What makes human a higher-functioning organism is fact that humans have aggregates of specialized cells that perform specialized functions. • Brain composed of many parts, which have multiple functions, larger components of brain are made up of individual cells. • Neurons and glia are specialized cells of nervous system; they are specialized in both structure and function. • Glia provide support and functions, and neurons are communicators. • Neurons react and respond to stimuli, they are basis of behaviors. • Neurons also learn and store information about their external environment. Neurons and Glia: Structure and Function Gross Anatomy of the Neuron • Most distinctive structural feature of neuron is its shape. • Neurons shape is closely related to its function: to receive, conduct, and transmit signals – to collect information and send it on (or not). • Neurons consists of three main components: 1. Dentrites: Which receive incoming information from other neurons. 2. Soma or cell body: Which contain the genetic machinery and most of the metabolic machinery needed for common cellular functions. 3. Axon: Which sends neural information to other neurons. • Information is passed from axon to dendrite across a gap called synapse. • Basis of their positions relative to synapse, events that occur in the axon referred to as presynaptic and events that happened in dendrite referred as postsynaptic. • Dendrites essentially increase surface area available for reception of signals from axons of other neurons. • Extent of branching of dendrites gives indication of number of connections or synapses it makes w/ incoming axon. • All of this information is sent to rest of neuron in form of electrical charge or action potential. • Dendrites often covered w/ tiny spines, which grow and retract in response to experience. • Spines themselves can form synapses w/ other neurons. • Axon is commonly thought of as information sender. • Neuron has only one axon, although axon can divide at its far end into many branches (increasing number of synapses it can form). • Axon essentially long thin fibre or wire that can pass its message along many different cells simultaneously. • Many axons in mammalian nervous system are covered w/ insulation, called myelin. It helps speed rate of information transfer and ensure message gets to end of axon. • End of axon is terminal button. Information send from terminal button across synapse to dendrite. • Information that passes from axon across synapse is in form of neurochemical message (by substances referred to as neurotransmitters), which may be transformed into electrical message w/in dendrite. Internal Anatomy of the Neuron • Neuron is covered w/ a membrane. • Nothing obvious sets neural cell membranes apart from other animal cell membranes. • Plasma Membrane consists of bilayer of continuous sheets of phospholipids that separate two fluid environments – inside (cytoplasm) and outside the cell. • W/in membrane are proteins and channels that allow passage of materials into and out of neuron. • Inside main cell body, small components of the cell (called organelles) form complex environment which organelles perform various genetic (nucleus), synthetic (ribosomes, endoplasmic reticulum), and metabolic (mitochondria) processes that keep neuron functioning. • Nucleus packages and controls genetic information contained in DNA (Deoxyribonucleic acid). Nucleus processes genetic information needed to complete series of events that form path from the recipe that genetic information provides to form proteins that neuron needs. Nucleus also contains all genetic information needed to code proteins such as for eye or hair color, as well as those thought to underlie complex processes such as linguistic ability Structure and Functions of Neurons • Neurons can be classified according to structure and function. • In nervous system, structure and function are related. • Some common neurons are labelled as unipolar, bipolar, and multipolar (most common). Unipolar neurons have only one process emanating from cell body; bipolar neurons have two processes; and multipolar have numerous processes extending from cell body. Neurons w/ no axons or only very short axons are called interneurons and they tend integrate information w/in a structure rather than sending information between structures. • Functionally, neurons can be classified by type of signals they process. For example, signals motor neurons convey muscle contraction. Sensory neurons process information elicited from sensory-type stimuli, interneurons make connections between cells, enabling sort of convergence and combination of behavioural responses. Type of information that represented by neural activity related to function of neuron. • Neurons can be classified as being afferent (bringing information to central nervous system or structure) or efferent (sending information from brain or away from structure). • Neurons vary in size, shape, and function and that neuron can change shape as result of experience. Glia • Glia performs an essential role in functioning of central nervous system. • Glia performs support functions, different types of glia providing different types of support. • Support cells outside of brain and spinal cord called satellite cells. • At least three diff. types of glia: astrocytes, oligodendocrytes, and microglia. • Astrocytes are largest glia cells and named astrocytes because tend to be star- shaped. • Astrocytes involved in blood-brain barrier, protective system that keeps brain separate from rest of body. Astrocytes also perform nutritive and metabolic functions for neurons. Astrocytes also essential for regulation of chemical content of extracellular space because the envelope synapse and can regulate how far neurotransmitters and other substances released by terminal button can spread. Astrocytes important in storage of neurotransmitters. Clear we do not know all of the functions of astrocytes. • Oligodendrocytes one very clear function: to make myelin. Oligodendrocytes wrap their processes around most axons in brain and spinal cord. These processes made of myelin, which fatty substance that acts to insulate axon. • Axons outside brain and spinal cord frequently myelinated, w/ myelin provided by Schwann cells. • Schwann cells provide only one segment of myelin to an axon, oligodendrocytes can contribute many segments to many axon. • Microglia named w/ reference to their size – smallest of the glia. • Microglia is phagocytes remove debris from nervous system. Debris can accumulate in brain as result of injury, disease, infection or aging. Microglia very different from other cells of nervous system: made outside of brain and spinal cord by macrophages. Excessive activation of microglia been implicated in neurodegenerative diseases. Communication within the Neuron: The Action Potential • Neurotransmitter diffuses across synapse to interact w/ postsynaptic site, series of electrical events occur, some of which act to send information to other neurons and some which inhibit sending information to other neurons. • Electrical events underlie transmission (or inhibition) of information rely balance of ions between inside of neuron (intracellular) and outside of neuron (extracellular). When neuron is at rest, maintains electrical charge of about -70 mV, means electrical charge on inside of neuron is 70 mV less than charge on outside. The initial state of neuron called the resting potential. • Resting potential of neuron depends on difference between concentrations of ions across neuron membrane. Neuron contains variety of ions; ones that are important for understanding electrical properties of neuron are sodium ions (Na )+ and potassium ion (K ). At rest, extracellular fluid contains high concentration of + + Na , intracellular fluid contains high concentrations of K . In brain, ions concentrated either extracellular or intracellular. • Neuron has two properties that promote uneven distribution of ions. First property related permeability of cell membrane that covers neuron. Membrane is not permeable to all types of ions. Ions cross membrane through proteins + embedded in membrane, which known as ion channels. At rest, K readly + crosses mem+rane, whereas Na cannot easily +nter neuron. Given enough time, enough Na sneak into cell and enough K leak out of neuron that would be homogeneous distribution of ions. Second property of neuron promotes uneven distribution of ions is the neuron active transport of ions by neuron. Neurons + + actively import K and actively export Na through transport of mechanism known as sodium-potassium pump. Sodium-potassium pump requires neuron to use energy, ensuring that uneven distribution of ions is maintained. Sodium- potassium pump exchanges three Na ions inside cell for two K ions outside cell. • When neurotransmitter diffuses across synapse, open ion channels that allow rapid influx (inflow) of Na into neuron and rapid efflux (outflow) of K from neuron. Opening of sodium channels allow Na to rapidly enter neuron, makes the intracellular space more positive. When change in membrane potential moves from resting state of -70 mV to about +50 mV (change in membrane called depolarization), action potential occurs. When action potential occurs, neurotransmitters released from terminal buttons. Although action potentials occur entirely w/in one neuron, result in neurotransmitter release that results in communication between neurons. • As neuron becomes depolarized, K channels open, K ions rapidly leave neuron. Efflux of K triggers closing of sodium channels, and neuron returns to resting + state of -70 mV, called repo+arization. Because K channels take longer necessary to close, some additional K leaks out resulting temporary change in membrane beyond -70 mV (called hyperpolarization). • There are times when an action potential cannot be triggered. When neuron strongly depolarized, sodium channels close and cannot be reopened. Action potentials require movement of Na , inability to open sodium channels result in period of time during which action potential cannot be triggered (known as absolute refractory period). Second feature of action potentials is they are “all or none”. Once neuron becomes sufficiently depolarized, sodium channels open and action potential occurs. • Myelin not uniformly located on axon; number of small gaps in the myelin, known nodes of Ranvier. In myelinated neurons, ion channels and sodium- potassium pumps occurs only nodes of Ranvier. • An action potential first reaches axon, passively propagated to the first node of Ranvier. Initial depolarization results in production of new action potential at node Ranvier. Depolarization jumps to next node of Ranvier, sequence of events occurs again. Jumping of action potential from one node of Ranvier to another is called salutatory conduction, this series of events occur down the entire length of axon. Because action potential is actively propagated, neural transmission in myelinated neurons is faster than transmission in neurons w/out myelination. Communication between Neurons: The Synapse • Between neurons, communication is largely chemical. • Although most synapses are axodendritic, they consist of axons that form synapses with dendritic spines. • Axosomatic synapses made up of axons forming synapses w/ soma of neurons, and they are also very common. • Dendrodendritic synapses are dendrites forming synapses w/ other dendrites • Axoaxonic synapses are axons forming synapses w/ other axons. • Terminal button of axon contains dozens of small packages (vesicles) that contains neurotransmitters. Neurotransmitters located next to active zones, areas of protein accumulation on membrane that allow vesicle deposit contents into synapse. Neurotransmitter release triggered by arrival of an action potential at terminal button of axon. Action potential causes Calcium (Ca ) channels to open, Ca rushes into neuron. Increase in concentration of Ca causes neurotransmitters released into synapse by process known as exocytosis. Membrane of vesicles fuses w/ axon membrane results opening in vesicle (or pore), allowing neurotransmitter flow into synapse. • Once neurotransmitter been released, diffuses across synapse to produce postsynaptic effects. • Postsynaptic effects occur when neurotransmitter binds protein embedded in postsynaptic membrane known as receptor. Most part, receptors are specific, only one type of neurotransmitter can bind to given receptor. • Two types of receptor are located on postsynaptic membrane: transmitter-gated ion channels and G-protein-coupled receptors. Often dendrites will have mixture of two types of receptors located on their membrane. • Transmitter-gated ion channels or ionotropic receptor are proteins that control an ion channel. When neurotransmitter binds to transmitter-gated ion channel, channel changes conformation (either opening or closing). Ionotropic receptors result in quick changes in ionic concentrations and often appear situation in which fast response is required. • Functional consequence of receptor binding often depends on ion that controlled by receptor. • When dendrite is depolarized (moved towards producing action potential) by release of neurotransmitter from presynaptic site, call electrical event an excitatory postsynaptic potential (EPSP). • When dendrite is hyperpolarized (moved away from producing action potential) the release of neurotransmitter from presynaptic site, call electrical event an inhibitory postsynaptic potential (IPSP). • Unlike action potentials, postsynaptic potentials are not actively propagated; get smaller the farther they travel, and they can differ in degree to which they depolarize or hyperpolarize neuron. • G-protein-coupled receptors or metabotropic receptors produce slower, more diverse, and more sustained response than transmitter-gated ion channel receptors do. • Metabotropic receptors also occur more frequently. Metabotropic receptors use a multistep process to produce their responses, which begins w/ neurotransmitter binding to receptor. • Once neurotransmitter is bound, subunit of G-protein breaks away or can either move along inside of membrane and bind to ion channel or trigger synthesis of other chemicals. Binding of G-protein receptors can result in IPSPs or EPSPs, or they can result in gene expression. • Also neurotransmitter receptors on presynaptic membrane (autoreceptors). Autroreceptors are metabotropic receptors that located on presynaptic cell membrane and bind neurotransmitter released by presynaptic axon. Their primary function to regulate and monitor amount of neurotransmitter in synapse. • Must be some mechanism to terminate action of neurotransmitter binding to receptor; otherwise, once neurons activated, they would remain active. Neurotransmitter must be removed from synaptic cleft, or will rebind w/ receptor. • Two mechanisms responsible for terminating activity of neurotransmitters: reuptake and enzymatic degradation. Reuptake more common and involves presynaptic neuron reabsorbing neurotransmitter from synapse and repacking it in vesicles to be used again. Enzymatic degradation is when neurotransmitter broken down into inactive form by enzyme present in synapse. Often inactive forms are absorbed into presynaptic neuron to be resynthesized into neurotransmitter. Neurotransmitters • Variety of neurotransmitters, observed in specific types of neurons and associated w/ specific behaviors. • Neurotransmitters divided into small and large molecule neurotransmitters. W/in small molecule of neurotransmitter group are four classes of neurotransmitters: acetylcholine, monoamines, soluble gases, and amino acids. W/in large molecule group, there is only one class of neurotransmitter: neuropeptides. Most neurotransmitters are either excitatory or inhibitory. • Small-molecule neurotransmitters associated w/ fast responses, whereas large- molecule neurotransmitters are associated w/ slower, longer-lasting responses. Acetylcholine • Acetylcholine, Ach, first neurotransmitter to be identified and neurons that release this neurotransmitter called cholinergic. • ACh is synthesized by enzymatic conversion from choline, commonly
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