Physiological Psychology Textbook Notes Chapter 2: Structure and Functions of Cells of the Nervous System Ultimately, movement ? or, more accurately, behavior ? is the primary function of the nervous system. Brain must know what is happening outside, in the environment Body contains cells that are specialized for detecting environmental events Information, in the form of light, sound waves, odors, tastes, or contact with objects, is gathered from the environment by specialized cells called sensory neurons: neurons that detects changes in the external or internal environment and sends information about these changes to the CNS Movements are accomplished by the contraction of muscles, which are controlled by motor neurons: neurons located within the CNS that controls the contraction of a muscle or the secretion of a gland In between sensory and motor neurons come the interneurons: neurons located entirely within the CNS Local interneurons form circuits with nearby neurons and analyze small pieces of information Relay interneurons connect circuits of local interneurons in one region of the brain with those in other regions Through these connections, circuits of neurons throughout the brain perform functions essential to tasks such as perceiving, learning, remembering, deciding, and controlling complex behaviors Central Nervous System: brain and spinal cord Peripheral Nervous System: the part of the nervous system outside the brain and spinal cord, including the nerves attached to the brain and spinal cord Cells of the Nervous System The neuron (nerve cell) is the information-processing and information-transmitting element of the nervous system. Most neurons have, in one form or another, the following four structures or regions Cell body, or soma Contains nucleus Shape varies in different kinds of neurons Dendrites Branched, treelike structure Attached to the soma of a neuron Neurons ?converse? with one another, and dendrites serve as important recipients of these messages Receives information from the terminal buttons of other neurons Axon Long, thin, cylindrical structure that conveys information from the soma of a neuron to its terminal buttons Often covered by a myelin sheath Basic message it carries is called an action potential Brief electrical/chemical event that starts at the end of the axon next to the cell body and travels toward the terminal buttons Like a brief pulse Action potential is always of the same size and duration Each branch receives a full-strength action potential Come in different shapes Classified according to the way in which their axons and dendrites leave the soma Multipolar neuron: most common type found in the CNS; neuron with one axon and many dendrites attached to its soma Somatic membrane gives rise to one axon but to the trunks of many dendritic trees. Bipolar neuron: neuron with one axon and one dendrite attached to its soma, at opposite ends Usually sensory their dendrites detect events occurring in the environment and communicate information about these events to the CNS Unipolar neuron: a neuron with one axon attached to its soma; the axon divides, with one branch receiving sensory information and the other sending the information into the CNS Transmit sensory information from the environment to the CNS The arborizations (treelike branches) outside the CNS are dendrites The dendrites of most unipolar neurons detect touch, temperature changes, and other sensory events that affect the skin The arborizations within the CNS end in terminal buttons The CNS communicates with the rest of the body through nerves attached to the brain and spinal cord Nerves are bundles of many thousands of individual fibers, wrapped in a tough, protective membrane Nerve fibers transmit messages through the nerve, from a sense organ, to the brain; or from the brain to a muscle or gland. Terminal buttons The bud at the end of a branch of an axon; forms synapses with another neuron; sends information to that neuron Very special function When an action potential traveling down the axon reaches these terminal buttons, they secrete a chemical, called a neurotransmitter: a chemical that is released by a terminal button, and has an excitatory or inhibitory effect on another neuron Either excites or inhibits the receiving cell Thus helps to determine whether an action potential occurs in its axon An individual neuron receives information from the terminal buttons of axons of other neurons ? and the terminal buttons of its axons form synapses with other neurons Internal Structure Typical multipolar neuron (Figure 2.5 on page 33) Membrane: a structure consisting principally of lipid molecules, that defines the outer boundaries of a cell, and also constitutes many of the cell organelles, such as the Golgi apparatus Embedded in the membrane Variety of protein molecules that have special functions Some detect substances outside the cell (such as hormones) and pass information about the presence of these substances to the interior of the cell Other proteins control access to the interior of the cell, permitting some substances to enter, but barring others Other proteins act as transporters, actively carrying certain molecules into or out of the cell Nucleus: a structure in the central region of a cell, containing the nucleolus and chromosomes Nucleolus: structure within the nucleus of a cell that produces ribosomes Ribosomes: a cytoplasmic structure, made of protein, that serves as the site of production of proteins, (protein synthesis), translated from mRNA Chromosomes: strand of DNA (deoxyribonucleic acid), with associated proteins Found in the nucleus Carries genetic information When the chromosomes are active, portions of the chromosomes (genes) cause production of another complex molecule, messenger ribonucleic acid (mRNA), which receives a copy of the information stored at that location. The mRNA leaves the nuclear membrane, and attaches to ribosomes, where it causes the production of a particular protein Deoxyribonucleic acid: (DNA) a long, complex macromolecule consisting of two interconnected helical strands Along with associated proteins, strands of DNA constitute the chromosomes Gene: the functional unit of the chromosome, which directs synthesis of one or more proteins Messenger ribonucleic acid: (mRNA) a macromolecule that delivers genetic information concerning the synthesis of a protein from a portion of a chromosome to a ribosome Proteins are important in cell functions As well as providing structure, proteins serve as enzymes, which direct the chemical processes of a cell body by controlling chemical reactions Enzyme: a molecule that controls a chemical reaction, combining two substances or breaking a substance into two parts Enzymes are special protein molecules that act as a catalyst; that is, they cause a chemical reaction to take place without becoming a part of the final product themselves. Because cells contain the ingredients needed to synthesize an enormous variety of compounds, the ones that cells actually do produce depend primarily on the particular enzymes that are present There are enzymes that break molecules apart There are enzymes that put them together Non-coding RNA (ncRNA): a form of RNA that does not encode for protein, but has functions of its own For example When most genes become active, segments of DNA are transcribed into molecules of messenger RNA And then other molecules cut the mRNA into pieces, discard some parts, and splice the remaining pieces together The protein is then made from the resulting chunk of mRNA The cutting and splicing are accomplished by molecular complexes called spliceosomes, and one of the constituents of spliceosomes is non-coding RNA. Molecules of ncRNA also attach to ? and modify ? proteins that regulate gene expression Thus, the human genome, more broadly defined to include ncRNA, is much larger than biologists previously believed Cytoplasm: the viscous, semi-liquid substance contained in the interior of a cell Bulk of the cell consists of cytoplasm It is complex, and varies considerably across types of cells Characterized as a jellylike, semi-liquid substance that fills the space outlined by the membrane Contains small, specialized structures called organelles: Mitochondria: organelle that is responsible for extracting energy from nutrients Shaped like oval beads Formed of a double membrane Inner membrane is wrinkled ? make up a set of shelves (christae) that fill the inside of the bead Perform vital role of economy of the cell Many of the biochemical steps that are involved in the extraction of energy from the breakdown of nutrients take place on the christae, controlled by enzymes located there. Mitochondria thought to be once free-living organisms that came to ?infect? larger cells Because the mitochondria could extract energy more efficiently than the cells they infected could, the mitochondria became useful to the cells, and eventually became a permanent part of them. Cells provide mitochondria with nutrients, and mitochondria provide cells with a special molecule ? adenosine triphosphate (ATP) ? that cells use as their own immediate source of energy Adenosine triphosphate (ATP): a molecule of prime importance to cellular energy metabolism; its breakdown liberates energy Mitochondria contain their own DNA and reproduce independently of the cells in which they reside. Endoplasmic reticulum: Parallel layers of membrane found within the cytoplasm of a cell. Rough endoplasmic reticulum contains ribosomes The protein produced by the ribosomes that are attached to the rough ER is destined to be transported out of the cell or used in the membrane Involved with production of proteins that are secreted by the cell Unattached ribosomes are also distributed around the cytoplasm Which appear to produce protein for use within the neuron Smooth endoplasmic reticulum provides channels for the segregation of molecules involved in various cellular processes Lipid (fatlike) molecules are produced here Golgi apparatus: special form of smooth ER; a complex of parallel membranes in the cytoplasm that wraps the products of a secretory cell Some complex molecules, made up of simpler individual molecules, are assembled here Serves as a wrapping or packaging agent For example, secretory cells (such as those that release hormones) wrap their product in a membrane produced by the Golgi apparatus When the cell secretes its products, it uses a process called exocytosis: the secretion of a substance by a cell through means of vesicles; the process by which neurotransmitters are secreted. Neurons communicate with one another by secreting chemicals by this means The Golgi apparatus also produces lysosomes: small sacs that contain enzymes that break down substances no longer needed by the cell Contains enzymes that break down waste products These products are then recycled or excreted from the cell If a neuron grown in a tissue culture is exposed to a detergent, the lipid membrane and much of the interior of the cell dissolve away, leaving a matrix of insoluble strands of protein This matrix is called the cytoskeleton: formed of microtubules and other protein fibers, linked to each other and forming a cohesive mass that gives a cell its shape Microtubules: a long strand of bundles of thirteen protein filaments arranged around a hollow core; part of the cytoskeleton and involved in transporting substances from place to place within the cell Axons can be extremely long relative to their diameter and size of the soma Because terminal buttons need some items that can be produced only in the soma, there must be a system that can transport these items rapidly and efficiently through the axoplasm (that is, the cytoplasm of the axon) This system is known as axoplasmic transport: an active process by which substances are propelled along microtubules that run the length of the axon Movement from the soma to the terminal buttons is called anterograde axoplasmic transport (?toward the front? from cell body toward terminal buttons) Remarkably fast (500mm per day) Accomplished by protein molecules called kinesin In the cell body, kinesin molecules, which resemble a pair of legs and feet, attach to the item being transported down the axon The kinesin molecule then walks down a microtubule, carrying the cargo to its destination Energy is supplied by ATP molecules produced by the mitochondria Movement from the terminal buttons to the soma is called retrograde axoplasmic transport (?toward the back? from terminal buttons to cell body) Half as fast as anterograde axoplasmic transport Accomplished by a protein called dynein Supporting Cells Neurons constitute only about half the volume of the CNS Rest consists of a variety of supporting cells Because neurons have a very high rate of metabolism but have no means of storing nutrients, they must constantly be supplied with nutrients and oxygen or they will quickly die The most important supporting cells of the CNS are the neuroglia, or ?nerve glue? Glia (glial cells): glue the CNS together; supporting cells of the CNS Surround neurons and hold them in place Control neuronal supply of nutrients and some of the chemicals needed to exchange messages with other neurons Insulate neurons from one another So that neural messages do not get scrambled Act as housekeepers ? destroying and removing carcasses of neurons that are killed by disease or injury Several types of glial cells Three most important types: Astrocytes: a glial cell that provides support for neurons of the CNS, provides nutrients and other substances, and regulates the chemical composition of the extracellular fluid ?Star cell? Clean up debris within the brain Produce some chemicals that neurons need to fulfill their functions Regulate chemical composition of fluid surrounding neurons by actively taking up or releasing substances whose concentrations must be kept within critical levels Involved in establishing structures responsible for communication between neurons Provide nourishment to neurons (the arm of the star) wrapped around blood vessels; other processes are wrapped around parts of neurons, so the somatic and dendritic membranes of neurons are largely surrounded by astrocytes Although neurons receive some glucose directly from capillaries, they receive most of their nutrients from astrocytes Astrocytes receive glucose from capillaries and break it down to lactate, the chemical produced during the first step of glucose metabolism Then they release lactate in the extracellular fluid that surrounds neurons, and neurons take up the lactate, transport it to their mitochondria, and use it for energy Astrocytes store a small amount of a carbohydrate called glycogen that can be broken down to glucose and then to lactate when the metabolic rate of neurons in their vicinity is especially high Astrocytes surround and isolate synapses, limiting the dispersion of neurotransmitters that are released by terminal buttons When cells in the CNS die, certain kinds of astrocytes take up the task of cleaning away the debris When they contact a piece of debris from a dead neuron, they push themselves against it, finally engulfing and digesting it We call this process phagocytosis: ?to eat?; the process by which cells engulf and digest other cells or debris caused by cellular degeneration. Once dead tissue has been broken down, a framework of astrocytes will be left to fill in the vacant area, and a specialized kind of astrocyte will form scar tissue, walling off the area. Oligodendrocytes: a type of glial cell in the CNS that forms myelin sheaths Myelin sheath: a sheath that surrounds axons and insulates them, preventing messages from spreading between adjacent axons 80% lipid and 20% protein, produced by the oligodendrocytes, in the form of a tube surrounding the axon Does not form a continuous sheath Series of segments Bare portion of axon between adjacent oligodentroglia or Schwann cells is called a node of Ranvier Microglia: smallest of glial cells; act as phagocytes and protect the brain from invading microorganisms Primarily responsible for the inflammatory reaction in response to brain damage In the CNS, the oligodendrocytes support axons and produce myelin In the PNS, the Schwann cells perform the same function Schwann cell: a cell in the PNS that is wrapped around a myelinated axon, providing one segment of its myelin sheath Most axons in the PNS are myelinated. The myelin sheath occurs in segments, as it does in the CNS; each segment consists of a single Schwann cell, wrapped many times around the axon In the CNS, the oligodendrocytes grow a number of paddle-shaped processes that wrap around a number of axons In the PNS, a Schwann cell provides myelin for only one axon, and the entire Schwann cell ? not merely a part of it ? surrounds the axon Schwann cells also differ from their CNS counterparts, the oligodendrocytes, in an important way A nerve consists of a bundle of many myelinated axons, all covered in a sheath of tough, elastic connective tissue If damage occurs to such a nerve, Schwann cells aid in the digestion of the dead and dying axons. Then the Schwann cells arrange themselves in a series of cylinders that act as guides for re-growth of the axons The distal portions of the severed axons die, but the stump of each severed axon grows sprouts, which then spread in all directions. If one of these sprouts encounters a cylinder provided by a Schwann cell, the sprout will grow through the tube quickly (at a rate of up to 3-4mm per day), while the other, nonproductive sprouts wither away. If the cut ends of the nerve are still located close enough to each other, the axons will reestablish connections with the muscles and sense organs they previously saved. The glial cells of the CNS are not as cooperative as the supporting cells of the PNS If axons in the brain or spinal cord are damaged, new sprouts will form, as in the PNS; however the budding axons encounter scar tissue produced by the astrocytes, and they cannot penetrate this barrier. During development, axons have two modes of growth The first mode causes them to elongate so that they reach their target, which could be as far away as the other end of the brain or spinal cord. Schwann cells provide this signal to injured axons The second mode causes axons to stop elongating and begin sprouting terminal buttons because they have reached their target. Liuzzi and Lasek found that even when astrocytes do not produce scar tissue, they appear to produce a chemical signal that instructs regenerating axons to begin the second mode of growth: to stop elongating and start sprouting terminal buttons Thus, the difference in the regenerative properties of axons in the CNS and PNS results from differences in the characteristics of the supporting cells, not from differences in the axons There is another difference between oligodendrocytes of the CNS and Schwann cells of the PNS: The chemical composition of the myelin protein they produce The immune system of someone with MS attacks only the myelin protein produced by oligodendrocytes; thus, the myelin of the PNS is spared. The Blood-Brain Barrier Blood-brain barrier: a semi-permeable barrier between the blood and the brain produced by the cells in the walls of the brain?s capillaries For example, if blue dye is injected into an animal?s bloodstream, all tissues except the brain and spinal cord will be tinted blue. However, if the same dye is injected into the fluid-filled ventricles of the brain, the blue color will spread throughout the CNS Some substances can cross the blood-brain barrier; others cannot It is selectively permeable In most of the body, the cells that line the capillaries do not fit together absolutely tightly. Small gaps are found between them that permit the free exchange of most substances between the blood plasma and the fluid outside the capillaries that surrounds the cells of the body In the CNS, the capillaries lack these gaps; therefore, many substances cannot leave the blood Thus, the walls of the capillaries in the brain constitute the blood-brain barrier Other substances must be actively transported through the capillary walls by special proteins Glucose transporters bring the brain its fuel, and other transporters rid the brain of toxic waste products What is the function of the blood-brain barrier? Transmission of messages from place to place in the brain depends on a delicate balance between substances within neurons and in the extracellular fluid that surrounds them If the composition of the fluid is changed even slightly, the transmission of these messages will be disrupted, which means that the brain functions will be disrupted The presence of the blood-brain barrier Makes it easier to regulate the composition of this fluid Many of the foods we eat contain chemicals that would interfere with the transmission of information between neurons Blood-brain barrier prevents these chemicals from reaching the brain Not uniform throughout the nervous system Several places where the barrier is relatively permeable For example, the area postrema: part of the brain that controls vomiting; region of the medulla where the blood-brain barrier is weak; poisons can be detected there and can initiate vomiting Poison stimulates area postrema to initiate vomiting organism expels poison from stomach before causing too much damage Communication Within a Neuron Electrode: a conductive medium that can be used to apply electrical stimulation or to record electrical potentials Microelectrode: a very fine electrode, generally used to record activity of individual neurons Membrane potential: the electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell Oscilloscope: a lab instrument that is capable of displaying a graph of voltage as a function of time on the face of a cathode ray tube Resting potential: the membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials; approximately -70mV in the giant squid axon Depolarization: reduction (towards zero) of the membrane potential of a cell from its normal resting potential Hyperpolarization: an increase in the membrane potential of a cell, relative to the normal resting potential Action potential: the brief electrical impulse that provides the basis for conduction of information along an axon Threshold of excitation: the value of the membrane potential that must be reached in order to produce an action potential The Membrane Potential: Balance of Two Forces The process whereby molecules distribute themselves evenly throughout the medium in which they are dissolved is called diffusion movement of molecules from regions of high concentration to regions of low concentration (when there are no forces or barriers to prevent them from doing so) When some substances are dissolved in water, they split into two parts, each with an opposing electrical charge Substances with this property are called electrolytes: an aqueous solution of material that ionizes ? namely a soluble acid, base, or salt The charged particles into which they decompose are called ions: a charged molecule Ions are of two basic types: Cations: positively charged ions Anions: negatively charged ions Particles with the same kind of charge repel each other Particles with different charges attract each other The force exerted by this attraction or repulsion is called electrostatic pressure The fluid within cells (intracellular fluid) and the fluid surrounding them (extracellular fluid) contain different ions The forces of diffusion and electrostatic pressure contributed by these ions give rise to the membrane potential Because the membrane potential is produced by a balance between the forces of diffusion and electrostatic pressures, understanding what produces this potential requires that we know the concentrations of the various ions in the extracellular and intracellular fluids. Several important ions in these fluids: Organic ions (A-) Negatively charged proteins and intermediate products of the cell?s metabolic processes Found only in the intracellular fluid Chloride ions (Cl-) Found in both intracellular and extracellular fluids Predominantly found in extracellular Sodium ions (Na+) Found in both intracellular and extracellular fluids Predominantly found in extracellular Potassium ions (K+) Found in both intracellular and extracellular fluids Predominantly found in intracellular **Easy way to remember which ion is found where:** Recall that the fluid that surrounds our cells is similar to seawater, which is predominantly a solution of salt, NaCl. Sodium-potassium transporters: a protein found in the membrane of all cells that extrudes sodium ions from and transports potassium ions into the cell Sodium-potassium transporters very effectively keep the intracellular concentration of Na+ low Transporters that make up the sodium-potassium pump use considerable energy: up to 40% of a neuron?s metabolic resources are used to operate them Neurons, muscle cells, glia ? in fact, most cells of the body ? have sodium-potassium transporters in their membrane The Action Potential The sodium-potassium transporter is a protein molecule embedded in the membrane, and actively pumps sodium ions out of the cell, and potassium ions into it Another type of protein molecule provides an opening that permits ions to enter or leave the cells Ion channel: a specialized protein molecule that permits specific ions to enter or leave cells; contain passages (?pores?) t hat can open or close When an ion channel is open, a particular type of ion can flow through the pore and thus can enter or leave the cell Neural membranes contain many thousands of ion channels Movements of ions through the membrane during the action potential Threshold of excitation is reached, sodium channels in membrane open, and Na+ rushes in, propelled by the forces of diffusion and electrostatic pressure. The opening of these channels is triggered by reduction of the membrane potential (depolarization); they open at the point at which an action potential begins threshold of excitation. Because these channels are opened by changes in the membrane potential, they are called voltage-dependent ion channels: an ion channel opens or closes according to the value of the membrane potential. The influx of positively charged sodium ions produces a rapid change in the membrane potential, from -70mV to +40 mV The membrane of the axon contains ?voltage dependent? potassium channels, but since these channels are less sensitive than the voltage dependent sodium channels, and require a greater level of depolarization before they begin to open up, they ultimately open up later than the sodium channels When the action potential reaches its peak, the sodium channels become refractory ? the channels become blocked and cannot open again until the membrane once more reaches the resting potential. At this time, no more sodium can enter the cell By now, the potassium channels are open, and K+ ions are moving freely through the membrane. At this time, the inside of the axon is positively charged, so K+ is driven out of the cell by diffusion and by electrostatic pressure. The outflow of these cations causes the membrane potential to return toward its normal value, and so, the potassium channels begin to close again. Once the membrane potential returns to normal, the sodium channels reset, so that another depolarization can cause them to open again The membrane actually overshoots its resting value (-70mV) and only gradually returns to normal as the potassium channels finally close. Eventually, sodium-potassium transporters remove the Na+ ions that leaked in, and retrieve the K+ ions that leaked out Conduction of the Action Potential Movement of the message down the axon conduction of the action potential All-or-none law: the principle that once an action potential is triggered in an axon, it is propagated, without decrement, to the end of the fiber This law states that an action potential either occurs or does not occur; and once triggered, it is transmitted down the axon to its end An action potential always remains the same size, without growing or diminishing. If the action potential is an all-or-none event, how can it represent information that can vary in a continuous fashion? The answer is simple A single action potential is not the basic element of information; rather, variable information is represented by an axon?s rate of firing (referring to the production of action potentials) A high rate of firing causes a strong muscular contraction, and a strong stimulus causes a high rate of firing Thus, the all-or-none law is supplemented by the rate law: the principle that variations in the intensity of a stimulus or other information being transmitted in an axon are represented by variations in the rate at which that axon fires Action potentials are not the only kind of electrical signals that occur in neurons We must see how signals other than action potentials are conducted To do so, we produce a weak, sub-threshold depolarization (too small to produce an action potential) at one end of an axon and record its effects from electrodes placed along the axon We find that the stimulus produces a disturbance in the membrane potential that becomes smaller as it moves away from the point of stimulation The transmission of the weak, sub-threshold depolarization is passive, neither sodium nor potassium channels open or close. The axon is acting like an electrical cable, carrying along the current that started at one end. Property of the axon follows laws discovered in the nineteenth century that describe the conduction of electricity through telegraph cables laid along the ocean floor As a signal passes through an undersea cable, the signal gets smaller because of the electrical characteristics of the cable, including leakage through the insulator and resistance in the wire Signal decreases in size (decrements) decremental conduction We say that the conduction of a weak depolarization by the axon follows the laws that describe the cable properties of the axon ? the same laws that describe the electrical properties of an undersea cable. And because hyperpolarizations never trigger action potentials, these disturbances, too, are transmitted by means of the passive cable properties of an axon How does the ?action potential? travel along the area of axonal membrane covered by myelin sheath? By cable properties Axon passively conducts to the electrical disturbance from the action potential to the next node of Ranvier Action potential gets retriggered at each node and is passed, by means of cable properties of the axon, along the myelinated area to the next node. Such conduction is called saltatory conduction: conduction of action potentials by myelinated axons. The action potential appears to jump from one node of Ranvier to the next. Two advantages: First is economic ? sodium ions enter axons during action potentials, and these ions must eventually be removed. Sodium-potassium transporters must be located along the entire length of unmyelinated axons because Na+ enters everywhere However, because sodium can enter myelinated axons only at the nodes, much less gets in, and consequently, much less has to be pumped out again. Therefore myelinated axons expend much less energy to maintain their sodium balance The second advantage to myelin is speed Conduction of an action potential is faster in a myelinated axon because the transmission between the nodes, which occurs by means of the axon?s cable properties, is very fast Increased speed enables an animal to react faster and to think faster Communication Between Neurons The primary means of communication between neurons is synaptic transmission ? the transmission of message from one neuron to another through a synapse. Neurotransmitters produce postsynaptic potentials ? brief depolarizations or hyperpolarizations ? that increase or decrease the rate of firing of the axon of the post synaptic neuron Neurotransmitters exert their effects on cells by attaching to a particular region of a receptor molecule called the binding site: the location on a receptor protein to which a ligand binds. Shape of the molecule of the neurotransmitter and the shape of the binding site are complementary A chemical that attaches to a binding site is called a ligand Neurotransmitters are natural ligands produced and released by neurons Synapses are junctions between the terminal buttons at the ends of the axonal branches of one neuron and the membrane of another Synapses occur in three places On the dendrites ? axodendritic Can occur on the smooth surface of a dendrite or on dendritic spines: a small bud on the surface of a dendrite, with which a terminal button of another neuron forms a synapse On the soma ? axosomatic Occur on somatic membrane On other axons - axoaxonic Consist of synapses between two terminal buttons Presynaptic membrane: the membrane of a terminal button that lies adjacent to the postsynaptic membrane and through which the neurotransmitter is released; located at the end of the terminal button, faces the postsynaptic membrane. Postsynaptic membrane: the cell membrane opposite the terminal button in a synapse; the membrane of the cell that receives the message; located on the neuron that receives the message These two membranes face eachother across the synaptic cleft: the space between the Presynaptic membrane and the postsynaptic membrane Contains extracellular fluid, through which the neurotransmitter diffuses A meshwork of filaments crosses the synaptic cleft and keeps the Presynaptic and postsynaptic membranes in alignment In the cytoplasm of the terminal button are mitochondria and synaptic vesicles; we also see microtubules, which are responsible for transporting material between the soma and terminal button The presence of mitochondria implies that the terminal button needs energy to perform its functions Synaptic vesicles: small, hollow, beadlike structures found in terminal buttons; contains molecules of a neurotransmitter. Found in greatest numbers around the part of the Presynaptic membrane that faces the synaptic cleft ? near the release zone, the region from which the neurotransmitter is released Small synaptic vesicles are produced in the Golgi apparatus located in the soma and are carried by fast axoplasmic transport to the terminal button Large synaptic vesicles are produced only in the soma and are transported through the axoplasm to the terminal buttons Action potentials conducted down an axon number of small synaptic vesicles fuse with the membrane and then break open spill their contents into the synaptic cleft Action potential causes synaptic vesicles to release neurotransmitter Population of synaptic vesicles become ?docked? against the presynaptic membrane, ready to release their neurotransmitter into the synaptic cleft Docking accomplished when clusters of protein molecules attach to other protein molecules located in presynaptic membrane Release zone contains voltage-dependent calcium channels Membrane of terminal button depolarized by arriving action potential, calcium channels open Three distinct pools of synaptic vesicles Release-ready vesicles ? are docked against the inside of the presynaptic membrane, ready to release their contents when an action potential arrives Recycling pool ? 10 to 15% of the total pool of vesicles Reserve pool ? remaining 85 to 90% of the total pool of vesicles If the axon fires at a low rate, only vesicles from the release-ready pool will be called on If the rate of firing increases, vesicles from the recycling pool and finally from the reserve pool will release their contents
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