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CHAPTER 1 NEUROSCIENCE PAST PRESENT AND FUTURE THE ORIGINS OF NEUROSCIENCE The nervous system is composed of 0 Brain 0 Spinal cord 0 Nerves of the body Trepanation boring holes in each other s skulls with the aim to cure The heart was initially considered to be the seat of the soul and the holder of memories Views of the Brain in Ancient Greece There is a correlation between structures and function The structure of the head shows that o The heard is specialized for sensing the environment o It contains ears eyes nose and tongue The brain is the organ of sensation Hippocrates believed that the brain was involved in sensation and was also the seat of intelligence Aristotle believed that the heart was the center of intellect and that the brain was a radiator for cooling the blood that was overheated by the head Views of the Brain During the Roman Empire Galen Greek physician and writer was a believer of the Hippocratic view of the brain and conducted most of his observations on the brain of a sheep He noted the cerebellum hard and cerebrum soft which were in the back and the front respectively This caused Galen to come to the conclusion that the cerebrum was the recipient of sensations and perceptions while the cerebellum was in control of the muscles movement control center He also discovered that the brain is hollow and contains ventricles with fluid in them Sensations were registered and movements initiated by the movement of humors tofrom the brain ventricles via the nerves Views of the Brain From he Renaissance to the Nineteenth Century Andreas Vesalius added more detail to the structure of the brain Hydraulically controlled mechanical devices supported the notion that the brain could be machinelillte in its function 0 Le fluid forced out of the ventricles through the nerves would pump up a person and cause limb movement Descartes was a chief advocate of the fluid mechanic theory 0 He proposed that the brain mechanisms control human behavior only up until a certain extent 0 He also believed that the mind was a spiritual entity that received sensations and commanded movements by communicating with the brain using the pineal gland The brain is divided into two parts gray matter and white matter The white matter is continuous with the nerves of the body and contains the fibers that bring information to and from the gray matter The nervous system has 0 A central division Central Nervous System that consists of the brain and the spinal cord 0 A peripheral division Peripheral Nervous System that consists of the network of nerves that course through the body and nerve cells that lie outside the brain and the spinal cord A pattern of bumps gyri and grooves sulci and fissures enable the parceling of the cerebrum into lobes 0 Led to the speculation that different functions might be localized to the different bumps on the brain The brain has three major parts 0 Cerebrum o Cerebellum 0 Brain Stem Lobes of the Cerebrum NineteenthCentury Views of the Brain Injury to the brain can disrupt sensations movement and thought and can even lead to death The brain and the body communicate via nerves The brain has different parts which perform different functions The brain operates like a machine and follows the laws of nature Nerves as Wires Galvani and Bois Reymond showed that muscles could be caused to twitch when the nerves were stimulated with electricity and that the brain itself could generate electricity o Displaced the notion that nerves communicate with the brain by the movement of fluid o Replaced by concept that nerves are wires that conduct electrical signals to and from the brain Each nerve contains nerve fibers which could serves as individual wires carrying information in different direction Just before the nerves attach to the spinal cord he fibers divide into two branches o The dorsal branch enters towards the back of the spinal cord 0 The ventral branch enters towards the front of the spinal cord Bell tested the possibility that the two spinal roots branches carried information in different directions by cutting each root separately and examining the consequences 0 Cutting only the ventral roots led to paralysis Magendie was able to show that the dorsal roots carry sensory information into the spinal cord 0 Bell and Magendie concluded that each nerve contains a mixture of wires that bring information into the brain and the spinal cord and others that send information out to the muscles o In sensory and motor nerve fiber the transmission is one way and they are bundled together for strength but are anatomically segregated when they enter or exit the spinal cord Localization of Specific Functions to Different Parts of the Brain 1811 Bell proposed that the origin of the motor fibers is the cerebellum and the destination of the sensory fibers is the cerebrum Experimental Ablation Method a method in which parts of the brain are systematically destroyed to determine their function 0 Technique employed by Bell and Magendie to identify the functions of the spinal roots 0 Used by Flourens to show that the cerebellum plays a role in the coordination of movement I He also concluded that the cerebrum is involved in sensation and perception Had solid experimental proof for his conclusions Gall believed that the bumps on the surface of the skull reflected the bumps on the surface of the brain and proposed that certain traits for related to the dimensions of the head 0 Phrenology correlating the structure of the head with personality traits Gall was heavily criticized by Flourens who performed experimental ablations to show that the particular traits weren t isolated to the potions of the cerebrum specified by phrenology o Flourens also maintained that all regions of the cerebrum participate equally in all cerebral functions I Later proven wrong Broca is credited with localization of function in the cerebrum 0 Presented with man that could understand language but not speak 0 Following his death Broca examined the brain and found a lesion in the left frontal lobe 0 Based on his case and others like it he concluded that this region of the cerebrum was responsible for production of speech There is a clear division of labor in the cerebrum with different parts performing different functions The Evolution of Nervous Systems DanNin articulated the theory of evolution 0 Theory of Evolution the species of organisms evolved from a common ancestor 0 Differences among species arise from natural selection I Due to reproduction the traits of the offspring are different from those of the parents and if they represent an advantage for survival the offspring will reproduce and pass down those traits He also noted that behavior among the heritable traits could evolve 0 Same reaction when frightened ie widening of the pupils heart racing hair standing on end Indicated that different species evolved from a common ancestor with similar behavioral traits Many behavioral traits are highly specialized for the environment a species occupies o Monkeys that swing from branch to branch have better sight than the rats that go through underground tunnels The Neuron The Basic Functional Unit of the Brain 1839 Schwann proposed the cell theory all tissues are composed of microscopic units called cells Even though cells in the brain had been identified and described it wasn t known for a fact if nerve cells were the basic unit of brain function Nerve cells have a number of think projections that extend from a central cell body Neuron individual nerve cell recognized as the basic functional unit of the nervous system NEUROSCIENCE TODAY Levels of Analysis Reductionist Approach breaking down a problem into smaller pieces for systematic experimental analysis Level of Analysis determined by the size of the unit of study In ascending order of complexity these are molecular cellular systems behavioral and cognitive Molecular Neuroscience Study of the brain at the foremost eve Brain matter consists of a variety of molecules which play different roles crucial to brain function Messengers allow neurons to communicate with one another Sentries control what materials can enter or leave neurons Conductors orchestrate neurons and are archivist of past experiences Cellular Neuroscience Focuses on how all the molecules work together to give the neuron its special properties Basic questions asked like how many different types of neurons are there and how do they differ in function How do neurons influence other neurons etc Systems Neuroscience Neurons form complex circuits that perform a common function 0 Vision or voluntary movement Neuroscientists study how different neural circuits analyze sensory information form perceptions of the external world make decisions and execute movements Behavioral Neuroscience How do neurons work together to produce behaviors Cognitive Neuroscience Studies how the activity of the brain creates the mind Neuroscientists Neuroscience research is divided into two types 0 Clinical mainly conducted by physicians 0 Experimental conduced by MD or a PhD The main medical specialties that are associated with the human nervous system are neurology psychiatry neurosurgery and neuropathology Neuroanatomists used sophisticated microscopes to trace connections in the brain Neurophysiologists use electrodes amplifiers and oscilloscopes to measure the brains electrical activity Neuropharmacologists use designer drugs to study the chemistry of the brain function Molecular neurobiologists probe the genetic material of neurons to find clues about the structure of brain molecules The Scientific Approach Consists of four steps observation replication interpretation and verification Observation Made during experiments that are designed to test a particular hypothesis Can also be derived by watching the world around us carefully from introspection or from human clinical cases Replication Repeating the experiment before it is accepted as fact It is repeated on different subjects and as many times as needed to rule out the possibility that the observation occurred by chance Interpretation Once the observation is believed to be correct the scientist makes an interpretation Depends on the state of knowledgeignorance at the time the observation was made and on preconceived notions of the scientist who made it Verification Means that the observation is sufficiently robust that any scientist who follows the protocols of the original observer wi reproduce it Successful verification means that the observation is accepted as fact Not all observations can be verified sometimes due to inaccuracies or insufficient replication The Use of Animals in Neuroscience Research Most of what we know about the nervous system has come from experimentation on animals The Animals Animals that are used for research education and testing are a small fraction of the total used for other purposes Neuroscience experiments are conducted using many different species which ranges from snails to monkeys 0 Choice of animal species is dictated by the question under investigation level of analysis and the extent to which the knowledge gained at this level can be related to humans The more basic the process under investigation the more distant the evolutionary relationship with humans Animal Welfare There are certain moral responsibilities towards animal subjects 0 Animals are used only for worthwhile experiments that promise to advance our knowledge of the nervous system 0 All necessary steps are taken to minimize pain and distress experienced by the experimental animals 0 All possible alternatives to the use of animals are considered Adherence to this choice is monitored in a number of ways 0 Research proposals must pass a review by the IACUC o Proposals are evaluated for scientific merit by a panel of expert neuroscientists 0 he papers are carefully reviewed by other neuroscientist for scientific merit and for animal welfare concerns Animal Rights Animal welfare must not be confused with animal rights The Cost of Ignorance Nervous System Disorders DISORDER Alzheimer s disease Cerebral palsy Depression Epilepsy Multiple scierosis Parkinson39s disease Schizophrenia Spinal paralysis Stroke DESCRIPTlON A progressive degenerative disease oi the brain characterized by dementia and always iatal A motor disorder caused by damage to the cere brum at the time oi birth A serious disorder oi mood charaCtenized by insom nia loss oi appetite and ieeiings oi dejection A condition diaracter39rzed by periodic disturbances oi brain electrical activity that can lead to seizures loss oi consciousness and sensory disturbances A prOgnessive disease that aiiects nerve conduction characterized by episodes oi weakness lack oi coordination and speech disturbance A progressive disease oi the brain that leads to diffi culty in initiating voluntary movement A severe psychotic illness characterized by delusions hallucinations and bizarre behavior A loss oi ieeling and movement caused by traumatic damage to the spinal cord A loss oi brain iunction caused by disruption oi the blood supply usually leading to permanent sensory motor or cognitive deficit Over 33 million people experience a major depressive illness at some point in their lives Schizophrenia strikes at the prime of life and can persist for life Stroke is the third leading cause of death in the US 0 Those that do not die are likely to be permanently disabled As many or more Americans are hospitalized with neurological and mental disorders than with any other major disease group including heart disease and caner Prevention of brain disorders requires an understanding of normal brain function which is the goal of neuroscience It has contributed to the increasingly effective treatments for Parkinson s Depression and Schizophrenia CONCLUDING REMARKS The goal of neuroscience is to understand how nervous systems function 10 QHAPTER 2 NEURONS AND GLIA INTRODUCTION 0 All tissues and organs in the body are made up of cells 0 The specialized function of cells and their interactions determine the functions of organs 0 The nervous system has two different types of cells 0 Neurons o Glia Neurons sense changes in the environment and communicate these changes to other neurons and command the body s response to these sensations Glia glial cells are thought to contribute to brain function by insulating supporting and nourishing the neighboring neuron o If the brain were a chocolate chip cookie then the neurons would be the chocolate chips and the glia would be the cookie dough that lls all the other space and makes sure that the chips are suspended in the appropriate locations THE NEURON DOCTRINE Most cells are in the rage of 001 005mm in diameter 0 Brain tissue has the consistency of jelly which makes it hared to make thin slices 3 Tissue can be hardened or xed by immersing them in formaldehyde and then be cut into thin slices using a microtome g Histology the microscopic study of the structure of tissues 3 Freshly prepared brain has a creamcolored appearance under a microscope Q Breakthrough was stains which could selectively color some parts of the cell 3 Franz Nissl introduced the Nissl Stain in which a class of basic dyes would stain the nuclei of all cells along with staining clumps of material that surrounded the nuclei of neurons 3 Nissl stain is useful for two reasons Q It distinguishes the neurons and the glia from each other Q Enables histologists to study the arrangement for crytoarchitecture of neurons in different parts of the brain Study of crytoarchitecture led to the realization that the brain has many specialized regions The Golgi Stain Golgi discovered that by soaking brain tissue in a silver chromate solution Golgi Stain a small percentage of neurons became darkly colored lt revealed that the neuronal cell body is actually a small fraction of the total structure of the neuron Q Neuronal cell body is the area of the neuron around the nucleus The Golgi stain showed that neurons have at distinguishable parts Q A central region that contains the cell nucleus Q Numerous thin tubes that radiate away from the central region least two Cell body soma perikaryon are all interchangeable names for the swollen region that contains the cell nucleus Neurites thin tubes that radiate away from the soma Q There are two types of neurites axons and dendrites Cell body gives rise to a single axon which is of uniform diameter with branches that extend at right angles Q They were recognized to behave like wires that carry the output of neurons Dendrites extend from the cell body and taper to a ne point Q It was recognized that due to dendrites coming in contact with many axons they acted as antennae of the neuron to receive input Dendrites ff Myefin sheath Nucleus quot Schwari quot m 39 EEIIS Eff quot Terminals r L Nudes of Ranvier Caial s Contribution Cajal used the Golgi stain to work out the circuitry of many regions of the brain Cajal argued that neurites of different neurons aren t continuous with one another and must communicate by contact not continuity Golgi argued that the neurites of different cells were fused together to form a continuous reticulum network like the arteries and veins of the circulatory system The idea that the neuron adhered to the cell theory came to be known as the neuron doctrine It was nally proven that the neurites of different neurons are not continuous with one another Q Starting point for the exploration of the brain is the individual neuron THE PROTOTYPICAL NEURON Neuron nerve cell is made up of several parts Q Soma Q Dendrites Q Axon The inside of the neuron is separated by the outside via the neuronal membrane The Soma The soma is the rough spherical central part of the neuron Cytosol the watery uid inside the cell potassium rich solution that is separated from the outside by the neuronal membrane Organelles number of membraneenclosed structures Cell body of the neuron contains the same organelles that are found in all animal cells Most important ones are the nucleus rough ER smooth ER the Golgi apparatus and the mitochondria Cytoplasm refers to everything in the cell including the organelles except for the nucleus membrane The Nucleus The nucleus of the cell is spherical and centrally located It is contained in the nuclear envelope which is perforated by pores Chromosomes contain doublestranded braid of DNA Q Although the DNA is the same all throughout the body what distinguishes the neuron from a liver cell and a kidney cell are the speci c parts of the DNA that are used to assemble the cell Genes segments of DNA Gene Expression reading of the DNA Q Final product of gene expression is synthesis of protein Protein synthesis the assembly of protein molecules in the cytoplasm Messenger Ribonucleic Acid mRNA consists of four different nucleic acids that are strung together in various sequences to form a chain Q Carries the genetic message to the sites of protein synthesis Transcription process of assembling the piece of mRNA that contains the information of a gene 1 Resulting mRNA is called the transcript At the one end of a gene is the promoter which is the region where RNA polymerase binds to initiate transcription At the other end is the terminator that the RNA polymerase recognizes as the end point for transcription Transcription factors regulate the binding of the polymerase to the promoter There are additional stretches of DNA that cannot be used for protein Q These regions are called sequences are called exons introns and the coding RNA splicing is a process in which the introns are removed and the exons are fused together Q In some cases speci c exons are also removed with the introns to leave an alternately spliced mRNA that encodes a different protein Q Transcription of a single gene can give rise to several different mRNAs and protein products FIGURE 13 tran nptlyffj mi Fillu r f hiigitslf i are swtlltrri5irecil b Hamlin TF i tl iT E i iH ErrE F T I39 39 nte FM EEEFEH HEWsf HE BIT Fll39lf EH l i j i f fwiz gm r tr TILEin mn the milk ifquot it Fragsa g l mriphm Eb d m alteri the Frattiter region iii the game Em wl ll itematriaquot migrant The With Milli rm use 39HF39LEIL39H 39r i rr 39u39e the letting tat car b pmiwk 3 mRNA transcripts emerge from the nucleus via the pores in the nuclear envelope and travel to the sites of protein synthesis elsewhere in the neuron Q The protein molecule is assembled by linking together many small molecules into a chain Q The building blocks are amino acids 20 different kinds Q The assembling of proteins from amino acids under the direction of the mRNA is called translation 3 Molecular Biology the study of the process that begins with the DNA of the nucleus and ends with the synthesis of protein molecules in the cell Itaizscnmpiitm1 39lrawisla itm flH r a MEIER 2 L4 Fraulein 0Molecular Neurobiology information in the genes is used to determine the structure and function of neuronal proteins Rough Endoplasmic Reticulum g Ribosomes are dense globular structures that dot the rough ER 3 The rough ER has more neurons than the glia or other non neuronal cells Q Rough ER are Nissl bodies ngH r Hugger FEE Rough ER is where protein synthesis occurs in neurons Q RNA transcripts bind to the ribosomes which translate the instructions contained in the mRNA to assemble a protein molecule Q They take raw material in the form of amino acids and manufacture proteins using the blueprint provided by the mRNA 3 A lot of the ribosomes are free ribosomes which basically means that they aren t attached to the rough ER Q Polyribosomes the free ribosomes that are attached by a thread 1 The thread is a single strand of mRNA and the ribosomes that are working on it are making copies of the same protein 3 Proteins that are synthesized on the free ribosomes are destined for the cytosol 3 Proteins that are synthesized on the rough ER are destined to be enclosed by or inserted into the membrane or an organelle Q They are inserted into the membrane as they are assembled J Eraaimmnm unu m b Fromm synrimming naughiEH mgath Endoplasmic Reticulum and the Golgi Apparatus 3 Smooth ER stacks of membranous organelles without the ribosomes g It performs many different functions in many different locations 3 Some smooth ER is continuous with rough ER and that is where the proteins that jut out from the membrane are carefully folded to give them heir 3D like structure 3 Some smooth ER regulate the internal concentrations of substances like calcium 3 Smooth ER is prominent in muscle cells where it is also known as sarcoplasmic reticulum g Golgi apparatus the stack of membraneenclosed dicks in the soma that lie furthest from the nucleus Q Site of extensive posttranslational chemical processing of proteins Q It sorts certain proteins that are to be delivered to different parts of the neuron like the axon and the dendrites The Mitochondria g Mitochondria site of cellular respiration g Cristae folds in the inner membrane of the mitochondria Littler Mambaam i Iquot e rmmnm me 1 rm l e Matrix V Hi liarareal minim irrigaer LEFUQHF A lib When a mitochondrion inhales it pulls inside pyruvic acid which enters into the Krebs cycle Q Pyruvic acid is made up of sugars and digested proteins and fat The Krebs cycle provides the energy that in another series of reactions with the cristae that results in the addition of phosphate to adenosine phosphate to give ATP Q ATP is the cells energy source Q 17 ATP molecules are released for every molecule of pyruvic acid that is taken in The energy stored in ATP is used to fuel most of the biochemical reactions of the neuron Special proteins in the neuronal membrane use the energy released by the breakdown of ATP into APD to pump certain substances across the membrane to establish concentration difference between the inside and the outside of the cell 10 The Neuronal Membrane iNeuronal Membrane serves as a barrier to enclose the cytoplasm inside of the neuron and to exclude certain substances that oat in the uid surrounding the neuron oThe membrane is studded with proteins Some of the membraneassociated proteins pump substances into the membrane while others form pores to regulate which substances are allowed inside the cell The protein composition of the membrane varies on whether it is in the soma the dendrites or the axon The Cytoskeleton The cytoskeleton gives the neuron it s shape The cytoskeleton is made up of o Microtubules o Micro laments o Neuro laments Elements of the cytoskeleton are in continual motion Microtubules Microtubules are big and they run longitudinally down the neurtes It has the appearance of a thickwalled hollow pipe which the wall of the pipe being composed of smaller strands that are braided around the hollow core The smaller strands are composed of tubulin oA single tubulin molecule is small and globular o The strands consist of tubulins that are stuck together Polymerization the process ofjoining small proteins to form a long strand 0 Polymer the resulting strand Microtubuleassociated proteins MAPS class of proteins that participate in the regulation of microtubule assembly and funcUon oThey also anchor the microtubules to one another and to other parts of the neuron Micro laments Micro laments are about the same thickness as the cell membrane and are found throughout the neuron o Particularly numerous in the neurites 11 They are braids of two thin strands which are made up of actin oActin is one of the most abundant proteins in cells oActin laments are critically involved in the mechanism of muscle contraction oActin micro laments are constantly undergoing assembly and disassembly 0 Process is regulated by signals in the neurons oThey also run longitudinally down the core of the neurites and are closely associated with the membrane Anchored to the membrane by attachments with a meshwork of brous proteins that like the inside of the membrane like a spider web Neuro laments oNeuro laments are intermediate in size between microtubules and micro laments oThey exist in all cells of the body as intermediate laments 0 Only in neurons are they known as Neuro laments oThe difference in names is representative of subtle differences in structure An example of an intermediate lament is keratin which when bundled together makes hair Neuro laments most closely resemble the bones and ligaments of the skeleton It consists of multiple subunits that are organized like a chain of sausages The internal structure of each subunit consists of three protein strands that are woven together 0 The strands consist of individual long protein molecules each of which is coiled in a tight spiral like con guration 0 Makes neuro laments mechanically very strong The Axon The axon is found only in neurons 0 Highly specialized for the transfer of information over distances in the nervous system o It begins with a region known as the axon hillock which tapers to form the initial segment of the axon proper 12 77 quot quot 1 i ll nillmh modquot I l collatemle l 11 o The axon differs from the soma in the sense that o No rough ER extends into the axon and there are barely any free ribosomes o The protein composition of the axon membrane is fundamentally different from that of the soma membrane 0 No ribosomes means no protein synthesis in the axon o All proteins in the axon must originate in the soma 0 Different proteins in the axonal membrane enable it to send information over great distances Branches of the axon are known as axon collaterals o Occasionally the axon collateral will return to communicate with the same cell that gave rise to the axon or with the dendrites of the neighboring cells 0 These axon branches are known as recurrent collaterals Variation in axon size is important 0 Speed of the electrical signal that sweeps down the axon nerve impulse varies depending on the axonal diameter 0 The thicker the axon the faster the impulse travels The Axon Terminal All axons have a beginning axon hillock a middle axon proper and an end axon terminal 0 Axon terminal is also known as terminal bouton Appearance of a swollen disk 13 o The terminal is the site where the axon comes in contact with other neurons and passes information on to them 0 Point of contact is known as synapse Terminal arbor refers to the branches that form a synapse on dendrites or cell bodies in the same region 0 Sometimes axons form synapse at swollen regions along their length and then continue on to terminate elsewhere 0 Known as boutons en passant When a neuron makes synaptic contact with another cell it provides innervation The cytoplasm of the axon terminal differs from that of the axon in several ways 0 Microtubules do not extend into the terminal 0 The terminal contains numerous small bubbles of membrane known as synaptic vesicles 0 The inside surface of the membrane that faces the synapse has a particularly dense covering of proteins 0 It has numerous mitochondria indicating a high energy demand The Synapse The synapse has two sides 0 Presynaptic generally consist of an axon terminal 0 Postsynaptic generally consists of a dendrite or soma or another neuron The names are indicative of the usual direction of information ow from pre to post 0 The space between the presynaptic and postsynaptic membranes is called the synaptic cleft The transfer of information at the synapse from one neuron to another is called synaptic transmission 0 Information in the form of electrical impulses travels down the axon and is converted in the terminal into a chemical signal that crosses the synaptic cleft On the postsynaptic cleft the chemical signal is converted again into an electrical one o Called a neurotransmitter o It is stored in and released from the synaptic vesicles within the terminal Synaptic transmission dysfunction accounts for certain mental disorders 0 Synapse is also the site of action for many toxins and most psychoactive drugs 14 Axoplasmic Transport The cytoplasm of axons doesn t have ribosomes o This means that the proteins of he axon need to be synthesized in the soma and then shipped down the axon Wallerian degeneration the degeneration of axons that occurs when they are cut 0 Axons cannot be sustained when separated from their parent cell body Wallerian degeneration occurs because the normal ow of materials from the soma to the axon terminal is interrupted Axoplasmic Transport the movement of material down the axon How does axoplasmic transport work 0 Material is enclosed within the vesicles 0 It then walks downquot the microtubules of the axon o The legs are provided by kinesin and ATP fuels the process 0 Kinesin moves the material from the soma to the terminal 0 All the movement in this direction is known as anterograde transport There is a mechanism for the movement of material p the axon from the terminal to the soma o This process provides signals to the soma about changes in the metabolic needs of the axon terminal 0 Movement from the terminal to the soma is known as retrograde transport The molecular mechanism is similar to anterograde transport except the legs for retrograde transport are provided by dynein Dend rites Dendrites resemble the branches of a tree and extend from the soma The dendrites of a single neuron are called a dendritic tree 0 Each branch from the tree is called a dendritic branch The variety of shapes and sizes of dendritic trees are used to classify different groups of neurons Dendrites are covered with thousands of synapses because they function as the antennae of neurons The dendritic membrane under the synapse has specialized protein molecules known as receptors 0 These detect the neurotransmitters in the synaptic cleft The dendrites of some neurons are covered with specialized structures called dendritic spines many 15 o The spines look like little punching bags that hang off the dendr e They are believed to isolate various chemical reactions that are triggered by some types of synaptic activation Spine structure is sensitive to the type and amount of synaptic activity The cytoplasm of dendrites resembles that of axons 0 Filled with cytoskeletal elements and mitochondria Polyribosomes can be observed in dendrites right under th spines Research suggests that synaptic transmission can direct oca protein synthesis in some neurons CLASSIFYING N JRQN gla All the neurons in the brain can be divided into a small number of categories 0 Within each category all neurons function identically gi cation Based on the Number of Neurites Neurons can be classi ed according to the total number of neurites that extend from the soma A neuron that has a single neurite is unipolar A neuron that has two neurites is bipolar A neuron that has three or more neurites is multipolar Most neurons in the brain are multipolar Classi cation Based on Dendrites Classi cation is unique to a particular part of the brain Q For example in the cerebral cortex there are two broad classes 1 Stellate ces starshaped 1 Pyramidal cells pyramidshaped Another way to classify neurons is whether or not the dendrites have spines Q Those that do are called spiny and those that don t are called aspinous 16 1 In the cerebral cortex all pyramidal cells are spiny and all stellate cells are either spiny or aspinous glassi cation Based on Connections OPrimary Sensory Neurons cells with connections that deliver information to the nervous system via neurons that have neurites in the sensory surfaces of the body 39 Other neurons have axons that form synapses with the muscles and command movements and are known as motor neurons Most neurons in the nervous system form connections only with other neurons Q These cells are called interneurons glassi cation Based on Axon Length O Golgi type I neurons projection neurons neurons with long axons that extend from one part of the brain to the other 1 Golgi type II neurons local circuit neurons neurons with short axons that do not extend beyond the vicinity of the cell body glassi cation Based on Neurotransmitter 39 Motor neurons that command voluntary movements a release acetylcholine at their synapses o Classi ed as cholinergic Collections of cells that use a common neurotransmitter make up the brain s neurotransmitter systems GLIA Glia contribute to brain function mainly by supporting neuronal funcUons Astrocytes Most numerous glia in the brain 0 They ll the spaces between neurons and in uence whether a neurite can grow or retract They regulate the chemical content of the extracellular space 17 0 They also have special proteins in their membranes that actively remove many neurotransmitter receptors that can trigger electrical and biochemical events inside the cell Tightly control the extracellular concentration of several substances that have the potential to interfere with proper neuronal function 0 Le regulation of concentration of potassium ions in the extracellular uid Myelinating Glia Oligodendroglial and Schwann cells provide layers of membrane that insulate axons A wrapping known as myelin spirals around axons in the brain 0 Myelin sheath describes the entire covering The sheath is interrupted periodically leaving a short length where the axonal membrane is exposed Known as node of Ranvier Myelin speeds up the propagation of nerve impulses down the axon Oligodendroglial and Schwann cells differ in their location and some characteristics 0 Oligodendroglia are found only in the central nervous system while Schwann cells are found only in the peripheral nervous system 0 One oligodendroglial cell will contribute myelin to several axons while each Schwann cell myelinates only a single axon cher NonNeuronal 0 Special cells called ependymal cells provide the lining of uid lled ventricles within the brain 0 Also play a role in directing cell migration during brain development Microglia function as phagocytes to remove debris left by dead or degenerating neurons and glia QONCLUDING REMARKS Structure correlates with function 0 Absence of ribosomes in the axon means that proteins in the axon terminal must be provided by the soma via axoplasmic transport 0 A large number of mitochondria in the axon terminal mean that there is a highenergy demand 18 The structure of the dendritic tree is suited for the receipt of incoming information and is where most of the synapses are formed with the axons of other neurons An important feature of neurons is the rough ER 19 QHAPTER 3 THE NEURONAL MEMBRANE AT REST INTRODUCTION 0 Even simple re exes require the nervous system to collect distribute and integrate information 0 An example of this is a person stepping on a pin The process of screaming occurs when the breaking of the skin by the pin is translated into neural signals that travel up the long sensory nerves of the leg In the spinal cord these signals are transferred to interneurons Some of the neurons connect with parts of the brain that interpret the signals as being painful Others connect to the motor neurons that control the leg muscles that withdraw the foot 0 The neuron conducts information over long distances by using electrical signals that sweep along the axon o The type of signal that is used by the neuron is constrained by the special environments of the nervous system 0 The electrical charge in the cytosol of the axon is carried by ions instead of free electrons 0 Makes the cytosol less conductive than a copper wire 0 The axon is not well insulated and is bathed in salty extracellular uid which conducts electricity 0 Electrical current passively conducts down the axon but it wouldn t get very far before leaking out The axonal membrane has properties that enable it to conduct a special type of signal action potential 0 Action potentials do not diminish over distance and are of xed size and duration Excitable membrane refers to the membrane of cells that are capable of generating and conduction action potentials including nerve and muscle cells 0 When a cell with excitable membrane isn t generating impulses it is at rest 0 In the resting neuron the cytosol along the surface of the membrane has a negative electrical charge compared to the outside 0 The difference in the electrical charge across the membrane is referred to as resting membrane potential Action potential is a brief reversal of this 0 Inside of the membrane becomes positively charged with respect to the outside THE CAST OF CHEMICALS There are three main parts of the resting membrane potential 0 Salty uids on either side of the membrane 0 The membrane 0 The proteins that are in the membrane Cytosol and Extracellular Fluid 3 The main ingredient that is inside the neuron the intracellular uid or the cytosol the neuron and the extracellular uid is water Q lons are dissolved in the water and are responsible for the resting and the action potentials VVater 3 Most important property is the uneven distribution of electrical charge 3 Hydrogen and Oxygen are bonded covalently Q ie they share electrons 2 The oxygen atom has a greater af nity for electrons in comparison to the hydrogen atom Q Shared electrons spend more time associated with the oxygen atom than with the two hydrogen atoms Q Oxygen acquires a net negative charge and the hydrogen atoms acquire a net positive charge 3 H20 is said to be a polar molecule that is held together by polar covalent bonds 3 The polarity makes water an effect solvent of other charged or polar molecules Q ie other polar molecules tend to dissolve in water Ions g Atoms or molecules with a net electrical charge 3 Ionic bond refers to the bond between ions of opposite charge Salt dissolves readily in water because the charged potions of the water molecule have a stronger attraction for the ions than they have for each other Q As each ion breaks away from the crystal it is surrounded by a sphere of water molecules 1 Each positively charged ion will be covered by water molecules The negatively charged ions will be surrounded by the hydrogen molecules The electrical charge of an atom depends on the difference between the number of protons and electrons When the difference is 1 the ion is monovalent When the difference is 2 the ion is divalent lons with a net positive charge are cations lons with a net negative charge are anions lons are the major charge carriers involved in the conduction of electricity in biological systems which includes the neurons lons of importance for cellular neurophysiology are Sodium monovalent Potassium monovalent Calcium divalent IO IO IO IO Chloride monovalent The Phospholipid Membrane Substances with uneven electrical charges will dissolve in water due to the polarity of the water molecule Q These substances are considered to be hydrophilic Compounds with nonpolar covalent bonds do not interact with water Nonpolar covalent bond occurs when the shared electrons are distributed evenly in the molecule which means that there is no net electrical charge Q These compounds do not dissolve considered to be hydrophobic Olive oil is an example of a hydrophobic compound in water and are Lipids class of waterinsoluble biological molecules that are important to the structure of cell membranes Q Lipids of neuronal membranes contribute to the resting and action potentials by forming a barrier to watersoluble ions and water The Phospholipid Bilayer g Phospholipids the main chemical building blocks of cell membranes 3 Contain log nonpolar chains of carbon atoms that are bonded to hydrogen atoms 3 A phospholipid has a polar phosphate group attached to one end of the molecule Q Polar phosphate group is a phosphorous atom bonded to three oxygen atoms Q They have a polar head that is hydrophilic and a nonpolar quottailquot that is hydrophobic g The neuronal membrane consists of a sheet of phospholipids that is two molecules thick 3 The hydrophilic head faces the outer and inner watery environment and the hydrophobic tails face each other Q Known as a phospholipid bilayer that isolates the cytosol of the neuron from the extracellular uid Protein 3 The type and distribution of protein molecules distinguish neurons from other types of cells The enzymes the cytoskeleton and the receptors are all made up of proteins Q Enzymes catalyze the chemical reactions in the neuron Q Cytoskeleton gives the neuron its special shape Q Receptors sensitive to neurotransmitters The resting potential and the action potential depend on special proteins that span the phospholipid bilayer g The proteins provide routes for ions to cross the neuronal membrane Protein Structure Proteins have widely different shapes sizes and chemical characteristics Proteins are molecules that are assembled from various combinations of 20 different amino acids All amino acids have a central carbon atom which is covalently bonded to four molecular groups Q A hydrogen atom Q An amino group Q A carboxyl group Q A variable group called the R group R for residue Rgraup or side chain Cit carbon I or hydragen K I l 1 Emma cgrbaxyi39 QFGUP 0 group The differences between the amino acids result from differences in the size and the nature of the R groups The properties of the R group determine the chemical relationships in which each amino acid can participate Peptide bond joins the amino group of amino acid to the carboxyl group of the next QAmino acids assemble polypeptide bonds into chains connected by Proteins that are made up of a single chain of amino acids are called polypeptides There are four levels of protein structure Q Primary structure a chain in which the amino acids are linked together by he peptide bonds Q Alpha helix coiling of the polypeptide chain into a spirallike con guration Example of a secondary structure of a protein molecule Q Tertiary structure threedimensional polypeptide into a globular shape folding of a Q Quaternary structure different polypeptides bonding together to form a larger molecule or protein 3 Each of the different polypeptides that contribute to a protein with a quaternary structure are called subunits Channel Prm Regions where nonpolar R groups are exposed on the surface of a protein are hydrophobic and will tend to associate readily with the lipid Regions with exposed polar R groups will be hydrophilic and will avoid a lipid environment Ion Channels 0 Made from membranespanning protein molecules 0 A functional channel across a membrane requires that 46 similar protein molecules assemble to form a pore between them 0 Ion selectivity is an important property of ion channels 0 Potassium channels are selectively permeable to K 0 Sodium channels are almost exclusively permeable to Na 0 Calcium channels are permeable to Ca2 Another important property of ion channels is gating 0 Channels can be gated by changes in the local environment of the membrane l n Pumps 0 Other membranespanning proteins come together to form ion pumps Ion pumps are enzymes that use the energy released by the breakdown of ATP to transport certain ions across the membrane THE MOVEMENT OF IONS A channel provides a path to cross from one side to the other Ionic movements through channels are in uenced by diffusion and electricity Diffusion g Diffusion refers to the movement of ions from regions of high concentration to regions of low concentrations 3 Ions don t usually pass through the phospholipid bilayer directly Q Diffusion causes ions to be pushed through channels in the membrane Concentration Gradient refers to a difference in concentration Ions go across a membrane via diffusion when Q The membrane possesses channels that are permeable to theions Q There is a concentration gradient across the membrane Electricity An electric eld can be used to induce a net movement of ions in a solution Q Ions are electrically charged particles Opposite charges attract and like charges repel Electrical current refers to the movement of electrical charge Q Represented by I and measured in amps There are two important factors that determine how much current will ow g Electrical potential voltage the force that is exerted on a charged particle re ective of the difference in charge between the anode and the cathode More current ows as the difference is increased Voltage V measured in volts Q Electrical conductance ability of an electrical charge to migrate from one point to another Represented by g and is measured in siemens S Depends on the number of particles that are available to carry electrical charge Electrical resistance the inability of an electrical charge to migrate Represented by R and measured in Q It is the inverse of conductance R 1g Ohm s Law I 9N VR Q If the conductance is zero then no current will ow even when the potential difference is large Q When the potential difference is zero no current ows even when the conductance is large Driving an ion across the membrane electrically requires that Q The membrane possesses channels that are permeable the n nn ion Q There is an electrical potential difference across the membrane Protein channels can be highly selective for speci c ions THE Movement of any ion through the channel is dependent on the concentration gradient and the knowledge in electrical potential across the membrane potential IONIC BASIS OF THE RESTING MEMBRANE POTENTIAL Eau Membrane Potential membrane at any moment 0 Represented by Vm Vm can be measured by inserting a microelectrode into the cytosol Microelectrode thin glass tube with an extremely ne tip that will penetrate the membrane of a neuron with minimal damage 0 Filled with an electrically conductive salt solution and connected to a voltmeter o The voltmeter measures the electrical potential difference between the top and a wire outside the cell Method reveals that the electrical charge is unevenly distributed across the neuronal membrane The inside of the neuron is electrically negative with respect to the outside 0 The resting potential is maintained whenever the neuron isn t generating impulses The resting potential of a neuron is about 65 mV 0 Absolute requirement for a functioning nervous system the voltage across the neuronal ilibrium PM The phospholipid bilayer is impermeable to charged hydrophilic atoms because it has no channel proteins If potassium channels were inserted into the bilayer due to the selective permeability of the channels potassium would be free to pass across the membrane but other anions wouldn t be g This would give the cell a net negative charge and an electric potential difference would be established across the membrane As the inside uid acquires a more negative charge the electrical force starts pulling positively charged potassium ions through the channels into the cell An equilibrium status is reached when the electrical force pulling inside the potassium ions is equal to the force of diffusion that is pushing them out The The Ionic equilibrium potential refers to the electrical potential difference that exactly balances an ionic concentration gradient also known as equilibrium potential Large changes in membrane potential are caused by miniscule changes in ionic concentrations The net difference in electrical charge occurs at the inside and outside surfaces of the membrane Ions are driven across the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential If the concentration difference across the membrane is known for an ion equilibrium potential can be calculated for that ion Nernst Equation Each ion has its own equilibrium potential By knowing the electrical charge of the ion and the concentration difference across the membrane we can deduce whether the inside of the cell would be positive or negative at equilibrium The exact value of equilibrium potential in mV can be calculated using the Nernst Equation 0 Takes into account the charge of the ion the temperature and the ratio of the external and the internal ion concentrations Allows us to calculate the value of the equilibrium potential foranion E 23O3 10g10n0 wquot zF ionL Eion ionic equilibrium potential R gas constant T absolute temperature 2 charge of the ion F Faraday s constant Log base 1 log iono ionic concentration outside the cell ion ionic concentration inside the cell where OOOOOOOO Distribution of Ions Across the Membrane The neuronal membrane potential depends on the ionic concentrations on either side of the membrane K is more concentrated on the inside and Na and Ca2 are more concentrated on the outside Ionic concentration gradients are established by the actions of ion pumps in the neuronal membrane o The sodiumpotassium pump is an enzyme that breaks down ATP in the presence of internal Na 0 Chemical energy released by this reaction drives the pump which exchanges internal Na for external K o The pump ensures that the potassium is concentrated inside the neuron and that sodium is concentrated outside 0 The pump pushes the ions across the membrane against the concentration gradient Requires the use of metabolic energy The sodiumpotassium pump expends as much as 70 of the total amount of ATP utilized by the brain The calcium pump is an enzyme that actively transports Ca2out of the cytosol across the cell membrane 0 There are additional mechanisms that decrease the intracellular calcium to a very low level 0 The mechanisms include intracellular calciumbinding proteins and organelles ie mitochondria and types of ER 0 Ion pumps work in the background to ensure that the ionic concentration gradients are established and maintained 0 Without ion pumps the resting membrane potential wouldn t exist and the brain wouldn t function Relative Ion Permeabilities of the Membrane at Rest o The pumps establish ionic concentration gradients across the neuronal membrane 0 The Nernst equation can be used to calculate the equilibrium potential for different ions 10 o The equilibrium potential for an ion is the membrane potential that results if a membrane is selectively permeable to that ion alone The resting membrane potential can be calculated using the Goldman equation 0 Takes into consideration the relative permeability of the membrane to different ions For a membrane that is permeable only to Na and K at 37 C Nana ZZiPNaZ Ki PKZ Na o 0 Z ZZOPNa Ki PKZ where c Vm6154mV log Vm membrane potential PKPNa relative permeabilities of K and Na The Wide World of Potassium Channels Selectivity for potassium ions derives from the arrangement of amino acid residues that line the pore regions of the channel There is an existence of a very large number of potassium channels including those that are responsible for the maintenance of the resting membrane potential in neurons Most potassium channels have four subunits that are arranged like the staves of a barrel to form a pore 0 They have common structural selectivity for potassium ions Pore loops contribute to the selectivity lter that makes the channel mostly permeable to the potassium ions Increased sodium permeability causes the membrane potential of the neurons to become less negative which disrupts neuronal funcUon features that lead to The Importance of Regulating the External atagium Concentraticm The neuronal membrane at rest is mostly permeable to K o The membrane potential is close to EK 11 o The membrane potential is particularly sensitive to changes in the concentration of extracellular potassium Depolarization refers to a change in membrane potential from the normal resting value to a less negative value 0 Increasing the extracellular potassium depolarizes neurons 0 Sensitivity of the membrane potential to KO has led to mechanisms that tightly regulate the extracellular potassium concentrations in the brain 0 BloodBrain barrier a specialization of the walls of the brain capillaries that limits the movement of potassium into the extracellular uid of the brain Glia possess ef cient mechanisms to take up the extracellular K when the concentration rises o Astrocytes ll up most of the space between neurons in the brain and have membrane potassium pumps that concentrate K in their cytosol and potassium channels 0 When KJ O increases K ions enter the astrocyte through the channels causing the membrane to depolarize o The entry of the potassium ions increases the internal potassium concentration KI which is dissipated over a large area by the extensive network of astrocytic processes 0 The mechanism for the regulation of KJ O by astrocytes is called potassium spatial buffering Not all excitable cells are protected from an increase in potassium 0 Muscle cells do not have a bloodbrain barrier or glial buffering mechanisms 12 Itas jum Epati l hill lri lb a r e Eyrt s f vr pull l l izlquot Ifquot l r u quot 39 ll l 33 7 In i it i t 1 ii i l Itquot i 5 quot w 397 It i l z th up Cl jig H i quot39Fl ll ii r rd i 77 quot CONCLUDING REMARK and the The activity of the sodiumpotassium pump produces maintains a large K concentration gradient across membrane The neuronal membrane at rest is highly permeable to the potassium due to the presence of membrane potassium channeb The movement of K ions across the membrane down the concentration gradient leaves the inside of the neuron negatively charged 13
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