Chapter 2: Structure and Functions of Cells of the Nervous System PDF

 The primary synaptic transmission means of communication between Ligands neurons, the transmission of ligand: chemical that attaches to a messages from...

 The primary synaptic transmission means of communication between Ligands neurons, the transmission of ligand: chemical that attaches to a messages from one neuron to another binding site across a synapse etymology presynaptic cells : sending cell Latin “Ligareˮ = to bind or to tie; : produced by postsynaptic potentials obligation (bound to a neurotransmitters; brief commitment); ligation (fallopian depolarizations or hyperpolarizations tubes are tied to prevent that increase or decrease the rate of pregnancy) firing of the axon of the postsynaptic molecules that bind to specific neuron receptors, triggering biological : particular region of a binding site responses receptor molecule where only bind to receptors neurotransmitters attach to, thereby affecting the cell cannot enter a neuron binding site + molecule = have can open ion channels complementary shapes ex: found in plants (salicylic acid for system acquired resistance against pathogens) and venomous animals too Phospholipase A2, enzyme that disrupts cell membranes and cause tissue damage)i can also be produced artificially in the laboratory (drugs) Neurotransmitters are naturally occurring ligands, produced and released by neurons Chapter 2  Structure and Functions of Cells of the Nervous System 34  Structure of Synapses : junctions between Synapses Terminal Buttons the terminal buttons at the ends of the axonal branches of one neuron and the membrane of another : small dendritic spines protrusions that stud the dendrites of several types of large neurons in the brain has cytoplasm has microtubules (transport material between soma and terminal button) Many synapses occur on the smooth surface of a dendrite. 2 prominent structures in cytoplasm: Some synapses can occur on the soma and on other axons.  Mitochondria  : small, rounded Synaptic Vesicles Basic Synapse structures made of membrane and filled : located presynaptic membrane with molecules at the end of the terminal most are found around the part of the button presynaptic membrane that faces the : located postsynaptic membrane synaptic cleft on the neuron that receives : the region from which release zone the message, across the neurotransmitter is released synapse; faces the presynaptic membrane 1 terminal button can contain from 100s to nearly a million synaptic extracellular fluid : fills the vesicle synapse, where neurotransmitter diffuses Chapter 2  Structure and Functions of Cells of the Nervous System 35  can be one of two types: small or large PSD Latin word "vesicula" postsynaptic density (PSD) is a protein dense specialization attached to meaning bladder or sac the postsynaptic membrane Chapter 2  Structure and Functions of Cells of the Nervous System 36  Small Synaptic V esicles contain molecules of the neurotransmitter (glutamate, GABA membrane made of 10k lipid molecules (clear) inside: 200 protein molecules produced in the soma, carried by fast axoplasmic transport can also be produced from recycled material in the terminal button locally a few dozen to several hundred; found in all terminal buttons transport proteins : fill vesicles with the neurotransmitter : involved in the release of trafficking proteins neurotransmitters and the recycling of the vesicles Large Synaptic V esicles has a dense-core contain one type of peptides produced only in the soma, transported through the axoplasm to the terminal buttons Release of Neurotransmitters Chapter 2  Structure and Functions of Cells of the Nervous System 37  Action potential causes release of neurotransmitters! a population of synaptic vesicles becomes “dockedˮ against the presynaptic membrane : when clusters of protein docking molecules attach to other protein molecules located in the presynaptic membrane. voltage-dependent calcium channels : the process when the cell exocytosis secretes molecules of neurotransmitter arriving action potential depolarizes the membrane of how neurons communicate with the terminal button one another calcium channels open membrane-wrapped product migrates to the inside of the outer Calcium has highest membrane of the cell, fuses with concentration in the the membrane, and bursts, spilling extracellular fluid its contents into the fluid Ca2 flows into the cell surrounding the cell through diffusion no Calcium = no release of a : a hole through both Fusion Pore neurotransmitter = no membranes that enables them to fuse communication between together. neurons cluster of protein molecules move : later Calcium Transporters apart to create this remove Ca2 from; caused by calcium ions binding to comparable to sodium- the proteins potassium pump The process of fusion takes Why does Calcium have a approximately 0.1 msec! charge of 2? Chapter 2  Structure and Functions of Cells of the Nervous System 38  The charge of an ion is determined by the number of electrons it loses or gains to achieve a stable electron configuration, usually similar to that of a noble gas. Calcium Ca²⁺) Calcium is in Group 2 of the periodic table, which means it has two electrons in its outermost shell. To achieve a stable electron configuration (like that of the noble gas argon), calcium loses both of these outer electrons, resulting in a 2 charge. Sodium Na⁺) and Potassium K⁺) Sodium and potassium are in Group 1 of the periodic table, each having only one electron in their outermost shell. To achieve a stable configuration (like neon for sodium or argon for potassium), they each lose one electron, resulting in a 1 charge. The difference in charges arises because calcium needs to lose two electrons to reach stability, while sodium and potassium only need to lose Chapter 2  Structure and Functions of Cells of the Nervous System 39  one. Thus, calcium forms a Ca²⁺ ion, and sodium and potassium form Na⁺ and K⁺ ions, respectively. other functions of calcium: bind and change characteristics of other proteins, join the membrane of synaptic vesicles with presynaptic membrane 3 Pools of Synaptic Vesicles 1. Release-ready 2. Recycling Pool 3. Reserve Pool vesicles 1015% of total vesicles 8590% of total called on when vesicles rate of firing last to be called on docked against the increases inside of the Merge and Bulk presynaptic Recycle Endocytosis membrane, ready to process endocytosis: “the release their process of contents when an  membranes entering a cellˮ action potential merge with the  Large pieces of arrives the membrane of Chapter 2  Structure and Functions of Cells of the Nervous System 40  1% of total number presynaptic the terminal of vesicles found in membrane button fold terminal inward, break off,  little buds of and enter the called on when axon the cytoplasm fires at a low rate membrane Kiss and pinch off  New vesicles are Run into the formed from small cytoplasm buds that break off of these  become pieces of synaptic membrane vesicles takes a few  proteins are minutes inserted into vesicle a process membrane Combined photo: used by  vesicle is Recycling Synaptic ready-release filled with Vesicle Membrane pool vesicles neurotransmitter  synaptic molecules vesicles takes a few release seconds most or all of their neurotransmitter  the fusion rizzoli2005.pdf pore closes  vesicles break away from the presynaptic membrane Chapter 2  Structure and Functions of Cells of the Nervous System 41  and get filled with neurotransmitter again takes less than a second After the synaptic vesicles have released a neurotransmitter into the synaptic cleft, the following takes place: In kiss and run, a vesicle fuses with the presynaptic membrane, releases the neurotransmitter, reseals, leaves the docking site, becomes refilled with the neurotransmitter, and mixes with other vesicles in the terminal button. In merge and recycle, the vesicle completely fuses with the postsynaptic membrane, losing its identity. Extra membrane from fused vesicles pinches off into the cytoplasm and forms vesicles, which are filled with the neurotransmitter. The membranes of vesicles in the reserve pool are recycled through a process of bulk endocytosis. Large pieces of the membrane Chapter 2  Structure and Functions of Cells of the Nervous System 42  of the terminal button fold inward, break off, and enter the cytoplasm. New vesicles are formed from small buds that break off of these pieces of membrane. Activation of Receptors  Neurotransmitter molecules diffuse across the synaptic cleft and attach to postsynaptic receptors (the binding sites of special protein molecules located in the postsynaptic membrane) they do not enter the postsynaptic cell, only bind to specific binding sites on the receptors  postsynaptic receptors open neurotransmitter-dependent ion channels (aka ligand- gated ion channels ) permitting the passage of specific ions into or out of the cell neurotransmitters open ion channels by at least 2 different methods: direct and indirect  local membrane potential changes Ionotropic Receptors Metabotropic Receptors simpler; direct Ion channel that is equipped with its own binding site when a molecule of the appropriate neurotransmitter attaches to it, the ion channel opens aka G-protein-coupled receptors more complicated; indirect does not open ion channels Chapter 2  Structure and Functions of Cells of the Nervous System 43  starts a chain of chemical events require the cell to expend metabolic energy take longer to begin and last longer located in close proximity to another protein attached to another protein G Protein  neurotransmitter molecule (first messenger) binds with metabotropic receptor  metabotropic receptor activates G protein next to it (both are inside the membrane)  G protein is activated and in turn activates an enzyme  Enzyme stimulates production of second messenger (a chemical)  molecules of second messenger travel through cytoplasm  second messenger attach to nearby ion channels and cause them to open Second Messenger : first one to be cyclic AMP discovered; chemical synthesized from ATP others have since been discovered play an important role in synaptic and nonsynaptic communication Chapter 2  Structure and Functions of Cells of the Nervous System 44  not just open ion channels, can travel to nucleus or other regions of the neuron to initiate biochemical changes that affection function of the cell can also turn on or off specific genes, controlling the production of proteins Postsynaptic Potentials Neurotransmitter doesnʼt determine whether postsynaptic potentials are excitatory or inhibitory. Determined by characteristics of postsynaptic receptors; type of ion channel they open 4 Major types of Ion Channels in Postsynaptic Membrane sodium Na+) potassium K+) chloride Cl–) calcium Ca2) EPSP IPSP excitatory postsynaptic potential inhibitory postsynaptic potential most important source: Potassium channels opening and neurotransmitter-dependent K leaving the cell, hyperpolarizing sodium channels the membrane and producing IPSP if Calcium channel open  Ca2 in addition, can also be Chloride ions will rush in → membrane is channels opening depolarized → produce EPSP Chapter 2  Structure and Functions of Cells of the Nervous System 45  Calcium also binds with and @ resting potential: nothing activates special enzymes for happens (balanced forces) the production of biochemical if membrane potential is and structural changes in the slightly more positive because postsynaptic neuron of excitatory synapses nearby, also, in the terminal button, then Cl- go into the neuron → calcium entry triggers the hyperpolarizing it → back to migration of synaptic vesicles resting potential → inhibitory and the release of the helps to neutralize EPSP! neurotransmitter Effects of Postsynaptic Potentials: Neural Integration  The interaction of the effects of excitatory and inhibitory neural integration synapses on a particular neuron rate that neuron fires = relative activity of the excitatory and inhibitory synapses on its dendrites and soma activity of excitatory synapses = rate of action potential firing increases activity of inhibitory synapses = rate of action potential firing will decrease Chapter 2  Structure and Functions of Cells of the Nervous System 46 Chapter 2  Structure and Functions of Cells of the Nervous System 47   release of a neurotransmitter  Na+ and Ca2 channels open Note that neural inhibition (that is, an inhibitory post-  several excitatory synapses synaptic potential) does not become active always produce behavioral  depolarizing EPSPs are produced inhibition. in the dendrites of the neuron Inhibiting the inhibitory  EPSPs (represented in red) are neurons makes the behavior transmitted down the dendrites more likely to occur.  across the soma Excitation of neurons that  to the axon hillock inhibit a behavior suppresses that behavior.  If the depolarization is strong ex: when we are dreaming, a enough to rise above the threshold particular set of inhibitory of excitation when it reaches this neurons in the brain point, the axon will fire an action becomes active and prevents potential. us from getting up and acting out our dreams. Termination of Postsynaptic Potentials : depolarizations or hyperpolarizations caused by the Postsynaptic Potentials activation of postsynaptic receptors with molecules of a neurotransmitter postsynaptic receptors only get a brief exposure to the neurotransmitter. terminated by two mechanisms Reuptake Enzymatic Deactivation more common enzyme destroys molecules of an extremely rapid removal of neurotransmitter neurotransmitter from the synaptic terminated for potentials produced by cleft by the terminal button acetylcholine (ACh) and for Chapter 2  Structure and Functions of Cells of the Nervous System 48  The neurotransmitter does not neurotransmitters that consist of return in the vesicles that get peptide molecules (large vesicles, pinched off the membrane of the longer lasting) terminal button ACh: for muscle activation! special transporter molecules draw on the cellʼs energy reserves to mediates: force molecules of the synapses on muscle fibers neurotransmitter from the synaptic some synapses between neurons cleft directly into the cytoplasm— in the CNS just as sodium–potassium transporters move Na+ and K myasthenia gravis, an immune across the membrane. system disease that destroys ACh receptors will reduce the amount of information conveyed from the ACh system to the muscles, and resulting in muscle weakness. drug that blocks AChE ▶ increase the amount of ACh in the synapse not broken down by AChE ▶ amplified message to muscles shortlived postysnaptic potentials because postsynaptic membranes at these synapses have acetylcholinesterase (AChE) “uh-SEE-til- KOH-lin-ES-ter-aysˮ destroys ACh by breaking it into its constituents: choline and acetate (cannot activate postsynaptic receptors) Autoreceptors Greek “autoˮ = self/same  neurotransmitter binds with autoreceptor Chapter 2  Structure and Functions of Cells of the Nervous System 49  located on the membrane of any  autoreceptor is stimulated part of the cell  regulates internal process receptors that respond to the synthesis of neurotransmitter neurotransmitter that they (production) themselves release release of neurotransmitter part of a negative feedback mechanism that allows presynaptic cells to monitor the amount of typically: decreases rate neurotransmitter they release into of synthesis or release of the synapse and adjust the neurotransmitter from the amounts to fine-tune their presynaptic cell chemical message If too little neurotransmitter is released, the rates of doesnʼt change membrane production and release go up. potential nor control ion channels prevents the presynaptic cell from releasing too much or too little neurotransmitter Other Types of Synapses Chapter 2  Structure and Functions of Cells of the Nervous System 50  Axoaxonic Synapses do not contribute directly to neural integration. they alter the amount of neurotransmitter released by the terminal buttons of the postsynaptic axon produce presynaptic modulation: presynaptic inhibition or presynaptic facilitation. : activity of the Presynaptic Inhibition axoaxonic synapse decreases the release of the neurotransmitter : activity of the Presynaptic Facilitation axoaxonic synapse increases the The release of a neurotransmitter by a release terminal button is initiated by an action potential. Normally, a fixed amount of Dendrodendritic Synapses neurotransmitter each is released each very small neurons that have time an action potential arrives. extremely short processes and > can be modulated by activity of apparently lack axons axoaxonic synapses synapses between dendrites do not transmit information from place to place within the brain. probably perform regulatory functions, perhaps helping to organize the activity of groups of neurons. Chapter 2  Structure and Functions of Cells of the Nervous System 51  can also be larger neurons Chemical vs Electrical Nonsynaptic Chemical Synapse Communication : presence of synaptic Chemical Synapse vesicles in one of the juxtaposed Neuromodulators dendrites and a postsynaptic chemicals released by neurons thickening in the membrane of the that travel farther and are other. dispersed more widely than use neurotransmitters neurotransmitters communication at each synapse is secreted in larger amounts and private and typically only involves diffuse for longer distances, the presynaptic and postsynaptic modulating the activity of many cell in the synapse neurons in a particular part of the Electrical Synapse brain. allow for instantaneous ex: neuromodulators affect transmission of electrical signals general behavioral states such between neurons, leading to as vigilance, fearfulness, and synchronized firing sensitivity to pain useful for reflexes (need rapid example communication) Dopamine (in certain brain : where membranes of the gap junction areas acts more as a cells meet and almost touch neuromodulator) Serotonin (can act as both a The membranes on both sides of a neurotransmitter in certain gap junction contain channels that synapses, causing immediate effect, and a neuromodulator, Chapter 2  Structure and Functions of Cells of the Nervous System 52  permit ions to diffuse from one cell influencing overall mood and to another arousal over a longer period of time by modulating how changes in the membrane neurons respond to other potential of one neuron induce neurotransmitters.) changes in the membrane of the other Most are peptides (chains of amino acids) Gap junctions are common in invertebrates Hormones most gap junctions in secreted by cells of endocrine vertebrate synapses are glands or by cells located in various dendrodendritic organs, such as the stomach, the function in vertebrates is still intestines, the kidneys, and the under investigation: Electrical brain. synapses appear to play released into the extracellular fluid important roles in the ➔ distributed to the rest of the vertebrate retina, olfactory body through the bloodstream bulb, cerebral cortex, hippocampus, suprachiasmatic affect the activity of cells nucleus, hypothalamus, (including neurons) that contain medulla, spinal cord, and parts specialized receptors located of the thalamus and enteric either on the surface of their nervous system membrane or deep within their nuclei gap junctions at postsynaptic dendrites and somas can also target cells: cells that contain occur. receptors for a particular hormone examples eg “Thyroid-stimulating hormone TSHˮ targets cells in the thyroid gland, stimulating the release of thyroid hormones; Follicle-stimulating hormone FSH in ovaries Chapter 2  Structure and Functions of Cells of the Nervous System 53  Adrenaline Epinephrine) target Noradrenergic neurons in the brain, particularly in the locus coeruleus, which regulate arousal, attention, and stress responses. only these cells respond to its presence affect behavior by stimulating neurons with hormone receptors and changing the activity of these neurons testosterone (sex hormone) increases the aggressiveness of most male mammals. Peptide Hormones  exert their effects on target cells by stimulating metabotropic receptors located in the membrane  second messenger is generated and travels to nucleus  initiates changes in the cellʼs physiological processes Second messengers can also turn specific genes on or off to initiate or terminate production of particular proteins. Steroid Hormones consist of very small fat-soluble molecule Chapter 2  Structure and Functions of Cells of the Nervous System 54   pass easily through the cell membrane  travel to the nucleus  attach themselves to specialized receptors located in the nucleus  receptor directs the machinery of the cell to alter its protein production ex: sex hormones secreted by the ovaries and testes and the hormones secreted by the adrenal cortex recently: discovered presence of steroid receptors in terminal buttons and around the postsynaptic membrane of some neurons steroid receptors influence synaptic transmission rapidly, but we do not know how Chapter 2  Structure and Functions of Cells of the Nervous System 55