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Nerve Fiber Classification Nerve fibers are classified according to: Diameter Degree of myelination Speed of conduction Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Synapses A junction that mediates information transfer from one neuron: To another neuron To an effector cell Presynaptic neuron – conducts impulses toward the synapse Postsynaptic neuron – transmits impulses away from the synapse Most neurons will function as both Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Synapses Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.17 Types of Synapses Axodendritic – synapses between the axon of one neuron and the dendrite of another Axosomatic – synapses between the axon of one neuron and the soma of another Other types of synapses include: Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrites to soma) PLAY InterActive Physiology ®: Nervous System II: Anatomy Review, page 5 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Electrical Synapses Contain protein channels made of connexin subunits that connect cytoplasm of adjacent neurons Electrically coupled Fast transmission across synapse Electrical synapses: Are less common than chemical synapses Correspond to gap junctions found in other cell types Are important in the CNS in: PLAY Arousal from sleep, Mental attention, Emotions and memory InterActive Physiology ®: Nervous System II: Anatomy Review, page 6 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Chemical Synapses Specialized for the release and reception of chemical neurotransmitters Typically composed of two parts: 1) Axonal terminal of the presynaptic neuron, which contains synaptic vesicles 2) Receptor region on the dendrite(s) or soma of the postsynaptic neuron 1 & 2 are separated by a synaptic cleft PLAY InterActive Physiology ®: Nervous System II: Anatomy Review, page 7 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Synaptic Cleft Fluid-filled space separating the presynaptic and postsynaptic neurons Prevents nerve impulses from directly passing from one neuron to the next Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one) Ensures unidirectional communication between neurons PLAY InterActive Physiology ®: Nervous System II: Anatomy Review, page 8 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Synaptic Cleft: Information Transfer Nerve impulses reach the axonal terminal of the presynaptic neuron and open Na+ channels as well as Ca2+ channels 1) Ca++ floods into the terminal from the extracellular matrix 2) Ca++ acts as an intracellular messenger directing synaptic vessicles to fuse with the axon membrane and empty neurotransmitter the synaptic cleft via exocytosis 3) Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron 4) Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect PLAY InterActive Physiology ®: Nervous System II: Synaptic Transmission, pages 3–6 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Synaptic Cleft: Information Transfer n tio ial Ac tent po Ca2+ 1 Neurotransmitter Axon terminal of presynaptic neuron Postsynaptic membrane Mitochondrion Axon of presynaptic neuron Na+ Receptor Postsynaptic membrane Ion channel open Synaptic vesicles containing neurotransmitter molecules 5 Degraded neurotransmitter 2 Synaptic cleft Ion channel (closed) 3 4 Ion channel closed Ion channel (open) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.18 Step 5: Termination of Neurotransmitter Effects Neurotransmitter bound to a postsynaptic neuron: Produces a continuous postsynaptic effect Blocks reception of additional “messages” Must be removed from its receptor Removal of neurotransmitters occurs when they: Are degraded by enzymes Are reabsorbed by astrocytes or the presynaptic terminals Diffuse away from the synaptic cleft Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Synaptic Delay Time taken for neurotransmitter release, diffusion across the synapse, and binding to receptors Synaptic delay – 0.3-5.0 ms Synaptic delay is the rate-limiting step of neural transmission (e.g. the slowest step) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Postsynaptic Potentials Postsynaptic membrane receptors are chemically gated and not voltage gated. Thus, they can not be self-amplifying nor self-generating Neurotransmitter receptors mediate changes in membrane potential according to: The amount of neurotransmitter released The amount of time the neurotransmitter is bound to receptors The two types of postsynaptic potentials are: EPSP – excitatory postsynaptic potentials IPSP – inhibitory postsynaptic potentials PLAY InterActive Physiology ®: Nervous System II: Synaptic Transmission, pages 7–12 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Excitatory Postsynaptic Potentials Here, neurotransmitter binding causes depolarization of the postsynaptic membrane At the synapse, a single type of chemically gated ion channel opens on postsynaptic membranes This channel allows Na+ & K+ to diffuse simultaneously through the membrane Na+ has steeper electrochemical gradient Thus, Na+ influx is greater than K+ efflux and net depolarization occurs Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Excitatory Postsynaptic Potentials If enough neurotransmitter binds, depolarization of the post synaptic membrane can reach 0mV However, post synaptic membranes DO NOT generate APs. Instead, Excitatory Postsynaptic Postentials (EPSPs) occur. EPSPs trigger an AP distally at the axon hillock Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings EPSPs EPSPs are brief, relatively weak graded depolarization events Currents created by EPSPs spread all the way to the axon hillock where they: Depolarize the axon to threshold Axonal VGICs open AP is generated Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Inhibitory Synapses and IPSPs Neurotransmitter binding to a receptor at inhibitory synapses: Causes the membrane to become more permeable to potassium and chloride ions Leaves the charge on the inner surface negative (hyperpolarization) Reduces the postsynaptic neuron’s ability to produce an action potential. How? This will now require an even larger depolarizing event to reach threshold and induce an AP Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Summation by the Postsynaptic Neuron A single EPSP cannot induce an action potential in the postsynaptic neuron But 1000 of them can! EPSPs must summate (add together) temporally or spatially to induce an action potential Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Temporal summation Occurs when one or more presynaptic neurons transmit impulses in high frequency. Successive EPSPs add on to one another Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Spatial Summation Spatial summation – postsynaptic neuron is stimulated by a large number of terminals at the same time (usually from different neurons) PLAY InterActive Physiology ®: Nervous System II: Synaptic Potentials Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Summation Both EPSPs and IPSPs can summate Most neurons receive both EPSPs and IPSPs as well as both chemical and electrical synapses The axon hillock responds to the most prominent summation, be it excitatory or inhibitory The axon hillock acts as a neural integrator It’s voltage potential reflects the sum of all incoming neural information The most effective synapses are those close to the axon hillock Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Summation Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.20 Neurotransmitters Chemicals used for neuronal communication with the body and the brain 50 different neurotransmitters have been identified Most neurons make and release more than one type Classified chemically and functionally Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Chemical Neurotransmitters Chemical classes are based on structure Acetylcholine (ACh) Biogenic amines Amino acids Peptides Novel messengers: ATP and dissolved gases NO and CO Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Acetylcholine First neurotransmitter identified, and best understood Released at the neuromuscular junction Synthesized and enclosed in synaptic vesicles Acetyl CoA + choline Ach + CoA (Acetyl CoA is acetic acid + Coenzyme A) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Acetylcholine Degraded by the enzyme acetylcholinesterase (AChE) into acetic acid and choline Released by: All neurons that stimulate skeletal muscle Some neurons in the autonomic nervous system Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Biogenic Amines Include: Catecholamines – dopamine, norepinephrine (NE), and epinephrine Indolamines – serotonin and histamine Broadly distributed in the brain Play roles in emotional behaviors and our biological clock Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Synthesis of Catecholamines Enzymes present in the cell determine length of biosynthetic pathway Norepinephrine and dopamine are synthesized in axonal terminals Epinephrine is released by the adrenal medulla Neurons possess only the enzymes needed to make their own neurotransmitter Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.21 Neurotransmitters: Amino Acids Include: GABA – Gamma (γ)-aminobutyric acid Glycine Aspartate Glutamate Found only in the CNS Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Peptides Include: Substance P – mediator of pain signals Beta endorphin, dynorphin, and enkephalins Act as natural opiates; reduce pain perception Bind to the same receptors as opiates and morphine E.g. increase in enkephalis during labor E.g. increase in endorphin release in athletes (extra boost of strength) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Novel Messengers ATP Is found in both the CNS and PNS Produces excitatory or inhibitory responses depending on receptor type Second messenger response depending on which type of receptor it binds to. Adenosine receptors: Adenosine is an inhibitor in the brain Caffeine blocks adenosine receptors Coffee drinkers get their “rush” Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Novel Messengers Nitric oxide (NO) Synthesized on demand and diffuses out of the cell that made it Binds to iron in guanydyl cyclase, the enzyme that makes cyclic GMP, and activates it Is involved in learning and memory Carbon monoxide (CO) is a main regulator of cGMP in the brain Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Functional Classification of Neurotransmitters Two classifications: excitatory and inhibitory Excitatory neurotransmitters cause depolarizations (e.g., glutamate) Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Functional Classification of Neurotransmitters Some neurotransmitters have both excitatory and inhibitory effects Determined by the receptor type of the postsynaptic neuron Example: acetylcholine Excitatory at neuromuscular junctions with skeletal muscle Inhibitory in cardiac muscle (norepinephren also has both excitatory and inhibitory effects, but are opposite of those of acetylcholine) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitter Receptor Mechanisms Direct: neurotransmitters that open ion channels Promote rapid responses Examples: ACh and amino acids Indirect: neurotransmitters that act through second messengers Promote long-lasting effects Examples: G-proteins, biogenic amines, peptides, and dissolved gases PLAY InterActive Physiology ®: Nervous System II: Synaptic Transmission Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitter Receptors: Channel-Linked Receptors Composed of integral membrane protein Ligand gated ion channels that mediate direct transmitter action (ionotropic receptors) Action is immediate, brief, simple, and highly localized Ligand binds the receptor, and ions enter the cells Excitatory receptors depolarize membranes (e.g. Ach, glutamate, aspartate, ATP are ligands for cation channels for Na+, K+, and Ca++) Inhibitory receptors hyperpolarize membranes (e.g. GABA and glycine are ligands for Cl- channels) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Channel-Linked Receptors Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.22a Neurotransmitter Receptors: G Protein-Linked Receptors Metabotropic receptors Responses are indirect, slow, complex, prolonged, and often diffuse These receptors are transmembrane protein complexes Examples: muscarinic ACh receptors, neuropeptides, and those that bind biogenic amines G-protein activation works by controlling production of second messengers such as cyclic AMP, cyclic GMP, diacylglycerol, or Ca++ which open or close ion channels or activate kinase enzymes that initiate an enzymatic cascade Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitter Receptor Mechanism Ions flow Blocked ion flow (a) Channel closed Ion channel Adenylate cyclase Channel open Neurotransmitter (ligand) released from axon terminal of presynaptic neuron 3 1 PPi 4 GTP 5 cAMP ATP 5 3 Changes in membrane permeability and potential GTP 2 GDP Protein synthesis Enzyme activation GTP Receptor G protein (b) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Nucleus Activation of specific genes Figure 11.22b Neural Integration: Neuronal Pools Functional groups of neurons that: Integrate incoming information Forward the processed information to its appropriate destination Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Neural Integration: Neuronal Pools Simple neuronal pool Input fiber – presynaptic fiber Discharge zone – neurons most closely associated with the incoming fiber Facilitated zone – neurons farther away from incoming fiber Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Simple Neuronal Pool Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.23 Types of Circuits in Neuronal Pools Divergent Circuits Converging Circuits Oscillating Circuits Parallel After-Discharge Circuits Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Types of Circuits in Neuronal Pools Divergent – one incoming fiber stimulates ever increasing number of fibers, often amplifying circuits Common in sensory and motor systems Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.24a, b Types of Circuits in Neuronal Pools Convergent Circuits– opposite of divergent circuits, resulting in either strong stimulation or inhibition Pool receives inputs from several presynaptic neurons and the circuit has a concentrating effect Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.24c, d Types of Circuits in Neuronal Pools Reverberating or Oscillating Circuits – chain of neurons containing collateral synapses with previous neurons in the chain Positive feedback, impulses are sent again and again Control rhythmic activities, e.g. breathing Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.24e Types of Circuits in Neuronal Pools Parallel after-discharge circuit– parallel arrays of post-synaptic neurons that stimulate a common output cell Signals reach the output cell at different times creating a burst of impulses (after-discharge) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.24f Patterns of Neural Processing Serial Processing One neuron stimulates the next, that neuron stimulates the next, and so on… Input travels along one pathway to a specific destination Works in an all-or-none manner Example: spinal reflexes (see sidenote) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Reflexes SideNote Reflexes are rapid, automatic responses to stimuli. The stimulus always causes the same response (stereotyped and dependable) E.g. jerking hand away upon touching something hot. E.g. eye blink when an object is too close Reflexes occur over reflex arcs that have 5 essential components: 1) receptor 2) sensory neuron 3) CNS integration center 4) motor neuron 5) effector Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Patterns of Neural Processing Parallel Processing Input travels along several pathways Information delivered by each pathway is dealt with simultaneously by different parts of the neural circuitry Thus, one stimulus promotes numerous responses E.g. a smell may remind one of the odor and associated experiences Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Development of Neurons The nervous system originates from the neural tube and neural crest The neural tube becomes the CNS There is a three-phase process of differentiation: Proliferation of cells needed for development Migration – cells become amitotic and move externally Differentiation into neuroblasts Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Axonal Growth: How do you get “wired” up? How do neurons find their correct target? The growing tip of the axon interacts with its environment Guided by: Extracellular and cell surface adhesion proteins direct growth cone into safe areas to grow Scaffold laid down by older neurons Orienting glial fibers Release of nerve growth factor by astrocytes Neurotropins released by other neurons “lure” the growing axon to approach or retreat Repulsion guiding molecules Attractants released by target cells Neurons that fail to make synapses die Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings