BIOL 2457 1st Edition Lecture 25Outline of Last Lecture Nervous Tissue Outline of Current LectureNervous Tissue Functions of the Nervous System Continuous versus Saltatory ConductionContinuous conduction (unmyelinated fibers) Step-by-step depolarization of each portion of the length of the axolemma Saltatory conduction Depolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channels Current carried by ions flows through extracellular fluid from node to nodeFactors That Affect Speed of Propagation Amount of myelination Axon diameter TemperatureSpeed of Impulse Propagation The propagation speed of a nerve impulse is not related to stimulus strength.  Larger, myelinated fibers conduct impulses faster due to size & saltatory conduction  Fiber types A fibers largest (5-20 microns & 130 m/sec) Myelinated somatic sensory & motor to skeletal muscle B fibers medium (2-3 microns & 15 m/sec) Myelinated visceral sensory & autonomic preganglionic C fibers smallest (.5-1.5 microns & 2 m/sec) Unmyelinated sensory & autonomic motor Signal Transmission at the Synapse Two types of synapses Electrical  Ionic current spreads to next cell through gap junctions Faster, two-way transmission & capable of synchronizing groupsof neurons Chemical One-way information transfer from a presynaptic neuron to a postsynaptic neuron Axodendritic -- from axon to dendrite Axosomatic -- from axon to cell body Axoaxonic -- from axon to axonChemical Synapses Action potential reaches end bulb and voltage-gated Ca+2 channels open Ca+2 flows inward triggering release of neurotransmitter Neurotransmitter crosses synaptic cleft & binds to ligand-gated receptors  The more neurotransmitter released, the greater the change in potential of the postsynaptic cell Synaptic delay is 0.5 msec One-way information transferExcitory & Inhibitory Postsynaptic Potentials The effect of a neurotransmitter can be either excitatory or inhibitory A depolarizing postsynaptic potential is called an EPSP It results from the opening of ligand-gated Na+ channels The postsynaptic cell is more likely to reach threshold An inhibitory postsynaptic potential is called an IPSP It results from the opening of ligand-gated Cl- or K+ channels It causes the postsynaptic cell to become more negative or hyperpolarized The postsynaptic cell is less likely to reach thresholdRemoval of Neurotransmitter Diffusion Move down concentration gradient Enzymatic degradation Acetylcholinesterase Uptake by neurons or glia cells Neurotransmitter transporters Prozac = serotonin reuptake inhibitorResponses of Postsynaptic Membrane Small EPSP occurs Potential reaches -56 mV only An impulse is generated Threshold is reached Membrane potential of at least -55 mV  IPSP occurs Membrane hyperpolarized Potential drops below -70 mV Summation If several presynaptic end bulbs release their neurotransmitters at about the same time, the combined effect may generate a nerve impulse due to summation Summation may be spatial or temporal.Temporal Summation Summation of effect of neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuronSpatial Summation Summation of effects of neurotransmitters released from several end bulbs onto one neuronNeurotransmitters Both excitatory and inhibitory neurotransmitters are present in the CNS and PNS; the same neurotransmitter may be excitatory in some locations and inhibitory in others. Important neurotransmitters include acetylcholine, glutamate, aspartate, gamma aminobutyric acid, glycine, norepinephrine, epinephrine, and dopamine.Neurotransmitter Effects Neurotransmitter effects can be modified Synthesis can be stimulated or inhibited Release can be blocked or enhanced Removal can be stimulated or blocked  Receptor site can be blocked or activated Agonist Anything that enhances a transmitter’s effects  Antagonist Anything that blocks the action of a neurotransmitterSmall-Molecule Neurotransmitters Acetylcholine (ACh) Released by many PNS neurons & some CNS Excitatory on NMJ but inhibitory on others Inactivated by acetylcholinesterase Amino Acids Glutamate released by nearly all excitatory neurons in the brain ---- inactivated by glutamate specific transporters GABA is inhibitory neurotransmitter for 1/3 of all brain synapses (Valium is a GABA agonist -- enhancing its inhibitory effect)Neural Circuits A neuronal network may contain thousands or even millions of neurons. Neuronal circuits are involved in many important activities Breathing  Short-term memory Waking upNeural Circuits Types Diverging – a small number of neurons in the brain that govern a particular body movement stimulate a much larger number of neurons in the spinal cord Converging – a single motor neuron that synapses with skeletal muscle fibers at neuromuscular junctions receives input from several pathways that originate in different brain regions Reverberating – impulses are sent back through the circuit time and time again Breathing, coordinated muscular activities, waking up, short-term memory Parallel after-discharge – a single presynaptic cell stimulates a group of neurons, which synapse with a common postsynaptic cell May be involved in precise activities such as mathematical