Lecture 10 - Nervous Tissue - Tagged PDF

NERVOUS TISSUE OVERVIEW OF THE NERVOUS SYSTEM Layout of the Nervous System Organization of the Nervous System  Sensory Sense changes through sensory receptors Functions of the Motor Nervous Respond to stimuli System Integrative...

NERVOUS TISSUE OVERVIEW OF THE NERVOUS SYSTEM Layout of the Nervous System Organization of the Nervous System  Sensory Sense changes through sensory receptors Functions of the Motor Nervous Respond to stimuli System Integrative Analyze incoming sensory information, store some aspects, and make decisions regarding appropriate behaviors HISTOLOGY OF NERVOUS TISSUE Neurons Neurons Electrically excitable Cellular structures Structural Classification of Neurons Neurons can be classified based on the number of processes extending from the cell body Example s of Dendritic Branchin g Functional Classification of Neurons Neurons can be classified based on the direction of nerve impulse propagation Sensory/afferent neurons Motor/efferent neurons Inter/association neurons  Neuroglia Not electrically excitable Make up about half the volume of the Neuroglia nervous system Can multiply and divide 6 kinds total (4 in CNS, 2 in PNS) Neuroglia of the CNS  Four Types of Neuroglia in the CNS 1. Ependymal cells: line the hollow regions of CNS, ventricles of the brain & CC/SC. Help to produce and circulate CSF. Neuroglia 2. Astrocytes: large cell bodies with many processes. Most numerous of neuroglia. They can guide axon regeneration. Impt in the Blood Brain Barrier (protection of cells in CNS).  3. Oligodendrocytes: myelinated axons of the CNS 4. Microglia: smallest and least numerous function as macrophages. Phagocytic nature. PNS Neuroglia: Neuroglia Schwann Cells - myelinate axons of the PNS and clear cellular debris for regrowth. Satellite Cells – surround neurons, similar to astrocytes. Contribute to chronic pain. Neuroglia. Myelinati on of Neurons The myelin sheath is produced by Schwann cells (PNS) and oligodendrocytes (CNS) and it surrounds the axons of most neurons Gray Matter vs. White Matter ELECTRICAL SIGNALS IN NEURONS: AN OVERVIEW Electrical Signals in Neurons Excitable cells communicate with each other via action potentials or graded potentials Action potentials (AP) allow communication over short and long distances whereas graded potentials (GP) allow communication over short distances only Production of an AP or a GP depends upon the existence of a resting membrane potential and the existence of certain ion channels  Graded Potential s& Action Potential s  5 Main Membrane Processes 1. Resting Membrane Potential (RMP) The transmembrane potential of resting cell. Voltage of a resting Transmembrane neuron -70 Mv Potential 2. Graded potential Temporary, localized change in resting potential Caused by stimulus A single graded potential will not initiate an action potential  5 Main Membrane Processes 3. Action potential Is an electrical impulse Produced by depolarizing graded potentials that are summed Transmembrane Action Potential occurs in an All or Potential None Fashion Propagates along surface of axon to synapse  5 Main Membrane Processes 4. Synaptic activity Releases neurotransmitters at presynaptic membrane Produces graded potentials in Transmembrane postsynaptic membrane Potential 5. Information processing Response of postsynaptic cell If the intensity of the depolarization spreads to the hillock then threshold is reached and an action potential will occur If the intensity of the depolarization doesn’t spread to the hillock then threshold is not reached and an action potential will not occur Ion Leakage channels alternate between open and closed Channels K+ channels are more numerous than in Neurons Na+ channels Ion Ligand-gated channels respond to Channels chemical stimuli (ligand binds to receptor) in Neurons Ion Channels in Neurons Voltage-gated channels respond to direct changes in membrane potential Ion Channels in Neurons  RESTING MEMBRANE POTENTIAL  Resting Membrane Potential The membrane of a non-conducting neuron is positive outside and negative inside. This is determined by: 1. Unequal distribution of ions across the plasma membrane and the selective permeability of the neuron’s membrane to Na+ and K+ 2. Most anions cannot leave the cell 3. Na+/K+ pumps  Resting Membrane Potential - RMP Three Requirements for Resting Membrane Potential (RMP) 1. Concentration gradient of ions (Na+, K+) 2. Selectively permeable through leak channels 3. Maintains charge difference across membrane (resting potential –70 mV)  Transmembrane Potential - RMP Passive Forces Acting Across the Membrane Chemical gradients Concentration gradients of ions (Na+, K+) Electrical gradients Separate charges of positive and negative ions Result in potential difference  Resting Membrane Potential: Voltage Difference Resting Membrane Potential -70mV Factors Contributing to Resting Membrane Potential  Active Forces Across the Membrane Sodium–potassium ATPase (exchange pump) Resting Is powered by ATP Membrane Carries 3 Na+ out and 2 K+ in Potential Balances passive forces of diffusion Maintains resting potential (–70 mV)  Graded Potentials Small deviations in resting membrane potential  Transmembrane Potential Graded Potentials Also called local potentials Produce changes in transmembrane potential They may be depolarization where inside of cell gets more positive They may be hyperpolarization where inside of cell gets more negative A single graded potential doesn’t spread far into the cell body Graded A graded potential occurs in response to the opening of a mechanically-gated Potentials or ligand-gated ion channel  Graded Potentials: Stimulus Strength The amplitude of a graded potential depends on the stimulus strength Graded Potentials: Summation Graded potentials can be added together to become larger in amplitude ACTION POTENTIALS  An action potential is a sequence of rapidly occurring events that decrease and eventually reverse the membrane Action potential (depolarization) and eventually restore it to Potentials the resting state (repolarization)  Initiating Action Potential All-or-none principle If a stimulus reaches threshold amount an action potential is triggered. (summed graded potentials) Action If the RMP is -70 mV and enough depolarization occurs to change Potential voltage to -55 mV (threshold) then the action potential takes place Action Potentials  Action Potentials: Stimulus Strength Action potentials can only occur if the membrane potential reaches threshold Action Potentials: The Status of Na+ and K+ Voltage-Gated Channels  Three Conditions of Voltage Regulated Channels 1. Closed, but capable of opening Voltage 2. Open (activated) 3. Closed, not capable of Regulated opening (inactivated) Found at the Hillock and along Channels the Axon  Action Potential 4 Steps in the Generation of Action Potentials Step 1: Depolarization to threshold Step 2: Activation of Na+ channels Rapid depolarization Na+ ions rush into cytoplasm Inner membrane changes from negative to positive Action Potential Step 3: Inactivation of Na+ channels, activation of K+ channels At +30 mV Inactivation gates close (Na+ channel inactivation) K+ channels open Repolarization begins Action Potential Step 4: Return to normal permeability K+ channels begin to close: when membrane reaches normal resting potential (–70 mV) K+ channels finish closing: membrane is hyperpolarized to -90 mV transmembrane potential returns to resting level: action potential is over Action Potenti al  Action Potential The Refractory Period The time period From beginning of action potential To return to resting state During which membrane will not respond normally to additional stimuli Absolute refractory period Sodium channels open or inactivated No action potential possible Relative refractory period Membrane potential almost normal Very large stimulus can initiate action potential Action Potenti al Transmembrane Potential Comparison of Graded & Action Potentials Propagation of Action Potentials In order for communication to occur Action potentials do not from one body part to die out, they keep their another, action strength as they spread potentials must travel across the membrane of from where they arise at a neuron the trigger zone to the axon terminals  Graded Potentials vs Action Potentials: Anatomy and Physiology Crack Tutorial https://www.youtube.com/watch?v=PBdSbGgn4hI You must be connected to the Internet and in Slideshow Mode to run this animation. Continuous vs. Saltatory Conduction Factors That Affect Propagation Speed AXON AMOUNT OF TEMPERATURE DIAMETER MYELINATION SIGNAL TRANSMISSIO N AT SYNAPSES  A synapse is the junction between neurons or between a neuron and an effector Signal Electrical Synapse Transmissio Gap junctions connect cells and allow the transfer of n at information to synchronize Synapses the activity of a group of cells Chemical Synapse One-way transfer of information from a presynaptic neuron to a postsynaptic neuron Synapses Between Neurons  Signal Transmissi on at a Chemical Synapse  Excitatory postsynapt ic A depolarizing postsynaptic potential potentials (EPSP) Postsynap Inhibitory tic postsynapt ic A hyperpolarizing postsynaptic potential potentials Potentials IPSP A postsynapt ic neuron can receive many signals at once  Neurotransmitters at chemical synapses cause either an excitatory or inhibitory graded potential Structure of Neurotransmitt er Receptors Neurotransmitter receptors have two structures Ionotropic Metabotrop receptors ic receptors Ionotropic & Metabotrop ic Receptors  Neurotransmitter can be removed from the synaptic cleft by: 1. Diffusion Removal of 2. Enzymatic degradation Neurotransmit 1. Example: Acetylcholinesterase ter breaks down Ach to choline where it gets recycled again in the synaptic end bulb. 3. Uptake into cells Summatio n If several presynaptic end bulbs release their neurotransmitter at about the same time, the combined effect may generate a nerve impulse due to summation Summation may be SPATIAL or TEMPORAL  Spatial Summati on Temporal Summati on Summatio n of Postsynapt ic Potentials Summary of Neuronal Structure and Function NEUROTRANSMITTERS Neurotransmitt ers Small molecule neurotransmitters Acetylcholine Amino acids Biogenic amines ATP and other purines Nitric oxide Carbon monoxide Neurotransmitters Regeneratio n & Repair of Nervous Tissue Although the nervous system exhibits plasticity, neurons have a limited ability to regenerate themselves Plasticity – the capability to change based on experience Regenerate – the capability to replicate or repair  In the CNS, there is little or no repair due to: Neurogene Inhibitory influences from neuroglia, particularly sis in the oligodendrocytes Absence of growth-stimulating CNS cues that were present during fetal development Rapid formation of scar tissue Damage and Repair in the CNS In the PNS repair is possible if the cell body is intact, Schwann cells are functional, and scar tissue formation does not occur too rapidly Steps involved in the repair process are: 1. Chromatolysis 2. Wallerian degeneration 3. Formation of a regeneration tube Damage and Repair in the CNS