Nervous Tissue CH. 12 Overview of the Nervous System Objectives List the structures and basic functions of the nervous system. Describe the organization of the nervous system. Structures Nervous system Smallest and most complex system Billions of neurons Includes: Brain Cranial nerves Nerves – bundle of axons Spinal cord – connects to brain Spinal nerves Functions Sensory – detect internal and external stimuli Sensory neurons carry information to brain Integrative – analyzes and stores information; makes appropriate responses Perception – conscious awareness of sensory stimuli Interneurons – participate in integration Function Motor – causes movement or gland secretion in response to stimuli Motor neurons – carry information to muscle or gland (effectors) Nervous System CNS – Central NS PNS – Peripheral NS All tissue outside the Contains brain and spinal cord (axial) Processes sensory info Source of thoughts, emotions, memories CNS (appendicular) Includes cranial nerves, branches, spinal nerves, sensory receptors Divided further SNS – somatic NS (body) ANS – autonomic NS (self) ENS – enteric NS (intestines) Nervous System Peripheral NS Branches Somatic NS – voluntary Consists of: Sensory neurons in head, body, limbs, vision, hearing, taste, and smell that send messages to CNS Motor neurons that conduct messages from CNS to skeletal muscles only Autonomic NS – involuntary Consists of: Sensory neurons in organs that send messages to CNS Motor neurons that conduct messages from CNS to smooth muscle, cardiac muscle, and glands ANS Branches Sympathetic division Parasympathetic division Supports exercise or “rest and digest” emergency actions “fight or flight” Ex: increases heart rate Ex: decreases heart rate Usually work in opposing actions ENS “brain of the gut” Controls all activities associated with digestion and the gastrointestinal (GI) tract Involuntary Overall Organization Organization Review What are the components of the CNS and PNS? What kinds of problems would result from damage of sensory neurons, interneurons, and motor neurons? What are the components and functions of the SNS, ANS, and ENS? Which subdivisions of the PNS control voluntary actions? Involuntary actions? Histology of Nervous Tissue Objectives Contrast the histological characteristics and the functions of neurons. Contrast the functions of neuroglia. Neurons vs. Neuroglia Neurons Neuroglia Provide unique Support, nourish, and functions Sensing, thinking, remembering, controlling muscle activity, regulating glandular secretions protect the neurons Maintain homeostasis in the interstitial fluid that bathes them Neurons Vocabulary: Neuron – nerve cell Electrical excitability Stimulus the ability to respond to a stimulus and convert it into an action potential any change in the environment that is strong enough to initiate an action potential Action potential – nerve impulse An electrical signal that propogates (travels) along the surface of the membrane of a neuron Can travel up to 280 mph Parts of a Neuron Three parts Cell body Main part of the cell Includes organelles, nucleus, and cytoplasm Dendrites Receiving parts of the neuron Short, tapered, and highly branched Axon Transmitting parts of the neuron Long, thin, cylindrical Parts of a Neuron Parts of a Neuron Synapse – site of communication between 2 neurons or a neuron and an effector cell Synaptic end bulb – swollen end of an axon where synaptic vesicles hold neurotransmitters Neural Diversity Multipolar neurons Several dendrites, one axon Found in brain and spinal cord Bipolar neurons One main dendrite, one axon Eye, ear, olfactory of brain Unipolar neurons Axon and dendrite fuse at beginning and then branch Occurs as an embryo Neural Diversity Others Purkinje cells – cerebellum Pyramidal cells – cerebral cortex of brain Neuroglia Actively participate in nervous tissue functioning Do not generate action potentials Can multiply and divide – neurons cannot Types of Neuroglia CNS Astrocytes – create blood-brain barrier, strength Oligodendrocytes – create myelin sheath around CNS axons Microglia – remove cellular debris during neural development Ependymal cells – assist with circulation of cerebrospinal fluid PNS Schwann cells – create myelin sheath around PNS axons Satellite cells – support, regulate exchange of materials Types of Neuroglia Types of Neuroglia Myelination Myelin sheath – multilayered lipid and protein covering around some axons Provides insulation Increases speed of nerve impulse If a cell has myelin we say that it is myelinated Gaps in the myelin sheath are called nodes of Ranvier Review Describe the parts of a neuron and the functions of each. Give examples of the structural diversity of neurons. Give examples of the different types of neuroglia. Where are each found? What do they do? What is myelin? Electrical Signals in Neurons Objectives Describe the cellular properties that permit communication among neurons and effectors. Compare the basic type of ion channels, and explain how they relate to action potentials and graded potentials. Describe the factors that maintain a resting membrane potential. Mini-Physics Lesson Potential energy – energy stored in a system (the body) as a result of its position or chemical composition Kinetic energy – energy being used for motion or force Physics to Anatomy Neurons are excitable because of a voltage difference across the membrane - potential “Potential” will initiate an impulse that can travel through the nervous system Graded potentials – used for short-distance communication Action potentials – allow communication over short and long distance within the body Potential Membrane potential – electrical voltage difference across the membrane Resting membrane potential – membrane potential in an excitable cell Like voltage stored in a battery If + and – ends connect, electrons flow in a current Current Flow of charged particles For the body – these are ions instead of elecrons Ion Channels Gradient – difference Electrochemical gradient – difference in charge and concentration Positive cations move toward negative areas, negative anions move toward positive areas Ion location can be controlled with gates that can open or close the pore Ion Channels Leakage channels Voltage-gated channels Ligand-gated channels Mechanically gated channels Ion Channels Leakage channels Randomly open and close most cells leak more potassium (K+) than sodium (Na+) voltage-gated channels Opens in response to a change in voltage (membrane potential) Generate and conduct action potentials Ion Channels Ligand-gated channels Opens and closes in response to chemical stimulus (nts, hormones, other ions – ligand molecule) Ex: Ach opens channels that allow Na+ and Ca2+ to go in and K+ to go out Work in 2 ways ligand molecule can open or close the channel itself by binding Ligand molecule activates another chemical messenger to open the channel Ion Chanenls Mechanically gated Opens or closes in response to stimulation by: Vibration: sound waves Pressure: touch Tissue stretching The channel is physically disrupted and opens Ion Channels Resting Membrane Potential Exists due to negative ions in cytosol (ICF) and equal positive ions in ECF The greater the difference in charge, the larger the potential Example: 5 Na+ on outside, 4 Cl- on inside – small potential 25 Na+ on outside, 4 Cl- on insdie – great potential RMP Most cells have potential between -40 to -90 mV; typical is -70mV Minus sign means the cell is negative – not negative potential! Any cell with potential is polarized The potential varies between +5 to -100 mV How does the potential get there? ECF Rich in Na+ and Cl ICF Rich in K+ Also has P-, amino acids, RMP Ion interaction There are many K+ leakage channels, so K+ diffuse out + ions exit, inside becomes more negative - ions can’t leave because they are bound to molecules - charges attract the K+ back in toward the cell Few Na+ ions leak inward This would destroy the membrane potential, so there are pumps that take care of this Na+/K+ pump To keep the RMP Pump out Na+ as it leaks in Pump in K+ as it leaks out 3 Na+ for each 2 K+ - this still maintains a negative charge in the cell RMP - Draw Review Define resting membrane potential. Describe each type of ion channel. Describe the cellular processes that create the resting membrane potential. Potentials Objectives Describe what causes a graded potential. Understand the process for creating an action potential. Explain depolarization and repolarization, including the relationship between them. Graded Potentials When a stimulus causes a channel to open or close in an excitable cell Causes more polarization (more - inside) Causes less polarization (less – inside) Hyperpolarized (much more – inside) Depolarized (much less – inside) Graded signals – vary in size Spread out along plasma membrane and die out Only used for short distance communication Generation of Action Potentials Action potential (AP) or impulse – sequence of rapidly occurring events that take place in two phases Depolarizing phase – negative membrane potential becomes Less negative Reaches 0 positive Repolarizing phase – membrane potential is Restored to the resting state of -70 mV Action Potential Depol. vs. Repol. Na+ channels open Na+ rushes into cell K+ channels open K+ flows out of cell All or none principle when depolarization reaches the threshold, the AP occurs AP is always the same size Like dominos – no matter how hard you push the first domino they will all fall or they won’t – same goes for the AP Depolarizing Phase Na+ channels open rapidly – Na+ rushes into the cell Potential goes from -55 to +30 (inside is 30 more positive than the outside) The more Na+ there is, the more channels open – this is a ____________-feedback mechanism Repolarizing Phase K+ channels open as the Na+ channels are closing Na+ inflow slows, K+ outflow increases Potential goes from +30 to -70 Depol. and Repol. Painkillers Local anesthetics – block pain and other body sensations Novacaine or Lidocaine Block the opening of Na+ channels The nerve impulse can’t go past the blocked region – CNS never gets the message that there is pain Review What causes a graded potential? Describe what happens to cause an action potential. What happens during depolarization? What does this cause? What happens during repolarization? What does this cause? Signal Transmission Synapses Objectives Explain the events of signal transmission at a chemical synapse. Synaptic vocabulary Presynaptic neuron – the one sending the signal Postsynaptic neuron – the one receiving the signal 3 types Axodendritic – axon to dendrite Axosomatic – axon to body cell Axoaxonic – axon to axon Electrical Synapse Action potentials conduct between cells through gap junctions Tunnels connect the two cells together and the electrical impulse can pass places: Smooth muscle, cardiac muscle, developing embryo advantages: Faster – don’t have to “jump the gap” Synchronization – allows groups of cell’s to work together – heart beating Electrical Synapse Chemical Synapse Pre- and post-synapses do not touch – separated by synaptic cleft Transfer of signal must occur Pre-synaptic neuron turns electrical signal into chemical signal (neurotransmitter) Post-synaptic neuron turns chemical signal into electrical signal this transfer takes more time than electrical signals Chemical Synapse 1. 2. 3. 4. 5. Nerve impulse arrives Depolarizing phase opens Ca++ channels, Ca++ flows in Ca++ causes release of NT from vesicles NT binds to receptors on postsynaptic neuron Action potential continues (Na+, K+, Cl) Review What are the differences between a chemical and electrical synapse?