Chapter 12! Chapter 12 Neural Tissue! Neural tissue! pp ; ! Nervous System Overview!

Transcription

1 Chapter 12! Neural tissue! pp ; ! Nervous System Overview! Nervous system! Provides swift, brief responses to stimuli! Endocrine system! Adjusts metabolic operations and directs long-term changes! Nervous system includes! All the neural tissue of the body! Basic unit = neuron! 2! SECTION 12-1! The nervous system has anatomical and functional divisions! 3! 1!

2 Divisions of the Nervous System! CNS (Central Nervous system)! Brain and spinal cord! PNS (Peripheral Nervous system)! Neural tissue outside CNS! Afferent division brings sensory information from receptors! Efferent division carries motor commands to effectors! a. Somatic NS: skeletal muscle! b. Autonomic NS: smooth, cardiac muscle, glands! 4! SECTION 12-2! Neurons are nerve cells specialized for intercellular communication! 5! Schematic Neuron! 6! 2!

3 Neuron Anatomy Figure 12-2! 7! Parts of a Neuron! Cell body (soma)! Cell membrane! Has different types of membrane channels! Transmits graded potentials, not action potentials! Cytoplasm! Perikaryon = cytoplasm surrounding nucleus! Neurofilaments, neurotubules = cytoskeleton! Nissl bodies (substance)! RER and free ribosomes Gray matter! 8! Parts of a Neuron (2 of 3)! Dendrites Receive information from other neurons! Carry information towards cell body! Transmit graded potentials, not action potentials! Most have very few voltage-gated Na + channels Axon Axolemma, axoplasm! Connects to soma at axon hillock! First part = initial segment! Initial segment generates action potentials! Has voltage-gated Na + channels 9! 3!

4 Parts of a Neuron (3 of 3)! Collaterals Major branches of an axon! Telodendria Small branches at the end of an axon! Synaptic terminals - ends of the telodendria! a.k.a. synaptic end bulbs, boutons, synaptic knobs! Store neurotransmitter in synaptic vesicles! Release neurotransmitter in response to electrical activity! 10! Axoplasmic Transport (1 of 2) kinesin movie! A. Slow stream transport = axoplasmic flow New cytoplasm (axoplasm) being produced! Moves materials 1-5 mm per DAY B. Fast stream transport = fast axonal transport Movement of:! Mechanism:! Moves materials 5-10 mm per HOUR Neurotransmitters, enzymes, organelles! Molecular motors move on tracks! Motors = kinesin, dynein (use ATP)! Tracks = neurotubules (microtubules)! 11! Fast Axonal Transport (2 of 2)! C. Directionality! Anterograde (kinesins)! Movement away from cell body! Retrograde (dynein)! Movement toward cell body! e.g. Metabolites, other chemicals! May affect gene activity! e.g. Rabies virus, herpes virus,! tetanus toxin! 12! 4!

5 ! Chapter 12 Neural Tissue! A. Structural Classification of Neurons! Based on number of processes attached to cell body! Anaxonic! Axon and dendrites not structurally different! Poorly understood! Brain, special sense organs! 13! Unipolar Neuron! Unipolar - one process! A.K.A. pseudounipolar! Sensory neurons! Cell body in dorsal root ganglion! Up to 1 m long! 14! Bipolar Neuron! Bipolar - two short processes! Relatively rare! Eye, ear, nose! Special sensory! 15! 5!

6 !! Chapter 12 Neural Tissue! Multipolar Neuron! Multipolar! More than two processes! Most common type! e.g. motor neurons! 16! B. Functional Classification of Neurons! Sensory = afferent! Carry information towards CNS! Motor = efferent! Carry information away from CNS! Interneuron = association = internuncial! Do not enter or leave CNS! (exception: some are in autonomic ganglia)! 17! Receptor Types! Interoceptors! Sense internal environment! e.g., body temp., heart rate, blood pressure, etc.! Exteroceptors! Sense external environment! e.g., ambient temp., light, touch, sound, etc.!! Proprioceptors! Sense position of muscles and joint! i.e., body position! 18! 6!

7 SECTION 12-3! CNS and PNS neuroglia support and protect neurons! 19! Introduction to Neuroglia Figure 12-5! 20! CNS Glial Cells Figure 12-6! 21! 7!

8 CNS Glial Cells Ependymal Cells! 1. Ependymal cells! Epithelial cells! Have cilia or microvilli! Contact other glial cells! Functions:! Assist in cerebrospinal fluid (CSF) production! Assist in CSF circulation! Monitor conditions in CSF! Form blood-csf barrier at choroid plexuses May be able to differentiate into neurons!! 22! CNS Glial Cells Astrocytes (1 of 2)! 2. Astrocytes - star cells! A. Maintain blood-brain barrier! Affect capillary permeability! B. Structural support for neurons! C. Repair damage scar! D. In embryo, direct growth and interconnections of developing neurons! 23! CNS Glial Cells Astrocytes (2 of 2)! E. Control internal environment! Regulate K +, Na +, etc.! Feed neurons nutrients, ions, gases! Regulate capillary blood flow! Absorb and recycle neurotransmitters! Modify synaptic activity! Inhibit axonal growth! F. May be involved in AP conduction by neurons! FYI: Courtney Sobieski, Xiaoping Jiang, Devon C. Crawford and Steven Mennerick, 2015! 24! 8!

9 CNS Glial Cells Oligodendrocytes! 3. Oligodendrocytes = few branches compared to astrocytes! A. Most processes contact neuron cell bodies! Function of these is currently unclear! B. Myelinate CNS axons! More than one at a time (in contrast to Schwann cells)! C. Provide structural support! 25! CNS Glial Cells Microglia! 4. Microglia - small! Derived from monocyte/macrophage stem cells! Can move through! neural tissue! Phagocytize:! Cell debris! Wastes! Pathogens! 26! PNS Glial Cells Satellite Cells! 1. Satellite cells (amphicytes)! Found in peripheral ganglia! Ganglion = a collection of neuron cell bodies outside of the CNS! Regulate internal environment (like astrocytes in CNS) 27! 9!

10 PNS Glial Cells - Schwann cells! 2. Schwann cells (neurilemmocytes)! Myelinate peripheral axons (myelin sheath)! Enclose unmyelinated peripheral axons! Myelin sheath functions in axon repair and speed of action potential conduction 28! Schwann Cells and Myelination 1 Figure 12-7c! A Schwann cell wraps itself around an axon, squeezing Schwann cell cytoplasm to the outermost end of the cell. This leaves many layers of Schwann cell membrane wrapped around the axon.! 29! Schwann Cells and Myelination 2 Figure 12-6! Myelinated! Unmyelinated! 30! 10!

11 ! Chapter 12 Neural Tissue! Myelination Schwann cells/oligodendrocytes! Unmyelinated! Myelinated! PNS! Schwann cell! Myelinated! CNS! Page 96 in Leboffe! Oligodendrocyte! 31! Neural Response to Injury! For repair to occur, neuron cell body must remain alive! Most mature neurons thought to be in G 0! Exceptions:! Active stem cells present in olfactory epithelium and hippocampus (brain region)!! CNS repair of axons very limited:! 1. Injury would destroy many axons at once! 2. Astrocytes produce scar - block axon re-growth! 3. Astrocytes release axon growth inhibitors! 32! PNS Repair of Axons! Repair involves Schwann cells! Axon injury Wallerian degeneration! 1. Axon distal to injury site degenerates! 2. Schwann cells divide, form tube! 3. Macrophages engulf debris! 4. Axon forms bud, grows into tube! 5. Schwann cells myelinate axon! See Figure 12-8! 33! 11!

12 PNS Axon Regeneration Figure 12-8! Assumes cell body is still alive! 34! SECTION 12-6! Axon diameter, in addition to myelin, affects propagation speed! 35! Action Potential Conduction Velocity! Action potential velocity is influenced by:! 1. Axon diameter! Fiber diameter velocity! 2. Presence of electrical insulation (myelin)! Electrical insulation velocity! 36! 12!

13 Axon Classification - Speed of Conduction! Type A fibers! Largest! Myelinated! Velocity to 140 m/sec (300 mph!!)! Type B fibers! Medium size! Myelinated! Velocity about 18 m/sec (40 mph)! Type C fibers! Smallest! Unmyelinated! Velocity about 1 m/sec (2 mph)! 37! Myelination and Saltatory Conduction! Your drawing here! 38! Refractory Periods! Refractory period!! Period of time between initiation of AP and restoration of normal resting potential! Membrane does not respond normally to stimulation! A. Absolute refractory period! All Na + gates are open or inactivated (locked)! No amount of stimulation will cause another AP! 39! 13!

14 Refractory Periods - 2! B. Relative refractory period! A greater than normal amount of stimulation is required to begin another AP! Na + gates closed, but not inactivated.! Voltage-gated K + channels open and/or! Membrane hyperpolarized! 40! Refractory Periods See Figure 12-14! All Na + channels open.! OR! All Na + channels inactivated.! Na + channels closed, but not inactivated.! 41! SECTION 12-7! At synapses, communication occurs among neurons or between neurons and other cells! 42! 14!

15 ! Chapter 12 Neural Tissue! Information Flow! Action potential travels along an axon! Information passes from presynaptic neuron to postsynaptic cell! Direction of information flow 43! Information Flow Is Unidirectional! Information flow is unidirectional.! 1. Refractory periods do not allow impulse to go back where it just came from.! 2. Neurotransmitter only released by one end of the neuron! 44! Types of Synapses 1! 1. Electrical! Pre- and postsynaptic membranes fused at gap junctions Relatively rare, but more are currently being found in certain brain regions! E.g. Some hormone-secreting cells in hypothalamus; all cells release hormone at same time! 45! 15!

16 Types of Synapses 2! 2. Chemical! Review cholinergic synapse from 251! Note that one dose of neurotransmitter is usually not sufficient to cause a new action potential in a postsynaptic neuron.! 46! Review of Cholinergic Synapse Figure 12-16! 47! Properties of Chemical Synapses! A. Synaptic delay: msec delay at synapse! Mostly due to time for Ca 2+ influx into bouton and neurotransmitter release! I.e. not the time for neurotransmitter diffusion! Is a short delay, but can add up! ü Reflex arcs have few synapses FAST! B. Synaptic fatigue! Possible to run out of neurotransmitter! (But not a factor in muscle fatigue!)! 48! 16!

17 Properties of Chemical Synapses - 2! C. Drug effects on synaptic transmission! 1. Block voltage-gated channels - TTX! 2. Demyelinate axons - lead, arsenic! 3. neurotransmitter release - spider venom! 4. Block binding sites - atropine, curare! 5. Stimulate receptors/mimic - nicotine! 6. transmitter inactivation - anti-ache, nerve gas! 7. Block transmitter release - C. botulinum toxin! 8. Depolarize axon hillock - caffeine! 49! Some Drug Effects! From 8 th! edition of text! 50! SECTION 12-8! Neurotransmitters and neuromodulators have various functions! 51! 17!

18 Neurotransmitters/Receptors Figure 12-17a! Mechanisms of action! Depend upon transmitter binding to different types of receptors! A. Ionotropic receptors - DIRECT EFFECTS! Receptor is an ion channel Binding of transmitter directly alters ion channel! 52! B. Metabotropic Receptors (1 of 2)! B. Metabotropic receptors - INDIRECT EFFECTS! Receptor is not an ion channel! Binding of transmitter:! a. Indirectly causes ion channel to open! - AND/OR -! b. Changes cellular metabolism! Involves G-protein! Usually involves a second messenger! (e.g. camp)! 53! Metabotropic Receptors (2 of 2)! Mechanism of action! 1. Neurotransmitter binds to receptor! 2. G-protein activated! 3. GTP and part of G-protein separate! a. Subunit opens ion channel - OR -! b. Subunit activates second messenger! Second messenger (e.g. camp):! Opens ion channel - OR! Activates intracellular enzymes that change cellular metabolism! 54! 18!

19 ! Chapter 12 Neural Tissue! Metabotropic Receptor Actions Figure 12-17b! Indirect effects via G protein! 55! C. Indirect Effects - No Membrane Receptor e.g. Nitric oxide (NO) and carbon monoxide (CO)! Lipid-soluble gases! Not stored - synthesized as needed! Easily enter cell! Bind to enzymes! Second messenger produced! Open ion channels - AND/OR -! Change cell metabolism! E.g. NO involved in erectile function! 56! Indirect Effects of NO and CO Figure 12-17c! No membrane receptor 57! 19!

20 ! Chapter 12 Neural Tissue! Neurotransmitter Types (1 of 2)! 1. ACh! 2. Biogenic amines! EPI, NE, dopamine, serotonin, histamine! 3. Amino acids! Glutamate, aspartate, glycine, GABA! 4. Neuropeptides! Enkephalins, endorphins, substance P! 58! Neurotransmitter Types (2 of 2)! 5. Purines! ATP, Adenosine, GTP! 6. Gases! Nitric oxide, carbon monoxide! 7. Hormones - ADH, insulin, glucagon! 59! Examples of Neurotransmitter Actions! Excitatory - direct effect! Open channels for Na + and close K + - e.g. ACh! Open Ca 2+ channels - e.g. Glutamate! Inhibitory - direct effect! Open Cl - channels - e.g. Glycine, GABA! Open K + channels - e.g. GABA! Block Ca 2+ channels - e.g. GABA! Whether direct or indirect, excitatory or inhibitory, the effect depends upon transmitter and receptor type. 60! 20!

21 ! Chapter 12 Neural Tissue! SECTION 12-9! Information processing! 61! Information Processing! Direction of information flow Post-synaptic cell receives many inputs! Effect of presynaptic cell activity on postsynaptic cell s membrane = postsynaptic potential! 62! Postsynaptic Potentials See Figure 12-19! Postsynaptic potential are graded potentials and can be:!! Excitatory - EPSP! Postsynaptic cell closer to threshold! More likely to fire action potential! Inhibitory - IPSP! Postsynaptic cell further from threshold! Less likely to fire action potential! 63! 21!

22 ! Chapter 12 Neural Tissue! EPSPs and IPSPs: Effects/Interactions Figure 12-19! Recording from a postsynaptic cell! 64! Summation! Postsynaptic potentials are added together! If initial segment reaches threshold action potential! If initial segment does not reach threshold no action potential! Temporal summation! Single synapse fires repeatedly! Spatial summation! Different synapses fire at same time! 65! Summation Scheme! Temporal summation:! Neuron A fires repeatedly! Spatial summation:! Neurons A and B fire at the same time! 66! 22!

23 EPSPs and IPSPs: Effects/Interactions Figure 12-19! Recording from a postsynaptic cell! 67! Presynaptic Inhibition Figure 12-20a! May occur at axoaxonal synapses! A. GABA released onto presynaptic axon! B. Inhibits opening of voltage-gated Ca 2+ channels! Ca 2+ entry into bouton! neurotransmitter release! effect on postsynaptic cell! 68! Presynaptic Facilitation Figure 12-20b! May occur at axoaxonal synapses! A. Serotonin released onto presynaptic axon! B. Inhibits closing of voltage-gated Ca 2+ channels! Ca 2+ entry into bouton! neurotransmitter release! effect on postsynaptic cell! 69! 23!

24 ! Chapter 12 Neural Tissue! Rate of Generation of Action Potentials! Information is interpreted on the basis of action potential frequency (AP/second)! Few impulses/sec - sensed as light touch! Many impulses/sec - sensed as pain!! Initial segment can fire second AP if axon hillock remains above threshold:! Fires when absolute refractory period is over - and -! Fires if enough depolarization exists during relative refractory period! 70! Environmental Factors Affect Nerve Function! 1. Hydrogen ion concentration (ph)! H + modulates (reduces) ion channel opening! Alkalosis - increased excitability! (less interference with Na + gate opening)! Acidosis - decreased excitability! (more interference)! 71! Effects of [Ca 2+ ] in ECF! 2. Ionic composition of ECF! [Na + ], [K + ], [Ca 2+ ] also very important! e.g. Effects of [Ca 2+ ]! Ca 2+ modulates Na + channel opening! Hypocalcemia seizures! Causes membrane permeability to Na +! Promotes depolarization, excitability! Hypercalcemia - CNS depression! 72! 24!

25 ! Chapter 12 Neural Tissue! Pressure and Temperature Effects! 3. Pressure! Pressure blood flow neurotransmission! 4. Temperature! Increased temperature increased excitability! Decreased temperature decreased excitability! (ice pack on injury)! 73! Metabolic Considerations! Brain = 2% of body weight! Brain uses 18% of resting oxygen consumption! Synthesis, release, recycling of neurotransmitters! Maintain resting potential (Na + /K + pumps)! Recovery of resting potential after action potential! ü More APs need more Na + /K + pump activity to restore resting conditions! Axonal transport of transmitters, organelles, etc.! 74! 25!