Neural Tissue PDF

12 Neural Tissue PowerPoint® Lecture Presentations prepared by Jason LaPres Lone Star College—North Harris...

12 Neural Tissue PowerPoint® Lecture Presentations prepared by Jason LaPres Lone Star College—North Harris 1 © 2012 Pearson Education, Inc. An Introduction to the Nervous System The Nervous System Includes all neural tissue in the body Neural tissue contains two kinds of cells 1. Neurons Cells that send and receive signals 2. Neuroglia (glial cells) Cells that support and protect neurons 2 © 2012 Pearson Education, Inc. 12-1 Divisions of the Nervous System Anatomical Divisions of the Nervous System Central nervous system (CNS) Peripheral nervous system (PNS) 3 © 2012 Pearson Education, Inc.  12-1 Divisions of the Nervous System Functional Divisions of the PNS Afferent division Carries sensory information From PNS sensory receptors to CNS Efferent division Carries motor commands From CNS to PNS muscles and glands 4 © 2012 Pearson Education, Inc.  12-1 Divisions of the Nervous System Functional Divisions of the PNS Receptors and effectors of afferent division Receptors Detect changes or respond to stimuli Neurons and specialized cells Complex sensory organs (e.g., eyes, ears) Effectors Respond to efferent signals Cells and organs 5 © 2012 Pearson Education, Inc. 12-1 Divisions of the Nervous System Functional Divisions of the PNS The efferent division Somatic nervous system (SNS) Controls voluntary and involuntary (reflexes) muscle skeletal contractions 6 © 2012 Pearson Education, Inc. 12-1 Divisions of the Nervous System Functional Divisions of the PNS The efferent division Autonomic nervous system (ANS) Controls subconscious actions, contractions of smooth muscle and cardiac muscle, and glandular secretions Sympathetic division has a stimulating effect Parasympathetic division has a relaxing effect 7 © 2012 Pearson Education, Inc. 12-2 Neurons Cell body (soma) Large nucleus and nucleolus Perikaryon (cytoplasm) Mitochondria (produce energy) RER and ribosomes (produce neurotransmitters dendrites axon 8 © 2012 Pearson Education, Inc. 12-2 Neurons Cytoskeleton Neurofilaments and neurotubules in place of microfilaments and microtubules Neurofibrils: bundles of neurofilaments that provide support for dendrites and axon Nissl bodies Dense areas of RER and ribosomes Make neural tissue appear gray (gray matter) 9 © 2012 Pearson Education, Inc.  Dendrites 12-2 Neurons axon Axoplasm Axon hillock Initial segment Collaterals Telodendria Synaptic terminals Synapse Presynaptic cell Postsynaptic cell The synaptic cleft 10 © 2012 Pearson Education, Inc. 12-2 Neurons Neurotransmitters Are chemical messengers Are released at presynaptic membrane Affect receptors of postsynaptic membrane Are broken down by enzymes Are reassembled at synaptic terminal 11 © 2012 Pearson Education, Inc.  12-2 Neurons Types of Synapses Neuromuscular junction Synapse between neuron and muscle Neuroglandular junction Synapse between neuron and gland 12 © 2012 Pearson Education, Inc.  Structural Classification of Neurons Anaxonic neurons Unipolar neurons Found in brain and sense organs Found in sensory neurons of PNS Bipolar neurons Multipolar neurons Found in special sensory organs Common in the CNS (sight, smell, hearing) Include all skeletal muscle motor neurons 13 © 2012 Pearson Education, Inc. 12-2 Neurons Three Functional Classifications of Neurons 1. Sensory neurons Afferent neurons of PNS 2. Motor neurons Efferent neurons of PNS 3. Interneurons Association neurons 14 © 2012 Pearson Education, Inc.  12-2 Neurons Three Types of Sensory Receptors 1. Interoceptors Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive) Internal senses (taste, deep pressure, pain) 2. Exteroceptors External senses (touch, temperature, pressure) Distance senses (sight, smell, hearing) 3. Proprioceptors Monitor position and movement (skeletal muscles and joints) 15 © 2012 Pearson Education, Inc. Think Pair Share Pair up with the person sitting next to you and answer the following question. Be prepared to present your answer to the class. Describe to me the Three Types of Sensory Receptors Describe to me the difference between the Efferent and Afferent divisions of the PNS Come up to the board and draw the basic parts of a nerve cell and label them © 2012 Pearson Education, Inc. 12-2 Neurons Motor Neurons Carry instructions from CNS to peripheral effectors Two major efferent systems 1. Somatic nervous system (SNS) Includes all somatic motor neurons that innervate skeletal muscles 2. Autonomic (visceral) nervous system (ANS) Visceral motor neurons innervate all other peripheral effectors Smooth muscle, cardiac muscle, glands, adipose tissue 17 © 2012 Pearson Education, Inc. 12-3 Neuroglia Neuroglia Half the volume of the nervous system Four Types of Neuroglia in the CNS 1. Ependymal cells Cells with highly branched processes; contact neuroglia directly 2. Astrocytes Large cell bodies with many processes 3. Oligodendrocytes Smaller cell bodies with fewer processes 4. Microglia Smallest and least numerous neuroglia with many fine-branched processes 18 © 2012 Pearson Education, Inc.  12-3 Neuroglia Ependymal Cells Form epithelium called ependyma Line central canal of spinal cord and ventricles of brain Secrete cerebrospinal fluid (CSF) Have cilia or microvilli that circulate CSF Monitor CSF Contain stem cells for repair 19 © 2012 Pearson Education, Inc.  12-3 Neuroglia Astrocytes Maintain blood–brain barrier (isolates CNS) Create three-dimensional framework for CNS Repair damaged neural tissue Guide neuron development Control interstitial environment 20 © 2012 Pearson Education, Inc.  12-3 Neuroglia Oligodendrocytes Myelination Increases speed of action potentials Myelin insulates myelinated axons Makes nerves appear white 21 © 2012 Pearson Education, Inc.  Oligodendrocytes Nodes and internodes Internodes - myelinated segments Nodes (also called nodes of Ranvier) Gaps between internodes Where axons may branch 22 © 2012 Pearson Education, Inc.  12-3 Neuroglia Myelination White matter Regions of CNS with many myelinated nerves Gray matter Unmyelinated areas of CNS 23 © 2012 Pearson Education, Inc. 12-3 Neuroglia Microglia Migrate through neural tissue Clean up cellular debris, waste products, and pathogens 24 © 2012 Pearson Education, Inc. 12-3 Neuroglia Neuroglia of the Peripheral Nervous System Ganglia Masses of neuron cell bodies Surrounded by neuroglia Found in the PNS 25 © 2012 Pearson Education, Inc.  12-3 Neuroglia Neuroglia of the Peripheral Nervous System Satellite cells Surround ganglia Regulate environment around neuron 26 © 2012 Pearson Education, Inc.  Neuroglia of the Peripheral Nervous System Schwann cells Form myelin sheath around peripheral axons One Schwann cell sheaths one segment of axon Many Schwann cells sheath entire axon 27 © 2012 Pearson Education, Inc. 12-3 Neuroglia Nerve Regeneration in CNS Limited by chemicals released by astrocytes that: Block growth Produce scar tissue 28 © 2012 Pearson Education, Inc.  Nerve Regeneration in PNS 29 © 2012 Pearson Education, Inc. Think Pair Share Pair up with the person sitting next to you and answer the following question. Be prepared to present your answer to the class. Describe to me the four different types of Neuroglia of the CNS Describe to me the difference between white and grey matter What type of cell makes of the myelination of a nerve in the PNS Does the CNS or PNS heal and why © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential Ion Movements and Electrical Signals All plasma (cell) membranes produce electrical signals by ion movements Transmembrane potential is particularly important to neurons 31 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Five Main Membrane Processes in Neural Activities 1. Resting potential The transmembrane potential of resting cell 2. Graded potential Temporary, localized change in resting potential Caused by stimulus 32 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Five Main Membrane Processes in Neural Activities 3. Action potential Is an electrical impulse Produced by graded potential Propagates along surface of axon to synapse 33 © 2012 Pearson Education, Inc. Five Main Membrane Processes in Neural Activities 4. Synaptic activity Releases neurotransmitters at presynaptic membrane Produces graded potentials in postsynaptic membrane 5. Information processing Response (integration of stimuli) of postsynaptic cell 34 © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential Passive Forces acting Across the Plasma Membrane Chemical gradients Concentration gradients (chemical gradient) of ions (Na+, K+) Electrical gradients Separate charges of positive and negative ions Result in potential difference 35 © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential The Electrochemical Gradient For a particular ion (Na+, K+) is: The sum of chemical and electrical forces Acting on the ion across a plasma membrane A form of potential energy 36 © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential Equilibrium Potential The transmembrane potential at which there is no net movement of a particular ion across the cell membrane Examples: K+ = –90 mV Na+ = +66 mV 37 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Active Forces across the Membrane Sodium–potassium ATPase (exchange pump) Is powered by ATP Carries 3 Na+ out and 2 K+ in Balances passive forces of diffusion Maintains resting potential (–70 mV) 38 © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential The Resting Potential Because the plasma membrane is highly permeable to potassium ions: The resting potential of approximately –70 mV is fairly close to –90 mV, the equilibrium potential for K+ The electrochemical gradient for sodium ions is very large, but the membrane’s permeability to these ions is very low Na+ has only a small effect on the normal resting potential, making it just slightly less negative than the equilibrium potential for K+ 39 © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential The Resting Potential The sodium–potassium exchange pump ejects 3 Na+ ions for every 2 K+ ions that it brings into the cell It serves to stabilize the resting potential when the ratio of Na+ entry to K+ loss through passive channels is 3:2 At the normal resting potential, these passive and active mechanisms are in balance The resting potential varies widely with the type of cell A typical neuron has a resting potential of approximately –70 mV 40 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Passive Channels (Leak Channels) Are always open Permeability changes with conditions Active Channels (Gated Channels) Open and close in response to stimuli At resting potential, most gated channels are closed 41 © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential Three Classes of Gated Channels 1. Chemically gated channels 2. Voltage-gated channels 3. Mechanically gated channels 42 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Chemically Gated Channels Open in presence of specific chemicals (e.g., ACh) at a binding site Found on neuron cell body and dendrites 43 © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential Voltage-gated Channels Respond to changes in transmembrane potential Have activation gates (open) and inactivation gates (close) Characteristic of excitable membrane Found in neural axons, skeletal muscle sarcolemma, cardiac muscle 44 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Mechanically Gated Channels Respond to membrane distortion Found in sensory receptors (touch, pressure, vibration) 45 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Transmembrane Potential Exists Across Plasma Membrane Because: Cytosol and extracellular fluid have different chemical/ionic balance The plasma membrane is selectively permeable Transmembrane Potential Changes with plasma membrane permeability In response to chemical or physical stimuli © 2012 Pearson Education, Inc. 46 Think Pair share A drug that blocks ATP production is introduced into an isolated axon preparation. The axon is then repeatedly stimulated, and recordings are made of the response. What effects would you expect to observe? 47 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Graded Potentials Also called local potentials Changes in transmembrane potential That cannot spread far from site of stimulation Any stimulus that opens a gated channel Produces a graded potential 48 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Graded Potentials The resting state Opening sodium channel produces graded potential Resting membrane exposed to chemical Sodium channel opens Sodium ions enter the cell Transmembrane potential rises Depolarization occurs 49 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Graded Potentials Depolarization A shift in transmembrane potential toward 0 mV Movement of Na+ through channel Produces local current Depolarizes nearby plasma membrane (graded potential) Change in potential is proportional to stimulus 50 © 2012 Pearson Education, Inc.  12-4 Transmembrane Potential Graded Potentials Whether depolarizing or hyperpolarizing, share four basic characteristics 1. The transmembrane potential is most changed at the site of stimulation, and the effect decreases with distance 2. The effect spreads passively, due to local currents 51 © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential Graded Potentials 3. The graded change in transmembrane potential may involve either depolarization or hyperpolarization For example, in a resting membrane, the opening of sodium channels causes depolarization, whereas the opening of potassium channels causes hyperpolarization 4. The stronger the stimulus, the greater the change in the transmembrane potential and the larger the area affected © 2012 Pearson Education, Inc. 52 12-4 Transmembrane Potential Repolarization When the stimulus is removed, transmembrane potential returns to normal Hyperpolarization Increasing the negativity of the resting potential Result of opening a potassium channel Opposite effect of opening a sodium channel Positive ions move out, not into cell 53 © 2012 Pearson Education, Inc. 12-4 Transmembrane Potential Graded Potentials Effects of graded potentials At cell dendrites or cell bodies Trigger specific cell functions For example, exocytosis of glandular secretions At motor end plate Release ACh into synaptic cleft 54 © 2012 Pearson Education, Inc. 12-5 Action Potential Action Potentials Propagated changes in transmembrane potential Affect an entire excitable membrane Link graded potentials at cell body with motor end plate actions 55 © 2012 Pearson Education, Inc. 12-5 Action Potential Initiating Action Potential Initial stimulus A graded depolarization of axon hillock large enough (10 to 15 mV) to change resting potential (–70 mV) to threshold level of voltage-gated sodium channels (–60 to –55 mV) 56 © 2012 Pearson Education, Inc. 12-5 Action Potential Initiating Action Potential All-or-none principle If a stimulus exceeds threshold amount The action potential is the same No matter how large the stimulus Action potential is either triggered, or not 57 © 2012 Pearson Education, Inc. Figure 12-14 Generation of an Action Potential Resting Potential –70 mV The axolemma contains both voltage- gated sodium channels and voltage- gated potassium channels that are closed when the membrane is at the resting potential. KEY = Sodium ion 58 = Potassium ion © 2012 Pearson Education, Inc. 12-5 Action Potential Four Steps in the Generation of Action Potentials Step 1: Depolarization to threshold Step 2: Activation of Na channels Step 3: Inactivation of Na channels and activation of K channels Step 4: Return to normal permeability 59 © 2012 Pearson Education, Inc.  12-5 Action Potential 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 60 © 2012 Pearson Education, Inc.  12-5 Action Potential Step 3: Inactivation of Na channels and activation of K channels At +30 mV Inactivation gates close (Na channel inactivation) K channels open Repolarization begins 61 © 2012 Pearson Education, Inc.  12-5 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 62 © 2012 Pearson Education, Inc. Think Pair share Explain how an action potential and graded potential are different. Action potentials are depolarization events that exceed the “threshold”. Graded potentials are depolarization events that do not exceed the “threshold.” Graded potentials include depolarizations that stay below threshold (less negative) or hyper-polarizations that dip below resting potential (more negative). 63 © 2012 Pearson Education, Inc. 12-5 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 64 © 2012 Pearson Education, Inc.  12-5 Action Potential Absolute Refractory Period Relative Refractory Period Sodium channels open or inactivated Membrane potential almost No action potential possible normal Very large stimulus can initiate action potential 65 © 2012 Pearson Education, Inc. 12-5 Action Potential Powering the Sodium–Potassium Exchange Pump To maintain concentration gradients of Na+ and K+ over time Requires energy (1 ATP for each 2 K+/3 Na+ exchange) Without ATP Neurons stop functioning 66 © 2012 Pearson Education, Inc. 12-5 Action Potential Propagation of Action Potentials Propagation Moves action potentials generated in axon hillock Along entire length of axon Two methods of propagating action potentials 1. Continuous propagation (unmyelinated axons) 2. Saltatory propagation (myelinated axons) 67 © 2012 Pearson Education, Inc. 12-5 Action Potential Continuous Propagation Of action potentials along an unmyelinated axon Affects one segment of axon at a time Steps in propagation Step 1: Action potential in segment 1 Depolarizes membrane to +30 mV Local current Step 2: Depolarizes second segment to threshold Second segment develops action potential 68 © 2012 Pearson Education, Inc.  69 © 2012 Pearson Education, Inc. 12-5 Action Potential Continuous Propagation Steps in propagation Step 3: First segment enters refractory period Step 4: Local current depolarizes next segment Cycle repeats Action potential travels in one direction (1 m/sec) 70 © 2012 Pearson Education, Inc.  71 © 2012 Pearson Education, Inc. 12-5 Action Potential Saltatory Propagation Action potential along myelinated axon Faster and uses less energy than continuous propagation Myelin insulates axon, prevents continuous propagation Local current “jumps” from node to node Depolarization occurs only at nodes 72 © 2012 Pearson Education, Inc.  73 © 2012 Pearson Education, Inc. 12-6 Axon Diameter and Speed Axon Diameter and Propagation Speed Ion movement is related to cytoplasm concentration Axon diameter affects action potential speed The larger the diameter, the lower the resistance 74 © 2012 Pearson Education, Inc.  12-7 Synapses Two Types of Synapses 1. Electrical synapses Direct physical contact between cells 2. Chemical synapses Signal transmitted across a gap by chemical neurotransmitters 75 © 2012 Pearson Education, Inc.  12-7 Synapses Electrical Synapses Are locked together at gap junctions (connexons) Allow ions to pass between cells Produce continuous local current and action potential propagation Are found in areas of brain, eye, ciliary ganglia 76 © 2012 Pearson Education, Inc.  12-7 Synapses Chemical Synapses Are found in most synapses between neurons and all synapses between neurons and other cells Cells not in direct contact Action potential may or may not be propagated to postsynaptic cell, depending on: Amount of neurotransmitter released Sensitivity of postsynaptic cell 77 © 2012 Pearson Education, Inc. Think Pair Share Pair up with the person sitting next to you and answer the following question. Be prepared to present your answer to the class. Describe to me the different types of refractory periods What is the difference between continues and saltatory propagation Describe to me the difference between Chemical Synapses and electrical synapses © 2012 Pearson Education, Inc.  12-7 Synapses Two Classes of Neurotransmitters 1. Excitatory neurotransmitters Cause depolarization of postsynaptic membranes Promote action potentials 2. Inhibitory neurotransmitters Cause hyperpolarization of postsynaptic membranes Suppress action potentials 79 © 2012 Pearson Education, Inc. 12-7 Synapses The Effect of a Neurotransmitter On a postsynaptic membrane Depends on the receptor Not on the neurotransmitter For example, acetylcholine (ACh) Usually promotes action potentials But inhibits cardiac neuromuscular junctions 80 © 2012 Pearson Education, Inc.  81 © 2012 Pearson Education, Inc. 12-7 Synapses Synaptic Delay A synaptic delay of 0.2–0.5 msec occurs between: Arrival of action potential at synaptic terminal And effect on postsynaptic membrane Fewer synapses mean faster response Reflexes may involve only one synapse 82 © 2012 Pearson Education, Inc. 12-7 Synapses Synaptic Fatigue Occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli Synapse inactive until ACh is replenished 83 © 2012 Pearson Education, Inc.  12-9 Information Processing Postsynaptic Potentials Graded potentials developed in a postsynaptic cell In response to neurotransmitters Two Types of Postsynaptic Potentials 1. Excitatory postsynaptic potential (EPSP) Graded depolarization of postsynaptic membrane 2. Inhibitory postsynaptic potential (IPSP) Graded hyperpolarization of postsynaptic membrane 84 © 2012 Pearson Education, Inc. 12-9 Information Processing Inhibition A neuron that receives many IPSPs Is inhibited from producing an action potential Because the stimulation needed to reach threshold is increased Summation To trigger an action potential One EPSP is not enough EPSPs (and IPSPs) combine through summation 1. Temporal summation 2. Spatial summation 85 © 2012 Pearson Education, Inc. Information Processing Temporal Summation Multiple times Rapid, repeated stimuli at one synapse 86 © 2012 Pearson Education, Inc. Information Processing Spatial Summation Multiple locations Many stimuli, arrive at multiple synapses 87 © 2012 Pearson Education, Inc. Think Pair Share Pair up with the person sitting next to you and answer the following question. Be prepared to present your answer to the class. Describe to me the different between synaptic delay and synaptic fatigue Describe to me the different between Temporal and Spatial Summation © 2012 Pearson Education, Inc.