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8/13/2009 Outline Nervous System - Neurons Biol 105 Lecture 9 Chapter 7 I. II. III. IV. V. VI. Nervous system function Central and peripheral nervous system Nervous system cells Myelinated neurons Nerve signal transmission Nerve Synapse Copyright © 2009 Pearson Education, Inc. Nervous Tissues Nervous System Includes nerve tissue and sense organs Nervous tissue functions to conduct messages throughout the body. When nerve cells are stimulated, an electrical signal quickly travels through the nerve cell to the nerve ending, triggering events Nervous system functions to: Senses environment – receives information from both outside and inside the body Processes the information it receives Respond to information – sends out orders Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Two Parts of the Nervous System Central Nervous System 1. Central Nervous System (CNS) Brain and Spinal Cord Peripheral 2. Peripheral Nervous System (PNS) Nervous tissue outside brain and spine Sense organs Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. 1 8/13/2009 Nervous System Cells Two types of nervous tissue cells Neurons – the cells that are responsible for transmitting messages Neuroglial cells – cells that support the neurons Copyright © 2009 Pearson Education, Inc. Neuroglial cells Microglia – immune system cells, engulf bacteria and cellular debris Astrocytes – provide nutrients to neurons Oligodenrocytes and Schwann cells – form myelin sheaths Copyright © 2009 Pearson Education, Inc. Figure 4.6 Parts of a Neuron Cell body – contains the nucleus, main body of cell Dendrites – projections from the cell body that carry messages to the cell body Axons – one large projection that carry messages away from the cell body Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Neuron Neurons Have Dendrites, a Cell Body, and an Axon Dendrites receive information from other neurons or from the environment. The cell body controls the cell’s metabolic activities. Axon endings release chemicals called neurotransmitters that affect the activity of nearby neurons or an effector (muscle or gland). Nucleus Cell body The cell body integrates input from other neurons. Receiving portion of neuron Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. An axon conducts the nerve impulse away from the cell body. Axon endings Sending portion of neuron Figure 7.2 2 8/13/2009 Neurons of the Peripheral Nervous System Neurons in the PNS are either carrying messages to or from the CNS Afferent = Sensory neurons = Neurons carrying messages to the CNS Efferent = Motor neurons = Neurons carrying messages from the CNS Copyright © 2009 Pearson Education, Inc. 12-12 Copyright © 2009 Pearson Education, Inc. Interneurons in the Central Nervous System Located between sensory and motor neurons within the CNS Interneurons integrate and interpret sensory signals Copyright © 2009 Pearson Education, Inc. Sensory Neurons Copyright © 2009 Pearson Education, Inc. Motor Neurons The efferent or motor neuron cell bodies are located in the gray matter of the spinal cord. The afferent or sensory neuron cell bodies are located in dorsal root ganglion. Copyright © 2009 Pearson Education, Inc. Their axons leave the CNS and go to the skeletal muscles Copyright © 2009 Pearson Education, Inc. 3 8/13/2009 Neurons of the Nervous System Muscle (effector) Sensory receptor for pain Impulse direction Sensory neuron Cell body Motor neuron Interneuron Copyright © 2009 Pearson Education, Inc. 12-5 Figure 7.1 Myelinated neurons Neurons that have axons covered with neuroglial cells that contain the protein myelin are called myelinated neurons Myelinated neurons are able to carry messages faster than non-myelinated neurons Copyright © 2009 Pearson Education, Inc. Functions of Myelin Sheaths 1. The main benefit of myelin sheaths is that myelinated neurons are able to carry messages faster than non-myelinated neurons Two Types of Cells that Myelinate neurons Schwann cells and Oligodenrocytes are wrapped around neuronal axons 2. Myelin sheaths from Schwann cells also help regenerate injured PNS neuron axons Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. 4 8/13/2009 Myelinated Neurons Myelinated neurons Nucleus Schwann cells are found in the PNS Oligodendrocytes are found in the CNS Dendrites Cell body In saltatory conduction, the nerve impulses jump from one node of Ranvier to the next. Nodes of Ranvier are spaces on the axon between the glial cells Node of Ranvier Schwann cell Myelin sheath (a) Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Myelin Sheath Copyright © 2009 Pearson Education, Inc. Multiple Sclerosis (MS) Caused by the destruction of the myelin sheath that surrounds axons found in the CNS Can result in paralysis and loss of sensation, including loss of vision Copyright © 2009 Pearson Education, Inc. Figure 7.3a Myelin Sheath Figure 7.3b Copyright © 2009 Pearson Education, Inc. Figure 7.3c Nerve Nerve contain Neuron axons are bundled together These bundles contain Axons Blood vessels Connective tissue Copyright © 2009 Pearson Education, Inc. 5 8/13/2009 Nerve Connective tissue surrounding one nerve Blood supply Axons within a connective tissue sheath One axon (d) The anatomy of a nerve Figure 8.9d Copyright © 2009 Pearson Education, Inc. The Nerve Impulse Is an Electrochemical Signal A nerve impulse, or action potential, involves sodium ions (Na+) and potassium ions (K+) that cross the cell membrane through the ion channels Each ion channel is designed to allow only certain ions to pass through Action Potential Cross section Axon membrane Neuron plasma membrane Extracellular fluid Cytoplasm Continually open ion channels “Gated” ion channels Ion channels Ion channels can be open continuously or opened and closed by a m olecular gate Copyright © 2009 Pearson Education, Inc. Membrane Potential The difference in charge between the inside and outside of the neuron is the membrane potential Sodium -potassium pump Sodium-potassium pump The sodium-potassium pump uses cellular energy (ATP) to pum p sodium ions out of the cell and potassium ions into the cell Copyright © 2009 Pearson Education, Inc. Figure 7.4 Resting Membrane Potential A neuron that is not conducting a message is said to be “Resting” When a neuron is resting there is more sodium (Na+) outside the neuron cell and more potassium (K+) inside the cell The inside of the cell has a negative charge compared to the outside the cell Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. 6 8/13/2009 Resting Membrane Potential Copyright © 2009 Pearson Education, Inc. Sodium Potassium Pump To maintain this resting membrane potential the neuron pumps Na+ out of the cell and K+ into the cell. The transport proteins take 3 Na+ ions out for every 2 K+ ions into the cell = Na+/K+ pump This is Active Transport – requiring ATP Copyright © 2009 Pearson Education, Inc. Steps of an Action Potential The Nerve Impulse Copyright © 2009 Pearson Education, Inc. Figure 7.5 (1 of 4) Action Potential An electrochemical signal conducted along an axon. It is a wave of depolarization followed by repolarization Depolarization is caused by sodium ions entering the axon Repolarization is caused by potassium ions leaving axon Copyright © 2009 Pearson Education, Inc. Action Potential 1. The axon is depolarized when voltage gated sodium ion channels open and Na+ comes rushing in, causing the inside of the neuron to become positively charged Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Figure 7.5 (2 of 4) 7 8/13/2009 Action Potential Steps of an Action Potential 2. The axon is repolarized when voltage gated potassium ion channels open up and allow K+ to go out of the axon This returns the membrane potential to be negative on the inside of the neuron The action potential travels down the axon Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Action Potential Figure 7.5 (3 of 4) The Nerve Impulse After the action potential, the sodium potassium pump restores the original conditions by pumping sodium (Na+) out of the cell and potassium (K+) back into the cell Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. The Nerve Impulse Figure 7.5 (4 of 4) Action Potentials It is an all or nothing response – if it is not a great enough stimulation the channels won’t open. The level of the action potential is always the same. The direction is always one way down the axon. The sodium channels are inactivated for awhile after the action potential passes = refractory period. PLAY Animation—The Nerve Impulse Copyright © 2009 Pearson Education, Inc. Figure 7.6 Copyright © 2009 Pearson Education, Inc. 8 8/13/2009 Nerve Synapse Components of the Synapse How are messages passed from one nerve to the next or from the nerve to a muscle? The junction between two neurons or between a neuron and a muscle is called a synapse 1. Presynaptic neuron is the transmitting neuron 2. postsynaptic neuron is the receiving neuron or the muscle 3. And the gap in between them = synaptic cleft Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Synaptic Transmission Presynaptic neuron Dendrites Axon Nucleus Presynaptic neuron has synaptic vesicles that contain neurotransmitters Cell body Impulse Impulse Step 1: The impulse reaches the axon ending of the presynaptic membrane. Synaptic knob Step 2: Synaptic vesicles release neurotransmitter into the synaptic cleft. Synaptic cleft Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Synaptic Transmission Membrane of postsynaptic neuron Figure 7.8 (1 of 3) Synaptic Transmission Step 4: Neurotransmitter molecules bind to receptors on the postsynaptic neuron. Neurotransmitter Synaptic vesicle Step 3: Neurotransmitter diffuses across synaptic cleft. Step 5: Sodium ion channels open. Step 6: Sodium ions enter the postsynaptic neuron, causing depolarization and possible action potential. Receptor (of sodium ion channel) on postsynaptic membrane Copyright © 2009 Pearson Education, Inc. Synaptic vesicle Figure 7.8 (2 of 3) Copyright © 2009 Pearson Education, Inc. Figure 7.8 (3 of 3) 9 8/13/2009 Transmission across synaptic cleft Transmission across synaptic cleft 1. The action potential gets to the end of the presynaptic axon 4. The synaptic vesicles release the neurotransmitters into the synaptic cleft 2. The action potential triggers Ca2+ to enter the presynaptic axon terminal 5. These neurotransmitters travel across the synaptic cleft to the postsynaptic neuron (or the muscle) 3. The Ca2+ triggers synaptic vesicles located at the axon terminal to merge with the neural membrane 6. Neurotransmitter binds to receptors on the postsynaptic neuron (or muscle) Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Transmission across synaptic cleft Neurotransmtters 7. These receptors are ligand gated sodium ion channels which allow Na+ to enter the postsynaptic neuron (or muscle) and triggers an action potential in the postsynaptic neuron (or muscle contraction) Acetylcholine 8. Once the neurotransmitters are released they need to be destroyed or contained quickly or they will continue to stimulate the nerve Myasthenia gravis is an autoimmune disease that attacks the acetylcholine receptors, resulting in reduced muscle strength Copyright © 2009 Pearson Education, Inc. Acts in both the PNS and the CNS as a neurotransmitter Causes voluntary muscles to contract Acetylcholinesterase Copyright © 2009 Pearson Education, Inc. Important Concepts Read Chapter 8 What are the functions of nervous system What are the two types of cells in nervous tissue (neuroglial cells and neurons). What are the three types of neuroglial cells and their functions What are the two main divisions of nervous system (CNS, PNS) and where each is found Copyright © 2009 Pearson Education, Inc. 10 8/13/2009 Important Concepts What are the parts and functions of a neuron What are the three types of neurons (sensory, interneuron and motor neurons) and their functions, and where are they located Where are the cell bodies are located for motor and sensory nerve cells Copyright © 2009 Pearson Education, Inc. Important Concepts How do ions pass through membranes What is the function of the sodium potassium pump Important Concepts What are schwann cells and oligodendrocytes and what are their function Where Schwann vs oligodendrocytes are found What is the cause and effects of multiple sclerosis What are the parts of a nerve Copyright © 2009 Pearson Education, Inc. Important Concepts What ions enter and the leave the neuron during the depolarization and repolarization steps of action potential, what is the relative charge of the inside vs the outside of the neuron during these events, what is the order of events. What are the steps of messages being conducted through a neuron, starting with the resting stage and ending with the next neuron or muscle being stimulated. Components of the synapse Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Important Concepts What is acetylcholine, where is it found, what effect does it have, how is acetylcholine removed from the synaptic cleft What is the cause and effect of Myasthenia gravis Copyright © 2009 Pearson Education, Inc. Function of neurotransmitters, how do they work, where do they work, know the ions involved and their functions. Definitions Afferent neurons, efferent neurons, dendrites, axons, sensory neurons, interneuron, motor neurons, myelin, myelin sheath, myelinated neurons, schwann cells, oligodendrocytes, nodes of ranvier, nerve, ions, ion channels, ligand gated ion channels, voltage gated ion channels, action potential, repolarization, depolarization, membrane potential, resting potential, sodium potassium pump, refractory period, synapse, synaptic cleft, synaptic vesicles, neurotransmitters, acetylcholinesterase, presynaptic neuron, postsynaptic neuron, stimulate, inhibit Copyright © 2009 Pearson Education, Inc. 11