The Nervous System: Neural Tissue Chapter 9a

Introduction Nervous system = control center & communications network Functions Stimulates movements Maintains homeostasis (with endocrine system)

Organization of the Nervous System Central nervous system (CNS) Brain & spinal cord Peripheral Nervous System (PNS) Afferent (Sensory) System Efferent (Motor) System Somatic Nervous System Autonomic Nervous System Sympathetic Nervous System Parasympathetic Nervous System Human Anatomy, 3rd edition Prentice Hall, © 2001

Organization of the Nervous System FG13_02.JPG Title: Functional Overview of the Nervous System Notes: This diagram shows the relationship between the CNS and PNS and the functions and components of the afferent and efferent divisions. Keywords: nervous system, central, information processing, sensory, afferent, motor commands, efferent, peripheral, somatic, autonomic, special sensory receptors, somatic sensory receptors, visceral sensory receptors, skeletal muscle, smooth, cardiac, glands Human Anatomy, 3rd edition Prentice Hall, © 2001

Histology of Nervous Tissue 2 types of cells Neurons Structural & functional part of nervous system Specialized functions Neuroglia (glial cells) Gli = glue Support & protection of nervous system

Human Anatomy, 3rd edition Neurons Function Conduct electrical impulses Structure Cell body Nucleus with nucleolus Cytoplasm Cytoplasmic processes Dendrites Axon Human Anatomy, 3rd edition Prentice Hall, © 2001

Human Anatomy, 3rd edition Anatomy of a Neuron FG13_09B.JPG Title: Anatomy of a Representative Neuron Notes: A neuron has a cell body (soma), some branching dendrites, and a single axon. (a) Detailed organization of the cell body. (b) A neuron may innervate other (1) neurons, (2) skeletal muscle fibers, or (3) gland cells. Synapses are shown in boxes. Keywords: neuroanatomy, multipolar neuron, skeletal muscle fibers, gland cells, dendrites, collateral branch, axon, axolemma, synapse, synaptic knobs, telodendria, neuroglandular junction, neuromuscular junction, perikaryon, axon hillock Human Anatomy, 3rd edition Prentice Hall, © 2001

Nerve Fibers of the PNS An axon and its sheaths Myelinated axon Axon is surrounded by a myelin sheath Unmyelinated axon Axon has no myelin sheath http://www.britannica.com/EBchecked/topic-art/409665/66781/Conduction-of-the-action-potential-In-a-myelinated-axon-the

Human Anatomy, 3rd edition Myelin White matter of nerves, brain, spinal cord Composed primarily of phospholipids Production Developing Schwann cells wind around axon Function Increases speed of impulse conduction Insulation and maintenance of axon Human Anatomy, 3rd edition Prentice Hall, © 2001

Human Anatomy, 3rd edition Myelin Nodes of Ranvier Unmyelinated gaps between segments of myelin Impulses “jump” from node to node Human Anatomy, 3rd edition Prentice Hall, © 2001

Human Anatomy, 3rd edition Nerve Fibers of the CNS Umyelinated Myelinated Production of myelin is from oligodendrocytes Nodes of Ranvier are less numerous Human Anatomy, 3rd edition Prentice Hall, © 2001

Classification of Neurons Functional – based on the direction of impulse transmission Sensory neurons Motor neurons Interneurons (association)

Functional Classification of Neurons FG13_11.JPG Title: A Functional Classification of Neurons Notes: Neurons are classified functionally into three categories: (1) sensory neurons that detect stimuli in the PNS and transmit information to the CNS, (2) motor neurons carrying information from the CNS to peripheral effectors, and (3) interneurons in the CNS Keywords: neuron, functional classification, peripheral nervous system, exteroceptors, proprioceptors, interoceptors, afferent, peripheral ganglion, somatic sensory neurons, efferent, central nervous system, ANS, SNS, somatic motor neurons, visceral motor neurons Human Anatomy, 3rd edition Prentice Hall, © 2001

Nerve Impulse A change in charge that travels as a wave along the membrane of a neuron Depends on the movement of K+ and Na+ across the nerve cell membrane. Called an action potential

Resting Neuron Sodium ions are in large concentration along the outside of the cell membrane Potassium ions are in large concentration along the inside of the cell membrane Membrane is polarized

Beginning of a Nerve Impulse Requires a stimulus of adequate strength Membrane is irritable Neuron may respond to a stimulus and convert it to an impulse. When? If above threshold = a critical level Minimum stimulus required to elicit a response

Starting a Nerve Impulse Depolarization – a stimulus begins the change in charge on the neuron’s membrane A depolarized membrane allows sodium (Na+) to flow inside the membrane The exchange of ions initiates an action potential in the neuron Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

The Action Potential If the action potential (nerve impulse) starts, it is propagated over the entire axon Potassium ions rush out of the neuron after sodium ions rush in Sodium and potassium are actively transported back to their original positions = repolarization Membrane is at rest again Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Nerve Impulse Propagation The impulse continues to move away from the cell body Impulses travel faster when fibers have a myelin sheath Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Continuation of the Nerve Impulse between Neurons Impulses are able to cross the synapse to another nerve Neurotransmitter is released from a nerve’s axon terminal The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter

How Neurons Communicate at Synapses Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Human Anatomy, 3rd edition Synapses FG13_12A.JPG Title: Structure of a Synapse Notes: A synapse is the site of communication between a neuron and another cell. (a) Diagrammatic view of a chemical synapse;color-enhanced TEM of a chemical synapse. (b) There may be thousands of chemical synapses on the surface of a single neuron. Keywords: synapse, synaptic knobs, telodendria, axon, dendrite, glial cell processes, myelin sheath, synaptic cleft, presynaptic membrane, postsynaptic membrane, synaptic vessicles Human Anatomy, 3rd edition Prentice Hall, © 2001

Human Anatomy, 3rd edition Neural Regeneration FG13_13A.JPG Title: Nerve Regeneration after Injury Notes: Steps involved in the repair of a peripheral nerve by the process of Wallerian degeneration. Keywords: nerve regeneration, injury, proximal stump, distal stump, axon, myelin, Schwann cell, macrophage, axon buds Human Anatomy, 3rd edition Prentice Hall, © 2001

Human Anatomy, 3rd edition Neural Regeneration FG13_13B.JPG Title: Nerve Regeneration after Injury Notes: Steps involved in the repair of a peripheral nerve by the process of Wallerian degeneration. Keywords: nerve regeneration, injury, proximal stump, distal stump, axon, myelin, Schwann cell, macrophage, axon buds Human Anatomy, 3rd edition Prentice Hall, © 2001

Human Anatomy, 3rd edition Neural Regeneration FG13_13C.JPG Title: Nerve Regeneration after Injury Notes: Steps involved in the repair of a peripheral nerve by the process of Wallerian degeneration. Keywords: nerve regeneration, injury, proximal stump, distal stump, axon, myelin, Schwann cell, macrophage, axon buds Human Anatomy, 3rd edition Prentice Hall, © 2001

Human Anatomy, 3rd edition Neural Regeneration FG13_13D.JPG Title: Nerve Regeneration after Injury Notes: Steps involved in the repair of a peripheral nerve by the process of Wallerian degeneration. Keywords: nerve regeneration, injury, proximal stump, distal stump, axon, myelin, Schwann cell, macrophage, axon buds Human Anatomy, 3rd edition Prentice Hall, © 2001

Structure of a Nerve Endoneurium surrounds each fiber Groups of fibers are bound into fascicles by the perineurium Fascicles are bound together by epineurium Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Classification of Nerves Mixed nerves Both sensory and motor fibers Afferent (sensory) nerves Carry impulses toward the CNS Efferent (motor) nerves Carry impulses away from the CNS