7 The Nervous System PART A Power Point

7 The Nervous System PART A Power. Point® Lecture Slide Presentation by Jerry L. Cook, Sam Houston University ESSENTIALS OF HUMAN ANATOMY & PHYSIOLOGY EIGHTH EDITION ELAINE N. MARIEB Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functions of the Nervous System § Sensory input – gathering information § To monitor changes occurring inside and outside the body § Changes = stimuli = sensory input § Integration § To process and interpret sensory input and decide if action is needed Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functions of the Nervous System § Motor output § A response to integrated stimuli § The response activates muscles or glands Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Structural Classification of the Nervous System § Central nervous system (CNS) § Brain § Spinal cord § Act as integrating & command centers (interpret sensory input & issue commands) § Peripheral nervous system (PNS) § Nerve outside the brain and spinal cord § Spinal nerves carry impulses to & from spinal cord § Cranial nerves carry impulses to & from brain Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functional Classification of the Peripheral Nervous System § Sensory (afferent) division § Nerve fibers that carry information to the central nervous system Figure 7. 1 Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functional Classification of the Peripheral Nervous System § Sensory fibers that carry impulses from skin, skeletal muscle & joints = somatic sensory fibers. § Those that carry impulses from the visceral organs = visceral sensory fibers Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functional Classification of the Peripheral Nervous System § Motor (efferent) division § Nerve fibers that carry impulses away from the central nervous system to effector organs, the muscles & glands Figure 7. 1 Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functional Classification of the Peripheral Nervous System § Motor (efferent) division § Two subdivisions § Somatic nervous system = voluntary § Autonomic nervous system = involuntary Figure 7. 1 Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functional Classification of the Peripheral Nervous System § Autonomic nervous system = involuntary § 2 parts § Sympathetic – mobilizes the body during extreme situations – called the “fight or flight” division § Parasympathetic – allows us to “unwind” & conserve energy – called the craniosacral division Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Organization of the Nervous System Figure 7. 2 Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Nervous Tissue: Support Cells (Neuroglia = literally “nerve glue”) § Astrocytes § Abundant, star-shaped cells § Brace neurons; anchor to capillaries § Form barrier between capillaries and neurons § Control the chemical environment of the brain Figure 7. 3 a Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Nervous Tissue: Support Cells § Microglia § Spider-like phagocytes § Dispose of debris (dead brain cells, bacteria) § Ependymal cells § Line cavities of the brain and spinal cord § Beating cilia circulate cerebrospinal fluid Figure 7. 3 b–c Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Nervous Tissue: Support Cells § Oligodendrocytes § Produce myelin sheath around nerve fibers in the central nervous system § Unable to transmit nerve impulses § Never lose their ability to divide § Most brain tumors are gliomas tumors formed by neuroglia Figure 7. 3 d Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Nervous Tissue: Support Cells § Satellite cells § Protect neuron cell bodies § Schwann cells § Form myelin sheath in the peripheral nervous system Figure 7. 3 e Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Nervous Tissue: Neurons § Neurons = nerve cells § Cells specialized to transmit messages § Major regions of neurons § Cell body – nucleus and metabolic center of the cell § Processes – fibers that extend from the cell body Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Neuron Anatomy § Cell body § Nissl substance – specialized rough endoplasmic reticulum § Neurofibrils – intermediate cytoskeleton that maintains cell shape Figure 7. 4 a Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Neuron Anatomy § Cell body § Nucleus § Large nucleolus § Lacks centrioles – confirms amitotic nature of most neurons Figure 7. 4 a–b Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Neuron Anatomy § Extensions outside the cell body – from microscopic to 3 -4 feet in length § Dendrites – conduct impulses toward the cell body – may number in the 100’s § Axons – conduct impulses away from the cell body – only one Figure 7. 4 a Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Axons and Nerve Impulses § Axons end in axon terminals § Axon terminals contain vesicles with neurotransmitters – impulses stimulate their release § Axon terminals are separated from the next neuron by a gap § Synaptic cleft – gap between adjacent neurons § Synapse – junction between nerves Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Nerve Fiber Coverings § Schwann cells – produce myelin sheaths in jelly-roll like fashion on axons outside the CNS § Neurilemma – part of Schwann cell external to myelin sheath § Nodes of Ranvier – gaps in myelin sheath along the axon Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings Figure 7. 5

Nerve Fiber Coverings § Oligodendrocytes form CNS myelin sheaths § CNS sheaths lack a neurilemma § Because the neurilemma remains intact (for the most part) when a peripheral nerve fiber is damaged, it plays an important role in fiber regeneration, an ability that is largely lacking in the CNS Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Neuron Cell Body Location § Most are found in the central nervous system § Gray matter – cell bodies and unmylenated fibers § Nuclei – clusters of cell bodies within the white matter of the central nervous system § Ganglia – collections of cell bodies outside the central nervous system § Bundles of nerve fibers in the CNS are called tracts; whereas in the PNS they are called nerves Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functional Classification of Neurons § Sensory (afferent) neurons § Cell bodies always found in the ganglion outside the CNS § Carry impulses from the sensory receptors to CNS § Cutaneous sense organs (skin) § Pain receptors – bare dendrite endings – most numerous § Proprioceptors – detect stretch or tension (muscles & tendons) – help maintain balance & normal posture Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functional Classification of Neurons § Motor (efferent) neurons § Carry impulses from the CNS to viscera, &/or muscles & glands § Cell bodies of motor neurons are always located in the CNS Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functional Classification of Neurons § Interneurons (association neurons) § Found in neural pathways in the central nervous system § Connect sensory and motor neurons § Cell bodies are always located in the CNS Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Neuron Classification Figure 7. 6 Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Structural Classification of Neurons § Multipolar neurons – many extensions from the cell body § All motor & interneurons (association neurons) are multipolar § Most common Figure 7. 8 a Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Structural Classification of Neurons § Bipolar neurons – one axon and one dendrite § Rare in adults - found only in some special sense organs (eye, nose) Figure 7. 8 b Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Structural Classification of Neurons § Unipolar neurons – have a short single process leaving the cell body Figure 7. 8 c Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Structural Classification of Neurons § Short, divides almost immediately into proximal (central) & distal (peripheral) processes § Dendrite = small process at end of peripheral process; remainder = axon § Axon conducts impulses both away from & toward cell body § Sensory neurons found in PNS ganglia are unipolar Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Functional Properties of Neurons § Irritability – ability to respond to stimuli § Conductivity – ability to transmit an impulse § The plasma membrane at rest is polarized § Fewer positive ions are inside the cell than outside the cell Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Starting a Nerve Impulse § Depolarization – a stimulus depolarizes the neuron’s membrane § A deploarized membrane allows sodium (Na+) to flow inside the membrane § The exchange of ions initiates an action potential in the neuron Figure 7. 9 a–c Copyright © 2006 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, which repolarizes the membrane § The sodium-potassium pump restores the original configuration § This action requires ATP Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

Nerve Impulse Propagation § The impulse continues to move toward the cell body § Impulses travel faster when fibers have a myelin sheath Figure 7. 9 d–f Copyright © 2006 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 § An action potential is started in the dendrite Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings

How Neurons Communicate at Synapses Figure 7. 10 Copyright © 2006 Pearson Education, Inc. , publishing as Benjamin Cummings