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Chapter 10 Nervous System I Quick Time™a nd a dec ompr esso r ar e nee ded to see this pictur e. 1 Divisions of the Nervous System • Central Nervous System •Brain and spinal cord •All sensations have to be relayed here to be acted on •Muscle & gland stimulation •Control center for the entire system 2 Divisions of the Nervous System • Peripheral Nervous System • Connection between and CNS & receptors, muscles, and glands • Nerves • Cranial nerves • Spinal nerves 3 Divisions of Peripheral Nervous System • Sensory Division •Afferent System • Picks up sensory information and delivers it to the CNS • Motor Division •Efferent System • Carries information to muscles and glands 4 Divisions of Peripheral Nervous System • Divisions of the Motor Division – Somatic (SNS) • Carries information to skeletal muscle • Voluntary – Autonomic (ANS) • Carries information to smooth muscle, cardiac muscle, and glands • Involuntary 5 Divisions Nervous System 6 Functions of Nervous System Sensory Function • sensory receptors gather information • information is carried to the CNS Integrative Function • sensory information used to create • sensations • memory • thoughts • decisions Motor Function • decisions are acted upon • impulses are carried to effectors Most rapid means of maintaining homeostasis in your body 7 Nervous System Cytology Two Types of Cells • Neurons • Conduct nerve impulses from one part of the body to another • Information Processing Units • Neuroglial cells 8 Neuron Structure 9 Neuron Structure • Cell Body • Large nucleus surrounded by granular cytoplasm • Dendrites • Thick branched divisions of cell body • Bring nerve impulses toward the cell body 10 Neuron Structure • Axon • Usually a single, longer process that conducts nerve impulses from the cell body • Terminates at another neuron, muscle or gland • May be up to a meter 11 Neuron Structure • Axon • Axon terminal • End of an axon with many branching fibers • End expands to form synaptic end bulb • Nerve Fiber • Common name for an axon and its myelin sheath 12 Myelination of Axons • Myelin sheath • Formed by neuroglial cell • Phospholipid segment that wraps around axon • Provides protection for axon • Increases speed of impulse along axon 13 Myelination of Axons • Myelin sheath • Schwann cells • Form myelin sheath • Found ONLY in peripheral nervous system • Assist in repair of damaged axons, provides tube for axon or dendrite to grow 14 Myelination of Axons • Myelin sheath • Production begins during 1st year of life • Amount increases from birth to maturity • This is why adults react quicker to certain stimuli 15 Myelination of Axons • Myelin sheath • Nodes of Ranvier • Segments on axon that are not myelinated • Gaps in sheath 16 Myelination of Axons White Matter • contains myelinated axons Gray Matter • contains unmyelinated structures • cell bodies, dendrites 17 Classification of Neurons – Structural Differences Bipolar • Two processes • One axon, one dendrite • Eyes, ears, nose Unipolar • One process that branches in two • Ganglia • Specialized masses of nerve tissue outside brain & spinal cord 18 Classification of Neurons – Structural Differences • Multipolar • Many processes • Only one axon • Most neurons of CNS 19 Classification of Neurons – Functional Differences Sensory Neurons • Afferent • Carry impulse to CNS • Dendrites act as sensory receptors • Most are unipolar • Some are bipolar Interneurons • Link neurons • Multipolar • In CNS Motor Neurons • multipolar • carry impulses away from CNS • Carry impulses to effectors (muscles or glands) 20 Classification of Neurons – Functional Differences 21 Types of Neuroglial Cells in the PNS Schwann Cells • Produce myelin found on peripheral myelinated neurons • Speed neurotransmission Satellite Cells • Support ganglia in PNS 22 Types of Neuroglial Cells in the CNS Astrocytes • CNS • Scar tissue in CNS • Regulate ion concentrations (K+) • Induce synapse formation • Connect neurons to blood vessels Oligodendrocytes • Form myelin in CNS • Myelinating cell 23 Types of Neuroglial Cells in the CNS • Microglia • CNS • Phagocytic cell • Proliferate w/ CNS injury • Ependyma • CNS • Cuboidal or columnar • Ciliated • Line central canal of spinal cord • Line ventricles of brain 24 The Synapse • Junction between two neurons • Also called synaptic clefts • Essential in homeostasis because of the ability to transmit some impulses while inhibiting others • Brain disease & many psychiatric disorders result from bad synaptic communication 25 The Synapse • 2 Types • Electrical & Chemical • Most in CNS are chemical • Function • Neuron secretes neurotransmitters across synaptic cleft • Post-synaptic neuron has receptors to match transmitter • When they match, impulse continues 26 Resting Membrane Potential • Inside is negative relative to the outside • Polarized membrane • Due to distribution of ions • Unequal distribution of Na+ and K+ ions • K+ 28x greater inside • Na+ 14x greater outside • Na+/K+ pump 27 Sodium/Potassium Pump • Fights osmosis • Transports 3 Na+ out & 2 K+ into a resting neuron • Active Process 28 Local Potential Changes • Caused by various stimuli • Temperature changes • Light • Pressure • Environmental changes affect the membrane potential by opening a gated ion channel •Allows Na+ to diffuse in & K+ to diffuse out 29 Local Potential Changes • If membrane potential becomes more negative, it has hyperpolarized • If membrane potential becomes less negative, it has depolarized • Summation can lead to threshold stimulus that starts an action potential • Multiple impulses often needed to reach threshold stimulus 30 Local Potential Changes 31 Action Potentials • At rest membrane is polarized • Threshold stimulus reached • Na+ channels open and membrane depolarizes • Na+ enters cell • K+ leaves cytoplasm and membrane repolarizes 32 Action Potentials 33 Action Potentials 34 All-or-None Response • If a neuron responds at all, it responds completely • A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon • All impulses carried on an axon are the same strength 35 Refractory Period • Absolute – Time when threshold stimulus does not start another action potential • Relative – Time when stronger threshold stimulus can start another action potential • Under normal conditions each fiber may conduct 10-500 impulses per second • Larger neurons conduct up to 2500 per second 36 Impulse Conduction • Nerve cell membrane maintains resting potential by diffusion of Na+ and K+ down their concentration gradients as the cell pumps them up the gradients • Neurons receive stimulation, causing local potentials, which may sum to reach threshold • Sodium channels in a local region of the membrane open • Sodium ions diffuse inward, depolarizing the membrane 37 Impulse Conduction • Potassium channels in the membrane open • Potassium ions diffuse outward, repolarizing the membrane • The resulting action potential causes an electric current that stimulates adjacent portions of the membrane • Series of action potentials occurs sequentially along the length of the axon as a nerve impulse 38 Saltatory Conduction 39 Saltatory Conduction • Impulse along myelinated fiber • Sheath inhibits movement of ions • Nodes of Ranvier allow generation of action potentials and conduction • Ionic current flows through extra-cellular fluid & triggers impulse at next node • Impulse mechanism is the same, BUT impulse skips from one node to the next. 40 Saltatory Conduction • Valuable to Homeostasis • Speed of impulse greatly increased • Low ATP expenditure by Na-K pump due to little exposed membrane 41 Clinical Application Drug Addiction • Occurs because of the complex interaction of neurons, drugs, and individual behaviors • Understanding how neurotransmitters fit receptors can help explain the actions of certain drugs • Drugs have different mechanisms of action • Several questions remain about the biological effects of addiction, such as why some individuals become addicted and others do not 42