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nervous tissue.docx

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University of Toronto Scarborough
Biological Sciences
Kenneth Welch

Nervous Tissue Nervous System • The master controlling and communicating system of the body • Functions: • Sensory input – monitoring stimuli occurring inside and outside the body • Integration – interpretation of sensory input • Motor output – response to stimuli by activating effector organs Organization of the Nervous System • Central nervous system (CNS) • Brain and spinal cord • Integration and command center • Peripheral nervous system (PNS) • Paired spinal and cranial nerves • Carries messages to and from the spinal cord and brain Peripheral Nervous System (PNS): Two Functional Divisions • Sensory (afferent) division • Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain • Visceral afferent fibers – transmit impulses from visceral organs to the brain • Motor (efferent) division • Transmits impulses from the CNS to effector organs Motor Division: Two Main Parts • Somatic nervous system • Conscious control of skeletal muscles • Autonomic nervous system (ANS) • Regulate smooth muscle, cardiac muscle, and glands • Divisions – sympathetic and parasympathetic Histology of Nerve Tissue • The two principal cell types of the nervous system are: • Neurons – excitable cells that transmit electrical signals • Supporting cells – cells that surround and wrap neurons Supporting Cells: Neuroglia • The supporting cells (neuroglia or glia): • Provide a supportive scaffolding for neurons • Segregate and insulate neurons • Guide young neurons to the proper connections • Promote health and growth Astrocytes • Most abundant, versatile, and highly branched glial cells • They cling to neurons and cover capillaries • Functionally, they: • Support and brace neurons • Anchor neurons to their nutrient supplies • Guide migration of young neurons • Control the chemical environment Microglia and Ependymal Cells • Microglia – small, ovoid cells with spiny processes • Phagocytes that monitor the health of neurons • Ependymal cells – squamous- to columnar-shaped cells • They line the central cavities of the brain and spinal column Oligodendrocytes, Schwann Cells, and Satellite Cells • Oligodendrocytes – branched cells that wrap CNS nerve fibers • Schwann cells (neurolemmocytes) – surround fibers of the PNS • Satellite cells surround neuron cell bodies with ganglia Neurons (Nerve Cells) • Structural units of the nervous system • Composed of a body, axon, and dendrites • Long-lived, amitotic, and have a high metabolic rate • Their plasma membrane functions in: • Electrical signaling • Cell-to-cell signaling during development Nerve Cell Body (Perikaryon or Soma) • Contains the nucleus and a nucleolus • Major biosynthetic center • Focal point for the outgrowth of neuronal processes • There are no centrioles (hence its amitotic nature) • Well developed Nissl bodies (rough ER) • Axon hillock – cone-shaped area from which axons arise Processes • Armlike extensions from the soma • Called tracts in the CNS and nerves in the PNS • There are two types: axons and dendrites Dendrites of Motor Neurons • Short, tapering, and diffusely branched processes • They are the receptive, or input, regions of the neuron • Electrical signals are conveyed as graded potentials (not action potentials) Axons: Structure • Slender processes of uniform diameter arising from the hillock • Long axons are called nerve fibers • Usually there is only one unbranched axon per neuron • Rare branches, if present, are called axon collaterals • Axonal terminal – branched terminus of an axon Axons: Function • Generate and transmit action potentials • Secrete neurotransmitters from the axonal terminals Myelin Sheath • Whitish, fatty (protein-lipid), segmented sheath around most long axons • It functions in: • Protection of the axon • Electrically insulating fibers from one another • Increasing the speed of nerve impulse transmission Myelin Sheath and Neurilemma: Formation • Formed by Schwann cells in the PNS • A Schwann cell: • Envelopes an axon in a trough • Encloses the axon with its plasma membrane • Concentric layers of membrane make up the myelin sheath • Neurilemma – remaining nucleus and cytoplasm of a Schwann cell Nodes of Ranvier (Neurofibral Nodes) • Gaps in the myelin sheath between adjacent Schwann cells • They are the sites where collaterals can emerge Unmyelinated Axons • A Schwann cell surrounds nerve fibers but coiling does not take place • Schwann cells partially enclose 15 or more axons Axons of the CNS • Both myelinated and unmyelinated fibers are present • Myelin sheaths are formed by oligodendrocytes • Nodes of Ranvier are widely spaced • There is no neurilemma Regions of the Brain and Spinal Cord • White matter – dense collections of myelinated fibers • Gray matter – mostly soma and unmyelinated fibers Neuron Classification • Structural: • Multipolar • Bipolar • Unipolar • Functional: • Sensory (afferent) • Motor (efferent) • Interneurons (association neurons) Neurophysiology • Neurons are highly irritable • Action potentials, or nerve impulses, are: • Electrical impulses carried along the length of axons • Always the same regardless of stimulus • The underlying functional feature of the nervous system Electrical Definitions • Voltage – measure (mV) of potential energy generated by separated charge • Potential difference – voltage measured between two points • Current (I) – the flow of electrical charge between two points • Resistance (R) – hindrance to charge flow • Insulator – substance with high electrical resistance • Conductor – substance with low electrical resistance Electrical Current and the Body • Reflects the flow of ions rather than electrons • There is a potential on either side of membranes when: • The number of ions is different across the membrane • The membrane provides a resistance to ion flow Role of Ion Channels • Types of plasma membrane ion channels: • Passive, or leakage, channels – always open • Chemically gated channels – open with binding of a specific neurotransmitter • Voltage-gated channels – open and close in response to membrane potential Operation of a Gated Channel • Example: Na -K gated channel • Closed+when a neurotransmitter is n+t bound to the extracellular receptor • Na cannot enter the cell and K cannot exit the cell Operation of a Gated Channel • Open when a neurotransmitter is attached to the receptor • Na enters the cell and K exits the cell Operation of a Volt+ge-Gated Channel • Example: Na channel • Closed when the intracellular environment is negative • Na cannot enter the cell Operation of a Voltage-Gated Channel • Open w+en the intracellular environment is positive • Na can enter the cell Gated Channels • When gated channels are open: • Ions move quickly across the membrane • Movement is along their electrochemical gradients • An electrical current is created • Voltage changes across the membrane Electrochemical Gradient • Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration • Ions flow along their electrical gradient when they move toward an area of opposite charge • Electrochemical gradient – the electrical and chemical gradients taken together Resting Membrane Potential (V ) r • The potential difference (–70 mV) across the membrane of a resting neuron • It is generated by different concentrations of Na , K , Cl , and protein anions (A ) • Ionic differences are the consequence of: + + • Differential permeability of the neurilemma to Na and K • Operation of the sodium-potassium pump Membrane Potentials: Signals • Used to integrate, send, and receive information • Membrane potential changes are produced by: • Changes in membrane permeability to ions • Alterations of ion concentrations across the membrane • Types of signals – graded potentials and action potentials Changes in Membrane Potential • Caused by three events: • Depolarization – the inside of the membrane becomes less negative • Repolarization – the membrane returns to its resting membrane potential • Hyperpolarization –the inside of the membrane becomes more negative than the resting potential Graded Potentials • Graded potentials: • Are short-lived, local changes in membrane potential • Decrease in intensity with distance • Their magnitude varies directly with the strength of the stimulus • Sufficiently strong graded potentials can initiate action potentials • Voltage changes in graded potentials are decremental • Current is quickly dissipated due to the leaky plasma membrane • Can only travel over short distances Action Potentials (APs) • A brief reversal of membrane potential with a total amplitude of 100 mV • Action potentials are only generated by muscle cells and neurons • They do not decrease in strength over distance • They are the principal means of neural communication • An action potential in the axon of a neuron is a nerve impulse Action Potential: Resting State • Na and K channels are closed + + • Leakage a+counts for small movements of Na and K • Each Na channel has two voltage-regulated gates • Activation gates – closed in the resting state • Inactivation gates – open in the resting state Action Potential: Depolarization Phase + • Na +ermeability increases;+membrane potential reverses • Na gates are opened; K gates are closed • Threshold – a critical level of depolarization (-55 to -50 mV) • At threshold, depolarization becomes self generating Action Potential: Repolarization Phase • Sodium inactivation gates close + • Membrane permeability to Na declines to resting levels • As sodium gates close, voltage sensitive K gates open • K exits the cell and internal negativity of the resting neuron is restored Action Potential: Undershoot + • Potassium gates remain open, causing an excessive efflux of K • This efflux causes hyperpolarization of the membrane (undershoot) • The neuron is insensitive to stimulus and depolarization during this time Action Potential: Role of the Sodium-Potassium Pump • Repolarization • Restores the resting electrical conditions of the neuron • Does not restore the resting ionic conditions • Ionic redistribution back to resting conditions is restored by the sodium-potassium pump Phases of the Action Potential • 1 – resting state • 2 – depolarization phase • 3 – repolarization phase • 4 – undershoot Propagat+on of an Action Potential (Time = 0ms) • Na influx causes a patch of the axonal membrane to depolarize • Positive ions in the axoplasm move toward the polarized (negative) portion of the membrane (bottom arrows in figure) • Sodium gates are shown as closing, open, or closed Propagation of an Action Potential (Time = 1ms) • Ions of the extracellular fluid move toward the area of greatest negative charge • A current is created that depolarizes the adjacent membrane in a forward direction • The impulse propagates away from its point of origin Propagation of an Action Potential (Time = 2ms) • The action potential moves away from the stimulus • Where sodium gates are closing, potassium gates are open and create a current flow Threshold and Action Potentials • Threshold – membrane is depolarized by 15 to 20 mV • Established by the total amount of current flowing through the membrane • Weak (subthreshold) stimuli are not relayed into action potentials • Strong (threshold) stimuli are relayed into action potentials • All-or-none phenomenon – action potentials either happen completely, or not at all Coding for Stimulus Intensit
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