Summary-fundamentals-of-the-nervous-system-nervous-tissue-ch11.pdf

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Unformatted text preview: lOMoARcPSD|3704854 Summary - Fundamentals of the Nervous System & Nervous Tissue (Ch11).pdf Introduction to Anatomy and Physiology I (University of Wollongong) StuDocu is not sponsored or endorsed by any college or university Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 CH 11 FUNDAMENTALS OF THE NERVOUS SYSTEM AND NERVOUS TISSUE Functions and divisions of the nervous system Functions: • Sensory input -­‐ Information gathered by sensory receptors about internal and external changes • Integration -­‐ Interpretation of sensory input • Motor output -­‐ Activation of effector organs (muscles and glands) produces a response Divisions: • Central nervous system (CNS) -­‐ Brain and spinal cord -­‐ Integrating and command centre of the nervous system -­‐ Interprets sensory input and dictates motor responses based on reflexes, current conditions and past experience • Peripheral nervous system (PNS) -­‐ Paired spinal and cranial nerves carry messages to and from the CNS -­‐ Consists mainly of the nerves (bundles of axons) that extend from the brain and spinal cord NOTE: spinal nerves carry impulses to and from the spinal cord Cranial nerves carry impulses to and from the brain -­‐ Two subdivisions: o Sensory (afferent) division: consists of nerve fibers (axons) that convey impulses to the CNS from sensory receptors located throughout the body (the sensory division keeps the CNS constantly informed of events going on both inside and outside the body) o Motor (efferent) division: transmits impulses from the CNS constantly to effector organs (muscles and glands) – the impulses activated muscles to contract and glands to secrete. The motor division has two main parts: 1. somatic nervous system (conducts impulses from the CNS to skeletal muscles – voluntary nervous system) 2. autonomic nervous system (regulate the activity of smooth muscles, cardiac muscles and glands – involuntary nervous system) à the ANS has two main parts – the sympathetic division and the parasympathetic division SHS111 Anatomy & Physiology I -­‐ 1 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 Histology of nervous tissue Nervous tissue is made up of two principal types of cells 1. neuroglia -­‐ supporting cells that surround and wrap the more delicate neurons (examples include astrocytes (CNS), oligodendrocytes (CNS), satellite cells (PNS) , Schwann cells (PNS)) 2. neurons – the excitable nerve cells that transmit electrical signals Neurons (nerve cells) • conduct messages in the form of nerve impulses from one part of the body to another à Other special characteristics: • extreme longevity (over 100 years) • amitotic (neurons are unable to divide) • high metabolic rate (depends on continuous supply of oxygen and glucose) à Cell body: • aka perikaryon or Soma • the major biosynthetic centre of a neuron (contains the usual organelles) • spherical nucleus with nucleolus surrounded by cytoplasm • rough endoplasmic reticulum is referred to as Nissl bodies (or chromatophilic substance) • microtubules and neurofibrils – bundles of intermediate filaments that are important in maintaining cell shape and integrity SHS111 Anatomy & Physiology I -­‐ 2 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 • • • most neuron cell bodies are located in the CNS, where they are protected by the bones of the skull and vertebral column nuclei – clusters of cell bodies in the CNS Ganglia – cell bodies that lie along the nerves in the PNS à Processes • Dendrites and axons – extend from the cell body of all neruons • Tracts – bundles of processes in the CNS • Nerves – bundles of processes in the PNS • CNS contains both neuron cell bodies and their processes, whereas the PNS consists mainly of neuron processes • Dendrites: -­‐ Short, tapering, diffusely branching extensions -­‐ Receptive or input regions – convey incoming messages toward the cell body (these electrical signals are not usually action potentials but are short-­‐distance signals called graded potentials) -­‐ Provide an enormous surface area for receiving signals from other neurons • Axons: -­‐ Conducting region of a neuron -­‐ Each neuron has a single axon -­‐ Generates and transmits nerve impulses (action potentials) away from the cell body -­‐ Vary in length – a long axon is called a nerve fiber -­‐ Conducting region – generates nerve impulses and transmits them away from the cell body -­‐ Rely on its cell body to renew the necessary proteins and membrane components and rely on efficient transport mechanisms to distribute them -­‐ Axons quickly decay if cut or severely damaged -­‐ Anterograde movement: movement toward the axon terminals -­‐ Retrograde movement: movement away from the axon terminals Process: the nerve impulse is generated at the junction of the axon hillock and the axon (called the trigger zone) and is transmitted along the plasma membrane. The impulse is conducted along the axon to the axon terminals (secretory region of the axon). When they impulse reaches the axon terminals, it causes neurotransmitters, signalling chemicals stored in cesicles there to be released into the extracellular space. • Myelin Sheath -­‐ Segmented protein-­‐lipid sheath around most long or large-­‐diameter axons It functions to: -­‐ Protect and electrically insulate the axon -­‐ Increase the speed of nerve impulse transmission -­‐ NOTE: myelin sheaths are associated only with axons. Dendrites are always unmyelinated. SHS111 Anatomy & Physiology I -­‐ 3 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 -­‐ -­‐ -­‐ -­‐ -­‐ -­‐ Myelin sheaths in the PNS: Schwann cells – wraps many times around the axon (the myelin sheath is concentric layers of Schwann cell membrane) Channel and carrier proteins are absent à exceptionally good electrical conductor Presence of specific protein molecules that interlock to form a sort of molecular Velcro between adjacent myelin membranes Neurilemma: the bulge just external to the myelin sheath (consists of the nucleus and most of the cytoplasm of the Schwann cell) and the exposed part of the plasma membrane Nodes of Ranvier: the gaps between adjacent Schwann cells Myelin sheaths in CNS: oligodendrocytes – have multiple flat processes that can coil around as many as 60 axons at the same time (adjacent sections of the myelin sheath are separated by nodes of Ranvier. CNS myelin sheaths do not have a neurilemma) White Matter: dense collections of myelinated fibers Gray matter: mostly neuron cell bodies and unmyelinated fibers Membrane potentials How do neurons become excited or inhibited and how do they communicate with other cells? Basic Principles of Electricity 1. Opposite charges attract each other 2. Energy is required to separate opposite charges across a membrane 3. Energy is liberated when the chargers move toward one another 4. If opposite charges are separated, the system has potential energy Voltage (V): measure of potential energy generated by separated charge Potential difference: voltage measured between two points Current (I): the flow of electrical charge (ions) between two points Resistance (R): hindrance to charge flow (provided by the plasma membrane) Insulator: substance with high electrical resistance Conductor: substance with low electrical resistance Ohms Law: Voltage = current x resistance Membrane Ion channels: 1. Non-­‐gated channels: -­‐ Leakage channels – always open 2. Gated channels: -­‐ Chemically gated channels – open whne th appropriate chemical (in this case a neurotransmitter) binds -­‐ Voltage-­‐gated channels – open and close in response to changes in the membrane potential -­‐ Mechanically gated channels – open in response to physical deformation of the receptor (as in sensory receptors for touch and pressure) • When gated ion channels are open, ions diffuse quickly across the membrane following their electrochemical gradients, creating electrical currents and voltage changes across the membrane • Ions move along chemical concentration gradients when they diffuse passively from an area of their higher concentration to an area of lower concentration SHS111 Anatomy & Physiology I -­‐ 4 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 • • Ions move along electrical gradients when they move toward an area of opposite electrical charge Electrochemical gradient – electrical and concentration gradients combined Resting Membrane potential: • Potential difference across the membrane of a resting cell + • Approximately -­‐70mV in neurons (cytoplasmic side of membrane is negatively charged relative to outside – Na on + outside, K on inside) • Generated by: -­‐ Differences in ionic makeup of ICF (intracellular fluid) and ECF (extracellular fluid) -­‐ Differential permeability of the plasma membrane • Differences in ionic makeup: + -­‐ + -­‐ Intracellular fluid – lower concentration of Na and Cl , higher concentration of K and negatively -­‐ charged proteins (A ) • At rest: + + -­‐ Membrane is highly permeable to K . The K leaks out through abundant leakage channels and establishes a negative membrane potential + + -­‐ Membrane is only slightly permeable to Na . Na entry through leakage channels reduces the negative membrane potential only slightly. + + -­‐ Na -­‐K ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential. The pump counteracts the leaks by transporting Na+ out and K+ in + 3 x Na OUT + 2 x K IN at one time + NOTE: main intracellular ion: K + + -­‐ Main extracellular ions: Na , Ca2 , Cl Membrane Potentials that Act as Signals Two types of signals: • Graded potentials -­‐ Incoming short-­‐distance signals • Action potentials -­‐ Long-­‐distance signals of axons Depolarization: the reduction in membrane potential. • Inside the membrane becomes less negative than the resting potential (less negative à moves closer to zero) • Increases the probability of producing a nerve impulse Hyperpolarization: an increase in membrane potential • The inside of the membrane becomes more negative (moves away from zero) • Reduces the probability of producing a nerve impulse – blocks the chance of an action potential occuring SHS111 Anatomy & Physiology I -­‐ 5 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 **These terms describe membrane potential changes relative to resting membrane potential. Graded Potentials: • Short-­‐lived localized changes in membrane potential that can be either depolarizations or hyperpolarizations • Called ‘graded’ potentials because their magnitude varies directly with the stimulus strength (the stronger the stimulus, the more voltage changes and the farther the current flows) • Graded potential spreads as local currents change the membrane potential of adjacent regions • Occur when a stimulus causes gated ion channels to open (eg. receptor potentials, generator potentials, postsynaptic potentials) • Magnitude varies depending on stimulus strength • Decrease in magnitude with distance as ions flow and diffuse through leakage channels • Short-­‐distance signals IF THE GRADED POTENTIAL BECOMES STRONG ENOUGH, A VERY RAPID DEPOLARIZATION OCCURS à (ACTION POTENTIAL) Action Potential: SHS111 Anatomy & Physiology I -­‐ 6 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 • • • • • • • • • A brief reversal of membrane potential with a total amplitude of about 100 mV (from resting at -­‐70mV to peak at +30mV) Only cells with excitable membranes – axons of neurons and muscle cells – can generate action potentials Does not decrease in magnitude over distance Principal means of long-­‐distance neural communication Three stages: depolarization, repolarization and hyperpolarization (this process occurs within a few milliseconds) Events of action potential generation and transmission are identical in skeletal muscle cells and neurons In a neuron, an AP is called a nerve impulse A neuron only transmits a nerve impulse only when it is adequately stimulated. The stimulus changes the permeability of the neuron’s membrane by opening specific voltage-­‐gated channels on the axon (the channels open and close in response to changing membrane potential In many neurons the transition from local graded potential to action potential occurs in the axon hillock è Generation of an action potential Ø Involves three consecutive but overlapping changes in membrane permeability resulting from the opening and closing of voltage-­‐gated ion channels Ø Phases of an action potential: 1. + + Resting state: all gated Na and K channels are closed. -­‐ Only leakage channels are open (to maintain resting membrane potential) + + -­‐ All gates Na and K channels are closed + -­‐ Each Na channel has two gates: a voltage-­‐sensitive activation gates that is closed at rest and an + inactivation gate that blocks the channel once its open (both gates must be open for Na to enter the cell but the closing of either gate closes the channel) + -­‐ Each K channel has a single voltage-­‐sensitive gate that is closed at rest and opens slowly with depolarization SHS111 Anatomy & Physiology I -­‐ 7 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 2. 3. + Depolarization phase: Na channels open + -­‐ Depolarizing local currents open voltage-­‐gated Na channels + -­‐ Na influx causes more depolarization + -­‐ When the voltage reaches the threshold (-­‐55 to -­‐50 mV) positive feedback leads to opening of all Na channels, and a reversal of membrane polarity to +30 mV (spike of action potential) + -­‐ SO: after being initiated by the stimulus, depolarization is driven by the ionic currents created by Na influx (the positive feedback cycle is responsible for the rising phase of action potentials + + Repolarization phase: Na channels are inactivating and K channels open. -­‐ Explosive rising of the action potential (depolarization) lasts about 1ms. -­‐ Repolarization is the retaining or returning of ions to original position -­‐ Repolarization restores the resting electrical conditions of the neuron + -­‐ The slow inactivation gates of the Na channels begin to close at this point + -­‐ Membrane permeability to Na declines to resting levels + + -­‐ The slow voltage-­‐gated K channels open and K rushes out of the cell and internal negativity is restored 4. + + Hyperpolarization: some K channels remain open and Na channels reset. + + -­‐ Some K channels remain open longer then needed so there is an excessive efflux of K (this causes after-­‐hyperpolarization -­‐ the undershoot seen on the AP curve) • • Repolarization restores the resting electrical conditions but it does not restore resting ionic conditions + + Ion redistribution is accomplished by the sodium-­‐potassium pump (an axon membrane has thousands of Na -­‐K pumps) – restores the resting ionic conditions è Propagation of an Action Potential Ø The AP must be propagated (sent or transmitted) along the axon’s entire length Ø AP is generated by the influx of Na+ through a given area of the membrane + Ø Na influx causes a patch of the axonal membrane to depolarize Ø Influx establishes local currents that depolarize adjacent membrane areas Ø AP propagates AWAY from its point of origin (in the body, APs are initiated at one end of the axon and conducted away from that point toward the axon’s terminals) Ø Depolarization opens voltage-­‐gated channels and triggers an AP Ø Following depolarization, each segment of axon membrane repolarizes (restores RMP) à the repolarization wave chases the depolarization wave down the axon Ø NOTE: action potential moves in one direction NOTE: this is describing the propagation process on unmyelinated fibers. Also note, neurons are fairly poor conductors. Nerve impulses are propagated rather than conducted. The AP is regenerated anew at each membrane patch, and very subsequent AP is identical to the one that was generated initially è Threshold and the all-­‐or-­‐none phenomenon SHS111 Anatomy & Physiology I -­‐ 8 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 Ø Ø Ø Ø Ø The depolarization must reach threshold values if an axon is to fire + + the threshold is the membrane potential at which Na influx exceeds K efflux (threshold is typically reached when the membrane has been depolarized by 15 to 20 mV from the resting value) local depolarizations are graded potentials – their magnitude increases with increasing stimulus intensity -­‐ Subthreshold stimuli: brief weak stimuli that produce subthreshold depolarizations that are not translated into nerve impulses -­‐ Threshold stimuli: stronger stimuli that produce subthreshold depolarizations currents that push through the membrane potential toward and beyond the threshold voltage (entering sodium ions exceed the outward movement of potassium ions) strong stimuli depolarize the membrane to threshold very quickly whereas weaker stimuli must be applied for longer periods to provide the crucial amount of current flow + the AP is an all-­‐or-­‐none phenomenon (if the number of Na ions entering the cell is too low to achieve threshold, no AP will occur) è Coding for stimulus intensity Ø Once generated, all APs are independent of stimulus strength (all APs are alike) – positive feedback cycle Ø How can the CNS determine whether a particular stimulus is intense of weak – information it needs to initiate an appropriate response? Strong stimuli cause nerve impulses to be generated more often in a given time interval than do weak stimuli Ø Stimulus intensity is coded by the number of impulses per second, that is, by the frequency of action potentials è Refractory periods Ø When a neuron is generating an AP, the neuron cannot respond to another stimulus Ø Absolute refractory period: the time from the opening of the Na+ channels until the resetting of the channels Ø It ensures that each AP is a separate, all-­‐or-­‐none event and enforces one-­‐way transmission of the AP Ø Relative refractory period: -­‐ Follows the absolute refractory period + -­‐ Most Na channels have returned to their resting state + -­‐ Some K channels are still open -­‐ Repolarization is occurring -­‐ During this time, the axon’s threshold for AP generation is substantially elevated (an + exceptionally strong stimulus can reopen the Na channels and allow another AP to be generated) è Conduction velocity Ø Nerve fibers that transmit impulses most rapidly are found in neural pathways where speed is essential (such as those that mediate reflexes) Ø Axons that conduct impulses more slowly typically serve internal organs, where slower responses are not a handicap Ø The rate of impulse propagation depends on: SHS111 Anatomy & Physiology I -­‐ 9 Downloaded by Lauren Myburgh ([email protected]) lOMoARcPSD|3704854 Axon diameter – the larger the axon’s diameter, the faster it conducts impulses (larger axons conduct more rapidly because they offer less resistance to the flow of local currents) 2. Degree of myelination -­‐ myelin sheaths insulate and prevent leakage of charge -­‐ Unmyelinated axons: the voltage-­‐gated channels are immediately adjacent to each other and conduction is relatively slow (continuous conduction) -­‐ Myelinated axons: current can pass through the membrane only at the nodes of Ranvier, where the myelin sheath is interrupted and the axon is bare (all the voltage-­‐ gated Na+ channels are concentrated at these nodes). The current does not dissipate through the adjacent membrane regions – the current is maintained and move rapidly to the next node where it triggers another AP. (APs are triggered only at the nodes – called saltatory conduction, because the electrical signal jumps from node to node along the axon) à Saltatory conduction is about 30 times faster than continuous conduction Nerve fibers may be classified according to diameter, degree of myelination and conduction speed Group A fibers: mostly somatic sensory and motor nerve fibers serving t...
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