Nervous Tissue - PowerPoint PPT Presentation

1 / 86
About This Presentation
Title:

Nervous Tissue

Description:

Motor(efferent) neurons serve this function. Principles of Human Anatomy and ... Motor (efferent) neurons. send motor nerve impulses to muscles & glands ... – PowerPoint PPT presentation

Number of Views:606
Avg rating:3.0/5.0
Slides: 87
Provided by: diete81
Category:
Tags: nervous | tissue

less

Transcript and Presenter's Notes

Title: Nervous Tissue


1
Chapter 12
  • Nervous Tissue
  • Lecture Outline

2
INTRODUCTION
  • The nervous system, along with the endocrine
    system, helps to keep controlled conditions
    within limits that maintain health and helps to
    maintain homeostasis.
  • The nervous system is responsible for all our
    behaviors, memories, and movements.
  • The branch of medical science that deals with the
    normal functioning and disorders of the nervous
    system is called neurology.

3
Chapter 12Nervous Tissue
  • Controls and integrates all body activities
    within limits that maintain life
  • Three basic functions
  • sensing changes with sensory receptors
  • fullness of stomach or sun on your face
  • interpreting and remembering those changes
  • reacting to those changes with effectors
  • muscular contractions
  • glandular secretions

4
Major Structures of the Nervous System
  • Brain, cranial nerves, spinal cord, spinal
    nerves, ganglia, enteric plexuses and sensory
    receptors

5
Structures of the Nervous System - Overview
  • Twelve pairs of cranial nerves emerge from the
    base of the brain through foramina of the skull.
  • A nerve is a bundle of hundreds or thousands of
    axons, each of which courses along a defined path
    and serves a specific region of the body.
  • The spinal cord connects to the brain through the
    foramen magnum of the skull and is encircled by
    the bones of the vertebral column.
  • Thirty-one pairs of spinal nerves emerge from the
    spinal cord, each serving a specific region of
    the body.
  • Ganglia, located outside the brain and spinal
    cord, are small masses of nervous tissue,
    containing primarily cell bodies of neurons.
  • Enteric plexuses help regulate the digestive
    system.
  • Sensory receptors are either parts of neurons or
    specialized cells that monitor changes in the
    internal or external environment.

6
Functions of the Nervous Systems
  • The sensory function of the nervous system is to
    sense changes in the internal and external
    environment through sensory receptors.
  • Sensory (afferent) neurons serve this function.
  • The integrative function is to analyze the
    sensory information, store some aspects, and make
    decisions regarding appropriate behaviors.
  • Association or interneurons serve this function.
  • The motor function is to respond to stimuli by
    initiating action.
  • Motor(efferent) neurons serve this function.

7
Nervous System Divisions
  • Central nervous system (CNS)
  • consists of the brain and spinal cord
  • Peripheral nervous system (PNS)
  • consists of cranial and spinal nerves that
    contain both sensory and motor fibers
  • connects CNS to muscles, glands all sensory
    receptors

8
Subdivisions of the PNS
  • Somatic (voluntary) nervous system (SNS)
  • neurons from cutaneous and special sensory
    receptors to the CNS
  • motor neurons to skeletal muscle tissue
  • Autonomic (involuntary) nervous systems
  • sensory neurons from visceral organs to CNS
  • motor neurons to smooth cardiac muscle and
    glands
  • sympathetic division (speeds up heart rate)
  • parasympathetic division (slow down heart rate)
  • Enteric nervous system (ENS)
  • involuntary sensory motor neurons control GI
    tract
  • neurons function independently of ANS CNS

9
Organization of the Nervous System
  • CNS is brain and spinal cord
  • PNS is everything else

10
Enteric NS
  • The enteric nervous system (ENS) consists of
    neurons in enteric plexuses that extend the
    length of the GI tract.
  • Many neurons of the enteric plexuses function
    independently of the ANS and CNS.
  • Sensory neurons of the ENS monitor chemical
    changes within the GI tract and stretching of its
    walls, whereas enteric motor neurons govern
    contraction of GI tract organs, and activity of
    the GI tract endocrine cells.

11
HISTOLOGY OF THE NERVOUS SYSTEM
12
Neuronal Structure Function
13
Neurons
  • Functional unit of nervous system
  • Have capacity to produce action potentials
  • electrical excitability
  • Cell body
  • single nucleus with prominent nucleolus
  • Nissl bodies (chromatophilic substance)
  • rough ER free ribosomes for protein synthesis
  • neurofilaments give cell shape and support
  • microtubules move material inside cell
  • lipofuscin pigment clumps (harmless aging)
  • Cell processes dendrites axons

14
Parts of a Neuron
Neuroglial cells
Nucleus with Nucleolus
Axons or Dendrites
Cell body
15
Cell membrane
  • The dendrites are the receiving or input portions
    of a neuron.
  • The axon conducts nerve impulses from the neuron
    to the dendrites or cell body of another neuron
    or to an effector organ of the body (muscle or
    gland).

16
Dendrites
  • Conducts impulses towards the cell body
  • Typically short, highly branched unmyelinated
  • Surfaces specialized for contact with other
    neurons
  • Contains neurofibrils Nissl bodies

17
Axons
  • Conduct impulses away from cell body
  • Long, thin cylindrical process of cell
  • Arises at axon hillock
  • Impulses arise from initial segment (trigger
    zone)
  • Side branches (collaterals) end in fine processes
    called axon terminals
  • Swollen tips called synaptic end bulbs contain
    vesicles filled with neurotransmitters

Synaptic boutons
18
Axonal Transport
  • Cell body is location for most protein synthesis
  • neurotransmitters repair proteins
  • Axonal transport system moves substances
  • slow axonal flow
  • movement in one direction only -- away from cell
    body
  • movement at 1-5 mm per day
  • fast axonal flow
  • moves organelles materials along surface of
    microtubules
  • at 200-400 mm per day
  • transports in either direction
  • for use or for recycling in cell body

19
Axonal Transport Disease
  • Fast axonal transport route by which toxins or
    pathogens reach neuron cell bodies
  • tetanus (Clostridium tetani bacteria)
  • disrupts motor neurons causing painful muscle
    spasms
  • Bacteria enter the body through a laceration or
    puncture injury
  • more serious if wound is in head or neck because
    of shorter transit time

20
Diversity in Neurons
  • Both structural and functional features are used
    to classify the various neurons in the body.
  • On the basis of the number of processes extending
    from the cell body (structure), neurons are
    classified as multipolar, biopolar, and unipolar
    (Figure 12.4).
  • Most neurons in the body are interneurons and are
    often named for the histologist who first
    described them or for an aspect of their shape or
    appearance. Examples are Purkinje cells (Figure
    12.5a) or Renshaw cells (Figure 12.5b).

21
Structural Classification of Neurons
  • Based on number of processes found on cell body
  • multipolar several dendrites one axon
  • most common cell type
  • bipolar neurons one main dendrite one axon
  • found in retina, inner ear olfactory
  • unipolar neurons one process only(develops from
    a bipolar)
  • are always sensory neurons

22
Functional Classification of Neurons
  • Sensory (afferent) neurons
  • transport sensory information from skin, muscles,
    joints, sense organs viscera to CNS
  • Motor (efferent) neurons
  • send motor nerve impulses to muscles glands
  • Interneurons (association) neurons
  • connect sensory to motor neurons
  • 90 of neurons in the body

23
Association or Interneurons
24
Neuroglial Cells
  • Half of the volume of the CNS
  • Smaller cells than neurons
  • 50X more numerous
  • Cells can divide
  • rapid mitosis in tumor formation (gliomas)
  • 4 cell types in CNS
  • astrocytes, oligodendrocytes, microglia
    ependymal
  • 2 cell types in PNS
  • schwann and satellite cells

25
Astrocytes
  • Star-shaped cells
  • Form blood-brain barrier by covering blood
    capillaries
  • Metabolize neurotransmitters
  • Regulate K balance
  • Provide structural support

26
Microglia
  • Small cells found near blood vessels
  • Phagocytic role -- clear away dead cells
  • Derived from cells that also gave rise to
    macrophages monocytes

27
Ependymal cells
  • Form epithelial membrane lining cerebral cavities
    central canal
  • Produce cerebrospinal fluid (CSF)

28
Satellite Cells
  • Flat cells surrounding neuronal cell bodies in
    peripheral ganglia
  • Support neurons in the PNS ganglia

29
Oligodendrocytes
  • Most common glial cell type
  • Each forms myelin sheath around more than one
    axons in CNS
  • Analogous to Schwann cells of PNS

30
Myelination
  • A multilayered lipid and protein covering called
    the myelin sheath and produced by Schwann cells
    and oligodendrocytes surrounds the axons of most
    neurons (Figure 12.8a).
  • The sheath electrically insulates the axon and
    increases the speed of nerve impulse conduction.

31
Schwann Cell
  • Cells encircling PNS axons
  • Each cell produces part of the myelin sheath
    surrounding an axon in the PNS

32
Axon Coverings in PNS
  • All axons surrounded by a lipid protein
    covering (myelin sheath) produced by Schwann
    cells
  • Neurilemma is cytoplasm nucleusof Schwann cell
  • gaps called nodes of Ranvier
  • Myelinated fibers appear white
  • jelly-roll like wrappings made of
    lipoprotein myelin
  • acts as electrical insulator
  • speeds conduction of nerve impulses
  • Unmyelinated fibers
  • slow, small diameter fibers
  • only surrounded by neurilemma but no myelin
    sheath wrapping

33
Myelination in PNS
  • Schwann cells myelinate (wrap around) axons in
    the PNS during fetal development
  • Schwann cell cytoplasm nucleus forms outermost
    layer of neurolemma with inner portion being the
    myelin sheath
  • Tube guides growing axons that are repairing
    themselves

34
Myelination in the CNS
  • Oligodendrocytes myelinate axons in the CNS
  • Broad, flat cell processes wrap about CNS axons,
    but the cell bodies do not surround the axons
  • No neurilemma is formed
  • Little regrowth after injury is possible due to
    the lack of a distinct tube or neurilemma

35
Gray and White Matter
  • White matter myelinated processes (white in
    color)
  • Gray matter nerve cell bodies, dendrites, axon
    terminals, bundles of unmyelinated axons and
    neuroglia (gray color)
  • In the spinal cord gray matter forms an
    H-shaped inner core surrounded by white matter
  • In the brain a thin outer shell of gray matter
    covers the surface is found in clusters called
    nuclei inside the CNS
  • A nucleus is a mass of nerve cell bodies and
    dendrites inside the CNS.

36
Electrical Signals in Neurons
  • Neurons are electrically excitable due to the
    voltage difference across their membrane
  • Communicate with 2 types of electric signals
  • action potentials that can travel long distances
  • graded potentials that are local membrane changes
    only
  • In living cells, a flow of ions occurs through
    ion channels in the cell membrane

37
Two Types of Ion Channels
  • Leakage (nongated) channels are always open
  • nerve cells have more K than Na leakage
    channels
  • as a result, membrane permeability to K is
    higher
  • explains resting membrane potential of -70mV in
    nerve tissue
  • Gated channels open and close in response to a
    stimulus
  • results in neuron excitability

38
Ion Channels
  • Gated ion channels respond to voltage changes,
    ligands (chemicals), and mechanical pressure.
  • Voltage-gated channels respond to a direct change
    in the membrane potential (Figure 12.10a).
  • Ligand-gated channels respond to a specific
    chemical stimulus (Figure 12.10b).
  • Mechanically gated ion channels respond to
    mechanical vibration or pressure.

39
Gated Ion Channels
40
Resting Membrane Potential
  • Negative ions along inside of cell membrane
    positive ions along outside
  • potential energy difference at rest is -70 mV
  • cell is polarized
  • Resting potential exists because
  • concentration of ions different inside outside
  • extracellular fluid rich in Na and Cl
  • cytosol full of K, organic phosphate amino
    acids
  • membrane permeability differs for Na and K
  • 50-100 greater permeability for K
  • inward flow of Na cant keep up with outward
    flow of K
  • Na/K pump removes Na as fast as it leaks in

41
(No Transcript)
42
Graded Potentials
  • Small deviations from resting potential of -70mV
  • hyperpolarization membrane has become more
    negative
  • depolarization membrane has become more
    positive
  • The signals are graded, meaning they vary in
    amplitude (size), depending on the strength of
    the stimulus and localized.
  • Graded potentials occur most often in the
    dendrites and cell body of a neuron.

43
How do Graded Potentials Arise?
  • Source of stimuli
  • mechanical stimulation of membranes with
    mechanical gated ion channels (pressure)
  • chemical stimulation of membranes with ligand
    gated ion channels (neurotransmitter)
  • Graded/postsynaptic/receptor or generator
    potential
  • ions flow through ion channels and change
    membrane potential locally
  • amount of change varies with strength of stimuli
  • Flow of current (ions) is local change only

44
Generation of an Action Potential
  • An action potential (AP) or impulse is a sequence
    of rapidly occurring events that decrease and
    eventually reverse the membrane potential
    (depolarization) and then restore it to the
    resting state (repolarization).
  • During an action potential, voltage-gated Na and
    K channels open in sequence (Figure 12.13).
  • According to the all-or-none principle, if a
    stimulus reaches threshold, the action potential
    is always the same.
  • A stronger stimulus will not cause a larger
    impulse.

45
Action Potential
  • Series of rapidly occurring events that change
    and then restore the membrane potential of a cell
    to its resting state
  • Ion channels open, Na rushes in
    (depolarization), K rushes out (repolarization)
  • All-or-none principal with stimulation, either
    happens one specific way or not at all (lasts
    1/1000 of a second)
  • Travels (spreads) over surface of cell without
    dying out

46
Depolarizing Phase of Action Potential
  • Chemical or mechanical stimuluscaused a graded
    potential to reachat least (-55mV or threshold)
  • Voltage-gated Na channels open Na rushes into
    cell
  • in resting membrane, inactivation gate of sodium
    channel is open activation gate is closed (Na
    can not get in)
  • when threshold (-55mV) is reached, both open
    Na enters
  • inactivation gate closes again in few
    ten-thousandths of second
  • only a total of 20,000 Na actually enter the
    cell, but they change the membrane potential
    considerably(up to 30mV)
  • Positive feedback process

47
Repolarizing Phase of Action Potential
  • When threshold potential of-55mV is reached,
    voltage-gated K channels open
  • K channel opening is muchslower than Na
    channelopening which caused depolarization
  • When K channels finally do open, the Na
    channels have already closed (Na inflow stops)
  • K outflow returns membrane potential to -70mV
  • If enough K leaves the cell, it will reach a
    -90mV membrane potential and enter the
    after-hyperpolarizing phase
  • K channels close and the membrane potential
    returns to the resting potential of -70mV

48
Refractory Period of Action Potential
  • Period of time during whichneuron can not
    generateanother action potential
  • Absolute refractory period
  • even very strong stimulus willnot begin another
    AP
  • inactivated Na channels must return to the
    resting state before they can be reopened
  • large fibers have absolute refractory period of
    0.4 msec and up to 1000 impulses per second are
    possible
  • Relative refractory period
  • a suprathreshold stimulus will be able to start
    an AP
  • K channels are still open, but Na channels have
    closed

49
The Action Potential Summarized
  • Resting membrane potential is -70mV
  • Depolarization is the change from -70mV to 30 mV
  • Repolarization is the reversal from 30 mV back
    to -70 mV)

50
Local Anesthetics
  • Local anesthetics and certain neurotoxins
  • Prevent opening of voltage-gated Na channels
  • Nerve impulses cannot pass the anesthetized
    region
  • Examples
  • Novocaine and lidocaine

51
Propagation of Action Potential
  • An action potential spreads (propagates) over the
    surface of the axon membrane
  • as Na flows into the cell during depolarization,
    the voltage of adjacent areas is effected and
    their voltage-gated Na channels open
  • self-propagating along the membrane
  • The traveling action potential is called a nerve
    impulse

52
Continuous versus Saltatory Conduction
  • Continuous conduction (unmyelinated fibers)
  • step-by-step depolarization of each portion of
    the length of the axolemma
  • Saltatory conduction
  • depolarization only at nodes of Ranvier where
    there is a high density of voltage-gated ion
    channels
  • current carried by ions flows through
    extracellular fluid from node to node

53
Saltatory Conduction
  • Nerve impulse conduction in which the impulse
    jumps from node to node

54
Speed of Impulse Propagation
  • The propagation speed of a nerve impulse is not
    related to stimulus strength.
  • larger, myelinated fibers conduct impulses faster
    due to size saltatory conduction
  • Fiber types
  • A fibers largest (5-20 microns 130 m/sec)
  • myelinated somatic sensory motor to skeletal
    muscle
  • B fibers medium (2-3 microns 15 m/sec)
  • myelinated visceral sensory autonomic
    preganglionic
  • C fibers smallest (.5-1.5 microns 2 m/sec)
  • unmyelinated sensory autonomic motor

55
Encoding of Stimulus Intensity
  • How do we differentiate a light touch from a
    firmer touch?
  • frequency of impulses
  • firm pressure generates impulses at a higher
    frequency
  • number of sensory neurons activated
  • firm pressure stimulates more neurons than does a
    light touch

56
Action Potentials in Nerve and Muscle
  • Entire muscle cell membrane versus only the axon
    of the neuron is involved
  • Resting membrane potential
  • nerve is -70mV
  • skeletal cardiac muscle is closer to -90mV
  • Duration
  • nerve impulse is 1/2 to 2 msec
  • muscle action potential lasts 1-5 msec for
    skeletal 10-300msec for cardiac smooth
  • Fastest nerve conduction velocity is 18 times
    faster than velocity over skeletal muscle fiber

57
SIGNAL TRANSMISSION AT SYNAPSES
  • A synapse is the functional junction between one
    neuron and another or between a neuron and an
    effector such as a muscle or gland.

58
Signal Transmission at Synapses
  • 2 Types of synapses
  • electrical
  • ionic current spreads to next cell through gap
    junctions
  • faster, two-way transmission capable of
    synchronizing groups of neurons
  • chemical
  • one-way information transfer from a presynaptic
    neuron to a postsynaptic neuron
  • axodendritic -- from axon to dendrite
  • axosomatic -- from axon to cell body
  • axoaxonic -- from axon to axon

59
(No Transcript)
60
Chemical Synapses
  • Action potential reaches end bulb and
    voltage-gated Ca 2 channels open
  • Ca2 flows inward triggering release of
    neurotransmitter
  • Neurotransmitter crosses synaptic cleft binding
    to ligand-gated receptors
  • the more neurotransmitter released the greater
    the change in potential of the postsynaptic cell
  • Synaptic delay is 0.5 msec
  • One-way information transfer

61
Excitatory Inhibitory Potentials
  • The effect of a neurotransmitter can be either
    excitatory or inhibitory
  • a depolarizing postsynaptic potential is called
    an EPSP
  • it results from the opening of ligand-gated Na
    channels
  • the postsynaptic cell is more likely to reach
    threshold
  • an inhibitory postsynaptic potential is called an
    IPSP
  • it results from the opening of ligand-gated Cl-
    or K channels
  • it causes the postsynaptic cell to become more
    negative or hyperpolarized
  • the postsynaptic cell is less likely to reach
    threshold

62
Removal of Neurotransmitter
  • Diffusion
  • move down concentration gradient
  • Enzymatic degradation
  • acetylcholinesterase
  • Uptake by neurons or glia cells
  • neurotransmitter transporters
  • Prozac serotonin reuptake inhibitor

63
Three Possible Responses
  • Small EPSP occurs
  • potential reaches -56 mV only
  • An impulse is generated
  • threshold was reached
  • membrane potential of at least -55 mV
  • IPSP occurs
  • membrane hyperpolarized
  • potential drops below -70 mV

64
Comparison of Graded Action Potentials
  • Origin
  • GPs arise on dendrites and cell bodies
  • APs arise only at trigger zone on axon hillock
  • Types of Channels
  • AP is produced by voltage-gated ion channels
  • GP is produced by ligand or mechanically-gated
    channels
  • Conduction
  • GPs are localized (not propagated)
  • APs conduct over the surface of the axon

65
Comparison of Graded Action Potentials
  • Amplitude
  • amplitude of the AP is constant (all-or-none)
  • graded potentials vary depending upon stimulus
  • Duration
  • The duration of the GP is as long as the stimulus
    lasts
  • Refractory period
  • The AP has a refractory period due to the nature
    of the voltage-gated channels, and the GP has
    none.

66
Summation
  • If several presynaptic end bulbs release their
    neurotransmitter at about the same time, the
    combined effect may generate a nerve impulse due
    to summation
  • Summation may be spatial or temporal.

67
Spatial Summation
  • Summation of effects of neurotransmitters
    released from several end bulbs onto one neuron

68
Temporal Summation
  • Summation of effect of neurotransmitters released
    from 2 or more firings of the same end bulb in
    rapid succession onto a second neuron

69
Summation
  • The postsynaptic neuron is an integrator,
    receiving and integrating signals, then
    responding.
  • If the excitatory effect is greater than the
    inhibitory effect but less that the threshold
    level of stimulation, the result is a
    subthreshold EPSP, making it easier to generate a
    nerve impulse.
  • If the excitatory effect is greater than the
    inhibitory effect and reaches or surpasses the
    threshold level of stimulation, the result is a
    threshold or suprathreshold EPSP and a nerve
    impulse.
  • If the inhibitory effect is greater than the
    excitatory effect, the membrane hyperpolarizes
    (IPSP) with failure to produce a nerve impulse.

70
Neurotransmitters
  • Both excitatory and inhibitory neurotransmitters
    are present in the CNS and PNS the same
    neurotransmitter may be excitatory in some
    locations and inhibitory in others.
  • Important neurotransmitters include
    acetylcholine, glutamate, aspartate, gamma
    aminobutyric acid, glycine, norepinephrine,
    epinephrine, and dopamine.

71
Neurotransmitter Effects
  • Neurotransmitter effects can be modified
  • synthesis can be stimulated or inhibited
  • release can be blocked or enhanced
  • removal can be stimulated or blocked
  • receptor site can be blocked or activated
  • Agonist
  • anything that enhances a transmitters effects
  • Antagonist
  • anything that blocks the action of a
    neurotranmitter

72
Small-Molecule Neurotransmitters
  • Acetylcholine (ACh)
  • released by many PNS neurons some CNS
  • excitatory on NMJ but inhibitory at others
  • inactivated by acetylcholinesterase
  • Amino Acids
  • glutamate released by nearly all excitatory
    neurons in the brain ---- inactivated by
    glutamate specific transporters
  • GABA is inhibitory neurotransmitter for 1/3 of
    all brain synapses (Valium is a GABA agonist --
    enhancing its inhibitory effect)

73
Small-Molecule Neurotransmitters
  • Biogenic Amines
  • modified amino acids (tyrosine)
  • norepinephrine -- regulates mood, dreaming,
    awakening from deep sleep
  • dopamine -- regulating skeletal muscle tone
  • serotonin -- control of mood, temperature
    regulation, induction of sleep
  • removed from synapse recycled or destroyed by
    enzymes (monoamine oxidase or catechol-0-methyltra
    nsferase)

74
Small-Molecule Neurotransmitters
  • ATP and other purines (ADP, AMP adenosine)
  • excitatory in both CNS PNS
  • released with other neurotransmitters (ACh NE)
  • Gases (nitric oxide or NO)
  • formed from amino acid arginine by an enzyme
  • formed on demand and acts immediately
  • diffuses out of cell that produced it to affect
    neighboring cells
  • may play a role in memory learning
  • first recognized as vasodilator that helps lower
    blood pressure

75
Neuropeptides
  • 3-40 amino acids linked by peptide bonds
  • Substance P -- enhances our perception of pain
  • Pain relief
  • enkephalins -- pain-relieving effect by blocking
    the release of substance P
  • acupuncture may produce loss of pain sensation
    because of release of opioids-like substances
    such as endorphins or dynorphins

76
Strychnine Poisoning
  • In spinal cord, Renshaw cells normally release an
    inhibitory neurotransmitter (glycine) onto motor
    neurons preventing excessive muscle contraction
  • Strychnine binds to and blocks glycine receptors
    in the spinal cord
  • Massive tetanic contractions of all skeletal
    muscles are produced
  • when the diaphragm contracts remains
    contracted, breathing can not occur

77
Neuronal Circuits
  • Neuronal pools are organized into circuits
    (neural networks.) These include simple series,
    diverging, converging, reverberating, and
    parallel after-discharge circuits (Figure 12.18
    a-d).
  • A neuronal network may contain thousands or even
    millions of neurons.
  • Neuronal circuits are involved in many important
    activities
  • breathing
  • short-term memory
  • waking up

78
Neuronal Circuits
  • Diverging -- single cell stimulates many others
  • Converging -- one cell stimulated by many others
  • Reverberating -- impulses from later cells
    repeatedly stimulate early cells in the circuit
    (short-term memory)
  • Parallel-after-discharge -- single cell
    stimulates a group of cells that all stimulate a
    common postsynaptic cell (math problems)

79
Regeneration Repair
  • Plasticity maintained throughout life
  • sprouting of new dendrites
  • synthesis of new proteins
  • changes in synaptic contacts with other neurons
  • Limited ability for regeneration (repair)
  • PNS can repair damaged dendrites or axons
  • CNS no repairs are possible

80
Damage and Repair in the Peripheral Nervous
System (Figure 19.a)
  • When there is damage to an axon, usually there
    are changes, called chromatolysis, which occur in
    the cell body of the affected cell this causes
    swelling of the cell body and peaks between 10
    and 20 days after injury.
  • By the third to fifth day, degeneration of the
    distal portion of the neuronal process and myelin
    sheath (Wallerian degeneration) occurs
    afterward, macrophages phagocytize the remains.
  • Retrograde degeneration of the proximal portion
    of the fiber extends only to the first
    neurofibral node.
  • Regeneration follows chromatolysis synthesis of
    RNA and protein accelerates, favoring rebuilding
    of the axon and often taking several months.

81
Repair within the PNS
  • Axons dendrites may be repaired if
  • neuron cell body remains intact
  • schwann cells remain active and form a tube
  • scar tissue does not form too rapidly
  • Chromatolysis
  • 24-48 hours after injury, Nissl bodies break up
    into fine granular masses

82
Repair within the PNS
  • By 3-5 days,
  • wallerian degeneration occurs (breakdown of axon
    myelin sheath distal to injury)
  • retrograde degeneration occurs back one node
  • Within several months, regeneration occurs
  • neurolemma on each side of injury repairs tube
    (schwann cell mitosis)
  • axonal buds grow down the tube to reconnect (1.5
    mm per day)

83
Neurogenesis in the CNS
  • Formation of new neurons from stem cells was not
    thought to occur in humans
  • 1992 a growth factor was found that stimulates
    adult mice brain cells to multiply
  • 1998 new neurons found to form within adult human
    hippocampus (area important for learning)
  • There is a lack of neurogenesis in other regions
    of the brain and spinal cord.
  • Factors preventing neurogenesis in CNS
  • inhibition by neuroglial cells, absence of growth
    stimulating factors, lack of neurolemmas, and
    rapid formation of scar tissue

84
Multiple Sclerosis (MS)
  • Autoimmune disorder causing destruction of myelin
    sheaths in CNS
  • sheaths becomes scars or plaques
  • 1/2 million people in the United States
  • appears between ages 20 and 40
  • females twice as often as males
  • Symptoms include muscular weakness, abnormal
    sensations or double vision
  • Remissions relapses result in progressive,
    cumulative loss of function

85
Epilepsy
  • The second most common neurological disorder
  • affects 1 of population
  • Characterized by short, recurrent attacks
    initiated by electrical discharges in the brain
  • lights, noise, or smells may be sensed
  • skeletal muscles may contract involuntarily
  • loss of consciousness
  • Epilepsy has many causes, including
  • brain damage at birth, metabolic disturbances,
    infections, toxins, vascular disturbances, head
    injuries, and tumors

86
  • end
Write a Comment
User Comments (0)
About PowerShow.com