Nervous Tissue

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Chapter 12

• Nervous Tissue
• Lecture Outline

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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.

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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

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Major Structures of the Nervous System

• Brain, cranial nerves, spinal cord, spinal
  nerves, ganglia, enteric plexuses and sensory
  receptors

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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.

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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.

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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

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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

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Organization of the Nervous System

• CNS is brain and spinal cord
• PNS is everything else

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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.

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HISTOLOGY OF THE NERVOUS SYSTEM
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Neuronal Structure Function
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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

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Parts of a Neuron
Neuroglial cells
Nucleus with Nucleolus
Axons or Dendrites
Cell body
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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).

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Dendrites

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

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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
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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

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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

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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).

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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

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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

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Association or Interneurons
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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

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Astrocytes

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

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Microglia

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

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Ependymal cells

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

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Satellite Cells

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

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Oligodendrocytes

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

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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.

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Schwann Cell

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

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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

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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

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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

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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.

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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

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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

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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.

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Gated Ion Channels
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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

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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.

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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

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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.

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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

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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

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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

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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

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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)

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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

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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

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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

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Saltatory Conduction

• Nerve impulse conduction in which the impulse
  jumps from node to node

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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

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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

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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

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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.

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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

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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

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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

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Removal of Neurotransmitter

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

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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

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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

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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.

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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.

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Spatial Summation

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

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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

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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.

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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.

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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

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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)

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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)

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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

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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

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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

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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

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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)

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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

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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.

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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

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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)

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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

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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

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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

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• end