THE NERVOUS SYSTEM: NEURAL TISSUE

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• THE NERVOUS SYSTEM NEURAL TISSUE

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Two organ systems coordinate and direct
activities of body

• Nervous system
• Swift, brief responses to stimuli
• Endocrine system
• Adjusts metabolic operations
• Directs long-term changes

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An Overview of the Nervous System
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Divisions of the nervous system
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Anatomical Classification of the Nervous System

• Central Nervous System
• Brain and spinal cord
• Peripheral Nervous System
• All neural tissue outside CNS

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Functional divisions of nervous system

• Afferent
• Sensory information from receptors to CNS
• Efferent
• Motor commands to muscles and glands
• Somatic division
• Voluntary control over skeletal muscle
• Autonomic division
• Involuntary regulation of smooth and cardiac
  muscle, glands

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Histology of Neural Tissue
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Cells in Nervous Tissue

• Neurons

• Neuroglia

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Neuroglia (Glia)

• about half the volume of cells in the CNS
• smaller than neurons
• 5 to 50 times more numerous
• do NOT generate electrical impulses
• divide by mitosis
• Four types in the CNS
• Astrocytes
• Oligodendrocytes
• Microglia
• Ependymal cells

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Astrocytes

• Largest of glial cells
• Star shaped with many processes
• projecting from the cell body
• Help form and maintain blood-brain barrier
• Provide structural support for neurons
• Maintain the appropriate chemical
• environment for generation of nerve
  impulses/action potentials
• Regulate nutrient concentrations for neuron
  survival
• Regulate ion concentrations - generation of
  action potentials by neurons
• Take up excess neurotransmitters
• Assist in neuronal migration during brain
  development
• Perform repairs to stabilize tissue

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Oligodendrocytes

• Most common glial cell type
• Each forms myelin sheath around the axons of
  neurons in CNS
• Analogous to Schwann cells of PNS
• Form a supportive network around CNS neurons

• fewer processes than astrocytes
• round or oval cell body

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Microglia

• few processes
• derived from mesodermal cells
• that also give rise to monocytes
• and macrophages

• Small cells found near blood vessels
• Phagocytic role - clear away dead cells
• protect CNS from disease through phagocytosis of
  microbes
• migrate to areas of injury where they clear away
  debris of
• injured cells - may also kill healthy cells

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Ependymal Cells

• epithelial cells arranged in a
• single layer
• range in shape from cuboidal
• to columnar

• Form epithelial membrane lining cerebral cavities
  (ventricles) central canal - that contain CSF
• Produce circulate the cerebrospinal fluid (CSF)
  found in these chambers
• CSF colourless liquid that protects the brain
  and SC against
• chemical physical injuries, carries
  oxygen, glucose and other necessary
• chemicals from the blood to neurons and
  neuroglia

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PNS Satellite Cells

• Flat cells surrounding PNS axons
• Support neurons in the PNS

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PNS Schwann Cells

• each cell surrounds multiple unmyelinated PNS
  axons with a single layer of its plasma membrane
• Each cell produces part of the myelin sheath
  surrounding an axon in the PNS
• contributes regeneration of PNS axons

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Neurons

• what is the main defining characteristic of
  neurons?

• have the property of electrical excitability -
  ability to produce
• action potentials or impulses in response to
  stimuli

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Representative Neuron
http//www.horton.ednet.ns.ca/staff/selig/Activiti
es/nervous/na1.htm
-neurofilaments or neurofibrils give cell shape
and support - bundles of intermediate
filaments -microtubules move material inside
cell -lipofuscin pigment clumps (harmless aging)
- yellowish brown
1. cell body or soma -single nucleus with
prominent nucleolus -Nissl bodies -rough ER
free ribosomes for protein synthesis -proteins
then replace neuronal cellular components for
growth and repair of damaged axons in the PNS
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Neurons
2. Cell processes dendrites (little trees) -
the receiving or input portion of the
neuron -short, tapering and highly
branched -surfaces specialized for contact with
other neurons -cytoplasm contains Nissl bodies
mitochondria
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• 3. Cell processes axons
• Conduct impulses away from cell body-propagates
  nerve impulses to another neuron
• Long, thin cylindrical process of cell
• contains mitochondria, microtubules
  neurofibrils - NO ER/NO protein synth.
• joins the soma at a cone-shaped elevation axon
  hillock
• first part of the axon initial segment
• most impulses arise at the junction of the axon
  hillock and initial segment trigger zone
• cytoplasm axoplasm
• plasma membrane axolemma
• Side branches collaterals arise from the axon
• axon and collaterals end in fine processes called
  axon terminals
• Swollen tips called synaptic end bulbs contain
  vesicles filled with neurotransmitters

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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 in the brain and SC
• bipolar neurons one main dendrite one axon
• found in retina, inner ear olfactory
• unipolar neurons one process only, sensory only
  (touch, stretch)
• develops from a bipolar neuron in the embryo -
  axon and dendrite fuse and then branch into 2
  branches near the soma - both have the structure
  of axons (propagate APs) - the axon that projects
  toward the periphery dendrites

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Structural Classification of Neurons

• Named for histologist that first described them
  or their appearance

• Purkinje cerebellum
• Renshaw spinal cord

• others are named for shapes
• e.g. pyramidal cells

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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 CNS

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The Nerve Impulse
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Terms to know

• membrane potential electrical voltage
  difference measured across the membrane of a cell
• resting membrane potential membrane potential
  of a neuron measured when it is unstimulated
• results from the build-up of negative ions in the
  cytosol along the inside of the neurons PM
• the outside of the PM becomes more positive
• this difference in charge can be measured as
  potential energy measured in millivolts
• polarization
• depolarization
• repolarization
• hyperpolarization

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The electric potential across an axonal membrane
can be measured

• the differences in positive and
• negative charges in and out
• of the neuron can be measured by
• electrodes resting membrane potential
• -ranges from -40 to -90 mV

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Ion Channels

• ion channels in the PM of neurons and muscles
  contributes to their excitability
• when open - ions move down their concentration
  gradients
• channels possess gates to open and close them
• two types gated and non-gated

1. Leakage (non-gated) or Resting channels are
always open, contribute to the resting
potential -nerve cells have more K than Na
leakage channels -as a result, membrane
permeability to K is higher -K leaks out of
cell - inside becomes more negative -K is then
pumped back in
2. Gated channels open and close in response to
a stimulus A. voltage-gated open in response to
change in voltage - participate in the AP B.
ligand-gated open close in response to
particular chemical stimuli (hormone,
neurotransmitter, ion) C. mechanically-gated
open with mechanical stimulation
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The resting potential, generated mainly by open
resting, non-gated K channels
-the number of K channels dramatically
outnumbers that of Na -however, there are a few
Na leak channels along the axonal membrane
ECF
AXON
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Action Potential

• Resting membrane potential is -70mV
• triggered when the membrane potential reaches a
  threshold usually -55 MV
• if the graded potential change exceeds that of
  threshold Action Potential
• Depolarization is the change from -70mV to 30 mV
• Repolarization is the reversal from 30 mV back
  to -70 mV)

• action potential nerve impulse
• takes place in two stages depolarizing phase
  (more positive) and repolarizing phase (more
  negative - back toward resting potential)
• followed by a hyperpolarizing phase or refractory
  period in which no new AP
• can be generated

http//www.blackwellpublishing.com/matthews/channe
l.html
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The action potential

• 1. neuron is at resting membrane potential
  (resting MP)
• 2. neuron receives a signal
• Neurotransmitter (NT)
• 3. NT binds ligand-gated sodium channel
• 4. LGNa channel opens
• 5. Na flows into neuron depolarization
• Inside of neuron (i.e. MP) becomes more positive
• 6. if neuron depolarizes enough to Threshold
  Action Potential (AP)
• 7. 1st stage of AP opening of voltage-gated Na
  channels
• 8. even more Na flows in through VGNa channels
  BIG depolarization
• Membrane potential goes from negative to positive
• 9. closing of VGNa channels opening of
  voltage-gated K channels
• 10. BIG outflow of potassium through VGK channels
  repolarization
• Inside of neuron (MP) becomes more negative
• 11. neuron repolarizes so much it goes past
  resting and hyperpolarizes
• 12. closing of VGK channels
• 13. all voltage-gated channels closed, Na/K pump
  resets ion distribution to resting situation

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Action Potential
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Continuous versus Saltatory Conduction

• Continuous conduction (unmyelinated fibers)
• An action potential spreads (propagates) over the
  surface of the axolemma
• as Na flows into the cell during depolarization,
  the voltage of adjacent areas is effected and
  their voltage-gated Na channels open
• step-by-step depolarization of each portion of
  the length of the axolemma

http//highered.mcgraw-hill.com/sites/0072437316/s
tudent_view0/chapter45/animations.html
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Saltatory Conduction

• Saltatory conduction
• -depolarization only at nodes of Ranvier - areas
  along the axon that are unmyelinated and where
  there is a high density of voltage-gated ion
  channels
• -current carried by ions flows through
  extracellular fluid from node to node

http//www.blackwellpublishing.com/matthews/action
p.html
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Rate of Impulse Conduction

• Properties of axon
• Presence or absence of myelin sheath
• Diameter of axon

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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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Synaptic Communication
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Synapses

• Synapse Site of intercellular communication
  between 2 neurons or between a neuron and an
  effector (e.g. muscle neuromuscular junction)
• Permits communication between neurons and other
  cells
• Initiating neuron presynaptic neuron
• Receiving neuron postsynaptic neuron
• You can classify a synapse according to
• 1. the action they produce on the post-synaptic
  neuron excitatory or inhibitory
• 2. the mode of communication chemical vs.
  electrical

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Synapses Excitatory vs. Inhibitory

• If the NT depolarizes the postsynaptic neuron
  excitatory
• The depolarization event is often called an
  excitatory postsynaptic potential (EPSP)
• Opening of sodium channels or other cation
  channels (inward)
• Some NTs will cause hyperpolarization
  inhibitory
• The hyperpolarization event is often called an
  inhibitory postsynaptic potential (IPSP)
• Opening of chloride channels (inward) or
  potassium channels (outward)
• Neural activity depends on summation of all
  synaptic activity
• Excitatory and inhibitory

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Synapses

• Electrical
• Direct physical contact between cells required
• Conducted through gap junctions
• Two advantages over chemical synapses
• 1. faster communication almost instantaneous
• 2. synchronization between neurons or muscle
  fibers
• e.g. heart beat

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Chemical Synapse

• Synapse
• Most are axodendritic axon -gt dendrite
• Some are axoaxonic axon gt axon

http//www.lifesci.ucsb.edu/mcdougal/neurobehavio
r/modules_homework/lect3.dcr
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Synapses Chemical vs. Electrical

• Chemical - Most common type of synapse
• Membranes of pre and postsynaptic neurons do not
  touch
• Space Synaptic cleft
• the AP cannot travel across the cleft release
  of neurotransmitters
• The neurotransmitter induces a postsynaptic
  potential in the PS neuron
• if the potential is an EPSP excitatory and an
  AP results
• If the potential is an IPSP inhibitory and NO
  AP results (e.g. glycine or GABA)
• Communication in one direction only

http//www.blackwellpublishing.com/matthews/nmj.ht
ml
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The Neuromuscular Junction

• end of neuron (synaptic terminal or axon bulb)
  is in very close association
• with the muscle fiber
• distance between the bulb and the folded
  sarcolemma synaptic cleft
• nerve impulse leads to release of
  neurotransmitter (acetylcholine)
• N.T. binds to receptors on myofibril surface
• binding leads to influx of sodium, potassium
  ions (via channels)
• eventual release of calcium by sarcoplasmic
  recticulum contraction

• Acetylcholinesterase breaks down ACh
• Limits duration of contraction

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The Events in Muscle Contraction

• AP generated at trigger zone in
• pre-synaptic neuron
• 2. AP arrives in end bulb causes entry
• of calcium into end-bulb release
• of Ach
• Binding of Ach to ligand-gated Na
• channels on muscle PM (Ach receptors)
• Na enters muscle cell depolarization
• Muscle membrane potential reaches
• threshold Action Potential
• 6. AP travels through PM of muscle cell into
• T-tubules
• 7. AP passes by sarcoplasmic reticulum
• release of calcium into muscle cell
• 8. Ca binds troponin-tropomyosin complex
• shifts it off myosin binding site
• 9. Cross-bridging between actin and myosin,
• pivoting of myosin head Contraction
• (ATP dependent)

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Neurotransmitters

• More than 100 identified
• Some bind receptors and cause channels to open
• Others bind receptors and result in a second
  messenger system
• Results in either excitation or inhibition of the
  target

1. small molecules Acetylcholine (ACh) -All
neuromuscular junctions use ACh -ACh also
released at chemical synapses in the PNS and by
some CNS neurons -Can be excitatory at some
synapses and inhibitory at others -Inactivated by
an enzyme acetylcholinesterase
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Neurotransmitters

• 2. Amino acids glutamate aspartate GABA
• Powerful excitatory effects
• Glutamate is the main excitatory neurotransmitter
  in the CNS
• Stimulate most excitatory neurons in the CNS
  (about ½ the neurons in the brain)
• Binding of glutamate to receptors opens calcium
  channels EPSP
• GABA (gamma amino-butyric acid) is an inhibitory
  neurotransmitter for 1/3 of all brain synapses

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Neurotransmitters

• 3. Biogenic amines modified amino acids
• catecholamines norepinephrine (NE), epinephrine,
  dopamine (tyrosine)
• serotonin - concentrated in neurons found in the
  brain region raphe nucleus
• derived from tryptophan
• sensory perception, temperature regulation, mood
  control, appetite, sleep induction
• feeling of well being
• NE - role in arousal, awakening, deep sleep,
  regulating mood
• epinephrine (adrenaline) - flight or fight
  response
• dopamine - emotional responses and pleasure,
  decreases skeletal muscle tone

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Removal of Neurotransmitter

• Enzymatic degradation
• acetylcholinesterase
• Uptake by neurons or glia cells
• neurotransmitter transporters
• NE, dopamine, serotonin

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Neuropeptides

• widespread in both CNS and PNS
• excitatory and inhibitory
• act as hormones elsewhere in the body
• -Substance P -- enhances our perception of pain
• -opioid peptides endorphins - released during
  stress, exercise
• -breaks down bradykinins (pain chemicals),
  boosts
• the immune system and slows the growth of
  cancer
• cells
• -binds to mu-opioid receptors
• -released by the neurons of the Hypothalamus
  and by
• the cells of the pituitary
• enkephalins - analgesics
• -breaks down bradykinins (200x stronger than
  morphine)
• -pain-relieving effect by blocking the
  release of
• substance P
• dynorphins - regulates pain and emotions

acupuncture may produce loss of pain sensation
because of release of opioid-like substances such
as endorphins or dynorphins