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THE NERVOUS SYSTEM: NEURAL TISSUE

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Title: THE NERVOUS SYSTEM: NEURAL TISSUE


1
  • THE NERVOUS SYSTEM NEURAL TISSUE

2
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

3
An Overview of the Nervous System
4
Divisions of the nervous system
5
Anatomical Classification of the Nervous System
  • Central Nervous System
  • Brain and spinal cord
  • Peripheral Nervous System
  • All neural tissue outside CNS

6
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

7
Histology of Neural Tissue
8
Cells in Nervous Tissue
  • Neurons
  • Neuroglia

9
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

10
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

11
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

12
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

13
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

14
PNS Satellite Cells
  • Flat cells surrounding PNS axons
  • Support neurons in the PNS

15
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

16
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

17
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
18
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
19
  • 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

20
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

21
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

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 CNS

23
The Nerve Impulse
24
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

25
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

26
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
27
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
28
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
29
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

30
Action Potential
31
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
32
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
33
Rate of Impulse Conduction
  • Properties of axon
  • Presence or absence of myelin sheath
  • Diameter of axon

34
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

35
Synaptic Communication
36
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

37
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

38
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

39
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
40
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
41
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

42
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)

43
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
44
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

45
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

46
Removal of Neurotransmitter
  • Enzymatic degradation
  • acetylcholinesterase
  • Uptake by neurons or glia cells
  • neurotransmitter transporters
  • NE, dopamine, serotonin

47
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
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