Transcript Lecture 8: Nervous System

I.
Overview
II.
Histology
III. Electrical Signals
IV. Signal Transmission at
Synapses
V.
Neurotransmitters
Nervous
System
VI. Neural Circuits
VII. Repairs
VIII.Pathology
1
I.
Overview
1.
Structures
2.
Functions
3.
Organization
II.
Histology
III.
Electrical Signals
IV.
Signal Transmission at
Synapses
V.
Neurotransmitters
VI.
Neural Circuits
VII.
Repairs
VIII. Pathology
Nervous
System
2
Nervous Tissue

Controls and integrates
all body activities within
limits that maintain life

Three basic functions
1. sensing changes with
sensory receptors
2. interpreting and
remembering those
changes
3. reacting to those
changes with effectors
3
Major Structures of the Nervous System
 Brain
 spinal cord
 cranial nerves
 spinal nerves
 ganglia
 enteric plexuses
 sensory receptors
4
Subdivisions of the PNS
1. Central nervous system (CNS)
2. Peripheral nervous system (PNS)
a) Somatic (voluntary) nervous system (SNS)
b) Autonomic (involuntary) nervous systems (ANS)
c) Enteric nervous system (ENS)
6
Organization
Sensory
Integration
SNS
(Motor)
SNS
(Sensory)
ANS
(Sensory)
Motor
Brain
Spinal
cord
ANS
(Motor)
ENS
(Sensory)
7
I.
Overview
II.
Histology
1.
Neurons
2.
Neuroglia
a)
CNS
b)
PNS
3.
Myelination
4.
Gray and White Matter
III.
Electrical Signals
IV.
Signal Transmission at Synapses
V.
Neurotransmitters
VI.
Neural Circuits
VII.
Repairs
VIII.
Pathology
Nervous
System
8
Neurons

Functional unit of nervous
system
1. Cell body
a) Nissl bodies
b) Neurofilaments
c) Microtubules
d) Lipofuscin pigment
clumps
2. Cell processes
a) Dendrites
b) Axons
9
Dendrites
impulse
 Conducts impulses towards
the cell body
 Typically short, highly
branched & unmyelinated
 Surfaces specialized for
contact with other neurons
 Contains neurofibrils &
Nissl bodies
11
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
12
Structural Classification of Neurons

Based on number of processes
found on cell body
1.
multipolar = several
dendrites & one axon

2.
bipolar neurons = one main
dendrite & one axon

3.
most common cell type
found in retina, inner ear
& olfactory
unipolar neurons = one
process only(develops from
a bipolar)

are always sensory
neurons
14
Structural Classification of Neurons

Based on number of processes
found on cell body
1.
multipolar = several
dendrites & one axon

2.
bipolar neurons = one main
dendrite & one axon

3.
most common cell type
found in retina, inner ear
& olfactory
unipolar neurons = one
process only(develops from
a bipolar)

are always sensory
neurons
15
Structural Classification of Neurons

Based on number of processes
found on cell body
1.
multipolar = several
dendrites & one axon

2.
bipolar neurons = one main
dendrite & one axon

3.
most common cell type
found in retina, inner ear
& olfactory
unipolar neurons = one
process only(develops from
a bipolar)

are always sensory
neurons
16
Association or Interneurons
 Named for histologist that
first described them or their
appearance
17
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
18
Neuroglial Cells (CNS): Astrocytes
 Star-shaped cells
 Form blood-brain barrier
by covering blood
capillaries
 Metabolize
neurotransmitters
 Regulate K+ balance
 Provide structural support
19
Neuroglial Cells (CNS): Oligodendrocytes
 Most common glial cell
type
 Each forms myelin sheath
around more than one
axons in CNS
 Analogous to Schwann
cells of PNS
20
Neuroglial Cells (CNS): Microglia
 Small cells found near
blood vessels
 Phagocytic role -- clear
away dead cells
 Derived from cells that also
gave rise to macrophages &
monocytes
21
Neuroglial Cells (CNS): Ependymal cells
 Form epithelial membrane
lining cerebral cavities &
central canal
 Produce cerebrospinal fluid
(CSF)
22
Neuroglial Cells (PNS): Satellite Cells
 Flat cells surrounding
neuronal cell bodies in
peripheral ganglia
 Support neurons in the PNS
ganglia
23
Neuroglial Cells (PNS): Schwann Cell
 Cells encircling PNS axons
 Each cell produces part of
the myelin sheath
surrounding an axon in the
PNS
24
Myelination
 Insulation of axon
 Increase speed of nerve impulse
25
Myelination: PNS
 All axons surrounded by a
lipid & protein covering
(myelin sheath) produced by
Schwann cells
 Neurilemma is cytoplasm &
nucleus
of Schwann cell
 gaps called nodes of
Ranvier
Node of
Ranvier
 Myelinated fibers
 Unmyelinated fibers
26
Myelination: 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
27
Myelination: 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
28
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)
29
I.
Overview
II.
Histology
III.
Electrical Signals
1.
Overview
2.
Ion Channels
3.
Resting Membrane Potential
4.
Graded Potential
5.
Generation of Action Potential
6.
Propagation of Nerve Impulses
7.
Encoding of Stimulus Intensity
8.
Comparison of Electrical Signals
IV.
Signal Transmission at Synapses
V.
Neurotransmitters
VI.
Neural Circuits
VII.
Repairs
VIII.
Pathology
Nervous
System
30
Electrical Signals in Neurons

Neurons are electrically excitable due to the voltage
difference across their membrane

Communicate with 2 types of electric signals
1. action potentials that can travel long distances
2. graded potentials that are local membrane changes only

In living cells, a flow of ions occurs through ion channels
in the cell membrane
31
Ion Channels
1. Leakage (nongated)
channels are always
open
Na
2. Gated channels can
open and close
a. Voltage-gated
b. Chemically
(ligand)-gated
c. Mechanically-gated
Na
32
Gated Ion Channels (Voltage-Gated)
K
K
ECF
ICF
K
K
K
K
34
Gated Ion Channels (Ligand-Gated)
K
K
NT
ECF
ICF
K
K
K
K
35
Action Potential


Na+
Series of rapidly occurring
events that change and then
restore the membrane potential
of a cell to its resting state
Ion channels open:
1.
Na+ rushes in
(depolarization)
2.
K+ rushes out
(repolarization)

All-or-none principal

Travels (spreads) over surface
of cell without dying out
K+
Depolarization
Na+
K+
Repolarization
39
Repolarizing Phase of Action Potential

Na+
When threshold potential of -55mV is
reached, voltage-gated
K+ channels open
K+
1. Na+ channel opening which caused
depolarization
Hyperpolarization
Repolarization
Depolarization
2. K+ channels finally do open, the Na+
channels have already closed (Na+
inflow stops)

K+ outflow returns membrane
potential to -70mV causing
repolarization
3. If enough K+ leaves the cell, it will
reach a -90mV membrane
potential and enter the afterhyperpolarizing phase
 K+ channels close and the
membrane potential returns to the
resting potential of -70mV
+30
0
-55
-70
40
Refractory Period of Action Potential
Na+
 Period of time during which
neuron can not generate another
action potential
K+
Repolarization
Depolarization
 Absolute refractory period
Hyperpolarization
 even very strong stimulus will
not begin another AP
+30
 Relative refractory period
 a suprathreshold stimulus will
be able to start an AP
0
-55
-70
41
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
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
A.P.
43
Continuous versus Saltatory Conduction
1. Continuous
conduction
(unmyelinated fibers)
2. Saltatory conduction
(myelinated fibers)
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
A.P.
45
Saltatory Conduction
Na
 Nerve impulse conduction
in which the impulse jumps
(Salta) from node to node
A.P.
Na
Na
Na
Na
46
I.
Overview
II.
Histology
III.
Electrical Signals
IV.
Signal Transmission at Synapses
1.
Overview
2.
Electrical synapses
3.
Chemical synapses
4.
Excitatoryand Inhibitory
Postsynaptic Potential
5.
Removal of Neurotransmitter
6.
Spatial and Temporal Summation
of Postsynaptic Potentials
V.
Neurotransmitters
VI.
Neural Circuits
Nervous
System
52
Signal Transmission at Synapses

2 Types of synapses
1. electrical

ionic current spreads to next cell through gap
junctions

faster, two-way transmission & capable of
synchronizing groups of neurons
2. 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
53
Chemical Synapses
 Action potential reaches end
bulb and voltage-gated Ca+ 2
channels open
 Ca+2 flows inward triggering
release of neurotransmitter
Presynaptic
Neuron
Na
Na
NT
 Neurotransmitter crosses
synaptic cleft & binding to
ligand-gated receptors
 the more neurotransmitter
released the greater the change
in potential of the postsynaptic
cell
Na
NT
Calcium
Synpatic
Cleft
 Synaptic delay is 0.5 msec
 One-way information transfer
Postsynaptic
Neuron
54
Excitatory & Inhibitory Potentials

The effect of a neurotransmitter can be
either excitatory or inhibitory
1.

it results from the opening of
ligand-gated Na+ channels
Na
NT
Calcium
the postsynaptic cell is more
likely to reach threshold
an inhibitory postsynaptic potential
is called an IPSP

Na
Na
a depolarizing postsynaptic
potential is called an EPSP

2.
Presynaptic
Neuron
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
NT
Synpatic
Cleft
Na
Postsynaptic
Neuron
55
Excitatory & Inhibitory Potentials

The effect of a neurotransmitter can be
either excitatory or inhibitory
1.

it results from the opening of
ligand-gated Na+ channels
Na
Na
a depolarizing postsynaptic
potential is called an EPSP

2.
Presynaptic
Neuron
Na
NT
Calcium
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
NT
Synpatic
Cleft
K
Postsynaptic
Neuron
56
Removal of Neurotransmitter
1. Diffusion

move down
concentration gradient
Presynaptic
Neuron
Na
Na
Na
NT
Calcium
2. Enzymatic degradation

acetylcholinesterase
NT
3. Uptake by neurons or glia
cells

Synpatic
Cleft
Na
K
neurotransmitter
transporters
Postsynaptic
Neuron
57
Summation
1. Spatial Summation of
effects of
neurotransmitters released
from several end bulbs
onto one neuron
NT
2. Temporal Summation of
effect of neurotransmitters
released from 2 or more
firings of the same end
bulb in rapid succession
onto a second neuron
58
Summation
1. Spatial Summation of
effects of
neurotransmitters released
from several end bulbs
onto one neuron
NT
2. Temporal Summation of
effect of neurotransmitters
released from 2 or more
firings of the same end
bulb in rapid succession
onto a second neuron
59
Summation: Three Possible Responses
1. Small EPSP occurs

potential reaches -56
mV only
2. An impulse is generated

threshold was reached

membrane potential of
at least -55 mV
3.
NT
IPSP occurs

membrane
hyperpolarized

potential drops below 70 mV
60
I.
Overview
II.
Histology
III. Electrical Signals
IV.
Signal Transmission at
Synapses
V.
Neurotransmitters
1.
NT Effect
2.
Small-molecule
3.
Neuropeptides
VI.
Neural Circuits
Nervous
System
64
Neurotransmitter Effects

Neurotransmitter effects can be
modified
1.
Synthesis
&
Release
2.
release can be blocked or
enhanced
3.
removal can be stimulated or
blocked
Removal
receptor site can be blocked
or activated
Receptor
4.

synthesis can be stimulated
or inhibited
NT
Categorized by size:
1.
Small – molecule
2.
Large
65
Small-Molecule Neurotransmitters
1. Acetylcholine (ACh)



released by many
PNS neurons &
some CNS
excitatory on NMJ
but inhibitory at
others
inactivated by
acetylcholinesterase
Neuron
Ach
Muscle
67
Small-Molecule Neurotransmitters
2. Amino Acids
Glutamate
(+)
a) Glutamate released by
nearly all excitatory
neurons in the brain
b) GABA is inhibitory
neurotransmitter for
1/3 of all brain
synapses

Valium is a GABA
agonist - enhancing
its inhibitory effect
GABA
(-)
Valium
68
Small-Molecule Neurotransmitters (2)
3.
Biogenic Amines modified amino acids
(tyrosine)

1.
norepinephrine -regulates mood,
dreaming, awakening
from deep sleep
2.
dopamine -- regulating
skeletal muscle tone
3.
serotonin -- control of
mood, temperature
regulation, & induction
of sleep
removed from synapse &
recycled or destroyed by
enzymes (monoamine
oxidase or catechol-0methyltransferase)
69
Small-Molecule Neurotransmitters (3)
4.
ATP and other purines (ADP,
AMP & adenosine)

excitatory in both CNS & PNS

released with other
neurotransmitters (ACh & NE)
5.
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
70
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
71
I.
Overview
II.
Histology
III. Electrical Signals
IV. Signal Transmission at
Synapses
V.
Nervous
System
Neurotransmitters
VI. Neural Circuits
72
Neuronal Circuits

Neurons in the CNS are organized into neuronal
networks


may contain thousands or even millions of
neurons.
Neuronal circuits are involved in many important
activities
1. breathing
2. short-term memory
3. waking up
73
Neuronal Circuits
1.
Diverging -- single cell
stimulates many others
2.
Converging -- one cell
stimulated by many others
3.
Reverberating -- impulses from
later cells repeatedly stimulate
early cells in the circuit (shortterm memory)
4.
Parallel-after-discharge -single cell stimulates a group
of cells that all stimulate a
common postsynaptic cell
(math problems)
74
Neuronal Circuits
1.
Diverging -- single cell
stimulates many others
2.
Converging -- one cell
stimulated by many others
3.
Reverberating -- impulses from
later cells repeatedly stimulate
early cells in the circuit (shortterm memory)
4.
Parallel-after-discharge -single cell stimulates a group
of cells that all stimulate a
common postsynaptic cell
(math problems)
75
Neuronal Circuits
1.
Diverging -- single cell
stimulates many others
2.
Converging -- one cell
stimulated by many others
3.
Reverberating -- impulses
from later cells repeatedly
stimulate early cells in the
circuit (short-term memory)
4.
Parallel-after-discharge -single cell stimulates a group
of cells that all stimulate a
common postsynaptic cell
(math problems)
76
Neuronal Circuits
1.
Diverging -- single cell
stimulates many others
2.
Converging -- one cell
stimulated by many others
3.
Reverberating -- impulses from
later cells repeatedly stimulate
early cells in the circuit (shortterm memory)
4.
Parallel-after-discharge -single cell stimulates a group
of cells that all stimulate a
common postsynaptic cell
(math problems)
77