Transcript Document

Chapter 8a
Neurons: Cellular
and Network
Properties
About this Chapter
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Organization of Nervous System
Cells of the nervous system
Electrical signals in neurons
Cell-to-cell communication in the nervous
system
• Integration of neural information transfer
Anatomically: Physiologically:
Central:
Afferent (Sensory)
Brain
Spinal Cord
Peripheral:
Nerves
Receptors
Ganglia
receptors
Efferent (Motor)
somatic
autonomic
Sympathetic
Parasympathetic
Organization of the Nervous System
Figure 8-1
The Neuron
Model Neuron
Input
signal
Dendrites
• Dendrites receive
incoming signals;
axons carry
outgoing
information
Integration
Cell
body
Nucleus
Axon hillock
Axon (initial
segment)
Myelin
sheath
Presynaptic
axon terminal
Synapse
Synaptic
cleft
Postsynaptic
dendrite
Output
signal
Postsynaptic
neuron
Figure 8-2
Anatomic and Functional Categories of Neurons
Sensory neurons
Somatic senses
• Neurons can be
classified according to
function or structure
Neurons for
smell and vision
Dendrites
Neurons can be
categorized by the
number of processes and
function
Schwann
cell
Axon
Pseudounipolar
(a)
Bipolar
(b)
Figure 8-3a-b
Anatomic and Functional Categories of Neurons
Interneurons of CNS
Axon
Dendrites
Axon
Anaxonic
(c)
Multipolar
(d)
Figure 8-3c-d
Anatomic and Functional Categories of Neurons
Efferent neuron
Dendrites
Axon
Axon
terminal
Multipolar
(e)
Figure 8-3e
Cells of NS: Glial Cells and Their Functions
GLIAL CELLS
• Glial cells provide
physical and
biochemical support
for neurons.
are found in
Peripheral nervous system
contains
Satellite
cells
Schwann cells
forms
Myelin sheaths
secrete
Support
Neurotrophic
cell bodies
factors
(b) Glial cells and their functions
Figure 8-5b (1 of 2)
Cells of NS: Glial Cells and Their Functions
GLIAL CELLS
are found in
Central nervous system
contains
Oligodendrocytes
forms
Microglia (modified
immune cells)
Astrocytes
Ependymal
cells
act as
Myelin sheaths
Scavengers
provide
Substrates for
ATP production
help form
Bloodbrain
barrier
secrete
create
take up
K+,
Neurotrophic
water,
factors
neurotransmitters
Source of
neural
stem cells
Barriers
between
compartments
(b) Glial cells and their functions
Figure 8-5b (2 of 2)
Amyotrophic Lateral sclerosis (ALS
• ALS has been linked to a mutation
on the gene coding for superoxide
dismutase.
• Microglia use reactive oxygen
species (superoxides) to destroy,
may lead to oxidative stress and
neurodegeneration
• A-myo-trophic comes from the
Greek language. "A" means no or
negative. "Myo" refers to muscle,
and "Trophic" means
nourishment–"No muscle
nourishment." When a muscle has
no nourishment, it "atrophies" or
wastes away.
Cells of NS: Glial Cells and Their Functions
Ependymal
cell
Interneurons
Microglia
Capillary
Astrocyte
Myelin
(cut)
Axon
Section of spinal cord
Node
Oligodendrocyte
(a) Glial cells of the central nervous system
Figure 8-5a
Cells of NS: Schwann Cells
• Sites and formation
of myelin
Nucleus
Schwann cell wraps around
the axon many times.
Axon
Schwann cell nucleus
is pushed to outside
of myelin sheath.
Myelin consists
of multiple layers
of cell membrane.
(a) Myelin formation in the
peripheral nervous system
Figure 8-6a
Cells of NS: Schwann Cells
Cell body
1–1.5 mm
Node of Ranvier is a section of
unmyelinated axon membrane
between two Schwann cells.
Schwann cell nucleus
is pushed to outside
of myelin sheath.
Myelin consists
of multiple layers
of cell membrane.
Axon
(b) Each Schwann cell forms myelin around
a small segment of one axon.
Figure 8-6b
Multiple Sclerosis
Nystagmus - involuntary
eye movement
Electrical Signals: Nernst Equation
• Describes the membrane potential that a
single ion would produce if the membrane
were permeable to only that ion
• Membrane potential is influenced by
• Concentration gradient of ions
• Membrane permeability to those ions
Electrical Signals: GHK Equation
• Predicts membrane potential that results from
the contribution of all ions that can cross the
membrane
Electrical Signals: Ion Movement
• Resting membrane potential determined
primarily by
• K+ concentration gradient leak channels open
• Cell’s resting permeability to K+, Na+, and Cl–
• Gated channels control ion permeability
• Mechanically gated
• Pressure or stretch
• Chemical gated
• Ligands, NTs
• Voltage gated
• Membrane potential change
• Threshold voltage varies from one channel type
to another (minimum to open or close)
Electrical Signals: Channel Permeability
Table 8-3
Electrical Signals: Graded Potentials
• Graded potentials
decrease in
strength as they
spread out from
the point of origin
Figure 8-7
Electrical Signals: Graded Potentials
• Subthreshold and
(supra)threshold
graded potentials in
a neuron
Figure 8-8a
Electrical Signals: Graded Potentials
Figure 8-8b
Electrical Signals: Action Potentials
5
6
4
1
Resting membrane potential
2
Depolarizing stimulus
3
Membrane depolarizes to threshold.
Voltage-gated Na+ channels open quickly
and Na+ enters cell. Voltage-gated
K+ channels begin to open slowly.
4
Rapid Na+ entry depolarizes cell.
5
Na+ channels close and slower
K+ channels open.
6
K+ moves from cell to extracellular
fluid.
7
K+ channels remain open and
additional K+ leaves cell, hyperpolarizing it.
8
Voltage-gated K+ channels close,
less K+ leaks out of the cell.
9
Cell returns to resting ion permeability
and resting membrane potential.
Threshold
3
1
2
7
8
9
Figure 8-9 (1 of 2)
Electrical Signals: Action Potentials
Figure 8-9 (2 of 2)
Electrical Signals: Voltage-Gated Na+ Channels
• Na+ channels have two gates: activation and
inactivation gates
Na+
ECF
ICF
Activation
gate
Inactivation
gate
(a) At the resting membrane potential, the activation gate
closes the channel.
Figure 8-10a
Electrical Signals: Voltage-Gated Na+ Channels
Figure 8-10b
Electrical Signals: Voltage-Gated Na+ Channels
Figure 8-10c
Electrical Signals: Voltage-Gated Na+ Channels
Figure 8-10d
Electrical Signals: Voltage-Gated Na+ Channels
Figure 8-10e
Electrical Signals: Ion Movement During an Action
Potential
Figure 8-11
Electrical Signals: Refractory Periods
Both
Na+
channels channels
closed
open
Na+ channels close and
K+ channels open
Na+ channels reset to original position
while K+ channels remain open
Na+
Na+
Both
channels
closed
K+
and
channels
K+
K+
K+
Absolute refractory period
Relative refractory period
Ion permeability
Membrane potential (mV)
Action potential
Na+
Excitability
K+
High
High
Increasing
Zero
Time (msec)
Figure 8-12
Electrical Signals: Coding for Stimulus Intensity
Na+ and K+ [ ]’s change very little
•1 in 100000 K+ leave to shift from +30 to 70mVolts
• Na/K pump will re-establish, but neuron without
pump can still 1000x
Figure 8-13a
Electrical Signals: Coding for Stimulus Intensity
Figure 8-13b
Electrical Signals: Trigger Zone
• Graded potential enters trigger zone
• Voltage-gated Na+ channels open and Na+
enters axon
• Positive charge spreads along adjacent
sections of axon by local current flow
• Local current flow causes new section of the
membrane to depolarize
• The refractory period prevents backward
conduction; loss of K+ repolarizes the
membrane
Electrical Signals: Trigger Zone
Figure 8-14
Electrical Signals: Conduction of Action Potentials
Trigger zone
1 A graded potential above
threshold reaches the
trigger zone.
Axon
2 Voltage-gated Na+ channels
open and Na+ enters the axon.
3 Positive charge flows into adjacent
sections of the axon by local current flow.
4 Local current flow from the
active region causes new sections
of the membrane to depolarize.
5 The refractory period prevents backward
conduction. Loss of K+ from the cytoplasm
repolarizes the membrane.
Refractory
region
Active region
Inactive region
Figure 8-15
Electrical Signals: Conduction of Action Potentials
Trigger zone
1 A graded potential above
threshold reaches the
trigger zone.
Axon
Figure 8-15, step 1
Electrical Signals: Conduction of Action Potentials
Trigger zone
1 A graded potential above
threshold reaches the
trigger zone.
Axon
2 Voltage-gated Na+ channels
open and Na+ enters the axon.
Figure 8-15, steps 1–2
Electrical Signals: Conduction of Action Potentials
Trigger zone
1 A graded potential above
threshold reaches the
trigger zone.
Axon
2 Voltage-gated Na+ channels
open and Na+ enters the axon.
3 Positive charge flows into adjacent
sections of the axon by local current flow.
Figure 8-15, steps 1–3
Electrical Signals: Conduction of Action Potentials
Trigger zone
1 A graded potential above
threshold reaches the
trigger zone.
Axon
2 Voltage-gated Na+ channels
open and Na+ enters the axon.
3 Positive charge flows into adjacent
sections of the axon by local current flow.
4 Local current flow from the
active region causes new sections
of the membrane to depolarize.
Refractory
region
Active region
Inactive region
Figure 8-15, steps 1–4
Electrical Signals: Conduction of Action Potentials
Trigger zone
1 A graded potential above
threshold reaches the
trigger zone.
Axon
2 Voltage-gated Na+ channels
open and Na+ enters the axon.
3 Positive charge flows into adjacent
sections of the axon by local current flow.
4 Local current flow from the
active region causes new sections
of the membrane to depolarize.
5 The refractory period prevents backward
conduction. Loss of K+ from the cytoplasm
repolarizes the membrane.
Refractory
region
Active region
Inactive region
Figure 8-15, steps 1–5
Electrical Signals: Action Potentials Along an Axon
Figure 8-16b
Electrical Signals: Speed of Action Potential
• Speed of action potential in neuron
influenced by
• Diameter of axon
• Larger axons are faster
• Resistance of axon membrane to ion leakage
out of the cell
• Myelinated axons are faster
Electrical Signals: Myelinated Axons
• Saltatory conduction
Figure 8-18a
Electrical Signals: Myelinated Axons
Figure 8-18b
Electrical Signals: Chemical Factors
• Effect of extracellular
potassium
concentration of the
excitability of neurons
Figure 8-19a
Electrical Signals: Chemical Factors
Figure 8-19b
Electrical Signals: Chemical Factors
Figure 8-19c
Electrical Signals: Chemical Factors
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Figure 8-19d