chapter 8a
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Chapter 8a. Neurons: Cellular and Network Properties. About this Chapter. 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. Physiologically : - PowerPoint PPT PresentationTRANSCRIPT
Chapter 8a
Neurons: Cellular and Network Properties
About this Chapter
• 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:Central:
BrainSpinal Cord
Peripheral:NervesReceptorsGanglia
Physiologically:Afferent (Sensory)
receptorsEfferent (Motor)
somaticautonomic
SympatheticParasympathetic
Figure 8-1
Organization of the Nervous System
The Neuron
Figure 8-2
Model Neuron
• Dendrites receive incoming signals; axons carry outgoing information
Dendrites
Cellbody
Nucleus
Axon hillock
Axon (initialsegment)
Myelinsheath
Synapse
Presynapticaxon terminal
Postsynapticneuron
Synapticcleft
Postsynapticdendrite
Integration
Outputsignal
Inputsignal
Figure 8-3a-b
Anatomic and Functional Categories of Neurons
• Neurons can be classified according to function or structure
Dendrites
Axon
Schwanncell
Somatic senses
Pseudounipolar Bipolar
Sensory neuronsNeurons for
smell and vision
(a) (b)
Neurons can be categorized by the number of processes and function
Figure 8-3c-d
Anatomic and Functional Categories of Neurons
Dendrites
Axon
Axon
Anaxonic Multipolar
Interneurons of CNS
(c) (d)
Anatomic and Functional Categories of Neurons
Figure 8-3e
Dendrites
Axon
Multipolar
Efferent neuron
(e)
Axonterminal
Figure 8-5b (1 of 2)
Cells of NS: Glial Cells and Their Functions
• Glial cells provide physical and biochemical support for neurons.
GLIAL CELLS
are found in
contains
forms
Peripheral nervous system
Myelin sheaths
Schwann cells
Neurotrophicfactors
Satellitecells
Supportcell bodies
secrete
(b) Glial cells and their functions
Cells of NS: Glial Cells and Their Functions
Figure 8-5b (2 of 2)
GLIAL CELLS
forms
contains
are found in
Central nervous system
AstrocytesOligodendrocytes Microglia (modifiedimmune cells)
provide help form secrete take up create
Scavengers
K+, water,neurotransmitters
Neurotrophicfactors
Blood-brain
barrierSubstrates for
ATP production
Ependymalcells
Barriersbetween
compartments
Source ofneural
stem cells
act as
Myelin sheaths
(b) Glial cells and their functions
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
Figure 8-5a
Interneurons
Astrocyte
Microglia
Oligodendrocyte
Capillary
Myelin(cut)
AxonNode
Section of spinal cord
(a) Glial cells of the central nervous system
Ependymalcell
Cells of NS: Schwann Cells
• Sites and formation of myelin
Figure 8-6a
Schwann cell nucleusis pushed to outside
of myelin sheath.
Nucleus
Axon
Myelin consistsof multiple layersof cell membrane.
(a) Myelin formation in theperipheral nervous system
Schwann cell wraps aroundthe axon many times.
Cells of NS: Schwann Cells
Figure 8-6b
Cell body
1–1.5 mm
Node of Ranvier is a section ofunmyelinated axon membranebetween two Schwann cells.
Schwann cell nucleusis pushed to outside
of myelin sheath.
AxonMyelin consists
of multiple layersof cell membrane. (b) Each Schwann cell forms myelin around
a small segment of one axon.
Multiple SclerosisNystagmus - 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
Figure 8-9 (1 of 2)
4
5
6
1 2
3
7 8 9
23
45
6
7
8
9
Threshold
Resting membrane potential
Depolarizing stimulus
Membrane depolarizes to threshold.Voltage-gated Na+ channels open quicklyand Na+ enters cell. Voltage-gatedK+ channels begin to open slowly.
Rapid Na+ entry depolarizes cell.
Na+ channels close and slowerK+ channels open.
K+ moves from cell to extracellularfluid.
K+ channels remain open andadditional K+ leaves cell, hyperpolarizing it.
Voltage-gated K+ channels close,less K+ leaks out of the cell.
Cell returns to resting ion permeabilityand resting membrane potential.
1
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
Figure 8-10a
ECF
ICFActivation
gateInactivationgate
Na+
(a) At the resting membrane potential, the activation gatecloses the channel.
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
Figure 8-12
Action potential
K+
K+ K+ K+
Absolute refractory period Relative refractory period
Na+
Na+
Na+ and K+
channels
Na+ channels close andK+ channels open
Na+ channels reset to original positionwhile K+ channels remain open
Na+
channelsopen
Bothchannels
closed
Bothchannels
closed
High
Zero
Increasing
High
Ion
perm
eabi
lity
Mem
bran
e po
tent
ial (
mV)
Exci
tabi
lity
Time (msec)
Electrical Signals: Coding for Stimulus Intensity
Figure 8-13a
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
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: Conduction of Action Potentials
Figure 8-15
1
Local current flow from the active region causes new sectionsof the membrane to depolarize.
Refractoryregion
Active region Inactive region
Trigger zone
Axon
Positive charge flows into adjacent sections of the axon by local current flow.
Voltage-gated Na+ channelsopen and Na+ enters the axon.
The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane.
A graded potential above threshold reaches thetrigger zone.
2
3
4
5
Electrical Signals: Conduction of Action Potentials
Figure 8-15, step 1
1
Trigger zone
AxonA graded potential above threshold reaches thetrigger zone.
Electrical Signals: Conduction of Action Potentials
Figure 8-15, steps 1–2
1
Trigger zone
Axon
Voltage-gated Na+ channelsopen and Na+ enters the axon.
A graded potential above threshold reaches thetrigger zone.
2
Electrical Signals: Conduction of Action Potentials
Figure 8-15, steps 1–3
1
Trigger zone
Axon
Positive charge flows into adjacent sections of the axon by local current flow.
Voltage-gated Na+ channelsopen and Na+ enters the axon.
A graded potential above threshold reaches thetrigger zone.
2
3
Electrical Signals: Conduction of Action Potentials
Figure 8-15, steps 1–4
1
Local current flow from the active region causes new sectionsof the membrane to depolarize.
Refractoryregion
Active region Inactive region
Trigger zone
Axon
Positive charge flows into adjacent sections of the axon by local current flow.
Voltage-gated Na+ channelsopen and Na+ enters the axon.
A graded potential above threshold reaches thetrigger zone.
2
3
4
Electrical Signals: Conduction of Action Potentials
Figure 8-15, steps 1–5
1
Local current flow from the active region causes new sectionsof the membrane to depolarize.
Refractoryregion
Active region Inactive region
Trigger zone
Axon
Positive charge flows into adjacent sections of the axon by local current flow.
Voltage-gated Na+ channelsopen and Na+ enters the axon.
The refractory period prevents backward conduction. Loss of K+ from the cytoplasm repolarizes the membrane.
A graded potential above threshold reaches thetrigger zone.
2
3
4
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
Figure 8-19d
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