Lecture11 15 13-1
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Transcript of Lecture11 15 13-1
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Neuroglia of the Peripheral Nervous System
1. Satellite cells -
2. Schwann cells -
In ganglia
Form myelin sheath around peripheral axons
“white matter”
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Neuroglia of the Central Nervous System
Figure 12–4
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Ependymal Cells- Line central canal of spinal cord and ventricles
of brain and secrete cerebrospinal fluid (CSF)
Astrocytes- Maintain blood–brain barrier (isolates CNS)
Oligodendrocytes
- Wrap around axons to form myelin sheaths
Microglia- Migrate and “clean up” cellular debris, waste
products, and pathogens
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Fig. 3-19, p. 91
Sodium-Potassium Exchange PumpActive Transport--requires energy (ATP)- not concentration gradient dependant.
-Usually ion pumps- ex. Na+, K+,Ca++, Mg++ and Cl-.
-Ex. Sodium-potassium exchange pump.
-[Na+] lower in cell than out-[K+] higher in cell than out-Both ions will diffuse down concentration gradient; pump re-establishes gradient-Rate depends on [Na+] in cell
Transmembrane potential
Inside of cell is slightly more negative than outside of cell:- more (+) ions outside and more (-) ions inside; - measured in volts (V)
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Resting Potential
- More passive “leaky” K+ channels than passive “leaky” Na+ channels
How does this affect overall charge inside and outside the cell?
Transmembrane potential of resting cell about —70 mV
• Na+ and K+ channels are either passive or active
- Large (-) charged proteins also “trapped” inside cell
What else contributes to resting potential?
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QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.
QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.
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Passive Forces Across the Membrane
• Chemical gradients:
• Electrical gradients:
Electrochemical GradientFor a particular ion = sum of chemical and electrical forces
- concentration gradients of ions (Na+, K+)
– separated charges of positive and negative ions
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Electrochemical Gradients
Figure 12–9a, b
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Equilibrium Potential
• The transmembrane potential at which there is no net movement of a particular ion across the cell membrane
• Examples:K+ = —90 mV
Na+ = +66 mV
Why?
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Active, or “Gated”, Channels
1. Closed, but capable of opening
2. Open (activated)
3. Closed, not capable of opening (inactivated)
One of 3 conditions:
How does the cell membrane change permeability??
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Active, or Gated, Channels
1. Ligand-gated channels:
3 kinds:
–open in presence of specific chemicals (e.g., ACh)
–on neuron cell body and dendrites
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2. Voltage-gated channels:
Active, or Gated, Channels
–respond to changes in transmembrane potential
–have activation gates (opens) and inactivation gates (closes)
–in axons, skeletal and cardiac muscle
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3. Mechanically-gated channels:
Active, or Gated, Channels
–respond to membrane distortion
–in sensory receptors (touch, pressure, vibration)
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Graded Potentials
Figure 12–11 (Step 2)
– caused by stimulus (eg, neurotransmitter)– local and temporary; effect decrease with distance from stimulus
Depolarization = shift in transmembrane potential toward 0 mV
• Change in potential is proportional to stimulus
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• The Action potential: – an electrical impulse– initiated by graded potential– propagates along surface of axon to
synapse
QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.
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Initiating Action Potential
• Initial stimulus: – graded depolarization at axon hillock large
enough (10 to 15 mV) to change resting potential (—70 mV) to threshold of voltage-regulated sodium channels (—60 to —55 mV)
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All-or-None Principle
• If stimulus exceeds threshold:– action potential is the same, no matter how
large the stimulus
• Action potential is either triggered, or not
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Steps in the Generation of Action Potentials
1. Depolarization to threshold
2. Activation of Na+ channels:
–Na+ rushes into cytoplasm
–rapid depolarization
–inner membrane changes
from negative to positive
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3. Inactivation of Na+ channels, activation of K+ channels:
Steps in the Generation of Action Potentials
–Na+ channel inactivation; gates close
–K+ channels open
–repolarization begins
At +30 mV:
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4. Return to normal permeability:
Steps in the Generation of Action Potentials
–K+ channels begin to close at
—70 mV
–K+ channels finish closing after membrane is hyperpolarized to
—90 mV
–transmembrane potential returns to resting level
Why?
Why?
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The Refractory Period
– from beginning of action potential to return to resting state
– membrane will not respond to additional stimuli; no action potential possible
– WHY??
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2 Methods of Propagating Action Potentials
1. Continuous propagation:
2. Saltatory propagation:
unmyelinated axons
myelinated axons
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Figure 12–14
Continuous Propagation
• action potentials along an unmyelinated axon
• Affects 1 segment of axon at a time
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Saltatory Propagation• along myelinated axon
Figure 12–15
• Myelin insulates axon
• Depolarization occurs only at nodes
• Current “jumps” from node to node
• Faster; uses less energy than continuous propagation
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Axon Diameter and Propagation Speed
• Ion movement is related to cytoplasm concentration
• Axon diameter affects action potential speed
– larger diameter, the faster the propagation
How do size and myelination effect nervous system?