This document discusses the types and functions of neuroglia cells in the peripheral and central nervous systems. In the peripheral nervous system, satellite cells surround ganglia and Schwann cells form the myelin sheath around peripheral axons. In the central nervous system, ependymal cells line the spinal cord and brain ventricles and secrete cerebrospinal fluid, astrocytes maintain the blood-brain barrier, microglia migrate to remove debris, and oligodendrocytes wrap around axons to form myelin sheaths. The document then discusses the resting potential of neurons and how sodium-potassium pumps and ion concentrations contribute to it.

1. Neuroglia of the
Peripheral Nervous System
1. Satellite cells In ganglia
2. Schwann cells Form myelin
sheath around
peripheral axons
“white matter”

2. Neuroglia of the Central Nervous System
Figure 12–4

3. Ependymal Cells
- Line central canal of spinal cord and ventricles
of brain and secrete cerebrospinal fluid
(CSF)
Astrocytes
- Maintain blood–brain barrier (isolates CNS)
Microglia
- Migrate and “clean up” cellular debris, waste
products, and pathogens
Oligodendrocytes
- Wrap around axons to form myelin sheaths

4. Transmembrane potential
Inside of cell is slightly more negative than outside of cell:
- more (+) ions outside and more (-) ions inside;
- measured in volts (V)
Sodium-Potassium Exchange Pump
Active 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 reestablishes gradient
-Rate depends on [Na+] in cell
Fig. 3-19, p. 91

5. Resting Potential
Transmembrane potential of resting cell about —70 mV
What else contributes to resting potential?
• Na+ and K+ channels are
either passive or active
- More passive
“leaky” K+ channels
than passive “leaky”
Na+ channels
How does this affect
overall charge inside
and outside the cell?
- Large (-) charged
proteins also
“trapped” inside cell

6. QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.

7. Passive Forces Across the Membrane
• Chemical gradients:
- concentration gradients of ions (Na +, K+)
• Electrical gradients:
– separated charges of positive and
negative ions
Electrochemical Gradient
For a particular ion = sum of chemical and electrical forces

8. Electrochemical Gradients
Figure 12–9a, b

9. 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?

10. How does the cell membrane change permeability??
Active, or “Gated”, Channels
One of 3 conditions:
1. Closed, but capable of opening
2. Open (activated)
3. Closed, not capable of opening (inactivated)

11. Active, or Gated,
Channels
3 kinds:
1. Ligand-gated channels:
–open in presence of
specific chemicals (e.g.,
ACh)
–on neuron cell body
and dendrites

12. Active, or Gated, Channels
2. Voltage-gated channels:
–respond to changes in
transmembrane potential
–have activation gates
(opens) and inactivation
gates (closes)
–in axons, skeletal and cardiac
muscle

13. Active, or Gated,
Channels
3. Mechanically-gated
channels:
–respond to membrane
distortion
–in sensory
receptors (touch,
pressure, vibration)

14. Graded Potentials
• Change in potential is proportional to stimulus
– caused by stimulus (eg, neurotransmitter)
– local and temporary; effect decrease with distance from
stimulus
Depolarization = shift in transmembrane potential toward 0 mV
Figure 12–11 (Step 2)

15. • The Action potential:
– an electrical impulse
– initiated by graded potential
– propagates along surface of axon to
synapse
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.

16. 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 voltageregulated sodium channels (—60 to —55 mV)

17. 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

18. 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

19. Steps in the Generation of Action
Potentials
3. Inactivation of Na+ channels, activation
of K+ channels:
At +30 mV:
–Na+ channel inactivation;
gates close
–K+ channels open
–repolarization begins

20. Steps in the Generation of Action
Potentials
4. Return to normal permeability:
–K+ channels begin to close at
—70 mV
–K+ channels finish closing after
membrane is hyperpolarized to
—90 mV
Why?
–transmembrane potential
returns to resting level
Why?

21. 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??

22. 2 Methods of Propagating
Action Potentials
1. Continuous propagation:
unmyelinated axons
2. Saltatory propagation:
myelinated axons

23. Continuous Propagation
• action potentials along an unmyelinated
axon
• Affects 1 segment of axon at a time
Figure 12–14

24. Saltatory Propagation
• along myelinated axon
• Myelin insulates axon
• Depolarization occurs only at nodes
• Current “jumps” from node to node
• Faster; uses less energy than continuous propagation
Figure 12–15

25. 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?