Neuroglia of the Peripheral Nervous System

1. Satellite cells -

2. Schwann cells -

In ganglia

Form myelin sheath around peripheral axons

“white matter”

Neuroglia of the Central Nervous System

Figure 12–4

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

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)

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?

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

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

Electrochemical Gradients

Figure 12–9a, b

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?

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??

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

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

3. Mechanically-gated channels:

Active, or Gated, Channels

–respond to membrane distortion

–in sensory receptors (touch, pressure, vibration)

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

• 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.

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)

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

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

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:

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?

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??

2 Methods of Propagating Action Potentials

1. Continuous propagation:

2. Saltatory propagation:

unmyelinated axons

myelinated axons

Figure 12–14

Continuous Propagation

• action potentials along an unmyelinated axon

• Affects 1 segment of axon at a time

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

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?