Neuronal Structure and Physiology

Review - Chapts 2 (Chem),

Chapts 3 (Cells - esp transport - active/passive)

 

The Nervous System - 3 Functions

Neuron

1) cell body

2) axon - membrane VIP

3) dendrites

Homeostasis - difficult

axonal transport

Glia

1) Schwann cells - wrap neurons peripherally

2) CNS types

• oligodendrocytes - wrap neurons

• astrocytes - support cells -

• microglia

Review

Diffusion

Active transport

Resting Potential - Neurophys

Membrane - phospholipids, channels

- charge separator

Energy - diffusion gradient

• channels

• Ions

• also Na+/K+ pump

Maintains RP - does not cause it!!

- measure difference - usually -50 - -90 mV depending on cell

HOW Measured?

• 2 electrodes

1) Reference electrode

2) Recording electrode

HOW maintain this difference?

1) impermeable to Na+ and K+ (less so)

2) pump which is usually running

--> maintains separation of charge

Action Potentials:

• Voltage-dependent changes in membrane permeability

Basis for AP?

1) Early Current - Na+ Hypothesis

Hypothesis states:

1) depol (V change) --> Na+ perm increase

2) Na+ ions flow into cell

3) inflow of Na+ further depolarizes membrane

Thus --> positive feedback loop (last semester)

2) Late current?

• appears to be carried by K+

Major conclusions of expts:

1) Depol increase Na+ conductance

2) Na+ permeability is shut off (slower but overlapping time course)

3) depol induces an increase in K+ conductance

Channels:

1) Pharmacology

A) TTX - a potent poison from puffer fish-->

B) TEA (tetraethylammonium ions)

C) pronase

2) Voltage dependence

• part of the channel that opens or closes must be charged in order to respond to changes in the electric field

---> protein changes shape

--> allows ions to pass

3) ION Selectivity

1) Na+ (MW 23) channel is thought to have

2) K+ (MW 39) channel structure must be diff

 

Other Characteristics of AP

1) Threshold

2) Regenerative (self-sustaining) impulse

3) All-or-none potential

4) Rapid repolarization

5) Refractory

• absolute refractory (appx 1 msec after 1st depol)

• relative refractory (2-4 msec after first depol)

6) ION Movements in APs

Large axon

Smaller axons

7) Conduction of APs - Patches

What prevents the AP from travelling backwards?

8) Conduction Velocity

1) Temperature

2) increasing diameter of axon

3) myelination axons