Neurobiology

text:
Principles of Neural Science
- Kandel, Schwartz and Jessell:
Read pages 105-123 for this lecture
end

VIII. Ion Channels back to VII. Membrane Potential

A. Ions (Na⁺, K⁺, Cl^-) are unevenly distributed across axonal membranes

1. Rapid movement of ions across the membrane creates
depolarization and action potential

2. ions cross the cell membrane through channels

B. Ion channels are proteins that stretch across the membrane

1. several helical peptide sequences form a
nearly cylindrical passage

a. connexin protein subunits form a ring

i. portions of individual subunits
may protrude into the passage

2. extracellular portion of this protein may bind hormones
or neurotransmitters

a. i.e.. channels may be receptors too

i. hormones/transmitters affect channel opening

3. intracellular portion may also bind substances,
such as ATP or ions, to control channel opening

C. Channels opening or closing results from
conformational changes in the protein

1. changes in membrane potential may change the shape
of the subunit sequence which blocks the channel passage

a. voltage-gated ion channels

i. e.g. Na⁺ channels open when axonal membrane
is depolarized to threshold potential

2. transmitter binding may also change the shape = transmitter-gated ion channels

3. intracellular biochemistry may affect channel opening

D. Channels discriminate between ions on the basis of charge

1. allowing cations or anions to permeate

2. some cation channels are non-selective (any cation Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺ can pass)

3. Most are selective for the specific cation

4. all known anion selective channels are permeable only to Cl^-

E. Ion channels are coded for by a family of genes

1. substantial amino acid sequence homology

2. family includes 3 groups

a. voltage gated channels

b. transmitter gated channels = receptors

c. gap junction channels