Lecture 6 - ion channels

Question 1

Identify Three Main Types of Ion Channels

Answer 1

Voltage-gated ion channels
Open or close in response to changes in membrane potential.

Neurotransmitter-gated ion channels
Open in response to binding of a neurotransmitter at the extracellular surface.

Second messenger-gated ion channels
Open in response to binding of intracellular second messengers (e.g., cAMP, cGMP, IP3).

Question 2

Structural Features of Voltage-Gated Ion Channels

Answer 2

6 Transmembrane Segments (TMS) per domain (labeled S1–S6).

1 Pore domain between S5 and S6.

• Forms the selectivity filter that determines which ions can pass.

S4 Segment = Voltage Sensor.

• Contains positively charged amino acids (like arginine or lysine).
• Moves in response to changes in voltage → causes channel to open or close.

N-terminal domain.

• In some channels (e.g., K⁺ channels), forms a “ball and chain” structure that inactivates the channel by physically blocking it after it opens (“fast inactivation”).

Question 3

Structure/Function of Neurotransmitter-Gated Ion Channels

Answer 3

Activation:

Neurotransmitters bind to the extracellular part of the channel.

Binding triggers a conformational change → channel opens.

Positive ions (cations) = excitatory → depolarize membrane.
Negative ions (anions) = inhibitory → hyperpolarize membrane.

Question 4

How are Second Messenger-Gated Ion Channels opened

Answer 4

Opened by intracellular signals, not extracellular neurotransmitters.

Question 5

Structure/Function of Second Messenger-Gated Ion Channels: Cyclic Nucleotide-Gated Channels (CNGCs)

Answer 5

Gated by: cAMP or cGMP.

Selective for: Cations (Na⁺ and Ca²⁺).

Functions:

Retina (Vision):
In darkness, cGMP levels are high, channels stay open, allowing Na⁺/Ca²⁺ to flow in.

Light activation of rhodopsin triggers cGMP breakdown → channels close, leading to a neural signal.

Olfactory Neurons (Smell):
Odorants trigger production of cAMP, which opens CNGCs → depolarization → smell perception.

Question 6

Structure/Function of Second Messenger-Gated Ion Channels: Inositol 1,4,5-Triphosphate (IP₃) Receptors

Answer 6

Located on: Endoplasmic Reticulum (ER) membrane.

Gated by: IP₃ (produced from signaling cascades).

Function:
Binding of IP₃ to its receptor opens the channel.

Ca²⁺ stored in the ER is released into the cytosol.

Elevation of cytosolic Ca²⁺ is crucial for multiple downstream signaling events (e.g., muscle contraction, secretion).