EFB325 Cell Physiology

Ion channels and the nervous system

Membrane bound transport proteins are the primary sites for the transport of molecules across the plasma membrane. Transporters can be classified into two types: channels and carriers.

Ion channels

Channels form hydrophilic pores through the membrane that allow ions to pass through (a pore in a protein rather than through the lipid bilayer)

• channels are usually very selective, based on charge of the ion and size of the atom
• channels do not bind to the ion, they simply form a selective pore
• transport through a channel is much faster than through a carrier (as much as 1000X faster)
• but channels cannot move ions against their electrochemical gradient, they only allow passive transport

Examples

Remember that the concentration of Na⁺ is much higher outside the cell than inside and that the membrane potential is negative on the inside of the cell

• this situation is maintained because the membrane is highly impermeable to Na⁺
• BUT, if a Na⁺ channel in the plasma membrane were to open, Na⁺ would diffuse into the cell, down its electrochemical gradient
• as Na⁺ enters the cell, the inside becomes more positive and the membrane potential becomes less negative

K⁺ concentration is much higher inside the cell than outside, but the membrane potential is negative on the inside

• usually, there are K⁺ channels open in the plasma membrane that allow K⁺ to diffuse out until the inside and outside concentrations are in equilibrium with the membrane potential

The pore in some channels can be opened and closed=gated

Changes in the shape of the protein result in the pore being fully open or fully closed

• this allows the cell to regulate the balance of ions across the membrane

The activity of a particular type of channel is regulated by one of three different factors

1) Voltage-gated channels: the activity of these channels is turned on or off at a particular threshold of the membrane potential (the voltage across the membrane)

• when the channel opens, the flow of ions may then change the membrane potential
• voltage-gated channels can be in one of three states: open, closed, or inactivated (channel is shut, even though the membrane potential would trigger it to open)

2) Ligand-gated channels: the activity of these channels is regulated depending on whether a signal molecule (the ligand) is bound to the channel protein or not these channels allow conversion of a chemical signal (the ligand) to an electrical one (change in membrane potential once the channel opens)

3) Stress-activated channels: the activity of these channels is changed by physical stress applied to the channel protein, such as sound vibrations in the auditory hair cells converts a physical stress to an electrical signal

Action potentials and signalling in nerve cells

The neurons (cells in the nervous system) transmit signals very rapidly and over long distances across an animals body. This signal is transmitted through a signal neuron as a wave of electrical excitation in the form of a localized change in membrane potential called the action potential. The signal is transmitted between two neuron cells by chemical signals called neurotransmitters.

A neuron receives signals through its branched dendrites, then transmit it down the long axon, and pass the signal on through branches in the axon ending in nerve terminals.

A neuron is stimulated by a signal that causes a local change in the membrane potential (making the inside less negative) called depolarization.

Typically, special sensory cells will initiate signals - like the stress-activated channels in auditory cells in your ear or olfactory receptor cells in your nose. Each olfactory cell produces only a single type of olfactory receptor - and we produce ~1000 different unique types of receptors.

• this change in membrane potential causes voltage-gated Na⁺ channels to open, and the initial signal is greatly amplified when the influx of Na⁺ causes the membrane potential to become positive on the inside (in this local spot of the cell)

The signal is conducted in a single direction down the axon as a wave of membrane depolarization

• the initial change in membrane potential causes adjacent voltage-gated Na⁺ channels to open and the depolarization spreads forward
• the original Na⁺ channels that were opened now switch to the inactive state- this allows that region of the membrane to return to its original membrane potential and prevents the signal from going backwards
• the wave of depolarization is conducted down the axon to the terminals

At the nerve terminal, the electrical signal is converted to a chemical signal, then back to an electrical signal by the next neuron

• when the wave of depolarization reaches the terminal, it triggers voltage-gated Ca²⁺ channels to open, and Ca²⁺ rushes in down its electrochemical gradient
• the increase in Ca²⁺ concentration in the nerve terminal triggers the cell to release chemicals called neurotransmitters to the outside of the cell
• the neurotransmitters are perceived by the adjacent neuron by binding to specific ligand-gated ion channels and triggering them to open, starting a new electrical signal (membrane depolarization) in this cell

Chemical interactions at the synapses can affect the nervous system

• There are excitatory neurotransmitters: acetylcholine, glutamate, serotonin; inhibitory neurotransmitters: GABA, glycine
• Drugs and neurotoxins can affect nerve signal transmission: Valium and Prozac affect neurotransmitter levels or sensitivity; some snake venoms, curare, and strychnine can block neurotransmitter signaling

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