Essential Cell Biology 5th edition

Essential Concepts
423
• Ion channels allow inorganic ions of appropriate size and charge to
cross the membrane. Most are gated and open transiently in response
to a specific stimulus.
• Even when activated by a specific stimulus, ion channels do not
remain continuously open: they flicker randomly between open and
closed conformations. An activating stimulus increases the proportion
of time that the channel spends in the open state.
• The membrane potential is determined by the unequal distribution of
charged ions on the two sides of a cell membrane; it is altered when
these ions flow through open ion channels in the membrane.
• In most animal cells, the negative value of the resting membrane
potential across the plasma membrane depends mainly on the K +
gradient and the operation of K + -selective leak channels; at this resting
potential, the driving force for the movement of K + across the
membrane is almost zero.
• Neurons produce electrical impulses in the form of action potentials,
which can travel long distances along an axon without weakening.
Action potentials are propagated by voltage-gated Na + and K +
channels that open sequentially in response to depolarization of the
plasma membrane.
• Voltage-gated Ca 2+ channels in a nerve terminal couple the arrival
of an action potential to neurotransmitter release at a synapse.
Transmitter-gated ion channels convert this chemical signal back
into an electrical one in the postsynaptic target cell.
• Excitatory neurotransmitters open transmitter-gated cation channels
that allow the influx of Na + , which depolarizes the postsynaptic cell’s
plasma membrane and encourages the cell to fire an action potential.
Inhibitory neurotransmitters open transmitter-gated Cl – channels in
the postsynaptic cell’s plasma membrane, making it harder for the
membrane to depolarize and fire an action potential.
• Complex sets of nerve cells in the human brain exploit all of the
above mechanisms to make human behaviors possible.
KEY TERMS
action potential
active transport
antiport
axon
Ca 2+ pump (or Ca 2+ ATPase)
channel
dendrite
depolarization
electrochemical gradient
gradient-driven pump
H + pump (or H + ATPase)
ion channel
K + leak channel
ligand-gated channel
mechanically-gated channel
membrane potential
membrane transport protein
Na + pump (or Na + -K + ATPase)
Nernst equation
nerve terminal
neuron
neurotransmitter
optogenetics
osmosis
passive transport
patch-clamp recording
pump
resting membrane potential
symport
synapse
synaptic vesicle
transmitter-gated ion channel
transporter
voltage-gated channel
voltage-gated Na + channel