584
14 Neural Networks and Brains
channel will use the energy of one ATP (→ADP) to transport two potassium ions
into the cell while carrying three sodium ions out. 6 The concentration of potassium
ions inside the cell can be twenty times that on the outside. A resting neuron will
build up a large reservoir of potassium ions within the cell, and expel sodium ions.
Only a very small fraction of these pass through channels in the membrane during
any one firing of the neuron. A neuron can fire hundreds of thousands of times
without a replenishment of the resting concentrations of these ions.
In addition to the slow-acting sodium-potassium pumps, there are voltage-gated
ion channels for the rapid diffusion of sodium and potassium ions during a nerve
impulse. There are membrane channels, as well, for the transport of calcium,
chlorine, and magnesium ions. (In smooth muscle cells, the calcium voltage-gated
channels, which act 10–20 times slower than the nerve cell sodium voltage-gated
channels, dominate the nerve pulse initiation. Calcium ion pumps keep a lower
concentration of calcium ions within the cell.)
The gates open and close depending on the electric field across the membrane.
In switching, gates draw energy available in the electric field across the membrane
working to affect conformational changes in channel proteins. The energy available in the membrane electric field comes from the slow but continual work done by
the ion pumps, which operate using ATP as their source of energy.
Activation (inactivation) voltage-gated channels open (close) with a probability
that increases with the depolarization of the cell membrane. Gates can also differ in
how quickly they respond to voltage changes.
Present also are so-called leakage channels. They let ions pass by the combined
passive effects of diffusion and electric force, although most let certain ions
pass preferentially in one direction. Some can be closed by the binding of a
ligand molecule, such as a neurotransmitter. Other nerve-membrane channels are
controlled by changes in the cell environment, such as temperature or pressure.
If a region of a nerve cell membrane experiences a depolarizing electric field
carrying its membrane potential from its resting value of −70 mV to a value above
about −55 mV, sodium diffusion channel gates open, letting positive charge into the
cell in this region of the membrane, and producing an ‘action potential’. The drop
in potential difference, in turn, polarizes the next region along the nerve membrane.
When the potential goes positive, the sodium diffusion channel gates begin to close,
and the potassium diffusion channel gates open, reversing the potential toward
negative values below −70 mV. This desensitizes the membrane for the refractory
period, so that a propagating nerve impulse traveling down a nerve will not cause
another impulse traveling backward.
6 The transport of both sodium and potassium ions is against the direction of their unforced
diffusion, but moving the potassium ions in the direction of the already established electric field
helps get back a little of the ATP→ ADP energy.
14 Neural Networks and Brains
channel will use the energy of one ATP (→ADP) to transport two potassium ions
into the cell while carrying three sodium ions out. 6 The concentration of potassium
ions inside the cell can be twenty times that on the outside. A resting neuron will
build up a large reservoir of potassium ions within the cell, and expel sodium ions.
Only a very small fraction of these pass through channels in the membrane during
any one firing of the neuron. A neuron can fire hundreds of thousands of times
without a replenishment of the resting concentrations of these ions.
In addition to the slow-acting sodium-potassium pumps, there are voltage-gated
ion channels for the rapid diffusion of sodium and potassium ions during a nerve
impulse. There are membrane channels, as well, for the transport of calcium,
chlorine, and magnesium ions. (In smooth muscle cells, the calcium voltage-gated
channels, which act 10–20 times slower than the nerve cell sodium voltage-gated
channels, dominate the nerve pulse initiation. Calcium ion pumps keep a lower
concentration of calcium ions within the cell.)
The gates open and close depending on the electric field across the membrane.
In switching, gates draw energy available in the electric field across the membrane
working to affect conformational changes in channel proteins. The energy available in the membrane electric field comes from the slow but continual work done by
the ion pumps, which operate using ATP as their source of energy.
Activation (inactivation) voltage-gated channels open (close) with a probability
that increases with the depolarization of the cell membrane. Gates can also differ in
how quickly they respond to voltage changes.
Present also are so-called leakage channels. They let ions pass by the combined
passive effects of diffusion and electric force, although most let certain ions
pass preferentially in one direction. Some can be closed by the binding of a
ligand molecule, such as a neurotransmitter. Other nerve-membrane channels are
controlled by changes in the cell environment, such as temperature or pressure.
If a region of a nerve cell membrane experiences a depolarizing electric field
carrying its membrane potential from its resting value of −70 mV to a value above
about −55 mV, sodium diffusion channel gates open, letting positive charge into the
cell in this region of the membrane, and producing an ‘action potential’. The drop
in potential difference, in turn, polarizes the next region along the nerve membrane.
When the potential goes positive, the sodium diffusion channel gates begin to close,
and the potassium diffusion channel gates open, reversing the potential toward
negative values below −70 mV. This desensitizes the membrane for the refractory
period, so that a propagating nerve impulse traveling down a nerve will not cause
another impulse traveling backward.
6 The transport of both sodium and potassium ions is against the direction of their unforced
diffusion, but moving the potassium ions in the direction of the already established electric field
helps get back a little of the ATP→ ADP energy.