Biology Reference
In-Depth Information
membranes. Nerve cells maintain a potential difference across the membrane
with the inside of the cell negative relative to the outside of the cell. The resting
nerve cell also maintains concentration gradients of sodium (Na), calcium (Ca),
and potassium (K) ions. Na and Ca ions are at a relatively high concentration
outside the cell, whereas K-ion levels are relatively high inside the cell. Signaling
involves a change in the resting-membrane potential brought about by charge
transfers carried by ionic fluxes through gated pores formed by transmembrane
proteins called
channels
.
Ion-channel proteins catalyze the transmembrane flow of ionic charge by
forming narrow, hydrophilic pores through which ions can diffuse (
Miller 1991
).
Ion channels
must open or close rapidly in response to biological signals (
=
gating).
Furthermore, the open pore is generally selective and will determine which ions
will permeate and which will not (
ionic selectivity
). Thus, a specific channel will
permit K but not Na to pass.
Stimuli from the environment are perceived by specialized nerve cells (sen-
sory cells). Each type of sensory cell responds to a particular stimulus such as
light, sound, touch, heat, or chemicals such as pheromones. These sensory
cells transform and amplify the energy provided by a stimulus into an electri-
cal signal (
=
sensory transduction).
Sensory transduction
is probably due to an
alteration in the ionic permeability of the sensory-cell membrane, which causes
a depolarization of the membrane of the sensory cell from its resting level.
The amplitude and duration of this departure generally increases logarithmi-
cally with the intensity of the stimulus. This signal is local and is not transmit-
ted along the nerve cell; however, it acts as a stimulus to the axon and if the
depolarization increases over a threshold level, the signal will trigger a change
in
action potential
in the axon. Action potentials are all-or-nothing electri-
cal impulses that propagate without distortion or attenuation along the entire
length of an axon.
The generation and propagation of an action potential alters ionic conditions
within the cell. When axonal membranes are depolarized, Na channels open and
allow Na ions to flow down their gradient into the cell, producing the depolar-
izing phase of an action potential. Within milliseconds after the Na channels are
opened, they are inactivated, but at about the same time the membrane depo-
larization activates K channels, and the reciprocal K flow repolarizes the cell and
restores the membrane-resting potential. During the course of an action poten-
tial, the Na currents in one region of the axon membrane cause the depolariza-
tion and firing of an action potential in an adjacent region of the membrane so
that the action potential is propagated along the full length of the axon.
