Final answer:
Hyperpolarization is when the membrane potential of a cell becomes more negative than its resting potential due to the activity of ion channels, particularly potassium channels. After an action potential, these channels remain open longer than necessary, causing an overshoot in the membrane potential. In photoreceptors, light-induced enzyme activity also causes hyperpolarization by closing sodium channels.
Step-by-step explanation:
Hyperpolarization of the Cell Membrane
During hyperpolarization, the membrane potential of a cell becomes more negative than its normal resting potential. This change is due to the activity of specific ion channels. When a stimulus leads to the opening of potassium channels after an action potential, there is an efflux of K+ ions from the cell. As these positively charged ions leave, they take their positive charge with them, making the inside of the cell membrane more negative.
While the resting potential of a neuron is typically around -70 mV, during hyperpolarization the membrane potential drops below this value. This hyperpolarized state is due to the potassium channels staying open a bit longer than necessary, even after the repolarization phase of the action potential, resulting in an overshoot of the membrane potential. Eventually, these ion channels will close and the Na+/K+ transporter will restore the membrane to its resting potential.
Additionally, in photoreceptor cells of the eye, when light strikes rhodopsin, a G-protein called transducin is activated. This leads to the conversion of cGMP to GMP by phosphodiesterase, which causes sodium channels to close, resulting in hyperpolarization of the membrane. Under these conditions, the hyperpolarized membrane does not release the neurotransmitter glutamate to the bipolar cell.