Final answer:
Upon stimulus detection, a cell's membrane receptor can trigger various responses such as opening ion channels, creating action potentials, or activating second messengers—depending on the stimulus and receptor type.
Step-by-step explanation:
Once a stimulus alters the receptor on a cell's membrane, several events could potentially follow. The response to the stimulus depends on the type of receptor and the nature of the stimulus itself. If the stimulus alters the membrane's permeability to sodium ions (Na+), these ions may enter the cell, changing the membrane potential and leading to an action potential. This electrical event is used for cellular communication, such as the interaction between nerves and muscles. For instance, neuron action potentials can prompt the release of neurotransmitters such as acetylcholine (ACh) into the synaptic cleft, binding to receptors on the motor end plate and resulting in the opening of sodium channels. Therefore, the correct answer to what happens next when a receptor on the cell's membrane is altered by a stimulus is: D. Any of these actions could happen next.