Assertion (A) : Neurons are excitable cells.

Reason (R) : The membranes of the neurons are in a polarised state and have the ability to produce action potentials in response to stimuli. 

Neurons are excitable cells because their membranes are polarised, meaning there is a difference in electrical charge between the inside and outside the cell. This polarisation is maintained by ion pumps and channels in the cell membrane, creating a resting membrane potential. When stimulated, such as by neurotransmitters or sensory signals, changes in membrane potential can trigger the generation and propagation of action potentials, which are rapid changes in membrane potential that allow for the transmission of signals along the length of the neuron. Therefore, the ability of neurons to produce action potentials in response to stimuli is a key characteristic of their excitability.