The overall job of sensory receptors is to act as transducers. That is, they convert (transduce) energy from one form, e.g. light, temperature, into electrical activity, ultimately action potentials. The first step in the process is a change in the ionic permeability of the membrane of the sensory receptor, thus altering its ionic conductance. This can be achieved in several ways. For example, photoreceptors, i.e. receptors which are responsive to light, trigger a series of reactions which alter the state of opening of ion channels. For example, in invertebrate photoreceptors, Na+ ion channels open causing a depolarizing response; in vertebrates, the opposite happens ― the Na+ channels are already open and light causes them to close as the photoreceptors hyperpolarize. Mechanoreceptors are sensory receptors which are sensitive to changes in pressure. These receptors may be sensitive to touch or pressure located on the general body surface or within deeper body tissues, or receptors sensitive to sound waves present in the external medium. Mechanoreceptors have ion channels which can be opened or closed by mechanical deformation, e.g. increased pressure. This physical deformation of the membrane causes specific ion channels to open. Imagine such deformation resulting in the opening of Na+ channels. This means that Na+ is free to enter the cell and can therefore produce a depolarizing response. This initial depolarizing change in membrane potential is converted into a local potential of the sort described in site. The greater the stimulus, the greater the local potential. For the animal to perceive any sensation, the local potential must be converted into action potentials which are the ‘currency’ of the nervous system. How this is done depends upon whether the receptor is a free nerve ending or whether it is a specialized cell.
In the case of free nerve endings, such as the Pacinian corpuscle (an example of a vertebrate pressure detector), the local potential, which in this situation is called a receptor potential, is transmitted by local currents to the first node of Ranvier. If the change in membrane potential is sufficient to bring the cell membrane to threshold at the first node, an action potential is generated. In the case of sensory receptors which are specialized cells, the local potential, which in this situation is called a generator potential, results in the release of neurotransmitter. The neurotransmitter is then able to influence the electrical activity of the neuron associated with the sensory receptor. A good example is in vertebrate hearing, where sound waves cause the hair cells of the inner ear to depolarize and release neurotransmitter onto the auditory neurons. The effect of this is to generate action potentials in the auditory neurons, which are conveyed back to the region of the brain that deals with auditory information. It is at this site that the action potentials are translated into sound and where the perception of sound occurs.
The relationship between the intensity of the stimulus and the response generated is a relatively complex one. In many cases the relationship is not a simple linear one. Many sensory receptors, when subjected to a maintained stimulus, respond by decreasing the frequency of action potential production. This phenomenon is called adaptation. Some receptors adapt very quickly (this is called phasic adaptation), whilst others adapt very slowly (tonic adaptation). Sensory receptors which display phasic adaptation include those which deal with pressure and touch, for example. This is easily demonstrated ― the perception of sustained pressure on your own skin by the weight of clothes is soon lost. In contrast to this, sensory receptors which display tonic adaptation include those which mediate the sensation of pain. Obviously, since pain is a protective response, it would be disadvantageous to animals to ‘switch off the activity of pain receptors in the presence of continual pain and its cause.