Sensory Receptors

Receptor is a structure in the body that responds to different changes (stimuli) and converts them into electrochemical energy which can then be transmitted to the central nervous system.

Receptors are classified by the type of stimulus that activates them. There are five basic types: 1) mechanoreceptors, 2) thermoreceptors, 3) nociceptors, 4) photoreceptors (electromagnetic receptors), 5) chemoreceptors.

Mechanoreceptors are activated by pressure or change in pressure. For example, there are baroreceptors in the carotid sinus, which react to blood pressure so that it can be regulated.  Thermoreceptors are activated by changes in temperature. There are receptors that detect cold and others that detect warmth.  Nociceptors are pain receptors, which detect extreme changes, extreme pressure, extreme temperature, or damage in the tissue. Photoreceptors are activated by light. Chemoreceptors are activated by chemicals. They are important for example for olfaction (smell in the nose), taste, they also detect the concentration of carbon dioxide, which is important for breathing, or the level of oxygen in the arterial blood.

Receptors are connected to a nerve which terminates somewhere in the central nervous system. The nerve can only transmit one type of sensation depending on where it terminates in the nervous system. So, for example, if the receptor is connected to a nerve that terminates in a centre of the brain involved in processing pain, it will only transmit the sensation of pain, but not any others. This important principle is called labelled line principle – a nerve can only transmit one type of sensation.

The process of the receptor converting a stimulus into electrochemical energy is called sensory transduction. This sensory transduction is carried out by ion channels. The process is as follows: first, there’s a stimulus and the stimulus excites the receptor (by light, change in temperature etc.). The excitation leads to a change in the receptor’s membrane permeability. This allows certain ions to diffuse through the membrane and change the receptor’s membrane potential. The change in the receptor’s membrane potential is called receptor potential. If the receptor potential reaches threshold – the value needed for eliciting an action potential - an action potential is elicited and transmitted through the nerve that is connected to the receptor, and the information that the receptor reacted to is transmitted to the central nervous system.

One important characteristic of sensory receptors is that they have a very broad range of response so that they can react to both weak and extreme stimulation. When there’s a very strong stimulus and the excitation of the receptor is strong, there’s a greater receptor potential. This causes the frequency of action potentials to increases considerably.

Receptors are also able to adapt, at least partially, if they are constantly stimulated. If the receptor is often stimulated, its rate of generating action potentials starts to decrease. Some receptors adapt more quickly than others and for that reason they are classified into two groups. The first group consists of slowly adapting receptors  called tonic receptors - these continue to transmit impulses as long as the stimulus is present or for a very long time until they adapt. The second group, the so-called phasic receptors, adapt rapidly to the stimulus. They are still, however, able to react to the stimulus if it’s stronger.

References:
Costanzo, L. S. (2018). Physiology. Elsevier.
Hall, J. E., Hall, M. E., & Guyton, A. C. (2021). Guyton and Hall Textbook of Medical Physiology. Elsevier.