Receptors: Definition, Function & Examples

Imagine that you couldn’t taste, see or hear and you didn’t feel any pain either. That would be rather unfavorable for you and your survival. You must be able to recognise, absorb and interpret all these stimuli from the environment, but also from your own body. In order to be able to perceive all these signals and to be able to pass them on in your body, you need receptors.

Receptors – definition

Sensory cells absorb chemical or physical stimuli, convert them into an electrical signal and transmit it to the organism’s central nervous system. The brain or spinal cord processes the information and then formulates a response from the organism.

Receptor-protein complexes serve the signal transductionso that an extracellular signal can be transmitted intracellularly across a cell membrane.

receptors sensory cells

Sensory cells can be distinguished based on different sensory modalities. The classic five sensory modalities are:

• See
• Smell
• Listen
• Taste
• Feel

The individual sensory modalities are only recorded via specific sensory receptors. The sensory receptors can be distributed in the tissue, as is the case with the pain receptors in the skin, or they can be organized in one sensory organ, such as the photoreceptors in the retina.

Receptors – transmission of stimuli

Receptors are highly specialized cells that only respond to a stimulus specific to the receptor. This specific stimulus is called adequate stimulus. When adequate stimuli arrive, receptors have a low threshold, so that even small amounts of energy are sufficient to send a signal to the central nervous system. Inadequate stimuli (= external stimuli or inadequate stimuli) are only converted into a signal by the sensory cell if the intensity of the incoming stimulus is very strong.

As soon as a strong stimulus, such as high levels of solar radiation, hits the receptor in the eye, the stimulus transmission begins. This is an adequate stimulus since sunlight strikes the light-sensitive photoreceptors.

types of receptors

Sensory receptors are highly selective and only respond to an adequate stimulus. They can therefore be classified into different categories based on the stimuli they receive. There is e.g. B. photoreceptors, mechanoreceptors, chemoreceptors and thermoreceptors.

On the one hand, receptors pick up external stimuli from the environment, but on the other hand they also absorb internal stimuli. The internal stimuli are chemical and physical stimuli. For example, a mechanoreceptor perceives external stimuli such as pressure, but at the same time internal stimuli such as changes in body tension.

You can see the individual receptor types and the stimuli they receive in the table below:

function of receptors

Sensory receptors are specialized cells that can pick up stimuli and convert them into electrical signals. These electrical signals are passed on to the next cell in the form of an action potential and thus to the central nervous system.

When at rest, the nerve cell has a resting potential of -45 to -70 mV. If a stimulus causes a small local voltage change, nothing happens in the nerve cell at first. Only if the stimulus is strong enough can the threshold voltage for an action potential be exceeded.

When the threshold voltage is reached, an action potential always occurs. More positively charged ions flow into the nerve cell and the cell membrane is depolarized (-70 mV to 0 mV). Since the influx of positively charged ions continues for a while, it comes to overshootthat is, the membrane potential briefly becomes positive (+30 mV) before repolarization begins.

During repolarization, positively charged ions are pumped out of the nerve cell so that the resting state can be reached again. This leads to a brief hyperpolarization, i.e. a more negative membrane potential than in the resting state, until the cell has restored the ion balance at -70 mV.

The potential of the cell increases to a value of up to +30 mV. Therefore, the cell has a greater electrical voltage than before. In this case one speaks of a depolarization.

The depolarization leads to a excitatory postsynaptic potential (EPSP) and thus to one Action potential in the next neuron. The electrical excitation of the receptor potential is passed on via signal transduction and transmitted to the central nervous system (CNS) via nerve tracts. only in Brain it is possible to perceive the stimulus.

stimulus strength

In general, the intensity of the receptor potential is proportional to the strength of the stimulus. Because it is true that the higher the stimulus strength, the higher the frequency of the successive action potentials. For example, you can distinguish different loud sounds from each other. In addition, the receptor potential lasts as long as the stimulus acts on the receptor.

Adaptation to stimulus

If a stimulus lasts for a longer period of time with the same intensity, your body gets used to it. You may know this yourself: After a certain time, you no longer perceive monotonous background noises or smells as active. This process is called adaptation. How and whether a stimulus adaptation takes place depends on the type of receptor.

Overall, the receptors can be divided into three different groups based on their different adaptation to long-acting stimuli:

1. the phasic sensory receptors show adaptation to a constant stimulus. Your stimulus response decreases and you get used to the stimulus.
2. at tonic receptors however, like pain receptors, the action potential frequency does not decrease with time. There is no adaptation to the stimulus.
3. The last group, the phasic-tonic receptors, represent a mixed form and are most frequently found in the human body. Even if dWhilst the pulse frequency is initially quite high, it falls to a low, constant value over time in phasic-tonic receptors.

In phasic sensory cells, however, the frequency of action potentials decreases significantly over time, which is why there is no longer any stimulus response over time. In the case of phasic-tonic sensory cells, the action potential frequency also decreases, but remains constant after a certain point in time. On the other hand, the stimulus response in tonic cells is constantly high over the entire period and corresponds to the stimulus strength.