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9.1. Perception: sensory receptors

Sense organs

The relationship function allows us to perceive information, both inside and outside of our body, to be able to analyze it and develop an appropriate response. In this way, living beings relate to their environment by responding to the information that reaches them.

stimulus is any information that reaches the body capable of triggering a response . This response can be slow, fast, immediate or long term, but necessary for the maintenance of life.

The sensory receptors are responsible for capturing this information. The receptors are usually neurons , although sometimes they are modified cells of another type. The receptors can function in isolation , such as those that detect heat, cold, pain, which are in the skin, or they can also constitute, aided by other cells, complex organs, forming the organs of the senses.

The sensory receptors are activated when a occurs stimulus, a change in the medium with an intensity (intensity threshold) capable of enabling receipt. When the stimulus is captured, neurons transform it into a nerve impulse to the central nervous system, where the sensation will originate.

When the stimulus persists over time, the sensory receptors adapt and stop sending nerve impulses with such intensity.



Although traditionally we have always been told about the five senses, receptors are very complex, since they are capable of capturing a wide variety of stimuli, such as light, sound, heat, cold, pressure, chemical molecules, position, etc.

Depending on where the stimulus  comes from, two types of sensory receptors are distinguished :

  • Internal receptors (enteroceptors or proprioceptors ): are nerve endings that are distributed throughout the body responsible for collecting information on the state of the body at all times. Thus, the brain knows the situation of the organism instantly.
  • External receptors (exteroceptors) : are the sense organs .

Depending on the type of stimulus they capture , several types of sensory receptors are distinguished :

  • Photoreceptors: they capture light stimuli and are located in the eyes.
  • Mechanoreceptors: they pick up mechanical changes, such as pressure or sound waves. They are located in the ear, on the skinmuscles, and joints.
  • Chemoreceptors: They capture chemicals, such as taste (tongue) and smell (pituitary membrane) receptors.
  • Thermoreceptors: they capture temperature variations. They are located on the skin.
  • Nociceptors: capture pain. They are located on the skin and in the organs.

Not all animals have all sensory receptors equally developed. Humans have  much more developed hearing and sight than other receptors. Other animals have more developed different ones, even some such as the lateral line of the fish (to detect the vibrations of the water) that we do not have, or those that allow bats to echolocate.

Activity: Function of the following elements.

Video: "Pa" and "Fa" perception experiment.

Answer in your notebook

9.1.- Define what is a stimulus.