Sensory receptors are essential for interacting with the external environment, acquiring resources from the environment, keeping the body safe, and maintaining constant, favorable conditions in the body called homeostasis. They detect information from the internal and external environment using sensory receptors or specialized cells. Sensory receptors are part of the afferent or sensory division of the peripheral nervous system (PNS). The information detected by the sensory receptors is called the stimulus. The stimulus activates the sensory receptor, which, in turn, sends electrochemical signals to the central nervous system (CNS) for interpretation and response. Besides detecting a particular stimulus, receptors can also provide information about the modality, duration, location, and intensity of the stimuli.
The body's response to a stimulus can occur at a conscious or unconscious level. For example, sensory inputs such as light, sound, and touch are interpreted on a conscious level, whereas changes in blood glucose levels and blood pressures are not. Habituation occurs when a nonthreatening stimulus persists for a long period of time and the nervous system "downgrades" the importance of the signal. Examples of muted or habituated stimuli include the feeling of clothes, background noises, and smells. The stimuli are still present, but the signal does not reach the conscious level of the brain.
Classification of Sensory Receptors
There are three different ways to classify a sensory receptor. The classification system based on receptor structure divides sensory receptors into two categories: general senses and special senses. The receptors for general senses are modified dendritic endings of sensory neurons that can be found in areas such as skin, mucus membranes, and connective tissues, and detect stimuli directly. These receptors are distributed throughout the body. Examples are receptors that detect pressure, pain, vibration, hunger, and temperature. The receptors for special senses are distinct receptor cells. These receptors are located in complex sense organs where they use an indirect means to gather information. They detect a specific stimulus and then relay that information to the sensory neurons. An example is a retinal cell detecting a particular wavelength of light and then transferring that information to the nervous system for processing. Receptors for special senses are found exclusively in the head region. The special senses are vision, hearing, equilibrium, smell, and taste.
The classification system based on stimulus location divides sensory receptors into three types: exteroceptors, interoceptors, or proprioceptors. An exteroceptor detects information from the external environment. Exteroceptors are found throughout the skin and in most of the special senses, such as vision, sound, touch, and smell. These receptors vary widely and include receptors that would detect the bright color of a shirt, a loud clap of thunder, or the sensation of wind moving over the skin. An interoceptor detects information from the internal organs, such as stretch, temperature, and pain. These receptors are primarily responsible for keeping the internal environment in homeostasis, or at relatively stable conditions. They would detect changes in blood pH, the stretch in the urinary bladder, the activation of the digestive system to stimulate hunger, and a change in blood pressure. A proprioceptor also receives signals from the internal environment, but it is found exclusively in muscles, tendons, or articular structures, such as hip joints, that provide information about body positions and movements. These receptors allow the body to correct any unwanted changes that could lead to losing balance or falling over.
The classification system based on the type of stimulus detected divides receptors into a number of different types: mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors, and nociceptors. A mechanoreceptor is stimulated by mechanical changes, or physical movement, in the nerve ending. These include pressures, touch, and stretch. A thermoreceptor detects changes in temperatures. A photoreceptor detects light. A chemoreceptor detects dissolved chemicals. These include chemicals in the blood (such as oxygen or carbon dioxide), chemicals in foods, and chemicals found in environmental odorous substances, as well as extracellular chemicals, which result in cell-to-cell messages being passed in the body. A nociceptor is a sensory receptor that detects potentially damaging stimuli, such as intense heat, to which the brain initiates a negative response.
These classification systems are not mutually exclusive, and any of them can be used to classify a receptor. For example, when retinal cells detect light, the special receptors activated include exteroceptors and photoreceptors. When ice is placed on the skin, the receptors that receive the information can be general exteroceptors and thermoreceptors.
Properties of Sensory Receptors
Sensory receptors have receptor proteins on the surface of their cell membranes that are specialized to respond to a specific stimulus. When these receptor proteins encounter the stimulus, the proteins change the membrane properties of the cell. Specifically, they will signal the cell to release a chemical signal or initiate an action potential , the result of ions crossing the neuron membrane. The energy of the stimulus is effectively transferred to electrical or chemical energy. This transfer is called transduction. For example, light energy changes the configuration of the receptor proteins in the eye. The protein structural change leads to the opening of ion channels in the membrane of the cell, which ultimately results in the photoreceptor releasing a chemical message to underlying sensory neurons.
The type of sensory information that is relayed to the brain includes the modality, location, intensity, and duration of the stimulus.
- The modality refers to the type of stimulus. The interpretation of the mode of stimulus is determined by the specific cell detecting it and the neural pathway the signal takes to the brain or spinal cord. Different stimuli are processed in discrete parts of the brain.
- The perceived location of the stimulus is also determined by the sensory nerves stimulated and their particular pathway to the brain. The skin has mechanoreceptors that detect touch and pressure. The density of receptors varies in skin from different body regions, with some regions being more sensitive than others. On the back, it is difficult to determine an exact location of touch because the same receptor is spread out across a wide region and sends the same information to the brain. On the other hand, a localized stimulus to the fingertips, toes, or facial structures produces a more precise determination of the stimulus source.
- The intensity of a stimulus can be perceived based on the number of receptors activated, the frequency of the firing action potentials, and the sensitivity of the receptors activated. A group of receptors may detect the same stimulus yet be activated only when the stimulus is more intense.
- The duration of a sensation generated by a stimulus is largely dependent on the type of stimulus and receptor.
- Stimulus intensity directly relates to the time it takes for receptors to produce a reaction. For example, a fingertip touching a burning ember produces a sharp, immediate response to prevent skin from serious burn damage. If the temperature is less intense, such as on a sunny day, receptors take longer to initiate a response. If a stimulus is persistent and does not change, the signals to the conscious level of the brain get slower and eventually stop. This phenomenon is called sensory adaptation, which is also called habituation.