What is the Difference Between Tonic and Phasic Receptors

The main difference between tonic and phasic receptors is that tonic receptors slowly adapt to the stimulus, whereas phasic receptors rapidly adapt to it. 

Tonic and phasic receptors are two sensory receptors with different adaptation rates to the stimulus.    

Key Areas Covered

1. What are Tonic Receptors
– Definition, Features, Importance
2. What are Phasic Receptors
– Definition, Features, Importance
3. Similarities Between Tonic and Phasic Receptors
– Outline of Common Features
4. Difference Between Tonic and Phasic Receptors
– Comparison of Key Differences

Key Terms  

Phasic Receptors, Tonic Receptors

What are Tonic Receptors

Tonic receptors are slow-response receptors that provide information about the strength of the stimulus. Importantly, they give information about how strong the particular stimulus is. Also, the occurrence of the nerve impulses of the tonic receptors is gradual. Therefore, tonic receptors are gradually stimulated. Additionally, the rate of firing is constant. Merkel’s discs are an example of tonic receptors. They occur on the skin and respond to pressure. Also, they allow us to distinguish objects from one another. Hence, they allow us to sense changes in texture. Ruffini corpuscles are another example of tonic receptors. They are mechanoreceptors that occur in the skin. They detect the stretches and movements of the skin. Also, they are involved in fine motor movements of the fingers. Further, nociceptors are another type of tonic receptors that detect pain.

Figure 1: Merkel’s Discs

Furthermore, hyperalgesia is a condition that occurs due to damaged nociceptors. In hyperalgesia, a person can feel extreme sensitivity to pain. It can occur due to illnesses such as rheumatoid arthritis or inflammation.    

What are Phasic Receptors

Phasic receptors are fast-adapting receptors that provide information about changes in the stimulus. They provide information about the changes in the rate of the stimulus. Therefore, they give information about the changes in the intensity of a particular stimulus. Also, they generate action potentials very rapidly in response to the stimulus. The frequency of the action potential decreases with time. However, the stimulus continues. Pacinian corpuscles are an example of phasic receptors that occur in the skin. They respond to the vibrations on the skin. Meissner’s corpuscles in the dermis are another example of phasic receptors in the dermis. They sense the initial touch of objects. Thermoreceptors are also phasic receptors that are initially phasic but become tonic with time. 

Figure 2: Pacinian corpuscles

Moreover, diabetes is a disease condition that causes damage to the phasic receptors. It causes damage to the phasic receptors, especially in feet, when it is poorly controlled. Pacinian corpuscle hyperplasia is another rare condition in which corpuscles enlarge to produce nodules under the skin. 

Similarities Between Tonic and Phasic Receptors 

• Tonic and phasic receptors are two sensory receptor types.
• They have different rates of adaptation to the stimuli. 

Difference Between Tonic and Phasic Receptors

Definition

Tonic receptors refer to a sensory receptor that adapts slowly to a stimulus, while phasic receptors refer to a sensory receptor that adapts rapidly to a stimulus.

Continuous Signal Transmission

Tonic receptors undergo continuous signal transmission for the duration of the stimulus, while phasic receptors cease firing if the strength of a continuous stimulus remains constant.

Importance

Tonic receptors are important for the monitoring parameters that need to be continually evaluated, while phasic receptors allow the body to ignore constant unimportant information.

Examples

Merkel’s discs, Ruffini corpuscles, and nociceptors are examples of tonic receptors, while Pacinian corpuscles, Meissner’s corpuscles, and thermoreceptors are examples of phasic receptors. 

Conclusion

In brief, tonic and phasic receptors are two sensory receptor types in the body. Importantly, they have different rates of adaptations to the stimuli. Tonic receptors adapt slowly to a stimulus. They undergo continuous signal transmission for the duration of the stimulus. Therefore, they are important in monitoring parameters that must be continually evaluated. Baroreceptors are examples of tonic receptors. In comparison, phasic receptors adapt rapidly to the stimulus. Also, they cease firing if the strength of a continuous stimulus remains constant. However, they are important in ignoring the constant unimportant information. Smell receptors are examples of phasic receptors. Therefore, the main difference between tonic and phasic receptors is their rate of adaptation to the stimuli. 

References:

1. Madrid R, Sanhueza M, Alvarez O, Bacigalupo J. Tonic and phasic receptor neurons in the vertebrate olfactory epithelium. Biophys J. 2003 Jun;84(6):4167-81. doi: 10.1016/S0006-3495(03)75141-8. PMID: 12770919; PMCID: PMC1302995.

Image Courtesy:

1. “Skin proprioception” By Thomas.haslwanter – Own Work (CC-BY 3.0) via Commons Wikimedia
2. “501 Structure of the skin” By OpenStax College : J. Gordon Betts, Peter Desaix, Eddie Johnson – Own work (CC BY-SA 4.0) via Commons Wikimedia