Weber Fechner and Steven's Power law

The concept of Just Noticeable Difference was given by Weber where he explained that we will be able to perceive the difference in intensity of the stimulus depending on the original strength of the stimulus. For higher strengths of stimulus, a greater chnage in intensity of stimulus is required is required to notice the difference in intensities. 

Fechner took this concept and tried to explain the percieved intensity of stimulus vis a vis the strength of stimulus. But Fechner law was applicable to only very high and very low intensities of stimulus. Later this was modified by Steven who gave Power law to explain the percieved intensity of stimulus.


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Classification of sensory receptors

Afferent neurons transmit action potential from the periphery to the central nervous system. These afferent neurons have sensory receptors at their peripheral endings. Receptors as the term indicates receive information or we can say characteristics of a stimulus from the surroundings. 

The stimuli are of various types depending on the type of energy change for e.g. there can be either mechanical stimulus , chemical stimulus or thermal stimulus etc. Body has receptors for detecting different types of energies. But once the receptors detect the stimulus, it has to be transmitted to the CNS as action potential, so receptors convert the stimulus energy into electrical signals. This is known as signal transduction.  

Sensory Receptors can be classified in 3 ways

1. Based on their location: exteroceptors, interoceptors and proprioceptors

2. Based on modality of stimulus they detect: mechanoreceptors, chemoreceptors, thermoreceptors, nociceptors, and electromagnetic receptors . 

3. Based on rate of adaptation: Slowly adapting receptors and fast adapting receptors.

In skin , their are various mechanoreceptors. However, they are located in various layers of skin e.g. Merke;'s disks are located superficially in dermis and detect superficial texture of the object while Ruffini ending is deeper in dermis and detects pressure. 

Also there are Meissner’s corpuscle versus Pacinian corpuscles. Both these receptors adapt very fast. Thus, they detect vibration sense. But, meissner’s corpuscle respond to slow vibration while pacinican corpuscle respond to fast vibration because pacinian corpuscle adapt very very fast. And pacinican corpuscles are located deep so they respond to deep pressure vibration while Meissner’s has to be superficial like a flutter on the skin. Also, meissner’s corpuscles detect the sensation from a small superficial area while Pacinian corpuscle respond to a stimulus even when it is present in a little larger area. This is known as receptive field of the receptor.