They are unencapsulated and exhibit little structural specialization. They are also present in connective tissue, bones, and joints. ![]() They often penetrate the epidermis and end in stratum granulosum there are FNEs that surround the hair follicles as well. Sensory pathways and their destinations adhere to strict topographic representation at each step to facilitate accurate generation of body schema.įree nerve endings (FNEs) sense pain and are ubiquitous in the skin. Tactile receptors show a central area of high sensitivity with surround or lateral inhibition, so that each receptor responds to stimulus occurring in one specific area. Surround inhibition of receptor field serves to increase contrast. From the firing rate of individual neurons and the number of neurons responding, the intensity of the stimulus is derived. Information regarding intensity of stimuli is transmitted by change in the rate and temporal codes. When a single receptor sends input to more than one ganglion cell, divergence occurs on the other hand, when a single ganglion cell receives input from several receptors, convergence occurs, leading to poor spatial resolution but allowing detection of relatively weak stimuli. Two techniques are used to modulate spatial resolution: divergence and convergence. Spatial resolution is essential for perception and recognition of complex stimuli. This general characteristic of receptors bestows an important attribute of being able to respond to an extreme range of stimuli, from very weak to very intense. As the stimulus strength increases, the amplitude of the receptor potential increases in proportion initially, along with an increase in the frequency of the action potential however, with an intense stimulation, after the initial increase, there is progressively less and less additional increase in frequency of action potential. 3.2 Development of receptor potential and subsequently action potential in the nerve terminal in a mechanoreceptor.ĭetermination of stimulus strength is crucial for meaningful sensory perception. When the receptor potential is large enough to cause depolarization of the nerve fiber terminating in the receptor, an action potential is triggered, which propagates along the axon toward the DRG (▶ Fig. This in turn leads to migration of sodium ions from the surrounding tissues, eventually leading to change in the membrane potential of the adjacent node of Ranvier. The usual change in the membrane is opening of the sodium channels, causing influx of positively charged sodium ions. Appropriate stimulus induces a change in the transmembrane potential (receptor potential) by changing the permeability of the ion channels of the cell membrane this may be triggered by mechanical deformation of the membrane causing opening of ion channels, by change in temperature altering permeability of the membrane, or by release of a chemical which may open the channels. Receptors are akin to transducers and generate electrical discharges in response to specific stimuli. Table 3.1 Innervation pattern of somatosensory receptors (From: Conduction of the Action Potential in Greenstein B, Greenstein A, ed. 3.1 Dorsal root ganglion showing afferent input from receptor and efferent input to spinal cord. ▶ Table 3.1 shows the various sensory units and the receptors. ![]() 3.1), with the peripheral portion of the axon terminating in receptors (either as bare nerve ending or in an encapsulated structure) and the proximal portion in the dorsal horn or more proximally in the central nervous system. The DRGs are pseudounipolar cells (▶ Fig. These needs have led to the evolution of receptors with unique characteristics, a variety of neurotransmitters, and axons with differing conduction velocities.Įach somatosensory unit consists of the dorsal root ganglion (DRG) cell, its axon, and all the receptors it innervates. It is crucial not only to sense and discriminate between different types of stimuli but also to acquire information regarding the intensity, topography, and frequency of the stimuli. To deal with different varieties of stimuli, it is necessary to have a variety of sensors (receptors) and separate populations of neurons to transmit them to the brain. From the various stimuli reaching the brain, an image of the body and of the external world is constructed and updated continuously, a process essential for generating prompt and appropriate responses to changes within and outside the body. 1The major categories of stimuli are visual, auditory, gustatory (taste), olfactory (smell), mechanical, thermal, and nociceptive (pain). ![]() Detection of a stimulus and recognition that an event has occurred constitute sensation, whereas interpretation and appreciation of that event constitute perception.
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