Thus, receptor is the first structure in the sensory path. A receptor may be a special structure (like a Pacinian corpuscle) or a bare nerve terminal

. A receptor may be a special structure (like a Pacinian corpuscle) or a bare nerve terminal (e.g. pain). Receptors are also called 'end organs'. The receptors are stimulated when excited by their specific stimulus (eg, rods and cones by ligtl or Merkel's disc by touch) 4 the receptors are now excited the nerve which emerges from the receptor is next excited and ultimately this nerve impulse reaches the brain. The specific sense is now perceived. HISTOLOGY OF RECEPTORS 1. Some receptors are encapsulated structures within which lies the beginning of the afferent nerve. The afferent nerve, therefore begins from here. Examples are, (i) Meissner s corpuscle; (ii) Pacimian corpuscle (fig 10B1.1.1). II. Some receptors are expanded structures which forms the beginning of an afferent sensory nerve. Examples are - (i) Merkel's disc, (ii) Ruffini's end organ. III. Some other receptors are free nerve endings. Pain receptor is the classical example. IV. Other receptors, like rods and cones, cristae and maculae, carotid body chemoreceptors, muscle spindle etc. have specialized structures and have been described elsewhere in this book. A Mechano receptors These are stimulated when they are deformed by the stimulating agent. As a result, the emerging afferent nerve is stimulated. Eg: baro receptors, cristae and maculae, Merkel's disc. Meissner's corpuscle. B .Thermo receptors. C Pain receptors. D. Chemoreceptors. They detect concentrations of some particular chemicals (Eg. carotid body chemo receptors, gluco receptors, central chemoreceptors). CLASSIFICATION Receptors are usually classified as