Review of Medical Physiology - William F. Ganong 2002

Physiology of Nerve and Muscle Cells
Initiation of Impulses in Sense Organs
Sensory Organs and Receptors

It is worth noting that in physiology, the term "receptor" is used not only with respect to Sense Organs, or sensory receptors, but also in a completely different sense—referring to Proteins that bind Neurotransmitters, Hormones, and other substances with high Specificity, which is the first step in initiating a specific physiological response (see Chapter 1). A sensory receptor can be part of a neuron or a specialized Cell that generates action potentials in Neurons. Such a receptor is often associated with non-neural Cells that surround it to form a sense organ. The forms of energy perceived by receptors include, for example, mechanical energy (Touch, pressure), thermal energy (degrees of warmth), electromagnetic energy (light), and chemical energy (smell, taste, Blood O₂ levels). Receptors in each sense organ are adapted to respond to one type of energy with a much lower threshold than other receptors responding to the same type of energy. The specific type of energy to which a receptor is most sensitive is called its adequate stimulus. For instance, the adequate stimulus for the rods and cones of the retina is light. Receptors are capable of responding to forms of energy other than their adequate stimulus, but the threshold for these nonspecific responses is much higher. For example, pressure on the Eyeball will stimulate the rods and cones, but the threshold intensity for pressure in these receptors is significantly higher than that for cutaneous receptors.