A neuron that responds to a non-neural, physical stimulus is a sensory receptor. Exteroreceptors sense stimuli that are external to the animal; interoreceptors respond to stimuli from within the body. Stimuli include, but are not limited to electromagnetic radiation — visible, infrared (IR), ultraviolet (UV); internal mechanical inputs such as muscle stretch and tension, joint rotation, and their first and second derivatives; chemical inputs including pH, pCO2, osmotic pressure, concentration of K+, amino acids, various odorants, pheremones, etc; sound and vibration; the Earth's magnetic field vector; electric field intensity; external mechanical inputs such as angular velocity and acceleration and linear acceleration including the Earth's gravity field.

There are many amazing sensory modalities that neurons and sensory systems respond to. The following sections consider some of the more unusual ones, as well as problems associated with threshold sensitivity. That is, what are the factors that determine the least resolvable stimulus (LRS)?

Most sensory receptors respond to an increasing stimulus by an increasing rate of firing, or if the receptor itself does not spike, a depolarizing (positive-going) generator potential. But, as will be seen, there are some receptors that fire at the cessation of a stimulus (OFF response), or, if nonspiking, by an hyperpolarizing membrane potential at ON of the stimulus.

Of interest is the dynamic responses of sensory receptors because they can be modeled mathematically, and they provide information about the information-processing properties of the nervous system. The following sections first examine external chemoreceptors in vertebrates and arthropods. The next section describes the properties of certain mechanoreceptors — insect trichoid hairs and campaniform sensillae, vertebrate muscle length receptors (spindles), muscle force sensors (Golgi tendon organs), invertebrate gravity and acceleration sensory organs (statocysts), and vertebrate internal pressure sensors (pacinian corpuscles).

There is a large body of behavioral evidence that certain vertebrates and invertebrates can sense their body orientation in the Earth's magnetic field. Section 2.4 reviews some of this evidence and examines putative magnetoreceptors and some theoretical models for animal magnetoreceptors.

Electroreceptors (sensory neurons that respond to an external electric field) are found both in certain saltwater and freshwater fish. These electroreceptors are found in arrays, an ubiquitous organizational modality that allows increased sensitivity over that for single receptors. Some electroreceptors are specialized to sense low-frequency changes in the electric field around the fish (~0.1 to 10 Hz), and are used to passively locate prey, and even for dc magnetic field sensing. Other electrorecep-tors respond to audiofrequency, amplitude, and frequency modulations in the electric field around mormyrid and gymnotid fish, which produce weak ac electric fields for navigation and communication.

The unique, gravity-sensing organs (tricholiths) found on the cerci of certain burrowing desert cockroaches are described in Section 2.6. (Insects as a rule do not have specific gravity-sensing organs or neurons; the cockroach Arenevaga sp. violates this principle.) The Arenevaga gravity-sensing system is also unique because it is an example of a simple sensory array that sends only four afferent axons to the brain, where central processing sharpens the detection of the animal's roll and pitch angles.

Dipteran flies generally have short, stubby, nonaerodynamic bodies, and two wings. Flight stabilization appears to be the result of the interaction of visual information, wind pressure on sensory hairs and the antennae, and the sensory outputs of mechanoreceptors in the bases of a pair of vibrating gyroscopes, the halteres. A mechanical model of the haltere vibrating gyroscope is analyzed, and it is shown that torques are generated at their bases proportional to roll, pitch, and yaw angular rate and accelerations.

Finally, the curious electrophysiological behavior of the simple, multireceptor eye of the plecypod mollusk Mytilus edulis is examined.

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