FIGURE 2.3-21 A block diagram of a dynamic model emulating the behavior of a PC. The time-varying pressure in the tissues around the PC is differentiated and then rectified (the PC fires to both increasing and decreasing pressure). It is then low-pass-filtered to emulate electrotonic conduction to form the generator potential, Vg(t). Vg is the input to an RPFM (leaky integrator) pulse generator. The instantaneous frequency of y is the PC output.
leading to depolarization of the membrane voltage and the generation of nerve spikes. Whether or not a mechanosensory neuron responds to length changes, tension force on a tendon, tissue pressure changes, or simple deflection of one or more cilia on the cell body depends on the mechanical support of the surrounding tissues and its connection to them. The neuron itself is soft (compliant), and responds to length changes or deflections as small as a micron. (To sense length changes on the order of cm, the 1 cm length change must be divided by about 104.) Thus a force sensor neuron (e.g., a GTO neuron) must be embedded in a very stiff, elastic substrate so that micron-level displacements occur for kg-level forces. In the case of insect trichoid hairs, the hair acts as a lever so that a large angular hair deflection produces a small deflection on the tip of the sensory neuron's dendrite (see Figure 2.3-1).
Probably one of the more amazing mechanosensory organelles is the vertebrate muscle spindle. Designed to respond to muscle stretch, either passive or under load with muscle fibers active, the spindle is loosely attached in parallel with the main (extrafusal) muscle fibers. Thus a major stretch of the muscle produces a small stretch of the spindle organelle. Inside the spindle are several intrafusal muscle fibers whose lengths are under CNS control. The active endings of the mechanosensory neuron are wrapped around the elastic midregion of the intrafusal muscles. Thus, if the main muscle shortens, so will the spindle, and any tension will be taken off the receptor neuron; it will cease to fire. The CNS then stimulates the intrafusal muscles to shorten, reestablishing some tension on the receptor. The CNS stimulation acts in this case as a kind of automatic sensitivity control. If the muscle is stretched, so is the spindle and the intrafusal muscles, causing the receptor neurons to fire briskly.
Now a CNS reflex inhibits the motor input to the intrafusal muscles, causing them to lose tension, and the firing of the receptor slows.
Statocysts were shown to be found in several invertebrate phyla, e.g., crustaceans (but not insects), annelids (segmented worms), and mollusks. The design of the statocyst permits it to sense the animal's body angle in the gravity field, and also respond to linear and angular acceleration. A statocyst is basically a fluid-filled cavity lined with ciliated, force-sensing receptor cells, and some sort of mass (statolith) that presses on the receptor cells. The animal has the job of integrating the outputs of the array of sensory cells that line the statocyst cavity to provide it with orientation information that will affect behavior.
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