Info

0 20 40 60 80 100 120 140 160 180 200 Pitch down, degrees

FIGURE 2.3-17 A graph showing the representative firing frequency of a single Type 1 lobster statocyst position receptor. The curve shows the response of the receptor to head pitched down. At 180°, the lobster is upside down; i.e., its back is down. Peak response occurs when the lobster is pointing down at 84°. Other Type 1 statocyst receptors around the cavity will have frequency peaks at other angles, depending on their positions. This curve can be modeled by the equation: r(0) s 31.5 - 28.5 cos[2(0 + 6°)], where r(0) is in pps and 0 is the pitch down angle in degrees. (Original data from Cohen, 1955.)

peak in spike frequency at about 60°. Not surprising, the same CW unit has a minimum response at about 60° + 180° = 240°. However, CCW rotation caused the same axon to fire maximally at about 160o of roll with a broad null at 340°. Note that the maximum firing rate of the statocyst nerve is only 5 pps. Nothing happens rapidly in gastropods.

In cephalopod mollusks (octopi and squids), the statocysts are well developed, each with four differentiated nerves projecting to the CNS (Bullock and Horridge, 1965, Ch. 25). One would expect the statocysts of cephalopods to be highly developed to provide the animals, which are agile swimmers, not only with gravity vector information but also neural information on linear acceleration, as well as signals on angular roll, pitch, and yaw and their derivatives (Young, 1960). Octopus statocysts are large, endolymph-filled cavities (~4 mm diameter in a 0.5-kg octopus), and are richly endowed with sense cells. The statolith is located in an ellipsoidal macula inside the statocyst structure. The macula has many hair cells. The details of their anatomy will not be described here, but it is worth noting that the sense cells in cephalopods have interneurons and lateral connections reminiscent of the neuropile underlying the Limulus compound eye and the lamina ganglionaris of compound eyes. Perhaps lateral inhibition acts here, too, to sharpen mechanical response. Also unique in the complex statocysts of the octopus is the presence of efferent nerve fibers, suggesting some sort of CNS feedback control of statocyst responses. Clearly much neurophysiology has yet to be done to understand the sense cell interactions and the role of efferent signals on the dynamic behavior of the octopus statocysts.

Statocyst Lobster

0 20 40 60 80 100 120 140 160 180 200 Pitch down, degrees

FIGURE 2.3-17 A graph showing the representative firing frequency of a single Type 1 lobster statocyst position receptor. The curve shows the response of the receptor to head pitched down. At 180°, the lobster is upside down; i.e., its back is down. Peak response occurs when the lobster is pointing down at 84°. Other Type 1 statocyst receptors around the cavity will have frequency peaks at other angles, depending on their positions. This curve can be modeled by the equation: r(0) s 31.5 - 28.5 cos[2(0 + 6°)], where r(0) is in pps and 0 is the pitch down angle in degrees. (Original data from Cohen, 1955.)

Statocyst LobsterStatocyst Lobster

FIGURE 2.3-18 Response of a single fiber from the statocyst nerve of the gastropod mollusk, Pleurobranchaea californica to roll around the longitudinal axis. Solid curves, average frequency in the first 10 s following position change. Dotted curves, average frequency in the period 110 to 120 s following position change. (From Wood, J. and von Baumgarten, R.J., Comp. Biochem. Physiol., 43A: 495, 1972. With permission from Elsevier Science.)

FIGURE 2.3-18 Response of a single fiber from the statocyst nerve of the gastropod mollusk, Pleurobranchaea californica to roll around the longitudinal axis. Solid curves, average frequency in the first 10 s following position change. Dotted curves, average frequency in the period 110 to 120 s following position change. (From Wood, J. and von Baumgarten, R.J., Comp. Biochem. Physiol., 43A: 495, 1972. With permission from Elsevier Science.)

The fact that statocysts of various degrees of complexity have apparently evolved independently in a number of invertebrate phylla argues for the robustness of its design. Also, the more complex (in terms of degrees of freedom and speed) the animal's means of locomotion, the more complex its statocysts (e.g., lobsters, octopi). Finally, compare the statocyst with the gravity receptors of the cockroach Arenivaga, in Section 2.6.

2.3.6 Pacinian Corpuscles

Pacinian corpuscles (PCs) are found in vertebrates. They are an excellent example of a phasic mechanosensory neuron that responds approximately to the absolute value of the time derivative of pressure applied to tissues surrounding the corpuscle. The exterior of the PC is an ellipsoid about 500 |m in length and 250 |m in diameter. The largest PCs are about 4 x 2 mm. The ellipsoid is made up from many thin cellular layers or lamellae, much like the structure of an onion. At the center of the corpuscle there is a cylindrical viscous fluid-filled region surrounding the tapered bare ending of the sensory axon. See Figure 2.3-19 for a schematic of a PC with its myelinated axon. Myelin beads are seen surrounding the axon at the distal end of the capsule and running down the axon to the spinal dorsal root ganglion where the soma of the receptor is located. The PC axons are from 8 to 14 |im in diameter, and conduct spikes at 50 to 85 m/s. They are classified as type Ap fibers (Guyton, 1991). The SGL for the PC is at the first node of Ranvier. A electron micrograph cross-section through a PC perpendicular to the axon is shown in Figure 2.3-20; note the many lamellae. The axon is in the center of the lamellae.

FIGURE 2.3-19 Schematic cross section through a PC. The onionlike lamellae are thought to give this pressure its highly phasic rectifying response.

PCs are located in deep visceral tissues (mesentaries), the joints, as well as in the hands and feet. Their spatial resolution is relatively poor compared with Merkel's receptors and Meissner's corpuscles. They evidently function to sense abrupt changes in tissue loading, and do not respond to steady-state (dc) pressure stimuli.

The PC response to a step of applied pressure to the surrounding tissues adapts completely to zero in about 0.3 s. When the step of pressure is released, the PC again fires a burst as it returns to mechanical equilibrium. If a steady-state sinusoidal displacement stimulus is used, the threshold skin indentation required to elicit spikes is seen to be a function of frequency. Maximum sensitivity (minimum displacement) is seen at 300 Hz, and sensitivity decreases as frequency is either raised above or lowered below 300 Hz (Kandel et al., 1991, Ch. 24). At 30 Hz, the sensitivity has decreased by a factor of about 0.01 (i.e., if the stimulus threshold is a skin indentation of 1 |im at 300 Hz, a 100 |im indentation is required at response threshold at 30

Continuous Circular Line Drawing

FIGURE 2.3-20 Electron micrograph of a transverse section through the bulb of a PC. The axon tip is at the center. Count the lamellae. (Edge-enhanced image available from www.udel.edu/Biology/Wags/histopage/colorpage/cne/cnepc.GIF.)

FIGURE 2.3-20 Electron micrograph of a transverse section through the bulb of a PC. The axon tip is at the center. Count the lamellae. (Edge-enhanced image available from www.udel.edu/Biology/Wags/histopage/colorpage/cne/cnepc.GIF.)

Hz). At the high-frequency end of the PC threshold sensitivity frequency response curve, the response is down by 0.01 times at about 1700 Hz.

If a square wave displacement stimulus is used at low frequency, the PC fires twice per cycle, generating an instantaneous frequency waveform at double the stimulus frequency. The generator potential from an isolated PC in response to sinusoidal vibrations has a nonlinear, asymmetrical, full-wave rectified form (Pietras and Bolanowski, 1995). The nonlinear, phasic response characteristics of the PC disappear if the outer lamellae are dissected away, and the nerve ending with one fluid-filled envelope is deflected. In this case, the generator potential (GP) decays slowly while the deflection is maintained. When the deflection is removed, the GP does not show a positive transient.

If an intact PC is deflected, a sharp transient depolarization is seen in the GP at the beginning of deflection, and another is seen when it is removed. From this behavior it can be concluded that the lamellar cover of the PC acts like a mechanical differentiator, transmitting the rate of change of pressure to the sensory neuron tip. The mechanically activated ion gates in the tip membrane also contribute to the PC phasic response. Apparently, they deactivate at a steady rate, even when pressure is maintained on the tip. However, most of the phasic behavior of the PC appears to be derived from its lamellar coating. A block diagram modeling the nonlinear dynamics of the PC is shown in Figure 2.3-21. The SGL has RPFM dynamics. This model does not have built-in saturation; but it does exhibit a firing threshold.

Was this article helpful?

0 0
Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Post a comment