Geotaxis Lobster

FIGURE 2.3-9 Block diagram of a theoretical type 0 control loop for a spindle designed to keep the steady-state output instantaneous firing rate, r2, constant. Note that the CNS output to the spindle ys motor fiber is proportional to some re = ro - r2, re = 0 if (ro - r2) < 0.

driving the intrafusal, ys motoneuron that acts to keep the static basal firing rate of the spindle type II afferent constant regardless of changes in muscle length. The type II afferent output acts inhibitorily in the CNS at a "neural differencer," which is also excited at a fixed rate, ro. The differencer output rate, re, ultimately determines the firing rate on the ys fiber to the spindle, hence the static tension of the spindle. The steady-state dc gain of the feedback system is easily shown to be r = x KlKo/ (Kl + K2) + r KcNsKmK0/(Kl + K2) 238

2SS 2o1 + KcNsKmKo/(( + K2) o1 + KcNsKmK^(Ki + K2) .

Assume 1 ^ KCNS K Ko/(K1 + K2). Then, r2SS = X2o Ki/(KcNS Km) + ro 2.3-9

In other words, ro produces a tonic bias firing rate to which signals from spindle stretch are superimposed. It allows the spindle to signal shortening of the spindle as a reduction in the tonic firing rate, as well as stretch as an increase in type II fiber frequency. Again, note that the firing frequency, r2, is non-negative.

Animals such as arthropods and mollusks also need muscle length and force proprioceptors. LaCourse (1977a, b) decribed mechanoreceptors located among the smooth (i.e., nonstriated) muscle fibers of the anterior byssus retractor muscles (ABRM) of the plecypod mollusk, Mytilus edulis L. (Mytilus is the common blue mussel that can be found in the intertidal zone along the coast of New England.) A Mytilus has two ABRMs, which are used to maintain tension on a web of tough biopolymer threads spun by the animal to support itself on its habitat substrate (rocks, pilings, etc.). The muscle is unusual because it is a smooth muscle, and because it exhibits the property of "catch." Catch is basically a controlled state of rigor whereby an ABRM contracted under load maintains its stiffness and shortened length without additional motor nerve input. There is evidence that the ABRM is unique in that it has a relaxing motor nerve (RMN) system that, when activated, releases a neu-rotransmitter that causes the prompt termination of the catch state (Northrop, 1964). (The relaxing nerves are not the same as inhibitory motor nerves that prevent contraction.) If no further motor stimulation is given, the ABRM relaxes upon stimulation of the RMNs. If both motor nerves and RMNs are simultaneously stimulated with a burst, the muscle produces a twitch; i.e., it contracts, then promptly relaxes without entering the catch state.

Using methyene blue stain and Rowell's silver stain (Rowell, 1963), LaCourse (1977a) identified structures in the ABRM that were similar in appearance to the annulospiral endings of vertebrate muscle spindles. Typically, a nerve fiber 0.5 to 1.1 |im in diameter forms a large spiral twisted around a smooth muscle fiber that is thinner (3.6 to 4.05 |im diameter) than a regular contractile smooth muscle fiber (4.0 to 4.5 |im diameter). Just before the axon joins the muscle, there is an apparent bipolar cell body about 3.5 to 4.1 m long by 0.75 to 1.8 |im in diameter. Figure 2.3-10 illustrates a section of the ABRMs and their associated nerves and ganglia.

FIGURE 2.3-10 Schematic of the Mytilus two ABRMs, showing the superficial nerve trunks recorded from by LaCourse (1977a). Key to abbreviations: CG, paired cerebral ganglia; PG, paired pedal ganglia; VN, visceral nerves; CVC, cerebrovisceral connective nerve; x, point of recording with hook or suction electrode; |, point where CPC nerve cut. (Courtesy of J.R. LaCourse.)

FIGURE 2.3-10 Schematic of the Mytilus two ABRMs, showing the superficial nerve trunks recorded from by LaCourse (1977a). Key to abbreviations: CG, paired cerebral ganglia; PG, paired pedal ganglia; VN, visceral nerves; CVC, cerebrovisceral connective nerve; x, point of recording with hook or suction electrode; |, point where CPC nerve cut. (Courtesy of J.R. LaCourse.)

Neurophysiological recording was made from one isolated cerebropedal connective (CPC) nerve connected to an ABRM though the branches of the visceral nerves. The neurally unstimulated ABRMs were placed under tension by applying a force to the byssus threads in the direction in line with the ABRMs. The force was modulated by a perpendicular displacement of the tensioning wire imposed by a small loudspeaker. Recorded along with the sensory nerve spikes on the CPC nerve was the tension applied to the ABRMs. The muscle length followed the applied force according to the force-length curve of the unstimulated muscle (Figure 2.3-11). LaCourse (1977) found two major classes of mechanoreceptor response: those that fired for various ranges of increasing tension and those that responded for various ranges of decreasing tension. In general, ABRM mechan-oreceptor responses were found to be nonhabituating. Mechanoreceptors did exhibit some rate sensitivity, however, as evidenced by adaptation to step stretch responses. LaCourse estimated the sinusoidal frequency response of ABRM mechanoreceptors to have its peak response at about 1 Hz. Response was down -40 dB at 2 Hz, and -7 dB at 0.1 Hz.

It is known that externally applied 5-hydroxytryptamine (5HT) or dopamine (DA) in concentrations of 10-4 or 10-6 M are effective in causing a contracted ABRM in the catch state to relax. LaCourse found that externally applied DA at 10-4 or 10-6 M caused all mechanoreceptors to totally cease firing. After a seawater rinse, mechanoreceptor activity revived. Externally applied 5HT at 10-4 or 10-6 M temporarily depressed ABRM mechanoreceptor sensitivity for about 8 or 4 s, respectively. Sensitivity spontaneously returned without washing, suggesting that 5HT was enzy-matically broken down in vivo.

The anatomy of Mytilus mechanoreceptors suggests that they may behave similarly to spindles. However, unlike spindles, some are unique in firing faster for decreasing length or tension. Spindles evidently signal muscle shortening by decreasing their tonic firing rate.

LaCourse (1977; 1979) proposed the theory that ABRM contraction and catch is mediated by motoneurons that release acetylcholine (ACh). The mechanoreceptors send spike signals back to the pedal ganglion regarding whether the muscle length

Force Length Curve
FIGURE 2.3-11 Approximate shape of the force-length curve of an unstimulated ABRM.

is increasing or decreasing. He postulates that the RMNs release 5HT at their endings and that 5HT terminates catch. A third set of efferent neurons specifically innervates the stretch receptors. These "R-neurons" release the neurotransmitter DA, which decreases the sensitivity of the receptors. The activity of the R-neurons coupled excitatorily to the RMNs in the pedal ganglion, so that when R-neurons fire, 5HT is also released, producing phasic ABRM behavior. Another scenario is that the activity of the receptors that sense increasing ABRM length stimulate the contractile motoneurons locally, producing a load-resistant constant length control loop.

What can be learned from the Mytilus ABRM stretch receptor story is that the simpler-appearing systems in nature turn out to have very complex physiology, challenging the modeler.

2.3.4 Muscle Force Receptors

Sometimes the distinction between muscle force (tension) and length receptors is not so clear. Striated skeletal muscles generally have static force-length curves of the form shown in Figure 2.3-12. Below the peak of the stimulated tension curve, there is a monotonic relation between muscle force and length. If a mechanoreceptor is located in the relatively inelastic end of a muscle where it joins the tendon, it is generally said that the receptor is behaving like a force or tension receptor. A stretch receptor located in the more compliant body of a muscle generally behaves like a length receptor (e.g., a spindle). However, active length changes generally require the muscle to generate force internally, which must equal any external load plus internal viscous and elastic forces associated with the contraction. Because of the high number of actin-myosin bonds, active muscle not only generates tension and can shorten, but it is also stiffer and has increased viscosity. If a muscle is not stimulated (i.e., it is passive), then it behaves statically like a nonlinear spring in which receptor stretch is a function of the externally applied force. A heuristic test

Motor Innervation
///////// Motor nerve

Stimulator

FIGURE 2.3-12 (A) The force-length curves of stimulated and unstimulated striated skeletal muscle. (B) Measurement setup to obtain the curves of A.

Stimulator

FIGURE 2.3-12 (A) The force-length curves of stimulated and unstimulated striated skeletal muscle. (B) Measurement setup to obtain the curves of A.

for a tension receptor is that it fires for an external force load with approximately the same sensitivity regardless of muscle length.

The best known and most widely studied muscle force sensor is the Golgi tendon organ (GTO), found in mammals. The GTO is an encapsulated structure, about 1 mm long and about 0.2 mm in diameter. Its output is carried by a single myelinated nerve fiber about 16 |im in diameter. The naked (unmyelinated) dendritic endings of the GTO fiber arborize and twist vinelike between the twisted collagen fibers making up the fine structure of the tendon. There is one GTO for a functional group of about 10 to 15 muscle fibers. When the muscle is force-loaded in passive or activated condition, the force causes the tendon collagen fibers to elongate and squeeze together, generating shear forces on the GTO dendrite membrane. The GTO spike frequency in response to muscle force has a proportional plus derivative component (see Figure 2.3-13). GTOs generally fire more when a muscle is active at a given force than when it is unstimulated at the same force load.

a Motor input a Motor input
Steps Muscle Contraction
FIGURE 2.3-13 Schematic of approximate firing behavior of GTOs and spindles when a muscle is passively stretched and allowed to shorten under load (isotonic contraction).

The anatomy of a GTO is shown schematically in Figure 2.3-14. Note that the muscle end of the tendon divides to attach to small groups of muscle fibers, the sum of which comprise the entire muscle. A GTO therefore responds so its instantaneous spike frequency, r, is proportional to r = Kp f + Kd[f]+ 2.3-10

where f is the positive (tension) force on the muscle fibers in series with the GTO, and [f]+ is the positive derivative of the force (the negative derivative is zero).

The myelinated fiber from a GTO on a flexor muscle tendon is called a type Ib axon. It projects into one of the many dorsal root ganglia where its cell body is located. The Ib fiber continues into the spinal cord, where it excites a type Ib

Spina Setae

FIGURE 2.3-14 Schematic drawing of the sensory endings of a GTO in intimate contact with tendon collagen fibers in the GTO capsule. The GTO type Ib afferent axon fires when tension on the muscle forces the twisted collagen fibers to squeeze the GTO nerve endings. (From Kandel, E.R. et al., Principles of Neural Science, 3rd ed., Appleton & Lange, Norwalk, CT, 1991. With permission from the McGraw-Hill Companies.)

FIGURE 2.3-14 Schematic drawing of the sensory endings of a GTO in intimate contact with tendon collagen fibers in the GTO capsule. The GTO type Ib afferent axon fires when tension on the muscle forces the twisted collagen fibers to squeeze the GTO nerve endings. (From Kandel, E.R. et al., Principles of Neural Science, 3rd ed., Appleton & Lange, Norwalk, CT, 1991. With permission from the McGraw-Hill Companies.)

inhibitory interneuron, which in turn inhibits the flexor a-motoneuron that causes the tension on the GTO. The same GTO afferent excites a type Ib excitatory inter-neuron, which excites the extensor a-motoneuron. The net effect of this local GTO reflex arc is to unload the flexor muscle if the load is sensed as too high for it. This reflex system is shown in a simplified schematic in Figure 2.3-15. (Not shown in the figure are the ascending afferent fibers from the spindles and GTOs, and the sensory neuron cell bodies. Circular synapses are inhibitory; arrow synapses are excitatory.)

Muscle force receptors are found throughout the animal kingdom. Eagles and Hartman (1975) described the properties of force receptors associated with the tailspine muscles of the horseshoe crab, Limulus polyphemus. Two classes of receptors were identified with light microscopy using methylene blue staining: (1) elaborately branched multipolar neurons (somas ~30 x 100 |im) associated with the flexor muscles where they insert upon the tailspine apodeme (tendon) and (2) smaller bipolar neurons (~20 | m diameter) located along the shafts of the tailspine apodemes bearing insertions of the flexor muscles. These receptors sent their axons to the

From CNS

From CNS

Renshaw Cell
FIGURE 2.3-15 Schematic of the role of the GTOs and spindles in the crossed, flexor/extensor reflex pathways. Key: R, Renshaw inhibitory interneuron; aF, flexor a-motoneuron; aE, extensor a-motoneuron; yaf, spindle length control motoneuron from CNS; FL, flexor load force.

animal's CNS through a mixed nerve (motor plus sensory fibers) called hemal nerve 16 (h.n. 16). Eagles and Hartman showed indirectly that the above-mentioned neurons behaved as muscle tension receptors. They did not measure muscle force directly, but recorded from the receptor axons while stretching the muscle a known xo + Ax. They also stimulated the motor nerves innervating the muscles having the receptors from which they recorded. In general, the larger xo, the larger the receptor response for a given Ax. When the muscle was stimulated, there was a big increase in receptor firing, because of the greatly increased tension. Eagles and Hartman found no evidence that there were intrafusal (accessory) muscles associated with the Limulus receptors, such as are found in mammalian spindle muscle length sensors. Isometric tension generation also showed that the putative tension receptors would respond without an external length change.

2.3.5 Statocysts

Statocysts are neuro-sensory organs that transduce the direction of the gravity vector relative to the animal's body. Statocysts are found in a number of invertebrate phyla:

Annelida (segmented worms and leeches), Arthropoda (crustaceans, but not insects), Coelenterata (jellyfish, sea anemones, etc.), and Mollusca (especially gastropods and cephalopods). They also respond to linear and angular acceleration of the animal's body, and perhaps to very low frequency vibrations of the animal. The basic design of a statocyst is simple; it consists of a fluid-filled, roughly spherical cavity lined with closely packed mechanoreceptor cells with sensory hairs (cilia or setae). Also in the cavity is a statolith, consisting of one or more dense particles such as grains of sand, an accretion of sand grains stuck together to form a pebblelike mass, or an internally secreted calcified "stone." When an animal is at rest, the mechanosensors in the statocyst respond to deflection of their hairs due to gravity pressing the statolith down on them and causing them to bend. If the animal undergoes roll or pitch, the statolith presses on a new region of sensory cells. The sense cells send nerve impulse data to the animal's CNS to apprise it of its orientation in the Earth's gravity field. Statocysts can also send data about acceleration; if the animal accelerates backward, such as a crayfish escaping a predator, the reaction force on the statoliths press them forward, presumably sending a false message that the animal is pitching head down. Angular acceleration around a statocyst produces complex forces acting on the internal hair cells. The statolith and endolymph both tend to remain at rest, so the hairs effectively move past them, deflecting. Once the fluid plus statolith gain the angular velocity of the animal, there are no further acceleration forces on the hairs until the angular velocity stops. Then the statolith and fluid tend to keep moving, again stimulating hair cells, this time in the opposite direction.

Figure 2.3-16 illustrates a schematic section through generalized invertebrate statocyst. The nearly spherical cavity is lined with ciliated mechanoreceptor cells that respond to force from the statolith mass resting on them, or reaction force when the statolith is accelerated.

In lobsters, the statocysts are located inside the basal joint of the left and right antennule. A crescentic sensory cushion for the statolith has four rows of hair cells, three of which are normally in contact with the statolith. In addition, there are thread hair cells located on the posteriormedial wall of the statocyst, which are apparently deflected by the flow of endocystic fluid past them as a result of angular acceleration. Lobster hair cells have been subdivided into three categories. Type I position receptors that respond to static pitch of the animal with a nonadapting spike frequency that is proportional to the angle of pitch. There is one statocyst nerve axon per hair cell, and each cell has a different range of peak sensitivity. Type I cells respond little to roll of the animal. Type II position receptors exhibit some adaptation to angle changes, and thus appear to be more angular velocity sensitive than Type I cells. Some Type II cells are sensitive to roll and others to pitch. The axons of the thin, "thread" hair cells on the medial wall of a statocyst fire when the endocystic fluid swirls due to yaw, pitch, or roll; they appear to signal the lobster angular acceleration information. Still other statocyst nerve fibers fire in response to underwater low-frequency vibrations, or large angular accelerations (Bullock and Horridge, 1965, Ch. 18). Figure 2.3-17 illustrates the typical averaged firing rate of a Homarus (lobster) Type I statocyst fiber as the animal is pitched head down (90° is straight

Geotaxis Lobster

FIGURE 2.3-16 Schematic cross section through a representative simple statocyst. SC, ciliated mechanosensory cells; CAV, fluid-filled statocyst cavity; SL, statolith. This type of organ responds to any motion or force that deflects the cilia on the receptor cells. Note the canal connecting the cavity to the water outside.

FIGURE 2.3-16 Schematic cross section through a representative simple statocyst. SC, ciliated mechanosensory cells; CAV, fluid-filled statocyst cavity; SL, statolith. This type of organ responds to any motion or force that deflects the cilia on the receptor cells. Note the canal connecting the cavity to the water outside.

down; 180° is upside down, facing to the rear). Note that the peak firing rate occurs for 85° tilt, indicating that the receptor recorded from is 5° from vertical.

Coelenterates (jellyfish, comb jellies, anemones, medusae) have very primitive statocyst organs. They have few hair cells and one or a few statoliths. Annelids (segmented worms and leeches) also have elementary statocyst organs. Polychaetes (marine tube worms) have reasonably well developed statocysts with sand grain statoliths. Because these worms are sessile, and often live in turbid water, they use their statocysts for geotaxis.

The phyllum Mollusca also has interesting statocysts. Wood and von Baumgarten (1972) recorded rotation and tilt responses from the 13 axons of the statocyst nerve of the marine gastropod mollusk, Pleurobranchaea californica. The two Pleurobranchaea statocycsts are located lateral to the left and right pedal ganglia within the connective tissue sheath that encloses the mollusk's CNS. Each statocyst is only about 200 |im in diameter, and contains a single statolith about 150 |im in diameter. Figure 2.3-18 illustrates the response of a single statocyst nerve fiber to roll of the animal through 360°, both clockwise (CW, right-side down) and counterclockwise (CW, left-side down). Note that for CW rotation, there is a sharp

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