Philip J. Stephens

REGIONAL DISTRIBUTION OF MUSCLE FIBER TYPES IN THE ASYMMETRIC CLAWS OF CALIFORNIAN SNAPPING SHRIMP

The properties of the opener and closer muscles in the asymmetric claws of Alpheus californiensis have been investigated using sarcomere length measurements and histochemical techniques. In the smaller pincer claw two types of muscle fibers are regionally distributed within the single closer muscle. A central band of fibers have short (2.5 µm) sarcomeres and high myofibrillar ATPase activity. Intermediate-type fibers have smaller diameters, sarcomeres 8.5 to 9 µm in length and low myofibrillar ATPase activity. The snapper closer muscle, by contrast, is composed of fibers with long (11-14 µm) sarcomeres and low myofibrillar ATPase activity. Opener muscle fibers in the pincer claw have shorter sarcomere lengths than their counterparts in the snapper claw.

The effects of temperature on the physiology of crustacean nerves and muscles

Abstract 1. 1.|The effects of temperature on crustacean motor nerve, muscle and synaptic physiology have been reviewed in terms of passive properties, threshold potentials and response characteristics. 2. 2.|The generation of additional spikes in the peripheral branches of certain axons has been discussed. 3. 3.|Temperature effects on behaviour have been reviewed.

The effects of temperature and ethanol on the properties of the fast excitatory axon to the crab limb bender muscle

Abstract 1. 1. Warming fast bender excitor (FBE) axons increased input resistance, hyperpolarized the resting potential and hyperpolarized the threshold potential for spike production. 2. 2. The reversal potential for the depolarizing afterpotential (DAP), which follows the FBE spike, was similar to the threshold potential at cool temperatures and depolarized at warm temperatures. 3. 3. Ethanol (⩽ 200mM) had little effect on FBE membrane potential and threshold potential, but increased input resistance. 4. 4. When FBE was depolarized to threshold a few spikes were recorded in normal saline, while a brief, high frequency spike discharge was recorded in the presence of ethanol. This was explained by ethanol increasing DAP amplitude.

THE EFFECTS OF TEMPERATURE AND ACCLIMATION ON CRUSTACEAN NERVE-MUSCLE PHYSIOLOGY

Crustaceans, like most ectotherms, have body temperatures that are close to ambient. Although some animals live in constant warm conditions, most crustaceans live in environments with both short- and long-term temperature fluctuations. Rapid temperature changes generally produce changes in the properties of the nerve and muscle membranes. As a result increases in temperature generally cause a decrease in the effectiveness of neuromuscular transmission. This is offset by an increase in the amount of synaptic facilitation, an increase in axon firing frequencies, and in some motor axons the production of additional responses in the peripheral branches. Although these changes act to overcome temperature-induced decreases in muscle tension, little is known about how the intact animal utilizes these changes to produce coordinated movements at different temperatures.Long-term changes in thermal conditions alter the properties of the motor nerves and the muscles. This results in a shift in the temperature range o...

Ethanol and temperature modify motor axon firing patterns.

At high temperature a single orthodromic action potential in the excitor (E) axon to the stretcher muscle of the crab Pachygrapsus crassipes provokes the generation of additional spikes in the peripheral E axon branches. This phenomenon exhibits a critical temperature threshold. Bathing preparations in crab salines made up with ethanol at levels as low as 0.5% (109 mM) reversibly decreased the temperature threshold for peripheral spike generation. Higher levels of ethanol resulted in the peripheral generation of E axon spikes at lower temperatures. These results, together with the observation that the temperature threshold is dependent upon the acclimation temperature, indicate that the peripheral generation of action potentials in the E axon is associated with an increase membrane lipid fluidity.

Fiber Types in the Limb Bender Muscle of a Crab (Pachygrapsus crassipes)

The bender muscle in the walking limb of the Pacific shore crab (Pachygrapsus crassipes) is composed of fibers with different structural (sarcomere length) and histochemical (NADH diaphorase and myofibrillar ATPase) properties. Slow fibers are located along the dorsal margin of the muscle and along the ventral margin in the distal portion of the muscle. The remaining bender muscle is composed of intermediate-type fibers, which can be differentiated into two groups based upon the pH sensitivity of the myofibrillar ATPase activity and the polysaccharide content of the fibers.

Spike afterpotentials in single, identified fast and slow motor neurons in the crab Pachygrapsus crassipes

Abstract 1. 1. In the fast (FBE) and most slow (SBE) bender excitor axons the spike was followed by a depolarizing afterpotential (DAP). The DAP reversal potential was about 25 mV above resting in FBE and 7 mV above resting in SBE. 2. 2. The resting potential and action potential amplitude were larger in FBE, while the input resistance was larger in SBE. 3. 3. In both axons, warming or addition of 200 mM ethanol to the saline decreased spike amplitude and increased DAP amplitude. 4. 4. In both axons there was a space between the axon membrane and the adaxonal glial cell.

An axonal spike in an identified fast crab motor neuron has sodium and calcium components.

1. High intensity intracellular stimulation of the FBE axon produced a spike discharge. The spikes divided into two components, each with a different frequency. 2. Ion replacement and blocking specific ion channels revealed that the FBE spike has calcium and sodium components. 3. The pronounced depolarizing wave that follows the FBE spike is not produced by changes in calcium conductance.

Temperature effects on a slow-crustacean neuromuscular system

The membrane potential of the E2 axon and the bender muscle fibers increased with temperature. The input resistance of the axon, the spike amplitude and time course declined with temperature. Excitatory junctional potentials (ejps) exhibited maximum amplitudes and minimum facilitation at about the same temperature. Ejp time course and muscle membrane input resistance declined with temperature. Tension produced by the muscle also declined but then increased when additional spikes were generated in the periphery of the E2 axon.

The effects of ethanol on intracellular potassium and the membrane potential of an identified crab motor axon

1. Observations were made on the fast bender excitor axon in autotomized crab walking limbs bathed in normal crab saline, and in salines made up with 0.2 M sucrose or 0.2 M ethanol. Microelectrode techniques were used to measure the resting membrane potential and the intracellular level of potassium. 2. Sucrose-saline had little effect on the membrane potential or the intracellular level of potassium. Ethanol-saline hyperpolarized the membrane potential by about 3 mV and increased the level of intracellular potassium. 3. The ethanol-induced changes in intracellular potassium levels and membrane potential took place with the same time course. Further, the changes in membrane potential could be accounted for by changes in the equilibrium potential for potassium, Ek as predicted by the Nernst equation.