Neural interfaces for regenerated nerve stimulation and recording.

A class of implantable, regeneration-type neural interfaces (NI's) for mammalian peripheral nerve recording and stimulation were developed using different fabrication processes and integrating purposely designed components. A typical NI comprises three main components: 1) a microfabricated silicon die incorporating a microelectrode array on multiple through-holes, 2) a polymer guidance channel housing the die, and 3) a flexible flat cable connecting the die to an external electronic circuitry. The design and fabrication of the NI's were aimed at achieving long term, reliable implants by taking into careful account the biological, electrical, and mechanical requirements of the specific implant site. Different versions of the NI were fabricated and implanted between the severed ends of the sciatic nerve in a mammalian animal model (rabbit). Morphological and histological evidence showed that nerves regenerated through the NI's and electrophysiological results demonstrated the recovery of electrical functionality. Moreover, the NI's allowed stimulation of the regenerated nerve producing a visible leg/foot contraction. The NI's presented in this paper are being further improved in the authors' laboratories with the ultimate goal of allowing the control of nerve motor and sensory functions in future prosthetic devices.

Absence of mechanical evidence for attached weakly binding cross-bridges in frog relaxed muscle fibres.

1. Passive force responses to ramp stretches at various velocities were measured in intact and skinned single muscle fibres isolated from the lumbricalis muscle of the frog. Force was measured using a fast capacitance transducer and sarcomere length was measured using a laser light diffraction technique at a point very close to the fixed end so as to avoid effects of fibre inertia. Experiments were performed at 15 degrees C with sarcomere length between 2.13 and 3.27 microns under high (170 mM) and low (20 mM) ionic strength. 2. The analysis shows that the force response is the sum of at least three components: (i) elastic (force proportional to the amount of stretch), (ii) viscous (force proportional to rate of stretch), and (iii) viscoelastic (resembling the response of a pure viscous element in series with an elastic element). 3. The amplitude of all these components increased progressively with sarcomere length in the whole range measured. 4. A further component, attributable to the short-range elasticity (SREC), was present in the force response of the intact fibres. 5. The amplitude of the force response decreased substantially upon skinning at high ionic strength but increased again at low ionic strength. The SREC was completely abolished by skinning. 6. None of the components of the force response was found to have the properties expected from the previously postulated 'weakly binding bridges'.

Development of stiffness precedes cross-bridge attachment during the early tension rise in single frog muscle fibres.

1. Force responses to ramp stretches were recorded in single muscle fibres isolated from the lumbricalis muscle of the frog. Stretches were applied at rest and at progressively increasing times after a single stimulus. 2. The increase of fibre stiffness that precedes tension development has a 'static' component that accounts for the whole fibre stiffness increase during the latent period and at very low tension at the beginning of the twitch. 3. Static stiffness increase was not affected by 2,3-butanedione-2-monoxime, a drug that almost completely inhibited twitch tension. 4. Static stiffness increased approximately 5-fold as the sarcomere length was increased from 2.1 to 2.84 microns. 5. These results suggest that static fibre stiffness increase is not attributable to the formation of non-force-generating cross-bridges.

Force response of unstimulated intact frog muscle fibres to ramp stretches.

The possibility that weakly binding bridges are attached to actin in the absence of Ca2+ under physiological conditions was investigated by studying the force response of unstimulated intact muscle fibres of the frog to fast ramp stretches. The force response during the stretching period is divided into two phases: phase 1, coincident with the acceleration period of the sarcomere length change and phase 2, synchronous with sarcomere elongation at constant speed. The phase 1 amplitude increases linearly with the stretching speed in all the range tested, while phase 2 increases with the speed but reaches a plateau level at about 50 x 10(3) nm/half sarcomere per second. The analysis of data shows that phase 1, which corresponds to the initial 5-10 nm/half sarcomere of elongation, is very likely a pure viscous response; its amplitude increases with sarcomere length and it is not affected by the electrical stimulation of the fibre. Phase 2 is a viscoelastic response with a relaxation time of the order of 1 ms; its amplitude increases with sarcomere lengths and with the stimulation. These data suggest that weakly binding bridges are not present in a significant amount in unstimulated intact fibres.

Are weakly binding bridges present in resting intact muscle fibers?

Several experimental results (Schoenberg, M. 1988. Biophys. J. 54:135-148) have shown that the force response of relaxed skinned muscle fibers to fast stretches arises from the presence of cross-bridges rapidly cycling between attached and detached states. These bridges were identified with the M.ATP<-->AM.ATP and M.ADP.Pi<-->AM.ADP.Pi states seen in solution and are commonly referred to as weakly binding bridges. In this paper we have investigated the possibility that weakly binding bridges are also present in resting intact muscle fibers. The force response to fast stretches can be accounted for by assuming the presence in the fiber of a viscous and a viscoelastic passive component arranged in parallel. None of these components has the properties previously attributed to weakly binding bridges. This shows that in intact resting fibers there is no mechanical evidence of attached cross-bridges, suggesting that, under physiological conditions, either the M.ATP or M.ADP.Pi states have a negligibly small affinity for actin or the AM.ATP and AM.ADP.Pi cross-bridge states are unable to bear tension and contribute to fiber stiffness.

Effects of 2,3-butanedione monoxime on the crossbridge kinetics in frog single muscle fibres.

The effects of 2,3-butanedione monoxime (BDM) on contraction characteristics were studied at 5 degrees C in single intact fibres isolated from the tibialis anterior muscle of the frog. The force-velocity relation was determined using the controlled-velocity method in either whole fibres or short fibre segments in which sarcomere shortening was measured by a laser light diffraction method. It is shown that 3 mM BDM decreases the speed of rise and the amount of tetanus tension, reduces the maximum velocity of shortening and increases the curvature of the force-velocity relation, as well as the value for the stiffness to tension ratio. BDM also slowed down the redevelopment of tetanus tension after a period of unloaded shortening both in fixed-end and in length-clamp conditions. In normal and in BDM-treated fibres length-clamping increased the speed of the initial rise of tetanus tension but not that of the recovery after shortening. The observed force-velocity data points were fitted by the Huxley (1957) equation. It was found that BDM produces a conspicuous decrease of the rate constant for crossbridge attachment. This effect, and also a reduction of the force per crossbridge, are responsible for the depression of the contractile characteristics produced by BDM.