Electrophysiological and morphological properties of neurons in the ventral horn of the turtle spinal cord.

In this report, we present recent findings on the electrophysiological and morphological properties of spinal motoneurons (MNs) and interneurons (INs) of the adult turtle which were studied in slices of the spinal cord. The range of values for the measured electrophysiological parameters in 96 tested cells included: resting potential, -57 to -83 mV; input resistance, 2.5-344 M omega; time constant, 2.5-63 ms; rheobase current, 0.04-5.3 nA; after-hyperpolarization (AHP) duration, 72-426 ms; AHP half-decay time; 11-212 ms; and, slope of the stimulus current-spike frequency relationship, 3.4-235 Hz/nA. For another 20 cells, we made both morphological and electrophysiological measurements (the latter values within the above ranges). Their ranges in morphological properties included: soma diameter, 20-54 microm; soma surface area, 299-2045 microm2; soma volume, 2.3-45 microm3 x 10(4); rostro-caudal dendritic projection distance, 150-1200 microm; and, sum of dendritic lengths, 1.5-16 microm x 10(3). The emphasized findings include: 1) the quality and robustness of the intracellular recordings, which enabled accurate measurement of the action potential's shape parameters (spike, afterhyperpolarization [AHP]); 2) the substantial AHP of the INs' AP; 3) no single action-potential shape parameter (nor combination of parameters) being cardinal for its (or their combined) changes matching the profile of the initial and later phases of spike-frequency adaptation; 4) the utility and flexibility of a cluster analysis (using varying combinations of passive, transitional and active cell properties) for providing a provisional classification of low (like cat S) and high (like cat F) threshold MNs, and groups of INs with non-spontaneous versus spontaneous discharge; 5) the clear-cut morphological confirmation of the provisional classification strategy; 6) the basis for testing the possibility that one of the provisionally classified MN types innervates non-twitch muscle fibers; and 7) the heuristic value of comparing the properties of MNs versus INs across vertebrate species, with an emphasis on the lamprey, turtle, and cat.

Properties of spinal motoneurons and interneurons in the adult turtle: provisional classification by cluster analysis.

The purpose of the present study was to compare, in motoneurons (MNs) vs. interneurons (INs), selected passive, transitional, and active (firing) properties, as recorded in slices of lumbosacral spinal cord (SC) taken from the adult turtle. The cells were provisionally classified on the basis of (1) the presence (in selected INs) or absence (MNs and other INs) of spontaneous discharge, (2) a cluster analysis of selected properties of the nonspontaneously firing cells, (3) a comparison to previous data on turtle MNs and INs, and (4) a qualitative comparison of the results with those reported for other vertebrate species (lamprey, cat). The provisional nomenclature accommodated properties appropriate for solely MNs (Main MN group) vs. nonspontaneously firing INs (Main IN-N) vs. spontaneously firing INs (IN-S) and for neurons with two degrees of intermediacy between the Main MN and the Main IN-N groups (Overlap MN, Overlap MN/IN). Morphological reconstructions of additional cells, which had been injected with biocytin during the electrophysiological tests, were shown to provide clear-cut support for the provisional classification procedure. The values for the measured parameters in the 96 tested cells covered the spectrum reported previously across adult vertebrate species and were robust in measurements made on different SC slices up to 5 days after their removal from the host animal. The interspecies comparisons permitted the predictions that (1) our Main MN and Overlap MN cells would be analogous to two MN types that innervate fast-twitch and slow-twitch skeletomotor muscle fibers, respectively, in the cat, and (2) the MNs in our Overlap MN/IN group probably innervate slow (nontwitch, tonic) muscle fibers whose presence has recently been established in the turtle hindlimb. In summary, the results bring out the utility of the SC slice preparation of the turtle for study of spinal motor mechanisms in adult tetrapod vertebrates, particularly as an adjunct to the in vivo cat, because of the ease with which robust measurements can be made of the active properties of both MNs and INs.

Contractile properties of the tongue muscles: effects of hypoglossal nerve and extracellular motoneuron stimulation in rat.

1. Stimulation of the whole hypoglossal (XIIth) nerve or its medial or lateral branch in the rat produced two major movements of the tongue as measured with a single force transducer attached to the tip of the tongue. Stimulation of the whole XIIth nerve or the lateral branch produced a retrusion of the tongue, whereas stimulation of the medial branch produced a protrusion. 2. The average retrusive twitch tension evoked by stimulation of the XIIth nerve (11.25 g) or the lateral branch (12.02 g) was significantly greater (P < or = 0.0001) than the protrusive twitch tension (1.05 g) elicited by medial branch stimulation. The tetanic tension produced by lateral branch stimulation (36.82 g) was significantly greater (P < or = 0.007) than the whole nerve tetanic tension (28.23 g). The greater tension elicited by stimulation of the lateral branch of the nerve when compared with the tension elicited by stimulation of the whole XIIth nerve was probably due to the absence of protrusive axons in the lateral branch of the nerve. Stimulation of the whole XIIth nerve activates axons innervating both protrusive and retrusive muscles, resulting in a weaker net retrusive force. 3. The contraction time of the tongue in response to medial branch stimulation (10.94 ms) was significantly faster than when the whole XIIth nerve (15.68 ms, P < or = 0.007) or lateral branch (13.36 ms, P < or = 0.05) was stimulated. The twitch contraction time of the tongue in response to whole XIIth nerve or lateral branch stimulation was not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of the mechanical properties of the cat inferior oblique muscle to the anatomy of its motoneurons and nerve branches.

Physiologically, the contractile characteristics and electromyography (EMG) of cat inferior oblique (IO) muscle fibers supplied by the medial and lateral IO muscle nerve branches were studied by direct nerve stimulation. Anatomically, the brain stem locations and sizes of IO motoneuron soma were evaluated after retrograde labeling by horseradish peroxidase (HRP) through whole IO muscle nerves and/or through each medial or lateral IO muscle nerve branch. Stimulation of the lateral nerve branch elicited significantly (p < 0.005) slower twitch contraction times (8.0 +/- 1.5 ms) and lower fusion frequencies (217 +/- 46 Hz) than when the medial branch of the IO nerve was stimulated (average twitch contraction time = 6.8 +/- 1.1 ms; average fusion frequency = 260 +/- 34 Hz). The EMG wave shape responses in the global and orbital layers could be differentiated when the individual nerve branches were stimulated, but the response differences were not consistent among animals. The average diameter of IO motoneuron soma with axons in the lateral branch of the nerve were significantly smaller (p < 0.001) than the average diameter of those IO motoneuron soma associated with the medial branch of the nerve (27.9 +/- 7.2 vs. 32.9 +/- 7.2 microns). Regardless of which nerve branch was labeled, the full range of motoneuron soma sizes was found, and these were distributed throughout the IO subdivision of the oculomotor nucleus. These findings showed that muscle contraction time and motoneuron soma diameter were correlated with the IO nerve branch subjected to stimulation or exposed to HRP. But no topographic organization of motoneurons was found within the IO division of the oculomotor nucleus.

Intracranial hypertension. Advances in intracranial pressure monitoring.

Research is continuing to find improved methods for monitoring intracranial pressure as well as the effect of intracranial hypertension on the patient's recovery. Each of the methods of monitoring has its advantages and disadvantages and the use of one method over others is usually the preference of the neurosurgeon. It is the responsibility of the nurse to be aware of the strengths and weaknesses of each device in order to strengthen patient outcome. Again, the major benefits of the bolt are a low infection rate and easy insertion. Problems associated with its use of which the nurse should be aware are a tendency for a dampened waveform that gives an inaccurate pressure reading requiring irrigation that may or may not clear the catheter, and the inaccurate readings given by the bolt at high pressures. The subdural catheter can be used for long-term monitoring; however, baseline drift has been reported. Ventricular catheters have a mixed bag of results according to research. They are the most accurate of the methods used and enable cerebrospinal fluid to be drained, thereby lowering ICP. However, the catheters appear to have a higher infection rate. This is debatable, however. Some researchers advocate the prophylactic use of antibiotics. A closed drainage system should be used and if the device is used for longer than 4 days, the device should be changed and care should be taken to prevent leakage of cerebrospinal fluid. The devices currently in use have been presented and patient outcomes discussed using medical research, as none was available from the nursing literature.(ABSTRACT TRUNCATED AT 250 WORDS)