Higher Conduction Velocity Research Articles

Investigation of the functional characteristics of reticulo-spinal fibers of the ventral funiculus

Functional characteristics of single reticulo-spinal fibers of the ventral funiculi were studied at the level of the 10th thoracic segment of the spinal cord in anesthetized (with chloralose and pentobarbital) and decerebrate cats after removal of the cerebellum. The reticulospinal tract of the ventral funiculus consists of a broad spectrum of rhythmically active and "silent" fibers, divided into three groups: fibers with a high (65–110 m/sec), medium (45–60 m/sec), and low (20–40 m/sec) conduction velocity. Spontaneous rhythmic activity is more characteristic of the fibers of the last two groups. The quantitative ratio between rhythmically active and silent fibers was about twice as high in the decerebrate as in the anesthetized animals. Depending on the character of distribution of interspike intervals the spontaneous activity of the reticulo-spinal fibers of the animals of both groups could be classified in three types: I) with a uniform distribution of interval; II) with a tendency toward grouping of spikes into volleys; III) with marked grouping of the intervals. Fibers with low and medium conduction velocities more often had spontaneous activity of types I and II, while fibers with a high conduction velocity more often had activity of types II and III. The possible functional significance of the reticulo-spinal fibers of the ventral funiculi with different conduction velocities and types of spontaneous activity is discussed.

Sensory receptors in the sheep's foot.

1. Median and ulnar nerves of anaesthetized Soay sheep have been investigated in vivo.2. The compound action potentials of these nerves indicate that the most rapidly conducting fibres had mean velocities of 87 m/sec in the median nerve and 57 m/sec in the ulnar nerve. The mean conduction velocities of the axons responsible for the Group A gammadelta wave in the median nerve were 23-30 m/sec and those responsible for the Group A delta wave in the ulnar nerve conducted at 23-26 m/sec.3. Single myelinated afferents were isolated by microdissection of median and ulnar nerves in the metacarpal region, and action potentials were recorded from hair follicle units, cutaneous mechanoreceptors, deeply situated proprioceptors, and from units of unknown function which were spontaneously active.4. The hair follicle units fell into two categories: ;diffuse' units which innervated many hairs in an area of about 1-3 cm(2) anywhere on the hairy skin above the hoof, and ;localized' units which innervated a few hairs within small areas of the skin adjacent to the hoof. The ;diffuse' units generally resembled the ordinary hair follicle units of other animals except that in the sheep the afferents had higher conduction velocities. The ;localized' units also had large myelinated axons, but their physiological properties differed in several aspects from the guard hair, tylotrich and carpal hair units which have been studied in other animals.

Electrophysiology of the heart of an isopod crustacean: Porcellio dilatatus. I. General properties.

ABSTRACT The electrical properties (recorded with intracellular microelectrodes) and the mechanical properties of the myocardium of the wood-louse Porcellio dilatatus (Brandt), a terrestrial isopod crustacean, have been investigated. The diastolic membrane potential varied from – 50 to – 70 mV. Several types of spontaneous electrical responses have been recorded. Their amplitude was usually between 30 and 45 mV and their duration varied from 150 to 400 msec. Except for very rare cases, overshoot did not occur. The phase of depolarization began slowly; it became faster but the maximum rate of rise never exceeded 1– 2 V/sec. The rising phase, devoid of steps, was followed by a partial repolarization leading to a more or less sustained smooth plateau. Superimposed changes of potential occurred very rarely. A jagged appearance of the plateau, as seen in numerous neurogenic hearts and interpreted as junction potentials, could not be observed. During spontaneous activity intracellularly applied currents modified the frequency, the time course and the amplitude of the responses. The junctional nature of the rising phase is suggested. At rest the myocardial membrane displayed a ‘normal’ rectifying property and a weak delayed rectification; in addition, some preparations showed an active graded response of the membrane. A complete regenerative activity was never triggered. A tentative explanation of the electrical response is proposed : the rising phase could chiefly correspond to a junctional potential, the plateau might be a response of the membrane to the synaptic depolarization. The value of the membrane space constant (about 1 mm) suggests that the small muscle fibres, as observed under optical microscope, are interconnected electrically. The impulses delivered by the heart ganglion would activate the whole myocardium almost simultaneously, resulting in a high apparent conduction velocity and a good synchronization from one end to the other. The degree of tension or of relaxation of the fibres closely depended on the value of the membrane voltage. The magnitude of the contraction depended on the absolute level of the potential and the length of time during which the depolarization was maintained. The functional importance of the plateau phase is considered. Intracellular depolarizing pulses led to a contraction of the heart; conversely, hyperpolarizing pulses led to its relaxation. GABA (10− 6 g/ml) inhibited the functioning of the heart and reduced the membrane resistance. Picrotoxin (10−4 g/ml) acted as an antagonist to GABA, L-Glutamic acid (10− 4 g/ml) strongly depolarized the membrane, leading to the cessation of the activity.

Excitation and inhibition in the visual center of Rana temporaria

Analysis of postsynaptic unit responses in the visual center ofRana temporaria showed that optic nerve fibers with high and low conduction velocities usually converge on a single neuron of the tectum opticum (TO). In response to stimulation of the optic nerve a complex depolarization potential consisting of 3 (or possibly 4) EPSPs was recorded in one group of neurons; these EPSPs were probably generated through excitation of several groups of afferent fibers. Either an increase or a decrease in the EPSPs can be observed in the TO neurons in response to repetitive and paired stimulation of the optic nerve. Postsynaptic inhibitory responses of some TO neurons, probably of direct and recurrent origin, are discussed.

Monosynaptic control over spinal motoneurons at different levels

The results show that direct monosynaptic connections with segmental motoneurons are formed in mammals not only by fibers of the pyramidal tract, but also by axons of descending systems arising from different levels of the brain. A characteristic feature of these monosynaptic projections is their high conduction velocity and the relatively small number of conducting fibers of the descending system compared with the remaining thinner and more slowly donducting elements. This feature, clearly distinguishable in higher vertebrates up to primates, possibly reflects the form of organization of suprasegmental control which appeared originally in the most primitive vertebrates with a fully formed brain (cyclostomes). With progressive encephalization of the nervous system in the course of evolution, the number of direct connections between the brain and segmental motoneurons progressively increases, the main source of these connections shifting into the motor cortex. An essential feature of monosynaptic suprasegmental control in higher vertebrates is evidently the increasing specification of the connections with particular functional groups of α-motoneurons.

Selective reinnervation of two cell populations in the adult pigeon ciliary ganglion.

1. Presynaptic fibres innervating the adult pigeon ciliary ganglion were cut 2 mm proximal to the ganglion. The modification of transmission in the ciliary and choroid cell populations was studied after periods of 6 hr-100 days.2. Transmission failed after 2 days and for the next 10 days there was no transmission through the ganglion. Long latency responses in both ciliary and choroid nerves were first observed at 13-15 days, and the latency decreased toward control values in 40 days. Electrical transmission reappeared in the ciliary population in about 26 days.3. Presynaptic fibres innervating the ciliary population have higher conduction velocities and lower electrical thresholds than those innervating the choroid group. This relation was maintained throughout the reinnervation process. Fibres innervating the extraocular muscles also regenerated and achieved conduction velocities similar to their control values.4. In two experiments out of seventeen, a few fast conducting, low threshold fibres, presumably stray ciliary fibres, innervated choroid cells and induced electrical transmission.5. It is concluded that each group of cells, ciliary and choroid, was reinnervated in a highly selective manner by its original class of presynaptic fibres, and that the presynaptic ciliary elements cause the specializations necessary for electrical transmission.

Electrophysiological demonstration that cockroach tibial tactile spines have separate sensory axons

The eighteen tactile spines of the left mesothoracic tibia of Blaberus craniifer were tested for axon fusion by comparing their individual action potentials and by observing their interactions. The test criteria were that sensilla sharing a common axon should produce action potentials identical in amplitude, waveform, and conduction velocity. They should also occlude one another when stimulated in pairs, whereas separately innervated sensilla could produce dissimilar action potentials and should sum rather than occlude. We found that action potentials of different sensilla on the same leg generally did differ appreciably but usually fell into a few groups with similar characteristics. However, all pairs of sensilla showed summation whether their action potentials were similar or not. Occlusion never occurred. We therefore conclude that all eighteen mesothoracic tibial tactile spines give rise to separate sensory axons. Thus axon pooling cannot account for the relatively high conduction velocities of these sensilla. Moreover, topographic representation of individual tactile spines in the central nervous system is possible.

Motor innervation, motor unit organization and afferent innervation of m. extensor digitorum communis of the baboon's forearm

1. One hundred and fifty efferent axons innervating m. extensor digitorum communis (EDC) were isolated in filaments of C7 and C8 ventral roots of baboons. Conduction velocities were measured antidromically by stimulating the muscle nerve and recording from the filaments, and fell into two groups: a fast (49-84 m/sec) and a slow (22-41m/sec), presumably fusimotor group. The threshold for these latter axons exceeded the strength needed to elicit the maximal motor twitch.2. Stimulation of ventral root filaments containing slow axons produced no contractile tension in EDC.3. Stimulation of ventral root filaments containing fast-group axons elicited all-or-nothing twitches of motor units of EDC. The twitch tensions of 66.3% of the units were < 2.0 g wt.; only 8.7% were > 5.0 g wt. Tetanus-twitch ratios were 1.4-4.7 in a sample of 14 units. Contraction times were between 15 and 35 msec in 97% of the units. There was no correlation between contractile properties and axonal conduction velocity.4. Afferent volleys from the stimulated EDC nerve were recorded from C6 or C7 dorsal roots. The threshold was below the threshold for a just-detectable motor twitch in ten out of eleven baboons. Conduction velocity of the earliest component of the muscle afferent volley was 67-83 m/sec.5. The conduction velocities of twenty-eight spindle afferents, identified by their responses to linear stretches of EDC and by their unloading by maximal twitches, were all < 70 m/sec. Higher dynamic sensitivity tended to be associated with higher conduction velocity.

Manner of stimulus conduction in atria.

(1) The manner of the stimulus conduction was investigated for both left and right atria of the canine heart through analysis of the configuration of unipolar electrograms and the arrival times of the activation wave. Langendorff's method of irrigating the heart was used.(2) In the right atrium, the stimulus spreaded from the S-A node to the orifice of the I.V.C. with higher conduction velocity along the sulcus terminalis. The stimulus coming along the sulcus terminalis spreaded over the atrium through the muscle bundles which branched from the sulcus terminalis. The apex of the atrial appendage was found to be the last portion to become activated.(3) In the left atrium, the stimulus originating in the S-A node, spreaded through Bachmann's bundle, first of all. The stimulus coming along Bachmann's bundle spreaded over the atrium through the muscle bundles which branched from Bachmann's bundle. The region between the left lower pulmonary vein and the A-V ring was found to be the last portion of activation in both left and right atrium.(4) In some cases, there was an area in which the stimulus originating in the S-A node might reach directly from the right atrium without passing through Bachmann's bundle. This area was located in a small region of the posteroinferior portion of the left atrium near the I.V.C.