SYNAPTIC DENSITY ON SPINAL NEURONS OF NORMAL DOGS AND DOGS WITH EXPERIMENTAL HIND-LIMB RIGIDITY

Ischemia-induced delayed-onset paraplegia is accompanied by an unusual form of synaptic degeneration in the lumbosacral segments: an experimental light and electron microscopic study in dogs

We studied the effect of high thoracic aorta cross-clamping, complete transverse section of the spinal cord at Th6 level, and combined hemisection at Th6 level followed later by high thoracic aorta cross-clamping upon the morphology and number of identified presynaptic knobs in lumbosacral segments in dogs. In animals surviving 48-72 hours after high thoracic aorta cross-clamping the occurrence of an unusual form of boutons accompanied by periboutonal halo in L3-S1 segments was found. According to the bouton size and light as well as electron microscopic appearance, four types, i.e., light giant (T1), dark enlarged (T2), light giant with periboutonal halo (T3), and giant disintegrating (T4) boutons were detected after 48 and 72 hour reperfusion. The appearance of four boutonal types in the lumbosacral segments is caused by spinal cord ischemia secondary to high thoracic aorta cross-clamping followed by 48 or 72 hour reperfusion. At the end of the sixth reperfusion day no signs of enlarged and giant boutons were detected in L3-S1 segments. A statistically significant increase of enlarged and giant boutons was noted at the end of the third reperfusion day in comparison with 48 hour survival. After spinal cord transection at midthoracic (Th6) level, followed by 72 hour survival, no such unusual synaptic knobs could be found in L3-S1 segments. The laminar distribution pattern of T1-T4 types based on light microscopic analysis and confirmed electron microscopically is characteristic and strictly bound to those spinal cord gray matter layers which serve as main termination sites of the descending cortical, brain stem, as well as long propriospinal projections in the lumbosacral segments (laminae V-VII). A statistically significant increase of enlarged and giant boutons was found in the intermediate zone (lamina VII). Hemisection at midthoracic level (Th6) followed later by 30 minute high thoracic aorta cross-clamping and 48 hour reperfusion caused a marked decrease of enlarged and giant boutons in L3-S1 segments on the hemisectioned side in comparison with the contralateral one. Large amounts of irregularly arranged round vesicles and tubular profiles were disclosed in the boutonal matrix of T1, T3, and T4 types in L3-S1 segments of animals subjected to 30 minute high thoracic aorta cross-clamping followed by 72 hour reperfusion. Accumulation of tubular and membranous materials was invariably seen in the bulbous enlargement of the terminal axonal branch.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of plasticity in the isolated spinal cord in paraplegia

In adult life a severe injury of the spinal cord results in total loss of locomotor functions of the hind limbs, i.e., paraplegia. However, after similar injury in neonatal life most hind limb functions are retained unaffected into adult life. Can such survival of locomotor function be produced in an adult paraplegic? Observations based on our previous studies suggest that sparing of function in the neonate is due to: 1) incomplete development of descending cord tracts 2) the presence of polyneuronal control of limb muscles by spinal motoneurons and 3) active growth of synaptic connections occurring in the cord while limbs are polyneuronally innervated. Such growth and remodelling ceases once mononeuronal (= adult) control of limb muscles is established. We suggest that recreation of conditions similar to neonatal life would be able to revive lost locomotor functions in the adult paraplegic. Experimental animal models are outlined here which may form a basis for future research.

Influences of morphology and topography of motoneurons and muscle spindle afferents on amplitude of single fiber excitatory postsynaptic potentials in cat

Excitatory postsynaptic potentials (e.p.s. p.s.) elicited by impulses in single muscle spindle afferent fibers from the medial gastrocnemius (m.g.) muscle were recorded intracellularly from homonymous and heteronymous motoneurons in order to study factors that influence the amplitudes of such responses. Impulses in large afferent fibers elicited larger single-fiber e.p.s.p.s. than those in smaller afferents. Mean e.p.s.p. amplitudes were related exponentially to afferent conduction velocities of both Ia and spindle group II fibers. The closer a motoneuron was to the spinal entry point of an afferent fiber, the larger was the mean e.p.s.p. amplitude evoked in it. Impulses in the same afferent fiber elicited larger e.p.s.p.s in small than large motoneurons when the two cells were located at the same craniocaudal levels. Other factors being equal, the single-fiber e.p.s.p.s evoked in homonymous and heteronymous motoneurons were approximately equal in amplitude. Relatively simple morphological and topographical explanations for the findings were advanced and their functional significance for orderly recruitment in partitioned and unpartitioned reflexes was described.

A theory on the lability and stability of spinal motoneuron soma size and induction of synaptogenesis in the adult spinal cord

There exists a dynamic relationship between the soma size of a motoneuron and its motor unit size. Adult motoneuron soma size can be experimentally increased if the neuron is allowed to innervate more muscle fibers than it normally innervates. In postnatal mammals a transition from polyneuronal to mononeuronal innervation of limb muscle fibers occurs which is temporally related to a plastic change in the perikaryal size. This lability of postnatal motoneuron size is temporally related to growth of synaptic connections on the motoneuron. In adult mammal, regenerating motor axons polyneuronally innervate the muscle fibers for a transient period. This hypothesis proposes that a plastic change in soma size occurs in these adult motoneurons. This short-lived labile state may revive the embryonic properties and evoke growth of synaptic boutons. Experimentally induced labile state in motoneuron pools and spinal ganglion neurons in the adult mammal should offer a basis for the study of mechanisms of synaptogenesis in the spinal cord.

Morphometric synaptology of gracilo-diencephalic relay cells: an electron microscopic study in the cat using retrograde transport of horseradish peroxidase

Gracilo-diencephalic relay cells were identified with the aid of retrograde transport of the horseradish peroxidase. Section embedding permitted light-microscopic identification and selection of relay cells and adjacent unlabelled neurons for ultrastructural study. Electron-microscopic analysis of the boutons which contacted the perikaryal surface of the cells revealed a statistically significant positive relationship between bouton covering ratio and cell body size for both labelled and unlabelled neurons. Estimates of the density of boutons also showed a positive correlation between the size of the cell body and the density of terminals contacting its surface. The functional implications of these findings are discussed.

How the size of motoneurones determines their susceptibility to discharge

The susceptibility of motoneurones to discharge by physiological inputs varies inversely with their size. This relationship has far-reaching consequences for the control of muscular tension, and also accounts for significant differences in the properties of muscle fibres and motor units. The probable basis for the relationship is that the amplitude of excitatory postsynaptic potentials (e.p.s.ps) is also correlated inversely with cell size, but no satisfactory explanation for this finding is known. We now describe experiments which indicate that afferent impulses invade the relatively simple ramifications of fibres going to small motoneurones more completely than the extensive arborisations going to large cells. This results in activation of a higher percentage of the synaptic endings on small cells, which produces larger e.p.s.ps in them and accounts for their greater susceptibility to discharge.

Effects of thiophene on the Purkinje cell dendritic tree: a quantitative Golgi study

A quantitative analysis was made of alterations in the dentritic organisation of Purkinje cells in the cerebellum of the adult rat following the administration of the degranulating agent, thiophene. This substance completely eliminated the granule cell population in a number of regions of cerebellar cortex. Purkinje cell dentdritic trees in these regions were markedly reduced in size when compared with control cells and provided evidence for a non-random loss of dentritic segments. Other dendritic abnormalities were noted. These results were discussed in relation to current theories of neuronal maintenance, and the feasibility of using toxic degranulating agents, such as thiophene, was considered.

Differences in occipital cortical synapses from environmentally enriched, impoverished, and standard colony rats

Initial and replication experiments were studied with littermate, S1, male rats 25-55 days of age placed in enriched (EC), standard colony (SC), and impoverished (IC) environments. Measurements of the length of postsynaptic thickenings were taken from asymmetrical axodendritic synapses in layer IV of the dorsal medial occipital cortex. This experiment differed from others of this nature, for animals from both experimental conditions were compared with the standard colony animals. The 8% (p less than 0.01) difference in postsynaptic thickening length between 675 synapses from EC and 680 synapses from IC was primarily due to the IC, as can be seen by comparing both with the SC. The number of synapses per unit area of neuropil was 15% (p less than 0.01) more in the IC than in the EC and this difference was primarily due to impoverishment. Both the original and the replication experiment verified this direction of change. Thus, environmental experience does significantly affect synaptic length and number, although the large magnitude previously reported by Mphillgaard et al. (1971) was not confirmed in the present experiments. The 7% cortical depth difference between the enriched and impoverished animals was seen to be attributed to both conditions when each was compared with the standard colony rats. The plasticity of synaptic dimensions is discussed in relation to our previous work and to work of others.

Analysis of synaptic efficacy in spinal motoneurones from 'quantum' aspects

1. Synaptic responses of triceps surae motoneurones of the cat following stimulation of single afferent fibres were examined by intracellular recording techniques.2. The mean quantum content (m) of monosynaptic excitatory postsynaptic potentials (EPSPs) was independent of the type of motoneurone recorded and of the afferent fibre stimulated. There was no significant difference in m value between homonymous and heteronymous synapses.3. A positive correlation was found between the amplitude of unit EPSPs and the input resistance of motoneurones. The difference in the amplitude of unit EPSPs appears to be responsible for the higher synaptic efficacy in slow-conducting motoneurones than in fast-conducting motoneurones.4. There was no significant difference in the time course of monosynaptic EPSPs evoked by impulses from homonymous and heteronymous afferent fibres.5. The ratio of monosynaptic connexions from a given afferent fibre was significantly greater on to homonymous than to heteronymous motoneurones. It is concluded that the difference in efficacy between homonymous and heteronymous synaptic transmission is due to the difference in the number of afferent fibres converging upon these motoneurones.

Group Ia synaptic input to fast and slow twitch motor units of cat triceps surae

1 The characteristics of the group Ia synaptic input to triceps surae motoneurones have been examined in pentobarbitone-anaesthetized cats, using intracellular recording techniques. The mechanical properties of the muscle units innervated by the cells were determined and the motoneurone input resistance values were also measured.2. A significant positive correlation was found between the maximum amplitudes of homonymous composite (electrically evoked) monosynaptic excitatory post-synaptic potentials (EPSPs) and the motoneurone input resistance values across the entire population of units sampled. The same correlation in the case of heteronymous EPSPs was also significant although somewhat less strong.3. The distribution of the amplitudes of unitary miniature EPSPs (mEPSPs) of presumed group Ia origin, elicited by small static stretches of the homonymous muscles, were also studied. A significant positive correlation was found between the median amplitudes of the mEPSP distributions and the input resistance values in the motoneurones studied. Positive correlation was also observed between the amplitudes of the median mEPSPs and the maximum homonymous composite EPSPs in the cells for which both data points were available.4. In each of these correlations, the synaptic potential amplitudes tended to be larger in the relatively high resistance type S (slow twitch muscle unit) motoneurones from both gastrocnemius and soleus motor pools, than in the lower resistance type F (fast twitch muscle unit) gastrocnemius cells.5. Examination of the shape of the homonymous monosynaptic EPSP wave forms in different motoneurones showed that these tended to be significantly longer in duration in type S cells than in type F. This difference could not be entirely accounted for by the relatively small difference in mean time constant values found in types F and S cells.6. The results suggest that the density of group Ia synaptic terminals tends to be higher on type S motoneurones than on the type F cells. Further, cells receiving a relatively high density of group Ia input apparently tend to have a greater proportion of this input distributed to distal membrane regions than is the case in motoneurones receiving a relatively low input density.

Intracellular recording from spinal motoneurones in cats with post-asphyxial rigidity

1. Intracellular recordings were obtained from lumbar spinal motoneurones in cats with post-asphyxial rigidity of the hind limbs.2. Membrane potentials, latencies and (or) appearance of excitatory post-synaptic potentials, initial segment responses and soma-dendritic spikes were not materially different from those observed in cells of normal cords.3. Dorsal root stimulation activated all the motoneurones examined through monosynaptic pathways in contrast to cells in normal cords in which such a stimulus sometimes elicits only a post-synaptic potential. In a number of cells subsequent polysynaptic activation caused a spike about 10 msec after the monosynaptic response, notwithstanding the serious interneuronal destruction which characterized these preparations.4. In a few preparations the effects of acute asphyxiation could be examined. The soma depolarized at a rate of 3-4 mV/min. Synaptic activation was more resistant to O(2) lack than antidromic and direct excitation, in contrast to the experience with normal cells. Survival times of 8.5, 11 and 16 min were found. At certain levels of depolarization ;spontaneous' spikes were observed, which, since they were preceded by post-synaptic potentials, could be considered as the result of synaptic activation.5. To account for the enhanced reflex activity of rigid preparations, it was postulated that the substantial loss of interneurones in the cord had caused denervation supersensitivity of the motoneurones to the transmitter compound without materially changing their electrical excitability.6. It was postulated that the early presynaptic failure during asphyxiation in normal preparations was dependent on a mechanism resembling presynaptic inhibition. The prolonged asphyxial survival of reflex activity in rigid preparations may be due to the destruction of interneurones involved in this form of inhibition.

Input resistance, electrical excitability, and size of ventral horn cells in cat spinal cord

Experiments on cat lumbosacral alpha motoneurones showed that, in comparison with cells possessing rapidly conducting axons, the cells with slowly conducting axons have the higher input resistance, that they need weaker stimulating currents to reach the threshold for repetitive firing, and that they need a relatively larger increment in current strength for a given increase in firing rate. Measurements of the number and diameters of dendritic trunks gave larger values for the larger cell bodies. The discussion deals with the interrelation between cell geometry, electrical properties, and the reflex action of alpha motoneurones.