Post-tetanic potentiation of response in monosynaptic reflex pathways of the spinal cord

Following tetanic afferent stimulation of a monosynaptic reflex pathway, the transmission through that pathway of isolated reflex volleys is enhanced for some minutes. Post-tetanic potentiation is comparable in the monosynaptic reflex arcs of flexor and extensor muscles. The facilitator and inhibitor actions of monosynaptic reflex afferent fibers, as well as the transmitter action, are potentiated following tetanization. Little post-tetanic change attends reflex transmission through plurisynaptic reflex arcs. Various tests for excitability change made independently of the tetanized afferent fibers reveal none or a slight depression. Hence the potentiating influence of a tetanus is limited to subsequent action on the part of the recently tetanized fibers themselves. Increase in the size of the individual impulses comprising an afferent volley such as might occur during positive after-potential, would accommodate the requirement for a limited process and provide for increased synaptic action. The proposed association between post-tetanic potentiation and positive after-potential (i.e. hyperpolarization) is supported by the following lines of evidence:— 1. Changes in intensity and duration of potentiation with change in frequency and duration of tetanic stimulation are characteristic of, and parallel to, the changes of positive after-potential in similar circumstances. 2. Afferent impulses are increased following a tetanus, and in a fashion that parallels the course of monosynaptic reflex potentiation. Post-tetanic potentiation, as here described, and after-discharge, whatever may be its mechanism, are unrelated phenomena.

A survey of dehydrogenases in various epithelial cells in the rat

Several different epithelial elements that have intense active transport or protein secretory functions were histochemically assayed in several dehydrogenase media by a recently perfected method. The mitochondria represented the only site of activity, not only when tested in the succinate and D-beta-hydroxybutyrate media, but also when tested in the lactate, malate, and isocitrate media. The reaction for D-beta-hydroxybutyric dehydrogenase in the mouse kidney was curiously limited to the mitochondria of the distal segment of the proximal convoluted tubule, a finding that most convincingly shows that dehydrogenase activity may be differentiated in certain instances from diaphorase activity by the ditetrazole methods and that D-beta-hydroxybutyric dehydrogenase is not present in all mitochondria. Tetranitro-BT is favored over nitro-BT in studies conducted on most organs prepared without fixation and on formalin-fixed tissues that consist of lipid-containing or active transport cells.

Inhibitory synaptic potential of frog motor neurons

Using isolated frog spinal cords treated lightly with pentobarbital, intracellular recordings from motor neurons revealed hyperpolarization of the membrane following selected dorsal root stimulation. The response showed 5.0 msec latency, 6.4 msec rise time, and 16.9 msec time constant of decay. Its reversal potential was 7.6 mv more negative than resting membrane potential. Threshold of the membrane for spike generation increased corresponding to the time course of the hyperpolarization. Spike potential generation following excitatory synaptic activation (dorsal root, lateral column, and adjacent ventral root) was depressed during the hyperpolarizing potential. Therefore, this hyperpolarization can be designated as inhibitory synaptic potential (IPSP). Pentobarbital sensitivity suggests a polysynaptic pathway. Evidence is presented which suggests that the IPSP is initiated from terminals on the somatic portion of the membrane.

Urea transport in the central nervous system

The kinetics of urea transport in the central nervous system have been studied in rabbits during sustained intravenous and intracisternal infusions of C12 and C14 urea. The steady state content of urea in the water phase of the white matter and cord was approximately equal to its content in plasma water. However, the water of whole brain and gray matter had levels of urea which exceeded those in plasma by 7 and 18%, respectively, whereas the urea in cerebrospinal fluid (CSF) was only 78% of the plasma level. Its rate of penetration into nervous tissue was approximately one-tenth as rapid as into muscle. The intravenous infusion of urea caused a significant decrease in water content of the brain and cord. It was estimated that urea infused into the subarachnoid space penetrated the central nervous system (CNS) tissues at four to five times the rate of transport from blood to CNS tissues. These studies suggest that intravenous infusions of urea lower CSF pressure by decreasing the volume of the brain and cord. The major barrier to urea penetration into nervous tissue is at the capillary level, and not the plasma membrane of the glial or neuronal cells.