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.