Firing behaviour in nerve cell models with a two-state pore system

The firing behaviour of simple nerve cell models with a two-state pore system was investigated by computer simulation. The pores were assumed to open and close randomly, with the probabilities for opening and closing calculated from the equations found by Hodgkin and Huxley for the squid giant axon. The cell models had a cell body and an axon with an initial segment with a smaller diameter. Both the case of a uniform membrane with constant pore densities all over the cell, and the case of a cell body membrane with only a leakage conductance (a ‘passive’ membrane) were investigated. The results indicate that the firing behaviour of a small nerve cell may be significantly influenced by the finite number of pores in the initial segment. In contrast to the original Hodgkin-Huxley equations which give a very non-linear frequency-current relationship in such nerve cell models, a fairly linear relationship over a large current range was found in many cases. It was estimated that the diameter of the initial segment must be less than approximately 1μm and the length larger than half a space constant, in order to obtain a current frequency relationship significantly different from that predicted by the original Hodgkin-Huxley equations.     

Susceptibility of the sympathectomized ear to noise-induced hearing loss

Unilaterally sympathectomiced rats were exposed to 100 dB L_eq frequency-modulated noise for I month. Normotensive as well as spontaneously hypertensive animals (with a blood pressure of above 200 mmHg) were investigated. Auditory sensitivity was determined by auditory brainstem responses to 1/3-octave filtered sine waves in the frequency range 0.8–20.0 kHz. In addition, a morphological analysis was carried out. It was found that the sympathetic innervation to the inner ear of the rat originated in, or passed through the ipsilateral superior cervical ganglion. Sympathectomy did not alter pre-exposure hearing thresholds nor influence the size of the noise-induced hearing loss either in 3 or 11 months old hypertensive rats, or in normotensive rats of 11 months. A slightly smaller loss was seen in the sympathectomized side in young normotensive rats. It was concluded that the sympathetic does not exert a protection of the inner ear against functional disturbances in hypertension, neither during basal metabolic condition nor during extreme conditions, i.e. during noxious noise exposure.     

Brain extracellular space during spreading depression and ischemia

The change of extracellular space volume of rat brain cortex during ischemia and cortical spreading depression, CSD (Leāo4) was evaluated by a new method. The cortical surface was imgated with isotonic CSF containing the extracellular markers 50 mM cholin or 50 mM trimethyltris(hydroxymethyl)methyl ammonium ion (N-TRIS), and their extracellular concentrations were monitored by ion-selective microelectrodes. When steady-state for the concentration of these markers was attained, CSD evoked a reversible increase of the concentration of the markers, indicating shrinkage of the interstitial volume of distribution. During ischemia an initial slow rate of concentration increase was observed, followed a few minutes later by a rapid increase concomitant with the sharp rise in extracellular potassium concentration. During CSD and ischemia, the maximal increases of choline and N-TRIS concentration reflected a shrinkage of the extracellular space amounting to about 50% of the initial volume.