title: Passive membrane properties, afterpotentials and repetitive firing of superior colliculus neurons studied in the anesthetized cat.
abstract: Intracellular recording and staining with HRP were used to characterize cat superior colliculus neurons with identified projection into the tecto-bulbo-spinal tract (TBSNs). TBSNs are large multipolar neurons with heavy stem dendrites. First and second order dendrites bifurcate with an average branch power n of about 3/2. More peripheral branch points have n less than 1.5. Input resistances of TBSNs range from 0.9 to 4.6 M omega. Most TBSNs display 'anomalous rectification'. Based on Rall's steady-state cable equations, input resistances were calculated for 3 TBSNs labelled with HRP. Assuming a specific membrane resistance of 2,300-2,600 omega cm2 the/calculated values agree well with the experimentally determined estimates from another set of non-stained TBSNs. Membrane time constants of TBSNs range from 3.0 to 5.6 ms. The electrotonic length was calculated using the ratio tau 0/tau 1. The respective average value was 1.13. TBSNs respond to orthodromic, antidromic and direct stimulation with action potentials of 60-80 mV, composed of IS- and SD-components. The critical interval for IS-SD-invasion was on average 1.6 ms. Spike decomposition occurs usually at M-level. The postspike conductance increase underlying hyperpolarizing afterpotentials (HAP) decays exponentially, with the time constants tau F = 1.5 ms and tau S = 13 ms. The HAP was equilibrated at membrane potentials of -73 to -90 mV. When tested by antidromic stimuli at varying intervals most TBSNs show very poor "summation" of HAPS. A pronounced depolarizing hump (DD) follows antidromic action potentials. Discharging at short intervals leads to a substantial increase and prolongation of DD. This apparent DD-potentiation is interpreted as a phenomenon secondary to the reduction of hyperpolarizing currents. In response to directly injected currents, TBSNs discharge with frequencies up to 1,100 imp/s. The frequency-current curves of TBSNs are characterized by 3 ranges. The average f-i-slopes of the adapted discharge were 19.2 imp/s/nA and 56.4 imp/s/nA for the 1st and 2nd range, respectively. At intermediate current intensities (2nd range) TBSNs discharge in groups of 2 to 7 action potentials, following each other at intervals of 1.0-2.8 ms. The spike groups are separated by pauses of 3.5-6.3 ms duration. The transition from 1st (low frequency continuous) discharge range to 2nd (grouped) discharge range is related to the appearance of extra-spikes. Extra-spikes are generated from a decreased firing level, from the peak of an enhanced DD.(ABSTRACT TRUNCATED AT 400 WORDS)
mesh_terms: D000200:Action Potentials; D000818:Animals; D002415:Cats; D002462:Cell Membrane / Q000502:physiology; D004553:Electric Conductivity; D005071:Evoked Potentials; D008526:Medulla Oblongata / Q000502:physiology; D009474:Neurons / Q000166:cytology; D013116:Spinal Cord / Q000502:physiology; D013477:Superior Colliculi / Q000166:cytology / Q000502:physiology*