No abstract

Field stimulation of the jejunum elicited successively an action potential of spike form, a slow excitatory depolarization, a slow inhibitory hyperpolarization, and a postinhibitory depolarization as a rebound excitation. The slow depolarization often triggered the spike. The inhibitory potential showed lower threshold than did the excitatory potential. Both the excitatory potentials were abolished by atropine and tetrodotoxin. Effective membrane resistance measured by the intracellular polarizing method was reduced during the peak of the excitatory potential, but the degree of reduction was smaller than that evoked by iontophoretic application of acetylcholine. Conditioning hyperpolarizafion of the muscle membrane modified the amplitude of the excitatory potential. The estimated reversal potential level for the excitatory potenialt was about 0 mv. No changes could be observed in the amplitude of the inhibitory potential when hyperpolarization was induced with intracellularly applied current. Low [K]o and [Ca]° blocked the generation of the excitatory potential but the amplitude of the inhibitory potential was enhanced in low [K]o. Low [Ca]° and high [Mg]o had no effect on the inhibitory potential.

Objective To determine the properties of the striated muscle of the greyhound (dog) urethra and to consider its role in maintaining continence. Materials and methods The thickness of the muscle layers and the muscle types were determined by examining sections stained with haematoxylin and eosin or Masson's trichrome. These factors were correlated with the mechanical and electrical responses of muscle strips to nerve stimulation, and compared with muscle from other breeds of dog and other parts of the animal. Results The striated muscle formed <70% of the membranous urethra and was predominantly (68%) type IIa muscle (i.e. fast but fatigue-resistant IntroductionThe lower urinary tract has as its primary function the storage and expulsion of urine at appropriate times. Constriction of the urethra prevents the passage of urine from the bladder to the exterior, but there is some controversy about which structures provide continence. The sphincteric action of the smooth muscle layers of the bladder neck and/or proximal urethra can provide a watertight seal despite wide¯uctuations in intravesical pressure [1,2]. The membranous urethra (which runs in the male from the apex of the prostate to the start of the penis) contains some smooth but mostly striated muscle that may be important in continence. The striated muscle is reported to contribute more than a third to a half of the maximum urethral pressure in humans [3,4]. To understand the role of the striated muscle, it is necessary to determine the extent and orientation of the muscle layers and relate the ®bre type to the electrical and contractile properties of the muscle. According to Cullen et al.[5] the striated muscle of the entire membranous urethra in adult male dogs occupies over half the urethral wall, whereas total connective tissue comprises <40%. It is not known if these percentages are similar in all breeds of dog. In comparison, in a human infant the striated muscle comprised 79% of the urethra; with increasing age this decreased to 35.5% in an elderly man [6]. This reduction probably explains why the incidence of incontinence after prostatectomy increases with age in men [7].From histochemical studies for the reaction of myo®brillar actomyosin ATPase, striated muscles can be divided into type I (slow twitch), type IIa (fast twitch fatigue-resistant) and type IIb (fast twitch fatiguable) ®bres [8,9]. The type IIb ®bres are absent in the dog but transitional type IIc ®bres (also fast, fatiguable) are present [10]. The striated muscle of the urethra of adult dogs of mixed breed has been reported to contain 35% type I, 52% type IIc and 13% type IIa ®bres [11]. Female dogs of mixed breed have been found to contain 76% type II ®bres [12]. Although Gosling et al. [13] stated that all ®bres in humans are type I, it has also been reported that about one-third of ®bres are type II [14,15]. It was suggested that the slow-twitch ®bres (type I) are likely to be responsible for continence at rest, whereas the fasttwitch ®bres would be recruited during stress [11]. T...

The individual muscle fibers of the anterior byssus retractor muscle (ABRM) of Mytilus edulis L. are uninucleate, 1.2-1.8 mm in length, 5 lim in diameter, and organized into bundles 100-200 Mm in diameter, surrounded by connective tissue. Some bundles run the length of the whole muscle. Adjacent muscle cell membranes are interconnected by nexuses at frequent intervals. Specialized attachments exist between muscle fibers and connective tissue. Electrical constants of the resting muscle membrane were measured with intracellular recording electrodes and both extracellular and intracellular current-passing electrodes. With an intracellular current-passing electrode, the time constant r, was 4.3 1.5 ms. With current delivered via an extracellular electrode r was 68.3 15 ms. The space constant, X, was 1.8 mm 4 0.4. The membrane input resistance, Ref, ranged from 23 to 51 M. The observations that values of r depend on the method of passing current, and that the value of X is large relative to fiber length and diameter are considered evidence that the individual muscle fibers are electrically interconnected within bundles in a three-dimensional network. Estimations are made of the membrane resistance, R, to compare the values to fast and slow striated muscle fibers and mammalian smooth muscles. The implications of this study in reinterpreting previous mechanical and electrical studies are discussed.