Impulse stimulus

Figure 2. Muscle stimulation, a) a single nerve impulse (stimulus) causes a single contraction (a twitch). There is a small delay following the stimulus before force rises called the latent period, b) A train of stimuli at a low frequency causes an unfused tetanus. Force increases after each progressive stimulus towards a maximum, as calcium levels in the myofibrillar space increase. But there is enough time between each stimulus for calcium to be partially taken back up into the sarcoplasmic reticulum allowing partial relaxation before the next stimulus occurs, c) A train of stimuli at a higher frequency causes a fused tetanus, and force is maximum. There is not enough time for force to relax between stimuli. In the contractions shown here, the ends of the muscle are held fixed the contractions are isometric.

Anregung,/. excitation stimulation stimulus, impulse suggestion. 

Anreiz, m. stimulus impulse incitement, anreizen, v.t. stimulate instigate, anrichten, v.t. prepare, dress perform, produce ... 

While this chapter is concerned primarily with the neurochemical mechanisms which bring about and control impulse-evoked release of neurotransmitter, some of the methods used to measure transmitter release are described first. This is because important findings have emerged from studies of the effects of nerve stimulation on gross changes in transmitter release and intraneuronal stores. The actual processes that link neuronal excitation and release of transmitter from nerve terminals have been studied only relatively recently. The neurochemical basis of this stimulus-secretion coupling, which is still not fully understood, is described next. The final sections will deal with evidence that, under certain conditions, appreciable amounts of transmitter can be released through Ca +-independent mechanisms which do not depend on neuronal activation. 

After an electrical impulse is initiated and conducted, there is a period of time during which cells and fibers cannot be depolarized again. This period of time is referred to as the absolute refractory period (Fig. 6-2),2 and corresponds to phases 1,2, and approximately half of phase 3 repolarization on the action potential. The absolute refractory period also corresponds to the period from the Q wave to approximately the first half of the T wave on the ECG (Fig. 6-2). During this period, if there is a premature stimulus for an electrical impulse, this impulse cannot be conducted, because the tissue is absolutely refractory. 

According to Fig. 6.17 the nerve cell is linked to other excitable, both nerve and muscle, cells by structures called, in the case of other nerve cells, as partners, synapses, and in the case of striated muscle cells, motor end-plates neuromuscular junctions). The impulse, which is originally electric, is transformed into a chemical stimulus and again into an electrical impulse. The opening and closing of ion-selective channels present in these junctions depend on either electric or chemical actions. The substances that are active in the latter case are called neurotransmitters. A very important member of this family is acetylcholine which is transferred to the cell that receives the signal across the postsynaptic membrane or motor endplate through a... 

A reflex occurs when a particular stimulus always elicits a particular response. This response is automatic and involuntary in other words, it occurs without conscious effort. Therefore, reflexes are specific, predictable, and, furthermore, often purposeful. For example, the withdrawal reflex causes a body part to be pulled away from a painful stimulus so that tissue injury is avoided. Spinal reflexes require no input from the brain because they are elicited entirely at the level of the spinal cord. However, while the reflex is underway, nervous impulses are also transmitted to the brain for further processing. In fact, input from the brain may modulate a reflex or alter the response to a stimulus through conscious effort. 

The mechanism of action of inhalational anesthetics is unknown. The diversity of chemical structures (inert gas xenon hydrocarbons halogenated hydrocarbons) possessing anesthetic activity appears to rule out involvement of specific receptors. According to one hypothesis, uptake into the hydrophobic interior of the plasmalemma of neurons results in inhibition of electrical excitability and impulse propagation in the brain. This concept would explain the correlation between anesthetic potency and lipophilicity of anesthetic drugs (A). However, an interaction with lipophilic domains of membrane proteins is also conceivable. Anesthetic potency can be expressed in terms of the minimal alveolar concentration (MAC) at which 50% of patients remain immobile following a defined painful stimulus (skin incision). Whereas the poorly lipophilic N2O must be inhaled in high concentrations (>70% of inspired air has to be replaced), much smaller concentrations (<5%) are required in the case of the more lipophilic halothane. 

The lethal oral dose in humans is probably around 100, but doses as low as 16 mg have reportedly been fatal whereas doses of 2 000 mg have been survived. After ingestion, effects usually occur within 10-30 minutes and include stiffness of the face and neck muscles and increased reflex excitability. Strychnine acts by altering nerve impulses in the spinal cord, resulting in a decreased threshold for stimulation, and, hence, a hyperexcitable state. Any sensory stimulus may produce a violent motor response that, in the early stages of intoxication, tends to be a coordinated extensor thrust and, in later stages, may be a tetanic convulsion with opisthotonos anoxia and cyanosis develop rapidly. Between convulsions, muscular relaxation is complete, breathing is resumed, and cyanosis lessens. Because sensation is unaffected, the convulsions are painful and lead to overwhelming fear. As many as 10 convulsions separated by intervals of 10-15 minutes may be experienced, but death often occurs after the second to fifth convulsion, and even the first convulsion may be fatal if sustained death is commonly due to asphyxia.If recovery occurs, it is remarkably prompt and complete despite the violence of the illness muscle soreness may persist for a number of days. ... 

The dynamics of the system under study can, in fact, be recovered from a variety of stimulus response tests. These include impulse and step response experiments, and frequency response and cross-correlation techniques. Descriptions of these methods and the interrelationships between them are discussed in many references, see, for instance, refs. 22—25 and Sects. 3.2.1—3.2.4 of this chapter. 

Control of attention. People may keep the opportunity to behave impulsively out of sight of out of mind, so that they cannot receive or process the information that the inferior reward is at hand. For instance, an overeater may keep food out of sight or keep busy so as not to think about food. Behaviour therapists have referred to similar activities as stimulus control (Kanfer and Phillips 1970 Goldiamond 1965), and psychoanalysts have called them suppression, repression or denial. 

Although there may be no demonstrable alteration in the function of the cardiovascular system or the kidneys of such patients, other than the sustained elevation of blood pressure when they are at rest, the extreme responsiveness of these systems to emotional stress is remarkable. There may be no alteration of the inherent ability of an organ or tissue to respond to a stimulus. Instead, their greater reactivity is to an increased transmission of nerve impulses in response to what may be a psychogenic stimulus. Wolf, Pfeiffer, Ripley, Winter, and Wolff (166) have demonstrated such changes in their patients admirably, and two of their figures from a recent publication illustrate this point. 

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