Excitability

“ In the preceding illustration, I have spoken of a tube, as constantly pouring in fresh fuel, because it was not easy otherwise to convey a familiar idea of the power which all living systems possess of renewing their excitability, when exhausted. The excitability is an unknown somewhat, subject to peculiar laws, some of which we have examined, but whose different states we are obliged to describe, though, perhaps, inaccurately, by terms borrowed from the qualities of material substances. ”

“ To maintain this state, the action of the stimuli should be strong enough to carry off from the body the surplus of this irritable principle. To obtain this end, a certain equilibrium is necessary between the excitability and the stimuli applied, or the sum of all the stimuli acting upon it must be always nearly equal, and sufficient to prevent an excess of excitability, but not so strong as to carry off more than this excess. It is in this equilibrium between the acting stimuli and the excitability, that the health, or tone of the living body consists. ”

“ And it is this readiness of organic tissues to respond to a suitable stimulus that physiologists designate by the term "excitability."
Nerves, no less than muscles, present the property of being excitable. If, together with the excised muscle, there had been removed from the animal's body an attached nerve, every time any part of this nerve is stimulated the attached muscle will contract as before. ”

“ Finally, instead of slow reaction there may exist excessive excitability, and this may be peculiar to the whole nervous system and brain, or only to a portion of the latter. A generally excessive excitability produces those morbidly-sensitive natures in whom the most insignificant phenomena create the most astonishing perceptions; who hear the ‘sobbing of the evening glow,’ shudder at the contact of a flower ”

“ Here the chime runs on the word excitability, which is not defined. If we call this property life, then we are only informed, that, as life is more or less vigorous, the body enjoys a greater or smaller degree of health; which we know without any medical instructor. If, instead of the accumulation and exhaustion of excitability, we take the original doctrine of excitement and debility laid down by Dr. Brown himself, we are nothing better. The whole theory is lost for want of the definition of a single word. As long as excitability remains an unknown property, we can explain nothing by it. ”

“ Bose found 'differential excitability' is widely present as a factor in determining the character of special responses and showed that many anomalous conclusions, with regard to the response of certain animal tissues, had arisen from the failure to take account of the 'differential excitability' of anisotropic organs. ”

“ When such an impression is thus conducted simply along a nerve fibre, and thence to a muscle, it induces or excites, as we have seen, the contraction of that muscle, and so exercises what is called a motor function.
The nerve cells appear to possess, beyond the simple excitability to general stimuli, conductility, and the peculiar receptivity which is essential to sensation, a special or more exalted kind of excitability which is called into play under mental or psychical stimuli by the changes produced in the gray matter [5] in the formation of ideas, emotions, and the will. ”

“ When a stimulus of any kind, whether mechanical, chemical, electrical, or vital, acts upon the living nervous substance, it produces an impression on that nerve substance and excites within it some particular change, and the property by which this takes place in the nerve substance has been called its excitability or neurility. ”

“ But the stimulant powers may act so powerfully, and exhaust the excitability to such a degree, that they may overstep the bounds of sthenic or inflammatory disease and bring on debility. Debility may therefore arise either from the stimuli acting too weakly, or from a deficient excitability, while the stimulus is not deficient. ”

“ Every stimulus causes a portion of the neural substance to pass from its normal state through a condition of increased excitability, which we may designate the positive phase, to a condition of diminished excitability, the negative phase. There is first an increase in the functional capacity of the tissues, then a decrease. If during the course of the change produced by a given stimulus a second stimulus becomes effective its result in reaction is determined by the particular phase of the tissues upon which it intrudes. ”

“ Besides, the subtraction of stimulants, which is one of the most certain means we have of accumulating excitability, if carried to a great extent, in diseases of indirect debility, would produce death, before the system had power to reproduce the lost or exhausted excitability. ”

“ Everything tends to a more violent motion. If the sensation be an agreeable one, all these parts will acquire a higher degree of harmonious activity; the heart’s beat will be free, lively, uniform, the blood will flow unchecked, gently or with fiery speed, according as the affection is of a gentle or violent description; digestion, secretion, and excretion will follow their natural course; the excitable membranes will pliantly play in a gentle vapor-bath, and excitability as well as sensitiveness will increase. ”

“ But it soon appeared that this was too narrow a way of looking at the matter, and that arrest is not so much the specific function of certain nerves as a general function which any part of the nervous system may exert upon other parts under the appropriate conditions. The higher centres, for example, seem to exert a constant inhibitive influence on the excitability of those below. The reflexes of an animal with its hemispheres wholly or in part removed become exaggerated. ”

“ The excitability of the body may, most probably, be conveyed by respiration, and the circulation of the blood, which tend continually to oxidate the different parts: and hydrogen or electricity may be secreted by the brain, and sent along the nerves, which are such good conductors of it, and by uniting with the oxygen of the muscle, may cause it to contract; but as the oxygen will, by this union, be diminished, if the contractions be often repeated, the excitability will thus be expended faster than it can be supplied by the circulation, and will become exhausted. ”

“ In this state many of the exciting powers cannot act upon us, unless applied with some violence, for we are insensible to their moderate action. A moderate degree of light, or a moderate noise, does not affect us, and the power of thinking, which very much exhausts the excitability, is in a great measure suspended. When the action of these powers has been suspended for six or eight hours, the excitability is again capable of being acted on, and we rise fresh and vigorous, and fit to engage in our occupations. ”

“ The excitability of a central element depends not only on its condition at the moment of excitation, but also on the condition of the other elements connected with it. An excitation occurring in adjacent parts lowers its own excitability, while a state of latency there favors an explosion of energy in the affected part. Ganglionic cells accumulate latent energy, and may, when excited, liberate and transmit energy. In sleep an immense amount of energy is accumulated. ”

“ The state of tension is increased by every stimulation which falls short of a discharge; that is to say, faint and frequent stimulation augments the excitability, whereas328 powerful stimulation exhausts it. When, therefore, one wave succeeds another in the same direction, it reaches a centre more disposed to discharge ”

“ It is a law of the system, that the absence of one natural stimulus is generally supplied by the increased action of others. This is more certainly the case where a natural stimulus is abstracted suddenly; for the excitability is thereby so instantly formed and accumulated, as to furnish a highly sensible and moveable surface for the remaining stimuli to act upon. Many proofs might be adduced in support of this proposition. ”

“ One of the most striking effects of external disturbance on certain types of living substance is a visible change of form. Thus, a piece of muscle when pinched contracts. The external disturbance which produced this change is called the stimulus. The body which is thus capable of responding is said to be irritable or excitable. A stimulus thus produces a state of excitability which may sometimes be expressed by change of form.
Mechanical response to different kinds of stimuli.—This reaction under stimulus is seen even in the lowest organisms ”

“ If, for example, a person should fall suddenly into the water, without bruising his body, and sink before his fears or exertions had time to dissipate his excitability; his recovery from apparent death might be effected by the gentle action of heat or frictions upon his body, so as to convert his accumulated excitability gradually into excitement. ”

“ But both forms of nervous tissue possess the same vital property, called excitability; by which term is meant, that when a nerve-cell or fibre is stimulated by some external agent, it is capable of receiving an impression and of being by it excited into activity. A ray of light, for example, falling upon one extremity of a fibre in the eye, excites it throughout its whole length; and its {160}other extremity, within the brain, communicating with a nerve-cell, the latter, in its turn, is excited, and the sensation of sight is produced. ”

“ Today we know with positiveness that a continued effect also exists during the uninterrupted flowing of a constant current in nerve or muscle, though much weaker, however, than in the case of the excitations produced by sudden fluctuations of the intensity. This is shown in the nerve by an altered excitability, which continues at the poles during the whole duration of the current. In the region of the anode the excitability is diminished, in that of the cathode it is increased. ”

“ The prepotent region at one end which usually dominates the common path is from time to time displaced by local increase of excitability at the other under local distension of the bloodsinuses there.
In judging of intensity of stimulus the situation of the stimulus in the receptive field of the reflex has to be remembered. One and the same physical stimulus will be weak if applied near the edge of the field, though strong if applied to the focus of the field. ”

“ Generally, the excitability of the vaso-motor nerves is the same in both halves of the body, and we all experience during strong emotions a feeling of cold, due to the contraction of the vessels, and spreading over the whole body, as though a cold sheet were being wrapped round our limbs and pressed upon our heart ”

“ A nerve preserves its excitability as long as it preserves its structural integrity, and recovers its power in recovering that integrity. The length of time varies.101 Gratiolet found the muscles in the leg of a tortoise, which had been amputated a week before, contract when the nerves were irritated; and Schiff found the divided nerve of a winter frog excitable at the end of three weeks. Even after all excitability has disappeared, it will reappear if arterial blood be injected ”

“ Considering the matter physiologically, we may say that the diminution of the general sensibility is compensated by an increase in special sensibility. If a large portion of the central organ is in a state of functional latency, the excitability of the functioning part is more intense. ”

“ The error of those who refer pain solely to intensity of stimulation is in considering objective conditions only; they forget the part played by the sentient subjects. Pains which depend on the quality of the stimulus are especially of subjective origin, because the degree of excitability in the patient’s nervous elements is the essential ruling factor. ”

“ If this is continued for a long time, the nerves of sensation lose their strength from over-action, and the nerves of motion lose their power from inactivity. In consequence, there is a morbid excitability of the nervous, and a debility of the muscular system, which make all exertion irksome and wearisome. ”

“ Indeed, the extreme nervous excitability which they produce sometimes becomes a real insanity, which, though it may, in many cases, be finally outgrown, may probably in many others lead to lasting and most deplorable results. ”

“ Minimally effective stimulus becomes effective by summation of stimuli. A maximum effect is produced, determined by the intensity of individual stimulation.
9. Hysteresis is exhibited in cyclic curves. The forward and return curves tend to coincide after several cycles. Previous annealing reduces hysteresis, and after one or two cycles the wire assumes a constant sensitivity.
10. Chemical reagents may profoundly modify the electric excitability. Some of them increase the excitability, while others depress or abolish it. ”

“ When we attend closely to a sensory impression, or to a train of thought, the excitability of every part of the brain except that actually engaged in the act is diminished by an inhibitory action of the working portion. ”

“ One mode of emotional excitability is exceedingly important in the composition of the energetic character, from its peculiarly destructive power over inhibitions. I mean what in its lower form is mere irascibility, susceptibility to wrath, the fighting temper; and what in subtler ways manifests itself as impatience, grimness, earnestness, severity of character. Earnestness means willingness to live with energy, though energy bring pain. The pain may be pain to other people or pain to one's self—it makes little difference; ”

“ When the stimulus is applied on the more excitable half of the organ, the result is a predominant contraction of that half, which cannot be neutralised by the excitation conducted to the less excitable half of the organ. ”