Monary Circulation

Conductivity.

The impulses which originate in the S-A node spread along the ordinary muscular tissue to every part of the auricles. The rate of conduction in the auricle ranges from 600–I,200 mm. per second. In its fan-like spread, the excitation wave reaches the A-V node. There is no indication of a prefer ential path of conduction between the two nodes, so that the im pulse travels at the same rate over the auricle and reaches the A-V node about 0.03 second after the origin of the impulse. In the A-V node, the rate of conduction is considerably slower, the impulse passing the node at about 150-200 mm. per second. Once it has passed the node, the impulse travels rapidly down the bundle and its ramifications at the rate of about 5,00o mm. per second.

The slow conduction in the A-V node and the rapid conduc tion in the bundle tissue ensure two important features of cardiac activity. On account of the former, auricular contraction is given time to end before the onset of the ventricular contraction, and on account of the latter the impulse arrives at every part of the ventricle at approximately the same time. Thus the whole ven tricular muscle contracts approximately at the same time, which is a condition necessary for the development of a high pressure in the ventricular cavities.

Irritability.

When a skeletal muscle is stimulated by an increasing strength of electrical stimulus, it responds by an in creasing strength of contraction, until a certain maximum is reached. A heart under the same conditions gives a contraction which is maximal for a given condition of the heart in response to the first effective stimulus ; further increase in the strength of the stimulus does not lead to an increase in the strength of the response. This difference between the skeletal and the cardiac muscle is not due to fundamentally different properties, but de pends on the fact that in cardiac muscle the muscle fibrils are in free inter-communication with each other, and in this way a stimulus which originates in one part spreads over the whole of the organ ; in the skeletal muscle, however, the contractile fibrils are collected into muscle fibres which are separated by the homogeneous coat of the sarcolemma. Since the threshold value of a stimulus which will evoke contraction is different for the dif ferent muscle cells, it is only to be expected that a greater num ber of these cells will contract in the skeletal muscle when the stimulus is strengthened, the result being a stronger total effect of contraction of the muscle. In the heart, when a stimulus evokes a contraction of a few fibres, this contraction will spread over the whole heart, so that with increase in the stimulus there is no further strengthening of the contraction. This behaviour of

the heart is known as the All or None Law. It is most evident in the cardiac muscle, but is not peculiar to it.

The Refractory Period.—The irritability of the heart undergoes rhythmical variations which are determined by the heart beat itself. Like all excitable tissues, the heart exhibits the phenomenon of the refractory period, which is a period of loss of irritability following each impulse which evokes a contractile response. In the heart this period is extremely prolonged. It lasts as long as the contraction of the heart. If a second stimulus is applied to the heart during the contraction which is evoked as a response to the first stimulus, it is found that it has no effect. In the skeletal muscle, on account of the fact that the refractory period is shorter than the time occupied by the contraction, a second stimulus evokes a second contraction, or a summation of the two con tractions. The period of complete loss of irritability is followed by gradual recovery, after which there is a phase of super-normal irritability before it returns to the normal.

The length of the refractory period depends in the first place on the strength of the contraction of the heart. Thus drugs and physiological conditions which strengthen the heart beat also in crease its refractory period. Of greater significance, however, is the relation of the refractory period to the rate of the heart beat. Lewis found that the duration of the refractory period of mam malian auricular muscle contracting ioo, 130 and 25o times per minute was 0.2, 0.15 and o•oi sec. respectively.

It is obvious that as a result of the shortening of the refractory period the heart is able to respond to more rapid rates of excita tion thah would otherwise be possible. As soon, however, as stimuli occur at intervals which are shorter than the refractory period alternate stimuli will fail to evoke any response, the heart beating at a half-rhythm or 2 :1 response. The change from a i :1 to a 2:1 response does not occur abruptly. Between these rates there exists a phase in which large and small beats alternate; with further acceleration of the stimulation, some beats are dropped, and with a still further acceleration a regular 2:1 rate sets in. This stage of irregular response is due to the fact that apparently all the parts of the cardiac muscle have not the same minimal refractory period, and consequently at some definite rate of excita tion parts of the heart will contract in response to every stimulus, while other parts will respond by a 2 : i rate.