Christopher Dawson 99139007 .
BM3501 12/02/02.
DESCRIBE THE FACTORS (INCLUDING CIRCULATION FACTORS AND NEURAL INPUTS) WHICH CAN ALTER THE INOTROPIC AND CHRONOTROPIC STATE OF THE HEART.
Cardiac output is defined as the frequency of beats per minute multiplied by the volume of blood expelled per stroke. If the cardiac output is to be transformed, then it stands to reason that there are only two methods of achieving this, altering the frequency of the beats (chronotropism), or varying the volume of blood expelled per stroke (inotropism).
The chronotropic state of the heart is regulated by the Sino Atrial Node (SAN) or pacemaker, the inotropic condition of the heart is dependant on the contractility of the myocardium. .
The SAN gives off an inherent frequency of 100 beats per minute (BPM) but the SAN is under constant influence of circulating drugs and neuronal control, therefore the heart has a great capacity to change its chronotropic value. Both sympathetic and parasympathetic neurones converge on the SAN and influence the heart rate, with the parasympathetic innervation far outnumbering the sympathetic. Parasympathetic signals via the Vagus nerve serve to slow the rate, while sympathetic nerve signals speed up the rate, the intrinsic rate is normally slowed to give a natural frequency of 70 BPM. .
Parasympathetic innervation of the heart originates in the Medulla Oblongata and travels via the Vagus nerve to the heart, where it splits into two sides of innervation. The right vagus nerve stimulates the SAN and causes Bradycardia or occasionally a short-lived complete block of all SAN impulses. The Left Vagus nerve induces the Atrioventricular node conduction tissue to become poorer at conducting impulses therefore creating a bigger refractory period between beats and causing bradycardia.
