Control of heart rate

autonomic means self-governing. The autonomic nervous system controls the involuntary activities of internal muscles and glands. It has two divisions:

• the sympathetic nervous system. This stimulates effectors and so speeds up any activity.  Acts like an emergency controller. It stimulates effectors when we excercise strenuously or experience powerful emotions. In other words, it helps us to cope with stressful situations by heightening our awareness and preparing us for activity. 
• parasympathetic nervous system. In general, this inhibits effectors and so slows down any activity. it contols activities under normal resting conditions. it is concrened with conserving energy and replenishing the bodys reserves. 

The actions of parasympathetic nervous systems are antagonistic. If one system contracts a muscle, then the other relaxes it. The activites of internalglands and muscles are therefore regulated by a balance of the to systems. Let us look at one such: the control of heart rate. 

Typical resting heart rate - 70 beats per minute.  
Changes to the heart rate are controlled by the medulla oblongata found in the brain.  This has 2 centres:

• a centre that increases heart rate which is linked to the sinoatrial node by the sympathetic nervous system.
• a centre that decreases heart rate, which is linked to the sinoatrial node by the parasympathetic nervous system.

Which of these centres is stumulated depends upon the information they receive from two types of receptor, which respond to one of the following:

• chemical changes in the blood.
• pressure changes in the blood. 

Chemoreceptors are found in the wall of the carotid arteries ( the arteries that serve the brain). They are sesitivie to changes in the pH of the blood that result from changes in carbon dioxide concentration. in solution, carbon dioxide forms an acid and therefore lowers pH. The process of control works as follows:

• when blood has a higher than normal concentration of carbon dioxide, its pH is lowered.
• The chemoreceptors in the wall of the carotid arteries and the aorta detect this and increase the frequency of nervous impulses to cenre in the medulla oblongata that increases heart rate.
• this centre increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node which in turn increases the heart rate.  
• this increased blood flow that this causes leads to more csrbon dioxide being removed by the lungs and so the co2 level of the blood returns to normal.  
• as a consequence the pH of the blood rises to normal and the chemoreceptors in the wall of the carotid arteries and aora reduce the frequency of nerve impulses to the medulla oblongata.
• the medulla oblonata reduces the frequency of impulses to the sinoatrial node, which therefore decreases the heart rate to normal.  

Pressure receptors occur within the walls of the arteries and aorta. They oprate as follows:

• when blood pressure is higher than normal - they transmit a nervous impulse to the medulla oblongata that decreases heart rate. This centre sends impulses via the parasympathetic nervous system to the sinoatrial nodeof the heart, which decreases that rate at which the heart beats. 
• when blood pressure is lower than normal - they transmit a nervous impulse to the centre in the medulla oblongata that increase,heart rate. This centre sends impulses via the sympathetic nervous system to the sinoatrial node, which increases the rate at which the heart beats.