3.6.1.3 Control of heart rate

Nervous organisation
Okay so I can't actually find this in the spec so feel free to skip it out, but there's a little bit about it in my textbook which I've found pretty useful to know for answering exam Qs etc. It basically lets us know about how the nervous system is split up...

So overall we have the nervous system. This has two major divisions:

• the central nervous system (CNS) made up of the brain and spinal cord
• the peripheral nervous system (PNS) made up of pairs of nerved that originate from either the brain or spinal cord. this can be divided into two further subdivisions:
• sensory neurones which carry electrical signals in the form of nerve impulses from receptors towards the central nervous system
• motor neurones which carry nerve impulses away from the CNS to effectors. The motor nervous system can again be divided into two more divisions:
• The voluntary nervous system carries nerve impulses to body muscles and is under conscious/voluntary control
• The autonomic nervous system controls our subconscious/involuntary activities and carries nerve impulses to internal muscles (cardiac and smooth muscle) and glands. You know what's coming... it can be broken down into two more nervous systems which work antagonistically:
• the sympathetic nervous system stimulates effectors therefore speeds up any activity. It helps us to cope with stressful situations thanku sympathetic nervous system) by heightening our awareness and preparing us for activity - many also studying psychology will know this as the flight or fight response.
• the parasympathetic nervous system inhibits effectors and therefore slows down any activity. It controls activity under normal resting conditions (when you're not stressed or anything) and is concerned with conserving energy and replenishing the body's reserves.

Control of the heart rate

Okay so I wanted to get all that said above because basically the activities of internal glands/muscles are regulated by the balance of the sympathetic and parasympathetic nervous systems and control of the heart rate is a good example of this.

Cardiac muscle is myogenic (contraction is initiated within the muscle as apposed to by the nervous system (neurogenic)). In the wall of the right atrium sits a group of cells that are collectively known as the SAN (sinoatrial node) - here the initial stimulus for contraction originates. This has a basic rhythm of stimulation that determines the heart beat rate. The sequence of events that leads to a heartbeat is as follows...

• A wave of electrical excitation spreads out from the SAN across both atria causing them to contract
• A layer of non-conductive tissue prevents the wave crossing to the ventricles. This is known as the atrioventricular septum
• The wave of excitation enters the AVN (atrioventricular node) - a second group of cells which lie between the atria
• After a short delay, this conveys a wave of electrical excitation between the ventricles along a series of specialised muscle fibres (Purkyne tissue) which make up the bundle of His
• The bundle of His conducts the wave through the atrioventricular septum to the base of the ventricles where the bundle branches into smaller fibres of the Purkyne tissue
• The wave of excitation is released from the Purkyne tissue causing the ventricles to contract quickly at the same time from the bottom of the heart upwards

We must of course be able to alter this heart rate e.g if we are running away from our parents. Changes to he heartrate are controlled by the medulla oblongata (a part of the brain). It has two centres. Which centre is stimulated depends on the nerve impulse they receive from two types of receptor which can respond to chemical or pressure changes within the blood:

• one increases heart rate linked to the sinoatrial note by the sympathetic nervous system
• one decreases heart rate linked to the sinoatrial node by the parasympathetic nervous system

Chemoreceptors are found in the wall of carotid arteries. They are sensitive to blood pH changes that result from CO2 concentration changes (more CO2 lowers the pH of blood as CO2 is acidic in solution):

• When the blood has a higher than usual CO2 concentration ifs pH lowers
• chemoreceptors in the walls of carotid arteries detect this and increase the frequency of nerve impulses to the centre in the medulla oblongata
• this centre increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node
• this increases the rate of production f electrical waves by the SAN which increases the heart rate
• the increased blood flow causes more CO2 to be removed by the lungs so CO2 concentration returns to normal
• pH of the blood rises to normal and the chemoreceptors reduce the frequency of the nerve impulses to the medulla oblongata and the medulla oblongata reduces the frequency of impulses to the sinoatrial node which therefore leads to a reduction in heart rate

Pressure receptors occur in the walls of carotid arteries and the aorta:

• when blood pressure is lower than normal pressure receptors transmit more nervous impulses to the medulla oblongata. The medulla oblongata increases the frequency of impulses via the sympathetic nervous system to the SAN which increases the rate at which the heart beats
• when blood pressure is higher than normal pressure receptors transmit more nervous impulses to the medulla oblongata. The medulla oblongata increases the frequency of impulses via the parasympathetic nervous system to the SAN which decreases the rate at which the heart beats

You can calculate cardiac output using the following equation...

cardiac output = heart rate x stroke volume