A basic control system

The basic elements of a control system are summarised in a block diagram (Fig. 13.1). The variable factor to be controlled must be measured by some sensor – in biological terms, a receptor. Some receptors monitor the variable directly, others indirectly. Thus the carotid body chemoreceptors assess directly the partial pressure of oxygen dissolved in the blood. The chemoreceptors of the medulla oblongata measure blood carbon dioxide, but they do this by measuring the hydrogen ion concentration in the cerebrospinal fluid, a related variable.

The information must be fed back to some controlling centre along an input pathway, usually a nerve, although in some instances this is not necessary because the sensor and the controlling centre are the same cell.

The controlling centre, sometimes called the comparator, is usually in the brain or spinal cord. Here the information from the sensor or sensors is compared with some internally set level and any departure from the set level causes appropriate instructions to be sent out. Sometimes the comparison of observed values with the set values may occur at the sensors themselves. A room thermostat provides one example, since the required temperature is set in a device which is the actual temperature sensing device and is also the control switch for the heating system. The hypothalamus acts both as a sensor and a controller for the osmotic activity of the extracellular fluid. The term controlling centre is therefore preferable for the area in which incoming information is translated into outgoing instructions and the term comparator reserved, if necessary, for the particular structures where the comparison takes place, whether in the control centre or elsewhere.

The output pathway may be via nerves or via hormonal messengers in the bloodstream. It may be so short as to have no separate identity, or it may be a clearly defined pathway of considerable length.

The effector organ is usually muscle or a gland, capable of changing the controlled variable.

These components make up the simplest possible feedback circuit. The circuit may be complicated by the duplication or multiplication of its components – several sensors, several input pathways, hierarchies of control centres, multiple output pathways, several effector organs. In the control of blood pressure, for example, the effector may be the heart or the blood vessels. In the control of rising body temperature, blood vessels may dilate and sweat glands may secrete.

Further complications arise in the possibility of control of the controlling centre itself, either by alteration of its comparator function or by directly altering its activity.

Direct and indirect sensors

Two kinds of sensor are distinguished – misalignment detectors and disturbance detectors. Misalignment detectors monitor the controlled variable either directly or indirectly, disturbance detectors respond to changes likely to affect the controlled variable. A room thermostat is a misalignment detector, an outdoor thermometer a disturbance detector. Disturbance detectors give early warning of an impending change in the controlled variable. Physiologically, skin temperature receptors are disturbance detectors, whilst hypothalamic deep temperature receptors are misalignment detectors. An example of the interaction of the two types of receptor can be seen in the control of blood osmolality. Osmoreceptors in the hypothalamus act as misalignment detectors to provoke a sensation of thirst which can be abolished by drinking. Intake of water will not immediately affect blood osmolality; the oral and pharyngeal receptors can function as disturbance detectors to cause suppression of the osmoreceptor signals so that the body is not overhydrated by continuing to drink beyond the necessary amount.

Variation of the set level

The set level of a feedback system in the body may be altered in a number of ways. For example, in exercise the temperature setting of the hypothalamic control centre is set at a much higher level.

Activity of the limbic brain may alter the setting of the blood pressure controlling system in the medulla oblongata by means of impulses passing through the hypothalamus. Stiffening of the arterial walls with increasing age reduces the sensitivity of the carotid sinus stretch receptors: as a result blood pressure is maintained at a higher level in the old than in the young.

The concentration of cortisol in the blood is monitored by the hypothalamus and, as the concentration falls, the hypothalamus secretes corticotrophin releasing hormone (CRH) into the portal bloodstream: when this reaches the anterior pituitary gland the secretion of adrenocorticotrophic hormone is stimulated. This in turn stimulates the adrenal cortex to secrete cortisol. The sensitivity of the hypothalamus to cortisol is reduced by stress: and so higher concentrations of cortisol are maintained in the bloodstream.

Normally the regular alternation of breathing, inspiration and expiration is controlled by the respiratory centres in the medulla oblongata but this control can be voluntarily over-ridden during breath-holding, or during speech.

Mathematical analysis of control systems