Homeostasis and Homeodynamism

Question 1

Internal environment was first coined by _______________________ in 1878 as “milleu interieure”. It is distinct from external environment and specific for a particular organism and for a particular level e.g. cell, organ, whole body.

Answer 1

Claude Bernard

Question 2

At the whole body level, state the internal and external environment.

Answer 2

Internal: temperature, fluid, electrolytes, oxygen
External: atmosphere, temperature and radiation

Question 3

Internal environment is relatively constant (has a range of normality). What benefit does the internal environment have especially in regard to choice of external environment?

Answer 3

It gives the organism a greater versatility and freedom of choice over external environment.

Question 4

Homeostasis was first coined by Walter Cannon in 1929. Define homeostasis.

Answer 4

It is the maintenance of internal environment within a narrow range of parameters.

Question 5

About 30-40 years later after homeostasis was first coined, the word “homeodynamism” came about. What does it imply?

Answer 5

It implies that the mechanisms and the maintenance of the internal environment are dynamic/in action rather than static.

Question 6

What is the necessity of the vast number of control systems?

Answer 6

To maintain the internal environment within its physiological range, therefore keeping the body operating in health. In the absence of any one of these controls, serious body malfunction or death can result.

Question 7

The assembly of processes that interact and result in a change in a measured quantity or variable is referred to as _________________.

Answer 7

system

Question 8

List the 2 main types of control systems.

Answer 8

☑ open loop
☑ closed loop

Question 9

How does an open loop system work? Examples of open loop systems?

Answer 9

➤ Control system is not dependent on the output
➤ No controlled variable (CV) measurement
➤ Value of CV based on a desired value, not on an actual value
➤ This is in non-vital situations e.g. hair growth, nail growth etc.

Further notes:
A simple reflex reaction, like the knee-jerk reflex, is an open loop system. When the tendon below the kneecap is tapped, sensory neurons transmit signals to the spinal cord, which immediately sends signals back to the muscles of the thigh to contract, causing the leg to be extended. This response occurs without any feedback to the brain to modify the action.
Open loop systems are generally simpler and faster than closed loop systems, as they do not require the extra step of feedback for the response. However, they are less precise because they cannot adjust for changes in conditions or errors in the response.

Question 10

Briefly describe closed loop system.

Answer 10

☑ Control system is dependent on the output
☑ Controlled variable measured
☑ Controlled variable based on actual values

Question 11

Closed loop systems can be further classified into 4 systems. Name them.

Answer 11

☑ Feed-back systems
☑ Feed-forward systems
☑ Adaptive control systems
☑ Combination of the above

Question 12

Feed-back systems are divided into 2; negative and positive feed-back systems. What is negative feed-back system? Give an example.

Answer 12

A system where the response of the system is in an opposite direction to the change in controlled variable level (response is negative to the initiating stimulus).
Therefore,
✓ if controlled variable rises above set-point, response is a lowering
✓ if controlled variable falls below set-point, response is a rise

A classic example of a negative feedback system is the regulation of blood sugar levels. Here’s how it works:
(1) Increase in blood sugar: After a meal, blood sugar levels rise, increasing the concentration of glucose in the bloodstream.
(2) Insulin release: In response to the high glucose levels, the pancreas releases insulin, a hormone that signals cells to absorb glucose from the blood.
(3) Glucose uptake: muscles and liver cells respond to insluin by taking up glucose. Liver cells can store excess glucose as glycogen.
(4) Normalization of blood sugar: as cells absorb glucose, the concentration of glucose in the blood decreases.
(5) Feedback to pancreas: with the decrease in blood glucose levels, the pancreas reduces the secretion of insulin.
(6) Restoration of balance: the reduction in insulin helps prevent a further decrease in blood glucose levels, maintaining the balance within a normal range.

Question 13

State the advantages and disadvantages of negative feedback.

Answer 13

Advantages:
📝 simple
📝 deals with one variable
Disadvantages:
📝 allows error to occur before correcting
📝 may result in fluctuation
📝 slow, as it awaits the consequences of disturbances

Question 14

The degree of effectiveness with which a control system maintains constant conditions is determined by the ____(a)____ of the negative feedback system.
(b) How is (a) of a control system calculated, and what can be inferred when those of physiological control systems are measured?

Answer 14

(a) gain
(b) Gain = Correction ÷ Error
The greater the gain, the more effective the control system.

Question 15

What happens in positive feedback?

Answer 15

☑ There is gradual build up of the controlled variable (CV).
☑ As CV value rises, it has to be cut-off, when the desired effect has taken place.
☑ Usually positive feed-back occurs in transient situations.

Question 16

State the advantages and disadvantages of positive feedback.

Answer 16

Advantage:
☑ allows an exponential build-up of CV to take place

Disadvantage:
☑ if there is no cut-off, then a ‘vicious cycle’ may develop causing damage to the system

NB: a mild degree of positive feedback can be overcome by the negative feedback control mechanisms of the body, and the vicious cycle then fails to develop.

Question 17

Give examples of situations where positive feedback can sometimes be useful.

Answer 17

☑ Blood clotting

☑ Childbirth
(1) Initiation: When labor begins, the fetus’s head presses against the cervix.
(2) Nerve stimulation: This pressure stimulates nerve endings in the cervix.
(3) Oxytocin release: The nerve signals reach the brain, which then prompts the pituitary gland to release the hormone oxytocin.
(4) Uterine contractions: Oxytocin travels through the bloostream to the uterus, causing it to contract more forcefully.
(5) Increased pressure: As the uterus contracts, it increases the pressure on the cervix, further stretching it.
(6) Enhanced response: This stretching intensifies the nerve signals, leading to even more oxytocin release and stronger uterine contractions.

☑ Generation of nerve signals: Stimulation of the membrane of a nerve fiber causes slight leakage of sodium ions through sodium channels in the nerve membrane to the fiber’s interior. The sodium ions entering the fiber then change the membrane potential, which, in turn, causes more opening of channels, more change of potential, still more opening of channels, and so forth until the nerve signals reaches the end of the fiber.

Further notes:
In each case in which positive feedback is useful, the positive feedback is part of an overall negative feedback process. For example, in the case of blood clotting, the positive feedback clotting process is a negative feedback process for the maintenance of normal blood volume. Also, the positive feedback that causes nerve signals allows the nerves to participate in thousands of negative feedback nervous control systems.

Question 18

What does feedforward systems do?

Answer 18

Unlike feedback system that react to changes, feedforward systems anticipate changes by using a model of the system to predict the output. This allows the system to adjust its behavior before an error occurs. Therefore it prevents change in the controlled variable and compensates for expected controlled variable.

Example 1: Some body movements occur so rapidly that there’s not enough time for signals to go from sensory receptors to the brain and then back to the effectors (in this case muscles). Hence the brain will use feedforward to cause required muscle contractions. Then later when the sensory signals reach the brain, they’ll inform it on whether the movement is performed correctly. If not, it corrects the feedforward signals and send the right ones next time when similar contractions are performed.

Example 2: Preadaptation for exercise
(1) Anticipation of exercise: before physical activity begins, the brain anticipates the need for increased muscle activity and blood flow
(2) Muscle adjustment: postural muscles adjust their tone in preparation for the expected exercise.
(3) Vascular response: The vascular system also prepares by dilating blood vessels to increase blood flow to muscles, readying the body for increased oxygen and nutrient demands.

Question 19

State the advantages and disadvantages of feedforward.

Answer 19

Advantages:
☑ fast
☑ anticipates the error
☑ prevents fluctuation

Disadvantages:
☑ complicated
☑ if there are multiple influences, it becomes more difficult to predict

NOTE:
Advantages and disadvantages of feedforward and negative feedback are complementary and combination of the two are very effective control systems.

Question 20

Briefly state what adaptive control systems are.

Answer 20

These systems change its characteristics in response to demand of the controlled variable. It “learns” from previous experience.

Question 21

State the various levels of control.

Answer 21

☑ Sub-cellular control
☑ Cellular control: electrolyte level, glucose level
☑ Organ control: pH, secretions
☑ Organ system control: blood pressure, respiratory rate
☑ Whole body control: temperature, fluid

Question 22

Give an example of an adaptive control system.

Answer 22

Tanning of skin when exposed to sunlight for long periods
(1) Exposure to sunlight: skin cells are exposed to UV radiation from the sun.
(2) Melanin production: in response, melanocytes in the skin increase the production of melanin, a pigment that absorbs UV radiation.
(3) Tanning effect: The increased melanin leads to a darker coloration of the skin, known as tanning.
(4) Protection: the tan serves to protect deeper layers of skin from UV damage by absorbing and dissipating the radiation.

Muscle growth
(1) Mechanical stimulus: When muscles are subjected to resistance training or mechanical overload, it creates micro-trears in the muscle fibres.
(2) Molecular signaling: The damage activates molecular signaling pathways which promote protein synthesis and inhibit protein degradation.
(3) Muscle repair and growth: The balance between protein synthesis and degradation shifts towards synthesis, leading to the repair of damaged fibers and the addition of new muscle proteins, which increases the size of the muscle fibres.