Homeostasis and Control Systems

Question 1

What are there large fluctuations in in the external environment?

Answer 1

• Temperature
• Diet
• Availability of water

Question 2

What are there small fluctuations in in the internal environment?

Answer 2

• temperature
• pH, [O2], [CO2], [glucose]
• blood pressure

Question 3

What is homeostasis?

Answer 3

The maintenance of a constant internal environment. Homeostasis aims to maintain a controlled variable at it’s set point

Question 4

What is a ‘controlled variable’ in homeostasis?

Answer 4

some aspect of the internal environment. E.g. blood pressure

Question 5

What is the ‘set point’

Answer 5

the ideal value at which the variable should be maintained

Question 6

How does a negative feedback loop work?

Answer 6

Change in variable -> detected by sensory (sensory receptor cells) which send signal through nervous pathway or hormone release -> processed by integrating centre (CNS, endocrine gland) which sends signal to -> effector (e.g. muscle or secretory cells)

Question 7

With what variable would we regulate through different set points?

Answer 7

Temperature:

• when we are active our body temperature is around 37.2 degrees
• at night our temperature is around 36.5 degrees.

Question 8

What is positive feedback and give an example?

Answer 8

When the initial stimulus causes a response which reinforces itself.

E.g. in clotting there is damage to blood vessel which leads to the activation of clotting factors which leads to the activation of thrombin and then thrombin leads to the activation of more clotting factors

Question 9

Why is positive feedback limited?

Answer 9

Because it can get out of control

Question 10

What are some examples of positive feedback

Answer 10

• action potential in nerves
• ovulation
• blood clotting

Question 11

What are the two types of control systems and their different effects?

Answer 11

• Nervous system: rapid short term effects (seconds)

- Endocrine system: Long term effects (minutes to days)

Question 12

what are the two branches of the nervous system and what are they made up of?

Answer 12

• Somatic nerves: skeletal muscle

- autonomic nerves: sympathetic and parasympathetic nerves

Question 13

What do both sympathetic and parasympathetic nerves affect?

Answer 13

Smooth and cardiac muscle and glands

Question 14

What is the basic principle in both the sympathetic and parasympathetic system?

Answer 14

The preganglionic neuron sends signals to the postganglionic neuron which sends signals to the target cell which triggers a response

Question 15

What are the preganglionic and postganglionic fibres like in the sympathetic nervous system

Answer 15

• preganglionic neurones come out of the spinal cord at the thoracic level but nothing comes out at the cervical
• preganglionic neurones come out of the lumbar part but not the sacral of the lower spine
• relatively short preganglionic fibres
• relatively long post-ganglionic fibres

Question 16

What are the preganglionic and postganglionic fibres like in the parasympathetic nervous system

Answer 16

• Preganglionic neurones come out of the cranial part of the spine but not the cervical or thorax
• preganglionic neurones come out of the sacral part of the lower spine but not the lumbar part
• relatively long preganglionic fibres
• relatively short post-ganglionic fibres

Question 17

How does the sympathetic nervous system work in regards to hormone release?

Answer 17

• releases neurotransmitter acetylcholine at the synapse between the pre and postganglionic fibres. This activates the N2 (nictic) receptor on the postganglionic fibre. Then norepinephrine/ noradrenaline is released which activates alpha and beta adrenergic receptors on the target organ.
• involved in the activation of adrenal gland
• adrenal medulla receives preganglionic sympathetic fibres into the internal adrenal medulla which releases acetylcholine that stimulates N2 receptors on Chromaffin cells and these chromaffin cells release Epinephrine/adrenaline into the circulation.
• Adrenaline has a very similar affect to noradrenaline

Question 18

How does the parasympathetic nervous system work in regards to hormone release?

Answer 18

• Releases neurotransmitter acetylcholine at the synapse between the pre and postganglionic fibres. The acetylcholine binds to the N2 receptor on the post ganglionic fibre which then releases acetylcholine which released acetylcholine M (muscarinic acetylcholine) receptor on the target organ

Question 19

what does the autonomic nervous system do?

Answer 19

• both sympathetic and parasympathetic divisions innervate most organs and tissues
• often have antagonistic effects
• sympathetic more active in stressful situations (fight or flight)
• parasympathetic more active in vegetative situations
• but we can’t generalise

Question 20

What are examples of negative and positive hormonal feedback systems?

Answer 20

• Hypothalamus released in the early morning CRH -> anterior pituitary releases ACTH -> adrenal cortex releases Cortisol (gives energy to start the day)
• As cortisol levels rise there is a negative feedback process to the hypothalamus leading to less release of CRH
• Suckling -> oxytocin -> milk
• Positive feedback loop: as infant continues to suckle more oxytocin is produces so more milk is ejected. Feedback loop broken when the infant stops suckling

Question 21

What is autoregulation?

Answer 21

A process that occurs when a cell, tissue, organ or organ system adjusts in response to some environmental change. E.g. when the oxygen levels decrease in a tissue the cells release chemicals that widen

Question 22

What is extrinsic regulation?

Answer 22

a process that results from the activities of the nervous or endocrine systems. These organ systems detect an environmental change and send and electric signal or chemical messenger to control or adjust the activities of another or many other systems simultaneously

Question 23

What does a homeostatic regulation mechanism consist of?

Answer 23

A receptor, a control centre and an effector

Question 24

What is thermoregulation?

Answer 24

The control of body temperature

Question 25

What is the control centre in the homeostatic control of body temperature?

Answer 25

The hypothalamus

Question 26

in thermoregulation what receptors does the hypothalamus receive information from?

Answer 26

two temperature receptors. One in the skin and the other within the hypothalamus

Question 27

What happens if the body temperature rises above 37 degrees?

Answer 27

The control centre targets two effectors:

1. the muscle tissue lining the walls of the blood vessels supplying blood to the skin. The muscle tissue relaxes to blood vessels dilate increasing blood flow through the vessels near the body surface
2. the sweat glands. They speed up the secretion of sweat
   - the skin then loses heat to the environment and the evaporation of sweat speeds up the process
   - as the body temperature returns to normal temperature in the hypothalamus decreases and the thermoregulatory centre becomes less active. Blood flow to the skin and sweat gland receptors return to previous levels
   - this is a negative feedback loop.

Question 28

What are our homeostatic set points determined by?

Answer 28

Genetic factors, age, gender, general health and environmental conditions.