5.1 Communication and Homeostasis

The need for communication systems

Effector - Cell, tissue or organ that brings about a response

Homeostasis - Maintaining a constant internal environment

Negative feedback - Mechanism that reverses a change, bringing system back to optimum

Positive feedback - Mechanism that increases a change, taking system further away from optimum

Sensory receptors - Cells/ Sensory nerve endings that respond to a stimulus in the internal or external environment of organisms and create action potentials

It is essential that the body maintains; temperature, pH, aqueous environment (water levels), toxins, inhibitors

Changing external environments

The stimulus changes in the surrounding environment can cause behavioural and physiological responses. These changes can be quick (eyes responding to light intensity, predator causing an animal to hide) or they can be gradual (thickening of coats in the winter, discolouring of leaves).

Changing internal environments

Cells and tissues are protected from the external environment by epithelial tissue and organs such as skin or bark. As cells undergo various metabolic activities they use of substrates and create new products, some of which could be unwanted or toxic. These substances move out of the cells into the tissue fluid. The accumulation of excess waste or toxins in this internal environment must act as a stimulus to cause removal of these waste products so that the cells can survive. This build-up of waste products in the tissue fluid may also act directly on the cells, which respond by reducing their activities so less waste is produced.

Any waste or toxins accumulating in the tissue fluid are likely to enter the blood and be carried away

Cell signalling

In cell signalling one cell will release a chemical that is detected by another cell. The second cell will respond to a signal released by the first cell.

Messages can travel through a network of neurones that signal across synapse junctions (neuronal system). The pre-synaptic neurone will exocytose a neurotransmitter in response to an action potential. The neurotransmitter diffuses across the synapse to the post-synaptic neurone and binds to receptors, initiating the action potential in the neurone.

Messages can also travel through hormones, which are released from glands into the blood. The hormone is transported throughout the body, but is only recognised by specific target cell. The hormonal system allows longer-term responses to be coordinated.

Homeostasis

Homeostasis is used to maintain conditions inside the body, despite changes in internal and external factors.

Standard response pathway = Stimulus → receptor → communication pathway → effector → response

Sensory receptors - these may be on the surface of the body, such as temperature receptors on the skin. These monitor changes to the external environment. Other receptors are internal for example, temperature receptors in the brain. When one of these receptors detects a change it will be stimulated to send a message to an effector.

A communication system such as the neuronal system or hormonal system acts by signalling between cells. It is required to transmit a message between the receptor cells and the effector cells via a coordination centre (the brain).

Negative feedback

In order to maintain a constant internal environment, any change away from the optimum conditions must be reversed.

When conditions change, the receptors detect this stimulus and send an input to the coordination centre. The coordination centre sends an output to the effectors and the effectors respond to this output. When the system moves closer to the optimum the receptors detect this and reduce input to the coordination centre.

When a stimulus occurs it may take some time to respond ad the response may cause a slight overshoot. However as long as this variation is not too great, the conditions will remain acceptable.

Positive feedback

Positive feedback is less common than negative feedback. When it occurs the response is to increase the original change, this destabilises the system and can be harmful.

One example is seen to bring about dilation of a cervix. As the cervix begins to stretch this causes the pituitary gland to secrete oxytocin. Oxytocin increases uterine contractions which stretch the cervix more, which causes the secretion of more oxytocin. Once the cervix is fully dilated the baby can be born, the birth ends the production of oxytocin

Temperature control in ectotherms and endotherms

Ectotherm - Organism that relies on external sources of heat to maintain body temperature

Endotherm - An organism that uses heat from metabolic reactions to maintain body temperature

Hypothalamus - Area of brain that conducts homeostatic responses

As temperature rises molecules have more kinetic energy, they move about more quickly and collide more frequently. This means essential chemical reactions occur more quickly (the reverse happens in colder temperatures).

The structure of proteins can also be affected by change in temperatures. If the temperature increases to much then the tertiary structure can change shape and then the enzyme won’t be able to function

Ectotherms

Ectotherms are not able to control their body temperature as effectively, they rely on external sources of heat and therefore body temperature fluctuates with external temperature.

Too cold:

• Move to a sunny area

• Lie on warm surface

• Expose larger surface area to sun

Too hot :

• Move out of sun

• Move underground

• Reduce body surface exposed to sun

Endotherms

Can use physiological adaptations and behavioural means to control their body temperature

Many chemical reactions in the body are exergonic - release energy in form of heat so can control the rate of respiration to control body temp.

Too cold :

• Less sweat

• Hair and feathers erect to trap air

• Vasoconstriction

• Increased respiration

• Shivering

• Limit blood flow to extremities

• Lie in sun

• Remain dry

• Move around to generate heat in muscles

Too hot :

• Sweat

• Hair and feathers lie flat

• Vasodilation

• Panting

• Less respiration

• Fewer muscle contractions

• Blood near extremities

• Hide from sun

• Remain inactive and spread limbs out

• Wet skin

Temperature receptors in the hypothalamus of the brain detect changes in temperature and consequently send out impulses to cause different responses that will reverse the change. The thermoregulatory centre in the hypothalamus monitors blood temperature and detects changes in the core body temperature. Peripheral temperature receptors (in the skin monitor temperatures in the extremities) can give an early warning that body temperature may change.