Maintaining a Balance Topic 4: Temperature Regulation and the Nervous System. Biology in Focus, HSC Course Glenda Childrawi , Margaret Robson and Stephanie Hollis. DOT POINT. Outline the role of the nervous system in detecting and responding to environmental changes. Introduction.
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Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
Any change in the external environment could affect the balance in the internal environment of the organism and so a mechanism is needed to ensure homeostasis. The mechanisms that allows this to occur are based on a negative feedback system, co-ordinated by the nervous system.
The function of the nervous system is co-ordination and this takes place in three steps:
The structures of the nervous system involved in the stimulus-response pathway of coordination are:
A stimulus is detected by a receptor, a message is carried by nerves to a control centre and a response is triggered. For example, if you touch a hot stove with your finger, receptors in your skin detect the heat and pain, and the result is that you withdraw your hand rapidly.
This rapid reaction requires a link between the receptors that detect the stimulus and the effectors, the muscles that carry out a response. The co-ordination is carried out by the nerves and the CNS of the body. The response usually counteracts the stimulus (change), reducing its
effect so that a balance is
maintained. This is a
negative feedback system.
Sensory cells called receptors detect stimuli (changes in the internal or external environment). In their most simple form, receptors consist of single cells, scattered over the body of an organism. In their more complex form, they are concentrated in particular areas to form sense organs.
In many animals, receptors in sense organs detect stimuli in the external environment. However there are also receptors that are sensitive to internal stimuli within the body. These are called interoreceptors and are important in detecting changes such as pH, body temperature, osmotic pressure and the chemical composition of blood related to homeostasis.
Receptors may be named according to the type of energy or molecules they detect. Those receptors important in our study of homeostasis are thermoreceptors and chemoreceptors.
The brain and spinal cord make up the central nervous system (CNS). The peripheral nervous system (PNS) consists of nerves, which carry information to and from the CNS.
The information carried by nerves is ‘messages’ transmitted in the form of electrochemical impulses. Incoming information passes from sensory receptors via sensory nerves to the CNS, which in turn transmits outgoing information to effector organs via motor nerves.
The role of the CNS is to process incoming information, analyse it and then initiate an appropriate response. Within the CNS, information is processed and analysed by a number of interconnecting nerve cells (neurons) and then a message is generated and transmitted, stimulating the effector organs.
Some actions involving the nervous system may take place voluntarily, but all those involved in homeostasis take place without any conscious thought. They are involuntary and many are inborn, unconditioned reflexed in response to a particular stimulus.
A response is a reaction in an organism or its tissues, as a result of receiving a stimulus. It is carried out by structures in the body known as effector organs. These are often muscles and/or glands. The response reaches the effectors from the CNS and causes the body to correct any deviation from the normal balanced state, thereby maintaining homeostasis.
Temperature is extremely important in maintaining homeostasis in living organisms. It’s the role of the nervous system to regulate body temperature. This is called thermoregulation. The body gains heat in many ways. For example:
The body also loses heat in many ways. For example:
Thermoreceptors are present both outside and inside the body. Peripheral receptors are located in the skin and central receptors monitor the temperatures of the blood as is circulates throughout the brain.
The central receptors are present in the hypothalamus of the brain and are sensitive to extremely small temperature changes (a fraction of a degree). Its also the control centre for temperature regulation in the mammalian body so the receptors don’t have to transmit the information very far to elicit a response.
The anterior hypothalamus has a heat-loss centre, which sends messages to effectors t cool the body down.
The posterior hypothalamus has a heat-gain centre, which initiates responses that help the body warm up.
The main homeostatic organ involved in temperature regulation in humans is the skin. If the body becomes to cold, the heat-gain centre of the hypothalamus stimulates responses in the effector organs to generate and/or retain heat within the body. On a cold day we get goose bumps on our skin, become pale and shiver.
Raised hairs on the body (goose bumps) are an attempt to trap a layer of warm air around the body to reduce the amount of heat lost by radiation, convection and conduction. The hypothalamus stimulates the erector muscles in the skin to contract, raising the hairs. This is more effective at trapping heat where the hair is thicker, for example on our heads.
Vasoconstriction is the narrowing of the arterioles to the skin. People who are very cold tend to appear pale-faced, with blue-tinged lips, fingers and toes due to ‘poor circulation’. This restricts blood flow to the skin and prevents heat carried by the blood throughout the body to be lost from the body's surface.
Shivering is brought about by rapid small muscle contractions, which generate heat in the body. The body can also speed up metabolism which generates some heat.
If the body becomes too hot, we become red, sweaty and sluggish. These are signs that our heat-loss mechanism has been activated to cool the body. The heat-loss centre of the hypothalamus stimulates the effector organs to lose heat.
Vasodilation: dilation (expansion) of the arterioles to the skin. Blood carrying heat is directed towards the surface of the body so that heat can be lost by conduction, convection and radiation to the surroundings.
Sweating: Sweat glands, the main heat-loss structures in the body, are activated by the heat loss centre in the hypothalamus. Liquid sweat is secreted through the sweat pores onto the surface of the skin and heat is removed from the body to evaporate the liquid.
Animals that do not have sweat glands still lose heat by evaporation. For example, dogs pant, rodents and kangaroos lick their bodies so that the saliva evaporates and cools them down. Plants also cool themselves this way, its called transpiration.
And finally, decreased metabolism can reduce heat. The heat-loss centre causes the thyroid gland to lower the rate of metabolism, generating less heat. This is why we feel tired and lethargic on hot days.
-Students are to complete DOT Point 1.7 and 1.8