2 1 6 Outline The Role Of The Nervous System In Detecting An

The nervous centre consists of the brain, spinal cord and all the nerves and nerve endings. The parts of the nervous system which are used in the stimulus-response model are the: receptors (sensory cells, such as olfactory cells in the nose), control centre (the central nervous system), effectors (muscles and glands) and nerves (link all the other parts and rely the messages in the form of electrochemical messages)

Stimulus-Response Pathway

Stimulus  receptor  control centre  effectors  response

The role of the nervous system in homeostasis is co-ordination. In homeostasis the response usually counteracts the stimulus, thus maintaining a balance – this is known as the negative feedback mechanism

Detecting change: sensory cells called receptors detect changes in the internal or external environment (stimuli). They generally consist of single cells scattered over the body of an organism, however in a more complex form thy can combine to form a sensory organ such as the eye, ear and tongue. Most receptors detects changes in the external environment, however interoreceptors detect changes in the internal environment (such as pH, body temperature and chemical composition). Receptors are named according to the energy or molecule they detect, examples include thermorecpetors and chemoreceptors.

Co-ordination: the central nervous system (CNS) consists of the brain and spinal cord, the peripheral nervous system consists of nerves which send information to and from the CNS. The information is carried in “messages” which are electrochemical nerve impulses. Information goes from the sensory receptors to the CNS via sensory nerves and information from the CNS to the effector organs pass through motor nerves. The role of the CNS is to process this information, analyse it and then initiate an appropriate response
Counteracting change: a response is a reaction in an organism or its tissues, as a result of receiving a stimulus. This is carried out by body structures known as effector organs – often these are glands or muscles. The response causes the body to correct any deviation from the normal balanced state, hence maintaining homeostasis.

Thermoregulation in humans

Heat gain may arise in a body from: metabolism (the oxidation process of respiration releases heat energy), muscle contractions (a large proportion of energy needed for muscle activity is converted into heat during muscle functioning), consumption of hot foods and drinks and radiant energy (external sources such as the sun)

Heat loss in a body results from: radiation (heat goes from the body into the cooler surroundings), convection (air currents surrounding the body replaces the warm air with cooler air), evaporation (heat is required to convert liquid perspiration into water vapor)

Detecting the change: thermoreceptors are present both inside and outside the body. Peripheral receptors are located in the skin and central thermoreceptors monitor the temperature of the blood as it circulates in the brain. Central receptors are present in the hypothalamus and are very sensitive to small temperature changes (fraction of a degree)

Coordination: the hypothalamus is the control centre for temperature regulation. The anterior hypothalamus controls heat loss and the interior hypothalamus controls heat gain.

Counteracting change: the main homeostatic organ involved is the skin. Some of the effectors are blood vessels, sweat glands, hair erector muscles and muscles of the body. The thyroid gland, which affects overall metabolic rate is also an effector

Warming the body: raised hairs on the body – attempts to trap a layer of warm air around the skin reducing heat loss through convection, radiation and conduction. Vasoconstriction (narrowing of the arterioles to the skin) this gives people their pale appearance as heat is lost in transporting blood around the blood. By narrowing the arterioles heat loss through the body surface, muscle walls and small blood vessels is reduced. Shivering is rapid contractions of muscles, which in turn generates heat. Increased metabolism by increasing the activity of the thyroid gland, metabolism is increased which produces heat.

Cooling the body: vasodilation (expansion of the arterioles) blood carrying heat is directed to the body surface so that excess heat can be lost through convection, conduction and radiation. Sweating liquid sweat is removed from the body through sweat glands and heat is removed from the body as it is used in evaporating the sweat away from the surface of the body which cools the body. Decreased metabolism reducing the activity of the thyroid gland, reduces the metabolic rates and hence the heat produced by these processes.