The Role Of The Nervous System Essay — страница 3
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behaviour can be seen in experiments with crickets and their song. The cross breeding of two species with different calls to produce hybrid calls, along with the subsequent backcrossing, shows that the neuronal network producing this behaviour is under rigid genetic control. However it is reasonable to say that in higher animals behaviours are a combination of both genetic and learned components. Hence the genetic code lays down the pattern of the nervous system on how to respond to the certain key stimuli. Responses are initiated by the stimuli, which need not be exact. Behaviour is also influenced by the nerves synapsing on glands of the endocrine system which cause changes in hormonal levels in the body, which can in turn cause mediation effects behavioural responses or cause behavioural responses themselves. Examples. Animals have evolved specialised sensory and motor capabilities that enable them to produce many varied and complex behaviours. Most can be caused to move with respect to a specific stimuli orientating. The role of the nervous system in this behaviour is the integration of sensory input and the co-ordination of motor output. Thus it depends on the properties of the sensory receptor neurons, the connections within the central nervous system, and the muscles which cause the body of the animal to move. A taxis is a movement that is directed with respect to a stimulus. Cockroaches will show a negative phototaxis (a movement away from light) when a light stimulus is applied. Jacques Loeb (1918). Suggested that these simple taxes occur by simple asymmetrical motor activation in response to simple asymmetrical sensory input. This would suggest that light landing on one eye would cause ipsilateral motor movement, causing movement away from the light. This hypothesis is corroborated by experiments with cockroaches blinded in one eye. Hence it is via the cockroach nervous system that this behaviour is brought about light stimulus caused a signal in an efferent neuron that via a control center brings about efferent innervation of muscle fibres causing movement. Behavioural responses are not just visually based, the are determined by a whole range of internal and external key stimuli, and can be moderated by other stimuli as well. Many animal can locate their prey by vibration a spider is alerted to prey on its web by vibrations through the strands of its web. Similarly desert animals can often detect sand-born vibrations giving information on potential prey. The manner in which some of these responses occur is quite complicated. Scorpions that detect movement in the sand require sensory receptors in each of their eight legs. Central neurons that receive the sensory inputs from the receptors appear to compare timings of the impulses that they receive from each of the vibration receptors in the legs. By the integration of timing of action potentials from the different legs the central nervous system calculates the direction of the stimulus source and then produces the appropriate motor taxis allowing the scorpion to orient and respond in the direction of the source. Navigation is a more complex behaviour which allows animals to find their way over long distances. These abilities usually rely on cues which humans cannot detect and hence the animals have specialised nervous systems to allow them to react to these cues. Many animals use a mechanism called a clock-compass which utilises an internal clock, part of the nervous system which allows the animal to fairly accurately tell the time. Thus the nervous system plays an important role in navigation whether it by knowing what time it is or by detecting magnetic fields in relation to the earth and using them in conjunction with internal maps. Summary. It can be seen that the nervous system is the major component controlling the behaviour of animals. Sensory receptors both internal and external control and modify behaviour directly and indirectly through responses effected by efferent nerves synapsing on motor / other units (Fig 6). Hence the role of the nervous system in an animal s behaviour is to initiate, modify control and effect behaviours that are pertinent to the current situation and to an internal schedule. Bibliography. Randal, Burggren & French, (1997); Animal Physiology Mechanisms and Adaptations. McFarland D., (1985); Animal Behaviour. Maning, A and Dawkins, M. S., (1998); An Introduction to Animal Behaviour.