The Role Of The Nervous System Essay — страница 2

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current status e.g. initiation of foraging behaviour if hungry. They are also involved in other behaviours which require specialised senses e.g. photoreception or sensitivity to magnetic fields which are used in orientation / navigation behaviours. Motor Networks. The sensory portion of the nervous system obtains and analyses raw data from the outside world, this allows the animal to respond in a manner pertaining to its current circumstances. The motor networks allow varying amount of flexibility allowing the animal to shape responses relative to information acquired about its surroundings. The pattern of walking, for instance, cane be modified to sensory feedbacks varying with features of the terrain. Control can also be exerted by higher centres which can modify the output.

Reflexes. Fig 2 A Simple Reflex. .- Randall, Burggren & French Animal Physiology. The simplest role the nervous system plays in behaviour is via reflexes. The most basic mechanism that controls the activity of on animal is a simple reflex, which can consist of as few as two or three neurons (Fig 3.). An afferent neuron carries a signal from a sensory receptor, in response to either an internal or external stimuli, to a co-ordinating unit (usually the spinal cord in vertebrates) which is separate to the brain. From this point an efferent nerve carries a signal to an effector organ which carries out an action in response to the stimulus. These reflexes hence control behaviour at a basic level. An example of this is nociception, whereby if an animal e.g. a frog was to feel pain,

for example in it s foot, then pain receptors carry a signal to the spinal cord where a response is sent via an efferent nerve to move the foot (Fig 4). The information is also relayed to the higher centres of the brain for further processing, where for instance behavioural modifications can be made. Fig 3 A Pain Reflex in a Frog Leg. – Randall, Burggren & French Animal Physiology. Invertebrates. Fig 4. Diagram of Cockroach Nervous System.- Randall, Burggren & French Animal Physiology Animals who lack a spine have a nervous system based upon the same kind of set-up. A nervous system can be defined as an organised constellation of nerve cells and associated non-nervous cells (Bullock, 1977). Hence although some animals do not posses a true nervous system they undoubtedly

have sensory capabilities. Nerve cells are organised in very simple to more complicated nerve nets which allow diffusion of information. The receptor cells can also be quite complicated, being receptive to touch, movement (wind Fig 4), temperature and chemical composition of water as in flatworms such as Planaria. The rate of transfer of information is slower but it still plays a role in the behaviour of the animal albeit at a lesser level. Learned responses to light in some flatworms has also been claimed but the results of this have proved to be controversial. Larger invertebrates have more complex nervous systems, especially Arthropods. Arthropods have evolved a greater variety of types of receptor than any other group, including vertebrates (Bullock, 1977). The sensory

neurons of these receptors have their cell bodies close to the sensory surface and not grouped into sensory ganglia. Some have many sensory neurons, some have only a few. More Complex Behaviours. Behaviours can be simplified to the responses that an animal should do when faced with a particular situation. 1973 Nobel Prize winners Konrad Lorenz and Niko Tinbergen broke down the responses in animal behaviour in to two main concepts: - 1. Fixed action patterns. The unit motor patterns. 2. Key Stimuli. The corresponding unit sensory elements Fixed action patterns have six key properties which help define them as definite behavioural responses: - 1. Relatively complex motor acts are involved, consisting of specific temporal sequences of components that are not less complicated

reflexes. 2. Typically activated by Key Stimuli as opposed to general stimuli. 3. Normally elicited by an environmental stimuli, but if this is removed before completion of response, it will usually continue to completion. 4. The stimulus required to initiate a fixed action pattern varies with the state of the animal. 5. When presented with the stimulus all similar members of a species should reliably elicit the same response. 6. Fixed action patterns should be demonstrated even by animals that have not experienced the stimulus previously, thus compounding the genetic nature of these behaviours (Although they can be modified with experience). Table Source: Randall, Burgenn and French. (1997); Animal Physiology, Mechanisms and Adaptations. Evidence for the genetic control of