The Role Of The Nervous System Essay

, Research Paper

The role of the nervous system in behaviour.

Introduction.

The nervous system is the fundamental control system for all behaviour. It is responsible for receiving incoming information from the surroundings, collating and processing it and then causing a relevant behavioural response. For an animal to survive it must be able to respond differing internal as well as external states and parts of the body must be able to act in a co-ordinated fashion to illicit behaviours that are favourable to the animal / species. The nervous system acts together with the endocrine system to initiate co-ordinated responses via the muscles and glands.

Nerves.

The way animal behaviour is brought about depends on precisely co-ordinated performance from many different cells. The most important of these in the nervous system are neurons (Fig 1), these communicate information using a combination of electrical and chemical signalling methods. These cells are specialised for transmitting information to one another, and although they vary greatly in size and shape, neurons typically have a cell body, responsible for maintaining the metabolic functionality of the cell, which has several processes extending from it. These processes are the dendrites and the axons, and are responsible for receiving signals from other cells, and carrying them away from the soma to be transmitted to other cells respectively. Hence the neurons is the component unit of the nervous system responsible for the transmission of signals which inform, control / mediate and effect behaviour.

Neuronal Connections.

The network that underlies a particular behaviour is reliant on subcircuits which have properties that effect the way the whole network operates. Sensory filter networks transmit only certain features of complex sensory input, while blocking out other features. Central pattern-generating networks produce a pattern of motor output that consistently generates stereotyped movements. Some of these operate on a cyclic fashion, such as breathing and locomotion behaviours. Others are non cyclic and occur only when necessary, some also have a motor command system superimposed upon them which allows moment to moment changes to motor output as changes occur in sensory input.

Neurons are combined into circuits in a number of different ways. A single neuron may receive input from many presynaptic terminals both excitatory and inhibitory in nature. This neuron may also branch several times to innervate other neurons (axon collaterals). Divergence is the repeated branching of an axon giving the neuron a widespread influence allowing a single receptor to have a large effect. Convergence allows a neuron to integrate signals from many presynaptic neurons. The effect of these is that most neurons are rarely depolarised, causing a signal to be produced, without summation of several inputs. Hence a behavioural response may not occur until the stimulus for that action is quite high.

Sensory Networks.

Sensory networks are the first step in the chain evoking a behavioural response. They sort and refine the mass of information that is made available from the external / internal environment. Sensory receptors can take the form of chemoreceptors (Chemical), Photoreceptors (Light), Mechanoreceptors (Deformation of receptor cell membrane touch stretch) and also nociceptors (pain) to name a few. Sensory networks are able to completely reconfigure the incoming stimulus by magnifying, amplifying, adding or subtracting from the sensory input. Their role in the nervous system is to make sure that only the required stimuli evoke the required behavioural patterns.

Information on internal condition comes from the somesthetic system, which relays data on joint position, temperature and pressure on internal organs. These systems allow modification of behavioural responses based on the animals 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 r