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.
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.
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 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.
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.
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.
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.
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 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.
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.
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.
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.