sensitivity

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In biology, the ability of an organism, or part of an organism, to detect changes in the environment. All living things are capable of some sensitivity, and any change detected by an organism is called a stimulus. Plant response to stimuli (for example, light, heat, moisture) is by directional growth (tropism). In animals, the body cells that detect the stimuli are called receptors, and these are often contained within a sense organ. For example, the eye is a sense organ, within which the retina contains rod and cone cells which are receptors. The part of the body that responds to a stimulus, such as a muscle, is called an effector, and the communication of stimuli from receptors to effectors is termed ‘coordination’; messages are passed from receptors to effectors either via the nerves or by means of chemicals called hormones. Rapid communication and response to stimuli, such as light, sound, and scent, can be essential to an animal's well-being and survival, and evolution has led to the development of highly complex mechanisms for this purpose.

Nervous systems
Most animals have a nervous system that coordinates communication between stimulus and response. Nervous systems consist of special cells called neurones (see nerve cell) which are fundamentally the same as other body cells in that each contains a nucleus, cytoplasm, and cell membrane. In addition, in order to receive and pass messages, they also have long thin fibres of cytoplasm extending out from the cell body termed ‘nerve fibres’. The longest of these, which can be more than a metre long, are called axons. The shorter fibres are called dendrites.

Nerve nets
Small animals, such as jellyfish, which do not need to coordinate complex messages between stimuli and response mechanisms, have simple nervous systems, termed ‘nerve nets’. In a nerve net, each neurone is connected by fibres to adjacent neurones, so that a message received in any one part of the nervous system is relayed from neurone to neurone throughout the whole of the organism's body.

Central nervous systems
The evolution of larger and more complex animals, such as humans, has necessitated the development of far more elaborate nervous systems, and most animals have a central nervous system (CNS). The main difference between a simple nerve net and a central nervous system is the addition of a brain and spinal cord to coordinate and relay messages between receptors and the appropriate effectors, without involving the whole body. Thus rapid responses to specific stimuli are triggered.

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