Sensitivity and coordination Organisms detect changes around them

All living organisms are sensitive to their environ­ment. This means that they can detect changes in their environments. The changes they detect are called stimuli and they respond to these stimuli in various ways which have the effect of helping them to survive. This capacity of living protoplasm to respond to stimuli is known as irritability.

Response and coordination in animals The response of many simple multi- cellular inverte­brates, such as millipedes, woodlice, insect larvae and adult ants, to stimuli, is a movement of the whole organism towards or away from the particular stimulus. ( I like to Move it, move it)

In small, simple, unicellular organisms like Amoeba there are usually no specialised structures set aside for receiving, passing on and responding to stimuli. Whole or parts of these organisms may respond in definite ways to certain stimuli. Amoeba, for example, responds to contact with food by enclosing it with the nearest pseudopodia (false feet) to form a food vacuole

Nervous system As organisms get larger and more complex, the need arises for some means of carrying sensations from one part of the organism to another, so that it can act as a unit. This is done by means of a nervous system.

Simple nervous system (nerve net) Found in the sea anemone and other members of the jellyfish or cnidarian group of animals Sense cells receive stimuli, and pass them on through special conducting cells or neurones which link up with each other to form a nerve net. Eventually the stimulus reaches muscle cells which respond by contracting.

Receptors & Effectors Cells that receive stimuli are known as receptors. Those which respond are known as effectors. The conducting cells or neurones have a number of thin fibres leading out from their cell bodies along which sensa­tions or messages can pass.

Any direction In the sea anemone, and its relatives, messages can travel in any direction along these fibres

Larger & active animals need a more efficient nervous system. Mammalian neurons contain the same basic parts as any animal cell Each has a nucleus, cytoplasm and a cell membrane. Their structure is specially adapted to be able to carry messages very quickly.

Nerve fibers They have long, thin fibers of cytoplasm stretching out from the cell body. Nerves fibers carrying impulses into the cell body are called dendrons or dendrites Usually there is one nerve fibre taking impulses away from the cell body. This is called the axon

Structure In many neurones, the axon is the longest fibre. Axons may be more than a metre long. The dendrites pick up messages from other neurones lying nearby. They pass the message to the cell body, and then along the axon.

Messages The axon might then pass it on to another neurone. The messages can pass in one direction only This helps to make the system more precise. Unlike what happens in the sea anemone