Sensitivity and coordination

1. Sensitivity and coordination Organisms detect changes around them

2. 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.

3. 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)

4. 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

5. 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.

6. 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.

8. 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.

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

11. 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.

12. 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

13. 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.

15. 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