HOMEOSTASIS The maintenance of a steady state in the body despite changes in the external environment The steady state is the optimum level for the body functions © 2016 Paul Billiet ODWS

A system in homeostasis needs Sensors to detect changes in the internal environment A comparator to fix the set point of the system Effectors which bring the system back to the set point Feedback control. Negative feedback stops the system over compensating (going too far) A communication system to link the different parts together. © 2016 Paul Billiet ODWS

A system in homeostasis needs Sensor Perturbation in the internal environment Return to normal internal environment Effector Comparator Sensor Negative feedback © 2016 Paul Billiet ODWS

Communication systems These should consist of the following components Linkage system Effector Response Sensor Stimulus © 2016 Paul Billiet ODWS

In animals there are two communication systems The endocrine system based upon hormones The nervous system based upon nerve impulses. © 2016 Paul Billiet ODWS

Hormones Organic substances Produced in small quantities Produced in one part of an organism (an endocrine gland) Transported by the blood system All over the body Target organs or tissues respond to the hormone. © 2016 Paul Billiet ODWS

The endocrine system Chemical signals Specific messages Effects may be very slow (e.g. growth hormone over years) Some are very fast (e.g. adrenaline which acts in seconds). © 2016 Paul Billiet ODWS

Nerve impulses All nerve impulses look the same Signals travel along nerves… …to specific parts of the body The nerve impulses travel very quickly Affect their target tissues in milliseconds. © 2016 Paul Billiet ODWS

The nervous system The nervous system is composed of excitable cells called neurons Neurons have long thin extensions which carry electrical nerve impulses The electrical signal of the nerve impulse needs to be converted into a chemical signal (a neurotransmitter) so that it can pass from nerve cell to nerve cell The neurotransmitter is destroyed after the signal has passed on. © 2016 Paul Billiet ODWS

The nervous system organisation A Central Nervous System (CNS) made of the brain and spinal cord… and peripheral nerves connecting it to sensors and effectors Stimulus Effector eg muscle or gland Central Nervous System Receptor or Sensor eg photoreceptor Sensory nerve Response Motor nerve © 2016 Paul Billiet ODWS

Hormone pathway Endocrine cell Hormone synthesis Hormone precursors Hormone stored in vesicles Stimulus acts on receptor site or directly inside cell Hormone secreted into the blood stream Chemicals stimulating the release of the hormone © 2016 Paul Billiet ODWS

Hormone pathway Hormone destroyed Hormone receptor site Metabolic effect Target cell © 2016 Paul Billiet ODWS