• Organisms must?respond to changes?in their?external?and?internal?environments in order to survive
  • Organisms need to
    • Find favourable external conditions e.g. avoiding locations that are too hot or cold
    • Find food
    • Avoid harm e.g. from predators or high blood glucose
• Changes in the environment, or?stimuli?(singular?stimulus) are detected by specialised?receptor cells
  • Receptor cells are located in the?sense organs?e.g. the nose and eyes
  • Receptor cells can also be found inside the body e.g. pressure receptors in the blood vessels
• Receptor cells send?signals?via either the?nervous system?or the?hormonal system?to the body's co-ordination centres in the?brain?or?spinal cord
• Signals are then sent on to the parts of the body which respond, known as the?effectors
  • Effectors can be either?muscles?or?glands?e.g.
    • An arm muscle would respond to a hot surface by contracting to move the hand away
    • The pancreas responds to high blood sugar by secreting insulin

Receptors are cells that detect stimuli in the internal and external environment

The nervous system

• The human nervous system consists of
  • Central nervous system?(CNS) – the?brain?and?spinal cord
  • Peripheral nervous system?(PNS) – all of the?nerves?in the body
• The nervous system allows detection of stimuli in our surroundings and the?coordination?of the body's?responses?to the stimuli
• Information is sent through the nervous system in the form of?electrical impulses?that pass along?nerve cells?known as?neurones
  • A?bundle of neurones?is known as a?nerve
  • There are different types of neurones?including?sensory neurones, relay neurones,?and?motor neurones
• The nerves connect the receptors in the sense organs with the CNS, and connect the CNS with effectors
  • The?CNS?acts as a?central coordinating centre?for the impulses that come in from, and are sent out to, any part of the body
• Nerve impulses pass through the nervous system along the following pathway

stimulus → receptor → sensory neurone → CNS →motor neurone →effector

• An example of this nerve pathway in action might be

hot surface → pain receptor in skin of hand → sensory neurone →CNS → motor neurone →?arm muscle

• • The muscle in the arm responds by contracting to move the hand away from the hot surface

The nervous system allows the detection of stimuli and the co-ordination of appropriate responses

The hormonal system

• Hormones are?chemical substances?produced by?endocrine glands?and carried by the?blood
  • Hormones are sometimes known as?chemical messengers
• Hormones?transmit information from one part of an organism to another?and bring about?change?by altering the?activity?of one or more?specific?target organs
  • Hormones can leave the blood and bind to?specific receptors?on the cell surface membranes of target organs
• Hormones are slower in action than nerve impulses and are therefore used to control functions that?do not need instant responses
• Endocrine glands that produces hormones in animals are known collectively as the?endocrine system
  • Endocrine glands secrete hormones directly into the?blood
  • Endocrine glands can be stimulated to secrete hormones by the action of?another hormone?or by the arrival of a?nerve impulse
• The pathway of hormone action is as follows

stimulus → receptor → hormone →effector

• An example of this pathway in action might be

high blood sugar → cells in the pancreas → insulin →?liver cells

• • The liver cells respond to insulin by converting glucose into glycogen

Hormones are secreted into the blood by the endocrine glands

Comparison of Nervous and Hormonal control Table