Co-ordination of Response (Edexcel A (SNAB) A Level Biology)

• 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