Nervous system

• Homeostasis


  The regulation of a cell or organism to maintain stable optimum conditions in response to internal and external changes
• Homeostatic mechanisms are so important for your survival that they are automatic
• Two main systems used to maintain a constant internal environment

  • Nervous responses
  • Hormonal responses
• Nervous responses

  Use nerves to transfer electrical signals from the receptors to the brain and spinal cord (co-ordinators) and then to effectors

  
• Hormonal responses


  Use hormones which are released from various organs and travel through the blood stream to various effectors
• Humans, like all mammals need to maintain a constant internal environment
• If core body temperature, blood glucose levels, or water levels change too much we can get very ill or even die
• High temperature or extreme pH can denature enzymes - break bonds causing their active sit to change shape
• Enzymes are the main reason for homeostasis
• Receptors


  • Detect a stimulus and send the information to coordinators where the information is processed
  • Coordinators do not actually bring about the change themselves – effectors do

  
• Effectors


  Any part of the body that produces a response
• Muscles

  Cause movement (through contraction)
• Glands


  Release hormones which bring about changes in the body
• The central nervous system is made up of the brain and spinal cord

  
• Nerves


  Branch out from the spinal cord to all parts of your body and are made of bundles of neurones (nerve cells)
• Receptors


  Cells which detect stimuli (changes in the environment)
• Sense organs and their stimuli


  • Sight - light sensitive cells in the retina of the eye, which react to light intensity
  • Smell - cells lining the inner surface in the nasal cavity, which react in the presence of chemicals
  • Taste - taste buds in the tongue and first part of the oesophagus which react in the presence of chemicals
  • Touch - pressure receptors in the skin which react to pressure changes
  • Sound - sound receptors in the inner ear which react to vibrations in the air
  • Stretch - muscle fibres which react to a change in length of cells if they are stretched

  
• How the nervous system enables you to respond to changes

  1. Stimulus

  2. Receptor

  3. Coordinator

  4. Effector

  5. Response

  
• Stimulus


  A change detected by a receptor
• Neurones


  Specialised cells which can carry electrical impulses along their long thin cytoplasm, called an axon
• Types of neurones

  • Sensory neurone
  • Relay neurone
  • Motor neurone

  
• Synapse


  The gap where a nerve ends and joins to another, where the electrical signal is transferred into a chemical signal that diffuses across the gap
• Reflex arc

  1. Receptor

  2. Sensory neurone

  3. Relay neurone in spinal cord

  4. Motor neurone

  5. Effector

  6. Response

  
• Reflexes are automatic responses which happen so fast you do not even have time to think
• Examples of reflex actions


  • Pupil changes size in response to light
  • Sudden jerky withdrawal of hand or leg when pricked by a pin
  • Coughing or sneezing, because of irritants in the nasal passages
  • Knees jerk in response to hitting the shin/lower leg
  • Sudden removal of the hand from a sharp object
  • Sudden blinking when an insect comes very near to the eyes
• Hormones

  Chemical messages secreted by the glands of the endocrine system and released directly into the blood stream to travel to cells with complementary receptors
• Sensory neurone

  Carries impulses to the central nervous system

  
• Motor neurone


  Carries impulses away from the central nervous system
• Types of effectors

  • Two types
• Cell body of sensory neurone

  Found in the central nervous system

  
• Cell body of motor neurone


  Found in the central nervous system

  
• Neurones involved in coordinating reflexes

  • Sensory
  • Relay
  • Motor

  
• Relay neurone


  Found in the reflex arc
• Reflex pathway


  1. Stimulus

  2. Sensory neurone

  3. Relay neurone

  4. Motor neurone

  5. Effector

  
• Synapses


  Junctions between neurones where information is passed
• Information passing through synapse


  By neurotransmitters
• Hormone

  Chemical message secreted by endocrine glands and released into the bloodstream
• Main organs of the endocrine system


  • Pituitary gland
  • Thyroid gland
  • Thymus gland
  • Adrenal glands
  • Pancreas
  • Ovaries
  • Testes

  
• Nervous control


  • Works by electrical signals in neurones
  • Travels fast and has an instant effect
  • Response is over quickly
  • Acts on individual cells (localised effect)
• Hormonal control


  • Works by chemicals transported in bloodstream
  • Travels slower and may take longer to have an effect
  • Response is long lasting
  • Widespread effect on different organs