chapter 13

• homeostasis


  maintenance of a stable equilibrium in the conditions inside the body
• cell signalling


  nervous and hormonal systems coordinate the activities of whole organisms, cell signalling occurs by one cell releasing a chemical which has an affect on another cell (target cell)
• cell signalling can transfer signs locally between neurones at synapses and the signal would be a neurotransmitter
• cell signalling can transfer signals across large distances using hormones such as the pituitary gland secretes ADH which acts on kidneys to maintain water balance in the body
• coordination in plants

  based on chemical communication systems including plant hormones
• stimulus

  detectable change in external or internal environment of an organism
• role of neurones

  transmit electrical impulses rapidly along the body so that the organism can respond to changes in its internal and external environment
• structure of a mammalian neurone

  • cell body
  • dendrons
  • axons
• cell body

  contains nucleus surrounded by cytoplasm, where there is endoplasmic reticulum and mitochondria which are involved with neurotransmitters
• dendrons


  short extensions which come from the cell body, they divide into smaller branches called dendrites and are responsible for transmitting electrical impulses towards the body
• axons


  singular, elongated nerve fibres that transmit impulses away from the cell body, cylindrical shape with a narrow region of cytoplpasn surrounded by a plasma membrane
• types of neurones

  • sensory
  • relay
  • motor
• sensory neurone

  transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain, have 1 dendron which carries impulse to the cell body, and 1 axon which carries impulse away from the cell body

  
• relay neurone


  transmit impulses between neurones, e.g between sensory and motor neurones, have many short axons and dendrons

  
• motor neurone


  transmit impulses from relay neurone or sensory neurone to an effector, such as a muscle or a gland, have 1 long axon and many short dendrites

  
• myelinated neurones

  neurones covered in myelin sheath
• myelin sheath


  membrane rich in lipid which surrounds the axon of some neurones, speeding up impulse transmissions
• schwann cells

  produce layers of membrane by growing around the axon many times, each time the grow around the axon, a double layer of phospholipid bilayer is laid down
• when the schwann cell stops growing, there may be more than 20 layers of membrane, the myelin sheath acts as an insulating layer and allows these myelinated neurones to conduct the electrical impulse at a faster speed
• myelinated neurones can transmit impulses at up to 100 metres per second , and non myelinated neurones can only conduct at 1 metre per second
• node of Ranvier

  2-3 micrometre gap between each adjacent schwann cell that creates gaps in the myelin sheath, in humans it occurs every 1-3 mm
• myelin sheath is an electrical insulator, and in myelinated neurones the electrical impulse jumps from one node to the next as it travels along the neurone, the sheath allows the jump to be faster
• in non-myelinated neurones the impulse does not jump, it transmits continuously along the nerve fibre so it is much slower
• sensory receptors

  all sensory receptors have 2 main features:

  • specific to a single type of stimulus
  • act as a transducer (convert a stimulus into a nerve impulse)
• sensory receptors role as a transducer


  detect a range of different stimuli including light, heat, sound or pressure, receptor converts the stimulus into a nervous impulse = generator potential
• pacinian corpuscle

  specific sensory receptors that detect mechanical pressure, located deep within the skin and are more abundant in the fingers and soles of feet, also found in joints to allow you to know what joints are changing direction

  
• the end of the sensory neurone is found in the centre of the pacinian corpuscle where it is surrounded by layers of connective tissue, each layer of tissue is separated by a layer of gel
• structure of pacinian corpuscle

  within the membrane of the neurone there are sodium ion channels, responsible for transporting sodium ions across the membrane, the neurone ending in a corpuscle has a special sodium channel
• pacinian corpuscle converting mechanical

  1. stretch mediated sodium ion channels are too narrow to allow ions through, the neurone is at a resting potential
  2. pressure applied so corpuscle changes shape and neurone stretch
  3. sodium ion channels now widen so ions can pass to membrane
  4. more ions changes the potential of the membrane making it depolarised
  5. depolarisation causes a generator potential
  6. generator potential causes an action potential that passes along a sensory neurone

  
• action potential generated by a pacinian corpuscle will be transmitted along neurones to the CNS
• resting potential

  potential difference across a neurone membrane when it is not transmitting an impulse, the outside of the membrane is more positively charged than the inside of the axon = membrane polarised = -70mV
• resting potential occurs because of the movement of Na+ and K+ ions across the axon membrane, the phospholipid bilayer prevents ions diffusing across the membrane so they have to be transported by channel proteins
• channel proteins can be gated as they only allow specific ions to pass through the, and others remain open all the time to allow ions to simply diffuse through
• creation of a resting potential:

  1. Na+ transported out of the axon and K+ transported into the axon by the sodium-potassium pump, every 3 Na+ out = 2 K+ in
  2. more Na+ outside the membrane than inside, so they diffuse back in down an electrochemical gradient
  3. they cannot diffuse back in as the sodium channels are gated, but potassium channels are open so there are more positive ions outside of the membrane = resting potential is -70mV

  
• when a stimulus is detected by a sensory receptor, the energy of the stimulus temporarily reverses the charges on the axon membrane, so it becomes polarised and has a charge of +40mV (depolarisation)
• depolarisation

  a change in potential difference from negative to positive
• as the impulse passes and the stimulus ends, the membrane repolarises and returns to its resting potential of -70mV
• action potential occurs when protein channels in the axon membrane change shape as a result of the change of voltage across its membrane, the change in protein shape results in the channel opening or closing = voltage-gated ion channels
• nerve impulse is an action potential that starts at one end of the neurone and is propagated along the axon to other other end of the neurone, an initial stimulus triggers an action potential in the sensory receptor, so the first region of the axon membrane is depolarised, acts as a trigger then the rest depolarise
• once sodium ions are inside the axon, they are attracted by the negative charge and the concentration gradient becomes steeper so they diffuse further along inside the axon, triggering the depolarisation of the next section