5.3.1

• sensory receptors

  • detect changes in the surroundings
  • are energy transducers that convert one form of energy to another
  • transducers are adapted to detect changes in a particular form of energy
  • receptors detect the presence of chemicals
• sensory receptor

  
• Pacinian corpuscles

  a pressure sensor that detects change in pressure on skin

  • oval-shaped structure that consists of a series of concentric rings of connective tissue wrapped around the end of a nerve cell

  → when pressure on skin changes this deforms the rings of connective tissue which push against the nerve ending

  • corpuscles is sensitive only to changes in pressure that deforms the rings of connective tissue therefore when pressure is constant they stop responding
• pacinian corpuscle

  
• Changing membrane permeability

  • if channel proteins are permanently open then ions can diffuse across the membrane + will do so until concentration on either side of membrane are in equilibrium

  → if closed then actions of active pumps can create a concentration gradient across membrane

  • cells associated with nervous system have specialised channel proteins + sodium channel are specific to sodium ions + potassium ions are specific to potassium channels
• Changing membrane permeability 2

  • the sodium channels are sensitive to small movement of the membrane

  → when membrane is deformed by changing pressure the sodium pump open allowing sodium ions to diffuse into cell, producing a generator potential (receptor potential)

  • they also have sodium/potassium pumps + actively pump sodium ions out of cell and potassium cells inside (for every 2 potassium ions pumped into cell, 3 sodium ions are pumped out)

  → when channel proteins are all closed, the sodium/potassium pumps work to create a concentration gradient
• Changign membrane permeability 3

  • concentration of sodium ions outside cell increases + the membrane is more permeable to potassium ions, so some leak out of cell

  → membrane is less permeable to sodium ions, so few of these are able to leak into cell

  • result of these ionic movements is a potential gradient across cell membrane ; cell is negatively charged inside compared with outside

  → negative potential is enhanced by the presence of negatively charged anions inside cell
• Creating a nerve impulse

  when cell is inactive the cell membrane is said to be polarised, that is the negatively charged inside compared with outside

  • nerve impulse is created by altering the permeability of nerve cell membrane to sodium ions + achieved by opening the sodium ion channels
  • as sodium ion channels open, membrane permeability is increased + sodium ions can move across the membrane down concentration gradient

  → the movement of ions across the membrane creates a change in potential difference (charge) across membrane
• Creating a nerve impulse 2

  • the inside of cell becomes less negative than outside than usual - depolarisation
  • if a small stimulus is detected only a few sodium channels will open; the lager the stimuli