Receptors

• In its normal (resting) state, the stretch mediated sodium ion channels in the membrane around the neurone of a corpuscle are too narrow to allow sodium ions to pass through them 

  In this state, the neurone of the Pacinian corpuscle has a resting potential because the inside is more negative than the outside. So there is a -70mV resting potential. 
• When pressure is applied to the Pacinian corpuscle, it deforms the membrane, the membrane around the neurone becomes stretched 

  This stretching widens the stretch mediated sodium ion channels in the membrane, which means they open. 

  Sodium ions diffuse across the membrane into the neurone by facilitated diffusion 
• The influx of sodium ions changes the potential of eh membrane (the membrane depolarises) and a generator potential is produced. 

  The generator potential creates an action potential that pases along the sensory neurone and via other neurones to the central nervous system 
• Stimulus = a change in the environment 

  Receptor = cells that detect changes in the environment
• Pacinian corpuscle = detects changes in pressure (mechanical energy) 

  Transducer= transfers energy from one form to another (e.g mechanical energy to electrical energy) 
• Pacinian corpuscles are located in the skin 

  Pacinian corpuscle cell membrane has stretch mediated sodium ion channels 
• Within the pacinian corpuscle there is an unmyelinated nerve cell ending 
• Pacinian corpuscle = detects changes in pressure (mechanical energy) 
• When stimulated a pacinian corpuscle creates a generator potential as:

  Increased pressure deforms the stretch mediated sodium ion channels 

  The channels open and sodium ions flow in 

  Depolarisation occurs leading to a generator potential 
• Generator potentials can be different sizes depending on the strength of he stimulus 
• Greater pressure = deforms the membrane more= more Na+ channels widen and become open = more na+ diffuse into neurone so neurone becomes more positive. If threshold is reached, an action potential will occur. 
• Light pressure = only a few stretch mediated na+ channels open, some na+ diffuse in to make a generator potential. If threshold is not reached by generator potential, an action potential will not occur in the sensory neurone
• The rate of ATP consumption of an unmyelinated neurone is greater than a myelinated neurone because there is greater entry of na+ ions and exit of k+ ions so more active transport will occur by the ion pumps to re establish resting potential 
• Action potentials are always the same value because they follow the all or nothing principle whereas the value of the generator potential depends on the strength of the stimulus