Receptors

• receptors are specific - they only detect on particular stimulus e.g. light or glucose concentration or pressure
• there are many different types of receptor that each detect a different type of stimulus
• • some receptors are cells e.g. photoreceptors are receptor cells that connect to the nervous system
  • some receptors are proteins on cell surface membranes e.g. glucose receptors are proteins found in the cell membranes of some pancreatic cells
• receptors in the nervous system convert the energy of the stimulus into the electrical energy used by neurones
• The resting potential:

  • when a nervous system receptor is in its resting state (not being stimulated)
  • there is a difference in charge between the inside and the outside of the cell - the inside is negatively charged relative to the outside
  • this means there is a voltage across the membrane
  • voltage is also known as potential difference
  • the potential difference when a cell is at rest is called its resting potential
  • the resting potential is generated by ion pumps/ channels
• The generator potential:

  • when a stimulus is detected, the cell membrane is excited and becomes more permeable
  • allowing more ions to move in and out of the cell
  • altering the potential difference
  • the change in potential difference due to a stimulus is called the generator potential
  • a bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change in potential difference
  • a bigger generator potential is produced
• potential difference across a cell membrane is usually measured in millivolts mV
• The action potential:

  • if the generator is big enough it will trigger an action potential
  • an action potential is an electrical impulse along a neurone
  • an action potential is only triggered if the generator potential reaches a certain level called the threshold level
  • action potentials are all one size
  • so the strength of the stimulus is measured by the frequency of action potentials (the number of them triggered during a certain time period)
  • if the stimulus is too weak the generator potential won't reach the threshold, so there is not action potential
• • lamellae
  • sensory nerve ending
  • inside of the membrane is negatively charged compared to the outside
  • all parts of the membrane have Na+ channels closed
  • stimulus applied
  • Na+ channels open here (where stim applied), so Na+ diffuses into cell
  • all other parts of the membrane still have Na+ channels closed
• Pacinian corpuscles only respond to mechanical stimuli, not to any other type of stimulus
• stretch-mediated sodium ion channels get there name bc they only open and let sodium ions pass through when they're stretched
• the bigger the stimulus i.e. the more pressure applied, the more sodium channels open.

  this creates a bigger generator potential which is more likely to reach the threshold and cause an action potential
• photoreceptors:

  • receptors in the eye that detect light
• • light enters the eye through the pupil,
  • amount of light that enters is controlled by the muscles of the iris
  • light rays are focused by the lens onto the retina, which lines the inside of the eye
  • the retina contains the photoreceptor cells
• • the fovea is an area of the retina where there are lots of photoreceptors
• • nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve
  • the optic nerve is a bundle of nuerones
  • where the optic nerve leaves the eye is called the blind spot
  • there are not any photoreceptor cells, so it is not sensitive to light
• How photoreceptors work:

  • light enters the eye
  • hits the photoreceptors and is absorbed by light-sensitive optical pigments
  • light bleaches the pigments - causing a chemical change and altering the membrane permeability to sodium ions
  • a generator potential is created and if it reaches the threshold, a nervous impulse is sent along a bipolar neurone
  • bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain
• light passes straight through the optic nerve and bipolar neurone to get to the photoreceptors
• The human eye has 2 types of receptors - rods and cones
• • rods are mainly found in the peripheral parts of the retina
  • cones are mainly found packed together in the fovea
• • rods and cones contain different optical pigments making them sensitive to different wavelengths of light
• • rods only give information in black and white (monochromatic vision)
  • cones give information in colour (trichromatic vision)
• There are three types of cones each containing a different optical pigment:

  • red-sensitive
  • green-sensitive
  • blue-sensitive
  • when they are stimulated in different proportions you see different colours
• also able to see yellow bc this frequency of light (which falls between the wavelengths for red and green) stimulates both red and green sensitive cones a bit - the brain converts this into seeing yellow
• • rod cells are very sensitive to light (they work well in dim light)
  • this is bc many rods join to one bipolar neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential
• • cones are less sensitive than rods (work best in bright light)
  • this is bc one cone joins one bipolar neurone, so it takes more light to reach the threshold and trigger an action potential
• Visual acuity: the ability to tell apart points that are close together
• rods give low visual acuity because many rods join the same bipolar neurone, which means light from two points close together can't be told apart

  the brain doesn't get separate information about 2 close points
• • cones give high visual acuity bc cones are close together and one cone joins one bipolar neurone
  • when light from 2 points hits 2 cones, 2 action potentials (one from each cone) go to the brain
  • so can distinguish between 2 points that are close together as 2 separate points
  • the brain gets separate information about 2 close points
• someone with a low visual acuity will have blurry vision
• rods:

  • mainly located in the peripheral parts of the retina
  • give info in black and white
  • many rods join one bipolar neurone
  • high sensitivity to light
  • give low visual acuity
• cones:

  • mainly located in the fovea
  • give info in colour
  • one cone joins one bipolar neurone
  • low sensitivity to light
  • give high visual acuity