Unit 5 Biology - Receptors

• Mechanoreceptors which detetct pressure and vibrations
• Contain the end of a sensory neurone that is wrapped in a lot of layers of connective tissue (lamellae)
• The lamellae is depressed when the Pacinian Corpuscle is stimulated and this presses on the nerve ending
• The stretch-mediated sodium channels in the sensory neurone's cell mebrane open and sodium ions diffuse into the cell, creating a generator potential
• When the potential reaches the threshold, an action potential is triggered

Explain how a tap on the arm is converted into a nerve impusle (6 marks)

A tap on the arm is a mechanical stimulus that's detected by pressure recpetors called Pacinian corpuscles. The stimulus deforms the lamellae which press on the nerve ending. This causes deformation of stretch-mediated ion channels in the neurone-cell membrane. Sodium ion channels open and sodium ions diffuse into the cell. This creates a generator potential  which, if it reaches its threshold it triggers an action potential.

• Light enters the eye through the pupil, the amount of light 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 and contains photoreceptor cells that detect the light
• The fovea is an area of the retina where a lot of photoreceptors are located
• Nerve impusles are carried from the retina to the brain by the optic nerve

There are two different types of photoreceptors, cone cells and rod cells

• Rod cells are very sensitive to light because many rod cells join to one neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential
• They give low visual acuity (the ability to tell apart points that are close together) because many rods join the same neurone, so the brain won't get seperate information
• Rods only give information in black and white
• Rods are found in the peripheral parts of the retina
• Cone cells are less sensitive as only one cone cell joins one neurone so it takes more light to trigger an action potential
• They give high visual acuity because cone cells are close together and one cone cell joins one neurone so seperate action potentials will be generated so the brain gets seperate information
• Cones are packed together in the fovea
• Cones give information in colour

How photoreceptors convert light into and electrical impulse

• Light enters the eye, hit the photoreceptors and is absorbed by light-sensitive pigments
• Light bleaches th epigments, causing a chemical change and altering the membrane permeability to sodium
• A generator potential is created and if it reaches the threshold,a nerve impusle is sent along a bipolar neurone
• Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain
• The human eye has two types of photoreceptors - rods and cones
• Rods are found in the peripheral parts of the retina and cones are packed together in the fovea
• Rods only give information in black and white but cones give information in colour