Receptors - Topic 6: Organisms respond to changes in their internal & external environments

Biology Paper 2

Define Pacinian Corpuscles

They are Mechanoreceptors. They detect pressure/vibration receptor on the skin

Where are Pacinian Corpuscles found?

Most abundant on the fingers, soles of the feet

What do Pacinian Corpuscles contain?

The end of a sensory neuron “sensory nerve endings”. This is wrapped in loads of layers of connective tissue called lamellae

What happens when a Corpuscle is stimulated (pressure is applied)?

The rings of connective tissue (lamellae) apply pressure onto the sensory neuron 

The sensory neuron has has stretch mediated Na+ channels in plasma membrane 

Applied pressure causes the stretch mediated sodium channels to open 

Na+ diffues into the cell, creating a generator potential. If it reaches a certain threshold, it triggers an action potential

Photoreceptors

convert light into an electrical impulse

Types of photoreceptor

Rods & Cones

Rods

Greater number than __

highly sensitive to dim light

evenly distributed on periphery of retina

absent in fovea

poor visual acuity

detects light and dark

are monochromatic (only detect 1 wavelength)C

Cones

Fewer number than __

sensitive to bright light

more concentrated at fovea

good visual acuity

detects colour

trichromatic - detect three different wavelengths of light

Rhodopsin

Pigment in Rod

Iodopsin

Pigment in Cone

Visual acuity

ability to distinguish between close objects or two points

Why rods give low visual acuity

many rods join the same neuron, which means light from two objects close together can’t be told apart

Why cones give high visual acuity

cones are close together and one cone joins one neurone

Photoreceptor that is sensitive to light

Rods (they fire action potentials in dim light)

Why rods are sensitive to light

many rods join one neuron so many weak generator potentials need to combine to reach the threshold and trigger an action potential

Less sensitive photoreceptor

cones (fire action potentials in bright light)

why cones are less sensitive

one cone joins one neuron, so it takes more light to reach the threshold and trigger an action potential