B3- Response to Internal Stimuli

what are receptors

• cells that detect stimuli

• they are specific so each type of receptor detects a different type of stimulus

what is a generator potential?

the change in electrical potential in a receptor when it is stimulated

what are Pacinain corpuscles

• type of mechanoreceptor found deep in the skin

• which responds to changes in pressure only; specific to pressure

where are Pacinian corpuscles found

at sensory neurone endings

describe the basic structure of Pacinian corpuscles

• made of many layers of membrane that are separated by gel

• membrane layers are sometimes referred to as connective tissue or lamellae

describe how a generator potential is established in a Pacinian corpuscle

1. Mechanical stimulus (eg. pressure) deforms lamellae and stretch-mediated sodium ion channels

2. so Na⁺ channels in membrane open and Na⁺ diffuse into sensory neurone

3. greater pressure causes more Na⁺ channels to open and more Na⁺ to enter

4. this causes depolarisation, leading to a generator potential

5. if generator potential reaches threshold it triggers an action potential

explain what the Pacinian corpuscle illustrates

1. receptors respond only to a specific stimuli

• Pacinian corpuscle only responds to mechanical pressure

2. Stimulation of a receptor leads to establishment of a generator potential

• when threshold is reached, action potential sent ( all or nothing principle )

1. name the 2 photoreceptors in our eye

2. where are they found

1. rods and cones

2. found in retina

what are more sensitive to light: rods or cones?

rods

why are rods more sensitive to light than cones?

• several rods are connected to a single neurone

• spatial summation to reach threshold ( as enough neurotransmitter released ) to generate an action potential

what are cones less sensitive to light than rods

• each cone connected to a single neurone

• no spatial summation

do rods or cones have a higher visual acuity / resolution

cones

explain how photoreceptors ( rods and cones ) detect light

1. light causes chemical breakdown ( bleaching ) of optical pigments inside rods and cones

2. the bleaching of these optical pigments results in a generator potential

3. if the general potential is large enough then a nerve impulse is sent along a bipolar neurone to the optic nerve

what is the light sensitive optical pigment in rods called

rhodopsin

what is the light sensitive pigment in cones called

iodopsin