| Pacinian corpuscles | - Responds to changes in mechanical pressure
- Occur deep in skin, most abundant on fingers, feet + genitalia
- Also occur in joints, ligaments + tendons, to enable organism to know which joints are changing direction |
| Features of the pacinian corpuscles | - Specific to single type of stimulus
- Produces a generator potential by acting as a transducer |
| Specific to single type of stimulus | - Responds only to mechanical pressure
- It will not respond to other stimuli like heat |
| Produces a generator potential by acting as a transducer | - All stimuli involve a change in some form of energy
- The transducer converts the change in form of energy by the stimulus into a form, like nerve impulses, that can be understood by the body |
| Generator potential (further info) | - The nerve impulse is also a form of energy
- Receptors therefore convert 1 form of energy into another
- Receptors in the nervous system convert energy of the stimulus into a nervous impulse known as generator potential |
| Structure of the Pacinian corpuscle | - Single sensory neuron is at the center of layers of tissue
- Each separated by a gel, looks like an onion
- Sensory neuron has special type of sodium channel in its plasma membrane, stretch-mediated sodium channel |
| The stretch-mediated sodium channel | - Their permeability to sodium changes when they are deformed, e.g. by stretching |
| Function of the Pacinian corpuscle (1) | - In its normal state, the stretch-mediated sodium channels of the membrane around the neuron are too narrow to allow sodium ions to pass along them
- In this state, the neuron has resting potential |
| Function of the Pacinian corpuscle (2) | - When pressure is applied to the PC, it is deformed + the membrane around its neuron becomes stretched |
| Function of the Pacinian corpuscle (3) | - Influx of sodium ions changes potential of the membrane (i.e. becomes depolarised)
- Producing a generator potential |
| Function of the Pacinian corpuscle (4) | - The generator potential creates an action potential (nerve impulse) that passes along the neuron
- Then via other neurons, to the CNS |
| Receptors working together in the eye | - Light receptor cells of the mammalian eye are found on its innermost layer, the retina
- Either rod cells or cone cells
- Both act as transducers by conserving light energy into electrical energy of a nerve impulse |
| Rod cells | - Cannot distinguish different wavelengths of light, images seen only in black + white
- Many connected to single sensory neuron in optic nerve
- Used to detect light of low intensity |
| Why rod cells respond to low-intensity light | - A certain threshold value has to be exceeded before generator potential is created in bipolar cells
(to which rod cells are connected)
- As a no. of rod cells are connected to a single bipolar cell (=retinal convergence), there is much greater chance that the threshold value will be exceeded than if only a single rod rod cell were connected to each bipolar cell
- This is due to summation
- In order to create a generator potential, the pigment in the rd cells (rhodopsin) must be broken down
- There is enough energy from low-intensity light to cause this breakdown |
| Why rod cell give low visual acuity | - Many rods are joined to a single bipolar cell
- Which means light from 2 points close together cant be told apart |
| Cone cells | - 3 different types, each responding to a different range of wavelengths of light
- Depending on the proportion of each type that is stimulated, we can perceive images in full colour |
| Why do cone cells respond to high-intensity light | - Each joined to specific separate bipolar cells connected to a sensory neuron in optic nerve
- So it takes more light to reach the threshold + trigger an action potential |
| Why do cone cells sensitive to different wavelengths | - CC contain iodopsin (pigment) needs higher light intensity for its breakdown + create a generator potential
- 3 different types of CC, has specific type of iodopsin
- So each CC sensitive to a different specific range of wavelengths |
| Why do cone cells have good visual acuity | - Each connected to single bipolar cell, they are close together
- When light from 2 points hits 2 cones, 2 action potentials (1 from each cone) go to the brain
- So you can distinguish 2 points that are close together apart |
| Where are cone cells and rod cells found exactly | - Fovea, lens on the retina opposite the pupil where light is focused
- The fovea receives the highest intensity of light
- So CC, are found at the fovea
- RC found at the peripheries of the retina, where LI is lowest |
| Differences between rod cells and cone cells (ROD) | - Rod-shaped
- Greater no. than cone cells
- Peripheries of the retina
- Give poor visual acuity
- Sensitive to low-intensity light
- One type only |
| Differences between rod cells and cone cells (CONE) | - Cone-shaped
- Fewer no. than rod cells
- At fovea
- Give good visual acuity
- Not sensitive to low-intensity light
- 3 types each responding to different wavelengths of light |
