• Recognise this receptor?• Remember how it works?

Starter

• The Pacinian corpuscle is an example of a primary receptor (a single neurone with ONE end detecting stimulus)

• Pacinian corpuscles function individually.

Pacinian corpusclePacinian corpuscle

Respond to change in mechanical pressure. As with all sensory receptors, a Pacinian corpuscle:1)Is specific to a single type of stimulus (e.g. pressure)2)Produces a generator potential by acting as a transducer – converts stimuli into a response that can be understood by the body, namely nerve impulses.

Pacinian corpusclePacinian corpuscle

The stimulus is always some form of energy, e.g. heat, light, sound or mechanical energy. The nerve impulse is a form of energy

All receptors convert the energy of the stimulus into a nerve impulse known as a generator potential.

Stretch-mediated sodium channels (Permeability change when shape changes)

Resting Potential:Positive outside – negative inside

Pressure:Distorts & opens Na+ channels

Action potential:Inflow of Na+ depolarises membrane

Pacinian corpusclePacinian corpuscle

In it’s normal (resting) state, the stretch-mediated sodium channels of the membrane around the neurone of a pacinian corpuscle are too narrow to allow sodium ions to pass along them. In this state, the neurone of the pacinian corpuscle has a resting state.

When pressure is applied to the corpuscle, it changes shape and its neurone becomes stretched

Pacinian corpusclePacinian corpuscle

This stretching widens the sodium channels in the membrane and sodium ions diffuse into the neurone.

The influx of sodium ions changes the potential of the membrane (depolarised), thereby producing a generator potential

The generator potential in turn creates an action potential (nerve impulse) that passes along the neurone and then, via other neurones to the CNS.

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Photoreceptors in the retina

Receptors working together in the eye

Receptors working together in the eye

Receptors respond to only one type of stimulus.It also only responds to a certain intensity of stimulus.

This means the body must have a range of receptors, each responding to a different intensity of stimulus.

Retinal Transduction• The eye is a transducer• What does this mean?

Transduction

• The eye is a transducer• What does this mean?• Converts light into a pattern of nerve

impulses• Transduction takes place in the retina

by a layer of photosensitive cells at the back of the eye

• Rods and cones attached to nerves

Electron Micrograph of Retina

PhotoreceptorsThese are found in the retina. There are two types Rods and Cones and they are arranged as shown:

Bipolar Neurones

Ganglion CellsTo Optic Nerve

Rod Cone

Pigmented Layer

Light

Inner Outer !

Rod cellsRod cells

Cannot distinguish different wavelengths of light and therefore produce images only in black and white.

Rod cells are more numerous than cones.

Rod cellsRod cells

Many rod cells share a single sensory neurone. Rod cells can therefore respond to light of very low intensity.

This is because a certain threshold value has to be exceeded before a generator potential is created in the bipolar cells to which they are attached.

Rod cellsRod cells

A number of rod cells are attached to a single bipolar cell (= retinal convergence), there is a much greater chance that the threshold value will be exceeded than if only a single rod cell were attached to each bipolar cell.

As a result, rod cells allow us to see in low light intensity (i.e. at night), although only in black and white.

Changes in the electrical potential of a receptor when stimulated by three separate stimuli. Only the third stimulus produces a generator potential high enough to trigger a nerve impulse.

A rod cell

Rhodopsin(pigment in rod cellsbroken down)

LIGHT

Signal from Bipolar cell

opsin

A rod cell As many rod cells are joined to the same bipolar cells, only a single impulse will be stimulated.

This means that they cannot distinguish between the separate sources of light that stimulated them.

2 dots close together will appear as a single blob. Rod cells therefore have low visual acuity.

Check!

•Discuss with the person sat next to you, the difference between visual acuity and nerve convergence.

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How do rod cells produce impulses?

Rod cells allow vision in dim light due to the presence of a pigment called rhodopsin, which is found in membrane-bound vesicles.

When rhodopsin absorbs light it splits into its constituent parts, opsin and retinal. This is called bleaching. Low intensity light is sufficient to cause this breakdown.

The presence of opsin causes a change in the permeability of the rod cell to sodium, which initiates a generator potential. Rhodopsin can reform in the absence of further light stimulation.

vesicles containing rhodopsin

Cone cellsCone cells

Cone cells are of three different types, each responding to a different wavelength of light. Depending on the proportion of each type that is stimulated, we can perceive images in full colour.

Each cone cell usually has its own bipolar cell connected to a sensory neurone. This means that often the generator potential is not exceeded. As a result, cone cells only respond to high light intensity and not to low light intensity.

Cone cellsCone cells

Cone cells contain a different pigment to rod cells (iodopsin). This requires a higher light intensity to be broken down and create a generator potential.

As cone cells are attached to their own bipolar cell, if 2 adjacent cells are stimulated, the brain receives 2 separate impulses.

Cone cells give very accurate vision, they have good visual acuity.

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How do cone cells produce impulses?

Cone cells are sensitive to high light intensities due to the presence of the pigment iodopsin.

The colour seen depends on the relative degree of stimulation of the three different types of cone cell.

In bright light, iodopsin is broken down into its constituent parts, generating an action potential in the ganglion cell.

There are three different types of cone cell, each containing a different form of iodopsin. Each form of iodopsin absorbs a different wavelength of light – green, blue or red.

vesicles containing iodopsin

Cone cellsCone cells

Light is focussed by the lens on a point known as the fovea. The fovea therefore receives the highest intensity of light.

Therefore cone cells, but not rod cells, are found at the fovea. The concentration of cone cells diminishes further away from the fovea. At the peripheries of the retina, where light intensity is at its lowest, only the rod cells are found.

Wavelengths of light absorbed by different cones

Colour Blindness

• If you have normal vision you will see a figure seven in reddish brown dots.

• People with red-green colour blindness will not see the 7, why?

• These people lack red sensitive cones, but the green stimulated cones are stimulated by the red light, so all dots appear green

Task Sheet and Booklet

Iodopsin pigment

Rod cells Cone Cells

Rod-shaped Cone-shaped

One type Red, green & blue types

Greater numbers

than cones

Fewer numbers than rods

Distributed more in the periphery

Fewer at periphery,

concentrated in fovea

Poor acuity Good acuity

High sensitivity

Low sensitivity

Rhodopsin pigment

Iodopsin pigment

Role of receptorsAn overview of the nervous system receive sensory information

WAL:

All

Most

What are the main features of sensory reception?

What is a Pacinian corpuscle and how does it work??

Plenary– Reflections: What did you learn?What do you want to find out? How might you find

this out? What skills did you use? How did your group function? What worked and what didn’t? What

connections did you make? How was your thinking pushed? Why did you choose the approach you did? What did you enjoy and why? How could you have

done it differently?

Some

What are the main features of sensory reception?