Role of Receptors

Sensory Reception

• CNS receives sensory information from internal and external environment through a variety of sense cells and organs – receptors.

• Each receptor responds to a different type of stimulus.

• Sensory reception = function of sense organs• Sensory perception = making sense of the

information from receptors.

Pacinian Corpuscule

• Respond only to changes in mechanical pressure. Will not respond to heat, light, sound etc.

• Acts as a transducer. It converts the information (energy) provided by the stimulus into a form that can be understood by the body (nerve impulses).

• Pacinian corpuscle converts energy of stimulus into nervous impulse called the generator potential.

Structure and Function of a Pacinian Corpuscle

• Occur deep in the skin.

• Most abundant in fingers, soles of feet and external genitalia.

• Occur in joints, ligaments and tendons to allow an organism to know which joints are changing direction.

• Sensory neurone of the Pacinian Corpuscle lies at centre of layers of connective tissue. Each layer is separated by gel.

Structure of Pacinian Corpuscle.

• Sensory neurone ending at centre has a sodium channel protein in its plasma membrane.

• Called the - stretch-mediated sodium channel

• Their permeability to Sodium changes when they change shape.

How it Works

• In resting state – the sodium channels of membrane around neurone are too narrow to allow sodium ions to pass along them.

• The neurone has a resting potential

• When pressure is applied to the Pacinian corpuscle, it changes shape and membrane is stretched.

• This widens sodium channels and sodium ions diffuse into the neurone.

• Influx of sodium ions changes potential of membrane (it becomes depolarised).

• This produces a generator potential

• This then creates an action potential (nerve impulse) that passes along neurone to CNS.

Is All Pressure Detected?• The lamellae and the gel filter out the stimuli, so that only rapidly

applied high pressures or rapid, high frequency vibrations result in the membrane being squashed.

• Slowly applied weak pressures or low-frequency vibrations just cause the gel within the Pacinian corpuscle to move slowly within the layers of lamellae, like faint ripples on the surface of a pond.

• They are not enough to deform the nerve cell membrane and allow the stimulus to be detected.

• The fluid gel and the lamellae, which effectively 'reset‘ the shape of the sensory nerve ending, as pressure is applied and removed.

Depolarisation• On either side of a neurone plasma membrane there are opposite

electrical charges (+/-).

• When depolarisation the charges either side of the neurone membrane swap (+ becomes – and - becomes +)

• Charges are a reflection of the amount of positive ions either side of the neurone membrane. When one side has more, it has a “+” charge.

• When the other side has less + ions, it is given the “-” charge. This can be misleading. It is not actually a negative charge BUT “less positive”.

• Movement of Na+ and K+ ions determine the charge.

Viscous gel inbetween layers

Stretch-mediated sodium channel opened

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Receptors Working Together in the Eye

• Receptors only respond to a certain intensity of stimulus.

• Therefore if the body is then able to distinguish between different intensities i.e. different intensities of light – it must have a range of receptors.

• Each receptor detects a different intensity of light.

Light Receptors

• In the mammalian eye, they are found in the innermost layer – the retina.

• Two types – rod cells and cone cells.

• They both convert light energy into electrical energy (in the form of a nerve impulse)

Rod Cells

• Cannot distinguish between different wavelengths of visible light (i.e. Colours)

• They produce images in black and white.

• More rod cells in retina than cone cells.

• Many rod cells share a single sensory neurone.

Retinal Convergence

• Several rod cells share a single neurone because before a generator potential can be produced, a threshold value has to be exceeded.

• Several rod cells linked together, all detecting energy from a stimulus, pool together.

• This creates more chance of a threshold value being exceeded and creating a generator potential.

• This enables rod cells to respond to low intensities of light and allow us to see in the dark (but only in black and white).

How do Rod Cells Detect Light?• Not the same as Pacinian Corpuscle

• Each rod cell contains a pigment – rhodopsin.

• When light hits rhodopsin, it breaks the pigment down and if enough is broken down – a generator potential is produced.

• Only low light intensity is needed to break down rhodopsin.

• Rhodopsin is produced when vitamin A (retinol) is oxidised and combines with the protein opsin.

• Carrots are a good source of Vitamin A – so they help produce rhodopsin and help keep rod cells working.

Visual Acuity (detail)

• As many rod cells are linked together, many different rod cells receiving light will only produce 1 generator potential.

• Rod cells find it hard to distinguish between separate sources of light that stimulated them so close objects in low light intensity merge into one.

Night Vision• The Tawny Owl can locate prey several metres away by the light of

just one candle about 1700 feet away!

• Its retina has about 56,000 rods per mm2

• Most mammals are nocturnal, and a maximum number of rods in the retina is an adaptation that gives these animals keen night vision.

• Cats, usually most active at night, have limited colour vision and probably see a pastel world during the day.

• Better night vision comes with having a larger eyeball, a larger lens, a larger optical aperture (the pupils may expand to the physical limit of the eyelids), more rods than cones (or rods exclusively) in the retina, and a tapetum lucidum

The Tapetum Lucidum

• A layer of tissue in the eye of many vertebrate animals.

• It lies immediately behind or sometimes within the retina.

• It reflects visible light back through the retina, increasing the light available to the photoreceptors.

• This improves vision in low-light conditions, but can cause the perceived image to be blurry from the interference of the reflected

Cone Cells

• 3 different types – each responding to different wavelength of light (red/green/blue)

• Depending on proportion of each that is stimulated – we see full spectrum of colours.

• Each cone cell is usually connected to one sensory neurone via ONE bipolar cell – so needs a high light intensity to create its own generator potential.

• This is why we cannot see colour in low light intensity (dark)

Cone Cells

• Cone cells contain different pigment than rod cells – they contain iodopsin.

• Iodopsin require higher light intensity to break it down.

• Because cone cells are connected to their own independent sensory neurone – two separate cone cells can receive light and send impulses to the brain separately – so brain can distinguish separate light sources.

• Cone cells see in detail (good visual acuity)

Distribution of Receptor Cells in the Eye

• Distribution is uneven

• Light is focused by the lens onto the part of the retina opposite the pupil (fovea).

• Fovea receives highest intensity of light.

• No rod cells are found here – only cones.

• At peripheries of retina (outside) there are more rods and no cones.

• This why we often see objects better in the dark if we look to the side of them.

Blind Spot

• One part of the retina does NOT contain any photoreceptors.

• This is our "blind spot". • Therefore any image that falls on this region

will NOT be seen. • It is in this region that the optic nerves come

together and exit the eye on their way to the brain.

Colour-Blindness

• Absolute colour blindness is almost unknown, but can occur in very rare cases.

• People that are effected by colour blindness, have less numbers of particular cones than normal, so they get colours confused.

• The genes that produce photopigments are carried on the X chromosome.

• Several forms – most common is Red/Green colour blindness

• The symptoms of colour blindness also can be acquired by physical or chemical damage to the eye, optic nerve, or the brain generally.

Protanomaly– red-green colour blindness – red cone cells behave more like green cone cells – they can’t distinguish.