Some Most All Role of receptors WAL: An overview of the nervous system receive sensory information WAL: All Most What are the main features of sensory reception? Some What is a Pacinian corpuscle and how does it work?? What are the main features of sensory reception? Starter – Use the following words to explain how heart rate is controlled: chemoreceptors, parasympathetic, SA node, pH

Today we are covering from the specification: Pages 150-152 of your textbook

Role of receptors Sensory reception is the function of sense organs, whereas sensory perception involves making sense of the information from the receptors. This is largely the function of the brain.

Receptors - Pressure A Pacinian corpuscle Yteach resource

Pacinian corpuscle Respond to change in mechanical pressure. As with all sensory receptors, a Pacinian corpuscle: Is specific to a single type of stimulus (e.g. pressure) 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 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.

Pacinian corpuscle Layers of connective tissue separated by a gel Blood capillary

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 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 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.

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.

Photoreceptors These are found in the retina. There are two types Rods and Cones and they are arranged as shown: Outer ! Inner Light Pigmented Layer Ganglion Cells Rod To Optic Nerve Bipolar Neurones Cone

Light receptors Both rod and cone cells act as transducers by converting light energy into the electrical energy of a nerve impulse.

Rod cells Cannot distinguish different wavelengths of light and therefore produce images only in black and white. Rod cells are more numerous than cones.

Rod 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 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 opsin Rhodopsin (pigment in rod cells broken down) LIGHT Signal from Bipolar cell

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.

Cone 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 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.

Greater numbers than cones Fewer numbers than rods 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 Iodopsin pigment

Cone 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

Plenary– Reflections: What did you learn? Role of receptors An overview of the nervous system receive sensory information WAL: All Most What are the main features of sensory reception? Some What is a Pacinian corpuscle and how does it work?? What are the main features of sensory reception? 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?