1. Section 46.1 pg. 970 Sensory receptors are structures that are specialized to respond to changes in their environment (stimuli) E.2.1 Outline the diversity of stimuli that can be detected by human sensory receptors, including mechanoreceptors, chemoreceptors, thermoreceptors, and photoreceptors

2. Activation of sensory receptors triggers nerve impulses along the afferent fibers coursing to the CNS There are 3 ways to classify sensory receptors: 1. by the type of stimulus they detect 2. by their body location 3. by the relative complexity of their structures

3. Sensory receptors named according to the stimuli that activate them are: 1. Mechanoreceptors Generate nerve impulses when they, or adjacent tissues, are deformed by mechanical forces such as touch, pressure (including blood pressure), vibration, stretch, and itch Examples include: hair follicle receptors, Meissner’s corpuscles (found on the surface of hairless skin), Pacinian corpuscles (deep pressure-sensitive receptors), and Ruffini and Merkel cells which are touch-sensitive; also, hair cell receptors of the inner ear and stretch receptors in tendons

4. Mechanoreceptors

5. 2.Thermoreceptors Sensitive to temperature changes Examples include: cold receptors located directly below the epidermis, and warm receptors which are located slightly deeper in the dermis They are also found in the hypothalamus where they regulate the temperature of circulating blood thus providing the CNS with information about the body’s “core” temperature

6. Thermoreceptors

7. 3.Photoreceptors Respond to light energy Examples include: rods and cones located in the retina; the photopigments contained within them break down when exposed to light, thus generating an action potential An action potential is an event that results in polarity reversal at the cell membrane of a nerve cell or muscle cell

8. Photoreceptors

9. 4.Chemoreceptors Respond to chemicals in solution (molecules smelled or tasted, or changes in blood chemistry) Examples include: taste buds, peripheral chemoreceptors of the aorta and carotid bodies which monitor plasma pH, central chemoreceptors of the medulla which monitor pH of CSF, and olfactory receptors located in the upper nasal passages

11. E2.2 Label a diagram of the structure of the human eye

12. The eye is protected inside a bony socket of the skull and is moved by a set of 6 muscles. Around the eye, we find eyelids and tear glands to keep the eye moist and clean, eyelashes to keep dust out, and eyebrows as a protector against sweat running down from the brow The eye is supported by the hydrostatic pressure of the aqueous and vitreous humours

13. Conjunctiva – thin transparent layer continuous with the epithelium of the eyelids Cornea – transparent front of the sclera; the curved surface is very important in refracting the light towards the retina Aqueous humour – clear solution of salts in the anterior chambers of the eye Pupil – opening in the center of the iris through which light enters the eye

14. Lens – transparent, elastic bi-convex structure which focuses light onto the retina Iris – the visible, colored part of the eye made up of 2 smooth muscles layers which will vary the pupil size in response to light intensity Ciliary body – a thickened ring of tissue made up of smooth muscles bundles called ciliary muscles that surrounds the lens and controls its shape Suspensary ligaments – attach the lens to the ciliary muscles

15. Vitreous humour – a clear gel the fills the posterior chamber Sclera – white, protective covering of the eye (fibrous) Choroid – the dark brown, vascular, middle layer of the eye that prevents light from scattering and reflecting within the eye Retina – the innermost layer of the eye which contains the photoreceptor cells (rods and cones) which play a direct role in vision

16. Fovea – a small pit located in the macula lutea (yellow spot) where light is allowed to pass directly to the photoreceptor cells without passing through several layers of retina; enhances visual acuity Blind spot (optic disc) – region of the eye that lacks photoreceptor cells; light focused on it cannot be seen; point where the optic nerve exits the eye Optic nerve – collection of axons from the retinal ganglion cells that exit from the back of the eyeball; carries impulses to the brain

17. E2.3 Annotate a diagram of the human retina to show the cell types and the direction in which light moves ***the direction of receptor potentials in the retina is opposite to the direction of light

18. E2.4 Compare rod and cone cells The photoreceptor cells become active when light is focused onto the retina Photoreceptor cells are modified neurons that resemble tall epithelial cells with their tips immersed in the pigmented layer of the retina Most of the cones are located in the central region of the retina called the fovea or fovea centralis, where the eye forms the sharpest image

19. Rods are almost completely absent from the fovea The rods and each of the 3 cone types (blue, red, and green) absorb different wavelengths of light and have different thresholds for activation Rods are very sensitive, responding to very dim light, making them best suited for night vision and peripheral vision Rods absorb all wavelengths of visual light but are perceived only in grey tones; they do not distinguish color

20. Cones need bright light to be activated (low sensitivity), but have pigments that furnish a vividly colored view of the world Cones absorb light in only 3 wavelengths; blue at 420 nm, green at or close to 530 nm, and red at or close to 560 nm In addition, rods and cones are “wired” differently to other retinal neurons

21. Rods participate in converging pathways, with as many as 100 rods feeding into each ganglion cell resulting in the rods’ effects being considered collectively with vision appearing fuzzy and indistinct The visual cortex has no way of knowing which rods of the large number influencing a particular ganglion cell are actually activated By contrast, each cone in the fovea (or at most a few) has a straight-through pathway via its “own personal bipolar cell” to a ganglion cell with information from each cone going directly to the higher visual centers This accounts for the detailed, high-resolution view of very small areas of the visual field provided by the cones

22. E2.5 Explain the processing of visual stimuli, including edge enhancement and contralateral processing The retina is made up of 3 layers of cells (See Fig. 46.21 Raven) Rods and cones are found in the layer closest to the external surface of the eyeball The next layer contains the bipolar cells and the layer closest to the eye cavity is made up of ganglion cells

23. Light must first pass through the ganglion cells followed by the bipolar cells before it reaches the rods and cones The rods and cones synapse with the bipolar cells and the bipolar cells synapse with the ganglion cells which will then transmit impulses to the brain by way of the optic nerve At the x-shaped optic chiasma, fibers from the medial aspect of each eye cross over to the opposite side and then continue on via the optic tracts

24. This is known as contralateral processing whereby the right brain processes information from the left visual field and vice versa Contralateral processing is often illustrated in the abnormal perceptions of patients with brain lesions Action potentials in the optic nerves are relayed from the retina via the optic tracts to the lateral geniculate nuclei located in the thalamus, and from there, the visual information is relayed to the visual cortex of the occipital lobes ( See fig. 46.22 Raven)

26. In the processing of visual information edge enhancement occurs Edge enhancement is the result of lateral inhibition whereby the capacity of an excited neuron reduces the activity of its neighbors leading to increased contrast and sharpness in the visual response particularly at the edges of images when there is contrasting background Edge enhancement can be demonstrated using the Hermann grid illusion which is a type of illusion that deceives a person’s vision

27. Hermann grid illusion

28. E.2.6 Label a diagram of the ear

29. E.2.7 Explain how sound is perceived by the ear, including the roles of the eardrum, bones of the middle ear, oval and round windows, and the hair cells of the cochlea The process of hearing can be divided into 6 basic steps: 1.Sound waves enter the external auditory canal and travel toward the tympanic membrane (eardrum) 2.Movement of the tympanic membrane causes displacement of the auditory ossicles (bones of the middle ear); when the tympanic membrane vibrates, so do the malleus and through their articulations, the incus and stapes, and the sound is amplified

30. 3.Movement of the stapes at the oval window establishes pressure waves in the fluid (perilymph) of the vestibular duct 4.Pressure waves distort the basilar membrane on their way to the round window of the tympanic duct; the lower the frequency of the sound, the longer the wavelength, and the farther from the oval window the area of maximum distortion will be. Frequency information is translated into position information

31. 5.Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane; this movement leads to displacement of the stereocilia (bend), which in turn leads to depolarization of the hair cells and stimulation of the sensory neurons 6.Information about the region and intensity of stimulation is relayed to the CNS over the cochlear branch of the vestibulocochlear nerve (auditory nerve) NVIII (Text 46.12, pg.981)

33. TOK: Other organisms can detect stimuli that humans cannot. For example, some pollinators can detect electromagnetic radiation in the non-visible range. As a consequence, they might perceive a flower as patterned when we perceive it as plain. To what extent, therefore, is what we perceive merely a construction of reality? To what extent are we dependent upon technology to “know” the biological world?