Sensations & Perceptions

1. Sensations & Perceptions • sensation - is an awareness of sensory stimuli in brain • perception - meaningful interpretation or conscious understanding of sensory data

2. 5 component of sensation 1- Sensory Receptors • Structures that detect changes in external & internal environment.  • Modified neurons or epithelial cells that have evolved to respond to stimuli (eye, ear, nose).  2- Reception - Ability of receptor to absorb energy of a stimulus.

3. 3.Transduction • conversion of stimulus energy into membrane potential*, i.e.,a Receptor Potential... RP (or generator potential... GP --> fires an AP)...   • sort of like an EPSP or IPSP...a change in permeability of a post-synaptic membran • often graded = proportional to strength of stimulus • may be amplified and/or may be summed. • may be strong enough (reaches threshold) to generate action potentials. 

4. 4- Transmission • Receptor potentials transmitted via AP's to CNS • 5-  Integration • Processing of frequency of receptor potentials.  sensory information is coded as frequency of Action potentials

5. Characteristic for receptor cells • Highly selective for specific stimuli, or modalities. • Have the ability to greatly amplify a signal. The sequence of events in a receptor cell from stimulus to generation of AP in a sensory axon is :

6. sequence from stimulus to AP

7. (1) Stimulus energy of the correct modality produces an alteration in receptor protein [in a membrane]. • (2) The receptor may be part of an ion channel or a receptor protein, usually located in a membrane. • (3) It may modulate the activity of ion channels indirectly through an enzyme cascade, which amplifies the cellular signal. • (4) the absorption of energy by the receptor protein eventually leads to a change in conductivity of the ion channels that carry the receptor current. • (5) The receptor current changes. • (6) The change in membrane potential, caused by an altered ion distribution across the receptor cell membrane, is called the receptor potential.

9. Sensory Adaptation • an attenuation of the stimulus...  • A decrease in responsiveness by receptors due to continual stimulation. • A uniformly maintained stimulus of constant intensity is perceived as progressively weaker with time, • while a variable intensity stimulus of shorter durations is perceived as a progressively stronger odor over time

11. Sense of Vision

12. Amount of light entering eye controlled by iris. • Sphincter muscle that lies between cornea and lens. • Light passes through pupil, zone in iris. • Enlarges in dim light to allow more light to enter eye. • Lenses limited by chromatic aberration. • a) Short wavelengths refracted or bent more than longer wavelengths • b) Short wavelengths focus at different point than long wavelengths. • c) Vertebrate eye thus filters out short-wavelength ultraviolet light • d) Insects do not focus light and can perceive ultraviolet light

13. Photoreceptors located on retina. • retina contains rods and cones • a) Rods used for black-and-white vision when illumination is dim. • b) Cones are used for color vision, are shorter than rods. • c) Humans have 100 million rods and 3 million cones in each retina. • d) Most cones found in fovea.            (1) Location where eye forms its sharpest image             (2) Almost no rods found here

14. Cellular structure of rods and cones very similar • Inner segment            (1) Rich in mitochondria             (2) Contains numerous vesicles filled with neurotransmitter molecule b) Outer segment: connected to inner segment by narrow stalk            (1) Packed with hundreds of flattened disks, stacked on one another             (2) Light-capturing photopigment molecules on membranes of these disks

15. Rhodopsin is rod cell photopigment          a) Opsin protein coupled to molecule of cis-retinal.       b) Cis-retinal produced from carotene.

16. Three kinds of cones, each has cis-retinal plus opsin with slightly different amino acid sequence • Sequence shifts absorption maximum from 500 nanometers of rhodopsin            (1) 455 nm is blue-absorbing.             (2) 530 nm is green-absorbing.             (3) 625 nm is red absorbing.

17. Sensory Transduction in Photoreceptors 1. Rod or cone contains many Na+ channels in plasma membrane of outer segment         a) In dark many channels are open          b) Na+ ions continually diffuse into outer segment, across stalk to inner segment          c) Small flow in absence of light called the dark current          d) Causes membrane to be somewhat depolarized in the dark

18. 2. In the light,Na+ channels in outer segment close rapidly a) Reduces dark current b) Causes photoreceptor to hyperpolarize c) Receptor respond by hyperpolarizing rather than depolarizing.

19.  3. Light causes Na+ channels to close         a) Cis-retinal is converted to trans-retinal when the photopigment absorbs light          b) Isomerization causes retinal to dissociate from opsin: bleaching reaction          c) Opsin protein changes shape          d) Shape change activates G protein          e) In turn activates hundreds of phosphodiester molecules          f) This breaks down intracellular messenger cyclic guanosine monophosphate (cGMP)

20. 4. Photopigments, G proteins and phosphodiesterase embedded in outer segment disks         a) cGMP found in cytoplasm between disks and plasma membrane          b) cGMP serves as link between events in disk membrane and Na+ channels in plasma membrane          c) cGMP is required to keep channels open          d) When light is absorbed by photopigment, cGMP is broken down          e) Channels close at rate of 1000 per second          f) Each photopigment coupled to many G proteins each to many phosphodiesterases          g) Absorption of one photon cascades to block entry of over a million Na+molecules          h) Photoreceptor thus hyperpolarizes

24. In the DARK,  Rhodopsin is inactive (CIS) and cGMP is bound to rod cell membranes;   Na channel is open; rod cell is DEPOLARIZED and releases neurotransmitter GLUTAMATE which excites or inhibits adjacent Bipolar cells = no sensation of light.

25. In the LIGHT  Rhodopsin changes conformation (TRANS), which activated enzyme TRANSDUCIN; transducin activates a PHOSPHODIESTRASE, converts cyclic-GMP to GMP; GMPcloses the Na channels, and HYPERPOLARIZES rod cells (RECEPTOR POTENTIAL); hyperpolarized rod cell stops releasing GLUTAMATE, allowing bipolar cells repsond

27. Two basic types of photoreceptor, rods and cones, exist in the vertebrate retina. • The rods are photoreceptors that contain the visual pigment -rhodopsin and are sensitive to blue-green light with a peak sensitivity around 500 nm wavelength of light.

28. Rods are highly sensitive photoreceptors and are used for vision under dark-dim conditions at night.

29. Cones contain cone opsins as their visual pigments • and, depending on the exact structure of the opsin molecule, are maximally sensitive to either long wavelengths of light [564nm],medium wavelengths of light [533nm] or short wavelengths of light [437nm]. • Cones of different wavelength sensitivity and the consequent pathways of connectivity to the brain are the basis of color perception in our visual image.

30. Three different cone mechanisms can be detected in behavioral, psychophysical and physiological testing. • These mechanisms are the basis of so called trichromatic vision which most humans have. • Where only one or two visual pigment bearing types of cone are present the vision is said to be monochromatic or dichromatic.

31. Most mammalian species are dichromatic containing as well as rods only middle and short wavelength sensitive cones in their retinas. • Primates and humans, birds, reptiles and fish are trichromatic, tetrachromatic and some even pentachromatic (the latter three vertebrate phyla).

32. Binocular Vision 1- Visual images of vertebrate eyes a) Eyes on opposite sides of head, each sees object at different angle          b) Parallax permits sensitive depth perception, stereoscopic vision 2- Predators have eyes set in front of head to increase stereoscopic vision 3- Prey have eyes set on sides of head to enlarge total receptive field. 4- Must learn to perceive distance, not inborn.

33. Chemical senses • Distinctions between gustation and olfaction. • Gustation used to detect and identify food-related chemicals in near environment. • Olfaction used not just to detect food, also detection of enemies and mates,

34. The gustatory system: Taste bud • Gustatory receptors found in clusters- taste buds, receptors anaxonal but generate AP's. • Gustatory receptors on tongue, throat, and mouth synapse with afferent fibers that travel to brain via 3 cranial nerves

35. Gustatory transduction • Two kinds of transduction mechanisms: • 1) Direct effect of chemical on one or more ion channels in receptor • 2) Second messenger pathway that indirectly changes flow of ions through channels.

36. Sour and salty stimuli Substances that evoke sour or salty taste exert effects by directly influencing ion channels in membrane of receptor • Weak acid vinegar ionize in water to produce protons (H+) and anions (- ions), in mud puppy, H+ ions block specific type of K+ channel in receptor • For salty substances like table salt Na+ and other cations act as stimuli, Na+ ions of salt enter receptor Na+ channels directly, depolarizing cell, anion exerts some other effect, taste of sodium chloride distinct from sodium bicarbonate

38. Sweet stimuli • Produce response via 2nd messenger, but many kind of receptors, even in single species

39. Binding of sucrose to receptor activates specific G protein, activates enzyme adelylyl cyclase, catalyses the conversion of ATP to cyclic adenosine monophosphate (cAMP), turns on cAMP dependent protein kinase A, protein kinase phosphorylates K+ channel in cell membrane, causes closing, reduction in K+ efflux, depolarization, possible AP.

40. Because variety of receptors, responses vary, not all sweet substances taste same • Rate of adaptation to different sweet stimuli not same, perception of sweet taste of saccharine declines more than twice as fast as sweet taste of sugar on successive samples

41. Bitter stimuli • 1) Direct effect on ion channels- blockage of K+ channels • 2) Receptor controlled ion pump- activation of chloride pump depolarizes • 3) Second messenger systems- example two pathway system, one path releases Ca2+ which directly causes transmitter release, other closes K+ channel causing depolarization, also some bitter receptors have pathway remarkably like rod and cone pathway, G protein 90% similar to transductin in rods and cones called gustductin, activates phosphodiesterase (PDE) that catalyses breakdown of cAMP, rod transductin itself also found in taste cells

43. Umami taste • Amino acids • Some animals have gustatory receptor cells that respond to amino acids, to humans, most amino acids taste bitter, alanine and serine sweet, a few such as salts of glutamic acid have unique taste, umami, neither sour, salty, sweet, or bitter

44. In humans, afferent fibers from tip of tongue travel via facial nerve (cranial nerve VII), back of tongue via glossopharyngeal nerve (IX), pharynx via vagus nerve (X) • After entering brainstem all gustatory afferents synapse in solitary nuclear complex of medulla. • In primates two cortical areas known to be involved in taste processing, facial part of somatosensory cortex, and insula

45. Olfactory transduction • In land animals, odorant must dissolve in mucus or fluid covering receptors • Odorant binding proteins bind with dissolved odorant, move to receptor cell membrane • Two transduction pathways in vertebrate olfactory neurons, each with G protein second messenger cascade and ligand gated ion channel, both pathways may be present in one neuron

46. A) G protein linked to adenylyl cyclase, promotes synthesis of cAMP, opens non-specific cation channel, depolarizes cell • B) G protein activates phospholipase C, stimulates production of IP3 and DAG, IP3 may act on Ca2+ channel

48. The olfactory system • Olfactory receptors line nasal epithelium, axon extends through pores in bones of skull, dendrite has branches, olfactory cilia that contain receptor proteins for odorants, protected by thin layer of mucus

49. Receptors send axons to olfactory bulb, in primates lies over nose, in other vertebrates posterior to nose • Axons from olfactory bulb enter brain via olfactory tract (cranial nerve I), project to primary olfactory cortex or pyriform cortex on anterioventral of telencephalon, no passage through thalamus