The Senses

1. The Senses Chapter 29

2. Sensory Input • All senses trigger the same type of action potential • The part of the brain that is activated discriminates between the types of stimuli • The brain detects sensations and interprets them • Action potentials from sensory receptors • Perceptions are constructed from sensations being integrated with other information • Results from neuronal communication in complex circuits involving multiple brain areas

3. Detecting a Stimulus • Sensory organs detect stimuli via specialized cells or neurons called sensory receptors • The cells convert 1 type of signal to an electrical signal, called sensory transduction • Produces a receptor potential, a graded change in response to opening or closing ion channels • Neurotransmitter release is increased to signal the CNS • The rate change in action potentials signals the brain

4. Sensory neurons from each type of sensory receptor synapse with different interneurons Sensory adaptation, tendency of some sensory receptors to become less sensitive when stimulated repeatedly Fewer action potentials which limits reactions to normal background stimuli

5. Types of Sensory Receptors • Pain receptors – tissue damage • All parts of the body but brain • Respond to excess heat, pressure, or chemicals • Thermoreceptors – hot and cold • Monitor both skin surface and blood temperature • Mechanoreceptors – touch and pressure • Bend or stretch PM of receptor cell changing permeability • Hair cells detect sound waves and other movements in H2O • Chemoreceptors – chemicals in the environment • Internal ones to monitor arteries and O2 changes • Osmoreceptors detect changes in blood solute concentration and can stimulate thirst • Electromagnetic receptors – energy wavelengths • Detect muscle movement in prey or help migrate • Photoreceptors detect light via pigments that absorb light

6. Receptors for discriminate stimuli Electromagnetic receptors for infrared light Chemoreceptors for pheromones Mechanoreceptors for sound waves

7. Homonculus Larger areas have more sensory receptors devoted to it. Necessary for greater control of mouth and fingers than shoulders.

8. Gross Anatomy of the Ear • 3 main regions • Eardrum separates outer and inner • Sound pressure moves ear drum to move ossicles in inner ear • Hammer, anvil, and stirrup amplify signal • Auditory tube equalizes pressure on either side of ear drum (‘pop’) • Semicircular canals for balance • Cochlea houses the hearing organ

9. Anatomy of the Inner Ear • Organ of Corti is the hearing organ • Contains hair cells embedded in a basilar membrane • Tectorial membrane bends hair cells as basilar membrane moves in response to sound waves • Signal to neurons to auditory nerve

10. Volume and Pitch • Sounds detected as increase in action potentials by the brain • Higher volume = higher amplitude of generated wave • Creates more vigorous vibrations in cochlea = more bending = more action potentials • Pitch depends on frequency • High pitch = higher frequency • Basilar membrane responsive to certain frequencies • Hair cells easily damaged due to prolonged exposure to certain frequencies

11. Balance • Body position and movement detected by fluid filled structures in the ears • All operate on principle of bending hairs of hair cells • Semicircular canals detect changes in head’s rate of rotation or movement • 3 canals, 1 in each plane of movement • Head movements cause fluid movement (slower) • Changes fluid pressure changing action potentials • Sudden stop, but fluid still moving = dizzy • Brain interprets and commands skeletal muscles

12. Motion Sickness • Results from conflict between eyes and equilibrium sensors in the inner ear • Feeling motion, but not seeing it • One system in hallucinating, implying toxins in system = vomiting • Dramamine inhibits input from equilibrium sensors • Some people can control normally subconscious body functions, like vomiting

13. The Evolving Eye • Eye cups provide information about light intensity and direction, but no image formed • 2 major types of image forming eyes have evolved • Compound eye: light-detecting units called ommatidia • Each has its own light focusing lens and photoreceptors • Acute motion detectors and color vision • Single-lens eye: evolved in invertebrates and vertebrates independently • Amount of light entering a pupil is regulated by an iris • Passes through a lens which focuses light on the retina

14. Lenses • Focus light onto retina • Squid and fish have rigid eyes so use muscle movement • Mammals use ciliary muscles to change lens shape • Near objects • Contract to round/thicken lens • Distance • Relax to flatten/thin lens

15. Correcting Vision • Vision tests measure visual acuity or ability to distinguish fine detail • 20/20 is normal – chart read at 20 feet • 20/10 better than normal – read at 20 instead of 10 • 20/50 worse than normal – read at 20 instead of 50 • Visual problems often are focusing problems • Nearsightedness: can’t focus at distances, eye longer • Farsightedness: can’t focus close, eye shorter • Astigmatism: blurred vision from misshaped lens • LASIK reshapes cornea to change focusing ability

16. Photoreceptors • 2 types • Rods: sensitive to light • Cones: detect color • Red, blue, and green • Number of each depends on activity level at day and night • Humans • Rods peripherally and cones centrally • Opsins in each absorb light • Rhodopsin in rods • Photopsins in cones • Cone type absorbs color • Light to back of eye forward • Multiple neurons to enhance edges • 3D images from 2 eyes

17. Olfaction • Olfactory receptors detects airborne molecules • Upper portion of nasal cavity • Impulses to olfactory bulb • Cilia from tips in mucous to detect substances • Odors bind to specific receptors that trigger membrane changes • Cause action potentials • Humans can detect 1000’s of odors • Animals can detect more, sensation is more important

18. Taste • Taste buds detect molecules in solution • Four familiar and 1 other taste • Sweet • Sour • Salty • Bitter • Umami –meats, cheeses, and protein-rich foods • Each receptor responsive to a particular type of substance • Many ‘tastes’ are really smell (head colds)

19. Supertasters • 3 times as sensitive to bitter tastes • Perceive more foods as bitter than normal, so more food dislikes • Typically avoid coffee, alcohol, vegetables like spinach, cabbage, and broccoli • Two tests • Hypersensitivity to propylthiouracil • Increased number of fungiform papillae, house tastebuds • Higher risk of obesity, colon cancer, and other health problems