Sensory Systems Dr. Audrey Ettinger April 10, 2006

Figure Page 755 Synapse: Chemical signal Receptors Axon: electrical signal Neuronal signaling

Examples of the Variety of Neurons Found in the Human Nervous System

Human (and animal) senses Vision Hearing Touch Taste Smell Proprioception (the secret sixth sense)

What is the purpose of having senses? Answer: Senses bring information about the outside world into the body

Sensory systems Convert sensory information into neural signals Process is called sensory transduction

What kind of information should the sensory system transduce for the rest of the nervous system?

Transduction process must include two kinds of information What kind of signal? –Red or blue sweater? –Salty or sweet taste? –More generally: a taste or a color? How much signal? –Loud or soft music?

General principles of sensation “What kind” information is transmitted by which neurons respond to the signal “How much” information is transmitted by the number of action potentials sent –The action potential is an “all or none” signal

Characteristics of sensory neurons Shape suited to function Receptor type (molecule) specific to sense –Chemoreceptors, mechanoreceptors, photoreceptors GRADED receptor potential –Not all-or-nothing action potential Synapse onto neuron that fires regular action potentials

Taste Chemoreceptors are similar to neurotransmitter receptors Only four (or five) tastes recognized by humans –Sweet, salty, bitter, sour, “umami” (MSG) One kind of receptor for each taste

Fig a,b Page 797 Papillae 50 µm Taste bud Epithelial cellsTaste receptor cellTaste pore Taste (Gustation)

G protein GTP K + channel open Adenylyl cyclase Protein kinase A Sugar molecule Receptor K + channel closes activates Figure 41-10c Page 797 Sugar binds to a taste receptor

Taste “Stronger” taste results from more sugar molecules binding Closing potassium channels depolarizes cell Gustatory neuron synapses onto another neuron to carry information into the brain

Smell Chemoreceptors are similar to neurotransmitter receptors 10,000 odorants recognized by humans 1,000 kinds of odorant receptors Each odorant activates a subset of receptors Pattern of receptors bound indicates odorant

Sinuses Neurons of the olfactory bulb Olfactory tract to brain centers for smell Wall of nasal cavity Cilia Nonsensory epithelium Receptor cells Olfactory bulb Olfactory bulb Fig a,b Page 798 Olfactory receptors

Figure 41-11c Page 798 GTP Na + channel opens Na + channel closed G protein Receptor Odor molecule Adenylyl cyclase Odorant binding to a receptor

Smell “Stronger” smell results from more odorant molecules binding Opening sodium channels depolarizes cell Olfactory neuron synapses onto another neuron to carry information into the brain

Touch (and pressure, and pain) Six kinds of mechanoreceptors Respond to different types of input Different adaptation patterns

Figure 41-2a,b Page 790 Ruffini corpuscle (pressure) Dermis 500 µm Pacinian corpuscle (deep pressure, touch) Hair follicle receptor (hair displacement) Merkel disc (touch, pressure) Meissner corpuscle (touch, pressure) HairFree nerve endings (pain) Epidermis Subcutaneous tissue Mechanoreceptors

Pressure directly opens sodium channels in the Pacinian corpuscle Pressure Sodium channel opens Sodium channel closed Figure 41-2c Page 790

Touch, pressure, pain Opening sodium channels depolarizes cell Mechanoreceptor neurons synapse onto another neuron to carry information into the brain

Vision Best understood of all the senses Retina contains light-sensitive cells Four types of photoreceptors : 3 cones, 1 rod Four additional types of neurons are present in the retina

The human eye Iris Lens Pupil Cornea Retina Pathway of light Optic nerve Fovea “Blind spot” Figure Page 801

The retina Light rays Bipolar cell Ganglion cell Cone cell Rod cell Optic nerve fibers Pigmented epithelium Retina Figure 41-16a Page 802

Electron microscopy of rods and cones Cone cellRod cell 10 µm Figure 41-16b Page 802

Vision Photoreceptor types are responsive to different light signals Brighter light causes bigger response More photoreceptors are recruited to see bigger objects

Figure Page 803 Na + channel closes Discs Rod Discs In the light, the rod cell becomes hyperpolarized In the dark, the rod cell is depolarized G proteinRhodopsinEsterase Na + channel open Plasma membrane of rod Plasma membrane of disc Disc interior Photon Light REDUCES signaling from the rods

Cells of the retina Cone Bipolar cell To optic nerve Light Amacrine cell Horizontal cell Ganglion cell Rod Discs Figure Page 802

The light signal travels into the brain Optic nerves Optic chiasm Lateral geniculate nucleus of the thalamus Right primary visual cortex Left primary visual cortex Figure Page 804

Vision Several neuron types process vision in the retina Visual information crosses to the opposite side of the brain Visual information travels to the thalamus (lateral geniculate nucleus) and then to primary visual cortex

Hearing Auditory receptors are complex mechanoreceptors Pitch and loudness signals are transduced by the same receptors

The Organ of Corti lies within the ear Oval window Organ of Corti Cochlear nerve, division of the vestibulocochlear (VIII) nerve Figure 41-9a Page 795

Auditory receptors Hair cell Force Fluid vibrations Tectorial membrane Organ of Corti Basilar membrane Tectorial membrane Stereocilia Basilar membrane Cochlear nerve Force Figure 41-9b,c Page 795

Auditory receptors Hair cell Force Fluid vibrations Tectorial membrane Organ of Corti Basilar membrane Tectorial membrane Stereocilia Basilar membrane Cochlear nerve Force Figure 41-9b,c Page 795

Hearing Receptors responsive to different wavelengths are arranged spatially Louder sounds move the stereocilia farther Ion channels are mechanically opened to depolarize the hair cells

If this material interests you... NEU/PSY/BIO 220 Sensation and Perception (Spring course; required for Neuroscience major, Biopsych concentration, elective for Biology majors) NEU 200: Introduction to Neuroscience (Spring course) Neuroscience Club!