Sensory Systems Dr Audrey Ettinger April 10 2006

Sensory Systems Dr. Audrey Ettinger April 10, 2006

Neuronal signaling Axon: electrical signal Synapse: Chemical signal Receptors Figure 39 -10 Page 755

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

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

Taste (Gustation) Epithelial cells Taste pore Taste receptor cell Papillae Fig. 41 -10 a, b Page 797 Taste bud 50 µm

Sugar binds to a taste receptor Sugar molecule 1 Receptor K+ channel open 2 3 G protein Adenylyl cyclase GTP 4 activates Figure 41 -10 c Page 797 5 Protein kinase A 6 K+ channel closes

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

Olfactory receptors Sinuses Olfactory bulb Olfactory tract to brain centers for smell Neurons of the olfactory bulb Olfactory bulb Receptor cells Nonsensory epithelium Wall of nasal cavity Cilia Fig. 41 -11 a, b Page 798

Odorant binding to a receptor Odor molecule 1 2 G protein Receptor 3 Adenylyl cyclase Na+ channel closed GTP 4 5 Figure 41 -11 c Page 798 Na+ channel opens

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

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

Pressure directly opens sodium channels in the Pacinian corpuscle Pressure Sodium channel closed Sodium channel opens Figure 41 -2 c 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 Figure 41 -14 Page 801 Retina Iris Lens Pathway of light Pupil Optic nerve Cornea “Blind spot” Fovea

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

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

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

Discs Light REDUCES signaling from the rods Na+ channel open Plasma membrane of rod Na+ channel closes Photon Plasma membrane of disc Rod Esterase G protein Rhodopsin Disc interior Figure 41 -18 Page 803 In the dark, the rod cell is depolarized In the light, the rod cell becomes hyperpolarized

Cone Rod Discs Cells of the retina Horizontal cell Bipolar cell Amacrine cell Figure 41 -17 Page 802 Ganglion cell To optic nerve Light

The light signal travels into the brain Optic chiasm Lateral geniculate nucleus of the thalamus Right primary visual cortex Optic nerves Left primary visual cortex Figure 41 -19 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 Cochlear nerve, division of the vestibulocochlear (VIII) nerve Oval window Figure 41 -9 a Page 795 Organ of Corti

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

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

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!