THE SPECIAL SENSES

Special senses: smell (olfaction), taste (gustation), vision, hearing (audition), and vestibular sensation • Special sensory neurons detect light, chemicals, and sounds present in environment and convert or transduce these stimuli • Many special senses rely on receptors that are not neurons THE SPECIAL SENSES

Figure 15.1 Comparison of general and special senses.

© 2016 Pearson Education, Inc. CONCEPT BOOST: TRANSDUCTION IN THE SPECIAL SENSES

© 2016 Pearson Education, Inc. PHYSIOLOGY OF SMELL STRUCTURES OF OLFACTION

Olfactory system allows for detection of odorants in air; transduces them into signals perceived as odors • Olfaction – initiated at olfactory epithelium: . Olfactory neurons (olfactory receptor cells) – modified bipolar neurons . Basal cells – stem cells that continually replace olfactory neurons . Supporting cells – columnar cells that surround olfactory neurons STRUCTURES OF OLFACTION

Olfactory epithelium and its anatomical structure: • Olfactory nerve (CN I) – combined axons of olfactory neurons • Olfactory bulb – structure that sits above cribriform plate of ethmoid bone, inferior to frontal lobe of brain • Olfactory tract – composed of axons that exit olfactory bulb; travel to other regions of CNS for interpretation STRUCTURES OF OLFACTION

Figure 15.2 Olfactory epithelium and receptor cells.

© 2016 Pearson Education, Inc. PHYSIOLOGY OF OLFACTION

1. Binding of an odorant to its receptor activates G- protein 2. Activated G-protein triggers enzyme adenylate cyclase to convert ATP into cyclic AMP (cAMP) 3. cAMP opens ion channels that allow sodium and calcium ions to enter cell; causes depolarization and action potential generation if threshold is reached 4. Once an action potential is generated, odorant has been transduced from chemical stimulus to electrical or neural signal PHYSIOLOGY OF OLFACTION

Figure 15.3 Transduction of olfaction in an olfactory neuron.

© 2016 Pearson Education, Inc. PHYSIOLOGY OF OLFACTION

Figure 15.4 The olfactory pathway.

© 2016 Pearson Education, Inc. ANATOMY OF THE EYE ACCESSORY STRUCTURES OF THE EYE • Lacrimal apparatus

. Lacrimal gland – in superolateral region of orbit posterior to conjunctiva; releases tears and mucus into tiny ducts that enter conjunctival sac when stimulated by autonomic neurons; lubricates and washes away debris

. Blinking sweeps tears medially and inferiorly across eye surface where they drain into passages that lead to nasal cavity ACCESSORY STRUCTURES OF THE EYE

Figure 15.9 The lacrimal apparatus.

© 2016 Pearson Education, Inc. ACCESSORY STRUCTURES OF THE EYE

Figure 15.10 Extrinsic eye muscles.

© 2016 Pearson Education, Inc. THE EYEBALL

• Eyeball – hollow sphere; consists of an outer wall that surrounds several chambers and supports lens (focuses light as it enters eye)

o Fibrous layer o Vascular layer o Neural layer THE EYEBALL

Figure 15.11 Sagittal section of internal structures of the eye.

© 2016 Pearson Education, Inc. THE EYEBALL

Figure 15.11 Sagittal section of internal structures of the eye.

© 2016 Pearson Education, Inc. THE EYEBALL

Figure 15.11 Sagittal section of internal structures of the eye.

© 2016 Pearson Education, Inc. THE EYEBALL

• Vascular layer (continued):

. Papillary sphincter muscle, found in iris, contracts during parasympathetic stimulation; reduces size of pupil; restricts amount of light that enters eyeball

. Papillary dilator muscle, also found in iris, contracts during sympathetic activation; allows pupil to increase in size; allows for entry of more light into eyeball THE EYEBALL

Figure 15.12 Constriction and dilation of the pupil.

© 2016 Pearson Education, Inc. THE EYEBALL

Figure 15.13 Photo of interior view of the eye.

© 2016 Pearson Education, Inc. THE EYEBALL

Figure 15.14 Demonstration of the blind spot and “filling in”.

© 2016 Pearson Education, Inc. THE EYEBALL

Figure 15.15 Cavities and chambers of the eye.

© 2016 Pearson Education, Inc. PHOTORECEPTORS AND THE RETINA • Cell types layered in inner layer of retina

o Cones – photoreceptors that function best in bright light for processing high-resolution color vision o Rods – photoreceptors that do not detect colors; instead, rods are most sensitive in low light and as component of peripheral vision o Photoreceptors synapse with bipolar cells, neurons that communicate with retinal ganglion cells o Retinal ganglion cells – in anteriormost region of retina; axons form optic nerve (CN II) o Horizontal cells and amacrine cells – involved in image processing PHOTORECEPTORS AND THE RETINA

Figure 15.21 Layers of the retina.

© 2016 Pearson Education, Inc. PHOTORECEPTORS AND THE RETINA

Figure 15.22a, b Cones and rods.

© 2016 Pearson Education, Inc. PHOTORECEPTORS AND THE RETINA . In absence of stimulation (in dark) photoreceptor cells are depolarized and continuously release neurotransmitters into synapses with other neurons; reverse of most neuron activation . In presence of light, photoreceptor becomes hyperpolarized and stops releasing neurotransmitter, altering activity of neighboring retina cells; sends information to brain PHOTORECEPTORS AND THE RETINA

Figure 15.23a Transduction of light in a photoreceptor cell.

© 2016 Pearson Education, Inc. PHOTORECEPTORS AND THE RETINA

Figure 15.23b Transduction of light in a photoreceptor cell.

© 2016 Pearson Education, Inc. PHOTORECEPTORS AND THE RETINA • Image processing by retina in dark consists of following steps :

. Photoreceptor depolarizes and releases glutamate onto bipolar cells

. Glutamate inhibits bipolar cell and reduces its release of neurotransmitters

. Retinal ganglion cell does not produce an action potential; no signals are sent to brain via optic nerve PHOTORECEPTORS AND THE RETINA

Figure 15.24 Image processing in the retina.

© 2016 Pearson Education, Inc.