Chapter 18: General & Special Senses Chapter objectives: Distinguish between general and specific senses Classify receptors according to stimulus detected, body location, and histological structure Describe the structures of the ear and eye Explain the pathways of sound in the ear and light in the eye Identify, describe, and discuss the receptors and neural pathways involved in each of the five special senses
4 general senses Nociceptors Thermoceptors Mechanoceptors Chemoreceptors 5 special senses Olfaction Gustation Hearing Equilibrium Vision Classification of Sensory System by structural Complexity
simplest receptor type: free nerve endings Sensory Receptors = specialized cells or cell processes monitoring conditions in/outside body (→ extero- and interoceptors) Receptors are specific for a certain type of stimulus → “receptor specificity” All sensory receptors are transducers, changing incoming stimulus of pressure, vibration, light, etc., into electro-chemical neuron impulses.
Area monitored by one receptor: = Fig 18-1 The larger the receptive field, the poorer ability to localize stimulus (2 pt. discrimination test)
Complexity Range of Receptors Specialized receptor cells Free nerve ending Encapsulated nerve ending
Four General Senses 1. Nociceptors Respond to heat, mechanical stress and chemicals – associated with tissue damage Most concentrated in skin Fast pain (to cortex, usually triggers reflex) Slow pain (later, persistent, indistinct source) Referred pain (visceral, "incorrect" source perceived)
2) Thermoreceptors Respond to changes in temperature In dermis, skeletal muscles, liver and hypothalamus Free nerve endings Cold receptors > warm receptors
3) Mechanoreceptors Respond to physical distortion of cell membrane (e.g.: stretching, twisting, compression) Subdivided into • BaroreceptorsSensitive to internal pressures: blood pressure, lung stretch, digestive tract tension • Proprioceptorsmonitors of muscle stretch • Tactile receptors- touch, pressure, vibration Unencapsulated: free nerve endings, Merkels dics - fine touch Encapsulated: Meissners corpuscles - fine touch; Pacinian corpuscles - deep pressure
4) Chemoreceptors Respond to small concentration changes of specific molecules (chemicals) Internal chemoreceptors monitor blood composition (e.g. Na+, pH, pCO2 ) Found within aortic and carotid bodies Very important for homeostasis Fig 18-5
Special Senses • Olfaction • Taste • Vision • Hearing • Equilibrium Organ responsible ??
Olfaction: Paired Olfactory Organs • Olfactory epithelium (10-20 Mio receptors / 5 cm2) • Responds to molecules dissolved in mucus or lipids • Easy to recognize – hard to categorize • (Only) neuron that can be replaced in adult • Through cribriform plate of ethmoid to olfactory bulb Type of receptor??
Olfactory Pathways Receptor neurons pass into cranium through cribiform plate and synapse in olfactory bulbs. Olfactory neurons are the only neurons known • to routinely replace themselves • to reach the cerebrum without synapsing in the thalamus Olfactory discrimination - Although difficult to describe, the number of different odors recognizable is immense.
Gustation 3 types of papillae 1) Filiform - thin, thread like projections 2) Fungiform - shaped like mushrooms. 3) Circumvallate - large target-shaped bumps near the back of the tongue Papillae contain taste buds Taste buds contain group of receptor & support cells How many 1o taste sensations? Fig 18.7
Gustatory Pathway Cranial nerves VII, IX and X to nucleus solitarius in medulla oblongata to gustatory cortex Fig 18.8
Hearing & Equilibrium 2 other names??
Middle Ear Function of the 2 muscles?
Inner Ear • Bony labyrinth vs. membranous labyrinth • Perilymph vs. endolymph • Cochlea & vestibular complex
Structure of cochlea: 2.5 turns of ducts central hub of cochlea
Organ of Corti Basilar membrane on which sit hair cells with stereocilia Tectorial membrane above the hair cells Sound causes hair cells to bounce and touch tectorial membrane causing transduction
Auditory Pathway Cochlear branch of CN VIII To inferior colliculus of opposite side of midbrain To thalamus To auditory cortex To cochlear nucleus of medulla
Vestibular Complex: • Semicircular canals with ampullae (mutually perpendicular) • Saccule and utricle (= fill up vestibule)
Two Receptor Organs:Maculae of Vestibule (or: macula of saccule plus macula of utricle)
Palpebrae = Eyelid • Continuation of skin • Eyelashes • Meibomian glands (on inner margin of lid) • lipid rich product, fu? • bacterial infection chalazion • Conjunctiva (= mucous membrane) • over cornea very thin (5-7 cells thick)
Lacrimal Apparatus Lacrimal gland with several ducts - superior and lateral to eye Lacrimal puncta (superior and inferior) - holes near nose to drain tears Lacrimal canaliculi - drain tears to Nasolacrimal duct - empties to nasal cavity Secretion contains lysozyme Compare to fig 18.18
4 recti 2 oblique Innervation? Extrinsic Eyemuscles (see p.272)
The Three Tunics: 1) Fibrous Tunic (tough outer layer) sclera - white part of fibrous tunic cornea - transparent avascular anterior part limbus - boundary between the above 2) Vascular Tunic (= Uvea) choroid - heavily vascular iris with pupil hole - inner sphincter and outer radial muscles ciliary body - muscle attached to suspensory ligaments, regulates focus of lens
Lens and Chambers of the Eye Ciliary body Suspensory ligaments Anterior and posterior chambers (= anterior cavity) with aqueous humor Posterior cavity with vitreous humor Cataract Glaucoma=? See fig 18.21
3) Nervous Tunic: Retina Outer layer pigmented - inner layer photoreceptors a) rods - black/white vision, dim light b) cones - color vision, intense light Bipolar cells - synapse with rods and cones Ganglion cells - synapse with bipolar cells Ora serrata - anterior edge of retina Macula lutea – fovea centralis - all cones, best vision Optic disc – blind spot, where optic nerve exits eye Optic nerve See Fig 18.22
Visual Pathway Optic chiasma - optic nerves partially cross (right side of the field of each eye combining and going to the lateral geniculate on the right, those from the left to the left) To superior colliculus and thalamus To visual cortex in __________ lobe THE END