Sensory function

1. Sensory function Chapter 17

2. Introduction • The general senses are pain, temp. pressure, touch and proprioception. Receptors for these are distributed throughout the body. • Special senses are: smell (olfaction), taste (gustation), sight (vision), hearing (auditory). Receptors for these are located in specialized cells called sense organs. • Receptor specificity allows each receptor to respond to particular stimuli. Simplest receptors are free nerve endings.

3. Introduction • Transduction: translation of a stimulus into an action potential. • Transferred to an afferent fiber which then travels to the CNS. • Tonic receptors-always sending signals. • Phasic receptors: only when the conditions they monitor change. • Fast-adapting receptors are phasic and slow-adapting are tonic.

4. The General Senses • Nociceptors: respond to stimuli that are associated with tissue damage. Two types: fast pain and slow pain. • Thermoreceptors: respond to changes in temperature. • Mechanoreceptors: respond to physical distortion, contact or pressure on their cell membranes. Three types of these.

5. General Senses • Mechanoreceptors: tactile receptors, baroreceptors and proprioceptors. • Tactile:respond to touch pressure and vibration. Six types are: free nerve endings, the root hair plexus, Merkel’s discs, Meissner’s corpuscles, Pacinian corpuscles and Ruffini corpuscles. • Baroreceptors:monitor changes in BP in the walls of major arteries and veins. Help coordinate reflex activities of digestion, monitor changes in the reproductive and urinary tracts.

6. General senses • Proprioceptors: monitor the position of joints, tension in tendons and ligaments,and the state of muscle contraction. • Chemoreceptors: respond to water-soluble and lipid-soluble substances dissolved in the surrounding fluid. They monitor the chemical composition of body fluids.

7. Special Senses-Olfaction • The olfactory organs contain olfactory epithelium with olfactory receptors. The epithelium is located in the roof of the nasal cavity. The receptors are ciliated bipolar neurons. • Individual neurons show a range of responsiveness to different chemicals. It can distinguish thousands of chemical stimuli. The CNS interprets by patterns of receptor activity.

8. Special Senses-Olfaction • Olfactory neurons are excited by volatile chemicals which bind to the receptors. • Action potentials of the nerve fibers are transmitted to the olfactory bulb and then via the olfactory tract to the olfactory cortex (at the inferior surface of each temporal lobe). • The highly sensitive olfactory system has extensive hypothalamic and limbic connections. • The total number of olfactory receptors declines with age.

9. Special Senses-Gustation • Gustatory receptors are clustered in taste buds. • The taste buds are scattered in the oral cavity and pharynx but most abundant in the papillae of the tongue (lingual papillae). • Gustatory cells, the receptor cells of the taste buds, have gustatory hair ( microvilli) that serve as the receptor regions. The cells are excited by the binding of chemicals to receptors on their microvilli.

10. Special Senses-Gustation • The four basic taste qualities-sweet, slat, sour and bitter-are sensed best at different regions of the tongue. Two other tastes:Umami and Water. • The taste sense is monitored by cranial nerves VII, IX and X which send impulses to nucleus solitarius in the medulla. From there the impulses are sent to thalamus and the taste cortex. (primary sensory cortex). • The number of taste buds declines with age.

11. Special Senses-Equilibrium and Hearing(auditory) • The senses of equilibrium and hearing are provided by the inner ear. • Basic receptors of inner ear are hair cells. The surfaces of each hair cell support stereocilia and a single kinocilium. • The ear is divided into the external ear, the middle ear and the inner ear. • The pinna or auricle and external auditory canal compose the outer ear. The tympanic membrane (eardrum), the boundary between the outer and middle ears transmits sound waves to the inner ear.

12. Special Senses-Auditory • The middle ear is a small chamber within the temporal bone, connected by the pharyngotympanic tube to the nasopharynx. The ossicles span the cavity and transmit sound vibrations from the eardrum to the oval window. • The inner ear consists of the bony labyrinth, within which the membranous labyrinth is suspended. Bony-perilymph and membranous-endolymph.

13. Special senses-Auditory • The bony labyrinth is further subdivided to vestibule, semicircular canals and cochlea. • The vestibule contains the saccule and utricle. The receptors provide sensations of gravity and linear acceleration. • The semicircular canals extend posteriorly from the vestibule in three planes. They contain the semicircular ducts. Sensation of rotation. • The cochlea house the cochlear ducts(scala media) which contains the organ of corti (hearing receptor). Within the cochlear duct, the hair (receptor) cells rest on the basilar membrane and their hairs project into the gelatinous tectorial membrane.

14. Sound and Hearing • Sound originates from a vibrating object and travels in waves consisting of alternating areas of compression and refraction. • Wavelength, frequency (pitch), energy. • Amplitude-height of the peaks. Reflects sound’s intensity. Measured in decibels. Perceived as loudness.

15. Sound and Hearing • Sound passing through the external auditory canal sets the eardrum into vibration at the same frequency. The ossicles amplify and deliver the vibrations to the oval window. • Pressure waves in cochlear fluids set specific basilar membranmes into resonance. At points of maximal vibration, the hair cells of the organ of Corti are alternately depolarized and hyperpolarized by the vibratory motion.

16. Auditory • Impulses generated along the cochlear nerve of the vestibulocochlear nerve (NVII) travel to the cochlear nuclei of the medulla. • from there through to the inferior colliculus of mesencephalon. • Then to medial geniculate nucleus of thalamus. • Then projection fibers take to auditory cortex in temporal lobe.

17. Mechanisms of Equilibrium and Hearing • The equilibrium receptors of the inner ear are called the vestibular apparatus. • The receptors for static equilibrium are the maculae of the sacculae and utricle. • The dynamic equilibrium receptor is the crista ampullaris within each semicircular duct. It responds to angular or rotatory movement in one plane. • Impulses form the vestibular apparatus are sent via vestibular nerve fibers to the vestibular complex of the brain stem and cerebellum.

18. The Eye and Vision • The eye has three layers, the fibrous tunic, a vascular tunic and an inner neural tunic. • Fibrous tunic:consists of the sclera (a dense fibrous connective tissue that covers most of the ocular surface) protects the eye and gives it shape, the cornea allows light to enter and the limbus. • Vascular tunic or uvea: includes the iris, the ciliary body and the choroid (provides nutrients). The iris contains blood vessels, pigment cells, and muscle fibers that change the diameter of the pupil. The ciliary body contains the ciliary muscle and the ciliary processes which attach to the suspensory ligaments of the lens.

19. The Eye and Vision • The neural tunic or retina consists of an outer pigmented layer and an inner neural retina. The inner neural retina contains photoreceptors (rods and cones), bipolar cells and ganglion cells. Ganglion cell axons form the optic nerve, which exits via the optic disc (“blind spot”). • The ciliary body and lens divide the interior of the eye into a large posterior cavity or vitreous chamber and a smaller anterior cavity. The ant. Cavity further subdivided to ant. And post. Chamber.

20. The Eye and Vision • The fluid aqueous humor circulates within the eye and reenters the circulation after diffusing through the walls of ant.chamber and into the canal of Schlemm. • The lens lies posterior to the cornea and forms the ant.boundary of the post.cavity. This cavity contains the vitreous body, a gelatinous mass that helps stabilize the shape of the eye. It also supports the retina. • Direct line to the CNS proceeds from-photoreceptor to bipolar cells, to ganglion cells, and to brain via optic nerve. Axons of ganglion cells converge at the optic disc or blind spot.

21. Accessory structures of the Eye • The accessory structures include the palpebrae (eyelids), which are separated by the palpebral fissure. The eyelashes line the palpebral margins.among the inner margin of the lid are Meibomian glands which secrete a lipid-rich product. • An epithelium called the conjuctiva covers most of the exposed surface of the eye. Ocular conjuctiva and palpebral conjuctiva. • The secretions of the lacrimal gland bathe the conjuctiva and contain lysozyme. Tears collect in the lacrimal lake. The tears reach the inferior meatus of the nose after they pass through the lacrimal puncta, lacrimal canals,sac and nasolacrimal duct.

22. Physiology of Vision • Light is made up of those wavelengths that excite the photoreceptors. • Light can be refracted when it goes from one medium to another or hen it strikes a curved surface. Concave -disperse light and convex converge light And bring its rays to a focal point. • As light passes through the eye, it is bent by the cornea and lens and focused on the retina. Cornea accounts for refraction and lens for active focusing for different dsistances .myopia, hyperopia and astigmatism.

23. Physiology of Vision • There are two types of photoreceptors-rods that respond to any radiation and cones which have characteristic ranges of sensitivity. Many cones are densely packed within the fovea centralis the site of sharpest vision. • Rod visual pigment, rhodopsin, is a combination of retinal and opsin. When struck by light, retinal changes shape-cis to all trans-and releases opsin. Freed opsin activates transducin-PDE-Na gates close. Hyperpolarization of receptor cells and inhibits release of neurotransmitter. Night and peripheral vision.

24. Physiology of Vision • The three types of cones all contain retinal but each has a different type of opsin. Each cone responds to one color-red, blue or green. Chemistry similar to rods. Provide for color vision. • Color blindness-inability to detect certain colors. • light adaptation-pigments are bleached and cones start responding. • dark adaptation-cones cease functioning and rods respond.