Understanding Sensory Receptors and Perception Pathways

1. Chapter 12Somatic and Special Senses Somatic (General) Senses • receptors that are widely distributed throughout the body • skin, various organs and joints Special Senses • specialized receptors confined to structures in the head • eyes, ears, nose, mouth

2. Senses Sensory Receptors • specialized cells or multicellular structures that collect information from the environment • stimulate neurons to send impulses along sensory fibers to the brain Sensation • a feeling that occurs when brain becomes aware of sensory impulse Perception • a person’s view of the stimulus; the way the brain interprets the information

3. Pathways From Sensation to Perception (Example of an Apple)

4. Receptor Types - General Chemoreceptors • respond to changes in chemical concentrations Pain receptors • respond to tissue damage Thermoreceptors • respond to changes in temperature

5. Mechanoreceptors • respond to mechanical forces proprioceptors –tensions of muscles and tendons baroreceptors – blood pressure stretch receptors – inflation in lungs Photoreceptors • respond to light

6. Sensory Impulses • Stimulation causes a change in membrane potential • Nerve fiberSensory receptors can be ends of nerve fibers or other cells close to them • generates an action potential and an impulse is sent • Peripheral nerves transmit the impulse to the CNS • CNS analyzes and interprets the impulse

7. Sensations • Feelings that occur when the brain interprets sensory impulses • All nerve impulses are the same • Sensation depends on which area of the • cerebral cortex receives the impulse • Receptors usually respond to specific • stimuli so the brain creates the correct • sensation

8. Cerebral cortex interprets the impulse to come from the receptors stimulated • Projection allows a person to pinpoint the region of stimulation Ex: eyes see and the nose smells things

9. Sensory Adaptation • Occurs when sensory receptors are continuously stimulated • As receptors adapt impulses leave at decreasing rates and may stop entirely • Impulses are only triggered if the strength of the stimulus changes Ex: adjusting to bad smell in a room

10. Somatic (General) Senses • senses associated with skin, muscles, joints, and viscera • three groups • exteroceptive senses – senses associated with body surface; touch, pressure, temperature, pain • visceroceptive senses – senses associated with changes in viscera; blood pressure stretching blood vessels, ingesting a meal • proprioceptive senses – senses associated with changes in muscles and tendons

11. Touch and Pressure Senses • common in epithelial tissues • simplest receptors • sense itching Senses mechanical forces that deform or displace tissue 3 types • Sensory nerve fibers – found in epithelial tissue associated with touch and pressure • Meissner’s corpuscles – small oval masses of connective tissue found in hairless part of the skin(lips, palms etc.) sense light touch and texture

12. Pacinian corpuscles – large masses of connective tissue fibers and cells found in subcutaneous tissues stimulated by heavy pressure

13. Touch and Pressure Receptors

14. Temperature Senses Heat receptors • sensitive to temperatures above 25oC (77o F) • unresponsive to temperature above 45oC (113oF) Cold receptors • sensitive to temperature between 10oC (50oF) and 20oC (68oF) • Both cold and heat receptors rapidly adapt Pain receptors • respond to temperatures below 10oC • respond to temperatures above 45oC

15. Pain Receptors • Consist of free nerve endings are located in the skin and internal tissues, except the nervous tissue of the brain • Stimulated when tissue is damaged • Usually specific to the type of pain • Adapt very little, if at all

16. Visceral Pain • pain receptors are the only receptors in viscera whose stimulation produces sensations • pain receptors respond differently to stimulation • not well localized • may feel as if coming from some other part of the body • known as referred pain • an example would be heartburn

17. Pain Nerve Pathways Chronic pain fibers • C fibers • thin, unmyelinated • conduct impulses more slowly • associated with dull, aching pain • difficult to pinpoint Acute pain fibers • A-delta fibers • thin, myelinated • conduct impulses rapidly • associated with sharp pain • well localized

18. An event may trigger impulses on both types of fibers • causing a dull sensation • a sharp pain then a dull aching one • aching pain is usually more intense and may worsen over time • Awareness of pain occurs when the impulse reaches the thalamus • Cerebral cortex judges the intensity and locates the source of pain

19. Midbrain, pons, and medulla oblongata regulate the flow of impulses from the spinal cord • Neuropeptides (endorphins) inhibit pain sensations • enkephalins and endorphins can suppress chronic and acute pain impulses • serotonin stimulates other neurons to release enkephalins

20. Stretch Receptors

21. Stretch Receptors • Proprioceptors that sense length and tensions of muscles - 2 main types Muscle spindles • found in muscles near junctions with tendons • responds to a muscle stretching (stretch reflex) • helps maintain the desired position of a limb

22. Golgi tendon organs • found in tendons close to muscle attachments • stimulated by increased tension • helps maintain posture and protects muscles from being torn away from attachments

23. Special Senses • sensory receptors are within large, complex sensory organs in the head • smell in olfactory organs • taste in taste buds • hearing and equilibrium in ears • sight in eyes

24. Sense of Smell Olfactory Receptors • chemoreceptors • respond to chemicals dissolved in liquids Olfactory Organs • contain olfactory receptors (bipolar neurons) and supporting epithelial cells • cover parts of nasal cavity, superior nasal conchae, and a portion of the nasal septum

25. Olfactory Receptors

26. Olfactory Nerve Pathways Once olfactory receptors are stimulated, nerve impulses travel through • olfactory nerves olfactory bulbs olfactory tracts limbic system (for emotions) and olfactory cortex in cerebrum(for interpretation) • The only nerve cells in direct contact to the outside environment • the only neurons that are regularly replaced (subject to damage) yet about 1% are lost every year • sense of smell diminishes with age

27. Olfactory Stimulation • olfactory organs located high in the nasal cavity above the usual pathway of inhaled air • olfactory receptors undergo sensory adaptation rapidly • sense of smell drops by 50% within a second after stimulation Olfactory Code • hypothesis • each odor may stimulate a specific set of receptor subtypes

28. Sense of Taste Taste Buds • organs of taste • located on papillae of tongue, roof of mouth, linings of cheeks and walls of pharynx Taste Receptors • chemoreceptors • taste cells – modified epithelial cells that function as receptors • taste hairs –microvilli that protrude from taste cells; sensitive parts of taste cells

29. Taste Receptors

30. Taste Sensations Primary Taste Sensations • sweet – most plentiful at tip of tongue • sour – margins of tongue • salty – tip & upper front part of tongue • bitter – back of tongue • umami(savory) – taste of glutamate (MSG) • Food must be dissolved in saliva to be tasted • Taste cells are modified epithelial cells that are replaced continually – taste does not diminish with age

31. Hearing Ear – organ of hearing Three Sections • External • Middle • Inner

32. External ear • outer structure called auricle - collects sound waves and directs them inward • external auditory meatus leads inward about 2.5 cm • lined with skin that has modified sweat glands that secrete wax • hairs guard the opening • sound waves pass to the end and alter the pressure on the tympanic membrane (eardrum)

33. Ear

34. Middle Ear • Includes the tympanic membrane, tympanic cavity, and 3 small bones called auditory ossicles (malleus, incus, stapes) • Tympanic cavity is an air-filled space in the temporal bone that separates the external and internal ears • Tympanic membrane covered by a thin layer of skin on outer surface and mucous membrane on the inside

35. Auditory ossicles transmit vibrations between the tympanic membrane and inner ear Malleus, incus and stapes • Vibration on the stapes moves fluid within the inner ear which stimulates hearing receptors • Oval window is an opening in the wall of tympanic cavity where the stapes is attached by ligaments

36. Tympanic reflex reduces pressure from loud sounds that might damage receptors – ossicles become more rigid • Auditory Tube (Eustachian tube) • Connects each middle ear to the throat • Allows air to pass between the tympanic cavity and outside the body

37. Helps maintain equal pressure on both sides of the tympanic membrane- needed for normal hearing • air pressure changes with altitude causing ears to pop • usually closed by valvelike flaps in the throat • swallowing, yawning, and chewing help open the valves and equalize the pressure

38. Inner Ear • Complex system of intercommunicating chambers called labyrinth – each ear has 2 regions • osseous labyrinth – bony canal in temporal bone • membraneous labyrinth – a tube within the osseous labyrinth • perylymph is fluid between the 2 chambers

39. endolymph is fluid in the membraneous labyrinth • cochlea contains the organ of Corti which contains the receptors (hair cells) that vibrations in the fluid stimulates • different frequencies stimulate different receptors • humans can detect sound waves with frequencies from 20 - 20,000 vibrations/second • semicircular canals provide a sense of equilibrium

40. Inner Ear

41. Organ of Corti

42. Auditory Nerve Pathways • Nerve fibers from hearing receptors travel in the cochlear branch of the vestibulocochlear nerves • Impulses travel to the medulla oblongata, midbrain, thalamus, and are interpreted in the temporal lobes of the cerebrum

43. Auditory Nerve Pathways

44. Summary of the Generation of Sensory Impulses from the Ear

45. Sense of Equilibrium Static Equilibrium • Maintains the stability of the head and body when they are motionless • Organs of static equilibrium are the vestibule (bony chamber between the semicircular canals and cochlea)

46. Sensory receptors are hair cells in a gelatinous material that sense changes in position • Movements tilt the gelatinous mass which stimulates the hair cells to signal nerve fibers • Brain responds by sending impulses to skeletal muscles to contract or relax in order to maintain balance

47. Vestibule

48. Macula • responds to changes in head position • bending of hairs results in generation of nerve impulse

49. Dynamic Equilibrium • Balances the head and body when they are moved or rotated suddenly • Sense organs (hair cells) are located in the ampullae of the semicircular canals • Sudden movements cause the hair cells to bend and send an impulse to the brain • Parts of the cerebellum also interpret information from the semicircular canals and help maintain balance • Visual information is also used to maintain balance

50. Semicircular Canals