Sensation

1. Sensation • Senses: Means by which brain receives information about environment and body • General: Distributed over large part of body • Somatic: Touch, pressure, temperature, proprioception, pain • Visceral: Internal organs and consist mostly of pain and pressure • Special senses: Smell, taste, sight, hearing, balance • Sensation or perception: Conscious awareness of stimuli received by sensory receptors

2. Sensation • Sensation or perception is the conscious awareness of stimuli received by receptor. • Receptors transduce (change) different forms of energy into nerve impulses • Nerve impulses are conducted to the brain • Stimulus must initiate and action potential in the cerebral cortex • The brain interprets these impulses as sound or sight even though the impulses themselves are identical in nature. • The cerebral cortex screens the information and ignores most of what it receives – subconscious • Our senses act as energy filters that perceive a narrow range of energy.

3. Sensation Requires: • A stimulus • Activation of a receptor, • Conduction of an action potential to a specific region of the CNS • Translation or interpretation of the signal. • Sensation or awareness of a stimuli occurs in the cerebral cortex.

4. General Properties of Receptors • A receptor is any structure specialized to detect a stimulus. • All receptors are transducers, changing stimulus energy into nerve energy. • Sensory receptors transmit four kinds of information: • Modality refers to the type of stimulus or sensation it produces (vision, hearing, taste, etc.). • Location is also indicated by which nerve fibers are firing. • Sensory projection is the ability of the brain to identify the site of stimulation. • Intensity of stimulous • Duration is encoded in the way nerve fibers change their firing frequencies over time. • Tonic vs Phasic receptor adaptation.

5. Classification of Receptors

6. Types of Sensory Receptors • Classification by Stimulus Modality • Mechanoreceptors: Compression, bending, stretching of cells • Chemoreceptors: Smell and taste • Thermoreceptors: Temperature • Photoreceptors: Light as vision • Nociceptors: Pain • Classification by Origin of Stimuli • Exteroreceptors: Associated with skin • Visceroreceptors: Associated with organs • Proprioceptors: Associated with joints, tendons

7. Sensory Nerve Endings • Unencapsulated Nerve Endings • Free nerve endings: Cold receptors and warm • Merkel’s disk: Light touch, superficial pressure • Hair follicle receptor: Light touch, bending of hair

9. Sensory Nerve Endings in SkinEncapsulated Nerve Endings • Pacinian corpuscle: Deep cutaneous pressure, vibration and proprioception • Meissner’s corpuscle: Two-point discrimination • Ruffini’s end organ: Continuous touch or pressure • Muscle spindle: Proprioception as to muscle stretch and control of muscle tone • Golgi tendon organ: Important in muscle contraction and tendon stretch proprioception

10. Two-Point Discrimination

11. Muscle Spindle and Golgi Tendon Organ

13. Responses of Sensory Receptors • Receptor: Interaction of stimulus with sensory receptor produces a local potential • Primary: Have axons that conduct action potential in response to receptor potential • Secondary: Have no axons and receptor potentials produced do not result in action potentials but cause release of neurotransmitters • Accommodation or adaptation: Decreased sensitivity to a continued stimulus • Proprioceptors • Tonic: Example is know where little finger is without looking • Phasic: Example is you know where hand is as it moves

14. Sensory Nerve Tracts • Transmit action potentials from periphery to brain • Each pathway involved with specific modalities • First half of word indicates origin, second half indicates termination

15. Spinothalamic System • Conveys cutaneous sensory information to brain • Unable to localize source of stimulus • Divisions • Lateral for pain and temperature • Anterior for light touch, pressure, tickle, itch

16. Dorsal-Column/Medial-Lemniscal System • Carries sensations of • Two-point discrimination • Proprioception • Pressure • Vibration • Tracts • Fasciculus gracilis • Fasciculus cuneatus

17. Spinocerebellar System • Carry proprioceptive information to cerebellum • Actual movements can be monitored and compared to cerebral information representing intended movement • Tracts • Posterior • Anterior

18. Sensory Areas of Cerebral Cortex

19. Somatic Sensory Cortex

20. Pain • Types • Referred: Sensation in one region of body that is not source of stimulus • Phantom: Occurs in people who have appendage amputated or structure removed as tooth • Chronic: Not a response to immediate direct tissue injury

23. Special Senses • Olfaction • Taste • Visual system • Hearing and balance

24. Olfaction • Sense of smell • Olfactory epithelium • 10-20 million neurons • Bipolar neurons project through cribiform plate. • Olfactory hairs • 10 – 20 Cilia per neuron. • Embedded in a mucous layer • Only neurons exposed to external environment • Replaced every 60 days.

27. Olfactory Physiology • Process of Olfaction • Airborne chemicals are dissolved in the fluid covering the olfactory epithelium. • Chemicals must be volatile and water soluble. • Odor molecule binds with a specific receptor • G-protein coupled membrane receptor) • a second messenger is produced, • Sodium channels are opened in the membrane. • The cell is depolarized creating an axon potential. • Olfactory Discrimination • We can discriminate between ~10,000 different odors. • There are between 7-50 primary classes of odors • A characteristic fingerprint of the odor is used to identify the odor. • Olfactory receptors adapt quickly • Some odors can stimulate nociceptors in the trigeminal nerve.

28. Olfactory Neuronal Pathways and the Cortex • Olfactory neurons project to the olfactory bulb. • Mitral cells project to the olfactory cortex. • Lateral olfactory area: conscious perception of smell • Medial olfactory area: visceral and emotional reactions • Intermediate olfactory area merges information from medial and lateral areas and projects back to olfactory bulb to modulate neuronal activity there.

29. Clinical Considerations of Olfaction • Anosmia – inability to smell (1.2% of the population) • Ability to smell decreases with age. • 98-99% of people can smell banana, rose and cloves. • 35% of the population cannot smell androstenone (body odor).

30. Papillae and Taste Buds • Taste results from the action of chemicals on the taste buds found on papillae. • ~10,000 taste buds • Papillae Types • Circumvallate • Fungiform • Foliate • Filiform • Taste Bud Structure • Supporting cells - Form an exterior supporting capsule • Gustatory or taste cells contain gustatory villi or hairs with surface receptors and are replaced every 7 to 10 days.

31. Physiology of Taste • Process of Taste • Molecules are dissolved in saliva. • Substance enters taste pore and attaches to chemoreceptor molecule • Depolarization of the taste cell. • Taste cells have no axons but release neurotransmitter • Neurotransmitter stimulates action potential in cells associated with the gustatory cells.

32. Four Primary Taste Sensations Exist: • Salty • lateral anterior of tongue • The presence of Na+ is detected • Sweet • tip of the tongue • most organic molecules (particularly sugars) are sweet. • Sour • posterior lateral portion of the tongue • H+ are detected • all acids taste sour. • Bitter • most posterior central portion of the tongue • most sensitive • protective function - most poisons are bitter • Umami (Glutamate) may also be considered.

33. Actions of Major Tastants

34. Neuronal Pathways for Taste

35. Visual System

41. Fibrous tunic: Outer Sclera: White outer layer, maintains shape, protects internal structures, provides muscle attachment point, continuous with cornea Cornea: Avascular, transparent, allows light to enter eye and bends and refracts light Vascular tunic: Middle Iris: Controls light entering pupil; smooth muscle Ciliary muscles: Control lens shape; smooth muscle Retina: Inner Contains neurons sensitive to light Macula lutea or fovea centralis: Area of greatest visual acuity Optic disc: Blind spot Compartments Anterior: Aqueous humor Posterior: Vitreous humor Lens Held by suspensory ligaments attached to ciliary muscles Transparent, biconvex Anatomy of the Eye

46. Compartments of the Eye • Posterior Compartment • Vitreous Humor • Anterior Compartment • Anterior Chamber • Posterior Chamber • Aqueous Humor • Produced by ciliary processes

47. Functions of the Complete Eye • Eye functions like a camera • Iris allows light into eye • Lens, cornea, humors focus light onto retina • Light striking retina is converted into action potentials relayed to brain

48. Light • Focal point: Point where light rays converge and cross • The more spherical the lens the more the light is bent. • Reflection: light rays bounce off a non transparent object • Visible light: Portion of electromagnetic spectrum detected by human eye • The visible spectrum ranges form ~400 to 700 nm • Refraction: Bending of light • Divergence: Light striking a concave surface • Convergence: Light striking a convex surface • The cornea, aqueous humor, lens and vitreous humor all refract light.

50. Focus and Accommodation • Focusing system of the eye creates a clear image on the retina. • Emmetropia: Normal resting condition of lens • Far vision: 20 feet + from eye. • Near vision: Closer than 20 feet • Accommodation • Occurs via changes in the shape of the lens. • Pupil constriction • Depth of focus • Convergence • The inverted image on the retina is detected by photoreceptors and passed via action potentials to the visual cortex.