Sensory Physiology

1. Sensory Physiology • The general purpose of the sensory systems is to convert a specific type of stimulus into electrical energy the CNS can interpret. Each of the sensory systems are unique in how they allow us to interact with our environment. • Somatosensation • Vision • Audition • Gustation • Olfaction

2. Sensory receptors detect change in the environment. • Stimulation of sensory receptors stimulates afferent impulse to the CNS. • The brain or spinal cord will interpret the afferent impulse and respond accordingly.

3. Sensory Receptors Types • Mechanoreceptors : respond to physical deformation such as vibration, pressure, stretch and tension • Thermoreceptors : respond to heat and cold • Photoreceptors: Located in the eyes respond to light • Chemoreceptors : respond to chemical change in the body such a taste smell and body fluid composition • Nociceptors : respond to pain • Proprioceptors: gives your body a sense of position and movements • Located in joint capsules, muscles and tendons.

5. Receptive Fields • A large receptive field will have reduced sensory acuity. The smaller the receptive the greater the sensory acuity. How does this relate to cortical mapping.

6. Overlapping Receptive Fields • Overlapping stimulation between neighboring receptive fields provides general information about the location of a stimulus. • The brain is able to determine contrast between the neurons.

7. Because neuron B is firing at the highest freq., it inhibits A and C via inhibitory pathways to a greater extent than A and C inhibit B. Lateral inhibition “sharpens contrast” in the pattern of action potentials received by the CNS, allowing a finer resolution of stimulus location.

8. CNS activity can screen out certain types of sensory information by inhibiting neurons in the afferent pathway.

9. Dermatomes • Area of the skin that is supplied by a single nerve root

11. Primary Sensory Cortex

12. The Eye • The eyes have binocular vision (we see with both eyes) • Sclera is the white of the eye provides protection and sight for muscular attachment. • The Iris contracts and dilates to allow light into the eye • Ciliary muscle attach to the lens allow us to focus

13. Extra ocular eye muscles and nerves

14. Extra Ocular Eye Muscles Movements of the eyes are tightly regulated by skeletal muscles whose neural controls are influenced by head position and operated in ways that assure convergent image formation.

15. The Eye

16. Retina • Light gets focused here. Photoreceptors are a specialized receptors that convert light waves into electrical impulses. • Rods allow for night and peripheral vision. Make up a majority of the retina. (view objects in with peripheral vision) • Cones : color vision and acuity. In highest concentration of in the fovea. ( turn our heads towards objects) • Optic disc is where the blood vessels enter and optic nerve exits the retina to project to the cortex.

17. The Retina

18. Images formed on the retina are upside down and are only a small fraction of the object’s actual size. Things in the upper visual field project to the inferior retina

19. Accommodation • The convex shaped lens will refract light medially. • Objects that are closer strike the lens at a greater angle (refraction) • This will project the image past the retina.( out of focus) • The PNS contacts ciliary muscles which results in the lens to become shorter and more convex in shape. • The more convex lens refracts the light medially at a greater angle allowing it to focus on the retina. • Presbyopia :near vision is dependant on the flexibility of the lens • With age the lens becomes stiffer making it more difficult to see objects up close

20. ANS and Pupils • Sympathetic stimulation causes radial muscles to dilate the pupils • The lens becomes less convex reducing the angle of refraction • Parasympathetic stimulation causes circular muscles to contract causing constriction of the pupil • This makes the lens more convex increasing the angle of refraction.

21. Light Transmission • Rods and cones function as Photoreceptors. • Absorb light are located on the back of the retina. • Bipolar ,Amacrine and Horizontal cells work as interneurons. • Retinal Ganglion cells (RGC) are the projection neurons that go to the brain. • RGC associated with cones provide information about visual clarity and contrast. • RGC on the peripheral retina associated with rods project information about movement

22. Photoreceptors • The cones have a 1:1 ratio to projection neurons. This allows us to see both color and contrast. • You turn your head to focus light on the fovea of the receptor • Rods are more numerous and tend to converge on horizontal cells. Because you have many rods projecting to only one horizontal cell; your cortex losses the ability to discriminate. • Objects are easier to see if you view the in your peripheral vision.

23. Interneurons of the Eye • Bipolar cells work as interneurons that synapse with the photoreceptors and retinal ganglion cells • Horizontal cells: Many Rods converge on them. • Amacrine and horizontal cells are interneurons that inhibit lateral bipolar cells to create contrast. This increases visual acuity.

24. Visual Pathways • Temporal field (peripheral vision): • Is projected to the medial retina. • Optic nerve fibers will cross at the optic chiasm a travel as optic tracts to the opposite LGN. • Nasal field: (binocular vision) • Project to the lateral retina of the eye. • These fibers continue to the thalamus in the same side.

27. The Ear

28. Outer, Middle and Inner Ear • Outer Ear • auricle • focuses sound into the auditory canal towards the tympanic membrane (ear drum) • Middle Ear • auditory (Eustachian) tube connects to nasopharynx • equalizes air pressure on tympanic membrane • ear ossicles • malleus • incus • Stapes • stapedius and tensor tympani muscles attach to stapes and malleus • Inner Ear • cochlea • organ of sound reception • vestibular apparatus • semicircular ducts, utricle and saccule • organs of equilibrium and balance

30. Anatomy of Inner Ear

31. Sound Production • The ear funnels sound through the external auditory meatus. • These waves cause the tympanic membrane to vibrate.

33. Ear Ossicles • The tympanic cavity contains three small bones: the malleus, incus, and stapes • Transmit vibratory motion of the eardrum to the oval window • Dampened by the tensor tympani and stapedius muscles

34. Physiology of Hearing - Middle Ear • Tympanic membrane • has 18 times area of oval window • ossicles transmit enough force/unit area at oval window to vibrate endolymph in scala vestibuli • Tympanic reflex – specific muscles controlled by cranial nerves (V3) and (VII) are designed to protect the hearing apparatus. • tensor tympani m. tenses tympanic membrane • stapedius m. reduces mobility of stapes • best response to slowly building loud sounds • occurs while speaking • Gun shots or entering a car with the radio on loud can damage the ear because this mechanism doesn’t have time to adapt.

35. Stimulation of Cochlear Hair Cells • Vibration of ossicles causes vibration of basilar membrane under hair cells. • as often as 20,000 times/second

36. Anatomy of Cochlea • A fluid filled tube divided into 3 segments • Scala media (cochlear duct) • spiral organ (organ of Corti) • contains hair cells that detect sound • filled with endolymph (ECF) • Scala vestibuli and Scala tympani • filled with perilymph • The vibrations in the stapes are transmitted to the oval window, which creates ripples (vibrations) in the cochlear fluid of the scala vestibuli

37. Spiral Organ

38. Spiral Organ • Stereocilia of hair cells attach to gelatinous tectorial membrane • Inner hair cells • hearing • Outer hair cells • adjust cochlear responses to different frequencies • increase precision

39. Cochlear Tuning • Increases ability of cochlea to receive some sound frequencies • Outer hair cells contract reducing basilar membranes freedom to vibrate • fewer signals from that area allows brain to distinguish between more and less active areas of cochlea • Pons has inhibitory fibers that synapse near the base of IHCs • increases contrast between regions of cochlea

40. Auditory Processing Centers

41. Auditory Pathway

42. Equilibrium • Control of coordination and balance • Receptors in vestibular apparatus • semicircular ducts contain crista • saccule and utricle contain macula • Static equilibrium – perceived by macula • perception of head orientation • Dynamic equilibrium • perception of motion or acceleration • linear acceleration perceived by macula ( moving in a car) • angular acceleration perceived by crista ( turning head)

43. Macula • Made up of the saccule and utricle • Require gravity to work. • Utricular hairs respond to horizontal movement( driving) • Saccular hairs respond to vertical movement ( elevator) • This bends the stereocilia and one kinocilium resulting in a specific direction would result in stimulation of the vestibulocochlear nerve

44. Saccule and Utricle • Referred to as the gravitational receptors. • hair cells with stereocilia and one kinocilium buried in a gelatinous otolithic membrane • Otoliths embedded in the membrane (CaC03 add to the density which increases inertia and enhance the sense of gravity and motion

45. Semicircular Channels • Crista ampullaris :consists of hair cells buried in a mound of gelatinous membrane (one in each duct) • Orientation causes ducts to be stimulated by rotation in different planes • The combination of 6 channels can provide sensory feedback in all planes of motions. • Each channel signals 2 direction. (Forward and backwards) • This gives us information regarding acceleration and deceleration.

46. Crista Ampullaris - Head Rotation • As head turns, endolymph lags behind, pushes cupula, stimulates hair cells • This results in stimulation of the vestibular division on the vestibulocochlear nerve

47. Bending of stereocilia in opposite directions has opposite effects on their membrane potentials.

48. Vestibular Projection Pathways