The PNS: Afferent Nervous System

1. The PNS: Afferent Nervous System • two kinds of pathways • 1. Somatic: sensory/afferent information from skeletal muscle • receptors are scattered at the body surface • can become specialized = Special senses • 2. Visceral: sensory information from the internal viscera • receptors are scattered throughout the viscera (organs located in a cavity) • e.g. blood pressure, body fluid concentration, respiratory gas concentration • never reaches a conscious level • although you can become aware of pain • this information is critical form determining the appropriate efferent output to maintain homeostasis

2. Perception & Sensation • Sensation: response to environment via generation of nerve impulse • -sensation occurs upon arrival of nerve impulse at cerebral cortex • -before nerve impulse is generated - sensory receptors integrate or sum up the incoming signals • -several types of integration: one type is adaptation - decrease in response to a stimulus • role of the thalamus?? (gatekeeper??) • -nerve impulses sent via ascending tracts in spinal cord to the brain • Perception: our conscious interpretation of the external world • created by the brain based on information it receives from sensory receptors • interpretation of sensation

3. Sensation • each type of sensation = sensory modality • one type of neuron carries only one type of modality • modalities can be grouped into two classes • 1. general senses – includes both the somatic and visceral senses • tactile (touch, pressure), thermal, pain and proprioception • 2. special senses: sight, sound, hearing, taste

4. Sensation • 1. stimulation of the sensory receptor • alters the permeability of the neuron’s PM • usually does this through non-specific opening of small ion channels • 2. transduction of the stimulus • increased influx of Na ions – depolarization – called a graded receptor potential • therefore the sensory receptor converts (transduces) the energy of the stimulus into a graded potential • 3. generation of the nerve impulse • increase in graded receptor potential past threshold -> Action Potential • AP propagates toward the CNS • 4. integration of the sensory input • receipt of sensory information by a particular region in the CNS • integration of sensation and perception

5. Sensory Pathways • these pathways consist of thousands of sets of neurons – grouped into threes • 1. first order neurons – conduct sensory information from the receptor into the CNS • cranial nerves conduct information from the face, mouth, eyes, ears and teeth • spinal nerves conduct information from the neck, trunk and limbs • 2. second order neurons – conduct information from the brain and SC into the thalamus • these neurons decussate (cross over) within the thalamus • 3. third order neurons – conduct information from the thalamus to the primary somatosensory areas within the cerebral cortex • for integration

6. Sensory Pathways

7. Sensory Pathways • sensory pathways enter the SC and ascend to the cerebral cortex via: • 1. the posterior column-medial lemniscus path • for conscious proprioception and most tactile sensations • two tracts of white matter: posterior column and the medial lemniscus • first order neurons from sensory receptors in the trunk and limbs form the posterior columns in the spinal cord • synapse with second order neurons in the medulla oblongata • these then cross to the opposite side of the medulla and enter the medial lemniscus in the thalamus – synapse with the third order neurons that travel to the cortex (primary somatosensory area) • fine touch • stereostegnosis – ability to recognize shapes, sizes and textures by feeling • proprioception • vibratory sensations

8. Sensory Pathways • 2. the anterolateral/spinothalmic path • first order neurons receive impulses from receptors in the neck, trunk or limbs • receptors end in dorsal root ganglion • synapse with the 1st order neurons are located in the dorsal root ganglion • synapse with second order neurons in the posterior gray horn • second order neurons than cross to the opposite side of the SC and pass to the primary somatosensory area in either the: • second order neurons pass through the brain stem as two possible tracts: • lateral spinothalmic tract: pain and temperature • anterior spinothalmic tract: information for tickle, itch, crude touch and pressure

9. Sensory Pathways • two tracts: posterior spinocerebellar and anterior spinocerebellar • major routes for proprioceptive impulses from lower limbs that reach the cerebellum • not consciously perceived • critical for posture, balance and coordination • posterior spinocerebellar routes are degraded upon advanced syphillis – severe uncoordination • first order neurons: muscle spindles and tendon organs • second order neurons: cell bodies in dorsal gray horn via thalamus to the cuneate nucleus of basal ganglia • third order neurons: thalamus to cerebellum (no decussation)

10. Primary Somatosensory area • specific areas of the cerebral cortex receive somatic sensory input from various parts of the body • precise localization of these somatic sensations occurs when they arrive at the primary somatosensory area • some regions provide input to large regions of this area (e.g. cheeks, lips, face and tongue) while others only provide input to smaller areas (trunk and lower limbs)

11. -sensory receptors: can either be a • 1) specialized ending of an afferent neuron • 2) a separate cells closely associated with an afferent neurons • -can classify a sensory receptor based on: • microscopic features: • free nerve endings: bare dendrites associated with pain, heat, tickle, itch and some touch • encapsulated nerve endings: dendrites enclosed in a connective tissue capsule - touch • e.g. Pacinian corpuscle • separate cells: individual receptors that synapse with first-order afferent neurons’ • e.g. gustatory cells (taste) • receptor location: • exteroceptors: located at or near the body surface, responds to information coming in from the environment (taste, touch, smell, vision, pressure, heat and pain) • interoceptors: located in blood vessels, visceral organs and the nervous system; provide information about internal environment • proprioceptors: located in inner ear, skeletal muscle and joints; provides information about position of limbs and head • type of stimulus: • 1. Chemoreceptors • 2. Mechanoreceptors • 3. Nociceptors/pain receptors • 4. Thermoreceptors • 5. Photoreceptors • 6. Osmoreceptors

12. Proprioceptive Sensation Proprioceptors -located in muscles, joints and tendons -position of limbs and degree of muscle relaxation -located in the inner ear – position of head -”hair cells” – position relative to the ground and movement -allow us to estimate weight and to determine how much muscular effort is needed for a task -high concentration in postural muscles (body position), tendons (muscle contraction) -Patellar reflex: muscle stretch, proprioceptor fires impulse to spinal cord, reflex arc results, muscle fiber response

13. Proprioceptive Sensation • three types of proprioceptors • 1. muscle spindles • monitor changes in muscle length • used by the brain to set an overall level of involuntary muscle contraction = motor tone • consists of several sensory nerve endings that wrap around specialized muscle fibers = intrafusal muscle fibers • very plentiful in muscles that produce very fine movements – fingers, eyes • stretching of the muscle stretches the intrafusal fibers, stimulating the sensory neurons – info to the CNS • IFMs also receive incoming information from gamma motor neurons – end near the IFMs and adjust the tension in a muscle spindle according to the CNS • also have extrafusal muscle fibers which are innervated by alpha motor neurons • response to a stretch reflex

14. Proprioceptive Sensation • 2. tendon organs • located at the junction of a tendon and a muscle • protect the tendon and muscles from damage due to excessive tension • consists of a thin capsule of connective tissue enclosing a few bundles of collagen • penetrated by sensory nerve endings that intertwine among the collagen fibers • 3. joint receptors (joint kinesthetic receptors) • several types • located in and around the articular capsules of synovial joints • free nerve endings and mechanoreceptors found – detect pressure within the joint • also can find Pacinian corpuscles which detect the speed of joint movement

15. Tactile Sensations • Cutaneous receptors • located in skin • -dermis: pressure, temperature, touch (fine and crude) and pain • -impulse sent to somatosensory areas of brain • -touch receptors: Meissner’s (fingertips, lips, tongue, nipples, penis/clitoris) – for fine touch • Merkel disks (epidermis/dermis) – fine touch, slowly adapting • Root hair plexus (root of hair) - crude touch receptors • -pressure receptors: Pacinian corpuscles – connective tissue capsule over the dendrites • -temp receptors: free nerve endings that respond to cold OR warmth - pain • -also: Krause end bulbs, Ruffini endings (also for stretching, slowly adapting)

16. Pain • analgesia: relief from pain • drugs: aspirin, ibuprofen – block formation of prostaglandins that stimulate the nociceptors • novocaine – block nerve impulses along pain nerves • morphine, opium & derivatives (codeine) – pain is felt but not perceived in brain (blocks morphine and opiate receptors in pain centers)

17. Taste -Taste requires dissolving of substances -taste buds: salty, sweet, bitter and sour -10,000 taste buds found on tongue, soft palate & larynx -found associated with projections called papillae -open at a taste pore -taste cells are associated with support cells and connect with sensory nerve fibers -tips of taste cells are microvilli - receptors proteins for specific chemicals salty bitter sour

18. Anatomy of Taste Buds • An oval body consisting of 50 receptor cells surrounded by supporting cells • A single gustatory hair projects upward through the taste pore • Basal cells develop into new receptor cells every 10 days. • taste buds: • foliate • fungiform • circumvallate • filliform (texture)

19. The Tongue & Papillae foliate fungiform filiform filiform fungiform circumvallate

20. Physiology of Taste • receptor-ligand interaction – ligand is the chemical from the food and the receptor is the taste cell • binding leads to a change in the receptor potential – action potential • stimulates exocytosis from the taste cell – binds to a first order neuron • pathway is distinct for different chemicals • e.g. salty foods – Na enters the gustatory cell via ligand-gated channels – depolarization – direct method • depolarization opens calcium channels – exocytosis • similar mechanism for sour foods – entrance of H+ ions which opens Na channels • other tastants do NOT enter the cell but bind to the PM – bind to G protein coupled receptors and trigger the production of a second messanger which than causes a depolarization and action potential – indirect methods • Complete adaptation in 1 to 5 minutes • Thresholds for tastes vary among the 4 primary tastes • most sensitive to bitter (poisons) • least sensitive to salty and sweet

21. Gustatory Pathway • gustatory fibers found in cranial nerves • VII (facial) serves anterior 2/3 of tongue • IX (glossopharyngeal) serves posterior 1/3 of tongue • X (vagus) serves palate & epiglottis • Signals travel to thalamus or limbic system & hypothalamus • Taste fibers extend from the thalamus to the primary gustatory area on parietal lobe of the cerebral cortex • providing conscious perception of taste • taste aversion – because of the link between the hypothalmus and the limbic system – conscious and strong connection between taste and emotion

22. Smell • -olfactory cells - located within olfactory epithelium in the nasal cavity • -Covers superior nasal cavity (superior nasal conchae) and cribriform plate • -are modified neurons • -end in microvilli with receptor proteins for odor molecules • -each olfactory cell is specific for one odor molecule - specific neuron types • -olfactory nerves make connections with the limbic system (emotions and memory)

23. Olfactory receptors • bipolar neurons with cilia or olfactory hairs • Supporting cells • columnar epithelium • Basal cells = stem cells • replace receptors monthly • Olfactory glands • produce mucus • Both epithelium & glands innervated by cranial nerve VII.

24. Olfaction: Sense of Smell • Odorants bind to receptors • Na+ channels open • Depolarization occurs • Nerve impulse is triggered • some odors bind the receptor and trigger the activation of a G protein – second messenger production, opening of Na channels and depolarization

25. Olfactory Pathway • has a very low threshold to trigger perception • Axons from olfactory receptors form the olfactory nerves (Cranial nerve I) that synapse in the olfactory bulb • pass through 40 foramina in cribriform plate • neurons within the olfactory bulb form the olfactory tract that synapses on the primary olfactory area of temporal lobe • conscious awareness of smell begins • Other pathways lead to the frontal lobe (Brodmann area 11) where identification of the odor occurs • hyperosmia – keener sense of smell then others • seen in women (time of ovulation) • opposite is hyposmia –reduction in the sense of smell

26. Adaptation & Odor Thresholds • Adaptation = decreasing sensitivity • Olfactory adaptation is rapid • 50% in 1 second • complete in 1 minute • Low threshold • only a few molecules need to be present • e.g. methyl mercaptan added to natural gas as warning

27. Vision Eye: tough outer covering - sclera (white, cornea) -middle choroid layer - vessels, melanin pigment (light absorption) -front of eye it becomes the iris (aperture), -inner nerve layer – retina -sight is generated by the bending and focusing of light onto the retina - done by the lens (shape changes controlled by tiny ciliary muscles) • Anterior cavity (anterior to lens) • filled with aqueous humor • produced by ciliary body • continually drained • replaced every 90 minutes • 2 chambers • anterior chamber between cornea and iris • posterior chamber between iris and lens • Posterior cavity (posterior to lens) • filled with vitreous body (jellylike) • formed once during embryonic life • floaters are debris in vitreous of older individuals

28. Accessory Structures of Eye • Eyelids or palpebrae • protect & lubricate • epidermis, dermis, CT, orbicularis oculi m., tarsal plate, tarsal glands & conjunctiva • Tarsal glands • oily secretions keep lids from sticking together • Conjunctiva • palpebral & bulbar • stops at corneal edge • dilated BV--bloodshot

29. Lacrimal Apparatus • About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called lysozyme.

30. Tunics (Layers) of Eyeball • Fibrous Tunic(outer layer) • Vascular Tunic (middle layer) • Nervous Tunic(inner layer)

31. Fibrous Tunic • CORNEA • Transparent • Helps focus light (refraction) • astigmatism • 3 layers • nonkeratinized stratified squamous (outer) • collagen fibers & fibroblasts • simple squamous epithelium • Nourished by tears & aqueous humor • SCLERA • “White” of the eye • Dense irregular connective tissue layer -- collagen & fibroblasts • Provides shape & support • Posteriorly pierced by Optic Nerve (CNII)

32. Vascular Tunic • Choroid • pigmented epithelial cells (melanocytes) & blood vessels • provides nutrients to retina • black pigment in melanocytes absorb scattered light • Ciliary body • choroid extends to the front of the eye as ciliary muscles and processes – for controlling the shape of the lens • ciliary processes • folds on ciliary body • secrete aqueous humor • ciliary muscle • smooth muscle that alters shape of lens • attach to the ciliary processes • -Suspensory ligaments attach lens to ciliary process • -Ciliary muscle controls tension on ligaments & lens • Lens: • Avascular • Crystallin proteins arranged like layers in onion • Clear capsule & perfectly transparent • Lens held in place by suspensory ligaments which attach to the ciliary processes • Focuses light on fovea (center of the retina)

33. Vascular Tunic Aqueous Humor • Continuously produced by ciliary body • Flows from posterior chamberinto anterior through the pupil • Scleral venous sinus • canal of Schlemm • opening in white of eyeat junction of cornea & sclera • drainage of aqueous humor from eye to bloodstream • Glaucoma • increased intraocular pressure that could produce blindness • problem with drainage of aqueous humor • Iris • is a coloured extension off the ciliary processes • Constrictor pupillae muscles (circular muscles) • are innervated by parasympathetic fibers while • Dilator pupillae muscles (radial muscles) • are innervated by sympathetic fibers. • Response varies with different levels of light

34. Major Processes of Image Formation • Refraction of light • by cornea & lens • light rays must fall upon the retina • Accommodation of the lens • changing shape of lens so that light is focused • Constriction of the pupil • less light enters the eye

35. Definition of Refraction • Bending of light as it passes from one substance (air) into a 2nd substance with a different density(cornea) • In the eye, light is refracted by the anterior & posterior surfaces of the cornea and the lens

36. Refraction by the Cornea & Lens • Image focused on retina is inverted & reversed from left to right • Brain learns to work with that information • 75% of Refraction is done by cornea -- rest is done by the lens • Light rays from > 20’ are nearly parallel and only need to be bent enough to focus on retina • Light rays from < 6’ are more divergent & need more refraction • extra process needed to get additional bending of light is called accommodation

37. Emmetropic eye (normal) • can refract light from 20 ft away • Myopia (nearsighted) • eyeball is too long from front to back • glasses concave • Hypermetropic (farsighted) • eyeball is too short • glasses convex (coke-bottle) • Astigmatism • corneal surface wavy • parts of image out of focus

38. Accommodation & the Lens • Convex lens refracts light rays towards each other • Lens of eye is convex on both surfaces • View a distant object • lens is nearly flat by pulling of suspensory ligaments • View a close object • ciliary muscle is contracted & decreases the pull of the suspensory ligaments on the lens • elastic lens thickens as the tension is removed from it • increase in curvature of lens is called accommodation

39. Nervous Tunic Retina • Posterior 3/4 of eyeball • Optic disc • optic nerve exiting back of eyeball • attachment of retina to optic nerve - optic disc (blind spot) • central depression in retina - fovea centralis • Detached retina • trauma (boxing) • fluid between layers • distortion or blindness View with Ophthalmoscope

40. Photoreceptors -rod and cone cells -rod cells: black and white, bright and dark -cone cells: color vision -visual pigment: opsin and retinal -visual pigment is folded into “discs” = outer segment of the photoreceptor -shape of the outer segment resulted in their name -inner segment - cell body -synaptic endings

41. Rods and Cones • Rods----rod shaped • shades of gray in dim light • 120 million rod cells • discriminates shapes & movements • distributed along periphery • Cones---cone shaped • sharp, color vision • 6 million • 3 types: blue, red and yellow/green colour (differences in opsin structure) • fovea of macula lutea (fovea centralis) • densely packed region of cones • at exact visual axis of eye • sharpest resolution or acuity • sharpest colour vision

42. Retinal cells • Pigmented epithelium • non-visual portion • absorbs stray light & helps keep image clear • 3 layers of neurons (outgrowth of brain) • photoreceptor layer • bipolar neuron layer • ganglion neuron layer • 2 other cell types (modify the signal) • horizontal cells – inhibits transmission to other bipolars • amacrine cells

43. photopigment – rhodopsin in rods, photopsin in cones • undergoes structural changes when it absorbs light • opsin – glycoprotein • responsible for the absorption of light wavelengths • e.g. red cones – opsin for the absorption of red wavelengths • loss of one cone type with one opsin type = color blindness • retinal – vitamin A derivative • in darkness –cis-retinal fits snugly with opsin • upon light – the cis-retinal conformation straightens out into trans-retinal = isomerization • results in the separation of trans-retinal from opsin – the opsin is colourless = bleaching • opsin now acts as an enzyme which acts on the molecular machinery underlying vision – inhibits this machine • the trans retinal gets converted back into cis-retinal by retinal isomerase • cis-retinal is free to rebind with opsin • vitamin A deficiency results in lower formation of rhodopsin = night blindness

44. Formation of Receptor Potentials • In darkness • Na channels open – Na ions flow through Na ligand-gated channels that bind cGMP • the photoreceptor becomes depolarized – release of NT which then binds its target – bipolar neurons • glutamate?? • IPSP results at the post-synaptic neuron (bipolar cell) • prevents transmission of signal from the retina to the optic nerve • receptors are always partially depolarized in the dark leading to a continuous release of inhibitory neurotransmitter onto bipolar cells • In light • isomerization of retinal from cis to trans • this activates enzymes that breakdown cGMP • closing of Na+ channels producing a hyperpolarized receptor potential (-70mV) • release of inhibitory neurotransmitter is stopped • bipolar cells become excited and a nerve impulse will travel towards the brain = image

45. Photochemistry mechanism • In the dark - Na channels in the outer segment are held open by cGMP • Na influx causes depolarization that triggers continual release of glutamate neurotransmitter in rods • Glutamate hyperpolarizes (inhibits) bipolar cells. • Inner segment has pumps that continuously pump Na out and K in, K diffuses out • In the light – photons pass through retinal layers and reaches rods • Cis-retinal is tightly attached to opsin • Cis-retinal absorbs light and shifts to trans-retinal form (isomerization) • Trans-retinal separates from opsin becoming colorless (bleaching) • Opsin activates transducin (a G protein) in the cell membrane • Transducin activates cGMP Phosphodiesterase • This enzyme breaks down cGMP – decrease in cGMP levels closes gated Na channels • This decreases Na influx into the rod while pump continues – more Na+ out than Na+ flowing in • Rod becomes hyperpolarized and ceases glutamate release • Bipolar cells are not inhibited and release neurotransmitter at synapse with ganglion cells resulting in action potential being sent along optic nerve • Retinal isomerase shifts trans-retinal back to cis-retinal form • Cis-retinal rebinds with opsin (regeneration) • Transducin is deactivated and Na channels are reopened • Rods regenerate at about same rate as bleaching occurs in daylight.  Cones regenerate very fast.

46. Dark vs. Light • Activated rhodopsin – bleached opsin and trans-retinal • Activation of transducin • Activation of cGMP phosphodiesterase • Decreased levels of cGMP within the photoreceptor • Closing of cGMP-gated ion channels (sodium) • NO Action potential and glutamate release • Action potentials by ON-Center bipolar cells and ganglion cells • IMAGE FORMATION • PC OFF – 1st, 2nd, 3rd order neurons ON • No activated rhodopsin • No activation of transducin • No activation of cGMP phosphodiesterase • Increased levels of cGMP within the photoreceptor • Opening of cGMP-gated ion channels (sodium) • Action potential and glutamate release • Inhibition of bipolar cell AP and ganglion cell AP • NO IMAGE FORMATION • PC ON – 1st, 2nd, 3rd order neurons OFF

47. Light and Dark Adaptation • Light adaptation • adjustments when emerge from the dark into the light • decreases its sensitivity • increases the bleaching of rhodopsin • decreases light sensitivity • Dark adaptation • adjustments when enter the dark from a bright situation • light sensitivity increases as photopigments regenerate • during first 8 minutes of dark adaptation, only cone pigments are regenerated, so threshold burst of light is seen as color • after sufficient time, sensitivity will increase so that a flash of a single photon of light will be seen as gray-white

48. Retinal Processes of Image Formation • bipolar cells: provide 30% of input to ganglion cells • rod bipolar cells – only one type • cone bipolar cells – ten forms classified as ON-center and OFF-center • ON responds to decreased glutamate upon light by depolarizing (action potential = eventual image) • responds in the dark to increased glutamate by hyperpolarizing (no action potential = no image) • OFF responds to decreased glutamate upon light by hyperpolarization (no action potential) • responds in dark to increased glutamate by depolarizing (action potential) • between 6 to 600 rods synapse with a single bipolar cells = convergence • increases the sensitivity of rod vision – but slightly blurs the image • usually only one cone synapses with a single bipolar cell – less sensitive but sharper vision

49. Retinal Processes of Image Formation • horizontal cells: inhibit the transmission of the visual signal to bipolar cells lateral to the targeted one • found in the outer plexiform/synaptic layer • concentrates the stimulation to a specific area of the retina - more contrast to the image and increases spatial resolution • three types – H1, H2, H3 • H2 converges rods • cones converge on all three types – cone-specific?? • light – photoreceptor hyperpolarization – reduction in glutamate release – hyperpolarization of bipolar cells and horizontal cells • inhibited horizontal cells decrease their release of GABA – reduction in GABA allows depolarization of photoreceptor (feedback) • amacrine cells: provide 70% of input to ganglion cells • other 30% comes from bipolar-ganglion synapses • regulate bipolar to ganglion transmission • 40 different types – most with no axons • laterally gather BP cell input • most are inhibitory to transmission • help supplement horizontal cell function

50. Visual Pathways • visual field of each eye is divided into two halves: nasal half (central half) and a temporal half (peripheral half) • ganglion cells synapse with the neurons of the optic nerve • the axons of the optic nerve enter the optic chiasma • some axons cross over within this structure (signals from the same side of the retina) • but some axons are processed by the same side (signals from the temporal half of the retina are processed in the same side of the brain) • after passing the chiasma- the axons are now part of the optic tract which enters the brain and ends at the lateral geniculate nucleus of the thalamus • the axons coming from the temporal half of the retina (i.e. nasal side of visual field) do NOT cross over in the chiasma – continue to the thalamus portion on the same side of the eye receiving the info • BUT the nasal axons (detecting temporal visual field) cross and continue to the opposite thalamus • information is processed by three areas of the cerebral cortex • one for color discrimination • one for object shape • one for movement, location and orientation Nasal half (right eye) -PCs “temporal” retina -first order - bipolar cells -second order – ganglion cells, end in thalamus NO CROSSING OVER -third order – thalamus to occipital lobe (right) Temporal half (right eye) -PCs “nasal” retina -first order - bipolar cells -second order – ganglion cells, end in thalamus CROSSING OVER -third order – thalamus to occipital lobe (leftt)