2 Function of the Nervous System The nervous system is a coordination and control system that helps the body maintain homeostasis. ItGathers information about the internal and external environment (sense organs, nerves)Relays this information to the spinal cord and the brainProcesses and integrates the informationResponds, if necessary, with impulses sent via nerves to muscles, glands, and organs
3 Divisions of the Nervous System Know all these subdivisions of the nervous system(Receives input)(Sends output)Nervous system, located in the dorsal cavity of the body, functions in rapid, short-term control of bodily systems. It does this through chemical and electrical messages that pass from nerve cell to nerve cell and the, ultimately to a target organ or tissue.**CNSPNS
4 Neuron StructureBe able to label structures on left- Dendrites bring impules TO the somaSoma is the ‘processing’ part of the neuronAxon carries impules AWAY from the somaSynaptic knobs contain ntx- Myelin is found on axons- Neurons conduct nerve impulses(soma)*Initial segmentMature neurons have no centrioles so they do not undergo cell division after adolescence. However, they are long-lived cells. There is some evidence that a pool of stem cells in the hippocampus (and possibly other regions?) can give rise to new neurons even into the 50s and 70s.About 1012 neurons in the nervous system (as many as 50 x this many glial cells!)Axons range from a few mm to more than a meter long! Fast axonal transport (anterograde) occurs at a rate of mm/day.*Initial segment – where action potentials (nerve impulses) begin
5 Structural Classification of Neurons Bipolartwo processessense organsUnipolarone processgangliaMultipolarmany processesmost neurons of CNS**Classification is based on the number of processes coming directly from the cell body
6 Functional Classification of Neurons Sensory Neuronsafferent, ascendingcarry impulse to CNSmost are unipolarsome are bipolarInterneuronslink neuronsintegrativemultipolarin CNSMotor Neuronsefferent, descendingmultipolarcarry impulses away from CNScarry impulses to effectorsNotice the directionality – one-way
7 Table of Neuroglia Name of Cell Location Function(s) Satellite Cells Ganglia of PNSRegulate microenvironment of neuronsAstrocytesCNSRegulate microenvironment of neurons; scar tissue in CNSSchwann CellsPNSMyelination of axons; structural support for non-myelinated axonsOligodendrocytesMyelination of axons; structural frameworkMicrogliaPhagocytes of the CNSEpendymal CellsAssist in producing and controlling composition of CSF
8 NeurophysiologyBe sure to look at the Supplemental Study Notes for Neurophysiology (on the Web site under Lecture 18 Supporting Materials)This should help if you are still a little ‘fuzzy’ about this material.You should also use these notes to address the points in your study guide.
9 Membrane Channel Proteins 1. Passive channels are ALWAYS openAlso called ‘leak’ channelsPassive K+ channels always allow K+ through2. Active (gated) channels open or close in response to signalsa. Mechanical – respond to distortion of membraneb. Ligand-gated (Chemically-gated)Binding of a chemical molecule, e.g., ACh on MEPPresent on dendrites, soma, sometimes on axonsc. Voltage-gatedRespond to changed in electrical potentialFound on excitable membranes, e.g., axons, sarcolemma
10 Transmembrane (Resting) Potential Responsible for establishing the resting transmembrane potential; flows OUT of the cell at restA potential difference of -70 mV exists in the resting neuron due to the electrochemical gradient = Transmembrane Potentialinside negative relative to outside*polarized at restNa+/K+-ATPase pump restores proper ion balance after its disturbed-3 mVThe Na-K ATPase pump uses about 70% of the ATP required by the nervous system, requiring a great deal of glucose and O2.Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
11 Postsynaptic Potentials Excitationdepolarizes membrane of postsynaptic neuronpostsynaptic neuron becomes more likely to become depolarized and generate its own action potentialInhibitionhyperpolarizes membrane of postsynaptic neuronpostsynaptic neuron becomes less likely to become depolarized and generate its own action potentialA typical EPSP is about 0.5 mV and lasts about 15 to 20 msec.One neuron acts on the next, postsynaptic, neuron by changing the resting membrane potential of the postsynaptic neuron; either de- or hyperpolarizing it
12 Changes in Membrane Potential If membrane potential becomes more positive than its resting potential, it has depolarizedA membrane returning to its resting potential from a depolarized state is being repolarizedIf membrane potential becomes more negative than its resting potential, it has hyperpolarized(Movement of ? charges causes this?)(Movement of ? charges causes this?)Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
13 Action Potential and Refractory Period Action Potential begins in initial segment of neuronARP = Absolute Refractory PeriodRRP = Relative Refractory PeriodInflux of Na+ (Depolarization)Outflow of K+ (Repolarization)Threshold; MUST reach this for AP to occur.ARPRRPGreat summary graphic to know for exam!
14 Action PotentialsShown at left is an example of continuous propagation (~ 1m/s)What keeps the action potential going in ONE DIRECTION, and not spreading in all directions like a graded potential?Absolute refractory period of the previously depolarized segment.Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007Action Potential
15 Local (Graded) Potential Changes Caused by various stimulichemicalstemperature changesmechanical forcesCannot spread very far (~ 1 mm max) – weaken rapidlyUses ligand-gated Na+ channelsOn membranes of many types of cells including epithelial cells, glands, dendrites and neuronal cell bodiesGeneral response method for cellsCan be summed (so that an action potential threshold is reached; change in membrane potential stimulus strengthStarting point for an action potential
16 Saltatory (Leaping) Conduction Myelin acts as an insulator and increases the resistance to flow of ions across neuron cell membrane(fast)Ions can cross membrane only at nodes of RanvierImpulse transmission is up to 20x faster than in non-myelinated nerves. Myelinated axons are primarily what makes up white matter.
17 Chemical Synaptic Transmission You should understand this process and be able to diagram/describe itNeurotransmitters (ntx) are released when impulse reaches synaptic knobThis may or may not release enough ntx to bring the postsynaptic neuron to thresholdChemical neurotransmission may be modifiedUltimate effect of a ntx is dependent upon the properties of the receptorHow is the neurotransmitter neutralized so the signal doesn’t continue indefinitely?
18 Postsynaptic Potentials EPSPexcitatory postsynaptic potentialdepolarizes membrane of postsynaptic neuronpostsynaptic neuron becomes more likely to become depolarizedIPSPinhibitory postsynaptic potentialhyperpolarizes membrane of postsynaptic neuronpostsynaptic neuron becomes less likely to become depolarizedA typical EPSP is about 0.5 mV and lasts about 15 to 20 msec.One neuron acts on the next, postsynaptic, neuron by changing the resting membrane potential of the postsynaptic neuron; either de- or hyperpolarizing it
19 Summation of EPSPs and IPSPs EPSPs and IPSPs are added together in a process called summationSummation can be temporal (over time) or spatial (within a certain space)Summation uses graded potentials
20 Neurotransmitters * * * Neuromodulators: Influence release of ntx or the postsynaptic response to a ntx, e.g., endorphins, enkephalins
21 Protection of the Brain The brain is protectedMechanically byThe skull bonesThe meninges (singular: meninx)The cerebrospinal (CSF) fluidBiochemically by the blood-brain barrierCapillaries interconnected by tight junctionsAstrocytes/ependymal cells control permeability of general capillaries/choroid capillariesMay be obstacle to delivery of drugsMay become more permeable during stressTwo points of entry to brain: general capillaries and capillaries of the choroid plexuses.
22 Meninges of the BrainBlood-brain barrier - Capillaries interconnected by tight junctions, astrocytes/ependymal cells control permeability of general capillaries/choroid capillaries- dura mater – outer, tough (anchoring dural folds)- arachnoid mater – web-like- pia mater – inner, delicateDura mater – thick, collagenous membrane (about thickness of rubber kitchen glove)Arachnoid mater – simple squamous epithelium; loose meshwork of collagenous and elastic fibers spanning the gap (subarachnoid space)Pia mater – delicate membrane, close follows contours of nervous tissue that it covers.- Subdural space – like interstitial fluid- Subarachnoid space – CSF*
23 Cerebrospinal Fluidsecreted by choroid plexus of ventricles (~500 ml/day)circulates in ventricles, central canal of spinal cord, and subarachnoid spacecompletely surrounds brain and spinal cordnutritive and protectivehelps maintain stable ion concentrations in CNSependymal cells are glial cells that play a role in generating CSFAbout 500 ml of CSF produced per day; about ml present at any time.
24 Overview of Cerebral Cortex The cerebrum can be divided into several functional areas:- Motor (frontal cortex) - Sensory (parietal, occipital, and temporal cortex) - Association (all lobes)Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
25 Cortex = Conscious Awareness The Homunculi shown here are associated with the CORTEX of the cerebrum
26 Functions of Parts of Brain Part of BrainMajor FunctionMotor areasPrimary motor cortexVoluntary control of skeletal musclesBroca’s area (motor speech area)Controls muscles needed for speechFrontal eye fieldControls muscles needed for eye movementSensory areasCutaneous Sensory AreaReceives somatic sensationsVisual areaReceives visual sensationsAuditory areaReceives auditory sensationsAssociation areasAnalyze and interpret sensory experiences; coordinate motor responsesmemory, reasoning, verbalization, judgment, emotionsBasal nucleiSubconscious control certain muscular activities, e.g., learned movement patterns (a nucleus is a collection of neuron cell bodies in the CNS); putamen, globus pallidus, caudateLimbic systemcontrols emotions , produces feelings, interprets sensory impulses, facilitates memory storage and retrieval (learning!)DiencephalonThalamusgateway for sensory impulses heading to cerebral cortex, receives all sensory impulses (except smell)HypothalamusVital functions associated with homeostasisBrainstemMidbrainMajor connecting center between spinal cord and brain and parts of brainstem; contains corpora quadrigemina (visual and auditory reflexes)PonsHelps regulate rate and depth of breathing, relays nerve impulses to and from medulla oblongata and cerebellumMedulla OblongataContains cardiac, vasomotor, and respiratory control centers, contains various nonvital reflex control centers (coughing, sneezing, vomiting)Reticular formation (system)Filters incoming sensory information; habituation , modulates pain, arouses cerebral cortex into state of wakefulness (reticular activating system)CerebellumSubconscious coordination of skeletal muscle activity, maintains posture
27 MemoryA “Memory” is the persistence of knowledge that can be accessed (we hope!) at a later time.Memories are not stored in individual “memory cells” or neurons; they are stored as pathways called engrams, or memory traces that use strengthened or altered synapses.Immediate memory lasts a few seconds, e.g., remembering the earliest part of a sentence to make sense of it.Short-term memory (STM) lasts a few seconds to a few hoursWorking memory is a form of this (repeating a phone number over to yourself just long enough to dial it – and then forget it!)Limited to a few ‘bits’ of information (about 7-9). So, ‘chunk up’!Long-term memory (LTM) can last a lifetimeCan hold much more information that STMDeclarative (events and facts); Procedural (motor skills)Remembering childhood events as an adult
28 Spinal Cord Structure Functions of spinal cord: is a center for spinal reflexesaids in locomotionFigure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007is a conduit for nerve impulses to and from the braincauda equina - Begins around L2 and extends to S5. Good area for lumbar puncture and collection of CSF.Spinal cord extends entire length of vertebral column early in fetal development, to L3 at birth, and to L1 in an adult. Thus, it occupies only the upper two thirds of the vertebral column in an adult. The lower third of the vertebral column is occupied by the cauda equina.
29 Organization of Spinal Gray Matter Cell bodies of sensory neurons are in dorsal root ganglionCell bodies of motor neurons are found hereFigure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
30 Tracts of the Spinal Cord Ascending tracts conduct sensory impulses to the brainDescending tracts conduct motor impulses from the brain to motor neurons reaching muscles and glandsTract: Contains axons that share a common origin and destinationTracts are usually named for their place of origin (1st) and termination (2nd)
31 1st, 2nd, and 3rd Order Sensory Neurons Examples of sensory (ascending) tracts (note how the names tell you where they’re coming from and where they are going to…)Spinothalamic - Spinocerebellar - Fasciulus cuneatus/gracilis1st order neuron – from receptor to the spinal cord (cell bodies are located in the dorsal root ganglion)2nd order neuron – from spinal cord to thalamus3rd order neuron – from thalamus and terminate in the cerebral cortex321
32 Descending Tracts Examples of descending spinal tracts corticospinal Upper motor – begin in precentral gyrus of cortexExamples of descending spinal tractscorticospinalreticulospinalrubrospinalDecussationLowerUpper MN – Cerebral cortex to spinal cordLower MN – Spinal cord to effector
33 Reflex ArcsReflexes – automatic, subconscious, quick, stereotyped responses to stimuli either within or outside the body, and occur in both the somatic and autonomic divisionThe 3 different somatic reflexes we discussed in class:1. Knee-jerk: monosynaptic, ipsilateral 2. Withdrawal: polysynaptic, ipsilateral 3. Crossed extensor: polysynaptic, contralateral
34 Peripheral Nervous System Cranial nerves arising from the brainSomatic fibers connecting to the skin and skeletal musclesAutonomic fibers connecting to visceraSpinal nerves arising from the spinal cordSomatic fibers connecting to the skin and skeletal musclesAutonomic fibers connecting to viscera
35 The Twelve Pairs of Cranial Nerves NumeralNameFunctionSensory, Motor, or Both (Mixed Nerve)IOLFACTORY (OLD)OLFACTION/SMELLSENSORY (SOME) IIOPTIC (OPIE)VISIONSENSORY (SAY) IIIOCULOMOTOR (OCCASIONALLY)MOVE EYEMOTOR (MARRY)IVTROCHLEAR (TRIES)MOVE EYE (superior oblique)MOTOR (MONEY)VTRIGEMINAL (TRIGONOMETRY)MAJOR SENSORY NERVE FROM FACEBOTH (BUT)VIABDUCENS (AND)MOVE EYE (lateral rectus)MOTOR (MY)VIIFACIAL (FEELS)MAJOR MOTOR NERVE OF FACEBOTH (BROTHER)VIIIVESTIBULOCOCHLEAR (VERY)HEARING AND EQUILIBRIUMSENSORY (SAYS) IXGLOSSOPHARYNGEAL (GLOOMY)MOVE MUSCLES OF TONGUE AND PHARYNXBOTH (BIG)XVAGUS (VAGUE)INNERVATE VISCERAL SMOOTH MUSCLE; MUSCLES OF SPEECHBOTH (BOOBS)XIACCESSORY (AND)MOVE NECK MUSCLESMOTOR (MATTER)XIIHYPOGLOSSAL (HYPOACTIVE)MOVE TONGUEMOTOR (MOST)You should know this table
37 Structure of a Peripheral Nerve A peripheral nerve is composed of bundles of nerve fibers (axons)Epineurium – surrounds entire nervePerineurium – surrounds a bundle of nerve fibers = fascicleEndoneurium – surrounds each axon (nerve fiber)Similar to the naming of the CT around muscle!!Nerve fiber (axon of one neuron)
38 Spinal Nerves spinal nerves contain mixed (motor/sensory) nerves 31 pairs8 cervical (C1 to C8)12 thoracic (T1 to T12)5 lumbar (L1 to L5)5 sacral (S1 to S5)1 coccygeal (Co)THIRTY ONEderful flavors of spinal nerves!Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
39 Nerves PlexusesNerve plexus – complex network formed by anterior (ventral) branches of spinal nerves; fibers of various spinal nerves are sorted and recombinedContains both sensory and motor fibersCervical Plexus - C1-C4supplies muscles and skin of the neckcontributes to phrenic nerve (diaphragm)Brachial Plexus - C5-T1supplies shoulder and upper limbsLumbosacral Plexus - T12 – S5supplies pelvis and lower limbs
40 Spinal Cord and Nerve Roots Ventral root - axons of motor neurons whose cell bodies are in spinal cordDorsal root - axons of sensory neurons in the dorsal root ganglionDorsal root ganglion - cell bodies of sensory neurons
41 Somatic vs. Autonomic Nervous Systems DualFigure from: Marieb, Human Anatomy & Physiology, Pearson, 2013
42 Review of Autonomic Nervous System Branch of ANSPARASYMPATHETICSYMPATHETICGeneralFunction* “rest and digest”* (SLUDD); Salivation, lacrimation, urination, digestion, defecation * 3 decreses; ↓ heart rate, ↓ pupil size, ↓ airway diameter* “fight or flight” * E situations: Emergency, exercise, embarassment, excitementOrigin of Preganglionic fibercranial region of brain or sacral region of spinal cord(craniosacral outflow)thoracic or lumbar region of spinal cord (thoracolumbar outflow)Divergence for widespread activation of bodyLocation of Gangliawithin or near effector organalongside or in front of spinal cord (paravertebral ganglia; collateral ganglia)NTx secreted by postganglionicfiberacetylcholineNorepinephrine (some acetylcholine; sweat glands, smooth muscle on blood vessels, brain)Good summary chart to know
43 Sympathetic Division of ANS *Paravertebral ganglionEffectors in head and thoracic cavityEffectors in muscles and body wall**(T5 – T12)1- Preganglionic fibers that synapse within the ganglion2 – Preganglionic fibers that extend up or down within the sympathetic trunk3 – Preganglionic fibers that synapse in collateral gangliaPrevertebral ganglionFigure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
44 Actions of Autonomic Neurotransmitters depend on receptorCholinergic receptorsbind acetylcholinenicotinicexcitatorymuscarinicexcitatory or inhibitoryAdrenergic receptorsbind norepinephrinealpha (Types 1 and 2)different responses on various effectorsbeta (Types 1 and 2)
45 Sensory Receptors Sensory Receptors specialized cells or multicellular structures that collect information (transduce information into nerve impulses)stimulate neurons to send impulses along sensory fibers to the brain (receptor vs. generator [action] potentials)Chemoreceptors (general)respond to changes in chemical concentrationsPain receptors or nociceptors (general)respond to stimuli likely to cause tissue damageThermoreceptors (general)respond to changes in temperatureSensory receptors txmit four kinds of info: 1) Modality (type of stimulus or sensation via labeled lines), 2) Location (which nerve fibers are firing), 3) Intensity (number and kind of fibers that are firing and time interval between potentials), 4) Duration (change in nerve fibers fire over time).Mechanoreceptors (general, special)respond to mechanical forcesPhotoreceptors (special)respond to light
46 MechanoreceptorsSense mechanical forces such as changes in pressure or movement of fluidTwo main groupsbaroreceptors – sense changes in pressure (e.g., carotid artery, aorta, lungs, digestive & urinary systems)proprioceptors – sense changes in muscles and tendons
48 Temperature Sensors (Thermoreceptors) Warm receptorssensitive to temperatures above 25oC (77o F)unresponsive to temperature above 45oC (113oF)Cold receptors (3-4x more numerous than warm)sensitive to temperature between 10oC (50oF) and 20oC (68oF)unresponsive below 10oC (50oF)Pain receptors are activated when a stimulus exceeds the capability (range) of a temperature receptorrespond to temperatures below 10oCrespond to temperatures above 45oC
49 Sensory Adaptationreduction in sensitivity of sensory receptors from continuous stimulation (painless, constant)stronger stimulus required to activate receptorssmell and touch receptors undergo sensory adaptationpain receptors usually do not undergo sensory adaptation (at level of receptor)impulses can be re-triggered if the intensity of the stimulus changes
50 The Middle Ear (Tympanic Cavity) Typanic reflex: Elicited about 0.1 sec following loud noise; causes contraction of the tensor tympani m. and stapedius m. to dampen transmission of sound waves
51 Auditory Tube eustachian, auditory, or pharyngotympanic tube connects middle ear to throathelps maintain equal pressure on both sides of tympanic membraneusually closed by valve-like flaps in throatWhen pressure in tympanic cavity is higher than in nasopharynx, tube opens automatically. But the converse is not true, and the tube must be forced open (swallowing, yawning, chewing).
52 Physiology of Hearing Know pathway for exam Figure from: Marieb, Human Anatomy & Physiology, Pearson, 2013Know pathway for examTympanic membrane malleus incus stapes oval window scala vestibuli scala tympani round window
53 Cochlea Scala vestibuli upper compartment Cochlea as it would look ‘unwound’Tympano = in relation to, in connection withOval window = base of scala vestibuli (vestibular duct); round window = base of scala tympani (tympanic duct).Scala vestibuli and s. tympani are filled with perilymph (like CSF); cochlear duct (s. media) is filled with endolymph (unique comp., but similar to interstitial fluid)Scala vestibuliupper compartmentleads from oval window to apex of spiralpart of bony labyrinthScala tympanilower compartmentextends from apex of the cochlea to round windowpart of bony labyrinth
54 Organ of Corti – in Cochlear Duct group of hearing receptor cells (hair cells)on upper surface of basilar membranedifferent frequencies of vibration move different parts of basilar membraneparticular sound frequencies cause hairs (stereocilia) of receptor cells to bendnerve impulse generated
55 VestibuleUtriclecommunicates with saccule and membranous portion of semicircular canalsSacculecommunicates with cochlear ductMaculacontains hair cells of utricle (horizontal) and saccule (vertical)Utricle and saccule provide sensations of: 1) gravity and 2) linear accelerationThese organs function in static equilibrium (head/body are still)
56 Macula & Static Equilibrium responds to changes in head positionbending of hairs results in generation of nerve impulseThese organs function in static equilibrium (head/body are still)
57 Semicircular Canalsthree canals at right anglesampulla (expansion)swelling of membranous labyrinth that communicates with the vestibulecrista ampullarissensory organ of ampullahair cells and supporting cellsrapid turns of head or body stimulate hair cellsAcceleration of fluid inside canals causes nerve impulseThese organs function in dynamic equilibrium (head/body are in motion)
58 Crista Ampullaris & Dynamic Equilibrium Semicircular canals respond to rotational, nonlinear movements of the head = Dynamic Equilibrium
59 Eyelids palpebrae = eyelids composed of four layers skinmuscleconnective tissueconjunctivaorbicularis oculi – closes eye (CN VII)levator palpebrae superioris – raises eyelid (CN III)tarsal (Meibomian) glands – secrete oil onto eyelashes; keep lids from sticking togetherconjunctiva – mucous membrane; lines eyelid and covers portion of eyeballFornixTarsal plates are CT and serve as insertion points for orbicularis oculi and lev. palpebrae sup. muscles. Blink reflex occurs every 3-7 sec. to spread oil, mucus, and saline across surface of eye.Eyelash roots are richly innervated and will cause blinking if disturbedTarsal (Meibomian) glands embedded in tarsal plates – modified sebaceous gl produce oily secrn to help keeps lids from sticking together. Ciliary gland are located between eyelashes (sebaceous and modified sweat gl).Infection of a tarsal gl = chalazion; infection of smaller glands = sty.Conjunctiva is a transparent mucus membrane containing blood vessels that produces a lubricating mucus that helps keep the eye from drying out.Sagittal section of right eyeFigure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
60 Lacrimal Apparatuslacrimal glandlateral to eyesecretes tearscanaliculicollect tearslacrimal saccollects from canaliculinasolacrimal ductcollects from lacrimal sacempties tears into nasal cavityLacrimal gland is visible through the conjunctiva when the upper lid is everted. Lacrimal puncta are visible as tiny red dots on the medial margin of each eyelid.Tears: - supply oxygen and nutrients to cornea (avascular) - are antibacterial (contain antibodies and lysozyme) - lubricate and bathe the conjunctiva
61 Extraocular Eye Muscles & their CN Which cranial nerves innervate each of the muscles in the diagram above?LR6SO4AO3
62 Outer (Fibrous) Tunic Cornea anterior portion transparent light transmissionlight refractionwell innervatedavascularFigure from: Hole’s Human A&P, 12th edition, 2010Scleraposterior portionopaqueprotectionsupportattachment site for extrinsic eye musclesCornea is covered by epithelial sheets on both surfaces; outside = str. Squamous epithelium that protects from abrasion and merges with the ocular conjunctiva (cells for renewal come from this layer), inside is a simple squamous epithelium that contains sodium pumps that maintain the clarity of the cornea by keeping water content low.Cornea is well supplied with nerve endings (mostly pain fibers). Touching of cornea causes reflexive blinking (corneal reflex; afferent V, efferent VII). Cornea has great capacity for repair.Transverse section, superior view
63 Middle (Vascular) Tunic = Uvea Figure from: Hole’s Human A&P, 12th edition, 20101. Irisanterior portionpigmented CTcontrols light intensity2. Ciliary bodyanterior portionpigmentedholds lensmuscles reshape lens for focusingaqueous humorBlood vessels of uvea provide nutrition to all eye tunics.Iris is most anterior portion of the uvea. Made up of two smooth muscle layers, one circular and one radial. Most babies irises are slate gray or blue because their iris pigment is not yet developed.3. Choroid coatprovides blood supplypigments absorb extra lightThis layer contains the intrinsic muscles of the eye - Regulate the amount of light entering the eye - Regulate the shape of the lens
64 Lens Loss of lens transparency = cataracts transparent, avascular biconvexlies behind irislargely composed of lens fibersenclosed by thin elastic capsuleheld in place by suspensory ligaments of ciliary bodyfocuses visual image on retina (accommodation)(Crystallins)Loss of lens transparency = cataracts
65 Aqueous Humor fluid in anterior cavity of eye secreted by epithelium on inner surface of the ciliary processesprovides nutrientsmaintains shape of anterior portion of eyeleaves cavity through canal of Schlemm (scleral venous sinus)Nomral intraocular pressure is about 16 mm Hg, maintained by constant production and drainage of aqueous humor. Aq humor supplies nutrients and oxygen to the lens and cornea and to some cells of the retina, and carries away metabolic wastes.Pressure build up in the eye compresses the retina and optic nerve (glaucoma). Late signs include halos around light and blurred vision.Normally about 1-2 ul / min of aqueous humor is secreted, the same amount being drained via the scleral venous sinus
66 Accommodation changing of lens shape to view objects nearby ciliary muscles (intrinsic) change shape of lensFar vision (emmetropia) (20 ft. or greater)The far point of vision is the distance beyond which no change in lens shape is needed for focusing. This is about 20 ft. for the normal (emmetropic) eye.Closer than 20 ft, the lens must round up (accommodate) in order to focus light acutely on the retina (and the pupils constrict and the eyeballs converge). This close vision reflex appears to be triggered by blurring of the retinal image. Pupils constrict to prevent the most divergent light rays from entering the eye since they would pass through the edges of the lens and not be focused properly. Convergence occurs in order to keep the object’s light rays falling on the foveae.Myopia = nearsightedness, i.e., NEAR vision is UN impaired. Hyperopia = farsightedness, i.e., far vision is unimpaired.Presbyopia is the loss of the ability to accommodate with ageNear vision
67 Iris How would viewing near objects affect pupil size? composed of connective tissue and smooth muscle (intrinsic muscles)pupil is hole in irisdim light stimulates (sympathetic) radial muscles and pupil dilatesbright light stimulates (parasympathetic, CN III) circular muscles and pupil constrictsmydriasisCycloplegia = paralysis of the ciliary smooth musclesmiosisHow would viewing near objects affect pupil size?
68 Visual Receptors Rods long, thin projections contain light sensitive pigment called rhodopsinhundred times more sensitive to light than conesprovide vision in low light/darknessproduce colorless visionproduce outlines of objectview off-center at nightoutward from fovea centralisConesshort, blunt projectionscontain light sensitive pigments called erythrolabe, chlorolabe, and cyanolabe (photopsins)provide vision in bright lightproduce sharp imagesproduce color visionin fovea centralisAbout 130 million rods and about 6.5 million cones – and only about 1.2 million nerve fibers in the optic nerve.Dark adaptation by the rods takes approximately 30 minutes. This adaptation can be destroyed by white light in just milliseconds
69 Optic Disc (Blind Spot) Optic disc(k) – Exit of optic nerve; no photoreceptors = no visionMacula lutea – area immediately surrounding fovea centralisFovea centralis – contains only cones; area of most accute visionFovea centralis is about 0.4 mm in diameter (about the size of the head of a pin).Figure from: Martini, Fundamentals of Anatomy & Physiology, Benjamin Cummings, 2004
70 Visual PathwayThe right side of the brain receives input from the left half of the visual fieldThe left side of the brain receives input from the right half of the visual fieldFigure from: Martini, Fundamentals of Anatomy & Physiology, Benjamin Cummings, 2004Hemidecussation – half the fibers of the optic nerve cross over to the other side of the brain.