are specialized to react to physical and chemical changes in

their surroundings     

an axon carries the information away from the cell in the form of bioelectric signals called_____

 bundles of axons

have many functions but typically do not send and receive messages

small spaces between neurons that are important centers for the transmission of electrochemical messages.

the carriers of these messages are biological messenger molecules called_____

consists of the brain and spinal cord

 at the ends of peripheral neurons gather information by

detecting changes in the environment; convert their information into nerve impulses, which are transmitted to the CNS

the nerve impulses from the sensory receptors are brought together, creating sensations, adding to memory, or helping produce thoughts

neurons carry information from the CNS to responsive structures called effectors; these include muscles and glands

 the somatic nervous system which is involved in conscious activities, such as skeletal muscle contration; the autonomic nervous system which controls viscera and various glands,

the subconscious activities

contains granular cytoplasm, mitochondria, a Golgi apparatus, and many microtubules.

scattered throughout the cytoplasm–consists of rough endoplasmic reticulum.

The large_____ is near the center of the cell body and has a conspicuous nucleolus. 

are usually highly branched, providing receptive surfaces to which processes from other neurons communicate

There is a single axon which arises from the axonal hillock; specialized to conduct nerve impulses away from the cell body; cytoplasm of axon includes mitochondria, microtubules, neurofibrils; may give off branches(collaterals); the distal end may have many axon terminals, each with a synaptic knob very close to the receptive surface of another cell, separated by the synaptic cleft; also conveys biochemicals that are produced in the cell body;this is axonal transport – involves vesicles, mitochondria, ions, nutrients, and neurotransmitters

which wind tightly around the axon; the larger axons are encased in lipid-rich sheaths formed by_____; the sheaths are composed of myelin (myelin sheath); the neurilemma(neurilemmal sheath) surrounds the myelin sheath; narrow gaps in the myelin sheath between the Schwann cells (nodes of Ranvier)

the smallest axons also have Schwann cells enclosure, but they do not wind around them, so they lack a myelin sheath; appear gray

axons with a myelin sheath;appear white- the white matter of the brain and spinal cord formed by oligodendrocytes, another type of neuroglial cell – these axons lack a neurilemma

has only two processes, one is an axon and the other a

  dendrite; specialized parts of the eyes, nose, and ears

a single process from the cell body which then divides into

 two branches, which really function as a single axon: one branch ( peripheral process ) is associated with dendrites near a peripheral body part; the other branch ( central process ) enters the CNS; cell bodies of some of these neurons aggregate in specialized masses of nerve tissue called ganglia, which are located outside the CNS

 have one axon and many dendrites; most neurons whose cell bodies lie within the CNS are of this type  

carry nerve impulses from peripheral body parts into the CNS; have specialized receptor ends at the tips of their dendrites, or they have dendrites that are near receptor cells; most sensory neurons are unipolar, though some are bipolar

– lie within CNS, are mulipolar and form links between other neurons.

 are multipolar and carry nerve impulses out of the CNS to effectors; two specialized groups of motor neurons innervate smooth and cardiac muscles: accelerator neurons increase muscular activities;  inhibitory neurons decrease muscular activities

shaped cells; provide support and hold structures together;

 aid metabolism of glucose, may help regulate concentrations of important ions, respond to injury of brain tissue, regulate movement of substances from blood vessels to neurons, play an important role in the blood-brain barrier, linked together

by gap junctions

form myelin in the CNS, can send out a number of processes

 so that one cell can provide myelin for many axons

are small and have fewer processes than other neuroglial cells, help support neurons and phagocytize bacterial cells and cellular debris

cuboidal or columnar cells that may have cilia, form the inner lining of the central canal, covers the inside of the ventricles, help regulate the composition of cerebrospinal fluid within the ventricles

If the cell body is injured, the cell usually dies and it is not replaced; Damaged peripheral axons may regenerate;  proximal ends develop sprouts; growth factors from glial cells are secreted; a sprout may grow into a tube formed by remaining connective tissue and basement membrane; Schwann cells may proliferate and form new myelin sheath around growing

      axon  growth is slow – 3-4 mm/day

The process by which the impulse in the presynaptic neuron signals the postsynaptic neuron; Axon terminals have synaptic knobs within which are synaptic vesicles containing

neurotransmitters; the synaptic vesicles fuse with the cell membrane when voltage-sensitive calcium channels open following a nerve impulse reaching the synaptic knob; the vesicles release their neurotransmitter by exocytosis into the synaptic cleft; receptors in the postsynaptic neuron membrane react with neurotransmitter; ions channels are opened or closed by this reaction – called  gated ion channels; these synaptic potentials created by chemically gated ion channels enable one neuron to influence another

stimulated by changes in the concentration of chemical substances.

sense mechanical forces, such as changes in pressure or movement of fluids

respond to light energy

 Stimulation of receptors causes local changes in membrane potentials – if threshold is reached, and action potential is generated, and a sensory impulse results on the afferent fiber

 Feelings that occur when the brain interprets sensory impulses – the sensation that results, depends on which region of the cerebral cortex receives the impulse;  When a sensation forms, the cerebral cortex interprets it to seem to come from the receptors being stimulated – this is called projection, because the brain projects the sensation back to its apparent source.

When sensory receptors are continuously stimulated, many of them undergo  adjustment called_____.  Impulses leave at decreasing rates, until the receptors fail to send signals at all.

small, oval masses of flattened connective tissue cells in connective tissue sheaths; abundant in hairless portions of skin – lips, palms, soles, nipples, external genitalia organs; sensations of light touch; used to judge texture

relatively large ellipsoidal structures composed of connective tissue fibers and cells; common in deeper subcutaneous tissues of hands, feet, breasts, and in tendons and ligaments;  stimulated by heavy pressure; many detect vibrations

most sensitive to temperatures above 77°F –become unresponsive at temperatures above 113°F ( pain receptors triggered )

most sensitive to temperatures between 50 - 68°F – pain receptors are stimulated if the temperature drops below 50°F ;adapt rapidly

simulated when tissues are damaged; can be stimulated by more than one type of change; mechanical damage;  chemicals – hydrogen ions, potassium ions, breakdown products of proteins, histamine, acetylcholine; deficiency of oxygen – hypoxia; deficiency of blood – ischemia; muscle cramp pain results from interruption of blood flow as the sustained contraction squeezes capillaries; pain-stimulating chemicals accumulate when blood flow is interrupted; adapt very little

pain results from stimulation of mechanical-sensitive ( stretch ) receptors and to decreased blood flow accompanied by lower tissue oxygen levels and accumulation of pain-stimulating chemicals; may feel as if it is coming from some other part of the body than that being stimulated – this is referred pain; may derive from common nerve pathways used by sensory impulses coming from skin areas and internal organs    

 are thin, myelinated fibers – conduct impulses rapidly, up to 30m/sec; associated with sharp pain, seems to originate in a local area of skin; usually stops when pain-producing stimulus stops

thin, unmyelinated fibers – conduct impulses more slowly, up to 2 m/sec; cause the dull, aching pain sensation that may be more widespread and difficult to pinpoint; pain may continue after stimulus is removed; likely to be felt in deeper tissues as well as in skin – visceral pain impulses are usually carried on C fibers

pain impulses from all tissues except the head pass into the spinal cord by way of dorsal roots – enter gray matter of posterior horn and processed – impulses cross over and ascend to brain in the lateral spinothalamic tracts; most terminate in the reticular formation  - from here are conducted to thalamus, hypothalamus, and cerebral cortex

 send information to CNS concerning the length and tensions

of muscles – no sensation results from their stimulation.

located in skeletal muscles near the tendon-muscle junction; if the whole muscle is stretched, the muscle spindle stretches, triggering sensory nerve impulses to spinal cord; impulses travel from spinal cord on motor fibers to the same muscle, contracting it – this is a stretch reflex that opposes the lengthening of the muscle and helps maintain the desired position of the limb

found in tendons close to their attachments to muscles; have high thresholds and stimulated by increased tension; impulses from them produce a reflex that inhibits contraction of the muscle; stimulate a reflex with an opposite effect of a stretch reflex; helps maintain posture and protects muscle attachments from being pulled away from their insertions by excessive tension

Potassium ions are the major intracellular positive ion ( cation ), and sodium ions are the major extracellular cations

the sodium/potassium pump actively transports Na⁺ ions out of the cell and K⁺ ions into the cell;  membrane proteins also act as channels for these ions; some channels are very selective; some channels are always open, others are gated; electrical and chemical factors can affect the opening and closing of these

A resting cell is more permeable to potassium ions than sodium, and the cytoplasm of the cell has many negatively charged ions, anions, which include phosphate, sulfate, and proteins, that cannot diffuse through the cell membrane; More positive ions leave the cell than enter it;  the outside of the cell membrane gains a surplus of positive charges;  The difference in the electrical charge between the two regions is called a

potential difference; it represents stored electrical energy that can be used to do work in the future

the potential difference across the membrane is called the______  and is measured in millivolts

The membrane potential of a resting neuron is called the_______and has a value of –70mV.

if the membrane potential becomes more negative

Neurons are excitable – they respond to changes in receptors or to stimuli that usually affect the membrane potential in the region of the membrane exposed to the stimulus; Typically, the environmental change affects the membrane potential by opening a gated ion channel; local potential changes are graded – the amount of change in the membrane potential is directly proportional to the intensity of stimulation; if the neuron is sufficiently depolarized, a threshold potential is reached, and an action potential will result

                   4.  Several stimuli may add together, in summation, to reach threshold.

The initial segment or trigger zone of the axon contains a great number of voltage- gated sodium channels; these channels remain closed at resting membrane potential, but open for an instant when threshold is reached; sodium ions rush inward, the membrane potential changes from its resting value to become positive on the inside ( still considered depolarization ) –  +30mV;  slower voltage-gated potassium channels open, potassium diffuses outward and the inside of the membrane becomes negative again – this is repolarization;  A series of action potentials along an axon is the nerve impulse

If a neuron responds, it responds completely;  A greater intensity of stimulation produces more impulses per second, not a stronger impulse.

 Following passage of a nerve impulse, a threshold stimulus will not trigger another impulse on an axon.  This brief period is the ______; the absolute refractory period lasts about 1/2500 of a second – the membrane is changing in sodium permeability and cannot be stimulated; the relative refractory period is when the membrane is reestablishing its resting potential – a threshold stimulus of high intensity may trigger an impulse; the return to the resting state usually takes from 10 to 30 milliseconds;  an action potential takes about 1 millisecond; the maximum theoretical frequency of impulses in a neuron is about 700 per sec; usually about 100 impulses per second found in the body

Unmyelinated nerve fibers conduct impulses over its entire surface; Myelinated nerve fibers have action potentials occurring only at the nodes of Ranvier – they appear to jump from node to node – saltatory conduction; conduction is much faster on myelinated fibers than unmyelinated;  The greater the diameter of axon, the faster the conduction; thick, myelinated axon – motor neuron – 120 meters/second; thin, unmyelinated axon – sensory neuron – 0.5 meter/second

If the membrane is depolarized, triggering an action potential, the membrane change is called an excitatory postsynaptic potential (EPSP).  Lasts about 15 milliseconds; If the membrane is hyperpolarized, with no action potential generated, the membrane change is called an inhibitory postsynaptic potential (IPSP);  Each neuron may receive the synaptic knobs of a thousand or more axons.  Some of the postsynaptic potentials are excitatory and some are inhibitory;  the sum of the EPSPs and IPSPs determine whether an action potential results; this summation commonly takes place at the trigger zone

which are formed by modifying amino acids

 such as glycine, glutamic acid, aspartic acid, and gamma-aminobutyric acid ( GABA )

large group of ____ such as enkephalins and substance P, which are short chains of amino acids

at least 30 different kinds; Some neurons release only 1, while others produce 2 or 3 kinds; most are synthesized in the synaptic knobs and stored in synaptic vesicles;  released by exocytosis into the synaptic cleft; Enzymes in synaptic clefts and on postsynaptic membranes rapidly decompose some neurotransmitters; acetylcholinesterase decomposes acetylcholine; Other neurotransmitters are transported back into the synaptic knob of the presynaptic neuron, a process called reuptake;  monoamine oxidase inactivates epinephrine and norepinephrine after reuptake; Destruction or removal of neurotransmitter prevents continuous stimulation of the postsynaptic neuron.

 These are synthesized by neurons in the brain and spinal cord and act as neuromodulators – substances that alter a neuron’s response to a neurotransmitter or block the release of a neurotransmitter;  Enkephalins consist of a chain of 5 amino acids.  Synthesized during periods of painful stress; bind to the same receptors in the brain as the narcotic morphine (opiate receptors); relieve pain sensations; Beta endorphin is found in the brain and cerebrospinal fluid;  acts longer than enkephalins and is a more potent pain reliever; Substance P consists of 11 amino acids and is widely distributed throughout the nervous system;  functions as a neurotransmitter in the neurons that transmit pain impulses into

the spinal cord and on to the brain  enkephalins and endorphins may relieve pain by inhibiting the release of substance P from pain-transmitting neurons

These are groups of neurons that make synaptic connections with each other and work together to perform a common function; Each pool receives input neurons and each pool generates output; These pools may have excitatory or inhibitory effects on other pools or on peripheral effectors; If the net effect is excitatory, threshold is reached, and outgoing impulse is sent;  If the net effect is excitatory but subthreshold, the neuron is more excitable to incoming stimuli and a condition called facilitation exists.

Axons from different parts of the nervous system lead to the same neuron in convergence; Allows the nervous system to collect, process, and to respond to information from various sensory receptors.

Axons may branch many times and thereby reach several other neurons in _______;  This leads to amplification of an impulse by spreading it to increasing numbers of

                             neurons within the pool.

  used to sense smells, and are similar to taste receptors – they are both chemoreceptors sensitive to chemicals dissolved in liquids;  about 75 – 80% of flavor actually derives from the sense of smell

 contain the olfactory receptors and epithelial supporting cells; yellowish brown masses in upper parts of nasal cavity, superior nasal conchae, and a portion of the nasal septum; olfactory receptor cells – bipolar neurons surrounded by columnar epithelial cells; knobs at distal ends of their dendrites covered with cilia; cilia projects into the nasal cavity and are the sensitive portion; each of the 12 million receptor cells has ten to twenty cilia;  odorant molecules enter nasal cavity as gases and are dissolved in the watery fluids surrounding the cilia; molecules bind to about 500 different types of olfactory receptors, depolarizing them and generating nerve impulses

nerve impulses travel along axons through tiny openings in cribiform plates of the ethmoid bone;  these fibers synapse with neurons located in olfactory bulbs, structures which

 lie on either side of the crista galli; impulses are analyzed in bulbs, and then impulses travel along olfactory tracts to portions of the limbic system, a brain center for memory and

    emotions; the olfactory cortex is located deep within the temporal lobes and at the bases; of the frontal lobes, anterior to the hypothalamus

leading hypothesis is that each odor stimulates a distinct set of receptor cells that in turn have distinct sets of receptor proteins – the brain then recognizes the particular combination as an olfactory code; olfactory receptors undergo adaptation rather quickly; olfactory receptor neurons are the only nerve cells in direct contact with the outside environment – are subject to damage, so are regularly replaced from stem cells; sense of smell diminishes with age

these are taste buds that are primarily located on the surface of the tongue in association with tiny elevations called papillae; also scattered in the roof of the mouth, the linings of the cheeks, and the walls of the pharynx

each taste bud includes a group of modified epithelial cells, which are the taste cells ( gustatory cells ) that function as receptors;  each of our 10,000 taste buds houses 50 – 150 taste cells; each bud has a taste pore on its free surface; microvilli called taste hairs project through the taste pore – these are the sensitive parts of the receptor cells; a network of nerve fibers is wrapped around the taste cells; chemicals in food are dissolved in saliva which then trigger a nerve impulse; mechanism of tasting probably includes combinations of chemicals binding  specific receptors on taste hair surfaces, altering membrane polarization, and thereby generating sensory impulses

four primary taste sensations – sweet, sour, salty, and bitter – some investigators recognize metallic, alkaline, and umami; current evidence suggests that all taste cells respond to more than one taste sensation, although for a given taste cell, one taste sensation is likely to predominate; responsiveness to particular sensations may vary from one region to another; taste receptors adapt rapidly; taste cells are very close to tongue surface and are therefore exposed to environmental wear and tear – divide continually – a taste cell functions for about three days before it is replaced

 sensory impulses from taste receptors travel to the medulla on nerves; VII, IX, and X – impulses then ascend to thalamus and are directed to the gustatory cortex, located in the parietal lobe along a deep portion of the lateral sulcus

consists of all of the structures that face the outside; auricle – outer, funnel-like structure ( also called the pinna );  external acoustic meatus – S-shaped tube ( also called auditory canal ) that leads inward about 2.5 cm and terminates with the tympanic membrane; passes into the temporal bone; hairs guard the tube; lined with skin that contains ceruminous glands; auricle collects sound waves traveling through air and directs them into external acoustic meatus; sound waves alter the pressure on the tympanic membrane; this semitransparent membrane is covered by a thin layer of skin on its outer surface and by mucous membrane on the inside; is cone-shaped, with the apex of the cone directed inward; moves back and forth in response to sound waves

tympanic cavity – is an air-filled space in the temporal bone that separates the external and internal ears – contains three auditory ossicles; the auditory ossicles are the malleus, incus, and stapes – they bridge the tympanic membrane and the inner ear, transmitting vibrations between these two parts; malleus is attached to the tympanic membrane – vibrates in unison with it; malleus vibrates the incus which then vibrates the stapes, which is held in place by ligaments to the oval window; the vibration of the stapes at the oval window, moves a fluid within the inner ear – these vibrations of the fluid stimulate hearing receptors; ossicles also amply the force of the vibrations as they pass from the tympanic membrane to the oval window; two small muscles in the middle ear are attached to the ossicles and controlled involuntarily; tensor tympani – inserted on medial surface of malleus and anchored to the cartilaginous wall of the auditory tube – its contraction pulls the malleus inward; stapedius – attached to the posterior side of the stapes and the inner wall of the cavity – pulls the stapes outward; are the effectors of the tympanic reflex, which is elicited in about 1/10 second following a loud, external sound

connects each middle ear to the throat; helps maintain equal pressure on both sides of tympanic membrane

complex system of interconnecting chambers and tubes called a labyrinth;  osseous labyrinth – bony canal in the temporal bone;  membranous labyrinth – a tube that lies within the osseous labyrinth, filled with a fluid called endolymph; perilymph is found between the two labyrinths; parts of the labyrinths include a cochlea that functions in hearing and three semicircular canals that provide a sense of equilibrium – a bony chamber  called the vestibule is located between the cochlea and semicircular canals, houses membranous structures that serve both hearing and equilibrium; cochlea – shaped like a snail shell, coiled around a bony core ( modiolus ) with a thin, bony shelf ( spiral lamina ) that wraps around the core like a spiral staircase; shelf divides osseous labyrinth into an upper compartment, scala vestibuli, that leads to the apex of the spiral, and a lower compartment, scala tympani,that extends from the apex of the cochlea to the round window – the two compartments are connected by the helicotrema; a portion of the membranous labyrinth called the cochlear duct ( scala media) lies between the two bony

compartments – ends as a closed sac at the apex of cochlea; separated from scala vestibuli by the vestibular membrane and from the scala tympani by the basilar membrane; basilar membrane forms floor of cochlear duct – contains many thousands of stiff, elastic fibers whose lengths vary; vibrations from the oval window pass along scala vestibuli, pass through the vestibular membrane to enter endolymph of cochlear duct where they move the basilar membrane – after passing through this membrane, the vibrations enter perilymph of scala tympani, and their forces are dissipated into the air in the tympanic cavity by movement of the membrane covering the round window; the organ of corti contains about 16,000 hearing receptor cells – located on the upper surface of basilar membrane the receptor cells, called hair cells, are in four parallel rows, with many hairlike processes ( stereocilia ) extending into the endolymph of the cochlear duct; a tectorial membrane is above the hair cells – contacting the tips of their

   hairs; when hairs bend against the tectorial membrane, its cell membrane depolarizes and becomes more permeable to calcium ions; the receptor cell releases neurotransmitter which stimulates the ends of nearby sensory fibers which in turn, transmit nerve impulses along the cochlear branch of the vestibulocochlear nerve to the brain; the range of greatest sensitivity is between 2,000 and 3,000 vibrations

cochlear branch of VIII extends to the medulla and then proceeds through the midbrain to the thalamus – then into the auditory cortices of the temporal lobes; some fibers cross over, so that impulses arising from each ear are interpreted on both sides of the brain

senses the position of the head, maintaining stability and posture when the head and body are still; organs are located within the vestibule – the membranous labyrinth consists of two expanded chambers – a utricle and a saccule; the utricle communicates with the saccule and the membranous portions of the semicircular canals; the saccule communicates with the cochlear duct; each organ has a small patch of hair cells and supporting cells called a macula

on its wall; hair cells contact a sheet of gelatinous material that has crystals of calcium carbonate ( otiliths ) embedded on its surface; hair cells have nerve fibers wrapped around them – associated with the vestibular portion of the vestibulocochlear nerve; gravity stimulates hair cells to respond – movements of the head tilt the gelatinous mass, which sags and bends the hairs; this action stimulates the hair cells – nerve impulses travel into the CNS on the vestibular branch of VIII informing the brain of the head’s position; maculae also participate in sense of dynamic equilibrium; aid in detecting movements such as falling and in maintaining posture while walking

senses motion when the head and body suddenly move or

rotate; aid in maintaining balance; the three semicircular canals which lie at right angles to each other are where the sense organs of dynamic equilibrium are located; a swelling suspended in the perilymph of each canal is called an ampulla(communicates with the utricle); each ampulla houses a sensory organ called a crista ampullaris – each of these has a number of sensory hair cells and supporting cells; hair cells extend upward into a dome-shaped gelatinous mass called the cupula; hair cells connected at their bases to vestibular branch of VIII;  rapid turns of the head or body stimulate hair cells; semicircular canals move with the body but endolymph in canals remains stationary because of inertia – this bends cupula in a direction opposite that of the head or body, and the hairs bend; parts of cerebellum are important in interpreting these impulses – allows the brain to predict the consequences of rapid body movement, and by modifying signals to appropriate skeletal muscles, the cerebellum can maintain balance

 is composed of four layers – skin, muscle, connective tissue, and conjunctiva

orbicularis oculi and levator palpebrae superioris; orbicularis oculi acts as a sphincter that closes the lids when it contracts; the levator palpebrae superioris raises the upper eyelids

connective tissue layer of eyelid contains _____that produce oily secretions along the borders of the lids; the conjunctiva is a mucous membrane that lines the inner surfaces of the eyelids and folds back to cover the anterior surface of the eyeball, except for the cornea

the lacrimal apparatus consists of the_____ , which secretes tears, and a series of ducts, which carry the tears to the nasal cavity; superior and inferior canaliculi collect tears – openings ( puncta ) can be  seen on the medial borders of eyelids; fluid moves from these ducts into the lacrimal sac, and then into the nasolacrimal duct, which empties into the nasal cavity

 _______of the conjunctiva and tears, moisten and lubricate the surface of the eye and lining of eyelids; tears contain an enzyme, lysozyme, that has antibacterial properties

 ______of the eye arise from the bones of the orbit and are inserted by broad tendons on the eye’s sclera – six such muscles; motor units of these eye muscles contain the fewest muscle fibers ( 5-10) of any muscles in the body – can move the eye with great precision

window of eye, helps focus entering light rays; contains few cells, no blood vessels; collagenous fibers form regular

 patterns; well supplied with nerve fibers, associated with many pain receptors with low thresholds, as well as cold receptors

the white portion of the eye; opaque due to many large, haphazardly arranged collagenous and elastic fibers; protects the eye; attachment for the extrinsic muscles

optic nerve and blood vessels pierce the sclera;  dura mater is continuous with the sclera

many blood vessels to nourish tissues; melanocytes give it a brownish- black color; melanin absorbs excess light and keeps inside of eye dark

ciliary body extends from choroid coat – forms inner ring around front of eye; suspensory ligaments hold the transparent lens in place

lens lacks blood vessels; composed of specialized epithelial cells;  lens fibers ( columnar cells ) make up substance of lens– are produced continually throughout life; lens capsule surrounds the lens – is elastic, so lens can change shape; acccomodation – lens changes shape to view a close object 

iris – thin diaphragm composed of connective tissue and smooth muscle fibers – lies between cornea and the lens, dividing this anterior cavity into an anterior chamber and posterior chamber this cavity is filled with aqueous humor – secreted by ciliary body into the posterior chamber, flows through the pupil ( a circular opening in the center of the iris ), and into the anterior chamber; provides nutrients and maintains shape of front of eye; leaves through veins and the scleral venous sinus ( canal of Schlemm); smooth muscles of iris form a circular set and a radial set – control the size of the pupil   

consists of the retina – contains the photoreceptors; continuous with the optic nerve; thin and delicate, but its structure is complex – distinct layers, including pigmented epithelium, neurons, nerve fibers, and limiting membranes; five groups of neurons: nerve fibers of receptor cells, bipolar neurons, and ganglion cells provide a direct pathway for impulses to the optic nerve and brain; nerve fibers of the horizontal cells and amacrine cells pass laterally between retinal cells and modify the impulses transmitted on the fibers of the direct pathway; the macula lutea is a yellowish spot on retina with a depression in its center, the fovea centralis – a region that produces the sharpest vision

the optic disc is where the nerve fibers leave the retina and become parts of the optic nerve; central artery and vein pass through here – supply blood to retina; lacks receptor cells, so commonly called the blind spot

the posterior cavity is the space enclosed by the lens, ciliary body, and retina; filled with transparent, jellylike fluid called vitreous humor, makes up the vitreous body – helps support internal structures of the eye and helps maintain its shape

 occurs when light passes obliquely from a less-dense medium into a denser medium – from the air into the cornea of the eye, the light is bent toward a line perpendicular to the surface between these two substances – if the surface is curved, a lens is formed; convex surface – light waves converge; concave surface – light waves diverge;  cornea provides about 75% of total refractive power of eye – light refracted  again by convex surface of the lens and to some extent by the surfaces of liquids within eye chambers; image formed on retina is upside down and reversed from left to right

modified neurons of two distinct types – rods and cones; found in deep layer of retina, closely associated with pigmented epithelium; projections from photoreceptors extend into pigmented layer and contain light-sensitive visual pigments; pigment layer of retina absorbs excess light – also stores vitamin A, used by receptor cells to synthesize visual pigments; rods have long, thin projections at their terminal ends; hundreds of times more sensitive to light than are cones; produce colorless vision;  about 100 million rods; provide general outlines of objects; cones have short, blunt projections at their terminal ends; provide sharp images- due to nerve fibers from a low degree of convergence of impulses from cones; about 3 million cones; fovea centralis lacks rods but contains densely packed cones with few or no converging fibers – a person moves eyes so that the important part of an image falls upon the fovea centralis

 light-sensitive pigments in rods and cones decompose when they absorb light;  rhodopsin or visual purple is found in rods – embedded in membranous discs that are stacked ( may have 2,000 interconnected discs in one rod cell ); in presence of light, rhodopsin breaks down into molecules of a colorless protein called opsin and a yellowish organic molecule called retinal that is synthesized from vitamin A; in bright light, nearly all rhodopsin decomposes, sharply reducing sensitivity of these receptors – cones function and we see color in bright light; in dim light, rhodopsin is regenerated using ATP, and rods allow us to see shades of gray;  dark-adapted eyes increase light sensitivity about 100,000 times – allows one to see in dim light ( such as going from daylight into darkened theater ); when reentering sunlight, rhodopsin decomposes in response to bright light; light sensitivity decreases greatly, and eyes become light; liight sensitive pigments of cones,(iodopsins) are composed of retinal and three different proteins; erythrolabe – cone pigment sensitive to red light waves; chlorolabe – cone pigment sensitive to green light waves; cyanolabe – cone pigment sensitive to blue light waves;  if all three types of cones are stimulated, light is perceived as white; if none are stimulated, black is seen

simultaneously perceives distance, depth, height, and width of objects; two images from the eyes are superimposed and interpreted by visual cortex, resulting in the perception of a single object in three dimensions

axons of ganglion cells in retina leave the eyes to form optic nerves – just anterior to the pituitary gland, the nerves give rise to the optic chiasma; fibers from the nasal half of each retina cross over here;  the optic tract contains fibers from both eyes – a few fibers leave the tracts and enter nuclei that function in visual reflexes prior to reaching the thalamus; most fibers enter thalamus and synapse in its lateral geniculate body – fibers enter nerve pathways called optic radiations, and the pathways lead to the visual cortex of the occipital lobes