The Senses

Anatomy & Physiology IChapter 11

The Senses

• Protect a person by detecting changes in the environment• Stimulus – an environmental change that initiates or

stimulate a nerve impulse• Sensation – when a specialized area of the cerebral cortex

interprets the nerve impulse and the stimulus then becomes something we experience

• Environmental change >> stimulus >> threshold stimulus >> sensory receptors >> sensory neuron>> CNS >> interpretation>> sensation (what we experience or sense)

• Threshold stimulus - minimum amount of stimulus required to generate a nerve impulse

Sensory Receptors – Types based on Structure

• Free dendrite of a sensory neuron; Example, pain, temperature

• End-organ - the modified dendrite ending of a sensory neuron enclosed in a capsule -Pacinian & Meissner corpuscles for pressure & touch receptors

• Specialized cell associated with an afferent neuron (afferent/ascending/to the CNS); rods & cones in the eye

Sensory Receptors

Sensory Receptors – Types based on Stimulus

• Chemoreceptors – detect chemicals in solution; taste & smell

• Photoreceptors – respond to light; eye retina• Thermoreceptors – detect temperature change; skin• Mechanoreceptors – respond to movement,

vibration, pressure, stretch; located in skin, joints (monitor body position), hearing & equilibrium in ear

Types of senses based on the distribution of their receptors

• Special senses; sensory receptors localized in a special sense organ– Vision – Hearing – Equilibrium – Taste – Smell

• General senses; sensory receptors widely distributed throughout the body– Pressure – Temperature– Pain – Touch– Position

The Eye - Protective Structures

• Skull bones form walls of eye orbit; over half of posterior eye

• Upper & lower eyelids; anterior eye • Eyelashes & eyebrows; anterior eye• Conjunctiva; lines inner surface of

eyelids & covers the visible portion of the white of the eye (sclera); produces mucus & is highly vascular

• Lacrimal glands; produce tears which lubricate & produce enzyme that protects against infection– Flow from superior lateral eye into

inferior medial nasolacrimal duct

Lacrimal Gland & Conjunctiva

Coats of the Eyeball• 3 tunics (coats) • Sclera – outermost tunic made

of tough connective tissue; white of the eye because made of collagen & no blood vessels

• Choroid – coat made of delicate connective tissue; extensive blood supply which are visualized during eye exam; prevents light from scattering throughout eye

• Retina – actual receptor layer; contains rods & cones which generate visual nerve impulses

What are some structures that protect the eye?

What are the names of the tunics of the eyeball?

Pathway of Light Rays & Refraction• Refraction – bending of light

rays as they pass from one substance to another substance of a different density

• Allows light from a very large area to be focused on a very small area of the retina

• Cornea• Aqueous humor• Lens• Vitreous body

Light Refraction in the Eye• Cornea – transparent, colorless

continuation of sclera that covers the pupil; the window of the eye

• Aqueous humor – watery fluid that fills the eye anterior to the lens; aids in refraction & maintains eye shape

• Lens – clear circular structure with biconcave surface made of firm, elastic material; can change in thickness & focus near or far

• Vitreous body – soft gel that fills entire space posterior to the lens; aids in refraction & maintains eye shape

Layers of the Retina

• Pigmented layer – deepest layer just anterior to choroid

• Rods & cones – receptors of the eye

• Connecting neurons that carry impulses toward the optic

nerve

Rods• Rods – highly sensitive to

light– function in dim light but do

not provide sharp image• Dark adaptation; the time it

takes for rods to begin working in a darkened area

– 120 million each retina– Distributed towards the

periphery of the retina– See shades of gray; no colors– Rhodopsin – visual purple

pigment that is sensitive to light; requires vitamin A; lack of this pigment leads to night blindness

Cones• Cones – sensitive to color

– Function only in bright light– 6 million per retina– Localized in center of retina

• fovea centralis, pit near the optic nerve; area of greatest visual acuity

• surrounded by the macula lutea• Optic disk; point where the optic nerve

arises in the retina; no rods or cones in this area; blind spot on the retina

– Sees sharp images– Pigments sensitive to red, green, blue

• Hereditary lack of pigment can lead to colorblindness in males

Fovea & Macula lutea Fovea (dark pink) & Macula lutea (yellowish)

Visual Impulses

• Light stimulates rods & cones which stimulate neurons that eventually merge to form the optic nerve (CN II)

• Some optic nerve fibers crosses at optic chiasma

• Visual center in the occipital cortex of the cerebrum interprets

Eye Muscles – Extrinsic & Intrinsic Groups

• Extrinsic – 6 voluntary on outer

surface of eye– Controlled by CN III, IV,

VI (oculomotor, trochlear, abucens)

– Convergence – pulling the eyeballs in a coordinated fashion so there is one visual field

Eye Muscles – Extrinsic & Intrinsic Groups

• Intrinsic– Involuntary muscles within the

eyeball– Controlled by CN III (oculomotor)– Iris; pigmented part of the eye

composed of 2 sets of muscles that control pupil size

• Circular muscle constricts in bright light

• Radial muscle constricts in dim light– Ciliary muscle; holds the lens of

the eye in place by suspensory ligaments

– Accomodation – ciliary muscle constriction changes the shape of the lens to allow for near & far vision

Intrinsic Eye Muscle Actions

• The light rays from a close object diverge (separate) more than do the light rays from a distant object >>the lens must become more rounded to bend the light rays more

• When the ciliary muscle is relaxed, tension on the suspensory ligaments keeps the lens in a more flattened shape. For close vision, the ciliary muscle contracts

• For close vision, the ciliary muscle contracts, which draws the ciliary ring forward and relaxes tension on the suspensory ligaments. The elastic lens then recoils and becomes thicker

Nerve Supply to EyeTwo sensory nerves supply the eye:• CNII – optic nerve; carries visual impulses from retinal

rods & cones to the thalamus (diencephalon) to the visual center in occipital lobe of cerebrum

• CNV – trigeminal nerve, opthalmic branch; carries pain, touch & temperature impulses from the eye to the brain

• There are no retinal rods and cones in the area of the optic nerve. Consequently, no image can form on the retina at this point, which is known as the blind spot or optic disk

Nerve Supply to the EyeThree nerves carry motor impulses to the eyeball muscles:• CNIII – oculomotor; largest motor nerve to the eyeball; supplies

voluntary & involuntary muscles to all but two eye muscles• CNIV – trochlear; supplies superior extrinsic eye muscle• CNVI – abducens; supplies lateral extrinsic eye muscles

Steps in Vision

• Light refracts (bends)• Muscles of the iris adjust the pupil• Ciliary muscle adjusts the lens (accomodation)• Extrinsic eye muscles produce convergence

(coordinate to allow one visual field)• Light stimulates rods & cones• Optic nerve transmits impulses to thalamus• Thalamus transmits impulses to occipital lobe• Occipital lobe cortex interprets impulses

Errors of Refraction• Hyperopia – farsightedness

– Usually due to abnormally short eyeball (flat cornea)– Light focuses behind retina– The lens can thicken only to a given limit to accommodate for near

vision– move an object away from the eye to see it clearly– Glasses with convex lenses that increase light refraction

• Myopia – nearsightedness– Usually due to abnormally long eyeball– Light focuses in front of retina– Distant objects appear blurred and may appear clear only if brought

near the eye– A concave lens corrects for myopia

• Astigmatism – blurred vision– Cornea or lens curves irregularly, bending light incorrectly

A. Normal B. MyopiaC. Hyperopia D. Astigmatism

Eye Disorders• Strabismus – deviation of the

eye due to lack of coordination of eye muscles

• In convergent strabismus, the eye deviates toward the nasal side

• In divergent strabismus, the affected eye deviates laterally.

• if not corrected brain will not develop to see properly

• Amblyopia – loss of vision in a healthy eye because it cannot work properly with the other eye

Eye Disorders• Conjuntivitis – inflammation of conjunctiva

– Pinkeye – conjunctivitis caused by infection; is usually caused by cocci or bacilli

– Inclusion conjunctivitis is an acute eye infection caused by Chlamydia (AKA Trachoma)

• Corneal laceration – most common eye injury & if untreated can result in blindness– Cornea is avascular so it is possible to receive transplant without

rejection– Enucleation – removes the eye due to traumatic injury

• Cataract – opacity of the lens which can lead to blindness • Glaucoma – excess pressure in the eyeball due to aqueous

humor not being reabsorbed into blood

Conjunctivitis & Cataract

Glaucoma

Disorders related to retina

• Diabetic retinopathy – retina damaged by vascular hemorrhages & overgrowth

• Retinal detachment – retina separates from underlying layer as the result of trauma or fluid accumulation between tunics of eye

• Macular degeneration – macula lutea deteriorates & distorts visual field

Diabetic Retinopathy

Detached Retina

Macular Degeneration

The Ear - Structure• Outer ear – external ear to the tympanic membrane• Middle ear – contains 3 bones (ossicles) of ear &

eustachian tube• Inner ear – contains sensory receptors for hearing &

equilibrium

Outer Ear• Pinna – aka auricle; the

external ear• External auditory canal

– aka external auditory meatus– Contains ceruminous

glands• Tympanic membrane –

aka eardrum– Vibrates as sound

waves enter the ear

Middle Ear - Ossicles• Small cavity containing 3

ossicles (bones) that amplify sound waves received by tympanic membrane

• Malleus (hammer) – attached to tympanic membrane by handle & head attaches to incus

• Incus (anvil) – connects to malleus & stapes

• Stapes (stirrup) – connects to oval window, the membrane of inner ear

The middle ear- ossicles

Middle Ear – Eustachian Tube

• Eustachian tube – connects middle ear to pharynx

• Allows pressure to equalize on the 2 sides of tympanic membrane

Inner Ear aka Bony Labyrinth• 3 separate divisions of sensory receptors

– Vestibule, semicircular canals & cochlea– Perilymph – fluid of inner ear– Membranous labyrinth – within bony labyrinth & filled with

endolymph

Inner Ear aka Bony Labyrinth• Vestibule – 2

bony chambers that contain equilibrium receptors

• Semicircular canals – 2 bony tubes that contain equilibrium receptors

Inner Ear aka Bony Labyrinth

• Cochlea – bony coil that contains hearing receptors

• Round window – membrane through which sound waves leave the inner ear

Hearing• Organ of Corti – sensory organ

of hearing– Consists of ciliated receptor cells

• Located inside membranous cochlea aka cochlear duct

• Wave against the roof of the cochlear duct (tectorial membrane)

• Stimulates cochlear nerve (auditory branch of vestibulocochlear nerve CN VIII)

• Sound waves leave inner ear through round window

Steps of Hearing• Sound waves vibrate tympanic

membrane• Amplified by ossicles in middle

ear• Reach the oval window &

create waves in the inner ear fluids

• Vibrating the chochlear duct • Causing cilia or organ of Corti to

wave against the tectorial membrane

• Cochlear nerve stimulated (branch of CNVIII)

• The temporal lobe of cerebrum interprets stimuli

Hearing• Organ of Corti differentiates both pitch (tone) & intensity (loudness)• Higher pitched tones near the base• Lower pitched tones near the top• Loud sounds stimulate more cells & produce more vibrations, sending more

impulses to the brain

Equilibrium - Balance

• Sensory receptors are ciliated cells located in vestibule & semicircular canals

• Shifting in position of cilia within the thick fluid that surrounds them generates a nerve impulse

• Send impulses to vestibular branch of CN VIII (vestibulocochlear)

Equilibrium - Balance• Static equilibrium-

Sensing the position of head or body when moving in a straight line– Due to vestibule

receptors known as macula

– Fluid surrounding maculae contains crystals called otoliths which drag the fluid & increase pull of gravity

Detail of Macula

Equilibrium

• Dynamic equilibrium – function during spinning or moving in different directions– Semicircular canal

receptors known as cristae

Ear Disorders

• Otitis media – inflammation of the middle ear– Children susceptible

because short eustachian tube

• Otitis externia – inflammation of external auditory canal - aka swimmer’s ear

Hearing Loss• Conductive hearing loss – interference with sound waves passing

through ear– Foreign body obstruction– Tympanic membrane damage– Otosclerosis – hereditary malfunction of stapes

• Sensorineural hearing loss – cochlea, CNVIII or temporal lobe of brain improperly transmit or interpret neural stimuli– Prolonged exposure to loud noises– Certain drugs or toxins– Cochlear implant can stimulate cochlear nerve directly, bypassing

receptor cells– Presbycusis – age related hearing loss due to atrophy of sensory receptors

an CNVIII fibers

Loud Noise Damage to Ear

Taste - Gustation

• Tongue receptors aka taste buds• Taste buds are stimulated only if

the substance to be tasted is in solution or dissolves in the fluids of the mouth– Sweet – tip of the tongue– Salty – middle of tongue– Sour – middle sides of tongue– Bitter – posterior tongue

• Transmitted along CN VII & IX (facial & glossopharyngeal) to frontal cortex; no special center in cortex

• Other tastes include water, alkaline, metallic

• Tastes can be combined with smells

Smell - Olfaction

• Important for protection, taste, triggering memory, sexual behavior

• Receptors located in superior nasal cavity

• Conduct impulses along olfactory nerve (CNI) to olfactory center in temporal lobe of cerebral cortex

General Senses

• Located throughout body

• Touch • Pressure• Temperature• Position • Pain

General SensesTouch – The touch receptors, tactile (TAK-

til) corpuscles, are found mostly in the dermis of the skin and around hair follicles

• Meissner corpuscles in skin, also near hair follicles; # of corpuscles determines sensitivity to touch– Lips, tip of tongue, fingertips

very sensitive– Back, back of hand less sensitive

• Pressure – Pacinian corpuscles in skin, also near joints– sensory end-organs– Can respond even when a

person is under anesthesia

General Senses• Temperature – free nerve endings; dendrite branches

– Widely distributed in skin– Separate branches for heat & cold– Stimulus travels to hypothalamus (diencephalon)

• Position – proprioceptors - receptors located in joints, muscles, tendons– Kinesthesia – sense of body movement – where the body is in

time & space– Inform brain about muscle contraction/tendon tension– Muscle tone, posture, coordination, complicated skills– Stimulus travels to cerebellum

Temperature & Proprioceptors

General Senses• Pain – free nerve endings;

widely distributed in skin, muscles, joints, and scattered sparsely in viscera & blood vessels

• Nocioceptors – nerve endings that transmit pain signals– Sharp – acute, fast Adelta

fibers– Dull – slow, chronic C fibers

Pain

• The 5th vital sign• An unpleasant sensory & emotional

experience associated with actual or potential tissue damage or described in terms of such damage

• Pain is always subjective • Pain is whatever the experiencing person says

it is, existing whenever he or she says it does

Neuroanatomy of Pain

• Nocioception – how noxious stimuli are perceived as pain

• 3 phases of nocioception– Transduction – a noxious stimulus occurs

peripherally and travels to the spinal cord– Transmission – pain impulse moves from the

level of the spinal cord to the brain– Perception – conscious awareness of a

painful sensation• Modulation – pain is inhibited

– The brain sends signals to slow down or impede the pain impulse

Sources of Pain• Visceral pain – from organs • Deep somatic pain – from structures like vessels, bones,

muscles, joints• Cutaneous pain – from skin• Referred pain – pain felt at a particular site but originates

at a different location innervated by the same spinal nerve• Neuropathic pain – an abnormal neurological processing

of pain signals– the most difficult to assess & treat– Pain often perceived long after the site of injury heals

Referred Pain

Different Pain Scales

Neonatal Pain Scoring

Pain Relief• Analgesics – drugs that relieve pain

– Nonnarcotic analgesics – act locally to reduce inflammation i.e. NSAIDs

– Narcotics – act on CNS to alter the perception of pain• Anesthetics – drugs that block pain • Endorphins – chemicals released from the brain to modulate

pain; massage can activate endorphins• Heat – reduces chronic pain as long as there is no inflammation• Cold – acts as an anesthetic; reduces inflammation; when in

doubt, ice• Relaxation or distraction techniques

Sensory Adaptation

• Sensory receptors exposed to continuous stimulus adjust so sensation becomes less acute; warmth, cold & light pressure adapt rapidly

• Pain receptors do not adapt; C fiber (slow pain) receptors become more sensitive if exposed to continuous stimulation

Chronic Pain

Chronic Pain