Chapter 7: The Senses

The Senses

• General senses are touch, pressure, stretch, heat, cold, and pain

  • Receptors for general senses are located throughout the body

• General senses are detected by:

  • Thermoreceptors - temperature

  • Mechanoreceptors - touch, vibration, stretch, and pressure

    • Fine touch 

  • Nociceptors - tissue injury

    • Pain, actual injury

• Special senses are taste, smell, hearing, equilibrium, and vision

  • Special sense organs are located in the head

Free nerve endings

found throughout skin, bare dendrites, can be thermoreceptors or nociceptors

• Not myelinated

• Located in the dermis

Tactile corpuscles

mechanoreceptors for light touch/texture

• Superficial 

Lamellar corpuscles

mechanoreceptors for deep pressure

• Deeper 

Pain Pathway

• The pathway can be the same as any other sensory input from the skin

  • Unipolar neuron → spinal cord → medulla oblongata → pons → midbrain → thalamus → parietal lobe 

• The signal may take a different pathway 

  • Unipolar neuron → spinal cord → reticular formation → parietal lobe 

→  or hypothalamus and limbic system after reticular formation

Taste (Gustation)

• Special sense organ for taste is the taste bud

• Rounded bumps we can see are called papillae

  • Underneath the papillae are the taste buds

• Sensory nerve fibers are connected to taste cells 

• Taste hairs are like dendrites since they receive 

• Taste buds are located mostly on the tongue and contain different cells:

  • Taste cells have taste hairs that are chemoreceptors

    • Binding to chemical molecules 

  • Basal cells are stem cells that replace dead taste cells

  • Support cells physically support the 50-150 taste cells in each taste bud

  • Five primary tastes: salt, sweet, sour, bitter, and umami

Smell

• Olfactory cells are bipolar neurons with chemoreceptors

• Olfactory cells access the olfactory mucosa of the roof of the nasal cavity through the foramen of the cribriform plate 

• Send signal to the olfactory bulb above

• Cilia are like dendrites 

  • Odor molecules bind to the cilia

External ear

Pinna, auditory canal (earwax is found)

Middle ear

Ossicles 

• Malleous (closest to eardrum)

• Incus

• Stapes

Inner ear

• Cochlea 

  • Snail shape

  • Main structure in hearing 

• Auditory nerve 

  • “Vestibucochler nerve”

• Vestibule/Vestibular branch 

  • Equilibrium

Round window

Opened

Oval window

Covered by stapes 

Tympanic membrane (eardrum)

• Separates external and middle ear 

• Closes the interior to the external auditory canal

• Externally covered in skin, internally covered in mucous membrane

• Sound waves cause the membrane to vibrate in and out at the same frequency

The Cochlea

• Cochlear duct extends almost to apex (where it stops and comes back around)  of cochlea

  • Contains endolymph

    • Made of perilymph 

  • Within cochlea 

  • Separated from scala vestibuli by the vestibular membrane (top of cochlear duct) 

  • Separated from scala tympani by basilar membrane (bottom of cochlear duct)

• Basilar membrane contains 20,000 cross hairs

  • Can vibrate when activated by vibrations made from sound

• Organ of Corti (organ of cochlea) is in upper surface of basilar membrane 

  • Where hair cells are found 

    • In the spiral organ

• Microvilli is like the dendrites

Pitch is determined by:

• Higher the frequency sound won't travel far into the cochlea

• Lower the frequency will travel further into the cochlea 

  • Ex. bass 

• Portion of basilar membrane and organ of Corti activated by a sound frequency

• The hearing centers receiving the impulses

Loudness depends on the intensity of the vibration of the basilar membrane and organ of Corti

• Intensity determines frequency of impulse formation

• Greater frequency means louder sound sensations 

• Lower frequency means quieter

Equilibrium 

Several sensory receptors are involved 

• Receptors in joints and muscles

• Receptors in the eyes

• Receptors in the inner ear

Static equilibrium

when the head is motionless

• Ex. elevator (vertical), driving (horizontally)

Dynamic equilibrium

when the head is moving 

• Ex. cartwheels (frontal), somersalt, moving in a chair (transverse)

Static Equilibrium (in depth)

• Mainly in the vestibule 

• The macula (bunch of hair cells and supporting cells) is the organ of static equilibrium

  • Inside the utricle and saccule 

  • Contains thousands of hair cells

• Hair cell cilia are embedded in a gelatinous mass containing otoliths (oto - ear, lith - stone) 

• Otoliths accomplish two tasks:

  • Increase the weight of the gelatinous mass

  • Make it more responsive to the force of gravity

Dynamic Equilibrium (in depth)

• Semicircular canals contain receptors that detect motion of the head

  • Canals are oriented at 90 degrees to each other

  • Each canal is attached to utricle by an ampulla 

• Each ampulla possesses a crista ampullaris 

  • Sensory organ for dynamic equilibrium

  • Contains hair cells with processes extending into a cupula (dome shaped)

  • Connected to fibers from the vestibulocochlear nerve to the brain

• Cerebellum

• Parietal 

• Corpus callosum

• Cranial XI, III, IV, VI

• Spinal cord 

Anatomy of the Eye/Orbital Region

• Eyebrow - gutter

• Pupil - blackhole

• Iris -  smooth muscles to constrict or dilate the pupil 

• The eye uses photoreceptors to detect light

• Rectus and oblique muscles stimulated by CN III, IV, and VI move the eye

• The wall of the eye has three layers:

  • The sclera

    • White part of the eye

      • Surrounds the entire eye

  • The uvea

  • The retina 

• The Orbital Region

  • Protected by surrounding bones

  • Supported by connective tissues

  • Cushioned by fatty tissues behind the eyes

Lacrimal Glands and Ducts

• Function: Tear production

  • Lacrimal gland produces tears

    • Above eye, around eyelids

  • Lacrimal ducts carry tears to eye surface

  • Lacrimal canals to lacrimal sac to nasolacrimal duct 

• Tear functions: 

  • Keep eyes moist

  • Wash away foreign particles

  • Contain lysozyme to reduce infection chances

Muscles of the Eye

• 6 muscles that originate on the back of the eye orbit and insert on the eyeball

  • Superior rectus muscle (rectus -  straight)

    • Eyeball moving straight up 

  • Medial rectus muscle

    • Anterior right eye- looking left

  • Inferior rectus muscle  

    • Eyeball looking downwards 

  • Lateral rectus muscle 

    • Anterior right eye - looking right 

  • Superior oblique muscle (oblique - wraps around diagonal way)

  • Inferior oblique muscle 

• Function as a coordinated group to enable eye movements

Sclera

tough, fibrous, opaque, white portion of the eye

• Provides protection for delicate internal portions of eye and optic nerve

Cornea

the anterior, convex (outward), clear window of the eye

• Bends light rays as they pass through it

• Lacks blood vessels and nerves

Choroid layer

(one of the main parts of the uvea)

• Has large blood vessels to nourish the eye

• Has melanin to prevent backscattering of light 

  • Reflections from back of the eye

  • Ex. blurry visions

Ciliary body 

(one of the main parts of the uvea)

• Has ciliary muscles (involuntary, smooth muscles) that surround the lens

• Can change shape of lens

• Suspensory ligaments between ciliary body and lens hold the lens in place

Iris

(one of the main parts of the uvea)

the colored portion of the eye

• It controls the amount of light entering the eye by controlling the size of the pupil

• Pupil is the opening in the center of the iris that allows light to pass into the eye

  • Constricted in bright light, dilated in dim light 

• Circular muscles

  • Constrict the pupil

• Radial (“eyelashes” around the circular muscle)

  • Dilate the pupil

Neurons of the Retina 

The retina lines the interior of the eye posterior to the ciliary body

• Possesses rods fo black and white vision

  • Sensitive only to presence of light

• Possesses cones for color vision

  • Requires bright light to function 

  • Different cone cells for different color

Optic disc (bright light)

• Blood vessels enter and exit the eye

• Axons exit the eye

• No receptor cells = blind spot

Macula Lutea

• Yellowish disc on the retina

• Contains the fovea centralis, is the area of sharpest vision

  • Cones are most concentrated at the fovea centralis

  • Rods are least concentrated at the fovea centralis

Accommodation

helps to focus light on the retina

• Involves adjusting the shape of the lens for distance, intermediate, and near vision

For distant vision (hyperopia)

• Ciliary muscles relax

• Tension on suspensory ligaments is high

• Lens flattens or elongated

For near vision (myopia)

• Ciliary muscles contract

• Suspensory ligaments have low tension

• Lens thickens

Pathway for Vision

• Photoreceptor impulses are transmitted along the optic nerve to the brain

• The two optic nerves meet at the optic chiasm

• Fibers from the medial half of the retina cross to the opposite side

Cataracts

• A cataract is a progressive, painless loss of vision due to the clouding of the lens of the eye

• Cataracts can be treated by surgically removing the lens and replacing it with an artificial lens