Special Senses

Joel G. Soria, MD

Special Senses

Sense organs carry messages about the environment to the central nervous system

General Senses

Widely distributed throughout the body and includes the senses of touch, pressure, pain, temperature, vibration, itch, and proprioception.

Receptors – sensory nerve endings or specialized cells capable of responding to stimuli

RECEPTORS

Mechanoreceptor – Respond to mechanical stimuli

Chemoreceptor – Responds to chemicals

Photoreceptors – Responds to light

Thermoreceptors – Responds to temperature changes

Nociceptors - Responds to sensationb of pain

Free Nerve Endings – Structurally the simplest and most common type of receptor nerve ending

Touch (pressure)

Mechanoreceptors

Free nerve endings

Pacinian corpuscles

Ruffini corpuscles

Merkel receptors

Meisaner's corpuscles

Barroreceptors

Cutaneous sense

Also known as tactile sense

The gate control theory of pain modulation

REFERRED PAIN

OLFACTION

OLFACTION

Sense of smell

Occurs in response to airborne odorants that enter the nasal cavity

Olfactory neurons – bipolar neurons within the olfactory epithelium lining the superior part of the nasal cavity.

CHEMOSENSES: Olfaction Routes of information

transfer through the CNS

(1) Cranial nerve I → olfactory bulb (mitral cells & glomeruli)

(2) From olfactory bulb to parts of paleocortex in the telencephalon, including the:

• Amygdala (emotions)

• Entorhinnal, pyriform, & orbitofrontal cortex.

(3) From there the information goes to

• Hypothalamus (emotions, motivations),

• Hippocampus (in temporal lobe: memory)

TASTE

How many different “tastes” are there?Four or Five

1. Salty2. Sweet3. Bitter4. Sour5. Kiamoy?

CN- VII

CN- IX

Primary ‘gustatory’ cortex (near region where the mouth is represented) Also to: amygdala, hypothalamus, orbital prefrontal cortex

Medulla

Pons

Thalamus

Vision

Physical information from the world (light)

Hue (“color”): wavelength Saturation: purity of the light wave Brightness: intensity of the electromagnetic radiation

Specialized sense organ (eye)

Specialized neural tissue (retina)

Specialized sensory receptors (rods,cones)

sensory transduction: light - neural activity

Anatomy of the Visual System

Pupil: Adjustable opening in the iris that regulates the amount of light that enters the eye Iris: Pigmented ring of muscles situated behind the cornea

Cornea: Transparent outer covering of the eye that admits light

Extra-occular muscle

• Ciliary body and lens divide the anterior cavity of the eye into posterior (vitreous) cavity and anterior cavity

• Anterior cavity further divided • anterior chamber in front of eye • posterior chamber between the iris and the lens

Fluids in the Eye

re-enters circulation Vitreous humor fills the posterior cavity.

Not recycled – permanent fluid

Aqueous humor circulates within the eye

diffuses through the walls of anterior chamber, passes through canal of Schlemm

Lens: Consists of a series of transparent, onion-like layers. Its shape can be changed by contraction of ciliary muscles.

Accommodation: Changes in the thickness of the lens, accomplished by the ciliary muscles, that focus images of near or distant objects on the retina

Accommodation

• Posterior to the cornea and forms anterior boundary of posterior cavity

• Posterior cavity contains vitreous humor

• Lens helps focus

• Light is refracted as it passes through lens

• Accommodation is the process by which the lens adjusts to focus images

• Normal visual acuity is 20/20

Photoreceptors:

Retina: The neural tissue and photoreceptive cells located on the inner surface of the posterior portion of the eye.

Rod: Photoreceptor cells of the retina, sensitive the light of low intensity.

Cone: Photoreceptor cells of the retina; maximally sensitive to one of three different wavelengths of light and hence encodes color vision.

Fovea:

Area of retina that mediates the most acute vision. Contains only color-sensitive cones.

Optic Disk:

Location on retina where fibers of ganglion cells exit the eye. Responsible for the blind spot.

Retina contains rods and cones

Cones densely packed at fovea (center of the macula lutea)

Retinal pathway

Photoreceptors to bipolar cells to ganglion cells, to the brain via the optic nerve

Axons of ganglion cells converge at blind spot (optic disc)

Horizontal cells and amacrine cells modify the signal passed along the retinal neurons

Photoreceptors

Lamella:

A layer of membrane containing photopigments; found in the rods and cones.

Photopigment:

A protein dye bonded to retinal, a substance derived from vitamin A; responsible for the transduction of visual information.

Opsin:

A class of protein that, together with retinal, constitutes the photopigments

Rhodopsin:

A particular opsin found in rods

Transducin:

A G-protein that is activated when a photon of light strikes a pigment. Activates phosphodiesterase molecules which destroy cyclic GMP and close cation channels in the photoreceptor

HEARING &

BALANCE

The Ear

The Ear

Organ of hearing and equilibrium Detects and analyses noises by transduction (or the conversion

of sound waves into electrochemical impulses) Maintains the sense of balance (equilibrium).

Intensity and pitch

• Sound characterised by intensity and pitch • Pitch is number of cycles per second • Intensity is the energy of the wave. High energy - high change

in atmospheric pressure - LOUD • The ear analyses the sound arriving at it - hair cells • Converts it to electrical energy • Brain decodes the info

Parts of the Ear

Ossicles

The MALLEUS (hammer) gets the vibrations from the eardrum, then sends

them to the anvil.

The INCUS (anvil) passes the vibrations to the

stirrup.

The STAPES (stirrup) passes the vibrations to

the inner ear.

Parts of the Ear

The inner ear is made of the cochlea and liquid.

The cochlea is in the inner ear. The cochlea looks like a shell.

The auditory nerve carries the hearing information to the brain and the brain tells us what we heard.

Cochlea

VESTIBULAR MEMBRANE – separates the cochlear duct from scala vestibuli. BASILAR MEMBRANE - separates the cochlear duct from scala tympani.

The cochlea is the organ where sound waves are converted first into fluid waves, then into chemical signals and finally into action potentials

The movement of the tectoral membrane by sound waves moves the cilia on the hair cells and effects neurotransmitters released by the hair cells.

Transmission of sound to the hair cells

Hair cells have steriocilia Vibrations are sent from the oval window

through the cochlea and vibrate Basiliar membrane - creates shearing force between the 2 membranes

Stereocilia are moved back and forward Change in hair cell properties and the creation

of electrical impulses which are sent to the brain via the auditory nerve

Hair cells responsible for transduction Inner and outer hair cells Coil round inside of Organ of Corti & attached to Basilar membrane

KKK

Summary on Transduction

1.Sound waves cause vibrations in basilar membrane 2.Travelling waves cause sheering between basilar and overlying

tectorial membrane 3.Bends stereocilia back and forward 4.Forward bending creates tension on the protein bridges 5.Tension opens K+ channels 6.Due to K+ gradient K+ floods into hair cells 7.Cells depolarise and transmitter is released 8.Transmitter stimulates auditory nerve

The Vestibular System

• Controls the sense of movement and balance • The first sensory system to fully develop by six months after

conception is the vestibular system

• This system is the sensory system considered to have the most important influence on the other sensory systems and on the ability to function in everyday life

• Unifying system in our brain that modifies and coordinates information received from other systems

The vestibular system

• Functions to sense movement (acceleration and deceleration) and static position

• Resides in the inner ear (labyrinth)

• Consists of 3 semi circular canals, otolith organs (utricle & sacule) and the superior & inferior nerves

• Semi circular canals and the otolith organs are filled with endolymph

• Perilymph fills the space between

Elements of the Vestibular Labyrinth

Continues with the cochlea

Three semicircular canals

Detect angular acceleration

Two otolith organs

Utricle & Saccule

Detect linear acceleration

Vestibular nerve fibers

synapse with hair cells

The Vestibular Labyrinth

Semicircular Canals

• Detect angular acceleration

• There are 3 canals

• Corresponds to the three dimensions in which you move

• Each canal detects motion in a single plane

• Paired on opposite sides of the head

• Each canal is a continues endolymph-filled hoop

• The actual hair cells sit in a small swelling at the base called the ampulla

The Ampulla

Bulge at end = ampulla

Contains a sheet of cells = cristae

The hair cells are arranged as a single tuft that projects up

Cilia of hair cells extend upward into the cupola

the Saccule & utricle Utricle and saccule signal linear

acceleration

Important for posture control

Contains a macule containing hair cells

Crystals of otolyths move in response to gravitational forces

This moves the otolthic membrane

Steriocilia move

Otolith organs & gravity

Each utricle and saccule has a macula

Macula contains hair cells Otolith crystals roll around when

the head tilts and this bend the microvilli

The otolithic organs sense motion according to their orientation.

The utricle is horizontal in the head, and largely registers accelerations acting in the horizontal plane.

The saccule is vertical in the head, and registers accelerations in the vertical plane

The Vestibular System