SENSORY SYSTEMS

By, Dr. Shamanthakamani

Narendran, M.D.

SENSORY ORGANS

Traditionally, there are five senses: sight, smell, taste, touch, and hearing.

Each of the senses consists of specialized cells that have receptors for specific stimuli.

These cells have links to the nervous system and thus to the brain.

Sensing is done at primitive levels in the cells and integrated into sensations in the nervous system.

Sight is probably the most developed sense in humans, followed closely by hearing.

There are million of neurons in the body that do not receive impulses from other neurons.  Instead these neurons which are called SENSORY RECEPTORS, REACT DIRECTLY TO STIMULATION FROM THE ENVIRONMENT.

Many Receptors that enable the Body to RECEIVE INFORMATION from the ENVIRONMENT are located in highly specialized Organs called SENSE ORGANS.

Examples of stimulation include: light, sound, motion, chemical, pressure, pain or changes in the temperature.

Once these Sensory Receptors are Stimulated, they TRANSFORM one form of ENERGY from the Environment (LIGHT, SOUND) into another form of ENERGY (ACTION POTENTIAL) that can be transmitted to other neurons.  These Action Potentials (IMPULSES) reach the Central Nervous System (CNS).

A Sensory Receptor is a Neuron that is Specialized to detect a Stimulus.  There are many kinds of Sensory Receptors, and they can be categorized on the Basis of the Type of Stimuli they Respond to:

1. MECHANORECEPTORS - Respond to Movement, Pressure, and Tension.

2. PHOTORECEPTORS (RODS AND CONES) - Respond to Variations in Light.

3. CHEMORECEPTORS - Respond to Chemicals.

4. THERMORECEPTORS - Respond to Changes in Temperature.

5. PAIN RECEPTORS - Respond to Tissue Damage - PAIN!

The sensory receptors are contained in the SENSE ORGANS.

Each of the five senses (sight, hearing, smell, taste, and touch) has a specific sense organ associated with it.

The most familiar Sense Organs are the EYES, EARS, NOSE, SKIN AND TASTE BUDS.

These Organs have RECEPTORS that can respond to Stimuli by producing NERVE IMPULSES in a Sensory Neuron.

The Receptors CONVERT the ENERGY of a Stimulus into ELECTRICAL ENERGY that can travel in the NERVOUS SYSTEM.

Receptors INSIDE the body inform the CNS about the CONDITIONS OF THE BODY.

Specialized cells (receptors) within each sense organ enable it to respond to particular stimuli.

Messages from Sense Organs to the CNS are all in the form of Nerve Impulses.

This Information is built into the "WIRING" in the Pathways of Neurons that Synapse with each other, and into the location in the Brain where the information arrives.

The Brain knows if the information received is from a Sensory Neuron that comes from LIGHT RECEPTORS CELLS when it gets the message.

HEARING AND BALANCE

The EAR is really TWO Sense Organs in ONE.  It not only detects Sound Waves, it also senses the Position of the HEAD, whether it is still, moving in a straight line, or rotating.

Sound is nothing more than Vibrations in the Air around us.

Deep LOW-PITCHED Sounds result from slow vibrations. HIGH-PITCHED Sounds are caused from faster Vibrations.

In addition to Pitch, sounds differ by their Loudness or Volume.

The Sense Organ that can distinguish both pitch and loudness of sounds are the EARS.

The External Ear consists of the visible fleshy part helps to COLLECT Sounds and FUNNEL them into the AUDITORY CANAL.

The Auditory Canal connects the External Ear with the TYMPANIC MEMBRANE, also called the Eardrum.

The Auditory Canal contains small Hairs and WAX Producing GLANDS that PREVENT Foreign objects from entering the ear.

The Auditory Canal extends into the bone of the head, but stops at the EARDRUM OR TYMPANIC MEMBRANE.

The Eardrum is the beginning of the MIDDLE EAR.

Sound Vibrations STRIKE the EARDRUM and are Transmitted through THREE TINY BONES:  THE MALLEUS (HAMMER), INCUS (ANVIL), AND STAPES (STIRRUP).

The Stirrup transfers the Vibrations to a thin membrane covering an opening called the OVAL WINDOW.

This Membrane transmits the vibrations to the COCHLEA, which begins the INNER EAR.

The COCHLEA is SNAIL SHAPED, consisting of Three FLUID FILLED Chambers that are separated by membranes.

The Middle Chamber contains the ORGAN OF CORTI, which is the organ of Hearing.

When the Fluid Vibrates, tiny Hair Cells lining the Cochlea are PUSHED back and forth, providing Stimulation that is turned into NERVE IMPULSES.

Interior structure of cochlea Detail structure of cochlea

These Nerve Impulses are carried to the Brain by the AUDITORY OR ACOUSTIC NERVE.

The EARS also contain structures for DETECTING STIMULI that make us aware of our MOVEMENTS and allow us to maintain our BALANCE.

Located within the INNER EAR just above the Cochlea are three tiny canals that lie at right angles to each other. Also called the SEMICIRCULAR CANALS because they each make half a circle.

The Semicircular Canals and the TWO Tiny Sacs located behind them help us to SENSE BALANCE OR EQUILIBRIUM.

Both the Canals and the Sacs are filled with Fluid and Lined with Hair Cells (Mechanoreceptors).

There are also Tiny Grains of Calcium Carbonate and Protein called OTOLITHS, Otoliths roll back and forth in response to gravity, acceleration, and deceleration.

The Movement of Fluid and Otoliths bend the hair on the Hair Cells, and in turn sends the impulses to the Brain that enable it to determine BODY MOTION AND POSITION.

VISION

The Sense Organ we use to sense Light is the EYES.

The EYE is composed of THREE LAYERS:

1. The OUTER Layer consists of the SCLERA AND CORNEA.

2. The MIDDLE Layer contains the CHOROID, CILIARY BODY, AND IRIS.

3. The INNER Layer consists of the RETINA.

The SCLERA (white of the eye) consist of tough white connective tissue.  The Sclera helps MAINTAIN the SHAPE OF EYE, and also provides a means of ATTACHMENT for the muscles that move the eye.

in the front of the eye, the sclera forms a transparent layer called the CORNEA.

The CORNEA is the part of the eye through which light enters. Just inside the Cornea is a small chamber filled with FLUID called AQUEOUS HUMOR.

At the BACK of this chamber, the pigmented CHOROID, which contains the blood vessels of the eye, becomes a disk-like structure called the IRIS.

The IRIS (A Diaphragm) is the portion of the eye that gives your eye its COLOR.  The Iris controls the amount of Light entering the eye by altering the Diameter of the Pupil.

In the MIDDLE of the Iris is a small opening called the PUPIL, through which light enters the eye.

The Pupil appears as a small black disk in the CENTER of the Eye.  Tiny muscles in the Iris REGULATE the SIZE of the Pupil, controlling the amount of Light to enter the Eye.

In DIM LIGHT the Pupil OPENS to INCREASE the amount of Light, In BRIGHT LIGHT the Pupil CLOSES to DECREASE the amount of Light entering the Eye.

Behind the Iris is the LENS.  Light is Focused by the Lens, which changes shape when pulled by muscles around its edges.

The CELLS that form the Lens contain a special PROTEIN called CRYSTALIN. CRYSTALIN is almost transparent and allows light to pass through.

Small Muscles attached to the Lens cause it to bend, this enables the eye to FOCUS on close and distant objects.

Behind the Lens is a Large Chamber called the VITREAL CHAMBER filled with a transparent Jelly-like Fluid called VITREOUS HUMOR.

Special Light Sensitive RECEPTOR CELLS, or PHOTORECEPTORS, are arranged in a layer in the RETINA, at the BACK of the EYE.

The PHOTORECEPTORS CONVERT LIGHT ENERGY INTO IMPULSES THAT ARE CARRIED TO THE CNS.

THERE ARE TWO TYPES OF PHOTORECEPTORS:  RODS AND CONES.  We have about 125 million RODS and 7 million CONES on a single Retina.

Photoreceptors contain a PIGMENT called RHODOPSIN, that can respond to most wavelengths of light.

RODS are extremely sensitive to ALL COLORS of LIGHT, but do not distinguish different colors.

CONES are less sensitive than RODS, but they do respond differently to light of different colors, producing color vision.

Humans have three kinds of cones.  Each type of cone contains a pigment that absorbs different wavelengths of light.  When the signals from these three kinds of cone are integrated, a person is able to see all the colors in the visible spectrum.

In DIM Light, when only RODS are activated, you may see objects clearly, but not their colors.

As the amount of Light INCREASE, the CONES are stimulated and the colors become clear.

The Impulses leave the Eye by way of the OPTIC NERVE, and CARRIED to the part of BRAIN Known as the OPTIC LOBE OR OCCIPITAL LOBE.  Here the Brain Interprets the visual images and provides information about the external world.

SMELL The Sense of Smell is a CHEMICAL SENSE, the Cells responsible for smell are Specialized Chemoreceptors called OLFACTORY RECEPTORS. 

These Cells are located in the Upper Part of the Nasal Cavity.

Chemoreceptors contain Cilia that extend into the air passageways of the nose and react to Chemicals in the Air.  Chemicals that come into contact with the Chemoreceptors Stimulate them, causing Impulses to be sent to the Brain by the OLFACTORY NERVE.

TASTE

The Sense of Taste is a Chemical Sense.

The Cells that are Stimulated by the Chemicals are called CHEMORECEPTORS.

The Sense Organ that detects Taste are the TASTE BUDS. NOT THE TONGUE!

Most of the 10,000 Taste Buds are embedded between bumps called PAPILLAE on the tongue, but can also be found on the roof of the mouth, on the lips, and in the throat.

Humans can detect FOUR main kinds of TASTE: 

SWEET

SALTY

SOUR

BITTER.

Each Taste Bud shows a particular Sensitivity to one of these taste. The Sense of Taste is converted to Nerve Impulses in the Taste Buds and transmitted to the Brain by TWO Nerves - The FACIAL NERVE & GLOSSOPHARYNGEAL NERVES.

Many of the Sensation associated with taste are actually SMELL Sensations, You depend on both Senses to detect Flavors in Food.

That is why when you have a Cold and Your Smell Receptors are blocked, food seems to have little or no taste.

TOUCH AND RELATED SENSES

All Regions of our bodies are sensitive to TOUCH.

Your largest sense organ is your SKIN.

MECHANORECEPTORS located throughout the Skin make it possible to sense touch, pressure, and tension.

In humans, the receptors for touch are concentrated in the face, tongue, and fingertips.

Body hair also plays an important role in the ability to sense touch.  Large numbers of Mechanoreceptors are found in the skin at the base of hair follicles.

Our Skin has several different types of Sensory Receptors that are just below the surface of the Skin.

Two Types respond to Heat or Cold – THERMORECEPTORS:

1. MECHANORECEPTORS

2. PAIN RECEPTORS.

Sensory receptors for Hot or Cold are scattered directly below the surface of the skin. There are THREE to FOUR WARM Receptors for every COLD Receptor.

Sensory Receptor can be more concentrated in different places of our bodies.

The MOST TOUCH-Sensitive areas are the FINGERS, TOES, AND LIPS.

Pain Receptors are located throughout the skin.  The Sensation of Pain can be experienced as either Prickling Pain (FAST PAIN) or Burning and Aching Pain (SLOW PAIN).

Pain receptors are Stimulated by mechanical, thermal, electrical, or chemical Energy.