The Sense Organs

Humans react to both internal and external stimuli – we can detect these stimuli because our bodies contain several types of sensory receptors

Sensory Receptors – neurons that respond to changes in the environment (stimuli) Classified by stimuli type: 1. Mechanoreceptors – detect mechanical force (movement, pressure, or touch)

3. Chemoreceptors – detect chemical changes 4. Thermoreceptors – detect temperature changes 5. Nociceptors – detect harmful stimuli, respond to pain 2. Photoreceptors – detect light energy

Sensory organs – these are specialized structures that contain sensory receptors There are 5 major sensory organs: Ear – contains mechanoreceptors Eyes – contain photoreceptors Nose – contains chemoreceptors

Mouth – contains chemoreceptors Skin – contains mechanoreceptors and thermoreceptors Nociceptors – are found everywhere except the brain

Classified by location: 1. exteroceptors – stimuli outside of the body found near or at the body surface include touch, pressure, pain, and temperature receptors in the skin and most receptors of sense organs

2. Interoceptors – stimuli within the body monitor chemical changes, tissue stretch, and temperature their activity can cause pain, discomfort, hunger, or thirst … usually unaware of their workings

3. Proprioceptors – specialized neurons found in skeletal muscles, ligaments, tendons and joints location is much more restricted constantly advise the brain of body movements

1. Ears are the sense organs responsible for hearing and balance (equilibrium) The ear is divided into 3 regions: outer ear middle ear inner ear

The outer ear is made up of: Auricle – projecting portion composed of cartilage Ear canal – tube that leads down to the eardrum

The outer ear is separated from the middle ear by the eardrum

The middle ear is an air-filled cavity that is connected to the pharynx by a tiny tube called the Eustachian tube

the function of this tube is to equalize pressure on both sides of the eardrum, preventing it from rupturing

Example – when you go up in an airplane, the outside pressure decreases Air leaves the middle ear through the Eustachian tube until the pressure in the middle ear matches the outside air pressure

The middle ear contains 3 tiny ear bones called ossicles the ossicles are the tiniest bones in the body they function to transfer vibrations from the eardrum to the inner ear

The inner ear contains a coiled structure called the cochlea and 3 semicircular canals

The cochlea contains hair cells (mechanoreceptors) that receive vibrations from the ossicles and transmits that information through the vestibulocochlear nerve to the region of the cerebrum that interprets sounds

The semicircular canals contain hair cells (mechanoreceptors) that detect rotation of the head

they send this information through the vestibulocochlear nerve to the region of the brain that maintains balance (cerebellum)

sometimes the hair cells of the semicircular canals are overstimulated – when this happens you experience nausea…you are suffering from motion sickness

2. Eyes are the sense organs responsible for sight

The eye is made up of 3 layers: Sclera – white of the eye (tough, outer part of the eye) Choroid – black layer that keeps interior of eye dark Inner retina – contains photoreceptors

There are 2 types of photoreceptors found in the retina: rods cones

Rods contain the pigment rhodopsin, which is stimulated by weak light rods enable you to see shades of gray but not colors they produce poorly defined images

Cones contain the pigment photopsin that is stimulated by bright light they enable you to see sharp images and in color If your cones are nonfunctional you are colorblind and see only in shades of gray

Pathway of light entering the eye: Cornea  Pupil  Lens  Retina The cornea is the front portion of your eyeball

It is normally transparent (clear) If it becomes cloudy, you have a cataract

The pupil is the tiny black opening in the center of your iris (colored portion of your eye) The iris constricts the pupil (gets smaller) when you are exposed to bright light

this reduces the amount of light entering the eye

The iris dilates the pupil when you are exposed to dim light this allows more light to enter the eye

The lens functions to focus images on the retina

if the lens fails to focus images properly, you need glasses You are: Hyperopia “Farsightedness” Farsighted if you can see far away, but objects close to you are blurred

Nearsighted if you can see close to you, but objects far away are blurred You are: Myopia “Nearsightedness”

Astigmatism – distortion of image by cornea or lens

the retina contains rods and cones

you have fewer cones than rods most cones are located in the back portion of the retina, while most rods are found around the periphery

The fovea is a tiny pit in the very back of the eye that contains cones only your vision is sharpest when the lens focuses the image on the fovea

There is one area of the retina which contains no cones or rods this area is called the blind spot there are not cones or rods in this area because this is where the optic nerve enters the eye

When the rods and cones have been stimulated, they send impulses through the optic nerve, to the occipital lobe of the cerebrum to be interpreted

3. Your tongue is the sense organ responsible for taste

The receptors for taste are chemoreceptors that are located in the taste buds

When food dissolves on your tongue, chemicals are released that stimulate chemoreceptors they send impulses to the cerebrum to be interpreted

Despite the many flavors we taste, there are only tastes that stimulate your taste buds: sweet sour salty bitter Other flavors are perceived by your sense of smell (80% of taste is smell) umami - savoriness as in beef or other protein rich foods

The taste buds that detect sweet, sour, bitter, and salty flavors are found in certain areas of the tongue sweet - tip sour - sides bitter - back salty – over the entire surface of the tongue

4. Your nose is the sense organ responsible for smell the receptors for smell are chemoreceptors which are located in the roof of the nasal cavity the odors we breathe contain numerous chemicals

these chemicals stimulate the chemoreceptors, which send impulses through the olfactory nerve to the cerebrum to be interpreted