Types of Receptors, Sensations and Sensory Adaption
Touch and Pressure Senses, Temperature Senses, and Sense of Pain
Olfactory Receptors, Olfactory Organs, Olfactory Nerve Pathways, and Olfactory Stimulation
Taste Receptors, Taste Sensations, and Taste Nerve Pathways
External Ear, Auditory Tube, Middle Ear, Inner Ear and Auditory Nerve Pathways
Static Equilibrium and Dynamic Equilibrium
Visual Accessory Organs and Structure of the Eye
Light Refraction, Visual Receptors, Visual Pigments, and Visual Nerve Pathways
Changes occurring within or one the body stimulate sensory receptors. This sensory information is then passes along sensory nerves to the brain for interpretation and processing. When the information is processed by the brain, a person experiences a sensation.
1) Chemoreceptors - activated by changes in chemical concentrations; smell and taste receptors are examples.
2) Pain receptors - activated by tissue damage.
3) Thermoreceptors - activated by changes in temperature.
4) Mechanoreceptors - activated by pressure or movements; touch and equilibrium receptors are examples.
5) Photoreceptors - activated by light; visual receptors are examples.
A sensation is a feeling that results from the brain interpreting sensory impulses. The resulting sensation depends on which region of the brain receives the impulse. For example, impulses reaching one area are always interpreted as sound, impulses reaching other areas are interpreted at touch.
At the same time that a sensation is interpreted, the cerebral cortex causes the sensation to seem to come from its source. This process is called projection. Because of this process, when you feel pain on your left pinky finger, you know exactly where the pain is coming from without even looking.
As some receptors are stimulated over and over again with the same stimulus, they quit sending the sensory information towards the brain. We call this adjustment to stimuli, sensory adaptation. For example, when you put on perfume, you smell it. As your smell receptors are stimulated by the perfume over and over again, they adapt and you no longer smell the perfume. Touch receptors also adapt well.
Somatic senses are associated with receptors in the skin, muscles, joints, and viscera.
Click here to see a summary table of somatic sense receptors
There are 3 types of touch or pressure receptors:
1) Sensory nerve fibers - common in epithelial tissues; pick up general touch sensations.
2) Meissner's corpuscles - common in hairless areas of skin such as the lips, fingertips, palms, soles, nipples, and external genital organs; they are stimulated by light touch.
3) Pacinian corpuscles - common in the skin, tendons, and ligaments; they respond to heavy pressure or deep touch.
Click here to view touch and pressure receptors found in skin.
There are two types of temperature receptors:
1) Heat receptors - only respond to moderately warm temperatures (25 degrees C to 45 degrees C). When you touch something very hot, your heat receptors are not activated, your pain receptors are activated!
2) Cold receptors - only respond to moderately cold temperatures (10 degrees C to 20 degrees C). When you touch something extremely cold, your cold receptors are not activated, your pain receptors are activated! This is why freezing is painful.
Pain receptors are widely distributed throughout the skin and internal tissues except in the nervous tissue of the brain which lacks pain receptors. Tissue damage or lack of blood flow to a tissue stimulates pain receptors.
Visceral pain receptors are found in visceral organs. Visceral pain does not feel very well localized. For example, if you have a stomach ache, your entire abdomen hurts not just a small area of the stomach.
Visceral pain often exhibits referred pain. Referred pain is pain that feels like it is coming from one area of the body but is actually coming from another area. For example, when a person has a heart attack, they often experience pain in the left arm. Referred pain is thought to occur when the cerebral cortex incorrectly interprets where the stimulus is coming from.
Click here to view common referred pain regions
There are two types of pain fibers:
1) Acute pain fibers - These pain fibers are thin and myelinated. They conduct nerve impulses rapidly and are associated with the sensation of sharp pain. These fibers are often found in the skin.
2) Chronic pain fibers - These pain fibers are thin but unmyelinated fibers. They conduct nerve impulses more slowly and are associated with dull, aching types of pain. Chronic pain is often associated with deeper body organs such as the stomach, heart, intestines, etc.
When pain information reaches the thalamus, a person is aware of the pain. However, it's not until the pain information reaches the cerebral cortex that the person is aware of the source and intensity of the pain.
Our bodies produce natural pain controlling substances such as serotonin, enkephalins, and endorphins. Enkephalins and endorphis block pain information from traveling up the spinal cord to the brain for interpretation. Serotonin causes cells to release enkephalins.
The special senses are smell, taste, vision, hearing and equilibrium. They are called special senses because their sensory receptors are localized within relatively large, sensory organs in the head.
Olfactory (smell) receptors are a type of chemoreceptor. Therefore, they respond to changes in chemical concentrations. Chemicals must be dissolved in mucus in order to activate olfactory receptors.
The olfactory organs are structures that contain olfactory receptors. They are yellowish in color and found primarily in the upper parts of the nasal cavity.
Click here to view olfactory receptors in the olfactory epithelium
Activated olfactory receptor cells send their nerve impulses along their axons which are in olfactory nerves. The olfactory nerves send their impulses to olfactory bulbs. The olfactory bulbs send their impulses to olfactory tracts. The nerve impulse is then transmitted to the olfactory cortex in the cerebrum where the information is interpreted as a smell.
Taste receptors are found on taste buds. Taste buds are found on "the bumps" of the tongue. These "bumps" are called papillae. Some students think these bumps are taste bud but they are not. Taste buds are microscopic. You cannot see them with the naked eye. Some taste buds are scattered on the roof of your mouth and in the walls of your throat.
Each taste bud is made of taste cells and supporting cells. The taste cells function as taste receptors and the supporting cells simply fill in the spaces between the taste cells
Taste cells (receptors) are activated by chemicals dissolved in saliva.
Click here to view taste receptors
There are basically four types of taste cells and each type is activated by a particular group of chemicals. Therefore, four primary taste sensations are produced which are:
1) sweet- taste receptors that respond to "sweet" chemicals are concentrated at the tip of the tongue
2) sour - taste receptors that respond to "sour" chemicals are concentrated on the sides of the tongue
3) salty - taste receptors that respond to "salty" chemicals are concentrated on the tip and sides of the tongue
4) bitter - taste receptors that respond to "bitter" chemicals are concentrated on the back of the tongue
When you eat spicy foods you are activating pain receptors on the tongue!
Sensory nerve impulses from taste receptors travel along several cranial nerves to the medulla oblongata. From the medulla oblongata the impulses travel to the thalamus and then to gustatory cortex in the parietal lobe of the cerebrum. The gustatory cortex is where the taste information is interpreted.
The organ of hearing is the ear. It is divided into three portions - external ear, middle ear, and internal ear.
Click here to view the structure of the ear.
The external ear is made of the auricle and the external auditory meatus. The auricle is the flap of skin and cartilage that hands off the side of your head. If functions to collect sound waves. The external auditory meatus is more commonly called the ear canal. When you stick your finger in your ear you are sticking it in your external auditory meatus. This meatus conducts sound waves to the tympanic membrane.
The middle ear begins with the tympanic membrane. The tympanic membrane is more commonly called the ear drum. This membrane is relatively thin and vibrates when sound waves hit it. On the other side of the tympanic membrane is a tiny bone called the malleus. When the tympanic membrane vibrates, it causes the malleus to vibrate. On the other side of the malleus is another tiny bone called the incus. The incus vibrates when the malleus vibrates. On the other side of the incus is the stapes. The stapes vibrates when the incus vibrates. When the stapes vibrates it hits a membrane called the oval window at the opening of the inner ear.
The malleus, incus, and stapes are collectively called the ear ossicles. They are the smallest bones of the body.
Each middle ear is connected to the throat by a tube called the auditory tube which is also known as the eustachian tube. The auditory tube helps maintain equal pressure on both sides of the eardrum which is needed for normal hearing.
Because the middle ear is connected to the throat by this tube, any throat infection can spread to the ear!
The inner ear is a very complex system of communicating chambers and tubes called a labyrinth which means maze. Each inner ear has an osseous labyrinth made of bone and a membranous labyrinth made of a delicate membrane. The membranous labyrinth lies inside the osseous labyrinth much like a balloon in a box. The shape of the membranous labyrinth follows the shape of the osseous labyrinth. Between the membranous labyrinth and the osseous labyrinth is a fluid called perilymph. Inside the membranous labyrinth is a fluid called endolymph.
The two labyrinths together made three shapes - 1) semicircular canals 2) cochlea and 3) vestibule.
There are 3 semicircular canals and they function to detect balance of the body or equilibrium. The cochlea is shaped like a snail's shell and it functions in hearing. The vestibule is the area between the semicircular canals and the cochlea; it also functions in equilibrium.
The osseous portion of the cochlea is further divided into an upper chamber called the scala vestibule and a lower chamber called the scala tympani. The membranous portion of the cochlea is called the cochlear duct.
The cochlear duct is separated from the scala vestibule by a vestibular membrane. The cochlear duct is separated from the scala tympani by the basilar membrane. There is a microscopic organ called the Organ of Corti that is located in the upper part of the basilar membrane.
The organ of Corti is where hearing receptors are found. As sound vibrations pass through the inner ear, fluids move in the inner ear and activate the hearing receptors in the organ of Corti. Sounds of different volumes and frequencies stimulate various different hearing receptors.
Click here to view the inner ear.
Once hearing receptors are stimulated, the nerve impulse travels along the auditory nerve to the auditory cortex in the temporal lobe of the cerebrum for interpretation.
Click here to view the auditory pathway.
The ear is also the organ of equilibrium.
Static equilibrium senses the position of the head which maintains stability and posture when the head and body are not moving.
Static equilibrium is detected by receptors in the vestibule of the inner ear. The membranous portion of the vestibule is divided into two chambers - 1) a utricle and 2) a saccule.
Both the utricle and the saccule contain a tiny structure called a macula. The macula contains the receptors for static equilibrium. When the head moves by bending forward or backwards, the receptors in the macula detect this movement. The receptors send the information along the vestibular nerve to the brain for interpretation. Now your brain knows if you are in balance or if it needs to do something to help you maintain your balance!
Dynamic equilibrium senses balance when you are moving. The semicircular canals contain the receptors for dynamic equilibrium. Within each semicircular canal is a tiny organ called a crista ampullaris. The crista ampullaris contains the receptors for dynamic equilibrium.
When the receptors in the crista ampullaris are stimulated, they send nerve impulses to the brain. This advises the brain of whether or not a person has their balance during body movements.
Visual accessory organs are those that assist the eyeball, the organ containing the visual receptors. They are as follows:
Click here to view the lacrimal apparatus and extrinsic eye muscles.
Each eye is a hollow, spherical shaped structure about 2.5cm in diameter. Its wall has three layers or tunics (outer tunic, middle tunic, and inner tunic). The chambers of the eyeball are filled with fluids.
Click here to view the structure of the eye.
Click here to view a summary table of structures of the eye
Fibrous tunic – outermost tunic; consists of the cornea and sclera; tough and fibrous; sclera is the white of the eye and does not allow light to enter the eye; cornea is anterior to the sclera and allows light to enter the eye. This entire tunic contains no blood vessels but is supplied with many sensory receptors.
Vascular tunic – this tunic is highly vascular and pigmented. It consists of the choroid that sits deep to the sclera; the choroid absorbs extra light in the eyeball. It also consists of the ciliary body which functions to hold and move the lens for focusing. It also contains the iris which is the anterior most structure of this layer. When you look at the color of a person's eye, you are looking at their iris. The iris controls the amount of light entering the eye. The hole in the iris is called the pupil.
Retina - innermost layer and the most delicate. This layer contains the photoreceptors (rods and cones)
Posterior segment of the eyeball is the segment behind the lens; it is
filled with vitreous humor (gel-like fluid).
Focusing
Focusing of light for distant vision- ciliary muscles are completely
relaxed; therefore the lens is stretched by tension in the suspensory
ligaments; the lens is flat.
Focusing of light for near vision – ciliary muscles contract; therefore the lens bulges because the suspensory ligaments are relaxed. Pupils constrict and the eyeballs converge (move medially).
Click here to view the lens and ciliary body during lens accomodation.
The bending of light waves is called light refraction. When we view an object the light (image) from the object is bent by our corneas. Then the light (image) is bent again by our lenses. By the time the image reaches the retina (where the visual receptors are) it has been bent to an extent that it is now sharply focused onto the retina. The image however is focused upside down and backwards onto the retina but the brain will interpret the image correctly.
Visual receptors are stimulated only by light. The following are two types of visual receptors:
Rods – used to see images in dim light; used to see general outlines of structures; used to see blacks, whites and grays.
Cones – used to see images in bright light; used to see details of structures; used to see colors.
Rods and cones contain light-sensitive pigments that break down when light hits them. When the pigment is broken down the visual receptors (rods and cones) send nerve impulses with visual information towards the brain.
The pigment found in rods is called rhodopsin. There are three pigments found in cones. Some cones contain the pigment erythrolabe which is sensitive to red light waves. Other set of cones contain the pigment chloralabe which is sensitive to green light wave. Finally some cones contain the pigment cyanolage which is sensitive to blue light waves.
If a person sees a bright white light then all sets of cones have been activated. When a person sees various colors, different combinations of cones have been activated. Color blindness results from a lack of pigments on cones.
The visual information travels to the brain through the following structures:
Photoreceptors à optic nerves à optic chiasm à optic tracts à thalamus à visual cortex in the occipital lobe of the cerebrum for interpretation
About half of the visual information detected in each eye is interpreted on the opposite side of the brain. Therefore, some of what you see in your right eye is interpreted in the left occipital lobe of the cerebrum.
Click here to view the visual pathway