Free nerve endings are the simplest sensory receptors located throughout the body and skin. They detect sensations like pain, touch, heat and cold. Other receptor types include Merkel disks for light touch, Meissner corpuscles for vibration, and muscle spindles and Golgi tendon organs in muscles and tendons that provide information on movement and positioning. The eye contains receptors for vision, with the retina containing rods and cones that detect light as the visual photoreceptors. Other receptors in the body are classified as exteroceptors on the skin, visceroceptors inside organs, and proprioceptors in muscles, tendons and joints.

1. Structure (5)
Christmas edition
 Free nerve ending: simplest, most common sensory receptors
located on the surface of the body known as exteroceptors and
in internal organs known as visce receptors. Nociceptor are slen-
der sensory fibers that are responsible for pain. Brain tissue lack It’s beginning to look a lot like
free sensory nerve endings and are incapable of sensing painful Christmas, everywhere you go…
stimuli. Nerve endings are also responsible for sensation and (page 6)
touch and also sensations of heat cold. We’ll tell you how
 Root hair plexuses: delicate, web-like arrangements of free to spot some
nerve endings that surround hair follicles that can detect any great deals with
type of hair movement. the help of your
 Merkel disks: disc-shaped or flattened and are responsible for vision! (Page 2-3)
mediating sensations of light or discriminative touch that is lo- The minty smell of
cated on the surface of the skin. (21)
that peppermint
 Meissner Corpuscle: when “deformed” by a mechanical type of candle… ah, it’s the
stimulus, this type of receptor, sometimes called a tactile corpus- smell of Christmas.
cle, mediates sensations of discriminative touch and low- (page 7-8)
frequency vibration. (especially numerous in hairless skin areas,
such as the nipples, fingertips, and lips)
 Pacinian Corpuscles: large mechanoreceptors, which when sec- We all love the
tioned, show thick laminated connective tissue capsules. (found cinnamon taste of
in deep dermis of the skin-especially in the hands and feet-and snickerdoodle
are also numerous in joint capsules throughout the body). cookies! (page 10-
 Muscle Spindles- the most important stretch receptors are asso- 11)
ciated with muscles and tendons and are classified as proprio-
ceptors. Two types of stretch receptors, called muscle spindles
and golgi tendon receptors, operate to provide the body with
information concerning muscle length and the strength of mus-
How are we able to see all the
cle contraction. (the result of stimulation is a stretch reflex that Remember to keep your bright Christmas lights? We’ll
shortens a muscle or muscle group, thus aiding in the mainte- balance while hanging tell you how! (page 4-5)
nance of posture or the positioning of the body or one of its ex- up your Christmas
tremities in a way that may be opposed by the force of gravity). lights! (page 9)
 Golgi tendon Organs- like muscle spindles, are proprioceptors.
They are located at the point of junction between muscle tissue Also in this issue...
and tendon. These receptors act in a way opposite that of mus- • Locations of receptors……………………….……page 12
cle spindles.
• Stimulus detected…………………………………..page 13
• Structure……………………………………………….page 14

2. HOW TO SPOT KILLER DEALS Stimulus Detected (5)
With the help of your vision
Page 2 Page 13
(25)
 Mechanoreceptors: are activated by mechanical stimuli that in
some way “deform” or change the position of the receptor, re-
sulting in the generation of a receptor potential. Examples of this
is pressure applied to the skin or to blood vessels, or cause by
stretch or pressure in the muscle, tendon, or lung tissue.
 Chemoreceptors: activated by either the amount or the changing
concentration of certain chemicals. Examples are our senses of
taste and smell depend on these.
 Thermoreceptors: activated by change in temperature. Examples
are when the outside temperature is colder than your body; your
brain tells your body to warm up.
Your vision plays an important part when spotting some  Nociceptors: the relatively unspecialized nerve cell endings that
good deals. There are 3 layers of tissue that compose the eyeball: initiate the sensation of pain are called Nociceptors. They trans-
the sclera (a tough outer coat), the cornea (the transparent anteri- duce a variety of stimuli into receptor potentials, which in turn
or portion that lies over the iris), and the canal of schlemm (a ring- trigger afferent action potentials. They arise from cell bodies in
shaped venous sinus found deep within the anterior portion of the dorsal root ganglia that send one axonal process to the periphery
sclera). and the other into the spinal cord or brainstem. (20)
The choroid is the middle coat of the eye and it contains  Photoreceptors: the retina is the back part of the eye that con-
many blood vessels and a large amount of pigment . The ciliary tains the cells that respond to light. These specialized cells are
body , suspensory ligament, and the iris are all part of this layer: called photoreceptors. There are two types of photoreceptors
 Ciliary body– the thickening of choroid, fits between anterior (rods and cones) in the retina. (9)
margin of retina and posterior margin of iris.
 Suspensory ligament– attached to the ciliary processes and
blends with the elastic capsule of the lens to hold it in.
 Iris– colored part of the eye.
The retina is the incomplete innermost coat of the eyeball.
Three layers of neurons make up the sensory retina. Photoreceptor
neurons are the visual receptors, highly specialized for stimulation
by light rays. Bipolar neurons are the second layer and ganglion
neurons are the third layer. All axons of these neurons extend back
to the optic disc; part of the sclera, which contains perforations
through which the fibers emerge from the eyeball as the optic
nerve.
(5)

3. Locations of receptors (24)
VISION CONT.
Page 12 Page 3
Cavities & Humors
The eyeball has a large interior space divided into two cavities. The
anterior cavity lies in front of the lens and has two subdivisions:
 Exteroceptors: are near the surface of the body. The extero-
 The anterior chamber– the space anterior to the iris and posterior
ceptors located in the skin provide the sensations of pain,
to the cornea.
touch, temperature, and pressure.
 The posterior chamber– the small space posterior to the iris and
 Visceroceptors: located in the blood vessels and viscera.
anterior to the lens.
 Proprioceptors: receptors located in muscles, tendons,
The posterior cavity is larger than the anterior cavity and it occupies
joints, and the external ear.
the entire space posterior to the lens, suspensory ligament, and ciliary
body.
Muscles
There are two types of eye muscles. The extrinsic eye muscle is the
skeletal muscles that attach to the outside of the eyeball and to the
bones of the orbit, and its named according to their position on the
eyeball. The muscles are superior, inferior, medial, and lateral rectus
muscles and the superior and inferior oblique muscles.
(5)
Accessory Structures
Eyebrows and eyelashes give some protection against foreign objects
entering the eye. Eyelids consist of voluntary muscle and skin with a
tarsal plate, lined with conjunctiva, a mucous membrane, and a pal-
pebral fissure where the upper and lower eyelids join together. Lacri-
mal glands are about the size and shape of an almond, and are locat-
ed at the upper, outer margin of each orbit. Approximately a dozen
small ducts lead from each gland and
drain tears onto the conjunctiva.
Lacrimal canals are small channels
that empty into lacrimal sacs.
Lacrimal sacs are located in a groove
in the lacrimal bone. Nasolacrimal
ducts are small tubes that extend
from the lacrimal sac into the inferior
meatus of the nose.
(5)

4. Take a gander at the Christmas
lights Process of seeing Taste cont. (23)
Page 4 Page 11
The cornea, pupil, iris, and lens work together to The Neural Pathway
refract light rays so that the rays from the Christmas
lights focus directly on the retina. The Taste Pathway
Light rays first touch the cornea. The cornea has Transduction occurs when different taste substances cause a
2 purposes: 1) it bends light rays as they pass through it change in the flow of ions across the membrane of a taste cell.
in order to focus them through the lens and towards the Different substances affect the membrane in different ways:
back of the eye. 2) its smooth, reflective surface serves  Bitter and sweet substances bind into receptor sites which
to protect the eye from external damage. release other substances into the cell.
Once light passes the cornea, it hits the aqueous  Sour substances contain H+ ions that block channels in the
humor, a clear watery fluid. The purpose of the aqueous membrane.
humor is to keep a constant pressure within the eye.  Salty substances break up into Na+ ions which flow through
Behind the aqueous humor are the iris and the pupil. the membrane directly into the cell.
They both work together to control the amount of light Electrical signals generated in the taste cells are transmitted in 3
that enters the eye. The iris consists of two muscles on pathways:
either side of the pupil that can dilate or constrict to ad-  The chorda tympani nerve conducts signals from the front and
just the size of the pupil. Behind the iris and the pupil is sides of the tongue.
the lens, and elastic structure that becomes thinner to  The glosso-pharyngeal nerve conducts signals from the back of
focus on distant objects and thick to focus on nearby the tongue.
 The vagus nerve conducts taste signals from the mouth and
objects.
the larynx.
All these parts of the eye work together to allow
These three nerves make connections in the brain stem in the nu-
you to see all the Christmas lights.
cleus of the solitary tract (NST) before going on to the thalamus
and then to two regions of the frontal lobe (the insula and the
(19)
frontal operculum cortex).
Why don’t deer see hunters who are
wearing bright orange?
Deer have under-developed cones. They have no
red-sensitive cones in their eyes, which prohibit
them from distinguishing red or orange from
green and brown.

5. All the tasty treats...yum! (23) PROCESS OF SEEING CONT.
Page 10 Page 5
Describe the role of Photopigments
Both rods and cones contain photopigments, or light sensitive pig-
ments compounds that are found in the distal area of both types of
photoreceptors near the pigments retina. A derivative called reti-
nal acts as the light-absorbing portion of all photopigments.
 Rods– the single photopigment found in rods is rhodopsin.
Rhodopsin is highly light sensitive that a dim light can cause a
rapid breakdown of the photopigment into its opsin and retinal
components. Light causes retinal to change shape and the op-
sin molecule to expand or open.
 Cones– 3 types of cones are present in the retina. Each con-
Taste buds- describe tains a photopigment different from the rhodopsin found in
rod cells. Each of the 3 primary colors (red, green, and blue)
Anatomy of taste reflect light rays of a different wavelength. Each wavelength
The tongue contains many ridges and valleys called papillae. There acts primarily on one type of cone, causing its particular pho-
are four types of papillae: topigment to break down and initiate impulse conduction by
1) Filiform papillae– cone shaped and found all over the tongue the cone. Brighter light is necessary for the breakdown of cone
(why gives the tongue its rough appearance). photopigments. (5)
2) Fungiform papillae– mushroom shaped and found at the tip
and sides of the tongue.
3) Foliate papillae– a series of folds along the sides of the tongue. Describe between nearsighted and farsighted.
4) Circumvallate papillae– shaped like flat mounds surrounded by How is it corrected?
a trench and found at the back of the tongue.
All papillae except filiform contain taste buds, so the very center of People with farsightedness (hyperopia) can see distant
your tongue (which only has filiform papillae) is “taste blind.” Each objects well, but have difficulty focusing on objects close up.
taste bud contains a number of taste cells which have tips that pro- People with nearsightedness (myopia) can focus well on objects
trude into the taste pore. close up, but have trouble seeing distant objects.
Glasses and contacts are the most common ways of cor-
recting either nearsightedness or farsightedness. Glasses help for
the abnormal shape of the eye so that
rays can hit the retina properly. Contact
lenses also work by refocusing light, and
may improve vision more than glasses
because the lenses sit directly on the
eyes.

6. CAN YOU HEAR THE SONGS OF THE SEASON? Remember to keep your balance! (5)
Page 6 Page 9
Oh the
Describe sense of balance
weather Balance is maintained by the visual system, the vestibule system,
outside is and proprioception. The vestibular system detects changes in static
frightful, but equilibrium, and the movements of fluid in the semicircular canals
the fire is so detect which direction we’re facing. The brain receives signals from
delightful... these parts of the body to determine how to keep balanced.
Dynamic equilibrium
Dynamic equilibrium is responsible for maintaining the position of
the head when the body rotates. This especially comes in handy
when hanging up Christmas lights on your house.
Describe mechanism of hearing
Sound is created by vibrations that can occur in air. Fluid or solid
material. For example, when we speak, our vocal cords create
sound waves that produce vibrations. Amplitude of a sound wave
determines its loudness or volume. The number of sound waves
during a specific time (frequency) determines pitch. This allows us
to hear about Elvis’ blue Christmas.
(5)
Detail Neuronal pathway of hearing
Dendrites of neurons terminate around bases of hair cells of the
organ name corti. Movement of hair cells against adherent tec-
torial membrane stimulates dendrites and initiates impulse con-
duction by cochlear nerve to the brainstem. Impulses pass (22) ©
through “relay stations” in the nuclei of the medulla, pons, mid-
brains, and thalamus before reaching auditory area of temporal
lobe.
(5)

7. The smells of Christmas time cont. The smells of Christmas time
Page 8 Page 7
(5)
Olfactory Pathways
When the level of odor-producing chemicals reaches a threshold
level, the following occurs: receptor potential, and then action po-
tential, is generated and passed to the olfactory nerves in the olfac-
tory bulb. The impulse then passes through the olfactory tract and
into the thalamic and olfactory centers of the brain for interpreta-
tion, integration, and memory storage.
Compare Olfaction in a human with a canine.
Genes between a dog and a human, suggest that our genes
evolved from a common mammalian ancestral repertoire by
successive duplications.
Olfactory Receptors
Olfactory receptors are unique because they are replaced on a regular
basis by germinative basal cells in the olfactory epithelium. They are
located in the most superior portion of the nasal cavity. They also con-
sist of epithelial support cells and specialized olfactory receptor neu-
rons.
 Olfactory cilia– located on olfactory receptor neurons that touch
the olfactory epithelium linin the upper surface of the nasal cavity.
 Olfactory cells– chemoreceptors; gas molecules or chemicals dis-
solved in the mucus over covering the nasal epithelium stimulate
the olfactory cells.
 Olfactory epithelium– located in the most superior portion of the
nasal cavity.
 Olfactory receptors– extremely sensitive and
easily fatigued
(5)