Sensory Pathway in Man

EXERCISE 3: SENSORY PATHWAY IN MAN Julienne Erika R. Alviar1, Jenny P. Concepcion1,

Kristienne Mae S. Diaz1*, Felimon A. Feliciano, Jr.1, Leonard Q. Guerrero1, and Cigno Adrien I. Pacheco1

1Department of Biology, College of Science

University of the Philippines Baguio *[email protected]

INTRODUCTION

Through billion years of life existence on Earth, mammals can be considered as lately existing group of organisms. But nonetheless, mammals had become the dominant animals of the world. This was possible primarily because they were able to acquire better body supports, enhance body processes (metabolism, regulation and reproduction), improve parental care, and develop larger brains. Especially important was the enlargement of the cerebral hemispheres which is concerned with higher brain functions such as thinking and reasoning (Cowan, 1994).

Belonging to the group of mammals, the human species-or Homo sapiens, to use the scientific name- was also able to enhance brain functions as well. In fact, modern humans were able to go even further and became the most successful of all living organisms. We, humans, have developed a variety of skills and attributes in order to master many aspects of the world (King, 1994). One skill/attribute in particular is our ability to acquire knowledge and keen awareness of the surroundings. A specific study of this skill would show that this was due to humans development of a higher brain level or a more advanced nerve center, referring to the central nervous system (CNS).

Associated with the advancement of the CNS is the specialization of the sensory receptors which composes of sensory cells that respond to specific stimulus or modalities. Examples of sensory receptors based on the kind of stimulus are mechanoreceptors, chemoreceptors, thermoreceptors, photoreceptors, nocireceptors, and many more. In a cellular level, a sensory cell works by converting an external stimulus into electrical signals via the opening and closing of ion channels (Sherwood, 2013). But in order to complete the body process through the nervous system, the electrical signal should enter a particular nerve pathway. An often used model of nerve pathways is the reflex arc which involves the relay of information from sense organs to the brain or spinal cord and the formation of response by way of an effector organ (Rabago, 2008).

The human body, together with other animals, has what is universally called as the special senses. These include primarily vision, hearing, taste, and smell. Each of these senses is specialized to respond to one type of stimulus, called its adequate stimulus. It means that there is a certain sensory receptor accompanying each special sense. However, there are some cases wherein a certain sense can be activated by various stimuli/modulator (Sherwood, 2013).

Olfactory Sensation (smell). Olfactory sensation may vary depending on several factors that include the odor concentration and the distance of the odorant from the site of olfaction, which is generally the nasal epithelium. In humans, smell is important for determining food flavors and signaling the presence of dangerous gases. It can also induce behavioral changes and play a powerful role in evoking memories and emotions (Hyman, 1942).

Gustatory Sensation (taste). Humans have thousands of taste buds located in bunches mainly on surface or edges of the tongue. Stimulated by chemicals in food, the gustatory

sensation of vertebrates- especially humans- are affected by factors such as temperature, food texture, and even past encounter with the same taste (Sherwood 2013).

Visual sensation (vision). Vision relies on a very complex receptor apparatus called the eye. It works on a variety of positioning and focusing mechanisms to form an image in the correct spot on the light-sensitive receptor cells inside the eye (Patton, 2007). These mechanisms involve muscles, lenses, and other structures present in the visual apparatus. The visual image perceived by humans has the qualities of resolution, brightness, color, and depth.

Auditory sensation (hearing). Hearing, the ability to detect a range of sound frequencies (pitches) and intensities, is mediated by the ear apparatus; and it varies among animals based on the strength of the receptors. Two types of equilibrium are also mediated by ear receptors. One type, static equilibrium, determines the body position of an organism relative to the center of gravity. Dynamic equilibrium, on one hand, gives information regarding the speed and direction of body motion (Patton, 2007).

On one side, there is what is called as the somesthetic or cutaneous sensation. Here, the skin serves as a large sensory organ. Nerve fibers conduct sensory messages from the surface of the body to the spinal cord and brain. When the skin is stimulated, any of five different sensations- touch, pressure, heat, cold, and pain- it may be aroused. All of these sensations depend on the presence of specialized sensory receptors at the end of nerve fibers (Sherwood, 2013).

Using the special and the somesthetic senses as background of research, the purpose of the study was to identify the location and functions of some sensory receptors present in the human body. And this should be able to give an overview of the mechanisms following the different sensory pathways.

MATERIALS AND METHODOLOGY

A. Olfactory sensation

To test the effect of distance of an odor to the site of olfaction, a perfume in a beaker was prepared, covered with an inverted funnel connected to a rubber tube as shown in the figure below.

Figure 1. Test for Olfactory Sensation Set-up (Image taken from: http://www.nuffieldfoundation.org/practical-chemistry/properties-hydrogen-

chloride)

The task was to place the rubber tube in 2 positions, one at the posterior chamber of the nose followed by placing at the anterior part inside the nasal chamber, resting for a minute between the two. In the experiment, the individual should be able to identify which position on the nasal chamber corresponded to stronger olfactory sensation.

B. Gustatory Sensation The objective of this test was to identify the effect of the texture of the food (solid or liquid) and its concentration to the rate of sensation. The test was done by using solid crystals of sugar and sugar dissolved in water in the following concentrations: 0.5%, 1.0%, 5.0%, 10.0%, 25.0%, and 50.0%. The test was done by placing each sample to the tip of the tongue and note the time for the individual to taste each. Before every test, the representative was ordered to rest for a minute and dry its tongue to prevent easier recognition due to familiarity with the taste.

C. Visual Sensations One light regulating mechanism of the eye is the dilation and contraction of the pupil termed, pupillary reflex which is affected by the amount of light that the eyes receive. To observe more on this, the group studied a representative tasked to stay in a dim area with very minimal light for several minutes followed by sudden exposure to light. The group observed carefully the contraction and dilation of the pupil of the eye, paying attention to the difference in size of the pupil in dim area from its size when there is light present. Another test was done that tested the eyes capacity of accommodation, or its adjustment of image formation depending on the distance of the object. The test was done by holding a pencil 50mm directly in front of the each eye of the tested while letting the other eye closed. As the pencil got nearer to the eye of the person, the writing on the pencil continued to blur. The distance where the person cannot read or identify the letters were noted.

The Blind Spot Test was done for the better understanding of the students of the position where the blind spot is located. In this test, the group representative was tasked to focus at the center of the two shapes in the figure below then a) to close the left eye and focus the right eye on the plus sign, moving closer to the figure until the circle disappears; and b) to close the right eye and focus the left eye on the plus sign, coming closer until the plus sign disappears. The distance from the figure when the blind spot was identified was recorded then.

Figure 2. Blind Spot Test

(Image retrieved from https://visionaryeyecare.wordpress.com/2008/08/04/eye-test-find-your-blind-spot-in-each-eye/)

D. Auditory Sensations To test humans sound localization, a watch was placed a distance away from the ear of a person. The objective was to note the distance it will take for the representative to hear the ticking of the clock while the ear opposite to the watch was plugged with cotton. This was tested by having the subject person to stand upright, placing the right foot closely in front of the left foot. After that, the persons movement was observed when a) the eyes were open and b) the eyes were closed.

E. Cutaneous Sensations The first test for cutaneous sensation is to test for pain reception or nociception. This was done by placing the elbow in ice cold water until the pain is felt somewhere else in the body besides the elbow. Skin receptors may vary depending on its sensitivity. These varying skin receptors also vary in location on the skin. A test was done to determine the presence of a specific type of receptor which is the most sensitive to even light touches. To do the said test, at the back of the hand of an individual, a 1x1 cm square was drawn and divided into 16 equal parts. Using a pin, a dull pencil, cold paper clip, and hot paper clip, the person tried to identify the occurrence of different types of stimuli like pain and temperature change in the square. The square would also determine the presence and abundance of different skin receptors in the sample.

RESULTS AND DISCUSSION

Olfactory Sensations

Olfaction is the capacity of an individual to smell involving specialized nerve cells and structures. The olfactory mucosa that is located in the upper tract of the respiratory pathway contains three types of cells: olfactory receptors, supporting cells and basal cells which perform different functions. Supporting cells secrete mucus which coats the nasal passage which works with the cilia to filter and dissolve particles in the breathed air, basal cells serves as precursors for new olfactory receptor cells and olfactory receptor cells which contains a large knob structure bearing several long cilia functions as the binding site of odorants in order to create the sensation of smell. For a substance to be smelled they must be sufficiently volatile so that some of its molecules could enter the nose via air or water and they must be sufficiently soluble in order to be dissolved in the mucus layer of the olfactory mucosa (Sherwood, et. al., 2013). Within the nose is the nasal cavity which is divided by the nasal septum, it contains projections called nasal conchae which increases the surface area that increases the area of heat and moisture exchange furthermore the nasal cavity is lined with mucous membrane which contains cilia and blood vessels. In the experiment the inner chamber of the nasal cavity showed stronger sense of smell than the outer part because smell receptors are found in the inner part of the nasal cavity, these receptors are special nerve cells with cilia which when stimulated by the binding of the odorants create nerve impulses that are sent to the nerve cells of the olfactory bulb and through the olfactory nerve these impulses reach the brain where interpretation, integration and memory storage occurs leading to the creation of the sensation of smell (Tucci, N. D).

Gustatory Sensations

Table 1. Test for Gustatory Sensations

Concentration of Sugar

Time before sweet taste was tasted (seconds)

Remarks

100% (Sugar Crystal)

90.6

0.5% 5.35 Tastes like H2O (Bland)

1% 3.81

5% 2.00 Sweeter than 1%

10% 3.74 Sweeter than 5%

25% 3.50 Similar sweetness to 10%

50% 3.09 Sweetest

Gustation is the detection of molecules in objects in solids or liquids in contact with the body where in exteroreceptors and chemoreceptors are utilized in detecting chemicals to generate neural signals (Sherwood et al, 2013). Chemoreceptors in higher vertebrates are packed in taste buds which are located in the human oral cavity, throat, digestive tract and even lungs but only the first two are actually involved in taste and flavor perception. Taste buds are made up of several spindle-shaped receptor cells each having a small opening called the taste spore where the microvilli protrude increasing the surface area exposed to the contents of the mouth. In order to taste something it should be in a solution either dissolved in ingested liquids or in saliva because it allows the chemicals to attach to the receptor cells thus evoking the sensation of tastes, this is true with the results obtained where in the sugar crystal had the slowest time before being recognized by the tongue, trying to taste something that isnt in a solution with a dry tongue is futile since it would be hard for the chemicals to interact with the receptor cells of the taste bud (Sherwood et al, 2013). Different concentration of the sugar also would affect the time as observed in the experiment the lowest concentration had the slowest time of recognition while the 10% to 50% sugar concentration had an average of 3.44 seconds before being recognized because with higher concentration more receptor cells would interact with the allowing easier recognition. Taste thresholds varies from person to person and the basic tastes of sweet, salty, bitter and sour would also have different concentrations at which they can be detected.

Visual Sensations The eye is a very complex organ responsible for vision. It has two fluid-filled sacs

separated by the lens: the anterior filled with aqueous humor and the posterior filled with vitreous humor. These fluids are further enclosed by three layers: the outermost sclera, the middle uvea and the innermost retina. The sclera is a tough connective tissue which differentiates as the cornea at the front of the eye. The cornea is a transparent tissue which allows the passage of light rays into the eyes. The uvea is composed of three regions: the choroid filled with blood vessels, the ciliary body enclosing the lens and the iris. The ciliary body

has a muscular component which contributes to the adjustment of the refractive index of the eye and a vascular component which is responsible in the production of the fluid filling the front of the eye. The iris is pigmented which determines the color of the eyes. The retina is where the rods and cones (photoreceptors), bipolar cells and ganglion cells, form layers and convert light energy into action potentials (Purves, 2008; Saladin, 2008; Sherwood et al., 2013). The axons of the ganglion cells form the optical nerve which transmits signals to the brain (Sherwood, 2013). The optic nerve exits the rear of the eye in the optic disc (Saladin, 2008).

The region of the retina where the axons of the retinal ganglion meet to form the optic

nerve and leave the eyeball is called the blind spot. It is located approximately 15 towards the nasal side of the retina and does not have any photoreceptors but it is accompanied by blood vessels which form the circulation of the eye (Winn, 2001). Field of vision is defined as the cone of space with its apex at the eye, which is seen by the subject, when the eye is kept fixed at one point (Ghai, 2013). If the vision is normal, the visual field extends ~100 temporally (laterally), 60 nasally, 60 superiorly and 70 inferiorly (Carroll and Johnson, 2013). The field of vision for an eye is charted in a field of vision of that eye (with the other eye closed). There are several factors that can affect it. a. Color of the object. For white objects, the visual acuity is better thus the field of vision is better delineated. b. Size of the object. If one wants a better visual acuity, then it is recommended that the size of the object is larger. However, in perimetry, only a standard size is used. c. Brightness of the object. There are several factors that affect visual acuity. This includes brightness, contrast and illumination. Since these factors affect visual acuity therefore these also affects the field of vision. d. Illumination. As said earlier, illumination is one factor that affects the visual acuity and field of vision. If theres a decrease in illumination then theres also a decrease in the visual field (Pal and Pal, 2005).

Analyzing ones field of vision is necessary in neurologic and ophthalmologic examinations for it is used to check gaps in ones side (peripheral) vision (Healthwise, Inc., 2013). Simulators which have a wide field of view tend to have greater incidents of simulator sickness. This is because field of view influences the subjects experiences of vection due to moving visual scenes (Riva, 1997).

Figure 3. Perimeter Chart

(Image retrieved from http://www.museyeum.org/info.php?page=0&v=1&s=perimeter&type=all&t=objects&f=&d=)

The circle and the cross disappear as they hit the blind spot of the left and right eye,

respectively. The photoreceptors in the retina have outer segments facing the back of the eye which

are specialized cilia to absorb light, inner segments housing the cellular organelles and processes at the base that synapse to the bipolar cells in the next layer which then synapse to the ganglion cells in the innermost retinal layer. The rods' outer segment is long and tube like composed of membranous discs stacked together while cones' have shorter and tapering. Rods provide a gray vision during nighttime or in dim conditions while cones are provide a color vision during the day or in bright conditions. A few ganglion cells directly absorb light and send signals to brainstem nuclei which regulate the body's circadian rhythms as well as the diameter of the pupil, an opening where light passes through (Saladin, 2008). In the experiment, when the subject was in an area with dim lighting, her pupils appeared dilated but when a light was shined upon her right eye, its pupil constricted. After 2 minutes with her eyes closed, the subject's pupil returned to its dilated form. The circular or constrictor muscle (muscle fibers arranged in ring-like manner around the pupil) and the radial or dilator muscle (muscle fibers radiate outwards similar to the spokes of a bike), two antagonistic muscles in the iris are responsible for these changes in the pupil's diameter. In bright conditions, the eye decreases the light coming in the eye by relaxing the radial muscles and contracting the circular muscles resulting in the constriction of the pupil. The circular muscle is innervated by the parasympathetic fibers through the oculomotor nerve. In dim conditions, on the other hand, the pupils dilate by the contraction of the radial muscles and relaxation of the circular muscles to permit more light to enter the eye. This dilation is innervated by the sympathetic fibers from the superior cervical ganglion (Saladin, 2008; Sherwood, 2013). A similar mechanism occurs when the eye focuses on near or distant objects and the ability of the eye to do this is called accommodation. Accommodation increases the power of the lens to focus on near objects and it does this with the help of the ciliary muscles (found in the ciliary body in mammals) which surrounds it. The lens is connected to the ciliary muscles through zonule fibers which act antagonistically with the ciliary muscles. The ciliary muscles are also controlled by the autonomic nervous system just like the iris muscles

are. The sympathetic nerve fibers cause the relaxation of the ciliary muscles which increases the tension in the zonule fibers, pulling the lens in a flattened weak shape for distant vision. The parasympathetic nerve fibers innervate the contraction of the ciliary muscles which relaxes the zonule fibers and increases the lens' curvature due to its elasticity. This happens when the eye needs to focus on near objects (Purves, 2008; Saladin, 2008; Sherwood et al., 2013). Accommodation is usually measured in diopters (meter-1) (Nuffieldfoundation.org, 2011). In this experiment, the subject's eye's near point accommodation was tested. At 5 cm (20 diopters), the letters on the pencil already appeared blurred which meant that the subject's near point accommodation may be lower. It is suggested that the normal range of accommodation for both younger and middle aged adults is 4 m to 70 cm (0.25 to 1.43 diopters) (Lockhart & Shi, 2010).

Auditory Sensations The process of hearing starts with the conduction of sound waves (vibrations) in the

outer ear through the auditory canal. It then moves to the middle ear where the tympanic membrane is located. The tympanic membrane vibrates freely when struck with sound waves and is innervated by the vagus and trigeminal nerves. Three bones, the malleus, incus and stapes, connected together then transfer the signal from the tympanic membrane, which is connected to the malleus, to the inner ear through the oval window which is connected to the stapes. Succeeding the oval window is a system of tubes which function for hearing and balance and equilibrium. In hearing, a spiral fluid filled structure called the cochlea receives the amplified vibrations from the oval window. The cochlea is divided into three longitudinal ducts: scala vestibuli and scala tympani filled with a fluid called perilymph and the organ of hearing, the cochlear duct housing a fluid called endolymph. The spiral organ or the organ of Corti generates auditory nerve signals and is located inside the cochlear duct on the basilar membrane. This organ of Corti is composed of stereocilia or hair cells on its apical surfaces. A tectorial membrane covering the stereocilia serves as a partition between the endolymph in the duct and the perilymph in the scala vestibuli and scala tympani. One row of inner hair cells (IHC) and three rows of outer hair cells (OHC) are neatly arranged on the length of the organ of Corti. The pressure generated from the amplification of the sound wave is dissipated through the perilymph. This dissipation then sends waves of pressure to the vestibular membrane, then to the cochlear duct then to the basilar membrane which causes the vibration of the basilar membrane, the organ of Corti and its hair cells. The IHC which is responsible for the sound we "hear" synapses with dendrites of sensory neurons. Then, through a chemical synapse, send signals to afferent nerve fibers of the vestibulocochlear nerve. The OHC synapses, meanwhile, with the dendrites of sensory neurons and the axons of motor neurons responsible for the transduction of signals from the brainstem to the hair cells.

It is suggested that the absolute threshold, the minimum amount of energy from a

stimulus that a person can detect, to hear the ticking of a clock is 20 feet (King, 2013). In this experiment, the absolute threshold of the subject was measured to be at 5.5 cm. It may be that the reason for this lower absolute threshold is that the ticking of the clock was loud enough and the place of experimentation was quiet enough.

Interference of the transmission of sound to the hearing apparatus or of the neural signal

to the auditory cortex can generate deafness. Two categories of it exist----conductive and sensorineural deafness. Conduction deafness results from the malfunction in the transmission of sound from the outer to the inner ear. Problems in the middle sometimes cause this to happen. Conduction deafness can happen in adults as ostoclerosis is the most frequent cause. In this kind of deafness, overgrowth of the labyrinthine bone surrounding the oval window is present which result to fixation of the stapes (Conn, 2008). This may be the reason why

conduction deafness is also called bone conduction deafness. In sensorineural or nerve deafness, results from damage of the cochlea, auditory nerve, or the central auditory system. Possible causes are numerous. It is very important to take note that nerve deafness results in decreased bone conduction and loss of high-tone appreciation while conduction deafness results in decreased air conduction and loss of low-tone appreciation (Ross, 2007). It is very important to differentiate conductive from nerve deafness as many types of conductive deafness can be treated (Conn, 2008). In cases of conductive deafness, bone conduction is more efficient (Ross, 2007). Bone conduction hearing has been used for several decades as a hearing rehabilitation technology although scientists have been in difficulty understanding the underlying mechanisms of bone conduction (Majdalaweih, 2008). The major step in bone conduction hearing aid development was the possibility to have direct transmission to the bone with a rigid system implanted to it. And thus a hearing aid system was created. A bone-anchored hearing aid system (BAHA) is a device which converts sound waves into sound vibrations. These vibrations are delivered directly to the inner ear through the skull bone. Bone conduction principle is used (Auditory Implant Service, 2012).

In the test for equilibrium, the subject was in equilibrium with her eyes closed and eyes opened but showed more ease with the eyes open.

The sensory system that is considered to have the most important influence on the other

sensory systems is the vestibular system. The vestibular system delivers information regarding on the spatial orientation which is significant for volitional movement control and it activates reflexes that help to stabilize the eyes, head, and body in space. It has three important reflexes, the vestibulospinal reflex (VSR), vestibulo- ocular reflex (VOR) and vestibulocollic reflex (VCR). Vestibulospinal reflex makes the muscle act to counter or oppose an unwanted movement thus stabilizes the body. An example of this is when suddenly one starts falling to the right, extensor muscles of the right leg will contract more slowly whereas the extensor muscles of the left will relax (Angevine and Cotman, 1981). Vestibulo- ocular reflex, on the other hand, stabilizes the vision during head rotations (Sunny, 2000). For example, a rightward head movement is associated with a leftward head movement and vice versa. This is to focus the image on the fovea of the eye and to compensate for the initial head rotation (Furman, et. al., 2010). Lastly, the vestibulocollic reflex which acts on the neck musculature to stabilize the head such as when the head starts to fall to the right, muscles on the neck contract to keep the head in its normal position ( Herdman and Clendaniel, 2014; Angevine and Cotman, 1981).

Aside from the vestibular functions, breathing, vision, musculoskeletal alignment and proprioception affect the equilibrium and balance in the body (Young, 2013).

1. Breathing. Oxygen is important for the brain. A relaxed deep breathing provides the brain oxygen which is essential for brain function and interaction with other sense organs that detect equilibrium and balance.

2. Vision. Vision is significant in attaining equilibrium because nerve fibers from the eyes interact with the vestibular system (~ 20%) (Politzer, N. D). The brain senses the bodys movement, orientation in space as well as the relationship to objects in the environment.

3. Musculoskeletal alignment. Groupings and alignment of muscles can affect equilibrium by affecting the whole skeletal system (Young, 2013).

4. Proprioception. Proprioception involves sensors, called proprioceptors, which are responsible for providing information concerning joint position, velocity, muscle tension, rate of

change in length, and other important information related in maintaining equilibrium (McLester and St. Pierre, 2008).

Cutaneous Sensations Various sensory receptors can be found in the skin and they can be differentiated in

terms of stimuli, sensitivity and location. There are 6 different types of free nerve endings in the skin. These are: Non- encapsulated Nerve Endings

a. Free nerve endings. These are not encapsulated nerve endings or endings without accessory structures which respond to light touch, pain, itching, and high and low temperatures. These are found in almost every tissue of the body and in the skin, they reach into the lower layers of the epidermis particularly the stratum germinativum and encircle the hair follicle (Alcamo, 2003; Kahle and Frotscher, 2003). b. Merkels cells or Merkels disks. These are modified epidermal cells which are sensitive to light touch (like free nerve endings), pressure and texture. Merkels cells are mostly found in highly sensitive skin like that of the fingertips and at the bases of some hair follicles (Mescher, 2013). c. Root hair plexuses. These are receptors which detect light touch that monitor the bending of hairs thus these are mostly found wrapping around hair follicles. Encapsulated Nerve Endings d. Meissners corpuscles. Meissners corpuscles initiate impulses when light- touch or low-frequency stimuli against skin temporarily deform their shape. These are prevalent in the fingertips, palms, and soles. e. Pacinian corpuscles. These are used in sensing coarse touch, pressure (sustained touch), and vibrations. These are found in the connective tissue of organs located deep in the body such as the wall of the rectum and urinary bladder. f. Krause end bulbs. These are like Pacinian corpuscles which sense vibrations however those vibrations which are low- frequency only. Krause end bulbs are the nerve endings found prevalent in the skin of the penis and clitoris. g. Ruffini corpuscles. The sensory axons of Ruffini corpuscles are stimulated by stretch (tension) or twisting (torque) in the skin. These are found anchored firmly to the surrounding of connective tissue (Mescher, 2013).

For this experiment, the skin on the back of the hand was used to examine these

receptors. A range of 1-5 was used to describe the sensation felt, with 1 being sensationless and 5 being the most sensed.

Tables 2-5. Individuals perception of the intensity of pain, pressure, heat and cold in a 1 cm2 area at the back of the hand

Pin

Tactile

Hot

Cold

2 3 3 4 1 2 2 2 2 2 2 1 2 2 1 2

3 3 4 4 2 2 3 3 1 1 2 2 2 2 2 2

2 2 2 3 2 2 2 2 2 1 2 2 2 2 2 2

3 3 2 3 3 2 2 2 2 1 2 1 1 2 3 3

Figure 4. Area measuring 1 cm2 at the back of the hand of the subject where the different

thermoreceptors and nociceptors were studied

These tests are prone to experimental errors since a lot of factors can affect the determination of the level of pain felt in a particular area, such as the differences of perception of individuals, adaptation of the receptor, acuity and reception fields of neurons, and the size of the source of stimulus (in this case, the tip of a paperclip). The adaptation of receptors to continuous exposure to pain, pressure, heat and cold can disrupt and affect the perception of the individual of the stimulus, thinking that it is milder than it really is. The ability of an object to conduct heat or cold which is related to its size and the material it is made of can also affect the sensation felt. Based on studies, receptors at the back of the hand have a mean receptive field diameter of 11.8 mm and the densities of the pain, pressure, heat and cold receptors are 130.5/cm2, 24.7/cm2, 3.4/cm2 and 9.1/cm2, respectively (Sato, et al., 1998). Knowing these, it can be said that the 10 mm2 is not enough to distinguish two different stimuli. Also, due to the density of the pain receptors, it may be possible that the sensations felt by the subject were all of pain and not of pressure, heat or cold.

Although sensory receptors found in the skin differ in almost all aspects, they

nonetheless follow similar sensory pathways. Dermatomes are regions in the skin which deliver signals to specific spinal nerves (Saladin, 2008). These signals are transmitted specifically to

the posterior (dorsal) horn of the spinal cord to first order neurons. It is then passed on to second order neurons (may be found in the spinal cord or the medulla) whose axons may directly or indirectly synapse to a third order neuron located in the thalamus. This third order neuron's axons ascend further and terminate in the somatosensory cortex, completing the pathway (Patestas & Gartner, 2006)

A few spinal nerves supplying dermatomes also supply visceral organs which leads to a

phenomenon where pain felt by nociceptors in the viscera is perceived by the brain to be coming from wrong locations such as the skin and other superficial sites (Patestas & Gartner, 2006; Purves, 2008; Saladin, 2008; Sherwood, 2013). In the experiment, when the subject's right elbow was placed in ice cold water, pain was centralized at first at the elbow. As the minutes passed, numbness was felt at the elbow while the pain spread 12 cm up the forearm and upper arm. No other pain was felt in other parts of the body. If the left elbow was the one placed in the ice cold water, pain would be felt in the upper chest wall, left shoulder and the withers region as characterized by anginal pain (Purves, 2008; Sherwood, 2013).

ANSWERS TO QUESTIONS

1. Make a diagram of the different sensory pathways.

Olfactory Pathway

*odorant receptor protein is a G-protein-coupled receptor - like most of the rest below

Neural Pathway for Taste

Neural Pathway for Vision

Neural Pathway of Hearing

Cutaneous Sensation

(Moyes and Schulte, 2008).

2. What is the role of the sense of smell and taste?

The sense of smell is called olfaction. Olfaction helps us interpret our environment by detecting molecules given off by different organisms and certain substances. Olfaction can create memory. Specific odors can trigger long term memory enabling an organism to recall emotions associated with a recalled experience. The sense of taste is gestation. Sense organs such as taste buds enable organisms to respond to taste. Olfaction and gustation are closely related. Both of its neural inputs travel along the same areas in the brain. Olfactory sensations are produced in the sensory cortex located in temporal lobes while gustatory sensations are produced in the sensory cortex found in parietal lobes. Both of these senses work frequently together. Flavor is a combined sense of smell and taste. Breathing out while we are chewing the food in the mouth triggers olfactory sensory receptors and the gustatory sensory receptors. Integration of information in the brain is formed and thus the sense of olfactory and gustatory is combined as a single experience (Patton and Thibodeau, 2009).

3. Diagram the pathway of salivary reflex and discuss mechanism involved. What is the significance of this type of reflex?

In vertebrates, the only digestive secretion that is under neural control is secretion of saliva. It is continuously secreted in mammals as it is an essential part in keeping the mouth and throat moist at all times. It can be enhanced by two different types of salivary reflexessimple salivary reflex and acquired salivary reflex. In the presence of food, chemoreceptors and pressure receptors found in the mouth are able to respond to it. These receptors generate impulses in afferent nerve fibers that transmit the information to the medulla oblongata specifically salivary center. Sending off signals to the salivary glands through extrinsic autonomic nerves is then observed to promote salivation. This is simple salivary reflex. In, acquired salivary reflex oral stimulation is not needed. An organism seeing the food initiates salivation and this is done by reflex. This reflex is learned based on experience. Inputs that are from outside the mouth and are mentally associated induce reflex through the cerebral cortex which will stimulate the medullary salivary center which will then enhance salivation (Sherwood, et.al., 2013). The diagram below shows and summarizes the pathway of both salivary reflexes

Figure 5. Pathway of Salivary Reflexes

(Image taken from Animal Physiology from Genes to Organisms, 2nd Edition)

4. Differentiate between nerve and bone conduction deafness. How can bone conduction remedy abnormalities.

Interference of the transmission of sound to the hearing apparatus or of the neural signal to the auditory cortex can generate deafness. Two categories of it exist----conductive and sensorineural deafness. Conduction deafness results from the malfunction in the transmission of sound from the outer to the inner ear. Problems in the middle sometimes cause this to happen. Conduction deafness can happen in adults as ostoclerosis is the most frequent cause. In this kind of deafness, overgrowth of the labyrinthine bone surrounding the oval window is present which result to fixation of the stapes (Conn, 2008). This may be the reason why conduction deafness is also called bone conduction deafness. In sensorineural or nerve deafness, results from damage of the cochlea, auditory nerve, or the central auditory system. Possible causes are numerous. It is very important to take note that nerve deafness results in decreased bone conduction and loss of high-tone appreciation while conduction deafness results in decreased air conduction and loss of low-tone appreciation (Ross, 2007). It is very important to differentiate conductive from nerve deafness as many types of conductive deafness can be treated (Conn, 2008). In cases of conductive deafness, bone conduction is more efficient (Ross, 2007). Bone conduction hearing has been used for several decades as a hearing rehabilitation technology although scientists have been in difficulty understanding the underlying mechanisms of bone conduction (Majdalaweih, 2008). The major step in bone conduction hearing aid development was the possibility to have direct transmission to the bone with a rigid system implanted to it. And thus a hearing aid system was created. A bone-anchored hearing aid system (BAHA) is a device which converts sound waves into sound vibrations. These vibrations are delivered directly to the inner ear through the skull bone. Bone conduction principle is used (Auditory Implant Service, 2012).

5. Of what importance are vestibular reflexes? Give other factors involved in equilibrium.

The sensory system that is considered to have the most important influence on the other sensory systems is the vestibular system. The vestibular system delivers information regarding on the spatial orientation which is significant for volitional movement control and it activates reflexes that help to stabilize the eyes, head, and body in space. It has three important reflexes,

the vestibulospinal reflex (VSR), vestibulo- ocular reflex (VOR) and vestibulocollic reflex (VCR). Vestibulospinal reflex makes the muscle act to counter or oppose an unwanted movement thus stabilizes the body. An example of this is when suddenly one starts falling to the right, extensor muscles of the right leg will contract more slowly whereas the extensor muscles of the left will relax (Angevine and Cotman, 1981). Vestibulo- ocular reflex, on the other hand, stabilizes the vision during head rotations (Sunny, 2000). For example, a rightward head movement is associated with a leftward head movement and vice versa. This is to focus the image on the fovea of the eye and to compensate for the initial head rotation (Furman, et. al., 2010). Lastly, the vestibulocollic reflex which acts on the neck musculature to stabilize the head such as when the head starts to fall to the right, muscles on the neck contract to keep the head in its normal position ( Herdman and Clendaniel, 2014; Angevine and Cotman, 1981).

Aside from the vestibular functions, breathing, vision, musculoskeletal alignment and proprioception affect the equilibrium and balance in the body (Young, 2013).

1. Breathing. Oxygen is important for the brain. A relaxed deep breathing provides the brain oxygen which is essential for brain function and interaction with other sense organs that detect equilibrium and balance.

2. Vision. Vision is significant in attaining equilibrium because nerve fibers from the eyes interact with the vestibular system (~ 20%) (Politzer, N. D). The brain senses the bodys movement, orientation in space as well as the relationship to objects in the environment.

3. Musculoskeletal alignment. Groupings and alignment of muscles can affect equilibrium by affecting the whole skeletal system (Young, 2013).

4. Proprioception. Proprioception involves sensors, called proprioceptors, which are responsible for providing information concerning joint position, velocity, muscle tension, rate of change in length, and other important information related in maintaining equilibrium (McLester and St. Pierre, 2008).

6. Define blind spot. Of what importance is the field of vision? Illustrate a perimeter chart.

The region of the retina where the axons of the retinal ganglion meet to form the optic nerve and leave the eyeball is called the blind spot. It is located approximately 15 towards the nasal side of the retina and does not have any photoreceptors but it is accompanied by blood vessels which form the circulation of the eye (Winn, 2001). Field of vision is defined as the cone of space with its apex at the eye, which is seen by the subject, when the eye is kept fixed at one point (Ghai, 2013). If the vision is normal, the visual field extends ~100 temporally (laterally), 60 nasally, 60 superiorly and 70 inferiorly (Carroll and Johnson, 2013). The field of vision for an eye is charted in a field of vision of that eye (with the other eye closed). There are several factors that can affect it. a. Color of the object. For white objects, the visual acuity is better thus the field of vision is better delineated. b. Size of the object. If one wants a better visual acuity, then it is recommended that the size of the object is larger. However, in perimetry, only a standard size is used. c. Brightness of the object. There are several factors that affect visual acuity. This includes brightness, contrast and illumination. Since these factors affect visual acuity therefore

these also affects the field of vision. d. Illumination. As said earlier, illumination is one factor that affects the visual acuity and field of vision. If theres a decrease in illumination then theres also a decrease in the visual field (Pal and Pal, 2005).

Analyzing ones field of vision is necessary in neurologic and ophthalmologic examinations for it is used to check gaps in ones side (peripheral) vision (Healthwise, Inc., 2013). Simulators which have a wide field of view tend to have greater incidents of simulator sickness. This is because field of view influences the subjects experiences of vection due to moving visual scenes (Riva, 1997).

7. Discuss the different types of nerve endings in the skin and their differences in terms of stimuli, sensitivity and location. There are 6 different types of free nerve endings in the skin. These are Non- encapsulated Nerve Endings

a. Free nerve endings. These are not encapsulated nerve endings or endings without accessory structures which respond to light touch, pain, itching, and high and low temperatures. These are found in almost every tissue of the body and in the skin, they reach into the lower layers of the epidermis particularly the stratum germinativum and encircle the hair follicle (Alcamo, 2003; Kahle and Frotscher, 2003 ). b. Merkels cells or Merkels disks. These are modified epidermal cells which are sensitive to light touch (like free nerve endings), pressure and texture. Merkels cells are mostly found in highly sensitive skin like that of the fingertips and at the bases of some hair follicles (Mescher, 2013). c. Root hair plexuses. These are receptors which detect light touch that monitor the bending of hairs thus these are mostly found wrapping around hair follicles.

Encapsulated Nerve Endings d. Meissners corpuscles. Meissners corpuscles initiate impulses when light- touch or low-frequency stimuli against skin temporarily deform their shape. These are prevalent in the fingertips, palms, and soles. e. Pacinian corpuscles. These are used in sensing coarse touch, pressure (sustained touch), and vibrations. These are found in the connective tissue of organs located deep in the body such as the wall of the rectum and urinary bladder. f. Krause end bulbs. These are like Pacinian corpuscles which sense vibrations however those vibrations which are low- frequency only. Krause end bulbs are the nerve endings found prevalent in the skin of the penis and clitoris. g. Ruffini corpuscles. The sensory axons of Ruffini corpuscles are stimulated by stretch (tension) or twisting (torque) in the skin. These are found anchored firmly to the surrounding of connective tissue (Mescher, 2013).

REFERENCES Alcamo, I. E. (2003). Anatomy Coloring Workbook, 2nd edition. Princeton Review Publishing,

LLC. United States. p134

Angevine, J. and Cotman, C. (1981). Principles of Neuroanatomy. Oxford University Press,Inc. United States of America. p81

Auditory Implant Service (2012). Bone Anchored Hearing Programme. Data retrieved from

http://ais.southampton.ac.uk/bone-anchored-hearing-aid-service/ on June 29, 2015

Bateman, J. (1994). The New Book of Popular Science: The Brain and Nervous System. Grolier

International, Inc. Danbury, Connecticut. p223, 226.

Carroll, J. and Johnson, C. (2013). Visual Field Testing: From One Medical Student to Another. Data retrieved from http://webeye.ophth.uiowa.edu/eyeforum/tutorials/VF-testing/ on June 28, 2015

Conn, P.M. (2008). Neuroscience in Medicine 3rd Edition. Springer Science & Business Media. p588.

Cowan, I. (1994). The New Book of Popular Science: Mammals. Grolier International, Inc. Danbury, Connecticut. p2- 4.

Furman, J., et. al. (2010). Vestibular Disorders: A Case-study Approach to Diagnosis and Treatment, 3rd edition. Oxford University Press, Inc. p10

Ghai, C. L. (2013). A Textbook of Practical Physiology. Jaypee Brothers Medical Publishers. New Delhi. p203

Healthwise, Inc. (2013). Vision Tests. Data retrieved from http://www.webmd.com/eye-health/vision-tests on June 29, 2015

Herdman, S. and Clendaniel, R. (2014). Vestibular Rehabilitation, 4th edition. F. A. Davis

Company. United States of America. p12 Hyman, L. (1942). Comparative Vertebrate Anatomy. Cacho Hermanos, Inc. Madaluyong City,

Philippines. p428, 491

Kahle, W. and Frotscher, M. (2003). Color Atlas and Textbook of Human Anatomy: Nervous system and sensory organs, Volume 3, 5th edition. Georg Thieme Verlag. p318

King, C. (1994). The New Book of Popular Science: The Human Species. Grolier International, Inc. Danbury, Connecticut. p152- 156

King, L. (2013). Experience psychology. Mcgraw-Hill. New York. p82

Lockhart, T., & Shi, W. (2010). Effects of age on dynamic accommodation. Ergonomics, 53(7), 892-903. doi:10.1080/00140139.2010.489968

Majdalaweih, O. (2008). Mechanisms, Testing, and Implementation of Bone Conduction Hearing: Clinical and Experimental Investigations. Dalhousie University. Canada

McLester, J. and St. Pierre, P. (2008). Applied Biomechanics: Concepts and Connections. Thomson Wadsworth. Canada. p215

Mescher, A. (2013). Junqueiras basic Histology: Text and Atlas, 13th edition. McGraw- Hill Education.

Moller, A. (2000). Hearing: Its Physiology and Pathophysiology. Academic Press. pp. 213-214.

Morrey, L. (1994). The New Book of Popular Science: Smell, Taste, and Touch. Grolier International, Inc. Danbury, Connecticut. p279- 281

Moyes, C. and Schulte, P. (2008). Principles of Animal Physiology, 2nd Edition. Pearson Education Inc. pp. 258-259, 262- 264, 266- 268, 283- 285, 273- 275

Nuffieldfoundation.org. (2011). The range of accommodation of the eye | Nuffield Foundation. Data retrieved from http://www.nuffieldfoundation.org/practical-physics/range-accommodation-eye on June 26, 2015

Pal, G. and Pal, P. (2005). Textbook of Practical Physiology, 2nd edition. Orient Longman Private Ltd. India. p325

Patestas, M., & Gartner, L. (2006). A textbook of neuroanatomy. Malden, MA: Blackwell Pub. Pp146-154

Patton, K., et.al. (2007). Laboratory Manual to Accompany Essentials of Anatomy and Physiology. 6th Edition. C&E Publishing, Inc. Quezon City, Philippines. p133, 155, and 161

Patton, K. and Thibodeau, G. (2009). Anthonys Textbook of Anatomy and Physiology 19th Edition. Elsevier Pte Ltd. Singapore. pp. 501-524

Politzer, T. (N. D). Balance & Illusions of Movement. Neuro-Optometric Rehabilitation Association, International, Inc. Data retrieved from https://nora.cc/balance-a-illusions-of-

movement-mainmenu-68.html on June 27, 2015

Purves, D. (2008). Neuroscience. Sunderland, Mass.: Sinauer. Pp 236, 253-258

Rabago, D., et. al. (2008). Functional Biology: Modular Approach. Vibal Publishing House, Inc. Quezon City, Philippines. p307- 313

Riva, G. (1997). Virtual Reality in Neuro- Psycho- Physiology: Cognitive, Clinical and Methodological Issues in Assessment and Rehabilitation. IOS Press. The Netherlands. p46

Ross, R. (2007). How to Examine the Nervous System. Springer Science & Business Media. p109

Sherwood, L., et. al. (2013). Animal physiology. Belmont, CA: Brooks/Cole. Pp 240-242, 245-246, 248

Sherwood, L., et. al. (2013). Animal Physiology: From Genes to Organisms. 2nd Edition. Brooks/Cole, Cengage Learning. United States of America. p208- 211, 665-666

Saladin, K. (2008). Human anatomy. Boston: McGraw-Hill. Pp 496- 500, 507-514

Sato, T., et. al. (1998). Distributions of sensory spots in the hand and two-point discrimination thresholds in the hand, face and mouth in dental students. Journal Of Physiology-Paris, 93(3), 245-250. doi:10.1016/s0928-4257(99)80158-2

Science Buddies (2013). Sensory Science: Testing Taste Thresholds - Scientific American. Data retrieved from http://www.scientificamerican.com/article/bring-science-home-taste-thresholds/ on June 29, 2015

Sunny, T. Y. F. (2000). The Human Vestibulo- ocular reflex: The effect of Vergence angle and Unilateral Lesions on Reflex Dynamics. National Library of Canada. Canada.

Tucci, D. L. (N.D). Nose and Sinuses - Ear Nose and Throat Disorders - Merck Manual Home

Edition. Data retrieved from https://www.merckmanuals.com/home/ear-nose-and-throat-disorders/biology-of-the-ears-nose-and-throat/nose-and-sinuses on June 29, 2015

Winn, P. (2001). Dictionary of Biological Psychology. Routledge. New York. p218

Young, J. (2013). Factors of Body Equilibrium & Balance. Data retrieved from

http://www.livestrong.com/article/49331-factors-body-equilibrium-balance/ on June 27, 2015