BSC NURSING SEM1 APPLIED PHYSIOLOGY UNIT 7 The sensory Organs
Functions of skin
Functions of the Skin
The skin, also known as the integumentary system, is the largest organ of the body. It serves multiple essential functions, ranging from protection to thermoregulation. Here’s an overview of its key functions:
1. Protection
Physical Barrier:
Protects against mechanical injury, chemical exposure, and microbial invasion.
Waterproofing:
Prevents water loss through the epidermis, maintaining hydration.
Microbial Defense:
Acidic pH (acid mantle) and antimicrobial peptides prevent the growth of harmful microorganisms.
UV Protection:
Melanin, produced by melanocytes, absorbs and dissipates ultraviolet radiation.
2. Regulation of Body Temperature (Thermoregulation)
Sweat Secretion:
Eccrine sweat glands produce sweat, which evaporates to cool the body.
Vasodilation and Vasoconstriction:
Blood vessels dilate (vasodilation) to release heat and constrict (vasoconstriction) to retain heat.
Insulation:
Subcutaneous fat acts as an insulating layer, reducing heat loss.
3. Sensory Function
The skin contains sensory receptors that detect:
Touch (Mechanoreceptors): Pressure, vibration.
Temperature (Thermoreceptors): Heat and cold.
Pain (Nociceptors): Injury or damage.
Itch and Tickling Sensations.
4. Excretion
Sweat glands excrete waste products such as:
Salts: Sodium, potassium.
Metabolic Waste: Urea, ammonia, lactic acid.
5. Synthesis of Vitamin D
The skin synthesizes Vitamin D3 (cholecalciferol) when exposed to ultraviolet (UVB) radiation from sunlight.
Vitamin D is essential for calcium absorption and bone health.
6. Immunological Defense
The skin acts as the first line of defense in the immune system.
Langerhans Cells:
Found in the epidermis, they process antigens and alert the immune system.
Antimicrobial Peptides:
Fight pathogens and prevent infections.
7. Storage
Fat Storage:
The hypodermis (subcutaneous layer) stores energy as adipose tissue.
Water and Electrolyte Reservoir:
The dermis retains fluids and electrolytes.
8. Communication
Emotional Expression:
Blushing or pallor conveys emotions such as embarrassment or fear.
Pheromones:
Secretions from sebaceous and sweat glands may play a role in non-verbal communication.
9. Wound Healing and Regeneration
The skin has the ability to repair itself after injury through:
Inflammatory Phase: Removal of pathogens and debris.
Proliferative Phase: New tissue formation.
Remodeling Phase: Restoration of skin integrity.
10. Prevention of Dehydration
The skin prevents excessive water loss by acting as a barrier, maintaining the body’s fluid balance.
11. Aesthetic and Identification
The skin’s appearance (color, texture) contributes to individual identity and social interactions.
Layers of the Skin and Their Role
Epidermis:
Outermost layer; acts as a protective barrier.
Contains keratinocytes, melanocytes, and Langerhans cells.
Dermis:
Middle layer; provides strength and elasticity.
Contains blood vessels, nerves, sweat glands, and hair follicles.
Hypodermis (Subcutaneous Layer):
Deepest layer; stores fat and connects the skin to underlying tissues.
Clinical Relevance
Burns: Disruption in skin integrity leads to loss of protection and dehydration.
Skin Diseases: Conditions like psoriasis, eczema, and acne disrupt normal skin functions.
Aging: Leads to reduced elasticity, thinning, and slower wound healing.
Vision
Vision: Overview
Vision is one of the primary senses, enabling the perception of light, colors, shapes, and depth. It involves the eye, optic nerve, and parts of the brain responsible for processing visual information.
Structure of the Eye
The eye is the organ of vision, consisting of several components that work together to focus light and transmit signals to the brain.
1. External Structures
Eyelids: Protect the eye from debris and excessive light.
Conjunctiva: A thin membrane covering the sclera and inner eyelids.
Lacrimal Apparatus: Produces tears to lubricate and protect the eye.
2. Internal Structures
Outer Layer:
Cornea: Transparent, curved structure that focuses light onto the retina.
Sclera: White, fibrous outer layer that provides structural support.
Middle Layer (Uvea):
Iris: Colored part of the eye; regulates the amount of light entering by controlling the pupil size.
Pupil: The central opening in the iris; adjusts to light intensity.
Ciliary Body: Produces aqueous humor and changes lens shape for focusing (accommodation).
Choroid: Vascular layer supplying nutrients and oxygen to the retina.
Inner Layer:
Retina: Contains photoreceptor cells (rods and cones) that detect light and color.
Optic Disc: Point where the optic nerve exits; no photoreceptors (blind spot).
Lens:
Transparent, flexible structure that focuses light onto the retina.
Changes shape for near and far vision (accommodation).
Vitreous and Aqueous Humors:
Aqueous Humor: Fluid in the anterior chamber, maintains intraocular pressure.
Light enters the eye through the cornea and pupil, with the iris regulating the amount of light.
Focusing:
The cornea and lens bend (refract) light rays to focus them on the retina.
Phototransduction:
Light hits the retina, where photoreceptors (rods and cones) convert it into electrical signals:
Rods: Sensitive to dim light; enable black-and-white vision.
Cones: Sensitive to bright light and color; provide sharp, detailed vision.
Signal Transmission:
Photoreceptors synapse with bipolar cells, which transmit signals to ganglion cells.
Ganglion cell axons form the optic nerve, which carries signals to the brain.
Processing in the Brain:
The optic nerves from both eyes converge at the optic chiasm, where some fibers cross.
Signals are relayed to the visual cortex in the occipital lobe of the brain via the lateral geniculate nucleus (LGN).
The brain interprets these signals to form a coherent image.
Functions of Vision
Detection of Light and Color:
Rods: Night vision and peripheral vision.
Cones: Daylight and color vision (red, green, blue-sensitive cones).
Depth Perception:
Achieved through binocular vision, which combines input from both eyes to perceive three-dimensional space.
Motion Detection:
Specialized retinal and cortical cells detect movement, aiding in navigating the environment.
Visual Acuity:
The ability to see fine details, primarily due to the concentration of cones in the fovea.
Adaptation to Light:
Pupillary Reflex: Adjusts pupil size for different light intensities.
Dark Adaptation: Rods increase sensitivity in low light.
Common Disorders Affecting Vision
Refractive Errors:
Myopia (Nearsightedness): Light focuses in front of the retina.
Hyperopia (Farsightedness): Light focuses behind the retina.
Astigmatism: Uneven curvature of the cornea or lens, causing distorted vision.
Presbyopia: Age-related loss of lens elasticity, reducing near vision.
Cataracts:
Clouding of the lens, leading to blurred vision.
Glaucoma:
Increased intraocular pressure damages the optic nerve.
Macular Degeneration:
Degeneration of the macula (central retina), affecting detailed vision.
Retinal Detachment:
Separation of the retina from the underlying layers, potentially leading to blindness.
Color Blindness:
Deficiency in cone function, often genetic.
Diabetic Retinopathy:
Damage to retinal blood vessels due to diabetes.
Key Visual Functions
Function
Key Structure Involved
Description
Focusing
Cornea and Lens
Refracts light to focus on the retina.
Light Detection
Retina
Converts light into electrical signals.
Color Perception
Cones
Enables recognition of colors.
Night Vision
Rods
Provides vision in low-light conditions.
Image Formation
Visual Cortex (Brain)
Interprets signals from the retina.
hearing
Hearing: Overview
Hearing, or audition, is the sense that allows us to perceive sound. It involves the conversion of sound waves into electrical signals that the brain can interpret. This complex process relies on the structure and function of the ear and the auditory pathways in the brain.
Structure of the Ear
The ear is divided into three main parts, each playing a distinct role in hearing:
1. Outer Ear
Auricle (Pinna):
The visible part of the ear that captures sound waves and funnels them into the ear canal.
External Auditory Canal:
Directs sound waves toward the eardrum and amplifies certain frequencies.
Tympanic Membrane (Eardrum):
Vibrates in response to sound waves, converting them into mechanical vibrations.
2. Middle Ear
Ossicles:
Three tiny bones (malleus, incus, stapes) that amplify sound vibrations.
The stapes connects to the oval window, transmitting vibrations to the inner ear.
Eustachian Tube:
Connects the middle ear to the nasopharynx, equalizing air pressure on both sides of the eardrum.
3. Inner Ear
Cochlea:
A spiral-shaped, fluid-filled structure that houses the organ of Corti, where sound vibrations are converted into electrical signals.
Organ of Corti:
Contains hair cells (sensory receptors) that respond to sound vibrations and generate nerve impulses.
Vestibular Apparatus:
Plays a role in balance, not directly related to hearing.
How Hearing Works
The process of hearing involves several steps:
Sound Wave Capture:
The auricle captures sound waves and directs them into the ear canal.
Vibration of the Tympanic Membrane:
Sound waves cause the eardrum to vibrate.
The frequency of vibrations corresponds to the pitch of the sound.
Amplification in the Middle Ear:
Vibrations are transferred to the ossicles (malleus → incus → stapes), amplifying the sound.
The stapes transmits these vibrations to the cochlea via the oval window.
Conversion of Vibrations into Electrical Signals:
Vibrations create waves in the cochlear fluid, which move the basilar membrane.
Hair cells in the organ of Corti bend in response to these waves:
Outer Hair Cells: Amplify sound vibrations.
Inner Hair Cells: Convert vibrations into electrical signals by releasing neurotransmitters.
Transmission to the Brain:
Electrical signals travel via the auditory nerve (Cranial Nerve VIII) to the brainstem and then to the auditory cortex in the temporal lobe.
The brain interprets these signals as recognizable sounds.
Functions of the Ear
Detection of Sound:
Converts sound waves into mechanical vibrations, then into electrical signals.
Frequency Discrimination:
The cochlea’s basilar membrane is tonotopically organized:
High frequencies activate hair cells near the base.
Low frequencies activate hair cells near the apex.
Localization of Sound:
The brain uses input from both ears to determine the direction and distance of a sound.
Protection Against Loud Sounds:
The acoustic reflex reduces ossicle movement in response to loud noises, protecting the inner ear.
Common Disorders of Hearing
Conductive Hearing Loss:
Cause: Obstruction or damage to the outer or middle ear (e.g., earwax, otitis media, perforated eardrum).
Effect: Reduced ability to transmit sound to the inner ear.
Sensorineural Hearing Loss:
Cause: Damage to the cochlea or auditory nerve (e.g., noise exposure, aging, infections).
Effect: Difficulty in perceiving sound, especially high frequencies.
Presbycusis:
Age-related hearing loss, typically affecting high-frequency sounds.
Tinnitus:
Perception of ringing or buzzing sounds without an external source.
Often linked to noise exposure, stress, or ear disorders.
Meniere’s Disease:
Affects the inner ear, causing vertigo, hearing loss, and tinnitus.
Clinical Tests for Hearing
Pure Tone Audiometry:
Measures hearing sensitivity at various frequencies.
Tympanometry:
Assesses middle ear function by measuring eardrum movement.
Otoacoustic Emissions (OAEs):
Evaluates cochlear function by detecting sounds emitted by the inner ear.
Brainstem Auditory Evoked Response (BAER):
Measures auditory nerve and brainstem activity in response to sound.
Key Terms in Hearing
Term
Description
Frequency (Pitch)
Determined by the speed of sound waves (measured in Hz).
Amplitude (Loudness)
Determined by the height of sound waves (measured in dB).
Tonotopy
Spatial organization of frequency detection in the cochlea.
Acoustic Reflex
Protective mechanism to reduce sound transmission.
taste and smell
Taste and Smell: Overview
Taste (gustation) and smell (olfaction) are closely related chemical senses that help detect and identify molecules in our environment, contributing to flavor perception and survival behaviors.
Taste (Gustation)
Structure of the Taste System
Taste Buds:
Located on the tongue, soft palate, pharynx, and epiglottis.
Contain taste receptor cells that respond to chemical stimuli.
Found in papillae on the tongue:
Fungiform Papillae: Tip and sides of the tongue.
Foliate Papillae: Sides of the tongue.
Circumvallate Papillae: Back of the tongue.
Filiform Papillae: No taste buds, involved in texture sensation.
Taste Receptor Cells:
Each taste bud contains 50–100 receptor cells.
These cells regenerate every 10–14 days.
Receptors respond to specific taste molecules.
Five Basic Tastes
Sweet:
Detected by sugars and artificial sweeteners.
Indicates energy-rich nutrients.
Sour:
Responds to hydrogen ions (acidity).
Detects spoiled or fermented foods.
Salty:
Detects sodium ions (Na⁺).
Essential for maintaining electrolyte balance.
Bitter:
Sensitive to alkaloids and toxins.
Acts as a warning against harmful substances.
Umami:
Detects glutamate and amino acids (savory flavor).
Found in protein-rich foods like meat and cheese.
Neural Pathway of Taste
Taste signals are transmitted from the tongue via three cranial nerves:
Facial Nerve (CN VII): Anterior two-thirds of the tongue.
Glossopharyngeal Nerve (CN IX): Posterior one-third of the tongue.
Vagus Nerve (CN X): Taste buds in the pharynx and epiglottis.
Signals travel to the solitary nucleus in the brainstem.
Relayed to the thalamus and then to the gustatory cortex in the brain for interpretation.
Supporting Cells: Provide structural and metabolic support.
Basal Cells: Regenerate olfactory receptor cells.
Olfactory receptor cells are bipolar neurons with cilia that detect odorants.
Olfactory Bulb:
Located in the brain above the nasal cavity.
Receives signals from olfactory receptor cells and processes them.
Mechanism of Smell
Odor Detection:
Odorant molecules dissolve in the mucus of the nasal cavity.
Bind to specific receptors on the cilia of olfactory receptor cells.
Signal Transmission:
Olfactory receptor cells send signals to the olfactory bulb via the olfactory nerve (Cranial Nerve I).
Signals are processed and relayed to the olfactory cortex, amygdala, and hippocampus for perception, memory, and emotional response.
Functions of Smell
Identification of Odors:
Detects food, environmental hazards, and pheromones.
Enhances Flavor Perception:
Smell works with taste to create the sensation of flavor.
Memory and Emotion:
Smell is closely linked to the limbic system, evoking memories and emotions.
Disorders of Smell
Anosmia: Loss of smell.
Hyposmia: Reduced smell sensitivity.
Parosmia: Distorted smell perception.
Phantosmia: Perception of odors that are not present.
Integration of Taste and Smell
Taste and smell combine to create the perception of flavor.
Flavor Perception:
Taste provides the basic qualities (sweet, sour, salty, bitter, umami).
Smell adds complexity and richness to the perception of food.
Retronasal Olfaction:
Odor molecules from food in the mouth reach the olfactory epithelium via the pharynx.
Clinical Importance
Loss of taste or smell can affect:
Nutrition: Reduced appetite and weight loss.
Safety: Inability to detect spoiled food or dangerous odors.
Quality of Life: Diminished enjoyment of food and beverages.
Comparison of Taste and Smell
Feature
Taste (Gustation)
Smell (Olfaction)
Receptors
Taste buds on the tongue
Olfactory receptor cells in the nasal cavity
Stimuli
Molecules dissolved in saliva
Molecules dissolved in nasal mucus
Pathway
CN VII, IX, X → Brainstem → Thalamus → Cortex
CN I → Olfactory Bulb → Cortex, Limbic System
Regeneration
Every 10–14 days (taste receptor cells)
Every 30–60 days (olfactory receptor cells)
Function
Basic nutrient detection
Complex odor recognition and memory
Errors of refraction, aging changes
Errors of Refraction
Refraction errors occur when the eye cannot properly focus light onto the retina, leading to blurred vision. This is often caused by abnormalities in the shape of the eye, cornea, or lens.
1. Myopia (Nearsightedness)
Cause: The eyeball is too long, or the cornea is too curved.
Effect: Light focuses in front of the retina.
Symptoms:
Difficulty seeing distant objects clearly.
Squinting and eye strain.
Correction: Concave lenses or refractive surgery (e.g., LASIK).
2. Hyperopia (Farsightedness)
Cause: The eyeball is too short, or the cornea is too flat.
Effect: Light focuses behind the retina.
Symptoms:
Difficulty seeing close objects clearly.
Eye strain and headaches, especially during reading.
Correction: Convex lenses or refractive surgery.
3. Astigmatism
Cause: The cornea or lens has an irregular shape.
Effect: Light is refracted unevenly, leading to distorted or blurred vision.
Symptoms:
Blurred or distorted vision at all distances.
Eye discomfort and headaches.
Correction: Cylindrical lenses or refractive surgery.
4. Presbyopia
Cause: Age-related loss of lens elasticity, reducing the ability to focus on close objects.
Effect: Difficulty focusing on nearby objects (similar to hyperopia).
Symptoms:
Difficulty reading small print.
Holding objects farther away to see clearly.
Correction: Reading glasses, bifocals, or progressive lenses.
Aging Changes in Vision
As we age, several structural and functional changes occur in the eye, impacting vision:
1. Decreased Lens Elasticity
Condition: Presbyopia.
Effect: Reduced ability to focus on near objects due to stiffening of the lens.
2. Cataracts
Cause: Clouding of the lens due to protein aggregation and oxidative stress.
Effect: Blurred or hazy vision, glare sensitivity, and reduced color perception.
Treatment: Surgical removal of the cloudy lens and replacement with an artificial intraocular lens.
3. Reduced Pupil Size
Condition: Senile miosis.
Effect: Less light enters the eye, reducing the ability to see in low-light conditions.
4. Dry Eye Syndrome
Cause: Reduced tear production or increased evaporation.
Effect: Eye irritation, redness, and blurry vision.
5. Loss of Peripheral Vision
Cause: Decreased retinal sensitivity and changes in the optic nerve.
Effect: Narrowed field of vision.
6. Age-Related Macular Degeneration (AMD)
Cause: Degeneration of the macula (central retina).
Effect: Loss of central vision, difficulty reading or recognizing faces.
Types:
Dry AMD: Gradual thinning of the macula.
Wet AMD: Abnormal blood vessel growth under the retina.
7. Glaucoma
Cause: Increased intraocular pressure damages the optic nerve.
Effect: Gradual loss of peripheral vision, eventually leading to blindness if untreated.
8. Vitreous Changes
Condition: Posterior vitreous detachment.
Effect: Floaters and flashes of light.
Risk: Retinal detachment in severe cases.
9. Color Vision Changes
Cause: Yellowing of the lens and reduced sensitivity of cone cells.
Effect: Difficulty distinguishing between similar shades of blue, green, and other colors.
10. Slower Adaptation to Light Changes
Cause: Decline in the efficiency of rod and cone cells.
Effect: Difficulty adjusting between bright and dark environments.
Preventive Measures and Management
Regular Eye Exams:
Early detection of conditions like glaucoma, AMD, and cataracts.
Proper Lighting:
Use adequate lighting for reading and other activities.
Protective Eyewear:
Protect eyes from UV radiation with sunglasses.
Healthy Diet:
Include foods rich in antioxidants (e.g., vitamin C, vitamin E, lutein, and omega-3 fatty acids).
Treat Dry Eyes:
Use artificial tears or humidifiers.
Corrective Lenses:
Use glasses or contact lenses tailored to specific refractive errors.