Special Senses

A&P1

Special Senses

the senses that provide specific types of sensory information and include: vision, hearing, taste, smell, and balance (equillibrium)

The eye

• Hollow, fluid-filled structure approx. 2.5cm in diameter

• Designed to respond to light stimuli

The eye: visibility

• only 1/6 of surface is visible

• remaining portion enclosed by bony orbit (eye socket)

The eye: protection

• eye socket provides structural protection

• cushioned by layer of fat within orbit

The eye: function

• Vision - focusing light onto the retina

• Photoreceptor cells in the retina convert light into neural signals

  • interpreted by brain to create vision

Eyebrows

• coarse, short hairs overlying the eye

• provide shade, prevent perspiration from reaching the eye

• important for nonverbal communication

Eyelids

• thin, skin-covered folds protecting anterior eye

• blink approx. every 3-7 seconds

• protect eye and maintain moisture

Eyelashes

• Hair follicles projecting from eyelid

• richly innervated by nerve endings, trigger blink reflex

• act as barrier for dust, dirt and foreign particles

Lacrimal Apparatus

The system responsible for producing and draining tears from the eye. Consists of the lacrimal gland, tear ducts, lacrimal sac, and nasolacrimal duct.

Lacrimal Gland

Located above the outer corner of the eye, it produces tears that lubricate and protect the eye

Tear Ducts

Small ducts through which tears flow from the lacrimal gland. These ducts distribute tears across the surface of the eye, keeping it moist, clean, and removing foreign particles.

Lacrimal Sac

A pouch-like structure that collects tears from the canaliculi. It serves as a reservoir for tears before they are drained.

Canaliculi

Small tubes that drain tears from the eye's surface into the lacrimal sac. They help facilitate tear drainage to keep the eye clean and lubricated.

Nasolacrimal Duct

Carries tears from the lacrimal sac into the nasal cavity, where excess tears drain out.

Tears

Watery fluid, produced by lacrimal glands, essential for lubricating, protecting, and cleaning the eyes

Tears: function

• Lubrication - preventing dryness and irritation

• Protection - cleaning the eye by washing away foreign particles

  • maintain healthy environment of eye’s surface

Tears: composition

• Mucus - helps spreads tears

• Antibodies - immune defense against infection

• Lysozyme - enzyme that destroys bacteria

External Eye Muscles

are the 6 muscles that originate from the bony orbit and insert onto the outer surface of the eye, allowing for controlled movement of the eye and maintain the shape of the eyeball

Oculomotor Nerve (III)

• Superior/Inferior Rectus - Moves eye upward and downward

• Medial Rectus - Moves eye inward (towards nose)

• Inferior Oblique - Rotates eye upward and outward

Trochlear Nerve (IV)

• Superior Oblique - Controls downward and outward eye movement.

Abducens Nerve (VI)

• Lateral Rectus - Moves eye outward (away from nose).

Conjunctiva

• Thin, transparent membrane covering the white part of the eyeball (Sclera) and inner surface of eyelids

Conjunctiva Functions

• Produces lubricating mucus

  • prevents drying of eye

• Traps debris and microorganisms

  • preventing infection

Conjunctival Sac

• The space between the eyelid and the eyeball that holds tears and allows for eye movement.

Clinical Application: Conjunctivitis (Pink eye)

is an inflammation of the conjunctiva, commonly resulting in redness, itching, and discharge from the eyes.

Clinical Application: 2 types of Conjunctivitis

• Viral - caused by viral infections, often associated with the common cold or respiratory infections that usually resolve on its own

  • Highly contagious and spreads through contact with infected eye secretions or respiratory droplets

• Bacterial - caused by bacterial infections, often due to Staphylococcus or Streptococcus bacteria which requires antibiotic treatment

  • Also, contagious and spreads through contact with infected eye secretions

Clinical Application: Symptoms of Conjunctivitis

Redness of the eye, itching or burning sensation, water (viral) or thick yellow green (bacterial) discharge, swollen eye lids and light sensitivity.

Internal Eye Structures

Include the sclera, cornea, choroid, ciliary body, iris, pupil, and retina.

Internal Eye Structures: Fibrous Layer

outermost layer, composed of dense, avascular connective tissue, has two main regions: the Sclera and Cornea.

Sclera

The white, outermost layer of the eye, composed of tough connective tissue that provides structure and protection.

• serves as an attachment point for the extraocular muscles, enabling eye movement

Cornea

The clear/transparent dome-shaped surface at the front of the eye that helps focus light as it enters the eye.

Internal Eye Structures: Vascular Layer

The middle layer of the eye, that helps nourish the eye and is subdivided into 3 regions: Choroid, ciliary body, and iris

Choroid

A layer of blood vessels that supplies nutrients to the eye

• contains pigment cells that help to absorb excess light, preventing glare and improving image clarity

Ciliary Body

A ring of muscles that controls the shape of the lens for focusing and produces aqueous humor

Iris

The colored part of the eye that controls the size of the pupil to regulate light entering the eye.

• Composed of two muscle layers the sphincter which constricts and the dilator muscle which dilates the pupil

• Pupil - the black circular opening in the center of the iris that lets light into the eye

• Lens - is a flexible structure behind the iris and pupil that focuses light onto the retina by changing its shape in a process called accommodation

Clinical Application: Cataracts

A condition where the lens of the eye becomes cloudy or opaque (white), leading to a gradual decrease in vision.

• Cause - often age-related, but can also result from injury, certain medications, or medical conditions like diabetes

Clinical Application: Symptoms of Cataracts

• Symptoms

  • Blurry or foggy vision

  • Difficulty seeing at night or in a low light

  • Sensitivity to glare, such as from headlights while driving

  • Faded colors

• Treatment

  • Surgery is the most common treatment, where the cloudy lens is removed and replaced with a clear, artificial lens

The Retina

The innermost layer of the eye. It is the light-sensitive layer that plays a key role in vision. It contains millions of photoreceptors (sensory receptor that responds to light) which send visual signals to the brain.

Types of Photoreceptors

Rods and Cones

Rods

Responsible for vision in low light (night vision) and detect shapes and movement, but they do not detect color

Cones

Function in bright light and are responsible for color vision and sharp, detailed central vision.

Optic Disk

This is the area where the optic nerve exits the eye. It lacks photoreceptors, creating a “blind spot”.

Fundus

The posterior part of the retina, which includes the optic disk, macula, and surrounding areas. It is the area seen during and eye exam.

Macula

A small, central part of the retina that contains a high concentration of cones. It is responsible for sharp, detailed central vision.

Optic Nerve

The nerve that transmits visual information from the retina to the brain for processing.

2 main Chambers of the Eye

Anterior Chamber and Posterior Chamber

Anterior Chamber

The front part of the eye, located between the cornea and the iris

• It is filled with a watery fluid called aqueous humor, which forms and drains continuously helping the eye maintain pressure and provides nutrients to the eye tissues, specifically the lens and cornea which lack blood supply

Posterior Chamber

It is located behind the iris and in front of the lens.

• Also filled with aqueous humor, helping to maintain intraocular pressure.

Vitreous Body (Vitreous Chamber)

Located behind the lens, filling the space between the lens and retina

• It is filled with vitreous humor, a gel-like substance that forms in the embryo and lasts a lifetime

• It reinforces the structure of the eye and helps to maintain intraocular pressure

Clinical Application: Glaucoma

A group of eye conditions that cause damage to the optic nerve, which can lead to permanent vision loss if left untreated.

• Cause - build-up of fluid (aqueous humor) in the eye, which increases intraocular pressure

• Treatment -

  • Medications (eye drops) to lower eye pressure

  • Laser therapy or surgery to improve fluid drainage

Clinical Application: 2 types of Glaucoma

• Open-angle

• Angle-closure

Open-angle Glaucoma

Most common form, where the drainage canals of the eye become blocked over time, causing gradual pressure increase

Angle-closure Glaucoma

Less common, more sudden type, where the drainage angle is completely blocked, leading to a rapid increase in pressure

Path of Light

Is essential for vision, as it travels through various structures that work together to focus and transmit visual information to the brain

Path of light steps

1. Light enters the eye through the cornea, which bends the light

2. The light passes through the pupil, and the iris controls the amount of light that enters by adjusting the pupil’s size

3. The light is further focused by the lens onto the retina at the back of the eye

4. The retina which contains photoreceptors (rods and cones) detect the light and convert it into electrical signals

5. These signals are sent to the brain via the optic nerve

6. The brain processes these signals to create a visual image

Visual Pathways

Describe the route taken by visual information from the eyes to the brain for processing

Visual Pathways steps

1. Light enters the eye through the cornea and lens, which focuses the light onto the retina at the back of the eye

2. The lens flips the image upside down and also swaps it from right to left. (This happens because the lens bends the light in a way that creates an inverted image on the retina)

3. The retina contains photoreceptor cells (rods and cones) that detect light and turn it into electrical signals

4. These signals travel through the optic nerve, which carries them from each eye toward the brain

5. At the optic chiasm, the nerves from each eye cross. The signals from the left half of each retina (which sees the right visual field) go to the right side of the brain, and the signals from the right half of the each retina (which sees the left visual field) go to the left side of the brain

6. The signals then travel to the visual cortex in the back of the brain, where the brain processes the information. The brain “flips” the upside-down and right-left reversed image back to its correct orientation, allowing us to see the world as it truly is.

Refractory Errors

Vision problems that occur when the eye doesn’t properly focus light on the retina, resulting in blurred vision. This can happen due to an irregular shape of the eyeball, cornea, or lens. Common types include myopia, hyperopia, and astigmatism.

Myopia

Also known as nearsightedness, occurs when light is focused in front of the retina

• Results in the ability to see close objects (near sighted), but distant objects appear blurred

Hyperopia

Also known as farsightedness, occurs when light is focused behind the retina.

• Results in the ability to see distant objects (far sighted) clearly, but close objects appear blurry.

Presbyopia

Is a natural age-related condition where the lens become less flexible

• Results in it becoming harder to focus on nearby objects

Astigmatism

Occurs when the cornea or lens has an irregular shape, causing light to focus on multiple points in the eye instead of just one

• Results in blurry or distorted vision at all distances

Chemical Senses

Refer to the senses of taste and smell, which are activated by chemical stimuli.

Chemoreceptors

The sensory organs for smell and taste. Sensory receptors that detect chemical stimuli play a crucial role in the senses of taste and smell.

Types of Chemoreceptors

Olfactory Receptors and Gustatory Receptors

Olfactory Receptors

Found in the nose, they detect smells (odors) by responding to airborne chemicals

Gustatory Receptors

Located on the tongue, they detect tastes by responding to chemicals in food and liquids

Sensory Transduction

When a chemoreceptor detects a chemical stimulus, it converts it into an electrical signal that the brain can interpret, allowing us to perceive smells and tastes.

Sense of Smell (Olfaction)

Detects and identifies odors in the environment, playing an important role in detecting food, danger (like smoke or spoiled food), and influencing emotions and memories.

Olfactory Epithelium

Specialized tissue located in the upper part of the nasal cavity where olfactory receptors are located

Olfactory Sensory Neurons

Neurons that are specialized to detect specific odor molecules.

Supporting Cells

Provide structural support and help regenerate olfactory neurons, which have a limited lifespan and are replaced regularly.

Specialty of Olfactory Receptors

Olfactory receptors are highly specific in detecting particular molecules or odors, allowing for precise identification of smells.

Olfactory Receptor Diversity

Each receptor is sensitive to a specific type of chemical structure, detecting a particular set of odor molecules or a small group of related compounds

• Humans have around 400 functional olfactory receptor genes, allowing us to recognize a wide variety of scents.

The Olfactory Pathway

Describes how the brain processes smells, from detecting odor molecules to perceiving scent. It involves a series of steps that allow us to recognize and respond to different smells

Olfactory Pathway: Odor Detection

Olfactory receptors in the olfactory epithelium (located in the nasal cavity) detect odor molecules in the air.

• When these molecules bind to specific olfactory receptors, they trigger sensory neurons to send electrical signals

Olfactory Pathway: Signal Transmission

The electrical signals from the olfactory neurons travel along the olfactory nerve (cranial nerve I) toward the brain

• These signals are directed to the olfactory bulb, a structure located at the base of the brain just above the nasal cavity

Olfactory Pathway: Olfactory Bulb Processing

The olfactory bulb processes the signals and sorts them based on the type of odor detected.

• From here, the signals are relayed to higher brain regions for further processing

Olfactory Pathway: Brain Processing

The signals are sent to the olfactory cortex (located in the temporal lobe), where the brain identifies and interprets the smell.

• Additional regions like the limbic system (involved in emotions and memory) also process smells, which is why certain odors can evoke strong emotional reactions or memories.

Anosmia

• Complete loss of smell

• Etiology: Nasal congestion, head trauma, damage to olfactory nerve

Hyposmia

• Reduces sense of smell

• Etiology: Aging, sinus infections, neurodegenerative disorders

Hyperosmia

• Increased sensitivity to odors

• Etiology: Migraines, pregnancy

Sense of Taste (Gustation)

Allows us to perceive the flavors of food and beverages. It helps identify safe and nutritional foods, avoid spoiled or harmful substances, and enhances the enjoyment of eating.

Taste Receptors

Also known as taste buds, are the sensory organs responsible for detecting taste. They are located in different types of papillae, which help detect different tastes. The 10,000 taste buds are primarily located on the tongue but can also be found in areas such as the roof of the mouth and back of the throat.

Fungiform Papillae

Are described as small, mushroom-shaped bumps located on the top and sides of the tongue.

• They contain taste buds that are sensitive to sweet, salty, and umami tastes.

Circumvallate Papillae

Are the large, circular bumps, that are usually arranged in a row at the back of the tongue in a “v” shape.

• Contain taste buds that are most sensitive to bitter tastes

Filiform Papillae

Are leaf-shaped and lines with folds on the top, sides and back of tongue.

• Responsible for detecting sour and salty tastes

Basic Taste Sensations

There are 5 primary tastes, each of which is sensitive to specific chemicals. Sweet, Salty, Sour, Bitter, and Umami.

Basic Taste Sensations: Sweet

Detects sugars or energy-rich compounds.

• Signaling the presences of carbohydrates which are needed to supply the body with energy

• This taste is generally associated with pleasant or desirable foods

• Ex: sugar, honey, maple syrup

Basic Taste Sensations: Salty

Detects sodium and other salts

• Plays a role in maintaining hydration and nerve function

• Ex: chips

Basic Taste Sensations: Sour

Responds to acids, such as citric acid in citrus fruits or acetic acid in vinegar

• Often signals ripeness in fruits or potential spoilage, helping us avoid consuming unsafe or overly acidic food

• Ex: Lemons, limes, vinegar

Basic Taste Sensations: Bitter

Often detects toxins or bitter compounds

• Acts as a warning system to avoid potentially harmful or poisonous substances

• Ex: Coffee, dark chocolate, kale

Basic Taste Sensations: Umami

Detects amino acids, particularly glutamate, found in protein-rich foods (e.g., meat, fish, and aged cheeses)

• often referred to as the “savory” taste

• plays a role in detecting protein and is associated with nourishing and satisfying foods

• Ex: soy sauce, parmesan cheese

Gustatory Pathway

Describes the route taken by the taste signals from the mouth to the brain for processing

Gustatory Pathway: Stimulus Detection

Taste buds on the tongue detect chemical compounds in food and drink

• These taste receptors are activated when chemicals in the food bind to the taste cells

Gustatory Pathway: Signal Transduction

When taste receptors are activated, they generate electrical signals that are carried by sensory neurons

Gustatory Pathway: Neural Transmission

The signals travel along 3 cranial nerves:

• Facial nerve (VII) - Carries taste information from the front 2/3 of the tongue

• Glossopharyngeal nerve (IX) - Transmits taste from the back 3rd of the tongue

• Vagus nerve (X): Transmits taste signals from the throat and parts of the palate

Gustatory Pathway: Relay to Brainstem

The signals travel to the medulla oblongata in the brainstem, where the gustatory information is processed

Gustatory Pathway: Processing in the Gustatory Cortex

From the brainstem, the signals are relayed to the thalamus (the brain’s relay station) and then to the gustatory cortex in the parietal lobe of the brain.

• Here, the brain interprets the signals, allowing us to perceive and identify different tastes

How Other Sensations Influence Taste

• Smell

• Texture

• Temperature

• Sight