Human Biology: The Senses of Sight, Hearing, and Equilibrium
Sense of Sight
Vision is arguably one of the most important senses, as it provides us with the most information about our surroundings.
The structures related to vision are:
- Accessory Structures
- Eyeball
- Optic Nerve
- Brain
Accessory Structures
Eyelids
The upper and lower eyelids cover the eyes during sleep, protect them from excessive light and foreign matter, and spread lubricating secretions over the eyeballs.
Eyelashes and Eyebrows
Eyelashes arise from the edge of each eyelid and are composed of numerous hairs. Together with the eyebrows, which are located in an arc above the eyelids, they help protect the eyes from the penetration of foreign bodies, sweat, and direct sunlight.
The hairs of the eyelashes are associated with their hair follicles, called glands of Zeis (sebaceous glands involved in the lubrication of the eye). When the glands of Zeis become infected, they form a stye.
Lacrimal Apparatus
The lacrimal apparatus is a set of structures that produce and drain tears.
- Lacrimal Gland: Almond-shaped, with 6 to 12 tear ducts that drain tears onto the surface of the conjunctiva of the upper eyelid.
- From there, tears move to two small openings, the tear ducts, then pass to the lacrimal sac to empty into the nasal cavity.
- Tear Fluid: An aqueous solution containing salts, mucus, and a bacterial enzyme called lysozyme (which breaks down bacterial cell walls).
The functions of tear fluid are to clean, lubricate, and moisten the eyeball.
- Tears are removed by evaporation and drainage through the tear ducts.
- If an irritant reaches the conjunctiva, the lacrimal glands are stimulated, tear secretion increases, and tears accumulate in the eye. This is called watery eyes.
- This is a protective mechanism, as the tears help flush out the irritant.
Conjunctiva
The conjunctiva is a thin, protective mucous membrane consisting of stratified epithelium with numerous goblet cells.
There are two types of conjunctiva:
- Palpebral Conjunctiva: Lines the inner surface of the eyelids.
- Bulbar Conjunctiva: Extends from the eyelids to the surface of the eyeball, covering the sclera.
When the blood vessels of the bulbar conjunctiva are dilated and filled with blood due to infection, the eyes appear red (as in conjunctivitis).
Eyeball
The eyeball is about 2.5 cm in diameter. Only one-sixth of it is exposed; the rest is hidden and protected by the orbit that houses it.
Anatomically, the wall of the eyeball is divided into three layers:
- Fibrous Layer (Fibrous Tunic)
- Vascular Layer (Vascular Tunic)
- Nervous Layer (Retina or Nervous Tunic)
Fibrous Tunic
This is the outer covering of the eyeball. It consists of the cornea in the anterior region and the sclera in the posterior region.
- Cornea: A transparent and avascular fibrous covering that covers the colored iris and helps focus light.
- Sclera (White of the Eye): A cover of dense connective tissue that covers the eyeball, giving it rigidity.
The sclera is pierced only by the optic nerve in its posterior region.
Vascular Tunic
This is the middle layer of the eyeball wall and is composed of:
- Choroid: Highly vascular, located in the back, and covers most of the back of the inner surface of the sclera.
The choroid provides nutrients and has a blackish color due to the presence of melanocytes. These melanocytes contain melanin, which is responsible for absorbing scattered light rays within the eyeball.
At the front, this layer thickens, forming:
- Ciliary Body: Formed by the ciliary processes (protrusions containing capillaries that secrete aqueous humor).
The ciliary body also contains the ciliary muscle, a circular band of smooth muscle that changes the shape of the lens to accommodate near or far vision.
- Iris: The colored portion of the eyeball. It is a flattened, donut-shaped structure suspended between the cornea and the lens.
The iris is composed of circular and radial smooth muscle cells. Its central hole is the pupil, which regulates the amount of light entering the eyeball. Sympathetic impulses open the pupil, while parasympathetic impulses close it.
Nervous Tunic (Retina)
This is the innermost layer of the eyeball and represents the beginning of the optic pathway.
The nervous tunic includes:
- Optic Disc: The spot where the optic nerve leaves the eyeball.
- Blood Vessels: Form a bundle with the optic nerve.
These include:
- Central Retinal Artery
- Central Retinal Vein
Both accompany the optic nerve at the optic disc and supply blood to the retina.
- Retina: Contains two layers:
- Pigment Epithelium: The non-visual portion. It consists of a layer of epithelial cells containing melanin, which absorbs scattered light rays, preventing reflection and ensuring a sharp, clear image.
- Albinos have less melanin in the pigment layer (fewer melanocytes) and therefore perceive moderate light as blinding.
- Neural Portion: The visual portion. It is composed of three layers of retinal neurons:
- Photoreceptor Layer
- Bipolar Cell Layer
- Ganglion Cell Layer
Between these three layers of cells are horizontal cells and amacrine cells.
Photoreceptor Layer
Photoreceptors are specialized cells that begin the process of converting light rays into nerve impulses.
There are two types of photoreceptors:
- Rods: Elongated and cylindrical, responsible for black and white vision (e.g., night vision). They are stimulated by different light intensities, allowing us to see in low light conditions.
- Cones: Small and slightly tapered, providing color vision and visual acuity. They are found mainly in the central fovea, a depression in the macula lutea. This depression is the center of the posterior chamber of the eye.
There are three types of cones, each stimulated by different wavelengths:
- Green Cones: Sensitive to green light (wavelengths around 575 nanometers).
- Blue Cones: Sensitive to blue light (wavelengths around 450 nanometers).
- Red Cones: Sensitive to red light (wavelengths around 700 nanometers).
Information from these photoreceptors is passed to the next layer of cells, the bipolar cells, and then to the ganglion cells. The axons of the ganglion cells extend into the optic disc and leave the eyeball to form the optic nerve.
- Amacrine Cells: Excited by bipolar cells and connect with ganglion cells, signaling changes in illumination on the retina.
- Horizontal Cells: Interneurons that connect photoreceptors and bipolar cells, modifying the information passed to bipolar and ganglion cells.
- The axons of the ganglion cells leave the eyeball as the optic nerve, cross at the optic chiasm, enter the brain, and project to the thalamus, which relays the visual information to the visual cortex.
Physiology of Vision
The first step in visual transduction is the absorption of light by photopigments, colored proteins that undergo structural changes when they absorb light.
- Rods: Contain a single photopigment called rhodopsin.
When light strikes rhodopsin, it breaks down into opsin (the protein component) and cis-retinal (the light-absorbing part, capable of isomerization – it changes from cis-retinal to trans-retinal).
Cis-retinal is a derivative of vitamin A.
- Cones: Contain three types of photopigments:
- Photopsin for green
- Photopsin for blue
- Photopsin for red
These photopsins break down into opsins (the protein component) and cis-retinal.
Stimulation of these photopigments triggers a cascade reaction that leads to the production of second messengers, which affect sodium channels in the photoreceptor cells.
The Lens
The lens is an avascular structure composed of a protein called crystallin, arranged in layers like an onion.
It is transparent and located between the pupil and the iris, held in place by suspensory ligaments. Loss of transparency of the lens is called a cataract.
The lens divides the eye into two cavities:
- Anterior Cavity: The space in front of the lens, divided into:
- Anterior Chamber
- Posterior Chamber
These chambers are filled with aqueous humor, which is secreted by the ciliary processes of the iris and contributes to the nutrition of the lens.
Intraocular pressure is due to the aqueous humor.
Changes in intraocular pressure are referred to as glaucoma.
- Posterior Cavity: Contains vitreous humor, a gel-like substance formed during embryonic development and not replaced throughout life. It helps prevent the eyeball from collapsing.
Formation of Images
When light travels from one medium (e.g., air) to another medium of different density (e.g., water), it is refracted, meaning it undergoes a change in direction.
When light enters the eye, it is refracted by the cornea, lens, and aqueous humor to be focused on the retina.
- Lens: Responsible for fine focus and shifting focus from distant to near objects. The lens is convex on both sides, refracting light rays so they converge and form a focused image on the retina.
The refractive power of the lens increases as its curvature increases.
When the eye focuses on a near object, the lens becomes more curved. This increased curvature of the lens is called accommodation.
Eyestrain or presbyopia is the loss of accommodation of the lens with age.
Pathology as a Result of Refraction and Accommodation
A normal eye is called emmetropic. However, there are conditions such as myopia (nearsightedness), in which the image forms in front of the retina. This requires diverging lenses to spread out the light rays before they reach the cornea.
Another condition is hyperopia (farsightedness), in which the image forms behind the retina. This can be corrected with converging lenses.
Myopia and hyperopia are caused by deformities of the lens or the shape of the eyeball.
In our visual field, we have a blind spot that corresponds to the exit point of the optic nerve from the eyeball. This is due to the absence of rods and cones (photoreceptor cells) in this region.
Hearing
One of the main functions of the ear is to convert sound waves into vibrations that stimulate mechanoreceptors, which then carry auditory sensations to the cortex of the brain.
The ear has three distinct parts, which are interconnected and each has a specific function in the processing of sound:
- External Ear
- Middle Ear
- Inner Ear
The ear contains receptors for both balance and sound waves.
External Ear
The external ear is responsible for collecting sound waves and directing them to the middle ear.
It consists of:
- Pinna (Auricle)
- Ear Canal
- Eardrum (Tympanic Membrane)
Pinna (Auricle)
The pinna is made of elastic cartilage covered by skin and is anatomically divided into:
- Helix: The upper, curled edge of the ear.
- Lobule: The fleshy, bottom part of the ear.
The pinna continues into the:
- External Auditory Canal: Contains hairs and ceruminous glands that secrete earwax (cerumen).
This combination of hair and wax prevents foreign substances from entering the ear canal.
Eardrum (Tympanic Membrane)
The eardrum is a thin layer of fibrous connective tissue located between the ear canal and the middle ear.
Middle Ear
The middle ear amplifies the signal from sound waves and transforms them into mechanical vibrations.
The middle ear consists of:
- Oval Window
- Round Window
- Eustachian Tube
- Ossicles (Malleus, Incus, and Stapes)
The middle ear is a small, air-filled cavity lined by epithelium.
It is separated from the outer ear by the eardrum and from the inner ear by a thin bony wall with two membrane-covered openings: the oval window and the round window.
The anterior wall of the middle ear has an opening that connects to the Eustachian tube (auditory tube), which links the middle ear to the nasopharynx.
Infections can travel from the throat to the ear through the Eustachian tube. It is normally closed but opens during swallowing or yawning. Its function is to equalize middle ear pressure with atmospheric pressure.
- Three small bones called auditory ossicles cross the middle ear and are attached to it by ligaments. Their names reflect their shapes:
- Malleus (Hammer): Attached to the inner surface of the eardrum.
- Incus (Anvil): Articulates with the malleus and the stapes.
- Stapes (Stirrup): Attached to the oval window, a membrane separating the middle ear from the inner ear. Below the oval window is another membrane-covered opening, the round window, which provides an outlet for sound vibrations.
- The middle ear also contains two small muscles:
- Tensor Tympani: Increases the tension of the eardrum to protect the ear from loud sounds.
- Stapedius: Dampens large vibrations caused by loud sounds.
Inner Ear
The inner ear is also called the labyrinth because of its complex series of tubes.
It is the region where sound is converted into nerve impulses.
The inner ear has three main areas:
- Semicircular Canals
- Vestibule (responsible for balance)
- Cochlea (responsible for hearing)
This entire labyrinth is contained within the bony labyrinth, which is covered by periosteum and filled with a fluid called perilymph.
The bony labyrinth is the hard outer wall surrounding the inner ear structures, including the semicircular canals, vestibule, and cochlea.
Within the bony labyrinth lies the membranous labyrinth, which is surrounded by perilymph and filled with endolymph.
The central portion of the bony labyrinth is oval-shaped and called the vestibule. It contains two structures of the membranous labyrinth:
- Utricle
- Saccule
The three semicircular canals project upward from the vestibule.
- The cochlea, a spiral-shaped bony structure, extends from the vestibule. It is internally divided into three channels by partitions:
- Scala Vestibuli: The upper channel, which connects to the oval window.
- Scala Tympani: The lower channel, which ends at the round window.
Both the scala vestibuli and scala tympani are filled with perilymph and connect at the apex of the cochlea through an opening called the helicotrema.
- Cochlear Duct (Scala Media): The middle channel, which contains endolymph.
The vestibular membrane separates the cochlear duct from the scala vestibuli, and the basilar membrane separates the cochlear duct from the scala tympani.
The Organ of Corti, responsible for hearing, rests on the basilar membrane.
The Organ of Corti consists of supporting cells that hold hair cells, the receptors for auditory sensations. These hair cells have long projections (stereocilia) that extend into the endolymph of the cochlear duct.
Above these projections is the tectorial membrane, a flexible, gel-like membrane that covers the hair cells.
The hair cells synapse with sensory neurons of the cochlear branch of the vestibulocochlear nerve (eighth cranial nerve).
Physiology of Hearing
Sound vibrations are transmitted from the ear canal and strike the eardrum.
These vibrations cause the eardrum to vibrate, which in turn moves the malleus. The malleus moves the incus, which then moves the stapes. The stapes strikes the oval window, creating pressure waves in the perilymph of the scala vestibuli. These pressure waves cause the vestibular membrane to vibrate, which in turn creates pressure waves in the endolymph of the cochlear duct and causes the basilar membrane to vibrate.
The wave is transmitted through the cochlear duct to the scala tympani, where it dissipates against the round window.
The movement of the perilymph and the pressure waves it creates cause the tectorial membrane to move against the hair cells’ stereocilia. Depending on the frequency of the sound vibrations, certain regions of the basilar membrane vibrate with greater intensity than others. Each segment of the basilar membrane is tuned to respond to a specific frequency.
The bending of the stereocilia stimulates the hair cells to release neurotransmitters, which excite the sensory neurons of the cochlear branch of the vestibulocochlear nerve. These neurons carry the auditory information to the thalamus, which relays it to the auditory cortex in the brain.
Physiology of Equilibrium
The vestibular apparatus is responsible for maintaining balance.
It consists of:
- Utricle
- Saccule
- Semicircular Canals
The utricle and saccule are involved in static equilibrium, sensing the position of the head relative to gravity and detecting linear acceleration and deceleration.
The semicircular canals are involved in dynamic equilibrium, maintaining balance during rotation or sudden movements.
Static Equilibrium
The utricle and saccule detect head movements in three dimensions and relay this information to the brain.
Each structure is filled with endolymph and contains sensory hair cells connected to receptor cells.
When the head moves, the endolymph pushes against the hair cells, which convert the pressure into electrical signals that are sent to the brain as nerve impulses.
Within the utricle and saccule are specialized regions called maculae. These are thickened areas containing two types of cells:
- Hair Cells
- Supporting Cells
Movements of the maculae provide information about head movements and linear acceleration, generating action potentials in the hair cells.
Hair cells have stereocilia on their apical surface.
Supporting cells secrete a gelatinous substance onto the apical surface, forming the otolithic membrane. This membrane contains protein and calcium carbonate crystals (otoliths).
The maculae of the utricle and saccule are oriented perpendicular to each other, so any change in head position alters the pressure on the otolithic membrane.
Movement of the otolithic membrane stimulates the hair cells, which send impulses to the vestibular branch of the vestibulocochlear nerve.
These nerve fibers carry the impulses to the brain, providing information about head position and changes in the force of gravity.
Stimulation of the maculae also triggers postural reflexes, muscle responses that help maintain balance.
Dynamic Equilibrium
Dynamic equilibrium is mediated by the semicircular canals, which detect rotational acceleration.
Each semicircular canal connects to the vestibule and has a dilated end called an ampulla.
Within each ampulla is a crista ampullaris, which contains hair cells and supporting cells.
The gelatinous layer covering the hair cells in the crista ampullaris is called the cupula.
The mechanism of dynamic equilibrium is similar to that of static equilibrium, with impulses also traveling through the vestibular branch of the vestibulocochlear nerve.
However, the cupula does not contain otoliths.
The semicircular canals are oriented perpendicular to each other, allowing them to detect movement in all three planes.
When the head rotates, the semicircular canals move with the body, but the endolymph lags behind due to inertia. This causes the cupula to bend, stimulating the hair cells and generating action potentials that are transmitted through the vestibulocochlear nerve to the brainstem and then to other brain areas for interpretation and response.
When a person stops rotating, the endolymph continues to move briefly, causing the cupula to bend in the opposite direction. This allows the dynamic equilibrium system to detect changes in the direction and speed of rotation.