Sensory Organs - Eye & Ear

 

The body's sensory organs serve as windows for the brain, keeping it aware of what is going on in the outside environment. There are five main senses: sight, hearing, taste, smell, and touch. There are organs associated to these senses that receive information and send it to the brain, allowing the body to act on it. Sensory organs detect changes in the environment and give messages to the central nervous system. These signals are processed and integrated with other information in the CNS to produce a perception that may result in a response change. Our sensory organs allow us to perceive changes in our surroundings and alter our behaviour accordingly.

Eye

The eyes are complicated sensory organs. They collect information about the surroundings, which the brain interprets to create an image of what appears in the field of vision. Each eye contains a layer of receptors, a lens system that focuses light on these receptors, and a network of nerves that transmit impulses from the receptors to the brain.

Anatomy of the eye:

The eyes are spherical, fluid-filled structures. Anatomically, the eye is composed of three layers: fibrous tunic, vascular tunic, and retina. The fibrous tunic is the outer layer of the eyeball, made up of the anterior cornea and posterior sclera. The sclera is a layer of dense connective tissue composed primarily of collagen fibers and fibroblasts. It gives shape and protects the inside parts. Light cannot enter through the sclera. It is changed anteriorly to generate the clear cornea, which allows light rays into the eye.

The cornea is a transparent layer that protects the coloured iris. Because it is covered, the cornea helps to focus light on the retina. The outer surface is made up of squamous epithelium. The cornea's middle coat is made up of collagen fibers and fibroblasts, whereas the inner surface is a simple squamous epithelium. The vascular tunic is the center layer of the eyeball. It consists of three parts: the choroid, ciliary body, and the iris. The iris, ciliary body, and choroid are collectively known as the uvea. The choroid is the circulatory layer that transports oxygen and nutrients throughout the eye.

The choroid layers create the ciliary body and iris in front of the eye. The iris is the pigmented and coloured part of the eye. The pupil is a circular aperture in the iris's center that allows light to enter the eye's interior. The iris contains both circular and radial muscle, which constrict and dilate the pupil. It functions similarly to the diaphragm of a camera, dilating and narrowing the pupil to admit more or less light into the eye. The autonomic nerve system controls the iris muscles.

The ciliary body is placed behind the iris and produces aqueous fluid, which fills the front of the eye. It consists of two key components: the ciliary muscle and the capillary network, which creates aqueous humor. The ciliary muscle controls the strength of the lens. The capacity to change the strength of a lens is known as accommodation. When the ciliary muscle relaxes, the suspensory ligaments pull the lens, causing it to flatten slightly. However, when the muscle contracts, the lens becomes more spherical. As a result, as the spherical lens's curvature rises, so does its strength. When the eye is focused on a nearby object, the ciliary muscles modify the focal length so that the picture is recreated on the retina and we can see the thing clearly.

The fundus refers to the eye's inner surface opposite the lens. It comprises the retina, optic disc, macula and fovea, and posterior pole. The retina is the eye's light-sensitive inner layer, covering approximately 65% of its internal surface. It contains three layers of excitable cells.

1. Rods and Cons - outermost layer containing photoreceptor cells
2. Bipolar cells - middle layer containing associated interneurons
3. Gnaglion cells - inner layer

Axons of the ganglion cells join to form Optic Nerve. The point on the retinal at which the optic nerve leaves and through which blood vessels pass is the optic disc. The region is often called the Blind Spot, where no image can be detected as it has no rods & cons.

Chambers of the eye:

The human eye is similarly divided into two primary chambers: anterior and posterior. Aqueous humor fills the gap in the anterior section. The anterior segment contains two chambers: the anterior chamber (between the cornea and the iris) and the posterior chamber (between the iris and the lens). The ciliary body's capillary network produces aqueous humor, a clear protein-free liquid that nourishes the cornea and iris.

The posterior portion includes The vitreous chamber lies between the lens and the retina. The vitreous chamber is filled with vitreous humor, a gelatinous material. It helps the eyeball keep its spherical form. Aqueous fluid bathes one side of the lens, while vitreous humor supports the other. It lacks blood circulation yet is metabolically active. It absorbs nutrients from the aqueous humor.

Protective mechanism:

Several systems serve to protect the exposed area of the eye from harm. The eyelids operate as shutters, shielding the exposed area of the eye from external injury sources. Frequent spontaneous blinking of the eyelids helps to disseminate the lubricating, cleaning, and antibacterial tears. The lacrimal gland produces tears continually. It's a fluid solution combining salts, mucous, and lysozyme. The fluid protects, cleans, lubricates, and moisturizes the eyeball. The eyes are also equipped with protective eyelashes, which collect small, airborne particles like dust before it enters the eye.

Photoreceptor cells:

There are two types of photoreceptor cells - Rods and Cones. Each photoreceptor cell is divided into two segments: the outer segment and the inner segment, which includes a nuclear region and a synaptic terminal. The outer segment detects light stimuli. The inner segment is densely packed with mitochondria, which is where the photosensitive chemicals are synthesized. Synaptic terminals store and release neurotransmitters. The outer segment is formed like a rod and a cone. Each rod has a stack of discs, which are flattened membrane-bound intracellular organelles. Cones often have thick inner and conical outer segments. The saccules of the cones are created by folding the membrane of the outer segment. The light-sensitive photopigments are stored in the saccules and discs.

Rods let us to see in low light. Because rods do not give color vision, we can only perceive black, white, and various hues of grey in low light. Brighter light activates cones, resulting in color vision. Three types of cones are present in the retina -

1. Blue cones, sensitive to blue light
2. Green cones, sensitive to green light
3. Red cones, sensitive to red light.

Colour vision resulte from the stimulation of various combinations of these three types of cones.

Rhodopsin is the name of the photosensitive pigment found in rod cells. It is made up of retinal (the light-absorbing component of photopigment), an aldehyde of Vitamin A, and a transmembrane protein called opsin. In cone cells, many types of opsins interact with retinal to generate pigments known as photopsins.

Ear

The human ear functions as an incredible transducer, transforming sound energy to mechanical energy and then to a nerve impulse that is relayed to the brain. The ear has three parts: the outside, middle, and inner ear. Each region of the ear has a specific function in the task of hearing and interpreting sounds.

The outer ear separates to collect and transport sound into the middle ear. The middle ear converts the energy of a sound wave into internal vibrations of the middle ear's bone structure, which are then transformed into a compressional wave in the inner ear. The inner ears convert the energy of a compressional wave within the inner ear fluid into nerve impulses that can be sent to the brain.

Outer Ear:

The external ear is made up of the pinna and the external auditory meatus, also known as the ear canal. The external auditory meatus is bordered with skin containing several modified sweat glands called ceruminous glands, which exude wax (cerumen). The outer ear collects sound and funnels it to the eardrum. The eardrum serves as the border between the outer and middle ears. It is a thin translucent connective tissue membrane covered by skin on the outside and a mucosa inside. Sound waves cause the eardrum to vibrate, which then distributes sound energy to the small bones of the middle ear, causing them to vibrate as well.

Middle Ear:

The middle ear is made up of an air-filled region within the temporal bone. It is separated from the external ear by the tympanic membrane and from the internal ear by a thin bone partition with two small membrane-covered holes. The middle ear enters the nasopharynx via the eustachian or auditory tube. The tube is generally closed, but it opens while swallowing, chewing, and yawning to keep the air pressure on both sides of the eardrum equal. Such apertures allow air pressure in the middle ear to equilibrate with ambient pressure, resulting in equal pressure on both sides of the tympanic membrane.

The middle ear has three auditory ossicles: malleus, incus, and stapes. The bones are the malleus, incus, and stapes, often known as the hammer, anvil, and stirrup, respectively. The tympanic cavity refers to the small cavities that surround the middle ear bones. The malleus, the first bone, is connected to the tympanic membrane, and the stapes, the last bone, to the oval window. Auditory ossicles are a series of levers connected together and powered by the eardrum. The movements of the eardrum will cause the hammer, anvil, and stirrup to move at the same frequency as the sound wave. The stirrup is attached to the inner ear, thus the vibrations of the stirrup are conveyed to the fluid of the inner ear, resulting in a compression wave within the fluid.

Internal Ear:

The internal or inner ear is also known as the labyrinth due to its complex network of canals. It is divided structurally into two parts: an exterior bony labyrinth and an interior membranous labyrinth. The bone labyrinth is a network of channels filled with a fluid called perilymph. The membranous labyrinth is located inside these bone tubes. The membrane labyrinth closely resembles the form of the bone channels and is filled with endolymph. The bone labyrinth is a series of cavities separated into two components -

1. Cochlea (contains hearing receptors)
2. Vestibular apparatus (contains receptors that respond to sense of equilibrium)

1. Cochlea: The cochlea is a coiled tube. Sections of the cochlea reveal that it is separated into three chambers: the cochlear duct, scala vestibuli, and scala tympani. The cochlear duct is the continuation of the membrane labyrinth into the cochlea, and it is filled with endolymph. The scala vestibuli is the channel above the cochlear duct that finishes with the oval window. The passage below is the scala tympani, and it ends at the round window. The vestibular membrane divides the cochlear duct from the scala vestubli, whereas the basilar membrane separates it from the scala tympani.The organ of Corti rests on the basilar membrane.

This structure houses extremely specialized auditory hair cells. The hair cells are organized in four rows, with three rows of outer hair cells and one row of inner hair cells. Stereocilia, or microvilli, are around 100 hairs that protrude from the surface of each hair cell. When fluid motions in the inner ear cause mechanical deformation of hair cells' surface hairs, neural impulses are generated.

Physiology of hearing -

• The auricle directs sound waves into the external auditory canal

• Sound waves strike the eardrum, the air causes the eardrum to vibrate back and forth. The vibration depends on the intensity and frequency of the sound waves.

• The central area of the eardrum connects to the malleus, which also starts to vibrate. The vibration is transmitted from the malleus to the incus and then to the stapes.

• As the stapes vibrate, it causes vibration in the membrane of the oval window.

• The vibration of the oval window sets up fluid pressure waves in the perilymph of the cochlea.

• Pressure waves are transmitted from the scala vestibuli to the scala tympani and eventually to the round window.

• As the pressure waves deform the walls of the scala vestibuli and scala tympani, they also push the vestibular membrane back and forth, creating pressure waves in the endolymph inside the cochlear duct.

• The pressure waves in the endolymph cause the basilar membrane to vibrate, which moves the hair cells of the spiral organ against the tectorial membrane. Bending of the stereocilia of hair cell ultimately leads to the opening of mechanically gated ion channels that allow cations (primarily potassium and calcium) to enter the cell. Unlike many other electrically active cells, the hair cell itself does not generate an action potential. Instead, the influx of cations generates a receptor potential (a type of graded potential). This receptor potential opens voltage gated calcium channels; calcium ions then enter the cell and trigger the release of neurotransmitters which generate action potentials in the nerve. The action potentials propagated to the brain. These neural signals are perceived by the brain as sound sensations.

The hair cells convert mechanical vibrations into electrical signals. High-intensity sound waves create bigger vibrations in the basilar membrane, resulting in a higher frequency of nerve impulses reaching the brain. The inner hair cells are the major sensory receptors in the auditory nerves that produce action potentials when triggered by fluid movements.

2. Vestibular appratus: The vestibular apparatus is made up of semilunar canals and otolith organs. The otolith organs comprise the oval central section of the bone labyrinth. It is made up of two sacs: the utricle and the sacculus, which are joined by a tiny duct. Both of these structures have a sensory epithelium known as the macula. The macula consists of two types of cells: hair cells and support cells. Hair cells' hairs consist of one cilium and a tuft of 20 to 50 microvilli.

Each ear has three semicircular canals, each of which is at roughly right angles to the outer two. At one end of each canal is a swelling protrusion known as the ampulla. The ampulla has a receptor component known as the crista ampullaris. Each crista contains a collection of hair cells and supporting cells. The semicircular canals sense the rotational acceleration and deceleration of the head. Hair cells are a type of mechanoreceptor.

Sense of equilibrium -

There are two sorts of equilibrium: static (gravitational) and dynamic (rotational). The mechanoreceptors in the semicircular canals sense dynamic balance, which is concerned with the head's rotational or angular movements. The mechanoreceptors in the vestibule's utricle and saccule perceive static balance, which refers to the movement of the head in vertical or horizontal planes.