Chapter 13: Special senses

NERVOUS SYSTEM: SPECIAL SENSES

Receptors are essential as they provide us with information about our body as well as our external environment. They are present in the skin, muscles, tendons, joints, viscera, blood vessels and sensory organs, and are sensitive or responsive to a stimulus. They can detect touch, pressure, stretch, heat, cold, blood chemistry, light and pain.

Classification of Receptors

Receptors can be grouped by the way they are distributed in the body, the type of stimulus they respond to, or by the origin of the stimulus.

Distributed in the body;

Receptor Type	Distribution in the body
Somatic Sense	Gives us information from the skin, muscles and joints.
Visceral sense	Gives us information from the internal organs and viscera.

Type of stimulus;

Receptor types	Stimulus
Mechanoreceptors	Touch, pressure, stretch.
Chemoreceptors	Chemicals.
Photoreceptors	Light.
Thermoreceptors	Hot and cold.
Nociceptors	Pain.

Origin of Stimulus

Receptor types	Origin of stimulus	Responsible for
Interoceptors	Internal organs.	Detecting heart rate, blood pressure, blood gas concentration and visceral pain.
Proprioceptors	Muscles, tendons and capsules.	Position and movements of the body.
Exteroceptors	External to the body.	Vision, hearing, taste, smell and skin.

Pain

Pain has its own pain receptors called nociceptors (a name derived from the word noxious). They occur in the entire body, except the brain, and are most numerous in the skin and mucous membranes.

The nociceptors can be broadly grouped into fast (A-delta fibres) or slow (C fibres) neurons. The fast pain neurons are myelinated and can conduct nerve signals at 30 meters a second and are responsible for 'acute pain' perceived at the time of injury. The slow pain neurons are unmyelinated and can conduct nerve signals at 2 meters a second and are responsible for the dull 'chronic pain' that follows an injury or is associated with such diseases as cancer or arthritis.

Nociceptors are unusual because they usually only respond to a strong stimulus, however, following an injury they become increasingly sensitive (hyperalgesia, Greek for "super pain”). When tissues become injured or inflamed they release chemicals such as histamine, that cause the nociceptors to become much more sensitive. This means that the affected nociceptors will now react to even a gentle stimuli.

Sensory organs are specialised receptors that are combined with other tissue to create an organ that provides us with specific information from our surroundings. They are innervated by the cranial nerves and deal with smell (chemoreceptors), taste (chemoreceptors), vision (photoreceptors), sound (mechanoreceptors) and balance (mechanoreceptors).

Special Senses	Receptors
Vision	Photoreceptors
Sound	Mechanoreceptors
Balance	Mechanoreceptors
Smell	Chemoreceptors
Taste	Chemoreceptors

Vision

Vision is important for our survival and communication. The paired eyes are the peripheral organs of vision. Their main function is to focus light onto the retina where there are specialisedphotoreceptors which respond to light within the visible spectrum (400-760 nm).

Anatomy of the Eye

Each eyeball is a sphere embedded in occular fat within the bony orbits of the skull; this provides protection as well as attachment points for its extrinsic muscles. It consists of tunica, optical and neural structures.

Tunica

The tunica form the outer and inner walls of the eyeball and consist of three layers: fibrous, vascular and neural. The outer fibrous tunica consists of the opaque sclera (white of the eye) behind, and the transparent cornea in front. The vascular tunica consists of the choroid behind, and the ciliary body and the iris in front. The neural tunica consists of the retina.

Name	Description	Function
Fibrous Tunica	Fibrous outer coat.
Sclera	This is the white of the eye found surrounding the entire eye except anteriorly at the cornea. The sclera becomes a perforated plate posteriorly and is pierced by the optic nerve as well as the retinal artery and vein. The sclera becomes continuous with the sheath around the optic nerve.	It is opaque preventing light from entering the eyeball anywhere other than at the pupil. It is strong and helps to maintain the shape of the eyeball. Provides attachment for the extra-ocular muscles.
Cornea	This projects from the anterior part of the eye and is transparent.	It bends light so that it enters the eye through the aperture of the pupil.
Vascular Tunica	Pigmented, vascular coat.
Choroid	This is a dark brown, rich network of blood vessels. It is present in the entire eye except anteriorly where it becomes continuous with the ciliary body.	It supplies all the other layers of the eye with oxygen and nutrients.
Ciliary body	This is an anterior thickened muscular portion of the choroid which sits just behind the iris forming a ring around the lens. From it project suspensory ligaments which attach it to the lens.	It supports the lens and iris. Adjusts the curvature of the lens. Secretes aqueous humour into the posterior chamber.
Iris	A pigmented muscular ring found around the aperture of the pupil.	It prevents light from entering the eyeball anywhere other than at the pupil. Changes the diameter of the aperture of the pupil.
Neural Tunica	Innermost, neural layer of the eye.
Retina	The inner most layer of the eye it is attached at the back to the optic disc and at the front to the ora serrata.	Contains the light receptors and associated neurons and fibres.

CROSS SECTION OF THE EYE

The retina is made up of four specialised layers; pigment, photoreceptor, inner nuclear and ganglion layers.

Name	Description	Function
Retina	The neural component of the eye.
Pigment layer	Dark brown outer layer of the retina.	It absorbs light to prevent it from reflecting inside the eye.
Photoreceptor layer	This consists of two types of light receptor, rods and cones.	Rods respond to black and white light only and only dim light. Cones respond to coloured light and only in bright light. Both receptors absorb the light and generate a chemical or electrical signal.
Inner nuclear layer	This consists of bipolar neurons.	They transmit the signals from the rods and cones to the ganglion cells.
Ganglion layer	The innermost layer of the retina containing ganglion cells. The axons of these cells converge at the optic disc to give rise to the optic nerve.	They receive the signals from the bipolar neurons and transmit them to the brain via the optic nerve. Some of the ganglion cells act as light receptors themselves, detecting light intensity for the control of pupil diameter.
Macula Lutea	A small area (3mm) of cells found on the retina at the back of the eye, directly in line with the lens. At its centre is the fovea centralis.	It is the area of the eye that provides the clearest vision.
Fovea centralis	A small area (1.5mm) occupying the macula lutea. The ganglion and inner nuclear layers lean away so that light can hit the cones (there are no rods present) directly.	The fovea is the point where vision is most accurate.
Optic disc	Found about 3mm medial from the macula lutea, this is where the axons of the ganglion cells of the retina converge.	It is the point where the ganglion cells leave the back of the eye to form the optic nerve.
Optic Nerve(II)	The optic nerve runs through the optic canal (foramen) to the middle cranial fossa to reach theoptic chiasma.	The optic nerve carries signals from the retina to the optic chiasma and tracts in the brain.

Optical structures of the eye

The optical structures of the eye are those that allow light to pass through and be focused on the retina.

Name	Description	Function
Cornea	This is the anterior part of the fibrous tunica of the eye. It is transparent and projects anteriorly.	It bends light so that it enters the eye through the aperture of the pupil.
Posterior Chamber	Found between the iris in front and the lens, suspensory ligament and ciliary body behind. It is filled with aqueous humour.	It allows light to pass through it.
Anterior Chamber	Found between the cornea in front and the iris behind. It is filled with aqueous humour.	It allows light to pass through it.
Aqueous Humour	A clear watery fluid secreted by the ciliary body and absorbed into the canal of Schlemm situated in the angles between the cornea and iris.	It fills the anterior and posterior chambers of the eye and allows light to pass through it.
Lens	A transparent elliptical structure, consisting of concentric layers of lens fibres (cells). It is held in place behind the aperture of the pupil by the suspensory ligament of the ciliary body.	Refracts light to a focal point on the retina. It is able to change shape to accommodate viewing objects at closer or further distances.
Vitreous body	Transparent jelly which fills the eyeball behind the lens.	Holds the retina against the wall of the eyeball while allowing light to pass through it.

Extrinsic Eye Muscles

The extrinsic muscles of the eye move the eyeball in almost any direction. They are innervated by the cranial nerves: trochlea (IV), abducens (VI) and oculomotor (III).

There are seven extraocular muscles:

Muscle	Innervated	Action
Levator palpebrae superioris	Oculomotor (III)	Elevates the upper eyelid.
Rectus superior	Oculomotor (III)	Moves the eye so that the cornea is directed upwards (elevation) and medially (adducted).
Rectus inferior	Oculomotor (III)	Moves the eye so that the cornea is directed downwards (depression).
Rectus medialis	Oculomotor (III)	Moves the eye so that the cornea is directed medially (adducted).
Rectus lateralis	Abducens (VI)	Moves the eye so that the cornea is directed laterally (abducted).
Obliquus superior	Trochlea (IV)	Depresses the posterior aspect of the eye, depressing the cornea.
Obliquus inferior	Oculomotor (III)	Depresses the posterior aspect of the eye, elevating the cornea.

Accessory Structures

These include the eyebrows, eyelashes, eyelids, conjunctiva and lacrimal apparatus, all of which function to protect the surface of our eyes. The eyebrows are thought to direct perspiration and rays of the sun away from the eyes, the eyelashes help to protect our eyes from airborne particles and the eyelids react rapidly to protect the eyes from injury. The conjunctiva and the lacrimal apparatus lubricate the eye as well as act as a bactericide.

Conjunctiva

The conjunctiva is a thin transparent film which covers the inside of the eyelids and is reflected onto the surface of the visible part of the sclera (it does not cover the cornea). The conjunctiva secretes oils and mucous that helps to lubricate the surface of the eye as well as keep it clean.

Lacrimal apparatus

This consists of the lacrimal gland, punctum, ducts, canal and sac. The lacrimal gland produces tears, which travel down tiny ducts in the upper eyelid to the surface of the eye, the conjunctiva. Here they lubricate the surface of the eye, supply it with nutrients and act as a bactericide. The tears begin to collect at the medial corner of the eye where there is a tiny hole on each eyelid called the lacrimal punctum. Each hole leads to a lacrimal canal, which drains the tears into the lacrimal sac which drains into the nasal cavity via the nasolacrimal duct.

LACRIMAL APPARATUS

Mechanism of Vision

Light hits the cornea where it is directed through the aperture of the pupil to the lens. The lens is convex and bends (refracts) the light so that it is focused on the retina at the back of the eye. The ciliary muscles adjust the curvature of the lens so that the light is focused correctly on the back of the retina, a process known as accommodation.

During the passage of light through the retina the visual field is reversed (image is flipped) so that the image appears upside down as well as the left side appearing on the right side of the retina.

Axons of the ganglion cells of the retina converge at the optic disc and form the optic nerve (II). The optic nerve runs through the optic canal (foramen) to the middle cranial fossa to reach theoptic chiasma. Here, half of the visual information crosses to the opposite side; fibres from the medial side of the retina enter the contralateral (opposite) optic tract whereas those from the lateral side of the retina remain uncrossed, travelling in the ipsilateral optic tract. These fibres continue in the optic tracts (which contain some fibres that have crossed-over and some that have not) to the brain. The fibres in the optic tracts that originate in the medial side of the retina terminate in the superior colliculi and pretectal nucleus. The fibres in the optic tracts that originate in the lateral side of the retina synapse at the thalamus (relay station). From here the fibres form the optic radiation which project to the visual cortex of the occipital lobe. Association tracts connect the visual cortex to the visual association area for interpretation.

Anatomy of the ear

External Ear

The external ear includes the auricle and the external acoustic meatus. The auricle sits on the side of the head and directs sound into the acoustic meatus. The external acoustic meatus is the opening into the s-shaped auditory canal, which directs sound onto the tympanic membrane (ear drum).

Name	Description	Function
External Ear	Consists of the auricle, external acoustic meatus and auditory canal.	Directs the sound to the middle ear.
Auricle	The auricles sit on either side of the head and are composed of and shaped by fibrocartilage which is covered with skin.	The shape of the auricles causes sound to be directed into the corresponding external acoustic meatus.
External Acoustic meatus	The external acoustic meatus is the opening between the auricle and the auditory canal.	It directs sound into the auditory canal.
Auditory Canal	The auditory canal is an S-shaped passage that begins at the external acoustic meatus. It travels about 4 cm through the tympanic bone to terminate at the tympanic membrane (ear drum).	It directs sound to the tympanic membrane (ear drum).

AURICLE

Middle Ear

The tympanic membrane separates the external ear from the middle ear; it is a thin, semitransparent concave sheet, fixed within a ring in the temporal bone and is commonly called the ear-drum. The middle ear, also know as the tympanic cavity houses the structures that amplify sound. These include the ossicles and auditory muscles. The ossicles are three tiny bones, themalleus (hammer), incus (anvil), and stapes (stirrup), which transmit sound from the tympanic membrane across the tympanic cavity. The tympanic membrane vibrates in response to a sound, this is transmitted to the malleus which it is attached to. The malleus articulates with the incus which then articulates with the stapes. Finally the footplate of the stapes bone articulates with the oval window which is the opening into the fluid filled inner ear. A round window in the inner ear can also be seen opening into the middle ear; this is closed off by asecondary tympanic membrane. This membrane moves out as the foot plate of the stapes moves into the oval window transmitting the pressure through the fluid of the inner ear. Theeustachian tube which connects the middle ear cavity to the throat (nasopharynx) allows the pressure in the cavity to be equalised.

Name	Description	Function
Middle Ear (tympanic cavity)	Houses the structures that function to amplify sound
Tympanic membrane	About 1 cm in diameter the tympanic membrane is a thin, semitransparent concave sheet, fixed within a ring in the temporal bone between the auditory canal and the middle ear.	It vibrates in response to sound.
Tympanic cavity	This is the cavity of the middle ear. It is connected to the pharynx (back of the throat) by the eustachian tube, which functions to equalize the pressure within the cavity.	It contains the ossicles.
Ossicles	Thee tiny bones; malleus, incus and stapes. The malleus is attached to the tympanic membrane and articulates with the incus. The incus articulates with the stapes and the stapes articulates with the oval window in the inner ear.	The ossicles transmit sound from the tympanic membrane across the tympanic cavity.
Oval window	An opening between the middle and inner ear; it articulates with the foot plate of the stapes.	Allows the foot plate of the stapes to transmit the sound vibrations from the middle ear to the inner ear.
Secondary tympanic membrane	Closes off the round window in the inner ear. It can be seen opening into the middle ear.	Transmits the pressure through the fluid of the inner ear.
Eustachian tube	Connects the middle ear cavity to the throat (nasopharynx).	Allows the pressure in the middle ear cavity to be equalised.

INTERNAL ANATOMY OF THE EAR

Inner Ear

The inner ear is embedded deep in the bony labyrinth of the temporal bone. This bony labyrinth can be split into three regions, the cochlear, vestibule and the semicircular canals. Within the bony labyrinth lies the membranous labyrinth, which also has cochlea, vestibule and semicircular components. The membranous labyrinth contains a fluid called endolymph and is separated from the bony labyrinth by a cushion of fluid called perilymph.

Name	Description	Function
Inner Ear	Bony and membranous labyrinth, containing the mechanoreceptors of hearing and balance.
Bony Labyrinth	This is a series of tunnels within the temporal bone; cochlea, vestibule and semicircular canals.	It contains the membranous labyrinth.
Cochlear	The bony part is a coiled tube of bone, which coils around a central pillar called a modiolus and resembles a snail shell.	It supports the membranous cochlear.
Vestibule	Central part of the bony labyrinth, its lateral wall containing the oval window.	Connects the inner ear with the middle ear and supports the membranous saccule and utricle.
Semicircular canals	Consists of superior, posterior, and lateral semicircular canals oriented at right angles to each other. They are connected to the vestibule via five ampullae.	Supports the semicircular ducts.
Membranous labyrinth	A series of membranous tubes containing endolymph; cochlea, vestibule and semicircular canals.	They contain the mechanoreceptors.
Cochlea	The membranous part is a membranous coiled tube which is split by two membranes into three chambers; scala vestibuli, cochlear duct and the scala tympani. The cochlear duct contains the organ of corti.	Organ of sound (the organ of corti detects the sound vibrations).
Vestibule	Two membranous sacs; saccule and utricle containing endolymph. The utricle is connected to the semicircular ducts via five openings.	Organ of balance, angular decelerations.
Semicircular canals	Three semicircular ducts, corresponding to the canals. They connect to the utricle via five openings where there are five groups of mechanoreceptors (cristae ampullaris).	Organ of balance, detects deceleration in all directions.

INNER EAR

Sound (Hearing)

Cochlea

The cochlea is the organ of hearing and consists of a bony and membranous part. The bony part is a coiled tube of bone, which coils around a central pillar called a modiolus and resembles a snail shell.

The bony cochlea supports the membranous cochlea which is spilt into three chambers, scala vestibuli, cochlear duct and the scala tympani by two membranes. The scala vestibuli is the superior chamber and is separated from the cochlear duct below, by the vestibular membrane. The scala tympani is the inferior chamber and is separated from the cochlear duct above, by the basilar membrane. The superior and inferior chambers both contain perilymph. The scala vestibuli begins near the oval window and spirals up to the apex where it communicates with the scala tympani via the helicotrema. The scala tympani spirals down from the apex to the round window, which is covered by the secondary tympanic membrane.

The cochlear duct is the middle chamber, sandwiched between the vestibular membrane above and basilar membrane below. It contains endolymph and the organ of corti. The organ of corti is located on the basilar membrane and consists of sound receptors (mechanorecptors) that detect sound along with supporting cells. The mechanoreceptors are composed of four rows of hair cells, each of which have numerous stereocilia projecting from them, with a gelatinous tectorial membrane lying above.

Mechanism of Hearing

When a sound is produced vibrations are transmitted from the tympanic membrane across the tympanic cavity via the ossicles to the inner ear. The vibrations are transmitted from the footplate of the stapes through the oval window to form waves in the perilymph of the cochlea. This causes the basilar membrane to move which causes the stereocilia on the hair cells of the organ of corti to move upwards and contact the overlying tectorial membrane. This stimulates the hairs cells to send a signal to the sensory neurons of the cochlear fibres below, which transmit the signal to the brain where the sound is interpreted and perceived.

Cochlear pathway

The dendrites of the cochlear neurons synapse with the hair cells at the basilar membrane, the axons of these neurons wind around the modiolus to form the spiral ganglion and become the cochlear nerve. The cochlear nerve travels to the cochlear nuclei in the medulla oblongata. There are ventral and dorsal cochlear nuclei which are functionally different and whose fibres follow a slightly different pathway.

The ventral cochlear nucleus enables us to determine the direction that sound is emanating from by comparing the timing and intensity (loudness) of the sound in each ear. The fibres from this nucleus project to the superior olivary nucleus of the pons. The fibres then travel in a tract called the lateral lemniscus to reach the inferior colliculus.

The dorsal cochlear nucleus enables us to distinguish between different frequencies, for example when listening to speech. The fibres from this nucleus project directly to the inferior colliculus via the lateral lemniscus.

From the inferior colliculus both sets of fibres project to the medial geniculate body of the thalamus. From the thalamus the fibres travel to the primary auditory cortex found on the anterior transverse temporal gyrus of temporal lobe.

NB Some fibres from both the ventral and dorsal nuclei cross to the opposite side of the brain via the trapezoid body where they then continue to travel in the lateral lemniscus to the inferior colliculus.

Equilibrium (Balance)

The vestibule and semicircular canals contain the mechanoreceptors responsible for balance and coordination also know as ‘equilibrium’. There are two types of equilibrium, static and dynamic. Static is how we perceive the head when the body is not moving and dynamic is the perception of motion (acceleration). Acceleration can be divided into linear and angular; linear is the change in velocity in a straight line and detected by the saccule and utricle. Angular acceleration/deceleration is a change in the rate of rotation and is detected by the semicircular canals.

Vestibule (static labyrinth)

The vestibule is the central part of the bony labyrinth and contains, in its lateral wall, the oval window connecting the inner ear with the middle ear. The saccule and utricle are two membranous sacs, containing endolymph, located within the vestibule. The saccule is the smaller sac and lies near the opening of the scala vestibuli; it has openings into the endolymphatic duct and the cochlear duct. The utricle lies in the posterosuperior part of the vestibule; it has five openings for the semicircular canals, and a duct connecting it to the saccule.

Both the saccule and utricle have a macula, a specialised oval patch of thickened cells, containing hair cells. The two macula lie at right angles to each other; that of the saccule (macula sacculi) lying vertically on the wall of the saccule and that of the utricle (macula utriculi) lying horizontally along the floor of the utricle. Each hair cell has many stereocilia as well as a single mobile cilia, all of which are embedded in the otolithic membrane. This is a gelatinous membrane containing otolithic granules (ear sand); tiny stones of calcium carbonate that give the membrane weight and enhance its sensitivity to gravity and motion.

The hair cells are the mechanoreceptors, which synapse with the vestibular fibres of the vestibulocochlear nerve.

Semicircular Canals

Posterior to the vestibule are the three bony semicircular canals; superior, posterior, and lateral. They are oriented at right angles to each other and connect to the vestibule via ampullae. Within the semicircular canals is the membranous part, the semicircular ducts, which all connect to the utricle via five openings. Within the ampullae are cristae ampullaris, which are mounds of mechanoreceptor hair cells. Each cell has many stereocilia as well as a single mobile cilia, all of which are embedded in the cupula. The cupula is a gelatinous projection that extends from the crista ampullaris to the roof of the ampulla.

Mechanism of Balance

Angular decelerations of the head cause a counterflow of endolymph, deflecting the cupola of each crista and bending the hairs (stereocilia/kinocilia on each hair cell). This causes a change in membrane potential of the affected receptor cell and causes the vestibular nerve to fire signals to the brain.

The semicircular canals (kinetic labyrinth) lie at right angles from each other meaning that deceleration is detected in all directions.

Vestibular pathways

The hairs cells of the saccule, utricle and crista ampullaris synapse at their base with fibres of the vestibular nerve. This nerve joins with the cochlear nerve to form the vestibulocochlear nerve, the vestibular component of which travels to the vestibular nuclei in the medulla oblongata as well as directly to the cerebellum.

There are four vestibular nuclei (superior, lateral, inferior and medial) that not only receive input from the three semicircular canals, the saccule, and the utricle, but also from the eyes,cerebellum and from the somatic sensory system. All of these signals are integrated together enabling us to coordinate head, eye and body movements so that we can maintain posture and be aware of our body movement and orientation so that we can coordinate motor functions.

TASTE

The surface (dorsum) of the tongue is split into oral and pharyngeal parts by a v-shaped sulcus (terminalis) near the back of the tongue. On the tongue surface the taste buds are associated with specialised areas called lingual papillae of which there are four types.

Taste cells are not neurons but chemosensory cells, which are capable of synaptic transmission, and synapse at their bases with fibres from one of the following cranial nerves; facial (VII),glossopharyngeal (IX) and vagus (X) nerves. The taste signal is relayed via the cranial nerves (VII, IX and X) to the solitary nucleus in the medulla oblongata which relays the signals to the cortex where we become conscious of taste.

Primary Tastes

We have 5 primary (basic) taste sensations: sweet, salty, sour, bitter and umami (meaty). Although all these tastes can be detected over the entire tongue surface, some regions are said to be more sensitive to certain tastes ( the tip of the tongue is supposed to be most sensitive to sweet, the anterior sides to salty, the posterior sides to sour and the back mainly to bitter). Many of the tastes we perceive as taste sensations are actually olfactory sensations. Taste is a combination of the primary taste sensations, smell, texture and temperature.

OLFACTORY

Our sense of smell is highly sensitive and the average person can distinguish between 2 to 4 thousand odours. An odour is sensed on the olfactory mucosa, a strip of mucous membrane located on the roof of the nasal cavity (superior concha and nasal septum). The olfactory mucosa has millions of olfactory neurons (cells). Olfactory cells are unusual as they are the only neurons that come into contact directly with the external environment; they are replaced every 40-60 days by stem cells.Smell

Olfactory cells are shaped like a mallet, and from its handle end project 10 to 20 cilia called olfactory hairs. These hairs are embedded in the olfactory mucosa and bind with the odour molecules that get trapped in the mucus layer as we breathe them into the nasal cavities. The mallet end of the cell tapers to become an axon which groups together with other axons to become a fascicle (nerve).

Each olfactory nerve ascends to leave the nasal cavity via small holes in the cribriform plate of the ethmoid bone and synapse in the olfactory bulb with the mitral cells. Mitral cell axons form the olfactory tracts, which travel to the brain.

The fibres of the olfactory tracts travel to the temporal lobe, the hypothalamus and limbic system, which control the autonomic reflexes such as salivating and vomiting. Fibres also travel to the thalamus, where they are relayed to an association area in the cerebral cortex (orbitofrontal) where the signals are integrated with those from taste and sight to give us an overall conscious perception of smell.