1 General Senses

Sensory receptors are located throughout the body. They may be free nerve endings or nerve endings surrounded by a capsule.

Free nerve endings (including the Merkel disc in the skin and hair follicles) respond to cold, heat, vibration, pressure, touch, itch, pain and stretch.

Encapsulated nerve endings include Meissner’s corpuscles (in hairless skin), Pacinian corpuscles (in skin, around the bones and joints), Ruffini’s corpuscles (dermis, hypodermis, joints), muscle spindles (skeletal muscle) and Golgi tendon organs (tendons).

2 The Eye

Our eye is the structure that detects visible em radiation (light). A benefit of having two eyes is stereoscopic vision (we perceive three dimensions), so we can estimate distances and the position of objects in space.

  • Fibrous tunic (sclera, cornea), aqueous humour (in the anterior cavity, between the lens and cornea)

  • Vascular tunic (choroid, ciliary body and suspensory ligament, pupil, iris, lens), vitreous humour (posterior cavity, post. to lens)

  • Sensory tunic (retina = pigmented layer, rods and cones, bipolar cells, ganglion cells), macula lutea, fovea centralis (cones), except at blind spot (optic disc)

(Light photons traverse the cornea, aqueous humour, pupil, lens, vitreous humour, axons, ganglion cells, amacrine cells, bipolar cells and horizontal cells to strike the rods and cones.)

Light enters the posterior chamber of the eye through the black pupil, then passes through the lens and then traverses the vitreous. The diameter of the pupil may be varied by the colourful iris (a sphincter muscle under autonomic nervous system (ANS) control), from about 3 mm in diameter in bright light to about 8 mm in the dark. This change will vary the amount of light entering the eye by a factor of 7; the retina is able to adapt to different lighting conditions also (the anaxonic horizontal cells and amacrine cells of the retina also have a role in the eye’s adjustment to dim or bright conditions).

The retina is the light-sensitive part of the eye. It converts light energy into electrical nerve impulses, which are sent to the brain. The retina covers the back half of the eyeball, enabling wide-angle vision.

Most vision is restricted to a small area called the macula lutea (yellow spot) and all detailed vision to a very small area called the fovea centralis (0.4 mm in diameter). At the fovea, the several layers of nerve tissue that overlie the retina are pushed aside so light strikes the cones directly (without having to traverse axons, ganglion cells and bipolar cells).

Rods and cones are the two types of photoreceptor cells in the retina.

The cones are used mainly for daylight (photopic) vision and colour vision and are primarily found in the fovea (but occur throughout the retina); each cone in the fovea has its own nerve connection to the brain (hence, vision is acute here), while in the rest of the retina, several cones share one nerve fibre.

The rods are used for low light (scotopic) vision (e.g. at night) and peripheral vision. They cover most of the retina and are much more numerous than cones. Hundreds of rods share the same nerve fibre, so rods have a poor ability to resolve close sources of light.

The blind spot on the retina is without rods or cones. Here, the blood vessels and axons of ganglion cells come together as the optic nerve and leave the eye at the “optic disc”.

Visual Pathway

Axons from retinal ganglion cells, issue from each eye as optic nerves (II). At the optic chiasma, fibres in the left (L) and right (R) optic nerves from the medial aspect of each eye cross over and continue (along with those from the lateral aspect of the contralateral eye) as L and R optic tracts and synapse with neurons in the lateral geniculate body (of the thalamus).

(Superior colliculi are involved in pupillary and eye movement reflexes via collateral connection.)

Many axons then continue as optic radiation of fibres into the primary visual cortex (in the occipital lobes).

(Note: Images of objects to our right fall onto the left half of the retina, and nerves from the left side of each eyeball go to the left side of the brain.)

Lenses

A lens is any transparent object with curved faces. If the curve is “outwards”, making the middle of the lens fat and the edges thin, the lens is convex; the opposite curve is concave.

The focal length of a lens is the closest distance to the lens that an image can be formed (~2 cm for our eye).

Our eye can be thought of as a two-lens system. Both the cornea and the eye lens have curved faces, so both refract (and aid in focussing) light that enters our eye.

Accommodation

This refers to our ability to alter our eye’s lens so that it can focus on objects at any distance away.

A thick, highly curved lens has a short focal length (for focussing light diverging from close objects); a thin, slightly curved lens has a long focal length (for focussing parallel light rays from distant objects).

Our lens is a “fat” biconvex shape, flexible and elastic, so it can be stretched thinly (by relaxing, the ciliary muscle retreats from the lens into a large circle, which pulls on the ciliary fibres, causing them to stretch the lens). This brings distant objects into focus.

Our lens can elastically recoil into a rounder, thick lens (by contracting ciliary muscles into a small circle, which releases the tension on ciliary fibres, allowing the lens to “ooze” back to shape). This brings close objects into focus.

2.1 Defects of Vision

  • Myopia – nearsightedness (too much refraction); it can be corrected with a concave lens.

  • Hyperopia – long-sightedness (too little refraction); it can be corrected with a convex lens.

  • Astigmatism – the cornea is unevenly curved, i.e. non-spherical.

  • Presbyopia (old-age vision) is caused by the lens losing its elasticity, so it cannot ooze into the short focal lengths of a youthful lens.

  • Corneal transplant: there is no rejection problem as there are no blood vessels to carry white blood cells (WBC).

  • Age-related macular degeneration: this refers to a blurred area near the centre of vision (the macula lutea).

  • “Colour blindness”: this happens when three pigments in the cones are sensitive to red, green or blue light and also have some sensitivity beyond their major colour.

  • If the red-sensitive cones are missing, then red is perceived as green, and the condition is called protanopia.

  • In the reverse situation, green cones are missing, so green light stimulates red cones only, a condition called deuteranopia.

  • Anomalous trichromats: in people with this condition, all of their three cone types are used to perceive light colours, but one type of cone perceives light slightly differently.

  • Cataracts: there is opacity in the lens. The lens is removed (by phacoemulsification) and replaced with a soft plastic trifocal intraocular lens (IOL).

Other eye problems include glaucoma, a retinal bleed, retinal detachment

3 Ear Structure

  • Outer ear: auricle, external auditory meatus, tympanic membrane

  • Middle ear (filled with air): eardrum, tensor tympani, malleus, incus, stapes, stapedius muscle, oval window (OW), Eustachian tube

  • Inner ear: vestibule, cochlea, semi-circular canals, endolymph

The Transmission of Sound Waves to the Auditory Nerve

The ear is an “impedance matching” device, thereby directing sound propagating through air, via the middle ear, into the liquid endolymph of the cochlea.

(Resonance in the ear canal, leverage by ossicles, area ratio of eardrum and OW)

The tympanic membrane (TM, eardrum) separates air of the external ear canal from air in the middle ear (air in the middle ear is vented to the atmosphere through the Eustachian tube). TM vibrates due to pressure variations associated with the compressions and rarefactions of sound waves. TM moves the malleus, which moves the incus, which moves the stapes (the three ossicles), which presses on the oval window (of the cochlea). Hence, vibrations of air pass through OW into the fluid of the cochlea (the snail-like inner ear). Vibrations in the cochlear fluid cause movement in a certain part of the basilar membrane (on which sits the organ of Corti). The o. of C. is covered by the tectorial membrane so that its hair cells rub against the TM, which causes nerve impulses to be generated in the o. of C., which are transmitted to the brain via the cochlear nerve (part of Chapter VIII).

3.1 Sensitivity of Human Hearing

Sound intensity (in watts per square metre, W/m2) is the objectively measurable amount of sound energy carried by a sound.

  • 10−12 W/m2 = silence (threshold of hearing).

  • 1 W/m2 = pain.

Sound level (in decibels (dB)) is a subjective measure of the loudness of a sound.

  • 0 dB = silence.

  • 0 dB does not mean that the amount of sound energy is zero. Just that we cannot hear it!

Two sounds that differ by 0.1 dB can barely be perceived as different in loudness; a sound that is 3 dB louder than another sounds twice as loud. Continuous noise at 90 dB will produce hearing damage without causing pain (→ encourage the wearing of ear-muffs – an occupational health requirement!)

The human ear is not equally sensitive to all audible frequencies.

Perceived loudness (in phons) – e.g. 60 dB “phon” (or equal loudness curve) is a graph of the sound level (in dB and over the range of audible frequencies) that we perceive to be the same loudness as a sound of 1000 Hz at 60 dB.

Audible range is ~20 Hz to ~20,000 Hz; infrasonic: <20 Hz, ultrasonic: >20 kHz.

The sounds of different frequencies stimulate our hearing in different amounts. The ear is most “sensitive” from 500 to 6000 Hz. It is not very sensitive below 200 Hz or above 12,000 Hz.

As we age, we progressively lose the ability to hear frequencies >~12,000 Hz (presbycusis).

For audible range test (Google “human audible range”). Try:

Other hearing problems include noise-induced hearing loss, Conductive deafness, Sensorineural loss.

4 The Sense of Equilibrium

(Involves input from the eyes, stretch receptors in muscles/tendons, and the vestibular apparatus)

The vestibule contains two membranous sacs (linked by a duct) called saccule and utricle. Each contains a sensory receptor called a macula, which is sensitive to linear acceleration forces.

The dense otoliths (embedded in the jelly-like otolithic membrane) have a relatively large inertia and, hence, respond more slowly than the body to changes in motion.

“Hairs cells” in the macula are bent (and stimulated) when the overlying otolithic membrane, in which they are embedded, moves.

The membrane in the saccule is vertical (responds to vertical acceleration). The membrane in the utricle is horizontal (responds to horizontal acceleration).

The three semi-circular canals each have an enlargement called an ampulla. The ampulla houses an equilibrium receptor called a crista ampullaris, with “hair cells” that respond to the rotation of the head. The endolymph in the ducts, because of inertia, remains stationary when the head (i.e. the “hair cells”) begins to move (or the endolymph continues to move when the hair cells suddenly stop).

When these hair cells are moved through the stationary endolymph (or the moving endolymph moves over the stationary hair cells), they are stimulated.

The maculae and cristae ampullaris send impulses via the vestibular nerve (part of Chapter VIII).

Motion sickness is an equilibrium problem

5 Taste and Smell

Five basic tastes:

  1. 1.

    Sweet (sugars, alcohols, saccharins)

  2. 2.

    Sour (acids, lemon juice)

  3. 3.

    Salty (metal ions, esp. Na+)

  4. 4.

    Bitter (quinine, caffeine, nicotine)

  5. 5.

    Umami (glutamate – beef taste, aged cheese, monosodium glutamate (MSG))

Also involved in “taste” are temperature, texture and pain, e.g. chilli. And “taste” is 80% smell!

Smell relies on volatile chemicals – molecules must enter the nose to be smelled. Olfactory receptors are neurons (replaced every ~60 days). Classifying “primary odours” is problematic.

6 Special Senses Revision: Homework Exercise 10

  1. 1.

    Describe the function of the (a) cornea, (b) lens, (c) retina, (d) canal of Schlemm, (e) choroid and (f) pupil.

  2. 2.

    Describe the role of the ciliary body and suspensory ligaments in the process known as accommodation.

  3. 3.

    Why does the retina have a blind spot?

  4. 4.

    What are the names commonly given to the two types of lenses?

  5. 5.

    Describe the effect on the light rays of each type of lens (diverge/converge).

  6. 6.

    Which one produces a magnified image when placed over some writing on a page?

  7. 7.

    Which one has the same curvature as your eye’s lens?

  8. 8.

    Define what is meant by hyperopia.

  9. 9.

    Describe the function of the (a) pinna, (b) ossicles, (c) Eustachian tube and (d) cochlea.

  10. 10.

    What are the average sound frequency endpoints of the audible range for humans?