The Special Senses

Taste We have about 10,000 taste buds on our tongue

and a few others scattered throughout the upper respiratory system. If you run your finger across your tongue you will feel a slightly rough surface. These are the taste buds. Look at this slide carefully to see the fungiform and vallate buds

Each bud contains up to 100 epithelial cells, which act as supporting cells, receptor cells and basal cells

Taste buds Supporting cells form the bulk of the taste

bud and these insulate receptor cells (which are also called gustatory cells) from each other. Long microvilli called gustatory hairs project from the tip of each cell through a taste pore bathed in saliva and out onto the epithelial surface. The basal cells act as stem cells to replace those cells which are destroyed when eating hot food

Tasting We have 4 basic tastes and these are sweet, sour, salty and

bitter. Some people have tried to map taste to different areas of the tongue but it is not accurate

When a chemical from our food is dissolved in saliva it can diffuse into the taste pore and come into contact with the gustatory hairs. This creates a neurotransmitter response, which generates an action potential in associated nerve fibres. The different taste sensations are achieved because of the different times it takes to generate nerve impulses. Nerve fibres then transmit the message to the brain, as we can see in this slide. Incidentally, chili peppers taste hot to us because pain receptors in the mouth are activated. And yes, 80% of taste is smell.

Smell This slide is an electron micrograph of a

small section of lavender. You can see the oil sac in the plant surrounded by thorns. When a person crushes the leaf they very quickly can smell the delightful odour of lavender. But how do we smell things?

The sense of smell in us As it turns out we may be able to distinguish

between 10,000 aromas but yet we only have about 700 genes for smell.

There may be sex and genetic differences in our capacity to smell not only different odours but the concentration of different odours. Its interesting that babies can smell their mother and there is some evidence that mothers can smell their babies;

Aromas and memory Most aromas can evoke strong memories and the Proust

effect describes the way that a smell can recall a memory. If you were to place an aroma under the nose of a blindfolded person then it probably would evoke powerful memories

Aromas can also influence physiological memory. An experiment on healthy young males showed that if you injected them with insulin and simultaneously exposed them to an aroma then naturally their blood glucose fell. After 4 days of this if you gave them the aroma only then their blood glucose also fell! So aromas clearly have some physiological effects

Aromas and gender It has been shown that females have greater

sensitivity to aromas than males. Some recent research suggests that females choose mates on the basis of smell. Whilst other research indicates that females can detect emotions associated with smell better than males (females can detect and distinguish tissues that have been placed under male and female armpits during scary movies)

Aromas and our senses Aromas can assist concentration. Some

Japanese industrialists use the citrus smell to stimulate workers in the morning and floral aromas around midday to help concentration. They use woody aromas in the afternoon to alleviate tiredness. However, our sense of smell declines with age

Olfactory structures The organ of smell is composed of millions

of ciliated pseudo stratified columnar epithelial cells located around a bend (sniffing improves air flow over these cells). Cilia are enclosed in mucus and this captures odour molecules. You can see these cells in this slide

Olfactory cells There are actually 3 types of olfactory epithelia

and these are basal cells, supporting cells and olfactory sensor cells. So the olfactory epithelia are really neuroepithelia and these are the only nerve cells that are known to be regenerated. Some modern research is looking at the basal cells as a way of regenerating other nerve cells. You can see the sensor cells in this slide as small bulbs amongst the cilia

Sensing aromas Air is swept over olfactory receptors in the nose. Odour

molecules bind to receptors cells and are dissolved by special proteins. These proteins are the result of about 700 genes. Dissolving and binding of the aroma causes chemical changes in the cells resulting in an electrical current transferred through axon.

Filaments of the olfactory nerves synapse with mitral cells. It is not known how the individual nerves transmit individual signals that differentiates different odour molecules but it is assumed that the glomeruli contain some sort of programmed file

The eye The eye is certainly a complex structure

and about 70% of all our body receptors are found in the eye so the brain must be busy processing a lot of visual information. Study this slide carefully so that you are familiar with the terms iris, pupil, sclera and lacrimal caruncle. This last one produces a white oily substance during the night

The eyelids I’m sure that you know where your eyelids

are but did you know that they close about 15 times a minute? In stressed people they close about 40 times a minute and this “blink rate” test is a good pointer to how stressed people are. Look carefully at this slide and note the lacrimal gland and canal. The lacrimal gland produces tears which are spread by blinking.

Eye muscles Movement of each eyeball is controlled by

6 eye muscles, which you can see in this slide. You can also see the conjunctiva, which is a transparent mucus membrane covering the white of the eye and over the eyelids. The whole membrane produces mucus to stop the eye from drying out. This can become inflamed and infected resulting in conjunctivitis.

Structure of the eyeball Study this slide carefully to ensure that you know

the terms ciliary body, cornea, iris, pupil, lens and retina. The posterior segment of the eye is filled with a clear gel called the vitreous humor whilst the anterior portion is filled with aqueous humor. This has a composition similar to blood plasma. Aqueous humor forms and drains continually through a sinus leading to the venous system. If the drainage is blocked then pressure will build leading to glaucoma. Cataracts result from thickening of the lens

Eye colour Eye colour is usually is lighter at birth and

becomes darker as the pigment production develops over 4 months. The pigment that gives eye its colour is actually two types of melanin. We can loose eye colour as we get older and one eye can have a different colour because of nerve damage. This slide is a microscope section of the iris and you can clearly see pigment deposits.

How do we see? This is obviously very complex and we will only

touch on it briefly. Light firstly passes through the cornea. This is well supplied with nerve endings and only a slight touch will result in the eye blinking to protect the rest of the eye. The coloured iris absorbs light and stops it from scattering. It’s round central opening called the pupil allows light to enter the eye. The iris actually has muscle layers which can expand or contract the pupil. The lens focuses the light onto the retina

The retina The retina is a multi-layer structure with 3 types

of neurons. The light receptor cells are rods and cones. Rods are used in dim light and for peripheral vision whilst cones provide very accurate and fine colour vision. Both rods and cones produce an electrical current when light hits them and this passes to the bipolar cells, which in turn activates the ganglion nerve cells and a message is sent to the brain

The Ear The mechanism for both hearing and balance is

relatively simple, although the structures are complex. Fluids are stirred and that stimulates mechanoreceptors of the ear. Sound vibrations move fluids to stimulate receptors and movement of the head disturbs fluids that surround the balance organs. To achieve this the ear is divided into outer, middle and inner ear.

The outer ear The outer ear consists of auricle and external

canal. The auricle directs sound into the canal, which is about 2.5cm long. The canal is lined with hair sebaceous glands and sweat glands, which produce cerumem or ear wax. It’s job is to trap foreign bodies

The outer ear (tympanic membrane) Sound waves hit the tympanic membrane, which is thin

connective tissue covered externally by skin and internally by mucosa. When you look at this slide you will note that the the eardrum is shaped like a flattened cone with the apex penetrating the middle ear. The eardrum transfers sound waves to the middle ear bones

The middle ear The Middle air is an air filled cavity. It is bounded laterally by the

eardrum and medially by a bony wall with two openings. These openings are the superior oval window and the inferior round window. The round window is closed by the secondary tympanic membrane. The mastoid antrum is a canal in the posterior tympanic cavity and this permits communication with mastoid air cells in the mastoid process. Note the anterior auditory tube (it used to be called the Eustachian tube. Normally this tube is flat and closed but if you yawn or swallow then it opens briefly to permit air pressure to equilibrate with external air pressure. This is essential for eardrum vibration. If pressures are unequal then the ear drum bulges causing hearing difficulties

Middle ear bones Three bones called ossicles are in the middle ear

(hammer, anvil and stirrup) and these transmits the vibration of the eardrum to the oval window which in turn sets the fluids in motion which in turn excites the hearing receptors

The inner ear (Vestibule) The vestibule is the central cavity and its lateral

wall is the oval window. It has 2 sacs (utricle and saccule). These contain equilibrium receptor regions called maculae. Each of these ducts has an enlarged swelling at one end called ampulla which also houses an equilibrium receptor. These receptors respond to rotational movements of the head. The cochlea (snail) is a small bony chamber about half the size of a split pea. It contains the spiral organ of Corti which is the receptor for hearing

How do we hear? Sound is a pressure disturbance originating from a vibrating object.

The pressure wave hits the tympanic membrane. It vibrates at the same frequency. The distance the membrane moves in is proportional to the sound intensity. The vibration is transferred to the ossicles. This sets fluid in motion in scala vestibuli (Cochlea) much like a wave moving back and forward. This makes the basilar membrane swing in response and the cochlear duct oscillates in time. The spiral organ of Corti has about 16,000 hearing receptor cells called cochlear hair cells. Notice that nerve fibres are twisted around these cells and vibration of the cells causes excitation of the nerve fibres and a sound message to the brain. Sound direction is worked out by the mid brain

Balance Keeping our balance relies on vision, information from stretch receptors in

muscles and tendons as well as the ear. Under normal circumstances equilibrium sensors in the vestibular apparatus sends signals to the brain that initiate reflexes. But if the system is damaged then our body can adapt so it is difficult to say exactly which receptors are responsible for what. But we do have sensors in the ear that respond to both static and dynamic balance. The sensory receptors for static equilibrium are macule (spots) in each saccule and utricle wall. They monitor the position of the head in space. They respond to straight line acceleration. Each macule is a flat epithelial patch containing supporting cells and scattered receptor cells called hair cells. The hair cells have numerous long supporting microvilli and are embedded in the otolithic membrane which is a jelly like mass studded with tiny stones. The function of the stones is to add weight and inertia. As we move the hairs of these cells move and this depolarises the cells causing a wave of electrical excitation to the brain

The maculae are horizontal in the utricle and when the head is upright the hairs are vertical. They respond to sidewards motion. In the saccule the macula is nearly vertical and the hairs are horizontal so these respond to vertical movement. The cells release neurotransmitters continuously but any movement causes an increase or decrease in the impulse generation by the vestibular nerve endings coiled around their base

Dynamic equilibrium The receptor for this is the cristae ampullaris. This is minute

elevation of in the ampulla of the semicircular canal. The cristae are excited by head movement. Since the semicircular canal is in 3 dimensions all rotatory movements of the head disturb the cristae. Each crista is composed of supporting cells and hair cells. The hair cells project into a gelled mass called a cupula. Nerve fibres encircle the base of the hair cells. The cristae respond to changes in velocity of rotating head. Because of its inertia the fluid in the semicircular canal goes in the opposite direction deforming cristae and sending a message to the brain. If the body continues to rotate at a constant rate the fluid comes to rest. If we are blindfolded we cannot tell if we are moving at a constant speed after a few seconds

Proprioceptors Proprioceptors occur in skeletal muscles, tendons, joints,

ligaments and connective tissue coverings of bones and muscles. They constantly advise the brain of our movement by monitoring the degree of stretch of the organs that they occupy. There are several types including Ruffini’s Corpuscle, Pacinian corpuscle, muscle spindles and Golgi tendon organs. In this slide you can see a muscle spindle. Stretching activates the spindle, which sends signals to the spinal cord and also to motor neurons that activate the muscle. The brain is informed and the muscle spindle relaxes until the next stretch

Stretch and deep tendon reflexes The brain needs to be kept informed constantly

about the current state of all skeletal muscles and of course if muscles are to work efficiently then they must have healthy tone. Muscle spindles and Golgi tendon organs keep the brain informed about muscle equilibrium whilst stretch reflexes, initiated by muscle spindles monitor changes in muscle length. All this information is crucial to normal muscle function, movement and posture. You can visualise this in this slide.