Chapter 12

Senses

Chapter Outcomes

Explain the difference between sensory reception, sensation, and perception Describe the process of sensory adaptation Distinguish between the major sensory receptors in the human body Describe the principal structures of the human eye and their functions

Chapter Outcomes

Observe the principal features of the mammalian eye and perform experiments that demonstrate the functions of the human eye Describe several eye disorders and treatments Describe how the structures of the human ear support the functions of hearing and balance

Chapter Outcomes

Explain how humans sense their environment through taste, smell, and touch Explain how small doses of neurotoxins can be used as painkillers

12.1- Sensory Reception, Sensation & Perception

What is the difference between these three?

Reception Sensation Perception

Sensory Receptors

Our sensory neurons are attached to receptors that are activated by specific stimuli These sensory receptors are highly modified ends (dendrites) of sensory neurons We have a number of different types of receptors in our body

Receptor

Taste Smell Pressure Proprioreceptors

Stimulus

Chemical Chemical Mechanical Mechanical

Information Provided

Taste buds identify specific chemicals Olfactory cells detect presence of chemicals Movement of skin or changes in body surface Movement of limbs Balance (ear) Outer Ear Eye Thermoregulators Mechanical Body movement Sound Light Heat Signals sound waves Signals change in light intensity, movement & colour Detect flow of heat

Groups of Receptors

Often receptors are grouped in specific organs which are specialized to respond to a single stimuli (such as organs for taste, smell, hearing and vision) The sensations that we receive from these receptors are actually produced in the brain – if transmission from the sensory neuron is blocked, the sensation stops

In general, the stimuli that we respond to are those most relevant to our survival For example, our range of hearing and vision is limited compared to other animals, even though the other stimuli are present Our senses can also undergo sensory adaptation This occurs when a receptor becomes accustomed to a particular stimulus being present

Sensory Adaptation Examples

Ever notice that some strong smells, over time, seem to disappear?

However, if you leave that environment and return, the smell has seemingly reappeared This phenomenon is due to your sense of smell becoming accustomed to that strong smell

We can also become accustomed to temperature changes For instance, before you step into a warm shower, the bathroom might seem relatively warm However, after the shower, you step out and feel very cold This is because your body becomes accustomed to the warmer temperatures of the shower

12.2 - The Eye

The eye consists of three layers: 1.

The Sclera Outermost portion of the eye Includes the cornea & aqueous humor

2.

Choroid Layer Contains pigments that prevent light from scattering Includes the: Iris Pupil Ciliary Muscles

3.

Retina Composed of three layers of cells: Rods & Cones Bipolar Cells Ganglion Cell Layer

The Retina

The Fovea Centralis

This is a region in the center of the retina that contains a dense bundle of cones The lens of the eye focuses the majority of the light on this area The fovea produces sharp colour images Surrounding this area are rods which pick up low-intensity black & white light

Vision – The Lens

Images form on the retina because of the focal length of the lens Unlike plastic or glass lenses, the lens of the eye can change its shape, which makes it able to focus on near and far objects Objects 6 m (20 ft) from the eye should be focused without any change to the lens’ normal shape

The Chemistry of Vision

Rods and cones contain a light-sensitive pigment known as rhodopsin In the absence of light, rods release inhibitory neurotransmitters that inhibits nearby nerve cells When light hits this pigment it is split into two components: Opsin (a protein) and retinene (a form of vitamin A) This division stops the release of the inhibitory transmitter, allowing transmission of an impulse to the optic nerve

Regeneration

As indicated by the previous diagram, the breakdown of rhodopsin is much faster than its regeneration This is responsible for the afterimages that are often seen after looking at a single object for a long time or at a bright light Bright light can cause temporary blindness because the rhodopsin is not regenerated in sufficient amounts to maintain vision

Colour Vision

The cones used for colour vision come in three varieties – red, green and blue Slight changes in the opsin component of rhodopsin are responsible for the various cones’ sensitivities to different colours of light The following diagram shows the subtle differences in the opsin molecules

As you can see, there are subtle changes to the amino acids that make up these proteins

Colour Blindness

Colour blindness is caused when one or more of the colour cones are defective This is caused by a mutation in the genes that create the opsin molecules These mutations alter the sequence of amino acids that make up the opsin, and therefore change its shape and function

Types of Colour Blindness

There are a number of different types of colour blindness A rare case, known as monochromacy, occurs when a person lacks all three colour pigments and can distinguish no colour at all More common is dichromacy, where one of the pigments is absent – this is often inherited and affects males more often than females

Types of Colour Blindness

A third type of colour blindness is anomalous trichromacy, where all three pigments are present, but have altered spectral sensitivity It often results in a difficulty in distinguishing between red and green hues (most common) or yellow and blue hues (very rare)

Types of Dichromacy

Protanopia – an absence of red colour receptors; red will appear dark Deuteranopia – green photoreceptors are absent, and it affects red-green colour distinction Tritanopia – total absence of blue receptors What does dichromacy look like?

Other Common Vision Defects

1.

There are a number of other common vision defects Glaucoma Caused by increased pressure in the aqueous humor This pressure causes the blood vessels in the retina to collapse The rods and cones die because of a lack of oxygen and other nutrients

Glaucoma can be treated with medication or surgery Medications aim at reducing the pressure within the aqueous humor by either helping it drain or reducing the production of the aqueous humor Laser or microsurgery can be used to cut a small hole to relieve the fluid pressure, but this is not a permanent solution

2. Cataracts Cataracts are caused by the lens becoming more opaque This prevents light from coming through and reaching the retina Cataracts can be treated by replacing the damaged lens with an artificial one using surgery http://upload.wikimedia.org

3.

Astigmatism Astigmatism occurs when the lens is irregularly-shaped and only correctly focuses in one plane This can be countered by using an external lens to compensate for the irregular shape of the lens in the eye

4.

Myopia (Nearsightedness) Myopia occurs when the eyeball is “too long” and the image from the lens focuses in front of the retina This is treated by using a biconcave lens to diverge the light rays before they reach the lens

5.

Hyperopia (farsightedness) The main contributing factor to hyperopia is an eye that is “too short”, resulting in the image being focused behind the retina A convex lens can be used to converge the light rays before they reach the lens, which refocuses the light on the retina As well, as we age, our lens becomes less elastic and we lose the ability to focus on near objects

The Blind Spot

Where the ganglion cells merge, they form the optic nerve At the point where the optic nerve enters the retina, it creates a region that has no rods or cones This is known as the blind spot

Visual Interpretation

Messages from the eyes travel through the optic nerves to the brain Once in the brain, the pieces of visual information are sorted, processed, and integrated to produce a 3-D image Aspects of sight such as movement, colour, depth, and shape are handled by different parts of the occipital lobe This speeds up the processing of the visual image

Note that images from the right eye are interpreted on the left side of the occipital lobe

12.3 - The Ear

The ear carries out two functions – it is used for balance and for hearing Both of these senses use specialized hair cells that are very tiny and respond to the movement of fluids in the ear

Anatomy of the Ear

The Outer Ear

The outer ear consists of: The pinna The auditory canal

The Middle Ear

The middle ear produces the sound nerve impulses that are sent to the brain It consists of several parts: The tympanic membrane (tympanum) The ossicles

The oval window The Eustachian tube

The Inner Ear

The inner ear contains: The cochlea The semicircular canals The vestibule

Hearing and Sound

Our hearing can detect sound energy as low as 1.0

×10 -12 Watts Sound travels as pressure waves through a material, and therefore will not pass through a vacuum Sounds travel most rapidly through solids, and most slowly through gases

The Organ of Corti

The Organ of Corti consists of three structures: The basilar membrane, which contains many hair cells The hair cells, which have many tiny projections known as stereocilia The tectorial membrane, into which are embedded the ends of the stereocilia

Production of a Sound Impulse

1.

2.

3.

The tympanic membrane vibrates as pressure waves hit it These vibrations are passed on to the ossicles, which amplify the sound The vibrations of the ossicles move the oval window; the round window moves as well, producing waves of fluid in the inner ear

4.

5.

6.

These waves of fluid travel through the cochlea The movement of fluid causes a thin membrane known as the basilar membrane to move. This membrane is attached to hair cells located in the organ of Corti The movement of the cilia of the hair cells against the tectorial membrane produces a nerve impulse which is sent to the brain Production of a Sound Impulse - Animation

Hearing and Pitch

Different pitches of sound can be heard by the human ear (a range of about 20 – 20,000 cycles per second) Low-pitched sounds stimulate the hair cells near the far end of the cochlea, while high-pitched sounds stimulate hair cells near to the oval window

Hearing Loss

Hearing loss generally results from nerve damage (generally damage to the hair cells) or damage to the sound-conduction system of the outer and middle ear Repeated loud noise destroys stereocilia Any noise over 80 dB can damage hair cells

Hearing Loss Treatment

For people who have conduction deafness, hearing aids are often used However, patients with nerve deafness can have a device implanted in the ear that picks up sounds and transmits them directly to the auditory nerve Scientists have also been able to use viruses to insert genes that allow the growth of new stereocilia in guinea pigs

Perception of Sound

Nerve transmissions from the ears eventually reach the temporal lobes Depending on the neurons stimulated, the brain interprets the sounds as specific pitches and intensities As well, neurons in our temporal lobes can also generalize the area from which the sound originated

The Inner Ear – Equilibrium

There are two types of equilibrium – static and dynamic equilibrium Static equilibrium refers to the position of the head, while dynamic equilibrium provides information regarding the direction of movement The inner ear registers equilibrium for the body

The Inner Ear

http://oto.ucsd.edu

Gravitational Equilibrium

Gravitational equilibrium is maintained by two fluid-filled sacs known as the utricle and the saccule Inside these sacs are tiny hairs suspended in a jelly-like substance, which contains calcium carbonate granules known as otoliths

When the head is in its normal position, the otoliths do not move If the head is tipped sideways or backwards, the otoliths are pulled by the force of gravity, and brush against the cilia The movement of the cilia produce nerve impulses that are sent to the brain, indicating the position of the head http://www.qmw.ac.uk

Rotational Equilibrium

Rotational equilibrium is maintained by the semicircular canals in the ear Each canal contains a fluid-filled pocket known as the ampulla Rotational stimuli causes the fluid in the canals to move This causes the ampulla to move, bending hair cells that are attached to them This produces a nerve impulse that is carried to the brain

The Ampullae

http://geoanalyzer.britannica.com/eb/art-538

Motion Sickness

Motion sickness is caused by contradictory messages being sent to the brain It is often caused by the balance centers of the ears sending a different message than what is perceived by the eyes This results in a nervous system response that often includes nausea

Preventing Motion Sickness

One way of preventing motion sickness is to ensure that the eyes and the ears receive the same information – you should be able to register the motion visually Certain drugs may be used to combat motion sickness

Taste

Our sense of taste enables us to differentiate between edible and non-edible matter Our taste buds are arranged into sections on our tongue http://www.diwinetaste.com

Our individual taste buds act in a similar manner to other selective receptors in the body Therefore, for a chemical to activate a nerve impulse from a taste bud, it must correctly fit into the receptor (the ‘lock & key’ principle)

Unlike other stimuli, taste requires water to operate The chemicals must be dissolved to enter the taste buds Therefore, saliva also plays a role in whether or not we can taste something that is put in our mouth

Smell

Our sense of smell is similar to our sense of taste Receptor sites on olfactory cells in our nose are designed to combine with molecules of a certain geometry The messages that are produced by the receptors are then sent to the olfactory bulb of the brain

Smell and Taste Together

You may be familiar with the fact that both taste and smell work together For instance, when you have a cold, the olfactory receptors in your nose do not work as effectively, and therefore you have a diminished sense of taste People such as wine tasters use both of these senses together

Touch

Mechanoreceptors for touch are located throughout the body Different receptors are sensitive to stimuli such as light touch, pain, and high and low temperatures The receptors that release pain signals, as we have seen, release impulses to the brain which can be blocked by pain medications

Sensation and Homeostasis

Our senses allow our bodies to maintain homeostasis Our senses of sight, touch, taste, smell, and hearing give us information on which to act

Neurotoxin Painkillers

Many animals (such as frogs, puffer fish, and cone snails) produce neurotoxins Many of these neurotoxins work by preventing the transmission of an impulse through a neural pathway Scientists are currently studying whether or not many of these compounds could be used to prevent the transmission of pain messages without the side-effects of morphine and other opiate-based drugs