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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 26. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What are the five senses?
  2. What are the three general types of sensory receptors that respond to the five different senses? 
  3. How are all of the environmental inputs into the sensory system transmitted to the central nervous system?
  4. How do the chemoreceptors found in your tongue discriminate between the different tastes?
  5. How does the ear take a sound wave and turn it into an action potential?
  6. How does your body detect balance and position?
  7. What are the components of the eye that allow us to see?
  8. How are the different sensory inputs such as taste and smell integrated together?

A Guide to the Reading

When exploring the content in Chapter 26 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

Essentials of Sensory Organs

Animals have evolved specialized organs that allow them to perceive and process information about the environment in which they live.  These sensory organs are involved in the five senses of smell, taste, touch, hearing, and vision.  Yet they are composed of only three sensory receptor types: chemoreceptors, mechanoreceptors, and photoreceptors.  These receptors in the sensory organs gather information about the external environment and relay the information to cells within the body.  Sensory organs range from simple to complex.  Each type of sensory receptor turns a property of the environment into an action potential that is transmitted to the central nervous system.  The ability of an animal to detect particular stimuli depends on the number of sensory receptor cells present in the sensory organ.  The greater the number of sensory receptors, the greater the sensitivity the animal will have to detect a specific stimulus.  The senses are important to animals because they allow the animal to detect the conditions of their environment and typically determine from which direction a stimulus is originating.

For more information on this concept, be sure to focus on:

  • In Section 26.1, Sensory structures share certain features
  • Table 26.1, Different Ways to Sense the World

Mechanoreceptors Allow Us to Touch and Hear the World

Mechanical sensors receive mechanical stimuli in the form of sound waves, gravity, motion, touch, and temperature. All mechanical sensors transduce mechanical stimuli into electrical stimuli through special sensory cells.  Touch mechanoreceptors depolarize in response to the stretching of the receptors.  Touch receptors that respond to slight changes in pressure often have hairs associated with the sensory neuron that help pick up small changes in pressure.  Hearing involves mechanoreceptors transforming pressure waves of sound into action potentials.  The ear is divided into three sections that help collect sound and produce a sensory response.  The outer portion contains the pinna that collects sound waves and an auditory canal that directs the sound waves to the eardrum.  The middle ear contains a eustachian tube that connects the middle ear to the throat, and three bones that help transmit vibrations of the eardrum to the membrane at the beginning of the inner ear.  The cochlea is found in the inner ear and contains the organ of Corti.  The organ of Corti contains a basilar membrane that is associated with hairlike mechanoreceptors.  A sound wave enters the outer ear and vibrates the eardrum.  This vibration is passed along the middle ear by the three bones to a membrane at the opening of the cochlea.  The vibration then moves down the organ of Corti where it vibrates the basilar membrane and hairs associated with the mechanoreceptors.  The stimulation of the mechanoreceptors sends a signal to the brain where it is processed as a sound.  Loud sounds move the basilar membrane more than soft sounds.  Additionally, different pitches of sound will stimulate different regions of the basilar membrane.

For more information on this concept, be sure to focus on:

  • In Section 26.3, Hearing is a highly refined form of mechanoreception
  • Figure 26.6, The Human Ear Detects Minute Pressure Changes
  • Figure 26.7, Our Ears Help Us Keep Our Balance


Light-sensing cells range from simple pigment spots to complex light-concentrating organs.   The human eye is a single-lens system that focuses light onto photoreceptors contained in the retina.  The pupil adjusts the amount of incoming light that the lens focuses onto the retina.  Humans have two types of photoreceptors in their retina, rods and cones.  The rods detect shades of gray in dimly lit conditions and the cones detect colors in bright light.  Our night vision is limited by the detection of shadows by the rods.  The ability of the eye to see depends on both the number of rods and cones and the distribution and density of these pigments.  When light stimulates photoreceptors, the stimuli are transmitted to the brain as an action potential where it is processed into an image.  With 100 million photoreceptors per retina sending input to the brain, humans are able to produce a sharp image.  Humans also possess depth perception because our eyes are positioned on the front of our face allowing for each eye to have a slightly different view of the world.  The brain processes these two different views to produce a three-dimensional image.  Animals such as cows have their eyes spread apart on either side of the head and have poor depth perception.   Unlike the single-lens system of the human, insects have a compound eye that contains many light-receiving units.  Other arthropods have only simple eyes that can determine between light and dark. 

For more information on this concept, be sure to focus on:

  • In Section 26.4, Forming images requires a way to focus light
  • In Section 26.4, Some vertebrates perceive their world three-dimensionally
  • Figure 26.8, Major Features of the Human Eye
  • Figure 26.9, How the Human Eye Forms Sharp Images

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

Sensory Structures: Making Sense of the Environment

  • Chapter 25 - Section 25.3, Making Sense of Action Potentials in the Body as a Whole

The Brain’s Role in Integrating Sensory Information

  • Chapter 25 – Section 25.4, Organization of the Human Brain

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