Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Sensory Systems in Animals

I. Generalized sensory system response
    The general system is diagrammed as follows:

stimulus → reception (stimulus received by a receptor) → transduction (causes change in membrane potential, resulting in action potential) → amplification (signal strength increased) → transmission (signal propagated through nervous system) → integration (processing of information by brain) → perception.

We’ll study each stage in more detail and then give some specific examples of the sensory system in action.

II. Integration - Or, Lessons from the newly sighted
    Consider an individual blind since birth who suddenly is able to see. What would s/he see? We certainly cannot learn the answer to this question by doing an experiment. However, we can study the experiences of individuals with cataracts whose sight was restored when suitable surgical techniques were developed.

    A passage will be provided from Goldstein and Goldstein (1984). The Experience of Science. Plenum Press.  I think this issue is also addressed in a film starring Val Kilmer (Love at First Sight?)

    Conclusion: the brain must learn to see and process information. Sight (and likely other senses) is not a purely mechanical processes. Thus, integration of sensations is crucial!

    As an aside, this brings up an interesting philosophical question about facts - things are only what they seem because that’s the way we learned and perceive them. How do we know that our senses haven't been "tricked" (i.e., optical illusions)?

III. Stimuli
    There are many different stimuli that activate the nervous system. These stimuli, which are essentially various types of energy, include:

1. chemical energy - associated with chemical bonds/structure; involved in taste and smell
2. mechanical energy - associated with movement such as physical deformations caused by sound, pressure, vibrations, fluids, touch; involved in touch and hearing
3. radiant or electromagnetic energy - such as light, heat, magnetism; involved in vision
4. damage to tissue is another type of stimulus.

IV. Receptors

A. Receptors are usually modified sensory neurons or epithelial cells. They can occur singly or in groups.

B. A receptor converts the energy of a stimulus into an electrochemical signal (action potential).  

C. Some receptors monitor stimuli coming from within the individual (i.e., from internal stimuli - internoreceptors), others monitor external stimuli (externoreceptors).

D. Receptors can be classified by the type of energy to which they respond. Thus, there are:

1. Photoreceptors - important in visual systems, respond to light (radiant energy) such as the eye.  Light alters membrane potential;
   
2. Chemoreceptors - important in taste and smell, have binding sites to recognize the shape of specific molecules (respond to chemical energy);
   
3. Thermoreceptors - respond to temperature (radiant energy). In humans, naked dendrites in the skin monitor external temperatures. The hypothalamus monitors internal body temperature;
   
4. Mechanoreceptors - monitor touch, pressure, stretch, motion (mechanical energy). One example includes the hair cells in the ear; and
   
5. Nocireceptors - pain receptors in the skin; naked dendrites; respond to tissue damage.
   

E. Receptors may: (1) occur in more than one location (i.e., somatic receptors such as pain receptors); or (2) be localized in specialized structures (i.e., the eye).

V. Transduction/Transmission/Amplification
    This refers to the ability of a receptor to trigger an action potential upon receiving a stimulus. The stimulus typically causes a change in membrane permeability resulting in an action potential.  This is the result of the stimulus causing ion channels in the membrane to open (or close) ultimately causing the release of neurotransmitters. 

    The stronger the stimulus the more action potentials (frequency) along a single neuron and the more axons that carry the message. Thus the response is graded.

    The response of the sensory system is "hard-wired". In other words, a given sensory system will react in a predictable way, even to "other" stimuli. For example, when you rub your eyes you see flashes of light. Thus, a mechanical stimulus causes the same response as an electromagnetic stimulus.

    Adaptation - the sensory system soon adapts to continued stimulation and doesn't respond to the stimulus. Just like the villagers who stopped coming to aid the "boy who cried wolf", the system becomes adapted to the repeated stimulus and no longer responds.


VI. Touch - monitoring mechanical stimulation
    Touch receptors are a type of mechanoreceptor. In humans, the fingertips and tongue are rich with receptors, but there are fewer in the back of the hand and neck. Touch receptors are free nerve endings or nerve endings that are surrounded by a capsule.

    In skin, there are receptors to monitor: (a) light touch; (b) deep pressure; (c) pain; (d) cold/hot (see below). Muscle spindles (stretch receptors) monitor muscle movements.


VII. Temperature - monitoring hot and cold 
    There are free nerve endings in skin to monitor: (a) heat; and (b) cold. The hypothalamus processes these nerve impulses.


VIII. Pain - monitoring tissue damage 
    Pain is recepted by free nerve endings (dendrites), the nocireceptors. Prostaglandins sensitize tissue to pain. Aspirin works by inhibiting prostaglandin synthesis.


IX. Taste - an example of a chemoreceptor system 
    Stimulus - chemical. Receptor - chemoreceptor. In humans, the chemoreceptors are "taste buds" located in circular papillae that are found in specific regions of tongue. Binding of chemicals to the receptors induces an action potential that is transmitted to the brain for perception as a "taste".

    There are four basic tastes - bitter, sweet, salty, and sour. The locations of these receptors can be mapped (bitter - back of tongue; sweet - front; salt - front; sour - sides). These tastes have different functions (sweet - suckling, bitter - avoid poisons, salt - obtain needed minerals, sour - maybe to avoid spoiled food?).


X. Odor - another chemoreceptor system  
    Odors are caused by the response of ciliated receptor cells that line the olfactory epithelium to different molecules. Humans have receptors for about 10,000 different chemicals. The signal is passed to olfactory bulb and olfactory cortex in the brain.

    As anyone who has ever had a head cold knows, things don’t taste very good when you can’t smell them. Thus, a large component of taste is related to odor.


XI. Hearing

A. Structure
    The vertebrate ear is made of three regions:  outer ear (receives signal), middle ear (amplifies signal) and inner ear (processes signal into action potential, transduction of signal).

    Major parts of the ear include: malleus (hammer), incus (anvil), stapes (stirrups), cochlea, basilar membrane, organ of Corti, oval window, round window, tympanic membrane.

    The middle ear is air filled - important for pressure equilibration. Middle ear is open to pharynx (via auditory tube = Eustachian tube) that is usually closed except when swallowing or yawning. Often need to equalize pressure as when scuba diving or changing altitude. Can be problematic for people with ear infections (i.e., when the middle ear is fluid-filled).

    The malleus/incus/stapes serve to amplify sound waves. They increase the pressure 20x.

B. A brief primer on the physics of sound
    No sound in a vacuum - sound is transmitted by the movement of air molecules. Consider a vibrating tuning fork that alternately compresses and releases air molecules. Expressed in the terminology of waves: (1) Volume is related to amplitude (degree of compacting). The units are decibels and humans can easily detect 10 db. (2) Pitch is related to frequency - number of waves that pass a given point per unit time. Frequency is expressed as Hertz. Humans can hear from 20-20,000 Hz, though we hear best from 500-4000 Hz.

C. Mechanism of sound perception
    Sound waves → tympanic membrane (ear drum) vibrates → malleus (hammer) → incus (anvil) → stapes (stirrups) → oval window → pressure waves in fluid of cochlea move from vestibular canal (scala vestibuli) around into tympanic canal (scala tympani) and then to round window → ultimately causes basilar membrane to vibrate → which pushes hair cells in organ of Corti to come into contact with tectorial membrane → hair cells bend → depolarization → initiates action potential → to brain → "sound"

D. Different regions of basilar membrane respond to different frequencies
    The distal region (nearest round window) narrowest and stiffest - responds to highest frequency sound; proximal region (nearest oval window) widest and most flexible - responds to lower frequency sound.

E. Hearing damage
    Loud sounds can damage hair cells. Permanent. Bummer.

F. Balance
    Semicircular canals are important for detecting position. Contain hair cells surrounded by a fluid that responds to gravity. When hair cells bend due to movement of the fluid it initiates an action potential that can be processed into info concerning body position. Like a gyroscope and similar to the gravitropic response in plants (statolith idea)

G. Video
    In class we saw a video clip concerning hearing and cochlear implants


XI. Vision

A. Camera model
    The vertebrate eye is analogous to a camera (but it does oh so much more!). Let's review the structure of a standard camera: UV/haze filter or other lens cover (protects the lens from damage), lens (bends light, focuses so the light converges at a single plane), diaphragm (regulates amount of light that enters camera), film (photosensitive layer), camera body (serves as housing and protection).  Check out Gink & Go Take Pictures.

B. Structure - the parts of the vertebrate eye include:

• Sclera - like camera body, serves to protect the eye, "whites of the eye";
• Cornea - clear protective layer that can be rapidly regenerated (i.e., when a child, my daughter Erin scratched her cornea.  After a quick trip to the pediatrician we realized there was no cause for concern.  The scratch was visualized with fluorescein and it's amazing how quickly it disappeared). Also serves to bend light and focus it onto photoreceptive surface. Thus, the cornea serves the function of both the UV/haze filter and lens of the camera;
• Iris - controls amount of light entering eye. Analogous to camera diaphragm;
• Retina - photoreceptive layer that absorbs and converts light, analogous to the film;
• Choroid layer - contains vascular tissue for nourishment and pigmented area to minimize light scattering. There is no good analogy to the camera except that cameras are typically black to minimize light scattering;
• Aqueous body - fluid inside the cornea, transmits light and maintains pressure; analogous to air inside lens of camera;
• Vitreous body - fluid inside eye, transmits light, supports lens and eye, analogous to air inside camera body;
• Pupil - opening into eye controlled by diaphragm/iris, can be narrow or wide letting less or more light into eye (like F-stops on camera);
• Optic disk - where optic nerve leaves eye, no photoreception ("blind spot" - can detect by making marks on paper 3" apart). Camera analogy?;
• Fovea - area of densely packed cones; area of best vision.

C. Image formation 
    Light reaches eye and hits cornea which causes light to bend (refract), focusing the image on the retina. The cornea is primarily responsible for focusing. The effect of distance is adjusted by the lens. In a camera the lens is used to focus - it moves toward and away from the film. In the vertebrate eye, the lens doesn't move with respect to the retina, rather the lens changes its thickness.  This is called accomodation.

    If the object is near, the lens is thicker (refracts light more) because the ciliary muscle contracts pulling the choroid toward lens thereby causing the ciliary fibers to become loose/slack (doesn't "pull" on lens so much).

    If the object is far away, the lens is thinner because the ciliary muscle relaxes, thus the choroid layer moves away from the lens, making the fibers tight pulling on the lens.

D. Photoreception - Cells
    The retina is packed with photoreceptive cells (named on the basis of the shapes of the cells):

1. Rods - sensitive to dim light; at periphery of eye, not sensitive to wavelength (color); approx. 100 million of them; important in coarse perception of movements; three regions - outer with lots of membrane stacks (another example of S/V ratios in action), middle region with the major organelles; and inner area (synaptic terminal);
   
2. Cones - respond best in bright light; conc. in center of eye; sensitive to light color (wavelength); approx. 6 million in the eye; responsible for color vision and sharp daytime vision. There are three types of cones, those that are responsive to red, green and blue, respectively. Color is determined by how strongly a group of cones are stimulated. Densely packed in fovea region (near center of retina).

E.  Photoreception - Visual Pigments
        Rhodopsin is the photosensitive visual pigment in the eye.  

• located the membranes of rods and cones. 
  
• aggregate molecule made of (a) 11 - cis - retinal (from vitamin A); and (b) the protein opsin. 
  
• Retinal undergoes an isomerization when exposed to light, changes from cis → trans form → causes a hyperpolarization → action potential.  Each type of photoreceptor has different type of opsin which results in response to different kind of light. 

E. Summary
    We can summarize visual sensory pathway: 

light → rod/cone → isomerization of rhodopsin in membrane → opsin changes shape → G protein (transducin) → activates phosphodiesterase → converts cGMP to 5GMP → sodium channels that were previously held open by cGMP close →  hyperpolarization of membrane → action potential → release neurotransmitter → stimulate other neurons (bipolar cells then ganglion cells) → optic nerve → thalamus → visual cortex in brain → sight