Sensory Physiology

Chapter Objectives

After studying this chapter, students should be able to . . . 1. explain how sensory receptors are categorized, give examples of functional categories and explain how tonic and phasic receptors differ. 2. explain the law of specific nerve energies. 3. describe the characteristics of the generator potential. 4. give examples of different types of cutaneous receptors and describe the neural pathways for the cutaneous senses. 5. explain the concepts of receptive fields and lateral inhibition. 6. describe the distribution of taste receptors on the tongue and explain how salty, sour, sweet and bitter tastes are produced. 7. describe the structure and function of the olfactory receptors, and explain how odor discrimination might be accomplished. 8. describe the structure of the vestibular apparatus and explain how it provides information about acceleration of the body in different directions. 9. describe the functions of the outer and middle ear. 10. describe the structure of the cochlea and explain how movements of the stapes against the oval window result in vibrations of the basilar membrane. 11. explain how mechanical energy is converted into nerve impulses by the organ of Corti and how pitch perception is accomplished. 12. describe the structure of the eye, and how images are brought to a focus on the retina. 13. explain how visual accommodation is achieved and describe the defects associated with myopia, hyperopia, and astigmatism. 14. describe the architecture of the retina, and trace the pathways of light and nerve activity through the retina. 15. describe the function of rhodopsin in the rods and explain dark adaptation is achieved. 16. explain how light affects the electrical activity of rods and their synaptic input to bipolar cells. 17. explain the trichromatic theory of color vision. 18. compare rods and cones with respect to their locations, synaptic connections, and functions. 19. describe the neural pathways from the retina, explaining the differences in pathways from different regions of the visual field. 20. describe the receptive fields of ganglion cells and state the stimulus requirements for simple, complex, and hypercomplex cortical neurons.

What are senses?

Sight, Smell, Hear, Taste & Touch
Sensation: Feeling or awareness of a bodily state or condition.
Perception: Creations of the brain; our ability to see, hear, tastes and smell.

Basic Info
-Sense organs are extensions of the nervous system
-Our awareness of the world and its contents through sensory experience
-There are more than the five "basic" senses: e.g., touch has many receptors for light, depth, etc.
-Receptors for the different senses go to different areas of the brain.

WE DON'T ALWAYS CORRECTLY PERCEIVE WHAT WE SENSE!!!
Reality is what we perceive, but what we perceive is not always reality!
-In order to "feel" a sense, you need:
*A Stimulus
*A Receptor
*A Conduction to the brain (carried all the way through)
*An interpretation of the impulse


Sensory Receptors

Can be Exteroceptic (Perceiving sense from OUTSIDE the body) or Interoceptor(Perceiving sense from WITHIN the body, though not usually consciously).

• Functional Categorizations
  • Nocioceptors for PAIN
    • Sparse or absent in viscera and brain
      
    • Plays a protective role
    • Continuously relays impulses to the brain
    • Skin grafts have no pain receptors
  • Photoreceptors for SIGHT
  • Chemoreceptors for CHEMICALS
    • Taste, Smell
    • Detect changes in environment
    • Olfaction
      • aka Smell, these receptors are not highly developed in humans.
      • Functions closely with gustation (taste)
    • Gustation
      • aka Taste
      • Functions closely with olfaction (smell)
        
  • Proprieceptors for BODY AWARENESS
    • Gives information on body position, joint stretch, etc.
      
    • Proprieceptors are developed further in the practicing of sports, typing, playing instruments, etc.
      
  • Mechanoreceptors for MECHANICAL STIMULI
    • Touch, Hearing, Equilibrium
    • 
  • Thermoreceptors for TEMPERATURE
    
    • Receptors for cold are more abundant than those for heat, and are located closer to the surface of skin
    • Brain actually perceives spiciness from capscaicin as actually being hot. Pain receptors in the mouth make you feel pain from this "heat" as well. The receptor is found in free nerve endings in the nerves, which binds to the capscaicin.
      
    • Brain also perceives menthol as being "cold". The same thing happens as with capscaicin.
      
  • Baroreceptors for BLOOD PRESSURE
    • Remember the karate chop from anatomy?!
      
    Tonic vs. Phasic Receptors

• • Tonic Receptors ("Slow Adapting")
    
    • The neuron continues to fire until the stimulus is removed
    • Example: You poke yourself with a needle. It will hurt until it is removed.
      
    • Tonic receptors are constantly on
      
  • Phasic Receptors ("Fast Acting")
    • Fire only when stimulus first is perceived or removed
    • Example: Putting your clothes on; you are only aware of them for a short period of time.
      
    • Phasic receptors are usually "off" until a stimulus turns them "on".
      
      • Example: You are not aware of the temperature until it becomes hot or cold.
        
  • Action Potentials
    • More stimulus = More frequent action potential= Perception of larger stimulus
    • The amplitude (depolarization) of the action potential DOES NOT change
      

• Types of Cutaneous Receptors - These vary in density depending on location
  
  • Free Nerve Endings - TONIC
    
    • Widely distributed
    • Non-specialized, can perceive touch, temperature, pain, etc.
      
    • Not all identical
    • Quite abundant
  • Merkel's Discs and Ruffini Endings - TONIC
    
  • Meissner's Corpuscle - PHASIC
    
    • Responds to light touch
    • Resides in dermal papillae, just beneath epidermis
      
    • Actual nerve ending embedded in other cells
    • Mechanoreceptor
    • Concentrated in areas sensitive to touch
    • Action potentials are generated based on actual physical change (i.e, a person's hand bends the Meissner's Corpuscles in your hand, so the action potential is generated and you can perceive it.)
      
  • Pascinian Corpuscle - PHASIC
    
    • Responds to deep touch
    • Resides in hypodermis
    • Mechanoreceptor
      

Referred Pain
-The impulses travel along the same pathway toward the brain, where they synapse with the same interneuron. This confuses the brain into thinking it is receiving sensory information from, say, the left arm, when it is really coming from the heart.



Phantom Pain
-May be experienced by amputees, when part of their nerve has been severed. The nerve can still transmit impulses, confusing the brain into "feeling" pain or sensation in a limb that is not actually present. 70 % of amputees suffer from phantom pain.




Two-Point Touch Threshold
-The minimal distance at which you are unable to discern two separate stimuli as such. Decreases where the skin is most sensitive, where the density of receptors is closest.

The Individual Senses

• Olfaction -Receptors not highly developed in humans
  Functions closely with gustation (taste)
  
  • Receptor cells are located in the nasal epithelium
    • Roof of nasal cavity
    • The cell bodies are bipolar
    • The dendritic endings of the cells have olfactory hairs
    • The cells respond to airborne molecules
  • Pathway of Olfaction
    • Nasal cavity --> Olfactory hairs --> Olfactory receptor cells --> Through cribriform plate of ethmoid bone --> Olfactory nerve fibers
    • 
      
  

(Above Picture is of the cribiform plate of the ethmoid bone showing the Olfactory nerve leading to the brain.)

Hardwired right into the brain---primal sense.


2nd messenger systems-Look at Figure 10-11 on page 269 cyclic AMP
The 2nd messenger opens ion channels it isn’t restricted to the nervous system. You must activate 2nd messenger…there can be many (up to 50) g-proteins. They will greatly amplify a signal. Sets off a chain reaction.
Pathway for Olfactory Sensation Olfactory nerve fibers, cribiform plate of ethmoid bone. Olfactory receptor cells. Supporting columnar epithelial cells Olfactory hairs.

Taste (Gustation)-salt and sour are directly letting Na+ (salt) or H+ (sour)ions in. The same cell type that is letting the Na+ or H+ in. (Using Lime and salt after tequila) Can make taste sensations less intense. Depolarizes and neurotransmitter is released at it’s dendrite.
Sweet receptors- (binds to membrane receptor, second messenger)
Sour receptors- (H+ through ion channel)
Salty receptors- (Na+ through ion channel)
Bitter receptors-evoked by quinine (Binds to membrane receptor, second messenger)
Glutamate (meaty savory flavor) receptors-
Fat receptors-yummy

Taste hair, taste bud, Gustatory (taste)cell, supporting cell, sensory nerve fiber are the anatomy involved in taste.
Pathway involves 2 nerves Facial and Glossopharyngeal nerves
Olfaction-cAMP is the 2ndary messenger that amplifies smell.


Hearing Equilibrium and Hearing-vibrations in the ear. Waves of pressure traveling through the ear, funneled into the ear through the bones and the Eustachian tube to the cochlear and sound is interpreted through the cochlea. The sense of equilibrium has to do with the semi-circular canals.

Different frequencies are interpreted in different parts of the cochlea. In the vestibule and Utricle and assoc. with gravity and linear motion up and down, forward and backward are picked up within the vestibule.
Circular movement is picked up in the semi-circular canals. They will pick up different types of circular motion. Horizontal, etc.

Mechanical receptors have hairs that get pushed one way or another that causes depolarization of the membrane which causes the action/receptor potential. The waves of the motion bends the cells and causes depolarization. Sound waves in the internal acoustic canal go to the tympanic membrane with vibration to cochlea.

Cupula-hair cells will get bent there is neural processing for equilibrium. Ampullae of the Inner Ear- as you twist and spin it catches up with your body. Then the perception is that you are still spinning. Vestibular nystagmus. And then stop them and hold their head after they spin and spin and spin. P.273

Vision
The Eye-Rods and Cones. The Rods are more sensitive to light than the Cones.
Color vision-cones respond to different colors. Some people are color blind. Different degrees of color-blindness. Short, medium and long wavelength. Blue cones, short wavelength. Medium perceive green. And long are for perception of the color red.

Each rod contains thousands of molecules of a purple pigment known as rhodopsin in these discs. The pigment appears purple because it transmits light in the red and blue regions of the spectrum, while absorbing light energy in the green region. Green cars (and other green objects) are seen more easily at night - when rods are used for vision - than are red objects. This is because red light is not absorbed well by rhodopsin, and only absorbed light can produce the photochemical reaction that results in vision. In response to absorbed light, rhodopsin dissociates into its two components: the pigment retinaldehyde which is derived from vitamin A, and a protein called opsin. this reaction is known as the bleaching reaction.

Dark Adaptation

The bleaching reaction that occurs in the light results in a lowered amount of rhodopsin in the rods and lowered amounts of visual pigments in the cones. When a light-adapted person first enters a darkened room, therefore, sensitivity to light is low and vision is poor. A gradual increase in photoreceptor sensitivity, known as dark adaption then occurs, reaching maximal sensitivity in about 20 minutes. The increased sensitivity to low light intensity is partly due to increased amounts of visual pigments produced in the dark. Increased pigments in the cones produce a slight dark adaptation in the first 5 minutes. Increased rhodopsin in the rods produces a much greater increase in sensitivity to low light levels and is partly responsible for the adaptation that occurs after about 5 minutes in the dark. In addition to the increased concentration of rhodopsin, other more subtle changes occur in the rods that ultimately result in a 100,000-fold increase in light sensitivity in dark adapted as compared to light adapted eyes.

Rod produces rhodopsin. Transmits light so that people are able to see in the dark. Red light is not absorbed well in the dark. You can use red light to see in the dark. Night vision is getting used to the dark.

Vitamin A is important in night vision. You can see 100,000 x’s better after you’ve been in the dark for over a half an hour than if you are just placed in the dark setting.
Cones are concentrated in the phobia centralis. They help you to be able to focus.

Floaters: Also known as "Muscae Volitantes" are deposits of various size, shape, consistency, refractive index, and motility within the eye's normally transparent vitreous humour. Floaters are suspended in the vitreous humour, the thick fluid or gel that fills the eye. Thus, they generally follow the rapid motions of the eye, while drifting slowly within the fluid. Floaters are visible only because they do not remain perfectly fixed within the eye. The shapes are shadows projected onto the retina by tiny structures of protein or other cell debris discarded over the years and trapped in the vitreous humour. They are also common after cataract operations or after trauma. In some cases, floaters are congenital. (Just thought this was interesting, because I wondered what those little things were myself!)

CHECK OUT THIS VIDEO!!
http://www.metacafe.com/watch/576490/amazing_story/

QUESTIONS

Note on formatting: Everyone has been very good about formatting questions, except for one minor thing (minor, except that I have to fix it for my test bank): make sure there is a spaceor two (but never a tab) between the period after a number or letter and the text. So "a.first answer" is incorrect while "a. first answer" is correct. Thanks! ~K

1. Tonic receptors:
a. are fast-adapting
b. do not fire continuously to a sustained stimulus.
c. produce action potentials at a greater frequency as the generator potential is increased.
d. are described by all of these.

2. Receptors for vision are located in which part of the eye?
a. cornea
b. lens
c. iris
d. retina

3. The receptors for taste are ________.

a. naked sensory nerve endings.
b. encapsulated sensory nerve endings.
c. neurons.
d. specialized epithelial cells.



4. Cutaneous receptive fields are smallest in ____________.

a. the fingertips.
b. the back.
c. the thigh.
d. the arms.
e. b and c


5. What is Olfaction?
a. taste
b. hearing
c. smell
d. touch

6. The loss of equilibrium that results in?
a. vertigo
b. cupula
c. kinocilium
d. stereocilia

7. Which of the following sensory modalities is transmitted directly to the cerebral cortex without being relayed through the thalamus?
a. taste
b. sight
c. smell
d. hearing
e. touch

8.The specialized epithelial cells of the taste bud are known as
a. Astrocytes
b. Red Blood Cells
c. Schawn Cells
d. Taste Cells
e. Water

9. Which of these sensory receptors are the pain receptors
a. chemoreceptor
b. photoreceptors
c. nociceptors
d. thermoreceptors

10.Parts of your tongue can only respond to certain tastes.i.e. sweet, sour, salty, bitter, savory, and fat.
a. true
b. false

11. The organ of hearing in the inner ear where nerve impulses are generated in response to sounds waves is the:
a. Semicircular canals
b. Cochlea
c. Utricle
d. Saccule
e. the lobe

12. Messissners corpussles are..
a. Bundles nerve endings
b. Sensitive to light touch
c. Phasic
d. All of the above

13. Pacinian Corpussles are..
a. sensitive to light touch
b. Sustain pressure
c. a different word for messiners corpusles.
d. phasic

14. What cells of the eyes perceive color?
a. cones
b. rods
c. cornea
d. iris

15. What area of the eye regulates the amount of light that gets in?
a. central fovea
b. pupil
c. iris
d. macula

16. Smell involves molecules in the air dissolving in nasal mucus and directly stimulating neurons.
a. true
b. false

17. What two cells are involved in perception of color and or black and white?
a. Central fovea, Larynx
b. ganglia, cone
c. rod, cone
d. cornea, ciliary body

18. What is the macula?
a. The white around the eye
b. The primary area for vision
c. A muscle that helps move the eye
d. A tear duct