Sensory Physiology

LECTURE OUTLINE (CHAPTER 10)

LECTURE OBJECTIVES

1. Establish how sensory neurons interpret specific stimuli.
2. How is this sensory information processed in the PNS and CNS?
3. Discuss some of the conditions that result from malfunction of sensory input.

LECTURE OUTLINE

I. INTRODUCTION

  A. Primary functions of the Peripheral Nervous System (PNS)
     1. Sensory input
     2. Motor output
  B. Types of sensory neurons
  C. General characteristics of sensory neurons
     1. Graded response results in an action potential
     2. Receptor and Generator Potentials    
          a. Dendritic action potentials
     3. Tonic verses Phasic firing
     4. Lateral inhibition 
     5. Receptive fields 
  D. Introduction to sensory pathways to brain
     1. Sensory systems often map to specific cortical locations 

II. PAIN

  A. Nociceptor structure and function
     1. Similarities between nociceptors and thermoreceptors
  B. Fast and slow pain
     1. Myelinated and unmyelinated fibers
  C. Pain pathway projections in the CNS
  D. The brain's analgesic system
     1. Periaqueductal gray, raphe, and pain suppression
          a. Role of endogenous opioids, such as enkephalin
     2. Referred pain

III. TASTE AND SMELL 

  A. How are taste and smell related?
     1. Summary of functional similarities 
     2. Functional differences
  B. Taste
     1. How a chemical stimulus activates its taste receptor?
          a. Signal transduction for taste
              1) Second messengers (e.g., Ca++) have a central role
     2. Integration of taste 
  C. Olfaction
     1. Functional organization of the olfactory epithelium
     2. Odor is a "lock and key" association 
     3. Integration of olfaction 

IV. HEARING AND BALANCE

  A. Introduction to the ear
     1. Relationship between hearing and balance
     2. Ear is divided into outer, middle, and inner ear
  B. How does the ear hear?
     1. Role of hair cells in basilar membrane
           a. Importance of K+ influx 
     2. Hearing frequency and loudness
           a. A tonotopic map exists in the auditory cortex 
     3. Summary (Animation on how we hear)
  C. Balance and sensing movement and position
     1. Roles of semicircular canals, cupula, kinocilium
     2. Motion of the head and body    
     3.  Position of the head
          a. Role of ototliths in the utricle and saccule

V. VISION 

  A. Structure/Function of the eye
     1. Lens
          a. Refraction and accomodation by the lens
          b. Nearsightedness and farsightedness
     2. Iris 
     3. Retina
          a. Structure and function of rods and cones
               1) color vision
          b. Fovea is packed with cones
               1) Dysfunction: Macular degeneration
  B. Neuronal organization and integration in retina
          a. Rods and cones, bipolar, ganglion cells, etc.
          b. Function of each
     1. Phototransduction: Light activates rhodopsin (Animation)
          a. Signal transduction mechanism 
          b. Hyperpolarization of photoreceptor
          c. Light disinhibits bipolar neurons
          d. Ganglion cell fires an action potential
     2. Receptive fields of ganglion cells (Animation on visual processing) 
     3. Integration beyond the retina 
          a. Projection to lateral geniculate nucleus (LGN) and visual cortex
     
Review of sensory input and motor output

Reading Assignment. Please read Chapter 11 and the first part of Chapter 12 for the next lecture. I will start to lecture on muscle physiology next Thursday.

STUDY QUESTIONS ON SENSORY PHYSIOLOGY (CHAPTER 10)

BASIC FACTS AND TERMS

Compare the slow and fast pain pathways with respect to 1) characteristics of the sensory neurons (type of fiber, etc.), 2) the ascending pathways to the brain, and 3) analgesic modulation of the pain pathways by the CNS.
You place the point of a pencil on the back of your hand. Trace the path that the sensory input takes from your hand to the cerebral cortex. In correct order list all of the structures that the signal passes through. List all of the synapses that occur en route.
What is referred pain? Explain the neuronal basis for referred pain.
Define and give the physiological significance (how or why it is important) for the following terms:
- Organ of Corti
- Cupula
- Cochlea
- Otoliths
- Olfactory glomeruli
- Semicircular canals
- Oval and Round windows
- Periaqueductal gray (PAG)
- Basilar membrane of the cochlea
- Phasic receptor firing
- Tonic receptor firing
- Receptive field (of a ganglion cell)
List six neuronal cell types which make up the retina. Where is each located? How does each function?
Using a flow chart format, outline the correct order of cellular events associated with light transduction (=signal transduction pathway) in a rod.
What is the optical basis for nearsightedness (myopia) and farsightedness (hyperopia)? How do glasses adjust these conditions? (See Fig. 10-33)?

CONCEPTS
What roles do the bones of the middle ear play in hearing? --roles of the oval and round windows? How does the cochlea sense differences in the 1) frequency and 2) loudness of a sound? Be sure to look at this animation on how the ear works, especially how sound activates the hair cells in the Organ of Corti. You are responsible for knowing the information presented in this animation.
In what ways are receptor potentials similar to Excitatory Post-synaptic Potentials (EPSPs)? Explain.
What is the receptive field of a ganglion cell? What are On-center Cells and Off-center Cells, and how do they function in vision? (see Text.) See this very good animation on how these cells work. You are responsible for knowning the information presented in this animation.
Diagram the visual paths from light input (rods and cones) to the primary visual cortex. What happens to this visual input at each step? How is light information coded at each step? See this animation for more details.
You are looking at a image that is far away. Where and how is the image altered as it passes through the eye to the retina? What change occurs in the eye that let you focus on a very close image (Read about accomodation in Silverthorn)? What is the default state for the lens?
When you first enter a dark room you can not see well, but with time your vision becomes better. What is the physiological basis for light adaptation? See Silverthorn.
What is the physiological basis for the following conditions?
- night blindness
- color blindness
Transduction of a stimulus usually causes depolarization of a sensory receptor. What is the cellular basis for depolarization of a sensory receptor? One exception to this rule is the photoreceptor which is hyperpolarized in the presence of light. Explain how this hyperpolarization is eventually realized as an action potential leaving the retina (See Fig. 10-39).
Explain pre-synaptic inhibition. Give an example and explain how it works.
How does the "lock and key" relationship between odorants and sensory receptors in the olfactory epithelium explain the multitude of complex smells that we can sense? Explain.
What are the cellular mechanisms for transduction of taste stimuli (salt, sweet, bitter, and sour)? You do not have to memorize these mechanisms, but look for generalizations, such as 1) how Ca++ is involved in signal transduction and neurotransmitter release, 2) how membrane depolarization is accomplished (Note that there is more than one way), and 3) how these mechanisms compare with what was covered in Figures 6-11, 6-12, and 6-13.
How is the internal structure of a rod and cone related to its function? Then, contrast rods and cones with respect to: 1) distribution in the retina, 2) sensitivity to light, 3) adaptation to light, and 4) visual acuity. At what point on the retina is vision the sharpest? Explain why this is case.
What is the neuronal basis for our ability to perceive subtle differences in the color spectrum?
How are mechanoreceptors involved in 1) hearing and 2) balance/head position? Draw each of these receptors. How are they alike in structure and function?
Which part of the cerebral cortex integrates 1) visual input, 2) auditory input, 3) gustatory input, 4) olfactory input, 5) somatosensory input, and 6) speech production and speech understanding. Which of these cortical areas have "point to point maps" for sensory input? Why have such "maps" in the cerebral cortex? Which sensory system apparently lacks a "point to point" map?
Afferent neurons often modulate their firing pattern to code specific sensory informaton. Give two different examples of how the nervous system encodes information about the intensity of a sensory input. What type of information does tonic firing convey? --phasic firing? Describe sensory adaptation and explain how it is used as a method of information coding. Describe lateral inhibition and explain how it is used as a method of information coding.
How does the vestibular apparatus sense 1) initiation of movement in a forward or backward direction and 2) turning the head?

REASONING AND PROBLEM SOLVING
Pain results from tissue destruction. What chemical stimuli at the tissue level excite pain receptors? Aspirin is an analgesic. Given your knowledge of how the pain pathways work, give three different avenues by which aspirin might inhibit pain. Click here for the correct answer.
Ganglion cells consist of either ON-CENTER CELLS (=firing increases) or OFF-CENTER CELLS (=firing decreases) when white light appears in the "center" of the receptive field. When white light appears in the "surround area" (rather that center) of the receptive field, the opposite response occurs for both the ON (=decreases) and OFF cells (=increases). Can you expand on this general principle to explain how color-senstive ganglion cells might code for color vision?
A person training for a marathon is often said to be "addicted" to running. Further, not running often leads to "withdrawal-like" symptoms. Given your knowledge of how endogenous opioids (e.g., enkephalin) act in the CNS, what might be happening interally that could explain these symptoms?
The endolymph, which surrounds both the the inner hair cells in the inner ear and the cupula in the semicircular canals, is rich in K+, rather than Na+. This difference in ionic concentration is unique to these fluids. Given your general understanding of how ion flux leads to depolarization, how might K+ current play a role in depolarization of the inner hair cells and the cupula?

Last revised: February 7, 2008