TABLE OF CONTENTS
KEY CONCEPTS IN THIS LECTURE 1. In mammals, the ear is responsible for hearing. It consists of 3 regions: outer ear or pinna (collects and directs sound waves), middle ear (transfers and amplifies the sound waves), and the inner ear (senses pitch/frequency, intensity/loudness, and quality/timbre). A snail-like structure called the cochlea is the sensory element of the inner ear. The cochlea is a fluid-filled chamber whose fluid is set into motion by movement of the oval window between middle and inner ears. The cochlea contains the basilar membrane that varies in stiffness along its length. High frequencies deflect the basilar membrane nearer to the oval window and low frequencies farther from the window. This deflection activate mechanoreceptors, called the hair cells which trigger nerve impulses. The sensory input travels along the VIIIth cranial nerve to the medulla, midbrain, thalamus, and cerebral cortex. At level of the midbrain, two ascending paths diverge. One projects to the auditory cortex in the temporal lobe where a tonotopic map exists, and the other projects to the somatosensory cortex in the parietal lobe. 2. The vestibular apparatus is anatomically associated with the inner ear, but its main functions are equilibrium and detection of head position and motion. The vestibular apparatus contains 3 fluid-filled semicircular canals, arranged at right angles to one another in the 3 dimensional planes. Fluid movement occurring within the canals due to head movement causes deflection of the cupula (a bundle of hair cells within a gelatinous cap). The saccule and utricle are also part of the vestibular apparatus. These structures also contain mechanoreceptors that are activated by an overlying layer containing calcium carbonates deposits (otoconia). These structures sense head position and acceleration. 3. The chemical senses are taste and smell. They are alike in that both require a moist surface for proper function. Taste receptors (salt, sour, sweet, and bitter) are found in taste buds on surface of the tongue. Each taste bud consists of several taste receptor cells which protrude through a common taste pore. Taste is not a highly discriminating sense, and there is functional overlap between taste receptor cells. In contrast, odor is highly discriminating. The olfactory receptors contain odor sensitive cilia which project into the mucous layer of the olfactory epithelium in the nasal passages. There are several types of olfactory receptor, each of which senses a particular odorant. Identification of an odorant is a "lock and key" relationship between the odorant and the membrane receptor. Sensory input from like olfactory receptors synapse at a structure called the glomerulus. Mitral cells transmit this olfactory information from the glomerulus to the olfactory nerve. Olfactory information then passes to either the limbic system or via the thalamus to the cerebral cortex where an olfactory map exists. 4. Other special senses exist in vertebrates, such as the ability to generate weak electric fields in some bony fish and sharks. These electric fields are used in courtship, territoriality, navigation, and prey capture. Some snakes, such as pit vipers, can sense infrared radiation (=heat) which they use to identify the presence of a warm blooded animal. Snakes and some mammals have a well-developed vomeronasal organ which is used to sense odorants, such as a scent trail or a pheromone. Finally, some vertebrates sense the earth's magnetic field which they use for navigation. LECTURE OUTLINE I. HEARING AND BALANCE
A. Introduction
1. Relationship between hearing and balance
2. Ear is divided into
a. outer (reception of sound)
b. middle (transfer and amplication of sound)
c. inner ear (senses the sound)
3. The hair cells in the Organ of Corti sense sound
a. hair cell function **
B. How does the ear hear?
1. Pitch (frequency of the sound)
a. K+ currents activate hair cells
b. Contraction of the outer hair cells increases frequency
sensitivity
2. Loudness (amplitude of the sound)
a. High noise levels and deafness
3. Timbre (sound quality and charactersitics--overtones) **
4. Integration of auditory input (1. cochlear nuc, inferior
colliculus, thalamus, cortex or 2. cochlear nuc, inferior
colliculus, superior colliculus/cerebellum)
a. Overlapping maps exist in midbrain
5. Tonotopic map in temporal cortex has a columnar organization
6. Summary (Animation)
C. Balance and the vestibular apparatus
1. Motion
a. Roles of semicircular canals, cupula, and kinocilium
2. Position
a. Roles of utricle, saccule, and otoliths **
D. Other vertebrate systems that sense vibrations
1. Lateral line system in fish
a. Lateral line system consists of neuromasts
1) Used to identify objects, monitor swimming, and determine
water depth
2) Used in polarized schooling behavior
2. Lower jaw, lung, and belly sense vibrations in snakes and
salamanders
II. HOW OWLS AND BATS FIND THEIR PREY: A ROLE FOR HEARING
A. Owl Prey Capture
1. Method of attack
a. Experimental evidence that hearing is important
b. How the barn owl hears
1) timing of a sound--along the horizontal axis
a) importance of coincidence detectors for timing
2) loudness of a sound--along the vertical axis
a) role for asymmetry between ears
2. Tonotopic map exists in the ICX (tectum)
3. Vision facilitates the learning of auditory cues during early
development only **
B. Bat Prey Capture
1. Introduction to bats and echolocation
a. Define frequency and harmonics
2. Hearing is central to prey capture
a. Early experimental evidence
b. FM Sweep (FM) and Constant Frequency (CF) signatures
3. A typical attack involves approach, attack, and the terminal phase
of prey capture
4. Using echolocation bats calculate: Distance to prey, size of prey,
lateral and vertical prey position, and the velocity at which the
prey moves
a. Doppler effects: What are they and how are they used by the bat?
5. Integration of sensory input
a. Inferior colliculus
1) pulse-echo delay neurons
b. Medial geniculate nucleus in thalamus
1) coincidence detectors
c. Tonotopic maps in auditory cortex (FM-FM, CF-CF, and DSCF
areas)
C. Sound communication is also important for aquatic mammals
1. Singing in humpback whales is probably related to courtship
2. Porpoise echolocation
III. CHEMICAL SENSES: TASTE AND SMELL
A. How are taste and smell related?
1. Summary of functional similarities
B. Taste
1. How chemical stimuli activate taste receptors
a. How do we sense taste?
2. Integration of taste
C. Olfaction
1. Functional organization of the olfactory epithelium
a. Roles of the olfactory epithelium, glomerulus, and mitral cells-
olfactory nerve in smell
2. Odor is a "lock and key" association
a. an example of how odorants work
3. Integration of olfaction (projections to olfactory bulb--
then to the amyygdala or entorhinal cortex)
4. Odorants and their role in animal behavior
a. Pheromones are special chemicals produced by one individual to
elicit a specific behavior in another individual
1) Examples of pheromones
b. The snake's "forked tongue" detects odors
c. Homing of migrating salmon
1) Getting back to the breeding grounds is done by smell **
IV. OTHER SPECIAL SENSES FOUND IN THE VERTEBRATES
A. Electric Organs in Fish
1. There are 1) strongly electric fish and 2) weakly electric fish
a. Strongly electric fish stun their prey before capture.
b. Weakly electric fish live in muddy waters, are nocturnal, and
have poor vision.
2. Discharge Organ is in tail (-); Electroreceptors in head (+) and
lateral line (+)
a. Electric Organ Discharge (EOD) is generated in the electric
organ by electrocytes. (Animation)
b. Nerve stimulation (Ach) causes an EPSP, activating the
Electric Organ to discharge.
c. Conductors and insulators in the environment deflect the
electrical field.
1) Changes in the electric field are used for navigation.
3. Other uses of weak electrical fields: Identify other individuals
(Jamming avoidance response, JAR), courtship, territorially, prey
identification, etc.
B. Magnetic Fields in Vertebrate Orientation
1. Characteristics of the earth's magnetic field
a. Latitude is determined by the inclination of the magnetic
field
b. Numerous vertebrates use the magnetic field for orientation
1) Newt's escape to the home pond; loggerhead turtles
return to breeding grounds; bird migration.
2. Example 1: Pigeon homing is knowing the latitude and longitude of
the home roost
a. Pigeons also use the sun's movement across the sky
b. On cloudy days, they use the magnetic field of the earth
c. Experimental evidence: Studies using magnets and flights over
magnetic anomalies (Iron Mountain in Rhode Island) **
C. Heat Sensors in Snakes
1. The Electromagnetic Spectrum: Relationship between infrared
(heat) and the visible spectrum
2. Using Heat (=Infrared, IR) Sensors in Pit Vipers
a. pit vipers use triangulation to find a prey
b. nasal pits contain IR sensory cells
c. nasal pits can sense a 0.03oC temperature
difference
3. Sensory IR and visual inputs are mapped onto the optic tectum
a. Optic tectum cells can be "or" and "and" cells sensitive to
IR/visible wavelengths
Seeing,
Hearing, and Smelling the World. An interesting, animated web page
from the Howard Hughes Medical Institute dealing with various aspects
of our senses, including illusions.
Somatosensory System. General discussion of the somatosensory pathways, including touch, pain, and temperature from the Washington University School of Medicine Vomeronasal Organ. An accessory olfactory system which senses intraspecific pheromones and other odorants. Animations on how we hear from the University of Western Ontario Navigating with a built-in compass. How magnets are used in animal orientation Electric fish: An introduction. The JAR of Electric Fish: A detailed account of the Jamming Avoidance Response (JAR). Weakly Electric Fish. Movies, biophysical considerations and more... The Neurobiology of Bat Echolocation Bat
Echolocation. Listen to the bats. Go to: |
