23: Physiology of Senses Special vs Somatic Senses Olfaction

Vision

Hearing/Balance

hedgehog tree shrew monkey

rhinal fissure

Estimating the size of a brain subunit: the neocortex of mammals:  

1) rhinal fissure placement

2) cerebral sulcation

3

Q: High-level cognition without selection (i.e., neocortex) of the forebrain?

Burish, Kueh, and Wang. 2002.

Q: What aspect(s) of bird behavior correlate most strongly with a large telencephalic (≈ forebrain) fraction of the brain?

Burish, Kueh, and Wang. 2002.

0.552

Pigeons have a telencephalic fraction of 0.552…..

…and can distinguish paintings by Picasso and Monet. Watanabe et al. 1995

Brain to body connection

Q. How does brain receive and send information?

Signal Transduction

Q: How do environmental stimuli initiate APs? See  text  box,  p.  338

CNS vs PNS **

*

General vs special senses

chemical energy electromagenetic

energy

mechanical energy

smell

hearing and balance

visionthermo-reception

General sensation enters the CNS via the sensory component of the general cranial nerves – V, VII, IX, X – as well as spinal nerves from the rest of the body Special sensation enters the CNS via the special sensory nerves – cranial nerves I, II, and VIII. They lack motor components and are located only in the head.

general or somatic senses special senses

touch

taste

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Afferent Pathways to/in brain

12

Senses: G Protein Coupled Receptors

Special Sensation

Electromagnetic energy

Mechanical energy

Chemical energy

The principle of “labeled lines”

Q: Does an incoming sensory AP reflect the environmental modality that generated it?

Northcutt 2002

Cranial nerves I II VIII Special senses

The same basic brain areas that: 1) are stimulated by a particular type of environmental energy 2) transduced in a specialized sense organ 3) reaches the brain via a “dedicated” special sensory cranial nerve

can be recognized in all vertebrate brains

Clark et al. 2001

Size comparison of of special sense referral areas often allows identification of dominant sensory modalities in non-mammals.

Secondary integration in the cerebrum obscures this pattern in mammals.

Special senses: Olfaction

The signal cascade of olfaction in mammals

APs generated by olfactory neurons are integrated in glomeruli of the olfactory bulb.

Shepherd 2006

Olfaction is used in near sensing,

remote sensing, and

to signal territories or sexual receptivity

Q: Why do some sharks have hammer heads?

Kaijura et al. 2005

Hammerheads sample a broader sample of water and have increased “klinotaxis” (but not larger olfactory epithelia) than other sharks

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Humans: 5-10 millionRodents: 15-20 millionDogs: 200 million

Each ON has one gene coding for receptor

Rodents: 1000 functional OR genes/1300 total genesChimps: 500 functional OR genes/1000 total genesHumans: 300 functional OR genes/1000 total genes

Q: How well do humans smell?

1. Relatively small olfactory bulbs (30% volume relative to brain size)3. Fewer functioning olfactory receptors (ORs)2. Fewer olfactory neurons (ONs)

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Q: Are our assumptions correct?

We’re bipedal

We don’t sniff deeply very often

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Porter et al. 2007

10 meter long scent trail: 67% successAble to detect direction (left vs right nose)

Q: How well do humans smell when we try?

23

VISIONTransduction in vision Accommodation

Color visionEcophysiology of vision

Terrestrial vs aquatic vision Diurnal vs nocturnal vision

Q: How does the retina generate receptor potentials?

1878 experiments by Wilhelm Kühne

Rhodopsin retinal + opsin

light

Rhodopsin is composed of

retinal

opsin

cell membrane

When exposed to light, rhodopsin

dissociates

changes color (“bleaches”)

Anatomy and physiology of rods

In the light:

cGMP is enzymatically degraded

Na+ channels close, and receptor cells hyperpolarize

In the dark:

Na+ channels are open(cGMP is bound)

Na+ entry and K+ exit is countered by the Na+/K+ pump

“dark current”

light

Integrative cell layers in the retina

Axons of ganglion cells exit as the optic nerve, creating a “blind spot.”

light

Eye structure

Pathways of visual information from the eye

Refractive indices air 1.0 water 1.3 cornea 1.3 lens of eye 1.4 glass 1.5 diamond 2.4

low refractive indexhigh refractive index

Accommodation (focusing light on the retina) requires:

1. materials of different refractive index

2. oblique orientation of light and refractor

Most vertebrate eyes have two refractors - one fixed and one variable

Close and distant vision require different degrees of refraction

The cornea is a fixed refractor The lens is a variable refractor

“Spectral tuning” in coho salmon ontogeny

Wavelength of maximum absorbance in cones of of different ontogenetic stages / environments. Temple et al. 2008

alevins freshwater

parr freshwater

smolts oceanic

Color vision

rods

rods and cones

Hearing Transduction in hair cells Lateral-line system

Electroreception Vestibular and acoustic systems

Characteristics of sound Inner, middle, and outer ears

Sound for communication and navigation

Transduction in hair cells

1) External structure deflected

2) Deformation opens cation channels

3) Change in cell membrane potential

4) Inhibition or promotion of APs in the integrative cells that synapse with the transducing cell.

Hair cell development and occurrence

Placodes “sink” into the dermis (lateral line and ampullae of Lorenzini) or dermal bone (inner ear) during development.

1. Lateral line

3. Vestibular and acoustic systems of the inner ear

2. Ampullae of Lorenzini

Blinded fish can still school if their lateral line system is intact.

…or in linear canals beneath scales (many fish).

1. Hair cells of the lateral line system :

located superficially in frogs…..

2. Ampullae of Lorenzini – passive electroreception

Scyliorhinus (catshark)

Kalmijn 1971

3. Acoustic (hearing) and Vestibular (balance) systems

located in the INNER ear = a WATER-BASED system based on fluid movement and the deflection of hair cell cilia.

bony fish bird mammal

VV V

AA

Vestibular system

Hair cells in the semicircular canals detect angular movements of the head.

Hair cells in the maculae of the utricle and sacculus detect linear acceleration.

otoliths

“X”“Y”

Acoustic System Mechanical disturbances of the environment, transferred to the endolymph of the cochlea, are detected by hair cells.

Deflection of stereocilia during basilar membrane vibration

Nerve VIII

Central nervous system referrals of acoustic system action potentials are interpreted as sound.

Q: is “ringing in your ears” really sound?