Download The Senses

The Senses
Chapter 29
Sensory Input
• All senses trigger the same type of action
potential
▫ The part of the brain that is activated discriminates
between the types of stimuli
• The brain detects sensations and interprets
them
▫ Action potentials from sensory receptors
• Perceptions are constructed from sensations
being integrated with other information
▫ Results from neuronal communication in complex
circuits involving multiple brain areas
Detecting a Stimulus
• Sensory organs detect stimuli via specialized cells or
neurons called sensory receptors
• The cells convert 1 type of signal to an electrical
signal, called sensory transduction
• Produces a receptor potential, a graded change in
response to opening or closing ion channels
• Neurotransmitter release is increased to signal the
CNS
▫ The rate change in action potentials signals the brain
o Sensory neurons from each type of sensory receptor synapse with different
interneurons
o Sensory adaptation, tendency of some sensory receptors to become less
sensitive when stimulated repeatedly
o Fewer action potentials which limits reactions to normal background
stimuli
Types of Sensory Receptors
• Pain receptors – tissue damage
▫ All parts of the body but brain
▫ Respond to excess heat, pressure, or chemicals
• Thermoreceptors – hot and cold
▫ Monitor both skin surface and blood temperature
• Mechanoreceptors – touch and pressure
▫ Bend or stretch PM of receptor cell changing permeability
▫ Hair cells detect sound waves and other movements in
H2O
• Chemoreceptors – chemicals in the environment
▫ Internal ones to monitor arteries and O2 changes
▫ Osmoreceptors detect changes in blood solute
concentration and can stimulate thirst
• Electromagnetic receptors – energy wavelengths
▫ Detect muscle movement in prey or help migrate
▫ Photoreceptors detect light via pigments that absorb
light
Receptors for
discriminate
stimuli
Electromagnetic receptors
for infrared light
Chemoreceptors for
pheromones
Mechanoreceptors
for sound waves
Homonculus
Larger areas have
more sensory
receptors
devoted to it.
Necessary for
greater control
of mouth and
fingers than
shoulders.
Gross Anatomy of the Ear
• 3 main regions
• Eardrum separates outer and
inner
▫ Sound pressure moves ear drum to
move ossicles in inner ear
 Hammer, anvil, and stirrup
amplify signal
• Auditory tube equalizes pressure
on either side of ear drum (‘pop’)
• Semicircular canals for balance
• Cochlea houses the hearing organ
Anatomy of the Inner Ear
• Organ of Corti is the hearing organ
▫ Contains hair cells embedded in a basilar membrane
• Tectorial membrane bends hair cells as basilar membrane
moves in response to sound waves
• Signal to neurons to auditory nerve
Volume and Pitch
• Sounds detected as increase in action potentials by the brain
▫ Higher volume = higher amplitude of generated wave
▫ Creates more vigorous vibrations in cochlea = more bending = more
action potentials
• Pitch depends on frequency
▫ High pitch = higher frequency
▫ Basilar membrane responsive to certain frequencies
• Hair cells easily damaged due to prolonged exposure to certain
frequencies
Balance
• Body position and movement detected by fluid filled
structures in the ears
▫ All operate on principle of bending hairs of hair cells
• Semicircular canals detect changes in head’s rate of
rotation or movement
▫ 3 canals, 1 in each plane of movement
• Head movements cause fluid movement (slower)
▫ Changes fluid pressure changing action potentials
▫ Sudden stop, but fluid still moving = dizzy
▫ Brain interprets and commands skeletal muscles
Motion Sickness
• Results from conflict between eyes and
equilibrium sensors in the inner ear
▫ Feeling motion, but not seeing it
▫ One system in hallucinating, implying toxins in
system = vomiting
• Dramamine inhibits input from equilibrium
sensors
• Some people can control normally subconscious
body functions, like vomiting
The Evolving Eye
• Eye cups provide information about light intensity
and direction, but no image formed
• 2 major types of image forming eyes have evolved
▫ Compound eye: light-detecting units called ommatidia
 Each has its own light focusing lens and photoreceptors
 Acute motion detectors and color vision
▫ Single-lens eye: evolved in invertebrates and
vertebrates independently
 Amount of light entering a pupil is regulated by an iris
 Passes through a lens which focuses light on the retina
Lenses
• Focus light onto retina
▫ Squid and fish have rigid eyes
so use muscle movement
▫ Mammals use ciliary muscles
to change lens shape
• Near objects
▫ Contract to round/thicken
lens
• Distance
▫ Relax to flatten/thin lens
Correcting Vision
• Vision tests measure visual acuity or ability to
distinguish fine detail
▫ 20/20 is normal – chart read at 20 feet
▫ 20/10 better than normal – read at 20 instead of 10
▫ 20/50 worse than normal – read at 20 instead of 50
• Visual problems often are focusing problems
▫ Nearsightedness: can’t focus at distances, eye longer
▫ Farsightedness: can’t focus close, eye shorter
▫ Astigmatism: blurred vision from misshaped lens
• LASIK reshapes cornea to change focusing ability
Photoreceptors
• 2 types
▫ Rods: sensitive to light
▫ Cones: detect color
 Red, blue, and green
• Number of each depends on
activity level at day and night
• Humans
 Rods peripherally and cones
centrally
• Opsins in each absorb light
▫ Rhodopsin in rods
▫ Photopsins in cones
 Cone type absorbs color
• Light to back of eye forward
• Multiple neurons to enhance
edges
• 3D images from 2 eyes
Olfaction
• Olfactory receptors detects
airborne molecules
▫ Upper portion of nasal cavity
▫ Impulses to olfactory bulb
▫ Cilia from tips in mucous to
detect substances
• Odors bind to specific
receptors that trigger
membrane changes
• Cause action potentials
• Humans can detect 1000’s of
odors
▫ Animals can detect more,
sensation is more important
Taste
• Taste buds detect molecules in
solution
• Four familiar and 1 other taste
▫
▫
▫
▫
▫
Sweet
Sour
Salty
Bitter
Umami –meats, cheeses, and
protein-rich foods
• Each receptor responsive to a
particular type of substance
▫ Many ‘tastes’ are really smell
(head colds)
Supertasters
• 3 times as sensitive to bitter tastes
▫ Perceive more foods as bitter than normal, so more
food dislikes
▫ Typically avoid coffee, alcohol, vegetables like spinach,
cabbage, and broccoli
• Two tests
▫ Hypersensitivity to propylthiouracil
▫ Increased number of fungiform papillae, house
tastebuds
• Higher risk of obesity, colon cancer, and other
health problems