Sensory Receptors

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• Sensory Receptors
• 8th ed 50.1 to 50.4
• 7th ed 49.1 to 49.4

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• Physiological basis for all animal activity
  processing sensory information and generating
  motor output in response to that information

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• This is a continuous cycle
• Sensory information can be from external or
  internal environment.

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• Sensory pathway
• Sensory reception
• Transduction
• Transmission
• Perception
• Amplification and adaptation

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• Sensory pathway
• Sensory reception
• Transduction
• Transmission
• Perception
• Amplification and adaptation

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• Sensory reception
• 1st step of sensory pathway
• Sensory receptors Specialized neurons or
  epithelial cells Single cells or a collection of
  cells in organs
• Very sensitive

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• Sensory pathway
• Sensory reception
• Transduction
• Transmission
• Perception
• Amplification and adaptation

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• Sensory transduction
• Conversion of physical, chemical and other
  stimuli to change in membrane potential
• Receptor potential
• change in membrane potential itself

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• Sensory pathway
• Sensory reception
• Transduction
• Transmission
• Perception
• Amplification and adaptation

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• Transmission
• Sensory information is transmitted through the
  nervous system as nerve impulses or action
  potential to the Central Nervous System (CNS).
• Some axons can extend directly into the CNS and
  some form synapses with dendrites of other
  neurons
• Sensory neurons spontaneously generate action
  potential without stimulus at a low rate

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• Magnitude of receptor potential controls the rate
  at which action potentials are generated (larger
  receptor potential results in more frequent
  action potentials)

Weak muscle stretch
Strong muscle stretch
Muscle
Dendrites
Receptor potential
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50
70
70
Stretch receptor
Membrane potential (mV)
Action potentials
0
0
Axon
70
70
0
1
2
3
4
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6
7
0
1
2
3
4
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Time (sec)
Time (sec)
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• Sensory pathway
• Sensory reception
• Transduction
• Transmission
• Perception
• Amplification and adaptation

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• Perception
• Action potential reach the brain via sensory
  neurons, generating perception of a stimulus
• All action potentials have the same property,
  what makes the perceptions different are the part
  of the brain they link to.

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• Sensory pathway
• Sensory reception
• Transduction
• Transmission
• Perception
• Amplification and adaptation

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• Amplification and adaptation
• Strengthening of stimulus energy during
  transduction (involves second messengers)
• Continued adaptation decrease in responsiveness
  upon prolonged stimulation

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• Types of sensory receptors
• Mechanoreceptors
• Chemoreceptors
• Electromagnetic receptors
• Thermoreceptors
• Pain receptors

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• Mechanoreceptors
• sense physical deformation caused by pressure,
  touch, stretch, motion, sound

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• Stretch receptors are mechanoreceptors are
  dendrites that spiral around small skeletal
  muscle fibers

Weak muscle stretch
Strong muscle stretch
Muscle
Dendrites
Receptor potential
50
50
70
70
Stretch receptor
Membrane potential (mV)
Action potentials
0
0
Axon
70
70
0
1
2
3
4
5
6
7
0
1
2
3
4
5
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Time (sec)
Time (sec)
in the axon of the stretch receptor. A stronger
stretch produces a larger receptor potential and
higher frequency of action potentials.
Crayfish stretch receptors have dendrites
embedded in abdominal muscles. When the abdomen
bends,
muscles and dendrites stretch, producing a
receptor potential in the stretch receptor. The
receptor potential triggers action potentials
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Hair
Light touch
Strong pressure

• Touch receptors (light and deep touch) are
  embedded in connective tissue

Epidermis
Dermis
Hypodermis
Connective tissue
Nerve
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LE 49-4

• Chemoreceptors
• General receptors respond to total solute
  concentrations
• Specific receptors respond to concentrations of
  specific molecules

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• Electromagnetic receptors
• Detect various forms of electromagnetic energy
  like visible light, electricity, magnetism
• Snakes can have very sensitive infrared receptors
  detect body heat of prey
• Animals can use earths magnetic field lines to
  orient themselves during migration (magnetite in
  body) orientation mechanism

Eye
Infrared receptor
This rattlesnake and other pit vipers have a pair
of infrared receptors, one between each eye and
nostril. The organs are sensitive enough to
detect the infrared radiation emitted by a warm
mouse a meter away.
Some migrating animals, such as these beluga
whales, apparently sense Earths magnetic field
and use the information, along with other cues,
for orientation.
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Cold
Hair
Heat

• Thermoreceptors
• Detect heat and cold
• Located in skin and anterior hypothalamus
• Mammals have many thermoreceptors each for a
  specific temperature range

Connective tissue
Nerve
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• Capsaicin triggers the same thermoreceptors as
  high temperature
• Menthol triggers the same receptors as cold
  (lt28oC)

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Hair
Pain

• Pain receptors (nociceptors)
• Stimulated by things that are harmful high
  temperature, high pressure, noxious chemicals,
  inflammations
• Defensive function

Connective tissue
Nerve
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• Sensing gravity and sound in invertebrates
• Hair of different stiffness and length vibrate at
  different frequencies and pick up sound waves and
  vibrations

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• Statocyts
• organ with ciliated receptor cells surrounding a
  chamber containing statoliths in invertebrates
  sense gravity

Ciliated receptor cells
Cilia
Statolith
Sensory nerve fibers
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Tympanic membrane
1 mm

• Tympanic membrane stretched over their ear help
  sense vibrations

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• Sensing gravity and sound in humans
• Human ear sensory organ for hearing and
  equilibrium
• Our organ for hearing hair cells are
  mechanoreceptors because they respond to
  vibrations
• Moving air pressure is converted to fluid
  pressure

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• Ear structure
• Outer ear pinna, auditory canal, tympanic
  membrane (separates outer and middle ear)

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• Middle ear three small bones malleus (hammer),
  incus (anvil) and stapes (stirrup) transmit
  vibrations from tympanic membrane to the oval
  window.
• Eustachian tube connects middle ear to the
  pharynx and equalizes pressure
• Inner ear consists of fluid filled chambers
  including semicircular canals (equilibrium) and
  cochlea (hearing)

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• Moving air travels through the air canal and
  causes the tympanic membrane to vibrate
• Three bones of the middle ear transmit the
  vibrations to the oval window, a membrane on the
  cochlear surface
• That causes pressure waves in the fluid inside
  the cochlea

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• The cochlea
• Upper vestibular canal and inner tympanic canal
  filled with perilymph middle cochlear duct
  filled with endolymph

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• Organ of Corti
• Floor of the cochlear duct is the basilar
  membrane
• Organ of corti is located on the basilar
  membrane, with hair cells which has hair
  projecting into the cochlear duct.
• Many of the hairs are attached to the overhanging
  tectorial membrane.
• Sound waves cause the basilar membrane to
  vibrate. This results in displacement and
  bending of the hair cells within the bundle.
• This activates the mechanoreceptors, changes the
  hair cell membrane potential (sensory
  transduction) which generates action potential in
  the sensory neuron.

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Cochlea
Stapes
Axons of sensory neurons
Vestibular canal
Perilymph
Oval window
Apex
Base
Tympanic canal
Basilar membrane
Round window
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• Equilibrium
• In the vestibule behind the oval window are
  urticle, saccule and three semicircular canals

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• Three semicircular canals arranged in three
  spatial planes detect angular movements of the
  head
• The hair cells form a cluster. They have a
  gelatinous capula.
• Fluid in the semicircular canals pushes against
  the capula deflecting the hairs, stimulates the
  neurons

Semicircular canals
Ampulla
Flow of endolymph
Flow of endolymph
Vestibular nerve
Cupula
Hairs
Hair cell
Vestibule
Nerve fibers
Utricle
Body movement
Saccule
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• Utricle (oriented horizontally), saccule
  (oriented vertically) tell the brian which way is
  up and the position of the body and acceleration
• Sheet of hair cells project into a gelatinous
  capula embedded with otoliths (ear stones).
  Movement of the head causes otoliths to in
  different directions against the hair protruding
  from the hair cells. This movement is detected
  by the sensory neurons
• Dizziness false sensation of angular motion

http//www.dizziness-and-balance.com/disorders/bpp
v/otoliths.html
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• Hearing and equilibrium in fish
• Vibrations in the water conducted by skeleton
  to inner ear canals, move otoliths which
  stimulate hair cells
• Swim bladder air filled, responds to sound
• Lateral line sense organ

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• Lateral line sense organ
• Water flows through the system
• Bends hair cells generates receptor potential
• Nerve carries action potential to the brain
• Helps them sense water currents, moving objects,
  low frequency sounds

Lateral line
Lateral line canal
Scale
Opening of lateral line canal
Epidermis
Neuromast
Lateral nerve
Segmental muscles of body wall
Cupula
Sensory hairs
Supporting cell
Hair cell
Nerve fiber
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• Vision
• Photoreceptors
• Planarians Ocelli or eye spots in the head
  region
• Light stimulates photoreceptors
• Brain compares rate of action potential coming
  form the two ocelli
• Brain directs the body to turn until sensation
  form both ocelli are equal and minimal
• Animal can move to shade, under a rock away from
  predators

Light
Light shining from the front is detected
Photoreceptor
Nerve to brain
Visual pigment
Screening pigment
Ocellus
Light shining from behind is blocked by the
screening pigment
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• Compound eyes
• Very good at detecting movement
• Very good at detecting flickering light (6 times
  faster than human eye)
• Some bees can see in the ultraviolet range of
  light

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• Several thousand omatidia (facets) in every eye
• Cornea and crystalline cone form the lens which
  focuses light on the rhabdom
• Light stimulates the photoreceptors to generate
  receptor potential which generates action
  potential

Cornea
Lens
Crystalline cone
Rhabdom
Photoreceptor
Axons
2 mm
Ommatidium
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• Vertebrate eye
• Single lens system (very different from
  invertebrate single eyes)

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Choroid
Sclera

• Eyeball or globe consists of
• Sclera tough white outer connective tissue
• Cornea clear part of sclera in the front of the
  eye lets light into the eye, acts as a fixed
  lens
• Choroid pigmented inner layer forms iris
  (doughnut shaped) can change size to regulate
  the amount of light coming in

Iris
Cornea
Optic nerve
Pupil
Central artery and vein of the retina
Optic disk (blind spot)
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• Retina innermost layer with neurons and
  photoreceptors
• Aqueous humor fluid that fills the anterior
  cavity (blockage of ducts increases pressure and
  causes glaucoma)
• Vitreous humor jellylike, fills the posterior
  chamber

Retina
Fovea (center of visual field)
Aqueous humor
Vitreous humor
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• Lens clear disk of protein

Ciliary body
Suspensory ligament
Lens
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Front view of lens and ciliary muscle

• Humans and other mammals
• spherical ( ciliary muscles contract, sensory
  ligaments relax near objects)
• flatter (ciliary muscles relax, edge of choroid
  moves away from lens, suspensory ligaments
  contract and pull the lens distant objects)

Lens (rounder)
Choroid
Retina
Ciliary muscle
Suspensory ligaments
Near vision (accommodation)
Lens (flatter)
Distance vision
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• Fishes, squids and octopuses focus by moving lens
  forward and backward

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• Photoreceptors
• Rods sensitive to light, do not distinguish
  colors
• Cones detect color, not very sensitive to light
• Nocturnal animals have a higher proportion of rods

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• Rods and cones have stacks of disks
• rhodopsin (retinal - vitamin A derivative
  opsin) in the membrane
• get activated and cause sensory transduction

Rod
Outer segment
Disks
Inside of disk
Cell body
Synaptic terminal
Cytosol
Retinal
Rhodopsin
Opsin
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• Information from the each eye is carried by the
  optic nerve (each with about a million axons)
• Optic nerves meet cross at the optic chiasm
• Information from right visual field of both eyes
  goes to the left side of the brain
• Information from the left visual field of both
  eyes goes to the right side of the brain
• Synapse with interneurons which take the
  information to the primary visual cortex

Left visual field
Right visual field
Left eye
Right eye
Optic chiasm
Optic nerve
Lateral geniculate nucleus
Primary visual cortex
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• Perception of gustation (taste) and olfaction
  (smell)
• Insects
• In insects taste sensation is located within
  sensory hairs called sensilla (on feet and
  mouthparts)
• Olfactory odorants are detected by olfactory hairs

Sensillum
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• Mammals
• In mammals specialized epithelial cells form
  taste buds
• Tastants detect five perceptions of taste
  sweet, sour, salty, bitter and savory (MSG)
• Chemoreceptors generate receptor potential by
  triggering a chain of reactions involving
  different proteins for different tastes in the
  receptor cells

Sugar molecule
Taste pore
Sensory receptor cells
Taste bud
Sensory neuron
Tongue
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• Sensory neurons lining the nasal cavity and
  extending into the mucus layer get stimulated by
  odorants. The stimulus is transmitted directly
  to the olfactory bulb of the brain

Brain
potentials
Action
Olfactory bulb
Nasal cavity
Bone
Odorant
Epithelial cell
Odorant receptors
Chemoreceptor
Plasma membrane
Cilia
Odorant
Mucus