Sensory Systems

1
Sensory Systems

• Brought to you by Zoe and Vince

2
Categories of Sensory Receptors and Their Actions

• Sensory information is conveyed to the central
  nervous system (CNS) in a 4-step process
• Stimulation
• Transduction - stimulus is transformed into
  receptor potential in sensory receptor
• Transmission - axon of sensory neuron conducts
  action potentials
• Interpretation - sensory perception created by
  brain.

3

• 3 classes of environmental stimuli
• Mechanical forces (stimulate mechanoreceptors)
• Chemicals (stimulate chemoreceptors)
• Electromagnetic/thermal energy (stimulate
  photoreceptors and others)

4

• Free nerve endings - the simplest sensory
  receptors, respond to the bending or stretching
  of sensory neuron membrane, change in temp, or
  chemicals like oxygen in the extracellular fluid.

5

• Exteroceptors - sense stimuli in external
  environment
• Exterior senses evolved before vertebrates came
  on land
• Some function in water and on land
• Mammalian hearing, uses similar receptors
  originally used in water
• Some cant function in air
• Electrical organs of fish
• Some cant function in water
• Infrared receptors

6

• Sensory systems provide levels of information
  about the external environment
• Determine an object is present
• Where the object is located
• Make a 3-D image of the object and its
  surroundings

7

• Interoceptors - sense stimuli from within body
• Muscle length/tension, limb position, pain, blood
  chemistry, blood volume/pressure and body temp
• Simpler than exteroceptors
• Close to primitive sensory receptors

8

• Sensory cells respond to simuli due to
  stimulus-gated ion channels in membranes
• Stimulus causes channels to open and close
  depending on the system
• Depolarization of receptor cell (receptor
  potential)
• Larger stimulus, greater degree of polarization
• Decrease in size with distance from source
• Prevents irrelevant stimuli reception
• If great enough, production of action potentials

9
Temperature and Pressure

• Cutaneous receptors - receptors of the skin
  (interoceptors)
• Respond to stimuli at border between external and
  internal environments

10

• Thermoreceptors - naked, dendritic endings of
  sensory neurons sensitive to temp changes.
• Found within the hypothalamus and monitor temp of
  circulating blood which provides the CNS with
  info on the bodys core temp.
• Skin has two populations of thermoreceptors

11

• Cold receptors and warm receptors - stimulated by
  corresponding temps and inhibited by opposite
  temps
• Cold receptors are right below the epidermis,
  warm receptors are slightly deeper in the dermis

12

• Nociceptors - transmit impulses perceived by the
  brain as pain
• Most consist of free nerve endings throughout the
  body near surfaces where damage is likely to
  occur
• Some sensitive to actual tissue damage, others
  respond before damage

13

• Mechanoreceptors - contain sensory cells with ion
  channels sensitive to mechanical force applied to
  the membrane
• Present in skin, dermis and subcutaneous tissue
• Concentrated on fingertips and face
• Phasic - intermittently activated. Tonic -
  continuously activated.
• Monitor duration of a touch and extent its
  applied
• Pacinian corpuscles- monitor onset and removal of
  pressure

14
Muscle Contraction

• Within skeletal muscles of all vertebrates except
  bony fish are muscle spindles
• Sensory stretch receptors
• Consist of thin fibers wrapped together,
  innervated by sensory neuron activated when
  muscle stretches

15

• Proprioceptors - muscle spindles and receptors in
  tendons and joints providing info about
  position/movement of body parts
• Muscle contracts, tension on attached tendon
• Monitored by Golgi tension organs
• If high, elicits reflex to inhibit active muscle
• Ensures muscles dont damage tendons by
  contracting too strongly

16
Blood pressure

• Monitored at 2 main sites
• Carotid sinus and aortic arch
• Walls of blood vessels have baroceptors
• Network of afferent neurons
• Detect tension/stretch in walls
• Blood pressure decreases, frequency of impulses
  decrease.
• CNS overcompensates for homeostasis and blood
  pressure increases, so does impulse frequency.

17
Taste/Smell/Body Position

• Taste buds - chemosensitive epithelial cells
• Fish have taste buds over their body, these are
  the most sensitive vertebrate chemoreceptors
• Important to bottom-feeders so they can sense the
  presence of food in murky environments

18

• Terrestrial vertebrates have taste buds on their
  tongue and oral cavity
• Papillae are raised areas bearing taste buds
• Taste buds are between 50-100 taste cells
• Microvilli on them called taste pore

19

• Salty/sour- directly through ion channels
• Sweet/bitter - bind to surface receptor proteins,
  trigger G proteins which changes interior of cell
  and open/closes ion channels
• Interact with other neurons carrying smell info
• Fly taste receptors are in sensory hairs on their
  feet

20
(No Transcript)
21

• Smell chemoreceptor is in upper nasal passages
• Since were surrounded by air, our sense of smell
  (olfaction) has specialized to detect airborne
  particles
• Extremely acute sense in mammals
• Humans can discern 1000s of smells in contrast to
  the 4 tastes.

22

• Internal chemoreceptors
• Peripheral - sensitive to plasma pH in
  aortic/carotid bodies
• Central - sensitive to cerebrospinal fluid pH in
  medulla oblongata
• Stimulation indirectly affects respiratory
  control center, increases breathing rate

23
Lateral Line System

• Provides fish with sense of distant touch
• Sense objects reflecting pressure waves and low
  vibrations
• Supplements hearing
• Longitudinal canal in skin along sides and in the
  head contain sensory structures
• Hair cells - hair-like processes project into
  capula (gelatinous membrane)
• Same length, stereocilia. Longer, kinocilium

24
(No Transcript)
25

• Vibrations move capula and hairs bend
• Stereocilia bent in direction of kinocilium,
  sensory neurons stimulated
• Opposite direction, activity inhibited

26
Gravity/Angular Acceleration

• Statocyst - helps invertebrates orient selves
  with gravity
• Ciliated hair cells in membane with crystals of
  calcium cabronate are statoliths (stones) that
  increase mass of membrane so cilia bend when
  position changes

27

• Similar structure in inner ear of vertebrates
• Membranous labyrinth of fluid-filled chambers and
  tubes
• Size of a pea

28

• Receptors of gravity in 2 chambers
• Utricle and saccule
• Similar to lateral line of fish
• Utricle - sensitive to horizontal acceleration
• Saccule - to vertical acceleration
• Continuous with 3 semicircular canals, which
  detect angular acceleration (head rotates)
• Together, all form vestibular apparatus
• Brain uses info to maintain balance and
  equilibrium

29
Hearing/Ears

• Works better in water because it transmits
  pressure waves more efficiently
• Auditory stimuli travel farther and quicker than
  chemical ones and provide better directional info

30
Structure of ear

• Vibration goes through ear canal to eardrum
  (tympanic membrane)
• Movement of 3 bones (ossicles) in middle ear
• Malleus (hammer)
• Ineus (anvil)
• Stapes (stirrups)
• Similar to Weberian ossicles of fish, which are
  small bones that vibrate to a fishs saccule

31
(No Transcript)
32

• Middle ear connected to throat by Eustachian tube
• Equalizes air pressure btw middle ear and
  external environment
• ear popping

33

• Inner ear consists of cochlea, a bony structure
  containing cochlear duct of membranous labyrinth
• Above is vestibular canal, below is tympanic
  canal
• Fluid-filled
• Stapes vibrate against oval window into inner
  ear, pressure waves to tympanic canal, pushing
  round window that transmits back to the middle ear

34
Transduction in Cochlea

• Bottom of duct, basilar membrane.
• Hair cells project into tectorial membrane
• Organ of Corti
• Basilar vibrates, cilia bend depolarizing hair
  cells which stimulate action potentials in
  neurons that project to the brain and are
  interpreted as sound

35
Frequency Localization in Cochlea

• Elastic fibers in basilar membrane
• Base of cochlea have short, stiff fibers
• At apex, 5x longer and 100x more flexible
• Resonant frequency higher at base (high pitches)
• Lower at apex

36

• Frequency range in humans between 20-20,000
  cycles per second (hertz) in kids
• High pitch detection ability down with age
• Dogs can hear 40,000 hertz

37
Sonar

• Our ears help determine direction of the source
  of sound, but not a reliable measure of distance
• Bats, shrews, dolphins, and whales perceive
  distance by sonar
• Echolation - emit sounds and determine time it
  takes sound to reach object and come back

38
Evolution of Eye

• Invertebrates have eyespots, sensitive to
  direction of light source but cant construct a
  visual image
• Annelids, mollusks, arthropod and chordates
  evolved image-forming eyes
• Believed to have evolved independently

39
Structure of Vertebrate Eye

• Sclera (white of eye) - tough connective tissue
• Transparent cornea - light enters and focus
  begins
• Iris- colored part of eye
• Pupil - opening in iris that contracts in bright
  light
• Lens - light passes through pupil to here where
  light is focused onto retina at back of the eye.

40
(No Transcript)
41

• Lens attached by suspensory ligament to ciliary
  muscles
• Shape influenced by amount of tension
• Ciliary muscle contracts, ligament slack, lens
  rounded and powerful
• Close vision
• Opposite reactions
• Far vision
• Near/far-sighted people dont properly focus
  image on retina
• Lens of fish/amphibians move in and out like
  camera to focus.

42
Vertebrate photoreceptors

• Retina has 2 photoreceptors (rods and cones)
• Rods - black and white vision when illumination
  is dim
• Cones - high visual acuity and color vision
• 100 million rods, 3 million cones in each retina

43

• Their light capturing molecules (or
  photopigments) are
• Rhodopsin in rods, photopsin in cones
• 3 kinds of cones in humans
• Different AA sequences causes shift in absorption
  maximum
• Region of electromagnetic spectrum best absorbed
  by pigment

44

• 500 max in rhodopsin
• In photopsin, 420 (blue), 530 (green) and 560
  (red)
• Cones also referred to as blue, green and red
  cones
• Most vertebrates have color vision
• Fish/turtles/birds, 4-5 cones see near
  ultra-violet light
• Many mammals have only 2 (squirrels)

45

• Retina is made of 3 layers
• 1. Rods and cones
• 2. Bipolar cells
• 3. Ganglion cells
• Flows of sensory info opposite to path of light
  through retina

46

• A rod or cone contains sodium channels in the
  plasma membrane, may of which are open in the
  dark
• Dark current
• Cyclic guanosine monophosphate (cGMP) required to
  keep channels open

47
Visual Processing in Vertebrate Retina

• Action potentials gathered along ganglion cells
  are relayed through the lateral geniculate nuclei
  of the thalamus and projected to the occipital
  lobe
• Binocular vision
• When both eyes are fixed on one object, each eye
  views object from different angle
• Useful for predators
• Prey have an enlarged overall receptive field
  rather than binocular vision

48
Diversity of Sensory Experiences

• Heat
• Electromagnetic radiation is too low in energy to
  be detected by photoreceptors
• Infrared radiation falls on membrane and warms
  it, thermal receptors are then stimulated
• In snakes this info is used as the visual center
  is used in other vertebrates

49

• Electricity
• All aquatic animals generate electric currents
  from muscle contractions
• Weak electrical discharges help construct a 3-D
  image of their environment
• Electroreceptors called ampullae of Lorenzini are
  used by sharks to detect muscle contractions of
  prey

50

• Magnetism
• There has been speculation of magnetic receptors
  in vertebrates, but it is still poorly understood
• However eels, sharks, bees, birds and some
  bacteria seem to navigate along magnetic field
  lines of the earth

51
Disease/Disorders

• Color blindness - due to an inherent lack of one
  or more types of cones
• Men are more likely than women to be color blind
  because the trait is on the X chromosome.
• Vertigo - an inner ear disturbance that causes
  one to feel extremely dizzy

52

• CIPA - congenital insensitivity to pain with
  anhidrosis
• Lack of sensory perception, inability to feel
  pain.
• Caused by a mutation on an autosomal chromosome
  which appears to control nerve growth
• Pain messages arent lost, but arent being sent
  to the brain.