Chapter 39
Learning Objectives
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•
•
•
Describe sensory circuit
List five basic type of sensory receptors
Define sensation in neurological terms
Identify the role of each of the various
mechanoreceptors for pressure, vibration,
etc.
• Explain the function of proprioceptors,
electroreceptors and magnetoceptors
Learning Objectives
• Describe how the ear transmits sound
• Describe the morphology and function of
the human eye
• Define chemoreceptors
• Map the human tongue and the process of
taste
• Diagram the human nasal cavity and the
process of olfaction
• Describe the function of nociceptors
Sensory Receptors (1)
• Formed by endings of afferent neurons or
specialized cells adjacent to neurons
• Detect stimuli (various forms of energy)
– Mechanical pressure
– Sound waves
– Light
– Specific molecules or chemical conditions
Sensory Receptors (2)
• Sensory transduction
– Axons of afferent neurons carry action
potentials generated by receptors to pathways
leading to specific parts of the brain
• Brain processes signals into sensory
sensations
a. Sensory receptor formed by dendrites of an afferent neuron
Stimulus
Stimulus
opens gated Action
ion channels potential
Afferent neuron (to
CNS)
Dendrites forming
sensory receptor
In sensory receptors
formed by the
dendrites of afferent
neurons, a stimulus
causes a change in
membrane potential
that generates action
potentials in the axon
of the neuron.
Temperature and
pain receptors are
among the receptors
of this type.
Fig. 39.1a, p. 886
b. Sensory receptor formed by a cell that synapses with an afferent neuron
Stimulus
Sensory receptor
cell or structure
Diffusion of
neurotransmitter
or first messenger
Action
potential
Neurotransmitter or
first messenger
opens gated ion
channels
In sensory receptors
consisting of separate
cells, a stimulus
causes a change in
membrane potential
that releases a
neurotransmitter from
the cell. The neurotransmitter triggers an
action potential in the
axon of a nearby
afferent neuron.
Mechanoreceptors,
photoreceptors, and
chemoreceptors are
examples of receptors
of this type.
Fig. 39.1b, p. 886
Types of Receptors
• 5 basic types
– Mechanoreceptors
– Photoreceptors
– Chemoreceptors
– Thermoreceptors
– Nociceptors
• Some animals have receptors that detect
electrical or magnetic fields
Sensory Perception
• Routing of information from sensory
receptors to particular brain regions
identifies a specific stimulus as a
sensation
• Intensity of a stimulus is determined by
– Frequency of action potentials along neural
pathways
– Number of afferent neurons carrying action
potentials
Sensory Adaptation
• In many systems, frequency of action
potentials decreases while a stimulus
remains constant
• Some sensory receptors (those related to
tissue damage) show little or no sensory
adaptation
39.2 Mechanoreceptors and the
Tactile and Spatial Senses
• Receptors for touch and pressure occur
throughout the body
• Proprioceptors provide information about
movements and position of the body
Mechanoreceptors
• Detect mechanical energy
– Touch, pressure, acceleration, vibration
• Touch and pressure receptors
– Free nerve endings
– Encapsulated nerve endings of sensory
neurons
39.2 Mechanoreceptors and the
Tactile and Spatial Senses
• Receptors for touch and pressure occur
throughout the body
• Proprioceptors provide information about
movements and position of the body
Mechanoreceptors
• Detect mechanical energy
– Touch, pressure, acceleration, vibration
• Touch and pressure receptors
– Free nerve endings
– Encapsulated nerve endings of sensory
neurons
39.2 Mechanoreceptors and the
Tactile and Spatial Senses
• Receptors for touch and pressure occur
throughout the body
• Proprioceptors provide information about
movements and position of the body
Mechanoreceptors: constant
feedback on tactile senses
• Detect mechanical energy
– Touch, pressure, acceleration, vibration
• Touch and pressure receptors
– Free nerve endings
– Encapsulated nerve endings of sensory
neurons
Shaft of hair inside Skin surface
follicle
Epidermis
Dermis
Myelinated
neuron
Free nerve
endings
around hair
root plexus
Free nerve
endings:
light touch
Pacinian
corpuscle:
deep pressure
and vibrations
Ruffini
endings:
deep
pressure
Meissner’s
corpuscle:
light touch,
surface
vibrations
Fig. 39.2, p. 888
Proprioceptors: constant feedback
on spatial position
• Proprioceptors provide information about
movements and position of the body
• The vestibular apparatus of the human ear
• Stretch receptors in muscles (muscles
spindles)
• Proprioceptors of tendons (Golgi tendon
organs) for balance
Vestibular apparatus
Anterior
semicircular
canal
Posterior
semicircular
canal
Utricle
Saccule
Lateral
semicircular
canal
Fig. 39.5a, p. 890
Ampulla of a semicircular canal
Direction
of body
rotation
Endolymph
pulls cupula
in this
direction
Cupula
Sensory
hair cells
Afferent
neurons
Fig. 39.5b, p. 890
Muscle Spindles and
Golgi Tendon Organs
Hearing
• Hearing relies on sensory hair cells in
organs that respond to the vibrations of
sound waves
Terrestrial Vertebrate Ear (1)
• Outer ear (pinna)
– Directs sound to the eardrum
• Eardrum (tympanic membrane)
– Transmits vibrations through one or more
bones in the middle ear to the fluid-filled
inner ear
Terrestrial Vertebrate Ear (2)
• Middle ear
– Malleus, incus, stapes
– Oval window
• Inner ear
– Transmits vibrations through structures that
bend stereocilia of hair cells
– Cochelea, organ of Corti, round window
– Bursts of action potentials determine
frequency of sound waves
Semicircular canals
Pinna
Bone of skull
Eustachian tube
leading to throat
Location of the human
ear in the head
Oval window (behind
stapes)
Auditory nerve
Stapes
Incus
Malleus
Auditory
canal
Round Cochlea
window
Eardrum
Outer ear
Middle
ear
Inner ear
Internal structures of the outer, middle,
and inner ear
Fig. 39.8a, p. 893
39.4 Photoreceptors and Vision
• Vision involves detection and perception of
radiant energy
• Mammalian retinas contains rods and
cones and a complex neuronal network
• Three kinds of opsin pigments underlie
color vision
• The visual cortex processes visual
information
Retina
Cornea
Lens
Pupil
Iris
Fig. 39.11, p. 895
Photoreceptors of the Eye
• Contain pigment molecules
– Absorb energy of light
– Generate changes in membrane potential
• Retinal
– Light-absorbing pigment in all animals
The Vertebrate Eye (1)
• Cornea
– Transparent, admits light
• Iris
– Behind the cornea
– Controls diameter of pupil
– Regulates amount of light that strikes the lens
The Vertebrate Eye (2)
• Lens
– Focuses image on the retina
• Retina
– Lines the back of the eye
– Photoreceptors and neurons integrate
information detected by photoreceptors
Sclera
Choroid
Ciliary
body
Iris
Lens
Retina
Fovea
Blind
spot
Pupil
Cornea
Aqueous
humor
Part of
optic
nerve
Ciliary muscle
Vitreous humor
Fig. 39.12, p. 896
Focus
• In birds, mammals, and most reptiles, light
is focused on the retina by the combined
effect of the cornea and adjusting lens
shape
Fig. 39.13a, p. 896
Fig. 39.13b, p. 896
Region of overlap of
the two visual fields
Visual field
of left eye
Visual field
of right eye
Region of overlap
of two visual
fields
Right
eye
Left
eye
Optic nerve
Optic chiasm
Lateral
geniculate
nucleus of
the thalamus
Visual cortex
Fig. 39.17, p. 900
Photoreceptors of the Retina
• Rods
– Specialized for detection of low-intensity light
• Cones
– Specialized for detecting light of different
wavelengths (colors)
Photopigment Molecules
• Absorb light in photoreceptor cells
– Consist of retinal combined with an opsin
protein
– 3 photopigments (photopsins)
Pigment absorbs light, retinal changes form
– Reactions alter amount of neurotransmitter
released by photoreceptor cells
a. Structure of cones and rods
Cone
Back of retina
Outer
segment
(houses discs
that contain
light-absorbing
photopigment)
Outer
segment
Rod
Discs
Lightabsorbing
photopigment
Discs
Inner
segment
(houses cell’s
metabolic
machinery)
Inner
segment
Synaptic
terminal
(stores and releases
neurotransmitters)
Synaptic
terminal
Front of retina
Fig. 39.14a, p. 897
b. How rhodopsin functions
Rhodopsin in the dark
(inactivated)
Rhodopsin in the light
(activated)
Light
absorption
Retinal
changes
shape
Enzymes
cis-Retinal
trans-Retinal
Fig. 39.14b, p. 897
The Retina and Visual
Processing
• Rods (low light) and cones( bright and
color)
– Linked to neurons in the retina
– Perform initial integration and processing of
visual information
• Processed signal is sent via the optic
nerve through the lateral geniculate nuclei
to the visual cortex
Color-blindness
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Genetic disorder
8% males, 0.5% females
Red-green is most common
Type of color deficiency depends on
wavelength of light that is not being
detected (long, medium, or short
wavelength detecting cone).
Retina: Initial Integration
Chemoreceptors
• Chemoreceptors respond to the presence
of specific molecules in the environment
• In vertebrates, they form parts of receptor
organs for taste (gustation) and smell
(olfaction)
Taste Receptors
• Detect molecules from food or other
objects that come into direct contact with
the receptor
• Are used primarily to identify foods
Papilla
(cutaway)
Taste
bud
Sensory
hair of
taste
receptor
Papillae
Tongue
Afferent nerve
Taste
buds
Papilla
Fig. 39.20, p. 902
Olfactory tract
from receptors
to the brain
Olfactory
bulb
Nasal cavity
Bone
Olfactory
receptors
Supporting
cells
Sensory
hairs of
olfactory
receptors
Mucus
Fig. 39.21, p. 902
39.6 Thermoreceptors and
Nociceptors
• Thermoreceptors occur in warm and cold
forms
• Nociceptors protect animals from
potentially damaging stimuli
Nociceptors
•
Detect stimuli that can damage body
tissues
•
Located on body surface and interior
•
Information from receptors is integrated
in the brain into the sensation of pain
Other Sensory Receptors
Found in Some Vertebrates
• Electroreceptors
– Detect electrical currents and fields
• Magnetoreceptors
– Detect magnetic fields