Chapter 41

Sensory Reception
Lecture Outline
Sensory receptors can be classified according to the source or type of stimulus
A. Exteroceptors receive stimuli from the outside environment
B. Interoceptors detect internal changes in chemistry or temperature
C. Mechanoreceptors respond to mechanical stimuli
D. Chemoreceptors respond to chemicals
E. Photoreceptors detect light
F. Thermoreceptors respond to changes in temperature
G. Electroreceptors respond to electrical stimuli
How sensory receptors function
A. Receptor cells work by producing receptor potentials
1. Receptor cells receive stimuli and convert that stimuli into electrical energy
2. Receptor cells then produce a receptor potential, a depolarization or
hyperpolarization of the membrane
3. The receptor potential is a graded response
4. Only if the receptor cell is sufficiently depolarized is an action potential
B. Sensation depends on transmission of a coded message
1. All action potentials transmitted by a sensory neuron are identical
2. The differentiation of different signals is accomplished in the brain
C. Receptors adapt to stimuli
1. Over time, the neuron produces fewer action potentials, a phenomenon
called sensory adaptation
a) Sensory receptors for pain or cold adapt slowly
b) Other receptors adapt rapidly, such as receptors for touch
Mechanoreceptors respond to touch, pressure, gravity, stretch, or movement
A. Mechanoreceptors are stimulated by deformation of the receptor
1. Mechanoreceptors include receptors for posture, gravity, texture, and
internal information, such as the fullness of the stomach or bladder
B. Touch receptors are located in the skin
1. Free nerve endings in the dermis perceive touch, pressure, and pain
2. Meissner corpuscles respond to light touch
3. Merkel discs respond to continued light touch
4. Pacinian corpuscles respond to deep pressure
5. Nocioceptors are pain receptors
C. Proprioceptors help coordinate muscle movement
Proprioceptors are mechanoreceptors within muscles, tendons, and joints
Proprioceptors enable animals to perceive the position of body parts
There are 3 types of proprioceptors: muscle spindles, Golgi tendon organs,
and joint receptors
a) Muscle spindles sense stretching in tendons
b) Golgi tendon organs sense stretching in tendons
c) Joint receptors sense movement in ligaments
D. Many invertebrates have gravity receptors called statocysts
1. Statocysts are composed of epidermal infoldings with receptor cells with
sensory hairs
2. Statoliths are particles that bend in response to movement of the animal
and stimulate the sensory hairs
E. Hair cells are characterized by stereocilia
1. Hair cells are mechanoreceptors that detect movement
2. Hair cells are important in maintaining balance and equilibrium, and they
are important in hearing
3. Hair cells typically have a single, long kinocilium and a number of shorted
4. Stimulation of the stereocilia in one direction causes depolarization of the
hair cells and release of neurotransmitters; stimulation in the opposite
direction results in hyperpolarization
F. Lateral line organs supplement vision in fish
1. Fish and aquatic amphibians have lateral lines that are composed of lateral
canals with receptor cells with sensory hairs
a) The sensory hairs have a gelatinous cupula, which responds to
changes in water pressure
G. The vestibular apparatus maintains equilibrium
1. All vertebrates have inner ears, while some may lack an outer or middle
2. The labyrinth is composed of the bony labyrinth, which encloses the
membranous labyrinth
3. In mammals, the membranous labyrinth is composed of the utricle, saccule,
and semicircular canals; all three together make up the vestibular
4. The utricle and saccule have granules of calcium carbonate called otoliths,
resting on the cupula of the hair cells
a) The receptor cells of the two structures lie in different planes
b) Linear acceleration causes the otoliths to stimulate the receptor cells,
allowing perception of the position of the head
c) The hair cells in the saccule and utricle have a single, long cilium and
multiple stereocilia (microvilli)
5. The semicircular canals are 3 canals lying in all 3 planes
a) Each canal has an expansion at the end called the ampulla
The ampullae have cristae (clusters of hair cells) but lack otoliths
Angular acceleration causes the fluid in the canal to move, stimulating
the hair cells in a particular canal
d) Humans are familiar with horizontal movements, and movement in
other directions may result in motion sickness
H. Auditory receptors are located in the cochlea
1. Hearing is best developed in tetrapods, specifically mammals and birds
2. The cochlea is a spiral-shaped tube consisting of two connected canals; the
upper vestibular canal and the lower tympanic canal that are continuous at
the apex of the cochlea
a) These two canals are filled with perilymph
3. The middle canal is the cochlear duct, and is filled with endolymph
a) The organ of Corti is located in the cochlear duct, and is composed of
hair cells resting on a basilar membrane
b) The basilar membrane separates the cochlear duct from the tympanic
c) The tectorial membrane lies above the hair cells
4. In humans, sound waves cause the tympanic membrane to vibrate; the
three middle ear bones (malleus, incus, and stapes) transmit and amplify
the vibration to the oval window, which transmits the vibration to the
perilymph in the vestibular canal
5. The pressure wave is transmitted through the vestibular canal to the
tympanic canal, and ultimately causes the basilar membrane to vibrate
6. The tectorial membrane stimulates the hair cells of the organ of Corti,
which send impulses to the brain via the cochlear nerve
7. Sounds of different frequencies resonate and stimulate the basilar
membrane in different ways and in different areas
8. Loudness is based on a greater number of hair cells being stimulated
9. The human ear typically responds to sounds between 20 and 20,000 cycles
per second (Hz), which is much more sensitive than the human eye
Chemoreceptors are associated with the senses of taste and smell
A. Taste buds detect dissolved food molecules
1. The taste buds of mammals, responsible for gustation, are primarily
located in the mouth
2. In humans, taste buds are located on papillae on the tongue
3. Taste buds sense dissolved chemicals, responding to the primary flavors of
sweet, sour, salty, and bitter
B. The olfactory epithelium is responsible for the sense of smell
1. In terrestrial vertebrates, the olfactory epithelium is located in the nose
2. The olfactory cells have olfactory hairs that respond to chemicals in the air
3. Humans can detect about 1000 types of olfactory chemicals, therefore there
must be at least that number of genes associated with olfactory detection
The sense of smell is very sensitive, but adapts very quickly
Odor molecules bind with receptors in the olfactory epithelium, and a
complex process occurs, involving cAMP and channels in the cell
Photoreceptors use pigments to absorb light
A. Rhodopsins are pigments that are photosensitive
1. Rhodopsin is found in cephalopods, arthropods, and vertebrates
2. Light that strikes cells with rhodopsin causes a chemical change in the
B. Eyespots, simple eyes, and compound eyes are found among invertebrates
1. Eyespots (ocelli) are found in some cnidarians and flatworms, and detect
light but do not form images
2. True eyes form images and typically have lenses
a) Vision requires a brain for integration of the action potentials
b) Crustaceans and insects have compound eyes with multiple visual
units called ommatidia
(1) Each ommatidium has a cornea, a lens, and the light sensitive
(2) Compound eyes adapt to light or dark conditions by migration of
the pigment within the ommatidium
(3) Compound eyes form a mosaic picture
(4) Compound eyes of many insects are sensitive to UV light
c) The eyes of cephalopods and vertebrates are camera-type eyes, which
resulted from convergent evolution
C. Vertebrate eyes form sharp images
1. Vertebrate eyes are similar to a camera
a) The iris regulates light passing through the pupil
(1) Circular muscles contract to decrease the size of the pupil
(2) Radial muscles contract to increase the size of the pupil
b) The lens is the light-sensitive layer located posterior to the iris and
focuses light
c) The choroid absorbs light passing through the retina
d) The sclera is the outer layer of connective tissue that helps to maintain
the rigidity of the eye
e) The sclera forms the "white" of the eye, and the cornea is the clear
anterior portion of this layer
f) The anterior cavity between the cornea and lens is filled with the
aqueous ("watery") fluid
g) Posterior to the lens, the posterior cavity is filled with the vitreous
h) The ciliary body is the anterior projection of the choroid, and consists
of the ciliary processes, which secrete the aqueous fluid, and the ciliary
muscle, which attaches to the suspensory ligaments and is involved in
i) In myopia, vision in the elongate eyeball may be corrected by concave
j) In hyperopia, vision in the short eyeball may be corrected by convex
The retina contains light-sensitive rods and cones
a) Rods function in dim light and are not sensitive to colors
(1) Rods are more abundant in the periphery of the retina
Cones function in bright light and are sensitive to color
(1) Cones are most abundant in the fovea, located in the center of the
c) The axons of the rods and cones synapse with several other layers of
neurons in the retina, and ultimately the axons exit the eyeball via the
optic nerve
3. A chemical change in rhodopsin leads to the response of a rod to light
a) Rhodopsin in the rods and related chemicals in the cones are
chemically changed by light
b) Rhodopsin is composed of opsin and retinal
c) When stimulated by light, retinal is transformed from the cis to the
trans form, and rhodopsin breaks down
d) An action potential results from these changes
4. Color vision depends on three different types of cones
a) The three types of cones respond to blue, green, or red light
D. Integration of visual information begins in the retina
1. The ganglion cells detect basic information about the image
2. At the optic chiasm, neurons from part of each visual field cross over to the
other side of the brain
3. Further processing is done in the lateral geniculate nuclei of the thalamus,
then in the primary visual cortex in the occipital lobe of the cortex