Chapter 17
The Sense Organs
Introduction
• To survive in a changing world vertebrates must
detect changes in their external and internal
environments and make appropriate behavioral
and physiological responses.
• The ability to detect and respond to change is a
basic property of life
– all cells possess this ability
– some cells have become specialized as receptors that
monitor the environment for the entire organism.
• Incoming sensory information is integrated by the
nervous system
– Responses are generated in the form of nerve
impulses or hormones to activate the appropriate
muscles, glands, or other effector organs.
• To understand the evolution and functional
morphology of a sensory system it is insufficient
to show that a particular type of cell responds to
a type of stimulus
– Because a certain cell may respond to several different
stimuli.
• Instead, it must be demonstrated that an animal actually
uses specific types of stimuli to cue behavioral or
physiological changes
Receptors
• A receptive region of a cell, may be stimulated
directly by environmental changes (within or
outside the body) and in response, generate a
nerve impulse.
– This method of reception is common among nonvertebrates.
– Vertebrates retain nerve endings of this type
• Free nerve endings
• Also, have many specialized types of receptor cells and
neurons
• Receptor cells act as transducers, which are
instruments that convert one form of energy into
another.
– They are specialized to detect minute energy changes
from a particular environmental signal, such as light or
pressure, and then initiate a nerve impulse in a
sensory neuron.
• A nerve impulse is said to be “all-or-nothing”
– Above threshold the nerve fires, below it does not.
– The magnitude of sensory stimulation above the
threshold level needed to initiate a nerve impulse is
encoded by the frequency of nerve impulses and
sometimes their pattern.
• The decoding and perception of a particular sensation
is not a function of the nerve impulses,
– It is a function of specificity within the receptors and with
the connections of their neurons to the nervous system.
• Vertebrates evolved receptor mechanisms that detect
environmental changes important for their survival
– Physiological changes that are less important for survival
go undetected.
• Some receptors respond to more than one type of
stimuli; therefore, it is convenient to classify receptors
based on their major sensory signal to which they
respond.
• Organization of receptor cells is based on the primary
source of incoming stimuli:
– chemical, mechanical, electrical, and electromagnetic.
Chemoreceptors
• All animals detect many chemical changes in their
external and internal environments.
• Vertebrates receive chemosensory signals in 4
ways
1. Internal chemoreceptors continuously monitor the
internal environment.
2. Many free nerve endings are present in serous and
mucous membranes of the eye, mouth, and nose
and detect noxious chemicals.
3. Receptors in the head detect odors;
•
olfactory receptors.
4. Receptors within the oral and nasal cavity detect
taste,
•
gustatory receptors.
• Vertebrates can detect a wide range of odors,
possibly as many as 10,000 different odors in
humans, but how the distinction in made is not
entirely understood.
• Perception of different odors probably depends
on;
– processing in the brain of a combination of
information from the stimulated olfactory neurons,
– the intensity of stimulation, and
– the synaptic pattern that their projections make
within the CNS.
• Olfactory receptor cells show considerable
sensory adaptation.
• Olfactory cells are sensitive to a new odor, but
their activity slows down or stops in the
continued presence of the same odor.
• Odors are less important for arboreal or flying
organisms because olfactory trails do not always
cross large open spaces.
– Only a few birds have been thought to have a well
developed sense of smell.
– This sense is poorly developed in primates because of
their ancestral arboreal lifestyle.
• Only vestiges of olfactory organs remain in
marine mammals which close their nasal
openings while under water.
Vomeronasal Organ
• In most tetrapods, the medioventral part of the
olfactory epithelium forms a pair of vomeronasal
organs, or Jacobson’s Organs.
– These organs are absent from most aquatic organisms
along with birds, bats, and many primates.
• The receptive neurons of the vomeronasal organs
resemble olfactory ones except that the cilia are
replaced by microvilli.
– The axons of the neurons terminate in the accessory
olfactory bulb of the brain.
• The paired vomeronasal organs are not
completely separated from the main olfactory
chambers in amphibians, turtles, snakes, and
lizards
– odorants can enter via either the external or internal
nostrils.
• In squamates, the vomeronasal organs usually
form a pair of distinct, sac-like structures that
have their own entrance to the mouth
– used in combination with the forked tongue.
• In mammals the vomeronasal organs are cul-desacs that open into the front of the mouth
through the nasopalantine duct, the nasal cavity,
or mouth.
Gustation
• Taste is detected by barrel shaped clusters of
20-30 receptor and sustentacular cells called
taste buds.
– The surfaces of these cells bear microvilli that
contain the molecules receptive to chemicals.
• Taste buds open to the surface by pores.
• The receptive cells are supplied by distinct
sensory neurons that return to the brain in
cranial nerves from the mouth and pharynx.
– The facial nerve carries fibers from taste buds in the
oral cavity
– The glossopharyngeal and vagal nerves carry fibers
from the pharynx.
• Although taste buds are used primarily to find
and recognize food, they are also important in
sexual and other behavioral interactions in many
species.
• Taste buds do not respond to as low a
concentration of substances as do the olfactory
cells,
– Also respond to a relatively narrower spectrum of
chemicals.
• The distinction among tastes probably results
from the particular combination of taste buds
that are activated and the pattern of their
projection in the brain.
– Our final perception of “flavor” depends in a
complex mixture of signals from not only taste
buds but also olfactory cells and tactile receptors.
• Taste buds are distributed throughout the oral
cavity and pharynx, and also spread onto the
skin, in fishes and amphibians.
• They occur over the entire body surface in
catfishes and minnows but are particularly
abundant on the barbels around the mouth.
• In amniotes, taste buds are limited to the oral
cavity and pharynx.
– Taste buds are more abundant in mammals and
most are associated with papillae on the tongue,
but some can be found on the pharynx, palate,
and epiglottis.
Cutaneous Receptors
• Cutaneous receptors are used for sensing the surfaces
of objects or other individuals.
• All vertebrates have branching free nerve endings in
their dermis, and these may penetrate the epidermis.
– These free nerve endings are activated by vibrations,
touch, injuries, abrupt temperature changes, and other
external stimuli.
• Their primary function in to alert the animal to cuts,
burns, and other injuries that are perceived as pain.
• Most cutaneous receptors are confined to the skin and
are supplied by spinal nerves.
Proprioreceptors
• Coordinated contraction and relaxation of
locomotor and other muscles, in the correct
sequence, and with the needed force and
velocity, requires some sensory feedback from
the muscle centers in the spinal cord and brain
that control their activities.
• A category of mechanoreceptors known as
proprioreceptors, located within the muscles,
tendons, and joints, continuously provide
information, although we are seldom aware of
their activity.
• Tendon organs consist of a group of
encapsulated collagen fibers that are
entwined by sensory nerve endings.
– They detect tensions developed within the
muscles and provide information on which
muscles are active and the magnitude of the
forces developed.
• If forces become dangerously large, then the
sensory information reaching the CNS will
initiate reflexes that reduce the number of
motor impulses going into the muscles.
• Tetrapods cope with gravitational forces by
continuously adjusting their posture.
– Adjustments are made continuously in the degree
and rate of muscle contractions as limb angles and
the loads on the muscle change.
• Tendon organs are more numerous in
tetrapods than non-tetrapods, and muscle
spindles are present within the skeletal
muscles of tetrapods.
– Muscle spindles provide info by which the degree
and rate of muscle contraction can be adjusted to
meet the changing forces to which each muscle is
subjected.
• As muscles stretch, because of increasing
loads on each of them, the muscle spindles
are passively stretched.
– This is detected by the sensory endings on their
nuclear chain fibers.
• Sensory info returning to the spinal cord
initiates nerve reflexes that increase the
motor output to the extrafusal fibers in the
stretched muscles
– This causes them to contract enough to
compensate for the increased load.
Lateral Line System
• The lateral line system is one of the most
highly variable sensory systems in craniates.
• In its basic form, it consists of a series of
mechanoreceptors in the skin organized as a
series of lines.
Basic Organization of the Lateral Line
System
• The lateral line system is present in living
hagfishes, lamprey, chondrichthyans,
actinopterygians, sarcopterygians, and larval
(and paedomorphic) amphibians.
– This somatic sensory system allows them to detect
water disturbances.
• It is entirely absent in all living amniotes, even
those that have returned to the aquatic
environments.
• The sense organs with the lateral line system are
small clusters of mechanoreceptors and
sustentacular cells called neuromasts.
– The individual receptor cells are termed hair cells,
because they bear a single, long kinocilium
• Gnathostomes generally have three lateral line
placodes rostral to the otic placode and three
caudal to it.
• Neuromasts enable the animal to detect water
movements in different directions
– Movements that bend the cupula toward the
kinocilium increase the rate of nerve impulses,
– and movements that bend them away slow the nerve
impulses.
• The polarity of the neuromast within the canal is
not the same.
• All of the kinocilia in one set of neuromasts may
be located on the caudal edges of the hair cells;
those in another set may be rotates 180° and be
on the rostral edge.
– This arrangement, together with the patterns of
distribution of the canals, enables the fish to detect
water movements and determine their source.
• Placement of neuromasts in canals allows fish to
better localize the source of the pressure wave.
Electroreceptor
• Water conducts electricity very well, and
electroreceptors occur in many groups of aquatic
craniates.
• Like the lateral line sense, electroreception is an
ancient sensory system of the vertebrates.
– Living hagfishes do not have electroreceptors or any
hindbrain nuclei to suggest they once had them.
– In contrast, lamprey are electroreceptive.
– Thus, it is believed that electroreception is a
vertebrate synapomorphy.
• The electroreceptor found in many living
gnathostomes are called ampullary organs.
– In chondrichthyans, groups of ampullary organs
are clustered on the head adjacent to the lateral
line canals.
• Each ampullary organ consists of a
subcutaneous tube that lies tangential to the
skins surface.
• One end of the tube opens by a pore onto the
skin’s surface the other terminates in a slight
enlargement, the ampulla, which contains
modified hair cells.
• Electroreception based on ampullary organs
was lost in the ancestor to neopterygians.
– This loss eliminated both the ampullary organs
and their connections with the brainstem.
• Electroreception was also independently lost
in frogs and the lineage that lead to amniotes:
– Air is a poor conductor of electrical currents.
• Electroreception has re-evolved in some
species (twice in teleosts, once in amniotes).
The Ear
• There are several similarities between the ear
and the lateral line system.
– The receptor cells of both systems are hair cells
that are stimulated when liquids or other
materials move across them and bend their cilia.
– The two systems develop from adjacent
neurogenic placodes
– Neurons that carry sensory information from the
ear terminate in the medulla adjacent to the
termination of the lateralis fibers.
• All vertebrates possess an inner ear
embedded in the otic capsule of the skull that
contains hair cells that are isolated from
external aquatic disturbances.
– Instead they are specialized to detect internal
liquid disturbances caused by changes in the
orientation of the head and body.
• Living tetrapods also have middle and external
ears that are specialized for the reception of
sound waves.
Equilibrium
• Hair cells within the ampullae, utriculus, and
sacculus of all vertebrates detect changes in
position and movement
– Provide information that helps an animal maintain its
position in space, or its equilibrium.
• The pull of gravity on the statoconia or, otolith,
registers static equilibrium, or the present
orientation of the body.
• Because hair cells in the maculae of the sacculus
and utriculus have different polarities, they can
detect displacement in different directions.
Sound in Water
• To fishes hearing is the detection of water
disturbances generated by sound sources under
water or at the surface.
• Sound waves generated by a vibrating object
spread more rapidly through a dense medium,
like water, then through air.
• Sound waves have two components:
1. Low frequency particle motion or displacement
waves, and
2. Higher frequency pressure waves, which result from
the alternate compression and rarefaction of
molecules in the water.
Hearing Mechanisms of
Chondrichthyans and Teleosts
• Because hair cells are stimulated by the
displacement of their stereocilia, a displacement
wave directly affects the hair cells that it can
reach, like those of the lateral line.
• Because the density of a teleost is similar to that
of water, sound pressure waves easily pass
through them at nearly the same amplitude and
frequency as they move through water.
– For a sound pressure wave to be detected it must
induce a movement over certain hair cells that differ
from that of the rest of the body.
• A potentially better method of sound
reception is to use a gas-filled chamber as a
hydrophone.
– Sound waves in the body do not compress watery
tissues, but the can compress the air in the swim
bladder.
– The changing pressure of the swim bladder
vibrates at the same frequency as the sound wave.
• Some fishes have a particularly elegant system for
detecting sound; Weberian ossicles, small bones
derived from the ribs, that connect the swim bladder to
the sacculus
Hearing Systems of Tetrapods
• Low frequency sound waves of sufficient energy
can travel through the ground as well as air and
can cause a slight disturbance in the superficial
skull bones that can be detected as sound.
– Caecilians, terrestrial salamanders, squamates, and
snakes all detect sound in this way.
• Most important sounds for a tetrapod, however,
are higher frequency and so travel only as
pressure waves in air.
• Terrestrial vertebrates that detect higher
frequency sound waves have a tympanic
membrane on or near the surface of the head.
– The term external ear may be applied to the
tympanum;
– The term also refers to accessory structures in
mammals like the pinna.
• An air-filled middle-ear, or tympanic cavity, lies
on the inside of the tympanic membrane and
connects to the pharynx by way of the auditory
tube, or Eustachian tube.
– The tympanic cavity and auditory tubes equalize the
air pressure on the two sides of the tympanic
membrane, which enable it to respond quickly to high
frequency sound.
• The conversion of a sound pressure wave in air
into a displacement wave in the perilymph and
endolymph of the inner ear is made by the
responses of the tympanic membrane and three
other essential structures.
1. At least one auditory ossicle, called the columella, or
stapes, must be present. The lateral end of which
connects to the tympanic membrane.
2. At least one specialized perilymphatic duct must be
present. It receives displacement waves from the
foot plate of the columella and carries them to
where they are dissipated
3. At least one sensory area within the membranous
labyrinth to receive the pressure waves.
• Different groups of tetrapods detect airborne
sounds in different ways, utilizing some
structures that are homologous and some that
are not.
– This suggests considerable independent evolution
in the evolution of auditory mechanisms in
tetrapods.
• In particular, the tympanic membrane was not
present in the earliest tetrapods and perhaps
its absence in some groups is a retention of
this trait.
– A tympanic membrane evolved independently
three time in tetrapods; in anurans, in turtles, and
in mammals
Photoreceptors
• The ability to detect changes in light is
important for nearly all vertebrates.
– Periods of feeding and reproduction and other
aspects of their physiology and behavior are
closely attuned to the diurnal cycle and to
seasonal changes in day length.
• The additional ability to detect the source of
light can provide info in the approach of
predators, location of shelter, etc.
• To receive light an animal must have
photoreceptive cells containing visual
pigments
– These absorb quanta of light energy & initiate
chemical changes that generate nerve impulses.
• The light energy most valuable the vertebrates
fall within what we call the visual spectrum,
380nm to 760nm.
Median Eyes
• For many vertebrates, the detection of light
and the physiological adjustments to changes
in light levels and day length are mediated by
photoreceptors different than those that form
images.
• In addition to image forming lateral eyes,
anamniotes and many diapsids have one or
two light sensitive median eyes on top of the
head.
• An adult lamprey has a nonpigmented spot of
skin on the top of its head, beneath which lies
a pineal eye.
• A second organ, known as the parietal eye,
lies deep to the pineal eye.
• Although not universally present, the median
eye complex is an ancient and widespread
feature of vertebrates
• Squamates have a well-developed parietal eye
complete with cornea-like and lens-like
structures and a reduced pineal organ
beneath the skull roof.
– The parietal eye monitors the level of solar
radiation and affects the animal’s orientation to
the sun and its movements into the sun or shade.
• The complex is represented in birds and
mammals by an endocrine pineal gland,
– The activities of which also are affected by light,
even through they are located deep within the
skull.
Evolutionary Adaptation of the Eye
• Eyes and their neuronal projections offer one
of the best anatomical predictors of
vertebrate behavior and function, so it is
important to consider ways in which the eyes
have become modified in different groups.
Eyes of Lamprey, Chondrichthyans, and
Actinopterygians
• These animals live in an aquatic environment,
and water continuously bathes the corneal
surface.
– Tear glands are unnecessary and never evolved.
• Most species lack moveable eyelids, a few have
stationary skin folds above and below the eye.
– In some chondrichthyans, a nictitating membrane is
present in the medial corner of each eye; it closes to
protect the eye during feeding.
• Many species of teleosts, including those that swim at
high speed, have recessed the eyes into the surface of
the head using clear adipose eyelids;
– These help to maintain streamlining of the head by limiting
the drag that would occur if the eyes protruded from the
head.
• Light levels change more slowly in water than on land,
and the pupillary response in fishes is slower that that
of tetrapods.
• Because the refractive index of water is close to that of
the cornea, light waves pass through the cornea
without being substantially bent or refracted.
– The lens is the primary refractive structure and it must be
sufficiently thick and nearly spherical to provide adequate
refraction.
• A fish’s eye cannot focus properly in air due to
the additional refraction that occurs when the
light passes from the air into the cornea.
• Light intensity is low in many bodies of water,
and long wavelengths of light, reds and
oranges, are readily absorbed.
• Correlated with this, the retina of many fishes
consist primarily of rods. Only those species
living in brightly illuminated habitats have
many cones.
• Many predatory fishes have a tapetum
lucidum that reflects light back into the retina.
The Eye of Terrestrial Vertebrates
• Because they are not constantly bathed in
water, the eyes of terrestrial vertebrates must
be protected and kept moist in other ways.
– A tetrapod eye usually has one or more eyelids
that can move across its surface and protect and
clean it.
• The eye of amphibians has a stationary upper eyelid
but a moveable and transparent lower one. They can
also retract the eyeball deeper into the orbit so that the
eyelid can completely cover it.
• The nictitating membrane is usually retracted into
the medial corner of the eye;
– In humans it is reduced to a vestigial, semilunar fold.
• Eyelashes are associated with the eyelids of
mammals.
• Tetrapods have also evolved tear glands
• Because its index of refraction is much greater
that that of air, the cornea of terrestrial
vertebrates refracts incoming light and plays an
essential role in focusing light on the retina.
– Because levels of illumination can be much greater on
land than in water, terrestrial vertebrates frequently
have many cones and correspondingly high visual
acuity, many also have color vision.
Thermoreceptors
• Vertebrates can detect thermal changes in
their environment.
• Such an ability is critical for thermal
regulations in in endothermic animals such as
birds and mammals, but the mechanism for
detecting environmental temperature changes
is not well understood.
• Specialized infrared detectors are present in
some groups of snakes.
– In boas and pythons, the receptors are in a series
of shallow pits in the scales bordering the mouth.
– Independently, rattle snakes and other pit vipers
evolved a set of deep pit organs on the face
between the eye and nostril.
• Highly branched free neuron endings with many
mitochondria lie in a delicate membrane within the pit.
• These pits are sensitive to slight differences in infrared
radiation between the warm-blooded prey and the
surroundings.
END