Download Investigating Nervous and Sensory Systems

Investigating Nervous and Sensory Systems
Student Prelab Preparation
Before doing this lab, you should read the relevant sections of your text.
You should be able to describe in your own words the following terms and concepts:
axon
cerebellum
cerebrum
cornea
cranial nerves
dendrite
iris
medulla oblongata
meninges
organ of Corti
pupil
reflex arc
semicircular canals
spinal cord
spinal nerves
cochlea
gray matter
myelin sheath
retina
white matter
As a result of this review, you might have questions about terms, concepts, or how you will do the experiments
in this lab. Write these questions in your lab notebook.
Learning Objectives
1.
Illustrate & describe the anatomy and physiological function of a typical nerve cell
2.
Describe the types of nervous systems found in Animals
3.
Identify the major structural and functional regions of the mammalian brain?
4.
Describe the anatomy of a reflex arc
5.
Describe how the eye “sees” light
6.
Describe how the ear “hears” sound
7.
Observe the microscopic structures of nerves, nerve cells, and neuromuscular junctions and describe the
significance of the major structures of each
8.
Learn the gross anatomy of the mammalian central nervous system, eye, and ear
9.
Determine experimentally some of the properties of rods and cones
Nervous System Lab – Must have in your lab notebook
Slides
1. Giant multipolar Neuron
a. Nuclei
b. Soma (cell body)
c. Neurites (Dendrites or axon)
2. Spinal cord/Ganglia
a. Grey matter
b. White matter
c. Dorsal horn
d. ventral horn
3. Cochlea @ 400x
a. hair cells
b. tectoral membrane
c. basilar membrane
4. Crista ampullaris
a. hair cells & cupula
5. Nerve cross section
a. Axon
b. Schwann cell/Oligodendrocite
6. Retina
a. Choroid
b. Photoreceptor cells (rods & cones)
c. Pigmented epithelia
d. Ganglion cells
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Diagram & Dissection
1. Spinal cord
a. Dorsal horn & ventral horn (afferent & efferent) neurons
b. gray matter
c. white matter
d. All 3 meninges
e. summary of spinal reflex arc
2. Cochlea or ear (ANY labels)
a. Summary of signal transduction
Dissections
1. Brain (labeled)
a. external anatomy
b. internal sagittal section
2. Eye (labeled)
a. Retina
b. choroid
c. lens
d. ciliary muscles & ligaments
e. Optic nerve
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Background
To survive and reproduce, animals must maintain a relatively constant internal state, often in the midst of
enormous environmental fluctuations. This constancy, called homeostasis, is maintained by the nervous and
endocrine systems, which interact to control an animal’s internal functioning (physiology) and external activity
(behavior). In the nervous system, sensors (affectors) monitor an animal’s internal and external environments.
When physiological adjustments are required, these are mediated by the nervous system and facilitated by
chemical messengers (hormones) secreted by the endocrine system, under close control of the nervous system.
When external action is called for, effectors - muscles and bones and glands - coordinate various behaviors. The
activities of the nervous and endocrine systems are directed or regulated by genetic information and learning.
In this laboratory you will study three levels of components of the mammalian nervous system: the structural
unit of the nervous system, the neuron, or nerve cell; several sensory receptors (sense organs); and the brain,
the major component of the central nervous system.
Coordination of various specialized tissues in multicellular animals is necessary for the organism to operate as an
integrated whole. The nervous system coordinates the body’s relatively rapid responses to changes in the
environment. The endocrine system regulates longer term adaptive responses to changes in body chemistry
between meals, as the seasons change, or as developmental changes occur during maturation. Because the
functions of these two systems often complement one another, biologists often speak of the neuroendocrine
system.
The nervous system has three functions:
1. to receive signals from the environment and from within the body through the sense organs
2. to process the information received, which can involve integration, modulation, learning, and memory
3. to produce a response in appropriate muscles or glands.
Receptors are usually specialized cells outside of the central nervous system that detect physical and chemical
changes. There are separate receptor cells for heat, cold, light, and so on. The function of receptor cells is to
convert an environmental signal into a change in the cellular membrane’s ion permeability, leading to a voltage
change across the cell membrane. If the voltage change is of sufficient magnitude, a nerve impulse will be
created in an adjacent nerve cell, and travel from the sensing zone to the central nervous system.
The central nervous system consists of neuronal networks, which are involved in decision making. A decision
may involve the simple passage of nervous impulses from an afferent sensory fiber to an efferent motor
(effector) fiber, assuring that perception results in a response, as in a reflex arc. In other cases, many
interneurons in the nervous system may become involved, tempering the inputs with memory or reasoning. The
response resulting from this interaction of nerve signals may differ according to conditions. For example, if a
person is frightened by a loud sound, he or she will gasp; but if this stimulus is repeated when the person is
under water, higher nerve centers inhibit the response.
To gain insights into how nervous systems operate, many biologists study the simpler systems of invertebrates.
These scientists have found that neurons are similar in form and function no matter what type of animal they
are taken from. The action potential, the electrical potential changes comprising a nerve impulse, also seems to
be universal in animals. The differences between simple and complex nervous systems involve the types of
receptors present, the organization of the central nervous system, and the neurotransmitter chemicals that
function in cell-to-cell impulse transmission.
One of the simplest nervous systems is found in the radially symmetrical cnidarians, such as Hydra. The nerve
net found in these organisms is not organized into tracts or centers (fig. l). Bilaterally symmetrical animals as
simple as the Platyhelminthes have tracts running the length of the organism, with the anterior end of the tract
serving as a coordinating center (fig. l). In higher phyla, the coordinating centers become more complex, creating
an evolutionary trend referred to as cephalization, in which sense organs and the coordinating centers are
concentrated at the animal’s anterior.
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The central nervous systems of annelids and arthropods consist of two ventral nerve cords with interspersed
ganglia, areas of the cord where the cell bodies of neurons are concentrated. The cords rapidly transmit signals,
whereas the ganglia serve as processing centers (fig. l). In arthropods, the ganglia are clustered in the head and
thorax regions, where most activity takes place.
In the vertebrates, it is obvious that the brain dominates the nervous system. It is estimated that the brain
contains between 10 and 100 billion neurons, which make over 100 trillion points of contact with each other.
Well over 99% of these neurons are interneurons; only 2 to 3 million are motor neurons.
Figure 1 Nervous systems in invertebrates: (a) nerve net in Hydra lacks central tracts; (b) earthworms have
longitudinal nerve cords with cross connectives; (c) ventral central nervous system of an arthropod shows
cephalization and has integrating ganglia in each segment.
(a)
(b)
(c)
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The nervous system receives its information about the environment and the position of an animal in its
environment from sensory cells. Sensory cells usually do not occur as isolated units but are part of a larger
organized unit, the sensory organ. The mammalian eye consists not only of light-sensitive rod and cone cells, but
also of a focusable lens, a light-regulating iris, a tear gland, and a covering eyelid. The proper functioning of a
sense depends not only on the sensory and neuronal cells, but also on the anatomical structures that collect,
amplify, and direct environmental signals to the receptor cells.
Senses are “hard-wired” into the organism. You feel cold because specific cold receptors in the skin are
stimulated, and impulses are carried by specific neuronal routes to specific areas of the brain where that
information is integrated with other sensory inputs and memory. You see something because light has
stimulated individual cells, and these cells each stimulate different neurons that carry nerve impulses to the
brain. The brain integrates those signals with memory. The perception of a face, especially a familiar face, is
actually a comparison of new neuronal inputs with a record of previous neuronal inputs. Sensory cells do not
perceive, they receive. Perception is an integrative process in which nerve impulses from receptors, not
environmental stimuli, are processed and may result in some action by the animal.
Microanatomy of the Nervous System
Nerve tissue is composed of two types of cells: (1) neurons, which are highly specialized for irritability and
conduction of nerve impulses, and (2) sheath cells and glial elements, which provide both nourishment and
support; some also help to conduct impulses.
On demonstration you will find preserved slides of several neurons. Examine these slides and diagram a
neuron. Label the neuron cell body, axon, and any dendrites that may be visible.
1. Obtain a slide of multipolar neurons and observe it through a compound microscope, comparing what you
see to figure 2. Small cells surrounding the neurons are neuroglial cells which supply nutrients for the
neurons and produce chemicals that modulate the functions of neurons. Find a neuron’s cell body called
the soma, and the cytoplasmic extensions called neurites. Axons are neurites that conduct impulses away
from the soma. Dendrites are neurites conducting impulses toward the soma. You cannot morphologically
distinguish between axons and dendrites on a slide like this. Why? Neurites often extend long distances. In
a 7-foot basketball player, axons extend from the base of the spine to the toes, a distance of 3 to 4 feet!
2. Obtain a slide of a spinal cord cross section through the dorsal root ganglion. Observe it at first under low
power with a compound microscope. Note the general organization and compare with figure 3. Sensory
nerves are dendrites and always enter the top (dorsal). Motor neurons are axons and always leave the
bottom (ventral) of the cord. Note that the nerve cord is hollow, containing a central canal, a vestige from
its embryonic development. The central canal is connected with the ventricle spaces of the brain.
3. Examine the peripheral white matter surrounding the central gray matter with a high-power objective.
Note that the white matter consists of neurites that have been cut in cross section. They are myelinated
nerves and are surrounded by layers of lipid which electrically insulates them, allowing rapid action
potential transmission. They extend up and down the spinal cord. When the spinal cord is severed in an
accident, it is impossible to rejoin these thousands of neurites and numbness and paralysis result.
4. Examine the central gray matter and find the cell bodies in this area. Intereurons here process information
and send motor axons out the ventral root to muscles. They lack the insulating myelin.
5. Examine the demonstration slide your instructor set up, showing a neuromuscular junction where a motor
nerve innervates muscle fibers. The axon of the motor nerve divides into fine branches which terminate as
motor end plates on the muscle fiber surface (see fig. 3). Nerve impulses arriving at the end plates cause
the release of acetylcholine, a neurotransmitter chemical that diffuses across the neuromuscular junction
cleft and triggers an action potential in the muscle cell. The action potential leads to the muscle
contracting. Sketch and label a junction below.
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Figure 2 Neurons in a smear from a bovine spinal cord.
Use your knowledge of nerve tissue to answer the
following questions.
Identify the functions of each of the following parts of a
neuron.
Cell body:
Axon:
Dendrite:
Distinguish among the functions of the following types of neurons. Where in the nervous system is each type
of neuron located?
Sensory (afferent) neurons
Interneurons
Motor (efferent) neurons
What is the difference between a neuron and a nerve?
The functional junction between two neurons is the synapse. What happens at this junction and why are these
events important? How does the synapse control the direction of information flow?
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Figure 3: Cross section of a spinal cord showing reflex arc components
Figure 4: Division of the Nervous System in Mammals
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Figure 5. The relationship of the spinal cord, spinal nerves, and sympathetic nervous system to the vertebrae
and meninges.
Mammalian Nervous System
Figure 4 provides an organizational overview of the mammalian nervous system. It consists of two major parts:
the peripheral nervous system and the central nervous system, which in turn can be further subdivided into
their component parts. As you work through the anatomy of the nervous system, refer to this chart to refresh
your memory of the relationships.
If you have not already done so, remove the skin on the dorsal side of your fetal pig. Expose the spine by
removing the muscles that cover and extend on each side of the vertebrae. Since it is time consuming to expose
the entire spine, lab groups should expose only short sections of about 5 - 8 vertebrae each. Coordinate your
efforts so that representative parts along the entire spinal cord are exposed.
Spinal Cord
Once the vertebrae are exposed, use a sharp scalpel to gradually remove the cartilaginous spines and neural
arches of several vertebrae, exposing the spinal cord (fig. 5). The spinal cord will be surrounded by three
membranes, or meninges. Slit the meninges to observe the spinal cord.
Note the spinal nerves branching from the cord. There are 33 pairs. Each nerve is composed of several small
roots or fibers. As discussed earlier, sensory neurons carrying impulses into the spinal cord enter by the dorsal
root, and motor neurons carrying impulses to effectors leave by the ventral root. These two roots come
together to form the spinal nerves. You may be able to better see the nerves by turning your animal over and
looking closely at the back wall of the body cavity. Each of these nerves consists of thousands of neurites
extending from different neurons in the spinal cord. They are bundled by surrounding connective tissues.
The cranial end of the spinal cord gradually enlarges to become the medulla oblongata. This region of the brain
contains the cardiac and respiratory centers as well as numerous sensory and motor nerve tracts, which
transmit impulses to and from the higher brain centers.
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Autonomic System
The autonomic nervous system consists of the sympathetic and parasympathetic systems, which are involved in
the control of glands, smooth muscle, and viscera. These two systems have opposing effects to slow or speed
muscle movements in the viscera. This system is part of the peripheral nervous system and is connected to the
central nervous system by many nerve fibers and ganglia.
The sympathetic trunks can be viewed by turning the fetal pig over and looking at the dorsal wall of the body
cavity on either side of the spinal column in the thoracic and abdominal cavities. They are thin strands of nerves
with small enlargements, the ganglia, along their length, running parallel to the spine.
Brain
This dissection takes some time to perfor:
Expose the skull by making a longitudinal cut from the base of the snout to the base of the skull. Make lateral
cuts from the ends of the first cut to the angle of the jaws at the anterior end and to the level of the ears at the
caudal end. Remove the muscle layers. Make a shallow longitudinal cut in the skull and then lateral cuts from
this incision at 2 cm intervals. Break off chips of the skull and gradually expose the brain. At the base of the skull,
the tough occipital bone will have to be dissected out separately. The spinal cord passes through this bone.
The brain is surrounded by three meninges as was the spinal cord. Remove the membranes and observe the
gross features of the brain (fig. 6). A longitudinal fissure separates the right and left hemispheres of the
cerebrum. Higher order functions, such as memory, intelligence, and perception, are associated with this part of
the brain. Caudal to the cerebrum is the smaller cerebellum, which coordinates motor activity and equilibrium.
The medulla oblongata lies ventral and caudal to the cerebellum and is continuous with the spinal cord.
The cerebrum has a convoluted surface. The ridges are called gyri and the valleys, sulci. The outer part of the
cerebrum, the cortex, is made up of gray matter, which is composed of nerve cell bodies and supporting, but not
conducting, neuroglial cells. The inner part of the cerebrum is made up of white matter, which is composed of
nerve-cell extensions or fibers encased in an insulating, fatty myelin sheath. In the nerve cord, the relative
positions of white and gray matter are reversed, with myelinated nerve fibers on the outside and cell bodies on
the inside.
The transverse fissure separates the cerebrum from the cerebellum. There are three parts to the cerebellum,
the two lateral hemispheres and a central vermis. If the caudal edge of the cerebellum is raised, a thin vascular
membrane called the posterior choroid plexus may be observed covering the medulla. The space beneath the
choroid plexus is the fourth ventricle of four ventricles, or cavities, found in the brain. Cerebrospinal fluid
created by filtration from the capillaries located in the ventricles passes into the spinal cord spaces. This fluid
carries nutrients to the cells and protects the nervous system from mechanical shock. The ventricles are best
seen in a sagittal section of the brain (fig. 7).
Remove the brain from the skull (or obtain demonstration material) and orient it with the ventral surface up (fig.
8). At the anterior end of the cerebrum, find the olfactory bulbs. In the midportion of the cerebrum, two large
nerve trunks, the optic tracts, cross to form the optic chiasm. Just posterior to the crossover is the pituitary, but
this small body is often broken off when the brain is removed from the cranium. Dorsal to the pituitary
comprising the ventral surface of the brain is a conspicuous oval area, the hypothalamus. It integrates many
autonomic functions, such as sleep, body temperature, appetite, and water balance. Posterior to this is a wide
transverse group of fibers, the pons, which serves as a passageway for nerves running from the medulla to
higher centers. Twelve pairs of cranial nerves directly enter the brain, bringing sensory information concerning
sight, sound, taste, equilibrium, and touch. Motor fibers to the head region also exit via these nerves. The nerves
are named in figure 8.
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Figure 6
Dorsal view of the sheep’s brain.
Figure 7
Sagittal section of the sheep’s brain.
Can you find all the names to the numbers?
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Figure 8. Ventral view of the sheep’s brain with cranial nerves indicated by standard numbering system.
Write a brief description of each structure and a summary statement of its function in the following table:
Spinal nerves
Sensory nerve tracts
Motor nerve tracts
Medulla oblongata
Pons
Cerebellum
Cerebrum
Meninges
Cranial nerves
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Sensory Systems
Before an organism can respond to its external or internal environment, it must receive relevant information
about its position within the environment and about the environment itself. Sensory receptors (sensors or
affectors) transmit this information to the central nervous system (the brain and spinal cord), thus enabling the
organism to coordinate its activities.
Prominent sensory receptors are sensitive to changes in particular forms of environmental energy such as heat,
pressure, light, and vibration (sound). Others are sensitive to various ions or chemical molecules, some are
sensitive to gravity, and others to the position of various skeletal elements and muscles. With the exception of
receptors for smell and the free nerve endings occurring in various regions of the body, sensory cells are not
derived from neurons. Sensors are specialized epithelial elements that synapse with one or more sensory
neurons.
Some sensory receptors are functional parts of conspicuous sense organs (for example, the eyes, ears, and
nose). However, diffuse receptors, such as those sensing touch, pain, temperature, and muscle or joint position,
are equally important to an organism.
Eye Anatomy
Photoreception: How We See
Light entering the eye passes through the chamber containing aqueous humor, then through the lens, and
finally through the chamber containing vitreous humor before it falls on the retina. The retina is the sensitive
layer of the eye and it is here that the energy of light is transduced into nerve impulses.
The light must pass through several layers of neurons on the retina before striking the photosensitive cells. In
the human retina, there are two types of photoreceptor elements: cones and rods.
Cones are responsible for vision in bright light and for the perception of fine detail and color. Cones are
concentrated in the fovea centralis at the center of the retina. Light-sensitive rods function in dim light and are
insensitive to colors. Rods are more numerous in the periphery of the retina. Information from the cones and
rods travels back toward the anterior of the retina through the layer of nerve cells. The first cell receiving
information from the rods or cones is a bipolar cell, the sensory neuron (Figure 9). Impulses from particular rods
and cones may be modified by impulses from other rods or cones or sensory elements by means of transverse
connections. Bipolar cells synapse with ganglion cells at the anterior of the retina. The axons of these neurons
join together to form the optic nerve, which relays information to the brain.
•
•
Which area has the sharpest vision-the fovea or the periphery, and WHY?
If you are trying to see in dim light, is it best to look at an object directly so that the image falls on the cones,
or to look at it out of the side of your eye so that the image falls on the rods and WHY?
Dissect the cow or sheep eye.
With a sharp razor blade, cut longitudinally through the eye to one side of the optic nerve and place the halves
in a dish of water. Identify the three layers making up the wall of the eye: the white sclera, the dark choroid, and
the retina (fig. 9). The front surface of the eye is covered by a transparent layer of connective tissue called the
cornea. The large white structure within the eye at the front is the lens. (It has turned white from the
preservative.) The colored material surrounding the lens is the iris, and the central opening in the iris is the
pupil. Muscles in the iris regulate the opening of the pupil, depending on the light intensity in the environment.
The large posterior space of the eye is filled with a gelatinous fluid, the vitreous humor, whereas the small
chamber anterior to the lens is filled with aqueous humor. Where the optic nerve enters the eye, the retina is
devoid of rods and cones. This area is called the blind spot (optic disk) since light falling on this area cannot
trigger nerve impulses. The small, yellowish spot in the center of the retina is called the macula lutea. The
depression (fovea) in the center of this spot has a very high density of cones, and is the area producing the
greatest visual acuity. Figure 9 shows the structure of the eye.
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Color Vision
In the human retina, there are two kinds of light-sensitive cells: rods and cones. These cells are located on the
outside of the retina, away from the vitreous humor and against the choroid layer, and are covered by a layer of
bipolar and ganglion cells as well as neurons, as shown in figure 10. Light enters the posterior chamber through
the lens and must pass through several cell layers before it is absorbed by the rods and cones. Look at the
demonstration of the mammalian retina and identify rods and cones.
Cones are involved in color vision at daylight intensities. Figure 10 shows that each cone is associated with one
bipolar cell. Generator potentials in a cone excite one neuron, giving high-level resolution vision. Cones will not
cause neuronal firing at low light intensities and are, therefore, nonfunctional at night.
Rods provide black-and-white vision at low light intensities. A bipolar cell associates with several rods, so that
generator potentials sum in their effect. This assures neuronal firing at low light intensities and provides a highly
sensitive vision system but with less resolution.
• How does this organization allow for heightened sensitivity but decreased resolution?
When light strikes a receptor cell, it triggers a photochemical reaction that leads to neuronal firing. If a rod or
cone receives high-intensity light, the visual pigments will be bleached temporarily and will not absorb any more
light. Within a matter of a few seconds, enzyme systems in the cells will restore the visual pigment to its lightabsorbing form. This can be easily demonstrated. If, after viewing a bright light, the eye is quickly shut or turned
to a dark wall, the bright image will persist as a positive afterimage because the high generator potential causes
the continued firing of neurons. If the eye is cast on a lighter background after a few seconds, a negative
afterimage, or a dark image of the object, will appear. This is because the still bleached cells are not receptive to
the light coming from the light background.
According to the Young-Helmholtz theory of color vision, there are three types of cone cells, which respond
respectively to red, green, and blue light. All other colors are perceived as the brain interprets impulses coming
from a mix of these receptors. If an object of one color is viewed for a long period of time at high light intensity,
the cones for that color will become bleached. The afterimage of the object will be “seen” in the complementary
color.
Study afterimages on your own. A small square of red light can be projected on the screen in the dark lab room.
After staring intensely at the square for 20 seconds or so without shifting your gaze, the slide will be changed to
a soft white. Continue staring at the screen as the change is made. What do you “see”? What was the color of
the afterimage?
Repeat this experiment using a blue square. Before doing the experiment, hypothesize what color the
afterimage will be according to the Young-Helmholtz theory.
•
•
State the hypothesis to be tested.
What did you see when the experiment was conducted?
•
•
Do you accept or reject your hypothesis?
Explain the colors seen in these afterimages using the Young-Helmholtz theory.
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Figure 9 Section of the eye.
Figure 10 Diagram of the cells in the retina. Note the light direction. Light must pass through the layer of nerve
cells before it reaches the rods and cones.
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Ear Anatomy
The mammalian inner ear is difficult to dissect because it is part of the bony skull. However, the anatomy is not
difficult to visualize. Study figure 11 and models of the ear provided in the laboratory.
Sound waves are collected by the external ear and pass down the auditory canal, causing the eardrum
(tympanic membrane) to vibrate. The movement of the eardrum is transmitted by the three auditory ossicles of
the middle ear to the flexible membrane covering the oval window of the cochlea or inner ear. Sound energy is
amplified in this process for two reasons: the area of the tympanic membrane is about 30 times larger than the
oval window membrane; and the mechanical leverage systems of the ossicles, the malleus, incus, and stapes,
greatly amplify any movement. The back-and-forth movement of the oval window causes the fluid in the cochlea
to move. The cochlea is divided lengthwise into three canals by soft-tissue walls.
Pressure waves generated at the oval window travel up the vestibular canal to the apex of the cochlea and
down the tympanic canal. The round window at the base of the tympanic canal serves as a pressure-release
valve, allowing the fluid of the cochlea to move back and forth. Thus, these gross anatomical structures of the
ear convert the movement of air molecules, or sound, into the movement of the fluid (perilymph) in the inner
ear.
The pressure waves cause the thin, longitudinal basilar membrane to vibrate. Sensory hair cells which are found
on this membrane in an area known as the organ of Corti (Fig. 11), are displaced upward and brush against a
stiff, overhanging structure called the tectorial membrane. Distortion of the hair cells causes generator
potentials, which in turn cause the firing of the cochlear neurons that travel along the auditory nerve to the
brain.
Loudness is encoded in the neurons by how frequently nerve impulses are generated in the optic nerve. Pitch is
detected in a different way. High-frequency sounds stimulate cells near the base of the cochlea and lowfrequency sounds stimulate cells near the apex. Separate neurons lead from hair cells at different points along
the cochlea to the brain. Location of receptors thus determines pitch discrimination.
Obtain a slide of a cross section of the cochlea. First look at the slide under the 10x objective. Identify the
structures shown in figure 11. Switch to the high-power objective and observe the hair cells and their
relationship to the tectorial membrane in the organ of Corti.
Pair off with another student and quiz one another about how the ear functions. Use the model and explain to
one another how sound passes into the inner ear and is transduced into a nerve impulse.
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Figure 11 Anatomy of the human ear: (a) the relationship between the external and inner ears and the cochlea;
(b) magnified view of the cochlear canal showing organ of Corti and the relationship of hair cells to the tectoral
membrane; (c) the organ of Corti in magnified view.
(a)
(c)
(b)
Chemoreception: The Sense of Taste
In humans, the specialized receptor organs for the sense of taste (gustation) are clusters of cells called taste
buds (Figure 12), located on the tongue, especially on the tip, edges, and posterior third.
Taste sensations are traditionally divided into four basic groups: sweet, sour, salty, and bitter. Receptors for
each group of substances are found scattered across the entire landscape of the tongue, but the densities of
receptor group do vary geographically. Receptors are thought to register different gradations of intensity
depending on the substance present.
Determine which areas of the tongue are especially sensitive to specific tastes.
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Figure 12 Human taste bud. Cells of
taste buds may be classified as hair
cells, responsible for transducing
sensory information, and supporting
(sustentacular) cells. The external
(distal) ends of the receptor cells
project from a pore in the taste bud,
enabling them to make direct
contact with chemical substances.
The internal (basal) ends of the
receptor cells synapse with sensory
neurons that enter the brain
through one of two cranial nerves.
Sustentacular cells can transform
into taste cells.
Chemoreception: Discrimination and Its Influence on Taste
Smell (olfaction) is another example of chemoreception. In this case, the receptors are part of a mucus secreting
membrane (hence, mucous membrane) in the upper part of the nasal cavity. The receptors, called hair cells,
have hair-like cilia on one end extending into a layer of mucus (Figure 13).
Before the odor of a substance can be detected, the substance must release molecules that diffuse through air
and into the olfactory epithelium. These molecules then dissolve in the mucus and bind to receptor molecules in
the cilia of the sensory cells. Exactly how these receptor cells and the brain distinguish odors is not fully
understood. It is known, however, that we can distinguish a far greater number of odors than tastes. In fact, our
enjoyment of food is actually the product of two sensations-the limited information conveyed by taste buds and
the broader range provided by the olfactory receptors.
Do you think that information obtained by our olfactory receptors can influence how we think a particular food
tastes? If you held your nose, would a cherry Life Saver taste the same as a lemon Life Saver? If you were blindfolded and given lemon Life Savers to smell, but were fed cherry Life Savers, what do you think you would report
when asked how the candy tasted?
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Formulate a hypothesis as a tentative explanation for how our sense of smell affects our sense of taste.
HYPOTHESIS:
NULL HYPOTHESIS:
What do you predict you will find if you test your hypothesis?
What is the independent variable?
What is the dependent variable?
Figure 13 (a) A patch of special tissue, the olfactory epithelium, arching over the roof of each nasal cavity, is
responsible for the sense of smell. (b) The olfactory epithelium is composed of three types of cells: supporting
cells, basal cells, and olfactory cells. The olfactory cells are the sensory receptors. Cilia protruding from their
upper surfaces are believed to be the odor receptors, although the way in which they function is not known.
Note that the sensory cells are neurons that give rise to extensions (axons) that lead directly to the olfactory
lobe of the brain.
(a)
Fill in the labels….
(b)
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Name
Location/ Composition/Structure
Functions
Prosencephalon-Telencephalon
Forebrain-Anterior Area
a Cerebral hemispheres (olfactoryMost dorsal and anterior portion of
lobes, basal nuclei, and cerebral
brain.
cortex)
Most prominent and well-developed
part of mammalian brain.
Two lateral hemispheres.
Highest integration center of the
nervous system.
Seat of psychic functions such as
consciousness, sensation, perception,
memory, and judgment (in humans).
Convolutions increase surface area.
Cortex
Outer gray matter of cerebrum
Controls speech and voluntary
(part of cerebral hemispheres)
contains cell bodies; inner white matter
composed of myelinated fibers and
movements.
Functions may be mapped by studies
organized fiber tracts that connect cell
of accidental injuries.
masses in cerebral cortex to other parts
Much of cortex is “silent,” involved in
of the brain.
associative activities.
Visual and auditory information is
projected to definite areas of the cortex.
b Corpus callosum
Located beneath the middle portion of
Fiber tracts interconnect the two halves
cortex.
of the cerebral hemispheres.
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Table 41K-lme
Location/ Composition/Structure
Functions
Olfactory bulbs
Much reduced in mammals, but they
Integrate olfactory sensory
(oldest part of the forebrain)
lead to an older, more primitive part of
the telencephalon where sensory
information.
information from the nose is
integrated.
Lateral ventricles
Prosencephalon-Diencephalon
Forebrain-Posterior Area
c Thalamus
Cavities in the cerebral hemispheres.
Contain cerebrospinal fluid.
Located posterior to cerebrum; forms
“Way station” for information
the side walls of this more posterior
traveling from lower brain centers to
region of the forebrain
cerebrum.
VISUal and auditory information relayed
to cortex in this area in mammals.
d Epithalamus
Non-nervous choroid plexus (tissue
Produces the cerebrospinal fluid that
layer)
fills the central cavities of the brain
and spinal cord.
Site of pineal gland.
Pineal gland is photoreceptive in some
vertebrates. Produces melatonin, a
hormone affecting rhythmic
phenomena.
e Hypothalamus
Floor of the brain posterior to the
cerebrum and below the thalamus.
Controls visceral nervous system,
particularly emotions, temperature
regulation, sleep, water balance, food
and water intake, metabolic activity,
and reproductive activity.
Median eminence
Endocrine gland-produces
neurohormones that regulate the
anterior pituitary and other body
functions.
Optic chiasma
Optic nerve enters the brain through
this area.
Mesencephalon
Midbrain
Tectum
Corpora quadrigemini-four small
Anterior bodies associated with
bodies, two anterior and two posterior,
pupillary reflexes. Posterior bodies
constitute the midbrain roof. The first
concerned with auditory reflexes (e.g.,
two correspond to a massive brain
pricking of a dog’s ears).
area, called the optic lobes, in
nonmammalian vertebrates.
Tegmentum
Floor and sides of the midbrain.
Relays motor fibers and information
traversing the ventral brainstem,
including pyramidal tracts which relay
information directly from the cerebral
cortex to the spinal column.
Controls a variety of involuntary
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processes.
Rhombencephalon-Metencephalon
Hindbrain
Cerebellum
Anterior part of hindbrain; relatively
large, rounded structure with a
convoluted surface. In mammals the
Concerned with equilibrium, posture,
and movement.
Receives input from most sense organs,
brain is folded so that the cerebellum
including proprioceptors in joints and
lies directly behind the cerebrum.
the inner ear.
Surface contains cell bodies forming
Keeps track of the orientation of the
gray matter. White matter under the
body in space and the degree of
surface contains nerve fibers
contraction of skeletal muscles.
connecting the medulla and other areas
Fine-tunes movements initiated in
of the brain.
other areas of the brain.
Injury to cerebellum results in
impairment of muscular coordination
but not in paralysis.
Pons
Anterior end of medulla. Composed of
Fiber tracts coordinate movements on
thick bundles of myelinated (white)
both sides of the body.
nerve fibers that carry impulses from
one side of the cerebellum to the other
in mammals.
Rhombencephalon-Myelencephalon
Medulla oblongata
(begins in the metencephalon)
Located beneath and posterior to
cerebellum; connects the spinal cord
and midbrain.
Roof has a second choroid plexus
i
Contains nerve centers that control
vital, largely subconscious activities
such as respiration, heart rate,
constriction and dilation of blood
vessels, swallowing, and vomiting.
Choroid plexus secretes cerebrospinal
fluid.
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