INTRODUCTION TO SENSORY SYSTEMS
Oral Sensation and Perception
In the first half of this course, we shall consider some of
the sensory mechanisms of the orofacial region. The distinction of
sensory from motor functions is somewhat arbitrary, as both
usually occur in the same regions of the body at the same time,
and often they involve common elements of the nervous system in
reflex actions. However, it is a useful way to compare the
different types of sensory systems, with respect to the
embryological origins of the sensory endings, the specific
mechanisms of transduction and the sensory pathways and central
projections involved. Oral sensory function has been the subject
of many studies in the past (Bosma, 1967, 1970, 1972, 1973; Dubner
et al., 1978) and continues to be examined (Kawamura, 1983; Finger
and Silver, 1987; Cagan, 1989). We shall discuss current research
on several specialized oral sensory systems in the following
chapters.
How Many Senses?
Aristotle taught that there are five senses - vision,
hearing, smell and taste and touch. The fifth sense, touch, did
not appear to later scientists to be unitary; it did not have an
organ like an eye, ear, nose or mouth, and the skin contained more
than one type of sensory ending. So Aristotle's "touch" was
subdivided into submodalities such as pressure, warmth, cold and
pain. These sensory endings are all designed to signal changes in
the outside environment, hence they are known as exteroceptors.
In addition, a little introspection convinces us of other
types of "body senses." These include proprioception, telling us
about the position of our limbs with respect to the body or of the
body as a whole, and kinesthesia, or the sense of movement. Then
there is deep pain sense, signaling distension or inflammation of
musculoskeletal or visceral structures, and the more subtle
awareness of hunger and thirst. Mostly unconscious signals about
blood glucose concentration, oxygen level, or blood pressure are
signaled by interoceptors, that detect changes occuring in the
respiratory system, circulatory system, or viscera.
When we pick up an object and turn it around in our hands to
help identify it or determine some property of the object, we are
using haptic perception (Gibson, 1967). This is an exploratory
method where the organism acts on the object and the object acts
on the organism: We squeeze, scratch or heft an object to see
"what it feels like." Our fingers deform the object, if it is
elastic, and are deformed by the object's weight. Some of the
qualities that may be perceived by haptic exploration of an object
are shape, edges, curves, points, texture, heaviness, and rigidity
or elasticity.
Organization of Sensory Systems
A schematic drawing of a generalized sensory system is shown
in Figure 1. It is somewhat simplified, but shows the main
elements present in all sensory systems. A stimulus consisting of
some form of energy impinges on a sensory ending, or receptor, and
gives rise to action potentials in the associated neural pathway.
The ending may respond to more than one form of energy, but that
which excites at the lowest energy level (the natural stimulus) is
called the adequate stimulus. The resulting action potentials
reach the central projection area, usually including the thalamus
and cortex, and evoke the appropriate motor responses. The sensory
signals also often result in an introspective awareness, or
sensation, produced by the application of the stimulus to the
body.
In a way, sensory systems perform the function of converting
analog signals such as forces, temperatures or concentrations into
digital sequences of nerve impulses. Action potentials in a single
nerve fiber are all the same size; they are either there or not.
What changes when a stimulus increases or decreases in intensity
is the frequency of firing of action potentials, and the number of
nerve fibers activated. When the digital information reaches the
central nervous system, neurotransmitters are secreted in an
analog fashion, the concentration corresponding to the frequency
of action potentials in the nerve terminals. We can see how this
is done by examining each of the steps outlined in Figure 1.
Peripheral Sense Organs
If one explodes the oval area in Figure 1 marked "Sensory
Endings," the internal mechanisms of this portion of a sensory
system may be shown as in Figure 2. The stimulus is applied at the
top, and the resulting nerve impulses are conducted into the
"Neural Pathway" of Figure 1. The first step in this process is
transformation of the stimulus energy into a form suitable for
exciting the sensory cell membrane. For instance, light is focused
by the cornea and lens before reaching the retina, and sounds are
converted from presure waves in air to fluid waves in the cochlea
before exciting the hair cells in the basilar membrane. The
transformed stimuli then directly excite the sensory cells. The
process of transduction occurs next; the internal stimulus causes
the opening of ionic channels in the sensory cell membrane,
producing a receptor current which gives rise to a receptor
potential. Through the encoding mechanism, this change in membrane
potential is converted to an outgoing train of sensory action
potentials.
In some sensory systems, such as that for cutaneous
sensation, the sensory cell itself is the first-order neuron, and
has an axon which connects to the central nervous system. In this
case the receptor potential is also called a generator potential,
as it encodes the production of action potentials within the same
cell. An example of a generator potential producing action
potentials in a stretch-sensitive ending is shown in Figure 3. The
membrane potential is negative at rest. Applying a stretch (first
arrow) causes a depolarization of the cell membrane and the
potential becomes less negative. When the potential passes the
threshold value, shown by the dashed line, a few action potentials
are produced. Further stretch (second arrow) increases the
frequency of firing. Release of the stretch (third arrow), again
increases the negative membrane potential below the threshold
value, and the receptor stops firing.
In other cases, such as vision, hearing, and taste, the
receptor potential of the primary sensory cell is converted by
synaptic transmission to membrane potential changes in a secondary
or tertiary cell, where the signals are then encoded into trains
of action potentials. In other words, the first-order neuron in
the system may not be the sensory cell itself, but a cell that
makes contact with it. In the sense of taste, the sensory cells
are derived from endodermal precursors, while the first-order
neurons connected to them arise from the neural crest.
Sensory receptors of various types have the property of
adaptation, or slowing of action potential frequency with a
maintained stimulus. This can result either from a gradual change
in the stimulus transformation process, such as a mechanical
slippage of a touch-sensitive ending, or an electrical change in
the sensory-cell membrane itself. Examples of adaptation are shown
in Figure 5. The muscle spindle ending maintains its frequency of
firing throughout a maintained stretch, as does the pressure
receptor in the skin of the cat. A rapidly-adapting touch receptor
or hair-follicle ending, however, stops firing soon after the
onset of the stretch, and a single nerve fiber responds only once
to a maintained electrical stimulus.
Neural Pathways
As mentioned above, the intensity of a stimulus applied to a
sense organ is encoded as the frequency of action potentials in
the associated sensory axons and the number of active axons.
Typical mammalian sensory axons do not come in all sizes; they are
grouped into classes, each with its own diameter and conduction
velocity, as shown in Figure 6. The inset shows the fiber-diameter
histogram obtained in a human sensory nerve when the axons were
stained and measured under the microscope. Two groups of fibers
are apparent, one with a mean diameter of about 3 m and the other
about 12 m.
When a section of this nerve was placed in a moist recording
chamber and stimulated at one end, the signal shown in the main
part of Fig. 5 was recorded a few cm away from the stimulating
electrodes; this is known as the compound action potential, as it
contains the activity of hundreds of individual axons. Here again,
two peaks are seen in the action potential, one due to the
rapidly-conducting group of axons and the other to the more
slowly-conducting group. In a series of experiments, Erlanger and
Gasser (1937) showed that the large-diameter fibers in nerves had
the fastest conduction velocities, as well as the lowest threshold
for electrical stimulation. Subsequently it has been shown that
large-diameter fibers are also less susceptible to blockage by
local anesthetics, which is why the sense of touch often remains
after a dental nerve block, when the sense of pain has been
eliminated.
Sensory signals in cutaneous and oral pathways are not
carried by single lines, as are telephonic or other
communications. Instead, they are carried by semiliquid
physicochemical systems, consisting of somewhat noisy axons and
synapses. To insure reliability of these important sensory
systems, mammalian organisms have used redundancy, or replication
of each piece of the equipment hundreds or thousands of times (a
natural strategy given the way the body develops from countless
mitotic divisions). Thus, the signals reaching the central nervous
system re really averages of the firings of these many axons.
First-order sensory neurons entering the spinal cord synapse
with cells in the dorsal horn, which send axons across the midline
and rostral toward the thalamus. Most sensory neurons in the
orofacial region relay in the nuclei indicated on the right side
of Figure 7. The trigeminal nucleus extends from the medulla
through the pons and into the midbrain. Orofacial trigeminal
inputs relay on neurons which then cross to the other side of the
brainstem and ascend toward the thalamus; these also make contacts
with other interneurons and motoneurons. The nucleus of the
tractus solitarius contains second-order sensory neurons for the
facial, glossopharyngeal, and vagus nerves. These afferents are
especially concerned with the sense of taste (Chapter 8) and the
initiation of swallowing (Chapter 15).
Central Projections
The next relay station for sensory afferents is the thalamus.
Here they make contact with third-order neurons which project to
the sensory cortex. Cell bodies for neurons in the spinal system
are located in the ventral posterolateral (VPL) nucleus of the
thalamus, and those for the orofacial system are in the ventral
posteromedial nucleus (VPM).
The highest level of projection of sensory pathways is to the
postcentral, or sensory cortex. The density of projection of
sensory neurons from various regions of the body on the sensory
cortex is shown in Figure 8. The amount of cortex representing
each area of the body is indicated by the drawings of each region
next to the cortex. This information may be gained by stimulating
the surface of the body electrically and observing the amount of
cortex excited (Marshall et al., 1941; Penfield and Rasmussen,
1950). Nowadays the technique of functional magnetic resonance
imaging can be used to reveal the same distribution (Hayman, 1992;
Orrison et al, 1995).
From psychophysical experiments it is seen that the areas of
skin with the greatest cortical representation are those that have
the greatest sensitivity, as measured with calibrated hairs or
two-point threshold testing. This sensitivity depends on the
density of innervation of the particular area. The back, which is
relatively insensitive to touch, has far less sensory axons per
mm2 of surface than the lips or the tips of the fingers, which are
much more sensitive to touch stimuli. Note that the head and face
occupy about one-half of the external surface of the sensory
cortex, and contain some of the most sensitive areas of epithelium
in the body.
Effects of cortical lesions
About 90% of people are right-handed, and in these the left
hemisphere of the brain controls speech and most intellectual
function. In many left-handed people there is shared dominance,
that is, both sides of the brain participate in intellectual
functions. So with right-handed individuals, the loss of a major
lobe (left side), from a cerebral hemorrhage or anoxia, leads to
loss of communicative abilities. Thus, major-lobe lesions are hard
to study. Minor-lobe events, however, do not block the ability to
speak, so patients can tell you what they are thinking or feeling.
Here is a story of a minor lobe lesion, or Left Hemiplegia, from
the book by Oliver Sachs titled, The Man Who Mistook His Wife For
A Hat, which consists of very unusual case histories of sensory
and perceptual dysfunctions. This chapter is called, "The Man Who
Fell out of Bed":
When I was a medical student many years ago, one of the nurses
called me in considerable perplexity, and gave me this singular
story on the phone: that they had a new patient--a young man--just
admitted that morning. He had seemed very nice, very normal, all
day--indeed, until a few minutes before, when he awoke from a
snooze. He then seemed excited and strange--not himself in the
least. He had somehow contrived to fall out of bed, and was now
sitting on the floor, carrying on and vociferating, and refusing
to go back to bed. Could I come, please, and sort out what was
happening?
When I arrived I found the patient lying on the floor by his
bed and staring at one leg. His expression contained anger, alarm,
bewilderment and amusement--bewilderment most of all, with a hint
of consternation. I asked him if he would go back to bed, or if he
needed help, but he seemed upset by these suggestions and shook
his head. I squatted down beside him, and took the history on the
floor. He had come in, that morning, for some tests, he said. He
had no complaints, but the neurologists, feeling that he had a
`lazy' left leg--that was the very word they had used--thought he
should come in. He had felt fine all day, and fallen asleep
towards evening. When he woke up he felt fine, too, until he moved
in the bed. Then he found, as he put it, `someone's leg' in the
bed--a severed human leg, a horrible thing! He was stunned, at
first, with amazement and disgust--he had never experienced, never
imagined, such an incredible thing. He felt the leg gingerly. It
seemed perfectly formed, but `peculiar' and cold. At this point he
had a brainwave. He now realized what had happened: it was all a
joke! ...It was New Year's Eve, and everyone was celebrating. Half
the staff were drunk....Obviously one of the nurses with a macabre
sense of humour had stolen into the Dissecting Room and nabbed a
leg, and then slipped it under his bedclothes as a joke while he
was still fast asleep. He was much relieved at this explanation;
but feeling that a joke was a joke, and that this one was a bit
much, he threw the damn thing out of the bed. But--and at this
point his conversational manner deserted him, and he suddenly
trembled and became ashen-pale--when he threw it out of bed, he
somehow came after it, and now it was attached to him.
`Look at it!' he cried, with revulsion on his face. `Have you
ever seen such a creepy, horrible thing? I thought a cadaver was
just dead. But this is uncanny! And somehow--it's ghastly--it
seems stuck to me!' He seized it with both hands, with
extraordinary violence, and tried to tear it off his body, and,
failing, punched it in an excess of rage.
`Easy!' I said. `Be calm! Take it easy! I wouldn't punch that
leg like that.'
`And why not?' he asked, irritably, belligerently.
`Because it's your leg,' I answered.
This is an example of loss of awareness of a hemiplegic limb, or
"hemineglect," a minor-lobe lesion in a right-handed person.
Measurement of Sensations
The field of psychophysics has as one component the study of how
sensations in the introspective world are related to stimuli in
the outside environment. One may study different properties of
these variables, such as the latency, duration, or magnitude. To
describe the changes of sensation magnitude with stimulus
intensity, an association is assumed between the stimulus S and
the resulting sensation :
Stimulus
Sensation
____________________________________
S1
1
S2
2
.
.
.
.
Sn
n
These are two continua, where the variable  changes with
different values of S. (This relation is valid only for static
conditions;  usually depends on the rate of change of S as well
as S.)
An early quantitative expression used to relate these two
variables was the Weber-Fechner relation,
 = K log S
Fechner, in 1860, had expanded on Weber's earlier theory to
produce this expression. It states that, as the stimulus intensity
is increased, the sensation goes up, but at a slower rate. This
expression is valid for some senses having a very wide dynamic
range of signals, such as sound intensity, but fails to describe
the variation of sensations in other systems such as temperature
or judgment of weights.
To encompass sensory systems that violated the Weber-Fechner
theory, S. S. Stevens (1975) used the method of magnitude
estimation. Subjects were asked to respond to varying intensities
of stimuli such as loudness, brightness, length of lines, etc., by
giving the investigator a number between certain limits (1-10 or
0-100, for instance), which they thought corresponded to the
magnitude of their perceived sensations. In some cases the
subjects were given a "standard" stimulus for comparison; they
might be told that a certain sensation was a "10" before being
asked to estimate the other sensations.
Stevens soon discovered that, on average, groups of subjects
gave reliable estimates of sensations that varied systematically
with the stimulus intensities. The unifying relationship for all
the sensory modalities studied was
 = K Sn
This is known as the Power Law of sensations and stimuli. The
exponent n is different for various modalities, for example,
Modality
n
__________________________
Loudness
.6
Brightness
.3
Smell
.5
Taste
1.3
Temperature
1.0
Vibration
.95
Heaviness
1.45
Electric shock
3.5
When n is less than 1, the relationship may be described fairly
well by the Fechner theory. When n equals 1 the sensation varies
linearly with the stimulus; for values of n greater than 1 the
sensation increases more rapidly than the stimulus; stronger
stimuli produce much stronger sensations, etc. This is a more
widely applicable relationship than Fechner's as it can describe
the behavior of systems where the sensation increases more rapidly
than the stimulus. By measuring a quantitative dependency such as
this, one can distinguish normal from abnormal functioning in a
sensory system, and study the effects of agents such as drugs that
change the system's behavior.
Sensory Illusions
Illusions are usually viewed as errors of judgment or
mistaken interpretations. However, they may tell us much about the
normal functioning of sensory systems. The hatched-line illusions
in Figure 8 are probably a result of the retinal circuitry;
placement of the background hatching in Part A makes the lines
appear to diverge, when they are actually parallel. The version in
Part B makes the vertical lines appear to bow outward at the
center, although they are parallel.
In cases where a familiar black-on-white pattern is replaced with
white-on-black, we must reorient our perceptions.
Confusion about whether the black or the white image predominates
can lead to oscillation from one perception to another.
Mach bands are produced at the intersection of dark areas, due to
increased off-center inhibition in the retinal circuitry.
After-images may change colors, due to saturation of pigmented
receptors in the retinal cones.
Our perceptions of various changes in the external world are
thus subject to modifying effects. These may include the
physiological behavior of the sensory system itself, or our
previous experience with sensations. We shall have more to say
about how perceptions are altered, especially in connection with
pain and taste.