Oral Neurophysiology
Dora Lee
April 8, 1998 10-11
Hey guys! The instructor’s name for this class is Dr. Junge (pronunciation? As in Jung,
the guy I learned about in AP English? I don’t know). Anyway, his room is in 63-078. As far as
reading material for the class, in your handouts, you will find a list of references. Please don’t
read them! If you must, he suggested Bradley’s Essentials of Oral Physiology. All questions on
the exams will come from the Main Lecture Points. Let’s get started…
I.
How many senses?
According to Aristotle, there are five senses : touch, smell, taste, sight, and hear.
Our body is aware of internal and external senses. External senses are perceived by
exteroceptors, receptors that feel things outside of the body. For example, proprioception feels
the body’s location in space and kinesthesia feels the body’s movements. Internal senses, such as
hunger and thirst, are picked up by interoceptors. Haptic perception tells us if an object has sharp
edges, curves, points, the weight, if it’s rigid or elastic.
II.
Organization of sensory systems.
Even though sensory systems are connected to motor systems, we can see this in every
sensory system:
central
stimulus-sensory ending---(pathway)- projection------causes sensation
area
Sensory systems convert analog signals (stimulus) to a digital signal (nerve impulses along axon).
(Digital, meaning that it either creates an action potential or it doesn’t). This then causes some
neurotransmitter to be secreted in an analog fashion, and becomes an analog signal.
A stimulus that is appropriate for a type of sensory system is called and adequate
stimulus. That means, a form of energy needed to excite at the lowest level. If you push on your
eyeball (or bash it or heat it), you will see light, b/c it is a light detector. But the form of energy
needed for the visual system at the lowest level is a photon, so light is an adequate stimulus.
Chemical are an adequate stimulus for the tongue, and mechanical stimulus is adequate for touch.
III. Peripheral sense organs
If you blow up the nerve ending :
Receptor
Stimulus-----transform------transduction-------------------encoding-----nerve impulses
Makes
internal
Stimulus
how sensory
cells detect
stimulus
current
how
current
generates AP
In vision, light is focused by the lens and gets onto the retina. The internal stimulus is the
focused light. In the ear, sound waves are converted into pressure waves in the cochlear fluid, so
there is a transformation of air waves into fluid waves. Transduction is how the sensory cells
detect the internal stimulus and make it into a current. Encoding is how current generates an AP
in the sensory axons.
Dr. Junge showed a slide of a generator potential in a touch-sensitive sensory cell. An
electrode was put in it and the stretch receptors were stretched. It reaches a threshold and starts to
fire. When stretched more, the frequency increases. When no longer stretched, it stops. This is
how most sensory cells work. The frequency tells us how strong the stimulus was.
In some sensory systems, (ie. Cutaneous) the sensory ending is the first-order neuron
(Meissner’s corpuscle or free-ending) they project into spinal cord. But in other systems, the
sensory ending is not even a neuron. For example, taste buds (sensory endings) are migrated
epithelial cells hooked up to the first-order neuron and are depolarized by the activity in the
sensory ending. In vision, the receptor cell (sensory cell) is a rod/cone, which are not neurons.
The bipolar neurons convert into nerve activity.
Sensory adaptation is when stimulus is continued but the AP frequency is lowered. He
showed a slide of a receptor potential from an electrode in a rat’s taste cell.
When a muscle spindle is stretched, it will fire continuously, it is slowly adapting. But in a
Meissner’s corpuscle, when you press on skin, it is rapidly adapting. For a nerve fiber, it fires a
little and slows down immediately, it is also rapidly adapting.
IV. Neural Pathways
If you put a sensory nerve in a recording chamber and shock it, you get two peaks: one
and two (see handout). So, what's the difference? Peak one conducts more rapidly, two is
slower. The rapidly conducting axons have a large median diameter (6.5-16) and the slower
ones are smaller (2.5-4.5 ). The rule: fast is large. Dr. Junge showed a slide of a fiber diameter
histogram obtained from staining the nerve. It shows that there are two populations of nerves:
small and large, they are not all sizes. Small fibers have a greater susceptibility to local
anesthetics. That is why C fibers are blocked before A fibers. (Pain fibers are blocked first).
They have a higher surface to vol ratio, so for a given conc of local, more gets in and makes a
bigger change in internal conc. For large, b/c more vol, not as much gets in and not much
change.
When stimulus is intense, freq of AP goes up, but amplitude does not. AP is all-or-none.
Also, when intense, there is recruitment. Not only is one axon firing more, but more axons are
firing. Those are two ways to incr sensory signal when stimulus incr.
V. Central projections
Dr. Junge showed us a slide of the brainstem just to give us an idea of the layout (see
handout). The right side has sensory nuclei. They run from midbrain, past the pons, into the
medulla. The main one for us is trigem. Facial signals from branches of V that cover the face go
into the spinal sensory nucleus of V. Solitary tract is for taste (VII, IX, X). The left side has
motor nuclei. V deals with mastication, swallowing. Facial nucleus deals with facial muscles.
Nucleus ambiguous is for IX, X, and XI.
(He had more slides to show us, but they mysteriously disappeared, they actually showed
up in the 2nd hour. If you’re curious, check out Netter’s – they were the same). The sensory
neurons come in and relay in the trigem nucl, cross over and go to thalamus. In the spinal cord,
they come in @ the dorsal horn, relay, and go to thalamus. In the spinal cord, they go to the
ventral posterior medial nucleus in the thalamus but the trigem nucl goes to the ventral posterior
lateral. So they both go to the thalamus, but the head goes to a more lateral area. (confused?)
The central projections are what happens after the thalamus. See sensory humunculus pic
in handout. Most of the body is represented in the upper half from the central sulcus to the
middle. The rest is the head (face and mouth). The amount of area covered depends on density
of ennervation. Hand has a big representation b/c lots of nerve axons vs. the back which is not.
About 90% of people are right-handed. The left brain is dominant b/c motor and sensory
all cross. It also has speech, math, intellectual stuff. (right brain/left brain theory). The right
brain does associations, seeing that things are correct, appropriate, familiar. A person with stroke
on left side can’t talk. People with a minor lobe lesion can only recognize half of things, ie. Draw
only half a daisy. Dr. Junge then went on to read an excerpt about a man with such a lesion from
The Man Who Mistook His Wife for a Hat, by Oliver Sacks. This person woke up one day and
did not recognize one of his legs as being his. He had hemineglect – loss of hemiplegic
awareness. These people don’t shave on one side, or put pants on one side, etc.
VI. Measurement of sensations
In psychophysics, there is the stimulus world and the sensation world.
S: S1, S2 ……S (stimulus)
: 1 2……. (sensation – what we feel)
To relate these quantities, there is the Weber-Fechner Law: =klogs
Log fns get less at high levels, so when stimulus gets bigger, sensation doesn’t get as big. At a
rock concert, although it is extremely loud, the sensation is only a little bigger. But this doesn’t
work with all systems. So, @ Harvard, they came up with the Stevens-Power Law: =ksn. For
diff. senses, there is a different power. For loudness, n=0.6, for brightness, n=0.3. you get a
specific curve. For temp sense, n=1. For heaviness, n>1. The heavier it gets, it seems even more
heavy. So, this covers more sensory systems than the Weber Law.
Magnitude estimation: for a given stimulus, a number b/n 1 – 100 is picked to tell
intensity. Sometimes, can be calibrated, one sound is a 10, a second, louder sound is 20, etc (like
the hearing test we took for health assessment before we all came to dental school). When the
perceived magnitude of sensation is plotted, we get a relation to the stimulus. If averaged with
enough people, we get a power log. This is just one way to measure sensation. Visual analog
scale is used when a subject is told to mark intensity on a given line from “none” to “worst
imaginable”. None------------------------------worst
VII.
Sensory Illusions
Shows how our sensory systems work. Some illusions affect the background, and thus, our
perception of the illusion ( parallel lines with irregular hatched lines make the lines appear to be
not parallel). Some show the effect of training. (We normally expect an image to be black and
white, so that’s the first thing we look for, but in illusions, we realize that it’s the background
that’s important. This refers to the oscillating images of two faces that becomes a vase). There is
also something called lateral inhibition in the retina. Light around the area we’re looking at
inhibits the retinal response in that area (off-center inhibition). This creates the illusion of dark
areas b/n dark boxes in a white background. There is also adaptation of receptors. We stared at a
green, black and yellow flag then looked at a white screen. We saw an after-image of a red, white
and blue flag. So what we see is based on how the sensory system is hooked up, and also by our
previous experience.
Main Lecture Points:
1. touch, vision, hear, smell, taste
2. temperature, proprioception (jaw opening), pain, kinesthesia, haptic perception, vibration,
3. kinesthesia
4. haptic
5. adequate
6. transduction
7. adaptation
8. lower velocity, higher threshold (a touch becomes painful when small fibers are excited),
greater susceptibility
9. density, ennervation
10. sensation, stimulus