(Nerve impulse / Contrast enhancement / Feature detectors)
In this section, Feynman discusses nerve impulse
and experiments on the nerve fibers of horseshoe crab (related to the principle
of contrast enhancement) and frog (feature detectors).
1. Nerve impulse:
“First of all, we must appreciate what kind of
information can come along nerves. A nerve carries a kind of disturbance which
has an electrical effect that is easy to detect, a kind of wavelike disturbance
which runs down the nerve and produces an effect at the other end: a long piece
of the nerve cell, called the axon, carries the information along, and a
certain kind of impulse, called a “spike,” goes along if it is excited
at one end (Feynman et
al., 1963, p. 36–9).”
The “spike” (disturbance) is also known as “nerve
impulse” or “action potential” that moves along a nerve fiber. A nerve impulse is an
electrical phenomenon that occurs due to differences in sodium ion-concentrations across
the membrane of a neuron (Hodgkin, 1964). In other words,
the nerve impulse is a sudden reversal of the electrical
charge across the membrane of the neuron. The Nobel Prize in Physiology or Medicine 1963 was awarded to
Alan Hodgkin and Andrew Huxley for their discoveries concerning the
ionic mechanisms involved in excitation and inhibition in the peripheral and
central portions of the nerve cell membrane. The
electrochemical nature of the nerve impulse was investigated by Hodgkin and
Huxley in their studies of squid giant axons.
“When one spike goes down the nerve, another cannot
immediately follow. All the spikes are of the same size, so it is not that we get higher spikes
when the thing is more strongly excited, but that we get more spikes
per second. The size of the spike is determined by the
fiber (Feynman et
al., 1963, p. 36–9).”
Perhaps Feynman could have included the “all or none
principle,” which means neurons will either transmit a nerve impulse
over the synapse to the next neuron completely or not at all. In short, the nerve impulse is not dependent on the
strength of the stimuli, but it is generated when the initial threshold is met
(see below). Specifically, the membrane potential of a neuron is about -70 mV
in the resting state (Patton & Thibodeau, 2018). If a stimulus is above the
threshold potential, the membrane depolarises, i.e., sodium-gated
ion channels open and allow sodium ions to move into the axon, making the
membrane potential more positive.
This principle underscores the binary nature
of action potential propagation: a neuron either fires a full-sized action
potential or it does not fire at all.
Source: Patton & Thibodeau, 2018
2. Contrast
enhancement:
“We can see, perhaps, if we
think about it awhile, that this is a device to enhance contrast at
the edges of objects, because if a part of the scene is light and a part is
black, then the ommatidia in the lighted area give impulses that are inhibited
by all the other light in the neighborhood, so it is relatively weak (Feynman et al., 1963, p. 36–10).”
Historically, Haldan Keffer Hartline (1949) identified the lateral
inhibition* phenomenon in his experiment on the compound eye of the horseshoe
crab. Furthermore, he showed that its photoreceptor cells are interconnected
and communicated via the mechanism of lateral inhibition: the ability of excited neurons to reduce the activity of neighboring
neurons. In essence, lateral inhibition of neurons can help to enhance contrast
at the edge of the objects perceived by the eye. Interestingly, Hartline discovered
the phenomenon accidentally when he turned on the room lights and found that
the neural response of a horseshoe crab eye decreased. Hartline was awarded the 1967 Nobel Prize in
Medicine or Physiology for his research in the neurophysiological mechanisms of
vision.
*The lateral inhibition phenomenon, which occurs in the retina was first
recognized by Ernst Mach, in the year 1866.
“Figure 36–11(a)
shows the compound eye of the horseshoe crab; it is not very much of an eye, it
has only about a thousand ommatidia. Figure 36–11(b) is
a cross section through the system; one can see the ommatidia, with the nerve
fibers that run out of them and go into the brain. But note that even in a
horseshoe crab there are little interconnections. They are much less elaborate
than in the human eye, and it gives us a chance to study a simpler example (Feynman et al., 1963, p. 36–10).”
It is worth mentioning that an ommatidium
is not an independent eye but a component of a compound eye; each ommatidium
functions as a part of the whole, providing information about a small segment
of the visual field. In addition, horseshoe
crab (or Limulus), has three different types of eyes: a pair
of image-forming lateral compound eyes, a pair of median ocelli, and three pair
of larval eyes—lateral, median, and ventral (Tanacredi et al., 2009). However,
horseshoe crabs are not really crabs (crustaceans), but are more closely
related to spiders and scorpions. More important, horseshoe crabs are “living fossils,” i.e.,
they have existed nearly unchanged for at
least 445 million years, even
before dinosaurs existed.
Hartline used the horseshoe crab as his first experimental model because of its
compound eye, easy-to-isolate optic nerve, and individual photoreceptor cells
help to understand the principle of contrast enhancement.
3. Frog’s feature detectors:
“The most recent picture of the operation of the
frog’s eye is the following. One can find four different kinds of optic
nerve fibers, in the sense that there are four different kinds of
responses. These experiments were not done by shining on-and-off impulses of
light, because that is not what a frog sees. A frog just sits there and his
eyes never move, unless the lily pad is flopping back and forth, and in that
case his eyes wobble just right so that the image stays put. He does not turn
his eyes. If anything moves in his field of vision, like a little bug (he has
to be able to see something small moving in the fixed background), it turns out
that there are four different kinds of fibers which discharge, whose properties
are summarized in Table 36–1 (Feynman et al., 1963, p. 36–11).”
Feynman
mentions that there are four different kinds of nerve fibers, but summarizes
five types of neural responses as Sustained
edge detection, Convex edge detection, Changing contrast
detection, Dimming detection, and Darkness detection. He adds that the neural responses seem to be rather complicated to
classify, however, Smoorenburg
(1987) identified the four responses as Sustained Contrast detector, Convexity
detector, Moving Edge detector, and Net-Dimming detector. Interestingly, toads,
a sub-classification of frogs, have only three primary types of responses in their visual processing
system: (1) Sustained edge detector, (2) Convexity detector, and (3) Moving
edge detector. While
frogs exhibit four types of neural responses, including dimming detectors, the
toads’ system has evolved for their specific ecological needs.
“Now these responses seem to be rather complicated
to classify, and we might wonder whether perhaps the experiments are being
misinterpreted. But it is very interesting that these same classes are very
clearly separated in the anatomy of the frog! By other measurements, after these
responses had been classified (afterwards, that is what is important
about this), it was discovered that the speed of the signals
on the different fibers was not the same, so here was another, independent way
to check which kind of a fiber we have found! (Feynman et al., 1963, p. 36–12).”
According to Lettvin and his collaborators’ (1959) study, there are four
types of nerve fibers or feature detectors in the frog’s eyes. The four types
of feature detectors can be classified as follows: (1) Contrast (Sustained Edge Detector): respond to the
presence of edges in the visual field and continue to signal as long as the
edge is visible. (2) Convexity (Convex Edge Detector): sensitive
to convex shapes, which might resemble preys. (3) Changing contrast (Change-in-Contrast
Detector): fire in response to moving edges or changes in contrast but
do not maintain the signal if the object is stationary. (4) Darkness (Dimming
Detector): increase their firing rate when the light intensity
decreases. They also suggested the idea of “bug detectors” that respond to
small, dark, and moving objects. However, the frog’s eye behaves like a quantum
sensor because it is possible to use a quantum light source to show that the
frog’s light-sensitive rod cells can detect single photon (Phan et al., 2014).
Review Questions:
1. How would you
explain the neural impulse using the all-or-none principle?
2. How would you relate
the lateral inhibition phenomenon to the principle of contrast enhancement?
3. How would you classify the frog’s feature detectors (or types of neural response in nerve fibers)?
The moral of the
lesson: The lateral inhibition (or contrast enhancement) in horseshoe crabs and
feature detections (three, four, or five) in frogs illustrate that brain-eye
systems and neural responses are dependent on how environments evolve.
References:
1. Ewert,
J. P. (1974). The neural basis of visually guided behavior. Scientific American, 230(3),
34-43.
2. Feynman,
R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading,
MA: Addison-Wesley.
3. Hartline, H. K. (1949). Inhibition of
activity of visual receptors by illuminating nearby retinal areas in the Limulus eye (abstract), Federation Proceedings, 8,
69.
4. Hodgkin, A. L. (1964). The ionic basis of nervous
conduction. Science, 145(3637), 1148-1154.
5. Lettvin,
J. Y., Maturana, H. R., McCulloch, W. S., & Pitts, W. H. (1959). What the
frog's eye tells the frog's brain. Proceedings of the IRE, 47(11),
1940-1951.
6. Patton, K. T., & Thibodeau, G. A.
(2018). Anthony's Textbook of Anatomy & Physiology-E-Book:
Anthony's Textbook of Anatomy & Physiology-E-Book. Elsevier Health
Sciences.
7. Phan, N.
M., Cheng, M. F., Bessarab, D. A., & Krivitsky, L. A. (2014). Interaction
of fixed number of photons with retinal rod cells. Physical Review Letters, 112(21),
213601.
8. Smoorenburg,
G. F. (1987). Discussion of physiological correlates of speech perception.
In The psychophysics of speech perception (pp. 393-399).
Dordrecht: Springer Netherlands.
9. Tanacredi,
J. T., Botton, M. L., & Smith, D. R. (Eds.). (2009). Biology and
conservation of horseshoe crabs. New York: Springer.
