this instructional model over open inquiry strategies, particularly if
students have limited prior knowledge in the area of study (
Kirschner et al., 2006). This structured/guided inquiry activity was combined with quantitative reasoning tasks, as it appears unlikely that
a deeper conceptual understanding of synaptic transmission involving kinetics aspects, such as diffusion time of transmitters, can be
acquired through qualitative treatment only. This instructional strategy is in line with the increasing recognition of quantitative reasoning as a core competency in undergraduate education (Elrod, 2014).
Inquiry-Based Activity
The following inquiry-based activity has been designed primarily
for lower-division college courses in biology or neuroscience. How-
ever, it might also be a useful exercise for high school students tak-
ing AP Biology. Its specific objectives are as follows:
(1) Students will learn how to determine the size of biological
structures on microphotographs or drawings using magnifi-
cation factors. Through this activity, they will develop some
basic familiarity with the cellular and subcellular dimensions
relevant to synapses.
(2) Students will learn to apply Einstein’s approximation equation to estimate the time it takes a neurotransmitter to diffuse across the synaptic cleft of chemical synapses, and
they will calculate the rate of diffusion.
(3) Students will learn to discuss how diffusion distance affects
both the diffusion time across the synaptic cleft and the concentration of transmitter molecules at the postsynaptic receptors, and thus the overall synaptic transmission process.
Materials & Resources Needed
For each group of two students, the following materials are needed:
• Printout of the model chemical synapses shown in Figures 1 and
2 (Figure 2 can be downloaded at https://commons.wikimedia.
org/wiki/ File:Synapse_diag1.svg)
• Metric ruler with millimeter markings
• Calculator
Part 1: Estimation of the Dimensions
of the Model Synapse
Procedure
In the first part of the guided inquiry activity, the students will estimate
the width of the synaptic cleft of the synapses shown in Figures 1A
and 2. In the following, I refer to the two models shown in these figures
as “Morphometric Synapse Model” and “Schematic Synapse Model,”
respectively.
By the time the students start Part 1, they should be familiar
with the relevant metric prefixes and the corresponding values
shown in Table 1. The students should also be able to relate the
results obtained through the inquiry activity to some fundamental
dimensions of nerve cells in the central nervous system, such as
the range of soma sizes (granule cells in the cerebellum: ~4 μm;
Betz cells of the motor cortex: 100 μm) and the range of diameters
of axons (0.1–10 μm) among cells.
For the estimation of the synaptic dimensions, the students are
provided with printouts of the model synapses shown in Figures 1A
and 2, including a description of the labeled components. The only
other information they receive is that the diameter of the presynaptic
axon is 0.5 μm. Based on this information, the students are asked to
estimate the dimensions of the following two components of the
two model synapses: (1) length of the presynaptic terminal (defined
as the dimension perpendicular to the axon shaft) and (2) width of
the synaptic cleft. To estimate these dimensions, they should follow
these steps:
(1) Measure (in millimeters) the diameter of the axonal shaft on
each of the two printouts.
(2) Assuming that the diameter of the axonal shaft is 0.5 μm,
use the results obtained in step 1 to calculate the magnification factor of the drawings shown in each of the two figures.
This is done by dividing the diameter measured on the
drawing by 0.5 μm. Remember to use identical units of measure for your calculations.
(3) Measure (in millimeters) the length of the presynaptic terminal (defined as the dimension perpendicular to the axon
shaft) on each of the two printouts.
(4) Use the magnification factors calculated in step 2 and the corresponding results obtained in step 3 to estimate the actual
lengths of the presynaptic terminals shown in the two
drawings.
(5) Measure (in millimeters) the width of the synaptic cleft on
each of the two printouts.
(6) Use the magnification factors calculated in step 2 and the
corresponding results obtained in step 5 to estimate the actual
width of the synaptic clefts shown in the two drawings.
Results
Let us assume that, on the printout of the Schematic Synapse
Model, the diameter of the axonal shaft is 40 mm (please note that
this value may vary among different printouts) – that is, this structure is 80,000 times enlarged, compared to the actual dimensions
of a typical axon, here assumed to be 0.5 μm. The length of the
Table 1. Metric prefixes.a
Prefix Symbol
Multiplication
Factor Power
(no prefix) 1 100
centi c 0.01 10−2
milli m 0.001 10−3
micro μ 0.000001 10−6
nano n 0.000000001 10−9
a The metric prefix is a modification of the basic unit of measure to indicate the
value of the unit. Examples: 1 μm (micrometer) = 1·10−6 m; 5 ms (millisecond) =
5·10−3s.
