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Systems Physiology Final Lab Report
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Full Lab Report for Systems Physiology Lab
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ABSTRACT:Introduction: Neurons which are excitable cells achieve communication via action potentials
(AP). Action potentials are transient changes in the voltage across the membrane and possess
the characteristics of resting membrane potential (Vm), threshold potential (Vt), and rates of
depolarization and repolarization. Action potentials follow the All-or-None principle.
Purpose: The aim of the experiment is to determine the threshold potential, the maximum
voltage that causes a change in the spike height, to assess the absolute and relative refractory
periods, and to calculate the conduction velocity of the sciatic nerve of a frog.
Methods: A sciatic nerve of a frog was isolated following the appropriate procedures.
Following the proper setup of the nerve in the nerve bath with corresponding cables and
electrodes, the LabChart8 was used to stimulate the nerve and record the data for 3 separate
experiments to test the 3 aims and the hypothesis.
Results: The experiment showed the threshold voltage of the sciatic nerve to be 20mV and
the maximum voltage to be 400mV with an AP amplitude of 47.8mV. It also showed the
absolute refractory period for the nerve is 1.3ms and the relative refractory period is 2.5 to
1.3ms. The final part showed the conduction velocity of the nerve is 90 m/s.
Conclusion: It is shown that each axon has a different threshold value. The first hypothesis
that there will be an increase in the compound action potential amplitude after the threshold
value was supported. The hypothesis about the refractory periods was supported as well. The
nerve wasn’t able to generate a second AP 1.3ms after the first AP, indicating the absolute
refractory period. It also showed the refractory period because when the inter-stimulus
interval (ISI) was higher than 2.5ms, the AP amplitudes were around 4mV but when the ISI
was in between 1.4-2.5ms, the AP amplitude was around 0.1-1mV. The conduction velocity
of the nerve was relatively high, supporting the third hypothesis, due to its large diameter of
the axons which counts for more myelination, which increases the conduction velocity.
, INTRODUCTION:
Neurons form the basis of the nervous system. They are excitable cells such that they
can transition from a resting state to an excited state when an electrical or chemical stimulus
is present. During their excited state, they conduct an action potential which is considered
their way of communicating. One measurement that determines whether a neuron is in an
excited or resting state is the membrane potential: the electrical potential difference across the
plasma membrane due to concentration differences of ions inside and outside of the cell [1].
When in rest, this potential is called resting membrane potential (Vm). In order to be excited
and thus conduct an action potential the resting membrane potential should reach a specific
value called threshold potential (Vt). This threshold potential is different for every single
axon of a neuron because it depends on the diameter and the degree of myelination of the
axon. This suggests that when stimulating a mixed neuron, which produces a compound
action potential (CAP), not every neuron will conduct an action potential with each stimulus.
Some may need higher voltages than the rest. In other words, stimuli that increase the
membrane potential to a value less than the threshold won’t cause any action potentials in the
neuron. This is called the All-or-none principle. Once the threshold is reached, potential
spikes will have a constant amplitude. Reaching the threshold potential opens certain ion
channels, resulting in the influx of the sodium ions which depolarizes the cell. This increase
in the membrane potential is followed by the increased permeability of the neuron to
potassium ions, resulting in the efflux of potassium and decreasing the membrane potential
back to its resting value [2]. This process of restoring the membrane potential is called
repolarization. In addition to individual threshold values, each neuron also possesses different
levels of myelination, resulting in different conduction velocities for each individual neuron.
During an action potential full of transitory changes of the permeability to different
ions, the neurons are incapable of generating a new action potential [3]. This period where
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