Nerve Impulse/Muscle Contraction Study Guide KEY
I.
Match the letters of the terms below with the region of the graph they represent.
 There may be more than one term used for any given region.
 Each term may be used more than once.
 For each portion of the graph, record the letter(s) of the primary factor(s) responsible for charge illustrated
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
threshold
– 55 mV
depolarization
– 70 mV
Na gated channels open in response to stimulus
Na voltage-gated channels open
K voltage-gated channels open
hyperpolarization
Na-K pump functional
resting potential
undershoot
refractory period
action potential
K+ ion channels open
repolarization
5
1
2
6
4
3
7
8
1. A, B
2. D, J
3. I, J, N, (D)
4. (A), (B), C, E
5. C, F, M
6. G, O, L, (M)
7. G, H, K, L
8. I, J, N, (D)
II.
Complete the following paragraph describing the transmission of a nerve impulse.
A resting neuron has a membrane potential of approximately - 70 millivolts, due primarily to two factors. First, the sodium potassium pump
establishes a relative negative charge inside the neuron as compared to outside by pumping 3 Na ions out of the neuron for every 2 K ions
pumped into the neuron. In addition, there are ion channels permeable to K that allow ions to flow freely out of the neuron according to the
concentration gradient in a type of transport known as facilitated diffusion. When a stimulus is initially received by the dendrite of a
neuron, it triggers the opening of gated-Na channels. This causes depolarization of the neuronal membrane. If the stimulus is strong
enough to trigger the opening of enough channels to reach threshold, a(n) action potential is initiated, stimulating the opening of
voltage-gated Na and K channels in the membrane of the axon. At this point, the events that follow are not reflective of the original intensity
of the stimulus, a principle known as all-or-none. Although both types of voltage-gated channels are stimulated at the same time, K
channels are slower-acting. Initially, Na ions move into the axon, resulting in depolarization. This is followed by the opening of the K
channels, resulting in repolarization of the membrane. This response is self-propagating … the change in charge in one point of the axon
triggers the opening of voltage-gated channels in the next region. As K ions continue to flow out of the neuron, there is a brief period of time
in which the charge is more negative than at resting potential, known as undershoot/hyperpolarization. This, combined with the
inability of voltage-gated Na channels to re-open immediately leads to a period known as the refractory period. This is important because
it maintains a unidirectional wave of depolarization/keeps the impulse moving in one direction. As the wave of
depolarization reaches the tips of the axons, known as axon terminals, voltage-gated Ca channels open. These ions move into the
neuron and allow for vesicles containing chemicals known as neurotransmitters to move to and fuse with the axonal membrane via
microtubules. These chemicals are “spit out” into the small space known as a synapse, between the neuron and the adjacent cell,
which requires ATP and the action of microfilaments of the cytoskeleton. They diffuse across the small space, where they bind with
ligand--bound receptors on the adjacent cell. The binding of chemicals to these receptors causes a conformational change to Na
channels if the response is excitatory, allowing Na ions to flow into the next cell, or K channels if the response is inhibitory. Finally, the
chemicals must be removed from the synapse, either through uptake by the neuron or degradation by enzymes.
III.
Label the following diagram of a motor unit . . . be as specific as possible!
1. Axon of motor/efferent neuron
2. Axon terminal
3. Vesicle
4. Synapse
5. Acetyl choline
6. Ligand-gated Na+ channels
7. Muscle fiber
8. Mitochondrion
IV.
Color the diagram with colored pencils using the following key.
actin - yellow
muscle fiber - blue
myofibril - orange
myosin - purple
Circle an individual sarcomere.
V.
Number the statements in their proper sequence to describe the contraction of a skeletal muscle fiber.
2
A. Acetylcholine is released into the neuromuscular junction by the axon terminal.
8
B. The hydrolysis of ATP allows the myosin heads to change from a low energy to high energy configuration.
5
C. The action potential, carried deep into the muscle fiber, triggers the release of calcium ions from the sarcoplasmic reticulum.
14 D. The muscle fiber relaxes.
6
E. Calcium ions bind to the troponin complex on thin filaments.
3
F. Acetylcholine diffuses across the neuromuscular junction and binds to receptors on the sarcolemma (muscle fiber cell membrane).
7
G. Tropomyosin undergoes a conformational change, revealing the myosin-binding sites on the actin filaments.
4 H. Depolarization of the muscle fiber occurs, and an action potential is generated.
13 I. Calcium is actively transported back into the sarcoplasmic reticulum.
9 J. Crossbridges are formed.
11 K. The myofilaments slide past each other, effectively shortening the sarcomere.
10 L. ADP and Pi are released, and myosin returns to a low-energy configuration.
12 M. The muscle fiber repolarizes.
1 N. An action potential moves through a motor neuron of the somatic division of the peripheral nervous system until it synapses with a skeletal
muscle fiber.