1. A motor unit includes a motor neuron and all of the muscle fibers it controls. When the motor neuron fires an action potential, it causes the release of acetylcholine at the neuromuscular junction, which then triggers an action potential in the muscle fiber. 2. The muscle action potential spreads throughout the fiber via t-tubules, causing the release of calcium from the sarcoplasmic reticulum. 3. The calcium binds to troponin, exposing actin binding sites for myosin, leading to cross-bridge cycling and muscle contraction.

1. Chapter 9, Section 2
Muscle Contractions

2. Synapse
Synapse: Functional (not physical) junction between an axon
of a neuron and another cell
The two cells are separated by a physical
space, called the synaptic cleft.
Neurotransmitters are stored within
synaptic vesicles of the presynaptic cell
and they’re released into the synapse.

3. Neuromuscular Junction
Neuromuscular Junction (NMJ) refers to the
synapse between an axon and a muscle fiber.
Motor End Plate is a highly folded region of muscle fiber
at NMJ that contain abundant mitochondria
Figure 9.8a.
General NMJ

4. Motor Unit
Motor neurons innervate
effectors (muscles or glands)
A motor unit includes a
motor neuron and all of the
muscle fibers it controls
1 motor unit may control
between 1 and 1000 muscle
fibers
Figure 9.9 two motor units. The muscle
fibers of a motor unit are innervated
(controlled) by a single motor neuron.

5. Stimulus for Contraction
Acetylcholine (ACh) is the only neurotransmitter
that initiates skeletal muscle contraction
Sequence of Actions
1. A nerve impulse (Action Potential)
reaches axon terminal
2. The impulse opens calcium channels
at the axon terminal
• Calcium diffuse into axon
3. The calcium triggers the release of
ACh from vesicles into synaptic cleft.

6. Stimulus for Contraction
Sequence of Actions…Continued
4. ACh diffuses across synaptic cleft &
binds to receptors on motor
endplate.
5. ACh opens Na+ channels on
muscle
6. Na+ floods into the muscle, initiating
a muscle impulse.
7. A muscle impulse (action potential)
is propagated across the entire
muscle.

7. Stimulus for a muscle
impulse. Corresponds to
steps 1-7 in the previous
slides.

8. The muscle impulse causes the release of calcium from the SR.
Calcium binds to troponin and tropomyosin is repositioned exposing the
actin filaments.

9. Stimulus for contraction continued…
8. The muscle impulse diffuses
across sarcolemma and down the ttubules into the cisternae of
sarcoplasmic reticula.
9. The sarcoplasmic reticula
release their calcium supplies into
the sarcoplasm.
10. Calcium binds to troponin and
the troponin repositions the
tropomyosin, so the myosin can
bind to actin.
11. Cross-bridge cycling causes the
muscle to contract.

10. Excitation-Contraction Coupling
Calcium released from sarcoplasmic reticulum binds to troponin.
Troponin moves tropomyosin, exposing actin filaments to myosin
cross-bridges.
myosin heads bind to actin, forming a cross bridge and cross-bridge
cycling causes the muscle to contract.
End of Chapter 9, Section 2