Muscle Contraction
Muscle Movement
• Muscle fiber must be stimulated:
– By an electrical signal called muscle action potential (AP)
– Delivered by motor neuron
• Motor neuron + muscle fibers = motor unit
• Simulation of 1 motor neuron causes all the
muscle fibers in that motor unit to act at the same
time.
– Precise movements = many motor units controlling few
muscles fibers
– Powerful movements = few motor units controlling
many muscle fibers.
Action Potential
Axon of Motor Neuron
Axon Terminal of NMJ
Nucleus
Sarcolemma of
Muscle Fiber
Myofibril
#1
NMJ
Nerve impulse (AP)
arrives at axon terminal
#2
Calcium ions (Ca2+) released
into axon terminal
#3
Ca2+ causes synaptic
vesicles to release
Acetylcholine (ACh) via
exocytosis
Ca2+
Ca2+
Vesicle
Mitochondrion
Synaptic Cleft
Sarcolemma
#4
ACh diffuses across
synaptic cleft and binds to
receptors in the
sarcolemma
#5
Na+
#6
ACh effects are stopped by
the enzyme
acetylcholinesterase (AChE)
-- Breaks down remaining
ACh in the synaptic cleft
Na+
K+
Binding of ACh to its
receptors opens ion
channels.
- Na+ moves into muscle
fiber (more positive)
- K+ moves out of
muscle fiber (more
negative)
Creates a new AP
#5 from NMJ
Binding of ACh to its receptors opens ion
channels.
- Na+ moves into muscle fiber (more
positive)
- K+ moves out of muscle fiber (more
negative)
Creates a new AP
Sarcoplasmic Reticulum (SR)
Network of tubules that surround
each myofibril
Terminal Cisternae of SR
Network of tubules that surround
each myofibril and store calcium
ions
T (Transverse) Tubules
Tunnel like extensions of the
sarcolemma, run between terminal
cisternae of the SR, and allow for AP
to reach deep regions of muscle cells.
#1
Action potential is
spread along
the sarcolemma and
down the T tubules
#2
Causes the terminal
cisternae of the SR to
release stored calcium ions
into the sarcoplasm.
-- within 1 millisecond!
#2
Causes the terminal
cisternae of the SR to
release stored calcium ions
into the sarcoplasm.
-- within 1 millisecond!
#3
Calcium binds to troponin and
removes the blocking action of
tropomyosin. When Ca2+ binds,
troponin changes shape, exposing
binding sites for myosin on the
actin myofilaments.
#4
Myosin binding to actin forms
cross bridges and results in the
sliding of the actin filaments
towards the center of the
sarcomere (power stroke) =
muscle contraction.