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The Peripheral Nervous System (PNS) > Motor Activity
Motor Activity
• Peripheral Motor Endings
• Overview of Motor Integration
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The Peripheral Nervous System (PNS) > Motor Activity
Peripheral Motor Endings
• A neuromuscular junction is the junction between the axon terminal of a motor
neuron and the plasma membrane of the motor end plate of a muscle fiber.
• With the arrival of an action potential to the axon terminal, voltage-dependent
calcium channels open and calcium infuses into the cell. The influx of calcium
ions causes the docking of acetylcholine-containing vesicles at the plasma
membrane of the neuron and exocytosis into the synaptic cleft.
• Acetylcholine is a neurotransmitter contained in the vesicles of the pre-synaptic
neuron. It is released into the synaptic cleft activating nicotinic acetylcholine
receptors on the motor end plate, causing local motor end plate depolarization
known as the end plate potential (EPP).
Neuromuscular Junction
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• The end plate potential propagates across the surface of the muscle fiber,
causing the fiber to contract and continuing the process of excitation-contraction
coupling.
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The Peripheral Nervous System (PNS) > Motor Activity
Overview of Motor Integration
• Motor units contain muscle fibers of all the same type which may be many muscle
fibers (as in the case of quadriceps) or few muscle fibers (as in the case of the
muscles which control eye movement).
• Groups of motor units often work together to coordinate the contractions of a
single muscle; all of the motor units that subserve a single muscle are considered
a motor unit pool.
• Motor units are generally recruited in order of smallest to largest (fewest fibers to
most fibers) as contraction increases. This is known as "Henneman's Size
Principle".
A graphed quadratic equation
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Appendix
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The Peripheral Nervous System (PNS)
Key terms
• alpha motor neuron Alpha motor neurons (α-MNs) are large lower motor neurons of the brainstem and spinal cord. They
innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor
neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.
• axon a nerve fibre which is a long slender projection of a nerve cell, and which conducts nerve impulses away from the body of
the cell to a synapse
• excitation-contraction coupling This process is fundamental to muscle physiology, whereby the electrical stimulus is usually an
action potential and the mechanical response is contraction.
• Henneman's size principle According to Henneman's size principle, motor unit recruitment is always in the same order from
smallest to largest motor unit. Additionally, the motor unit action potential is an all-or-none phenomenon - once the recruitment
threshold (the stimulus intensity at which a motor unit begins to fire) is reached, it fires fully.
• motor unit A neuron with its associated muscle fibers.
• nicotinic acetylcholine receptor Nicotinic acetylcholine receptors, or nAChRs, are cholinergic receptors that form ligand-gated
ion channels in the plasma membranes of certain neurons and on the postsynaptic side of the neuromuscular junction.
• presynaptic neuron the neuron that releases neurotransmitter into the synaptic cleft
• synaptic cleft a small space between neurons
• voltage-dependent calcium channels Voltage-dependent calcium channels (VDCC) are a group of voltage-gated ion channels
found in excitable cells (e.g., muscle, glial cells, neurons, etc. ) with a permeability to the ion Ca2+.
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The Peripheral Nervous System (PNS)
The Orbicularis Oris Eye Muscle
These small motor units may contain only 10 fibers per motor unit. The more precise the action of the muscle, the fewer fibers innervated.
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Wikipedia. "Gray379." CC BY-SA http://en.wikipedia.org/wiki/File:Gray379.png View on Boundless.com
The Peripheral Nervous System (PNS)
Rectus femoris
The rectus femoris muscle is one of the four quadriceps muscles of the human body. These muscles may have as many as a thousand fibers in each
motor unit.
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Wikipedia. "Rectus femoris." Public domain http://en.wikipedia.org/wiki/File:Rectus_femoris.png View on Boundless.com
The Peripheral Nervous System (PNS)
A graphed quadratic equation
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Wikipedia. "Quadratic function." GNU FDL http://en.wikipedia.org/wiki/Quadratic_function View on Boundless.com
The Peripheral Nervous System (PNS)
Muscle Contraction and Actin-Myosin Interactions
Skeletal muscle contracts following activation by an action potential. Binding of Acetylcholine at the motor end plate leads to intracellular calcium release
and interactions between myofibrils, eliciting contraction.
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Wikipedia. "Muskel-molekulartranslation." CC BY-SA http://en.wikipedia.org/wiki/File:Muskel-molekulartranslation.png View on Boundless.com
The Peripheral Nervous System (PNS)
Neuromuscular Junction
Electron micrograph showing a cross section through the neuromuscular junction. T is the axon terminal and M is the muscle fiber. The arrow shows
junctional folds with basal lamina. Postsynaptic densities are visible on the tips between the folds. Scale is 0.3 µm.
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Wikipedia. "Motor end plate." Public domain https://en.wikipedia.org/wiki/Motor_end_plate View on Boundless.com
The Peripheral Nervous System (PNS)
Detailed View of a Neuromuscular Junction
Detailed view of a neuromuscular junction: (1) Presynaptic terminal; (2) Sarcolemma; (3) Synaptic vesicle; (4) Nicotinic acetylcholine receptor; (5)
Mitochondrion.
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Wikipedia. "Motor end plate." CC BY-SA https://en.wikipedia.org/wiki/Motor_end_plate View on Boundless.com
The Peripheral Nervous System (PNS)
Which of the following describes what happens at the
neuromuscular junction upon acetylcholine release into the
synapse?
A) Acetylcholine binds to its receptor and calcium ions flow into the
muscle fiber.
B) Acetylcholine binds to its receptor and sodium ions flow into the
muscle fiber.
C) Acetylcholine binds to its receptor and potassium ions flow into the
muscle fiber.
D) Acetylcholine binds to its receptor and chloride ions flow into the
muscle fiber.
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The Peripheral Nervous System (PNS)
Which of the following describes what happens at the
neuromuscular junction upon acetylcholine release into the
synapse?
A) Acetylcholine binds to its receptor and calcium ions flow into the
muscle fiber.
B) Acetylcholine binds to its receptor and sodium ions flow into the
muscle fiber.
C) Acetylcholine binds to its receptor and potassium ions flow into the
muscle fiber.
D) Acetylcholine binds to its receptor and chloride ions flow into the
muscle fiber.
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Saylor OER. "Biology « Saylor.org – Free Online Courses Built by Professors." CC BY 3.0
http://www.saylor.org/majors/Biology/
The Peripheral Nervous System (PNS)
The notion of a motor unit is important in describe muscle
function. What is a motor unit?
A) A single alpha motor neuron and the single muscle fiber it innervates
B) All the alpha motor neurons and muscle fibers in a muscle
C) A single alpha motor neuron and all muscle fibers it innervates
D) All of the muscles that have been recruited at any given point in time
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The Peripheral Nervous System (PNS)
The notion of a motor unit is important in describe muscle
function. What is a motor unit?
A) A single alpha motor neuron and the single muscle fiber it innervates
B) All the alpha motor neurons and muscle fibers in a muscle
C) A single alpha motor neuron and all muscle fibers it innervates
D) All of the muscles that have been recruited at any given point in time
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Boundless - LO. "Boundless." CC BY-SA 3.0 http://www.boundless.com/
The Peripheral Nervous System (PNS)
Attribution
• Wikipedia. "Motor unit recruitment." CC BY-SA 3.0
https://en.wikipedia.org/wiki/Motor_unit_recruitment#Neuronal_mechanism_of_recruitment
• Wikipedia. "Motor unit." CC BY-SA 3.0 https://en.wikipedia.org/wiki/Motor_unit
• Wikipedia. "Henneman's size principle." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Henneman's%20size%20principle
• Wikipedia. "alpha motor neuron." CC BY-SA 3.0 http://en.wikipedia.org/wiki/alpha%20motor%20neuron
• Wiktionary. "motor unit." CC BY-SA 3.0 http://en.wiktionary.org/wiki/motor+unit
• Wikipedia. "Motor end plate." CC BY-SA 3.0 https://en.wikipedia.org/wiki/Motor_end_plate
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//physiology/definition/synaptic-cleft
• Wiktionary. "presynaptic neuron." CC BY-SA 3.0 http://en.wiktionary.org/wiki/presynaptic+neuron
• Wikipedia. "axon." CC BY-SA 3.0 http://en.wikipedia.org/wiki/axon
• Wikipedia. "voltage-dependent calcium channels." CC BY-SA 3.0 http://en.wikipedia.org/wiki/voltagedependent%20calcium%20channels
• Wikipedia. "nicotinic acetylcholine receptor." CC BY-SA 3.0 http://en.wikipedia.org/wiki/nicotinic%20acetylcholine%20receptor
• Wikipedia. "excitation-contraction coupling." CC BY-SA 3.0 http://en.wikipedia.org/wiki/excitation-contraction%20coupling
• Saylor. CC BY http://www.saylor.org/site/wp-content/uploads/2010/11/Smooth-Muscle-Contraction.pdf
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