BIOL 1051H Chapter Notes - Chapter 11: Muscle Fascicle, Neuromuscular Junction, Myocyte

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14 May 2018
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Muscle Tissue (Chapter 11)
Skeletal muscle cells are multinucleated tube-shaped cells also called muscle fibers. The
plasma membrane of the muscle fiber is called the sarcolemma. This is the location of the
neuromuscular junction.
The endomysium is the innermost layer of connective tissue that surrounds each muscle
fibre individually. The perimysium wraps around a group of muscle fibres creating a muscle
fascicle. This is the unit that carries nerves and blood vessels to the fibres. The next step up is
the epimysium which is a connective tissue sheath that surrounds a group of fascicles. The
fascia is the thicker layer of connective tissue that separates larger groups of muscle into
compartments and surrounds the muscle as a whole. A single muscle may contain multiple
compartments separated by fascia. It is the fascia that is the toughest part of muscle and the
part that will form tendons that attach muscles to bones.
A motor neuron generally innervates more than one muscle fiber. The motor neuron
along with all the muscle fibers it innervates is called a motor unit. Innervation takes place at the
neuromuscular junction. A single synaptic ending of the motor neuron innervates each muscle
fiber and underlying specializations of sarcolemma allow for efficient transmission of the signal
to the relatively large muscle cell. The place on the sarcolemma where the neuromuscular
junction occurs is the motor end plate. Activation of the one neuron will cause contraction of all
the muscle fibers in the motor unit. The number of muscle fibers innervated by a neuron can
vary substantially (1 neuron to 100 muscle fibers up to about 1 neuron to 2000 muscle fibers).
Muscle movements are made smooth by recruitment of motor units in an increasing
fashion. For example, imagine that you are all the muscle cells in your elbow flexor muscles
were activated at once your arm would flex almost instantly. Also, the strength of the contraction
would probably dislocate your elbow. If for instance you have hundreds of motor units, each
working independently you can smoothly increase the number of motor units involved until the
elbow flexes into position. The strength of contraction would only be enough to lift your forearm.
Each muscle fiber is packed with collections of filaments called myofibrils. The myofibrils
are only about 1 micrometer in diameter but they extend the entire length of the muscle fiber.
The myofibrils are packed with myofilaments which are composed of thick and thin filaments
that give rise to the striations visible in a skeletal muscle fiber.
The thick filaments are composed mostly of myosin while the thin filaments are mostly
actin. Together with a few other proteins they make up a sarcomere which is the contractile unit
of skeletal muscle. The figure below highlights the visible striations of a sarcomere. The I bands
are made up of an elastic protein that anchors thin filaments. This structure is called a Z-disc.
The sarcomere is defined as the structure extending from one Z disc to the next.
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Document Summary

Skeletal muscle cells are multinucleated tube-shaped cells also called muscle fibers. The plasma membrane of the muscle fiber is called the sarcolemma. This is the location of the neuromuscular junction. The endomysium is the innermost layer of connective tissue that surrounds each muscle fibre individually. The perimysium wraps around a group of muscle fibres creating a muscle fascicle. This is the unit that carries nerves and blood vessels to the fibres. The next step up is the epimysium which is a connective tissue sheath that surrounds a group of fascicles. The fascia is the thicker layer of connective tissue that separates larger groups of muscle into compartments and surrounds the muscle as a whole. A single muscle may contain multiple compartments separated by fascia. It is the fascia that is the toughest part of muscle and the part that will form tendons that attach muscles to bones. A motor neuron generally innervates more than one muscle fiber.

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