Chapter 5.1 Notes
Muscle Categories
1. Skeletal muscle – voluntary, striated
2. Smooth muscle – involuntary, nonstriated
3. Cardiac muscle – involuntary, striated
Voluntary = consciously controlled
Involuntary = not consciously controlled
Striated = appears to have stripes when viewed under a microscope
Nonstriated = has a smooth appearance (no stripes)
Skeletal Muscle
 Skeletal muscle fibers (cells) contain multiple nuclei.
 The number of skeletal muscle fibers in a person in genetically determined and doesn’t
change throughout life.
 Muscle fibers do, however, change in length and diameter throughout life.
 Resistance training (ex. lifting weights) can add diameter to muscle fibers.
 Skeletal muscle is highly organized.
 The cell membrane of the muscle fiber is called a sarcolemma.
 Surrounding each sarcolemma is a fine, protective sheath called an endomysium.
 Groups of fibers are bundled together by a strong, fibrous membrane called a
perimysium. Each bundle is called a fascicle.
 All the fascicles in a muscle are enclosed in a thick, though connective tissue called an
epimysium. The epimysium connects to a tendon or aponeurosis on each end of the
muscle. The tendon and/or aponeurosis is what attaches to the bone.
Smooth Muscle
 Small and spindle-shaped
 Also known as “visceral muscle”
 Found in the walls of many internal organs such as the stomach, intestines, urinary
bladder, and respiratory passages
 Smooth muscle cells are arranged in layers with each layer of cells running in a different
direction from the other layers (allows for coordinated, alternate contracting and relaxing
of the layers which changes the size and shape of the organ and can aid in moving the
contents of the organ (peristalsis - food through the intestines )
 Smooth muscle can work for longer periods of time than skeletal muscle without
becoming fatigued (tired)
Cardiac Muscle
 Found only in the walls of the heart
 Arranged in interconnected network of figure-eight or spiral-shaped bundles that joint
together at structures called intercalated discs
Muscle Functions
All three muscle tissue types share four behavioral characteristics:
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4.
Irritability – ability to respond to a stimulus
Extensibility – ability to be stretched
Elasticity – ability to return to normal after being stretched
Contractility – ability to contract or shorten
Tension = the pulling force of a muscle on a bone
Consult Figure 5.4 on textbook page 160
1. Concentric contraction – when a muscle shortens as tension develops
2. Eccentric contraction – when a muscle lengthens as tension develops
3. Isometric contraction – when a muscle does not shorten or lengthen as tension develops
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Antagonistic pairs – skeletal muscles that oppose each other to produce movement
Agonist – the muscle that produces the main movement when contracted; also called the
prime mover
Antagonist – the muscle that opposes the agonist; relaxes when the agonist contracts
Chapter 5.2 Notes
Motor Unit
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Muscle tissue must be stimulated by nervous tissue in order to produce tension.
Because of this dependent relationship between the muscular and nervous systems, they
are sometimes collectively referred to as the neuromuscular system.
A nerve cell is a neuron. A nerve cell that stimulates a muscle tissue is a motor neuron.
A single motor neuron and all the muscle cells (muscle fibers) it stimulates is called a
motor unit.
A motor unit is considered to be the functional unit of the neuromuscular system.
One motor neuron may connect to 100 to 2000 skeletal muscle fibers.
Small muscles have small motor units with a few fibers per motor unit. (ex. muscles of
the eyes and fingers)
Large muscles have large motor units with many fibers per motor unit. (ex. glutes)
Consult Figure 5.5 on textbook page 163.
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An axon is a fiber on a neuron that carries an impulse to the muscle fiber. When it gets
close to the muscle fiber it branches into axon terminals which branch out to individual
muscle fibers.
The link between the axon terminal and the muscle fiber is called the neuromuscular
junction.
The space between the axon terminal and the muscle fiber is called the synaptic cleft.
When an impulse reaches the end of the axon terminal, a chemical called a
neurotransmitter (a chemical that carries a nerve impulse across a synapse) discharges
and diffuses across the synaptic cleft to attach to receptors on the sarcolemma (cell
membrane of the muscle cell).
What is a synapse? The place where a signal passes from one nerve cell to another.
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The neurotransmitter that stimulates muscle is called acetylcholine. Acetylcholine is
what causes an electrical charge in the muscle cell. This electric charge is what causes
the muscle to contract.
The electric charge that causes contraction is called action potential.
Motor units always develop maximum tension when stimulated. (All-or-none principle)
The more motor units that are stimulated is what determines how much strength or power
can be created.
What is the difference between fast-twitch and slow-twitch muscles?
Fast twitch
 Type IIa and Type IIb muscle fibers
 These contract much faster than slow-twitch fibers but fatigue much more quickly.
 Type IIb are the fastest. They contract in about 1/7 the time of the slow twitch fibers.
Slow twitch
 Type I muscle fibers
 These contract slower but have better lasting potential.
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All fibers within one motor unit are of the same type (Type I, Type IIa, or Type IIb),
however, all three types are usually found within one muscle. The ratio of fast-twitch vs
slow-twitch varies from person to person and is genetic.
Muscle Fiber Arrangement
There are two major categories of fiber arrangements.
1. Parallel fiber arrangement – fibers run parallel to each other along the length of the
muscle
a. Fusiform – wide in the middle and tapers at both ends (biceps brachii)
b. Bundled – (rectus abdominis)
c. Triangular – (pectoralis major)
2. Pennate fiber arrangement – fibers attach obliquely to a central tendon
a. Unipennate – fibers that are aligned in one direction to a central tendon (certain
muscles of the hand)
b. Bipennate – fibers that attach to a central tendon on both sides (rectus femoris)
c. Multipennate – fibers that attach to a central tendon in more than two directions
(deltoid)
Muscular Strength – amount of force a given muscle can produce
Muscular Power – force x velocity (how fast can you generate a force)
Muscular Endurance – ability of a muscle to produce tension over a period of time (how long
you can sustain a force)