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Chapter 5
Muscular System
5.2 Skeletal Muscle Actions (outline)
Muscle tissue develops tension only when stimulated by one or more nerves.
Muscles and nerves working together is called the neuromuscular system
A. Motor Unit – functional unit of the neuromuscular system
- Composed of one motor neuron (nerve that stimulates skeletal
muscle) and all muscle fibers stimulated by it (100 – 2,000 fibers)
1. Generating Action Potentials (nerve to muscle communication)
- Motor neuron cell body (located in the spinal cord) is connected to
the muscle cell by a long, thin fiber – the axon
- The axon terminals (branches) lie close to a muscle fiber at the
neuromuscular junction. The gap between is the synaptic cleft
- When a nerve impulse reaches the axon terminals, a neurotransmitter
(acetylcholine) is released and attaches to receptors on the
sarcolemma (plasma membrane of the muscle fiber)
- The attached acetylcholine changes the Na+ and K+ permeability of
the sarcolemma (called depolarization). The flood of positive ions
causes an action potential
2. Contractions of the Sarcomere
- Actin and Myosin are contractile proteins that compose functional
units called sarcomeres (contractile units found in myofibrils)
- When an action potential sweeps across a muscle fiber, it allows
Ca++ ions to be released from storage in the muscle cells. The
calcium ions remove inhibitors (tropomyosin) from the actin fibers.
- Myosin fiber heads are free to attach to the actin fibers and form
cross bridges (ATP breakdown allows the cross bridges to be formed,
released, and reformed) The myosin heads pull the actin fibers across
them creating a muscle contraction.
- Small action potentials create slow, gentle movements, while a large
number of action potentials create, fast or forceful movements.
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- 1. https://www.youtube.com/watch?v=7wM5_aUn2qs
- 2. https://www.youtube.com/watch?v=CepeYFvqmk4
- 3. https://www.youtube.com/watch?v=BMT4PtXRCVA
3. Maximum Tension and Return to Relaxation
- When a motor unit receives an action potential it will develop
maximum tension (sarcomere contractions) = all-or-none response
- A single muscle contains many motor units. In order to obtain
maximum tension on a muscle it needs multiple actions potentials on
multiple motor units. Sustained, maximum stimulation = Tetanus
- Muscles develop tension in a twitch fashion, as the action potential
completes its travel the permeability of the membranes returns,
calcium is returned to storage and the muscle rests.
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B. Skeletal Fiber Types
1. Twitch types – skeletal muscle include motor units of both slow & fast
- Each motor unit contains only one type of twitch, but the ratio of slow
to fast in each muscle varies muscle to muscle, person to person
- slow- twitch (Type I) – contract slowly, up to 7 times slower than fast
twitch (Type IIb)
- fast-twitch (Type II) – intermediate (Type IIa) – faster contraction
- fast (Type IIb) – fastest contraction, but
fatigue quickly
2. Fiber Architecture – the arrangement of fibers in the muscle
a. Parallel Fiber Architecture – fibers run parallel to each other along
the length of the muscle (not entire muscle)
1. fusiform – wide in the middle, tapered on each end (bicep brachii)
2. bundled – also known as bundled (rectus abdominis)
3. triangular – also known as convergent (pectoralis major)
- parallel fibers allow for the muscle to move body segments in large
ranges of motion
b. Pennate Fiber Architecture -fibers attach obliquely to a central tendon
1. unipennate – fibers aligned in one direction, attached to a central
tendon
2. bipennate – two sets of fiber, both attach to a central tendon
3. multipennate – fibers attach to central tendon in two different
Directions
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C. Muscular Strength, Power, and Endurance
1. Muscular Strength – dynamometers measure joint torque
- The assessment of strength of a muscle group at a given point is
called torque (rotary force)
- Torque is a product of a size of a force and the perpendicular distance
of that force from an axis of rotation
Fm x d F = W x d W
Fm = muscle force
dF = perpendicular distance of muscle attachment from the joint
W = weight
dW = perpendicular distance of weight from joint
2. Muscular Power – involves the speed of contraction
- defined as muscle force multiplied by muscle shortening velocity
- can not be measured outside the body but some dynamometers measure
resistance moved and movement speed
3. Muscular Endurance –
- defined as the ability of a muscle to produce tension over a period of
time
- Tension can be continuous or cyclical
- The faster a muscle fatigues, the less endurance it has