Chapter 7
The Muscular System
1
1
Functions of Skeletal Muscle
Skeletal muscles perform five functions:
1) Produce movement of skeleton
2) Maintain posture and body position
3) Support soft tissues
4) Guard entrances and exits
5) Maintain body temperature
To understand how skeletal muscle contracts, we
must study the structure of skeletal muscle.→
2
Anatomy of Skeletal Muscles
 Gross Anatomy
 Three Layers of connective tissue are part
of each muscle:
1)
Epimysium
Fibrous covering of whole muscle
2)
Perimysium
Divides the skeletal muscle into bundles of muscle
fibers called fascicles
3)
Endomysium
Within a fascicle, the endomysium surrounds each
skeletal muscle fiber
(Each cell in skeletal muscle tissue is a single muscle fiber!)
3
Anatomy of Skeletal Muscles
•At each end of the muscle, the collagen fibers of all
three layers come together to form either a bundle
known as a tendon, or a broad sheet called an
aponeurosis.
•Tendons – attach skeletal muscles to bones
•Aponeuroses – connect different skeletal muscles
•The tendon fibers are interwoven into the periosteum
of the bone, providing a firm attachment. Any
contraction of the muscle exerts a pull on its tendon
and in turn on the attached bone.
4
Fig. 7-1. Organization of Skeletal Muscle
5
Microanatomy of a Muscle Fiber
• Sarcolemma
- Cell membrane of muscle cell (fiber)
• Sarcoplasm
- Cytoplasm of muscle cell (fiber)
• Sarcoplasmic reticulum (SR)
- Like smooth ER; forms a tubular network around each myofibril. On
either side of a T tubule lie expanded chambers of the SR called
terminal cisternae, which contain high levels of calcium ions.
• Transverse tubules (T tubules)
- Form passageways through the muscle fiber
• Myofibrils (contraction organelle)
• Sarcomeres
6
Fig. 7-2a. Organization of a skeletal muscle fiber
7
Anatomy of Skeletal Muscles
• Sarcomere—Repeating structural unit of the myofibril
Components of a sarcomere:
➢ Myofilaments
Thin filaments (mostly actin)
Thick filaments (mostly myosin)
➢ Z lines at each end
Anchor for thin filaments
8
Fig. 7-2c,d,e. Organization of Skeletal Muscle Fiber
9
Fig. 7-3. (1 of 2)
Changes in the
Appearance of a
Sarcomere During
Contraction of a Skeletal
Muscle Fiber
10
Fig. 7-3. (2 of 2)
Changes in the
Appearance of a
Sarcomere During
Contraction of a Skeletal
Muscle Fiber
11
The Neuromuscular Junction
• Each skeletal muscle fiber is controlled by a nerve cell called a motor
neuron.
• A kind of communication occurs as a motor neuron “sends” a command
to a skeletal muscle fiber.
• Neuromuscular junction – the site where the communication takes
place; a specialized intercellular connection.
12
The Neuromuscular Junction
• Neurons control skeletal muscle fibers by stimulating
the production of an action potential, or electrical
impulse, in the sarcolemma.
13
Events of Neuromuscular Junction
1) Arrival of action potential at
the synaptic terminal
2) Release of acetylcholine
3) Binding of Ach at the motor
end plate
4) Appearance of an action
potential at the sarcolemma
5) Return to the initial state
ACh
Sarcolemma of
Motor End Plate
Ach has been broken down by AChE and the sequence of
events can now be repeated with another action potential.
14
14
So, a command (in the form of an action potential) was sent
to the muscle fiber…
Now what?
1. The action potential spreads across the surface of the
sarcolemma and then along the T tubules.
2. This triggers a massive release of calcium ions by the
terminal cisternae of the SR.
3. Active sites become exposed on thin filaments, crossbridge interactions occur, and a contraction begins.
Let’s look at the molecular events involved…
15
15
Fig. 7-5. Molecular Events of the
Contraction Process
Thin Filament
(actin molecules)
Thick Filament
(myosin molecules)
16
Fig. 7-5. Molecular Events of the
Contraction Process
Ca2+ ions bind to troponin
Changes the shape of the
troponin-tropomyosin
complex and uncovers the
myosin-binding sites on actin
17
Fig. 7-5. Molecular Events of the
Contraction Process
Mysosin heads hydrolyze ATP
and bind to actin, forming
crossbridges
18
Fig. 7-5. Molecular Events of the
Contraction Process
Myosin crossbridges
rotate toward center of the
sarcomere (power stroke)
Overall Idea:
center of
sarcomere
19
Fig. 7-5. Molecular Events of the
Contraction Process
Cross-bridges detach when
the myosin head binds
another ATP molecule
20
Fig. 7-5. Molecular Events of the
Contraction Process
The detached myosin head
is reactivated as it splits
the ATP and captures the
released energy
…Cycle can now be
repeated, starting with
step 2
21
Table 7-1. Summary of Contraction Process
http://www.youtube.com/watch?v=DeTen4GVb-Q
Muscle Tension
•Muscle tension—The pulling force on the tendons
that muscle cells generate when contracting
•Before movement can occur, the applied tension
must overcome the object’s resistance, a passive
force that opposes movement.
•The amount of tension produced in the skeletal
muscle as a whole is determined by
(1) the frequency of neural stimulation and
(2) the number of muscle fibers activated.
23
Fig. 7-8. Motor Units
24
Muscle Tone
• Muscle tone—Tension in a “resting” muscle produced by a low level
of spontaneous motor neuron activity. Distinct from resting tension
produced by passive stretching.
• Function of muscle tone
• Stabilizes bones, joints
• Prevents atrophy (muscle wasting )
• Isotonic contraction
The tension (load) on a muscle stays constant (iso = same, tonic = tension) during a
movement. (Example: lifting a baby)
• Isometric contraction
The length of a muscle stays constant (iso = same, metric = length) during a
“contraction” (Example: holding a baby at arms length)
25
Muscle Performance
Two Types of Skeletal Muscle Fibers
Fast fibers
Large diameter, abundant myofibrils, ample glycogen, scant
mitochondria. Produce powerful, brief contractions
Slow fibers
Smaller diameter, rich capillary supply, many mitochondria,
much myoglobin. Produce slow, steady contractions
26
Physical Conditioning
• Anaerobic endurance
Time over which a muscle can contract effectively under
anerobic conditions.
• Hypertrophy
Increase in muscle bulk. Can result from anerobic training.
• Aerobic endurance
Time over which a muscle can contract supported by
mitochondria.
27
Muscle Performance
• What you don’t use, you lose!
• When motor units are inactive for days or
weeks, muscle fibers break down their
contractile proteins and grow smaller and
weaker. If inactive for long periods, muscle
fibers may be replaced by fibrous tissue.
28
Origins, Insertions, & Actions
•Origin – where muscle begins
•Insertion – where muscle ends
•The muscle contracts to perform a specific action (joint
movement). A muscle’s origin remains stationary while the
insertion moves.
Ex. The gastrocnemius muscle (in the calf) has its origin on
the distal portion of the femur and inserts on the calcaneus.
Its contraction results in plantar flexion (pointing your toes)
29
Actions of Muscles
• Muscles can be described by their primary actions:
• Prime mover (agonist)
• Main muscle in an action
• Synergist
• Helper muscle in an action
• Antagonist
• Opposed muscle to an action
30