1. The Muscular System rev 12-12
Muscle cells are involved in every movement that our bodies perform.
They are found in every body organ and tissue
Muscles can:
Shorten or contract to produce movement (prime mover or agonist)
Relax or be pulled back to their original length by gravity or by opposing muscle groups, called antagonistic muscles
Work with other muscle groups, called synergistic muscles, to produce movement
Resist movement to maintain our posture
Generate heat to maintain our body temperature
Shiver, sweat
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2. BIO 102 Muscle Lecture HANDOUT
Muscle tissue is made up of tightly packed cells called muscle fibers. The muscle fiber cytoplasm contains proteins which allow the cell to contract
BIO 102 Muscle Lecture HANDOUT

3. BIO 102 Muscle Lecture HANDOUT
There are 3 types of muscles:
Skeletal muscles
Cardiac muscles
Smooth muscles
attach to the bones of our skeleton and provide strength and mobility (movement) for our body
Cardiac muscle
found in the heart; pumps blood throughout the body
found in most internal organs; generally they work to push something (fluids or other body substances) through a body part
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4. BIO 102 Muscle Lecture HANDOUT
Muscles may also be classified as
voluntary (muscles over which we have conscious control and requires peripheral nervous stimulation to contract)
involuntary (muscles over which we have no conscious control; require autonomic nervous stimulation to contract)
The term sarcolemma refers to the cell membrane of a muscle cell
BIO 102 Muscle Lecture HANDOUT

5. BIO 102 Muscle Lecture HANDOUT
Skeletal Muscles
are multinucleated
cells are arranged in a parallel fashion
are responsible for all locomotion and manipulation
enable us to respond quickly to changes in the external environment
compared to other muscle types, their speed of contraction is fast
BIO 102 Muscle Lecture HANDOUT

6. BIO 102 Muscle Lecture HANDOUT
Skeletal Muscle
Is also called Striated or Voluntary muscle
they have striations (or stripes) which are caused by alternating dark and light “bands”
bands are composed of tightly packed contractile proteins called myofilaments which are made up of thicker myosin filaments and thinner actin filaments
BIO 102 Muscle Lecture HANDOUT

7. BIO 102 Muscle Lecture HANDOUT
Cardiac Muscle
Cells are striated, short, fat, branched and interconnected
Have specialized areas called intercalated disks where the cells connect with each other
Intercalated disks contain gap junctions that permit one cell to electrically stimulate the next one.
because of these connections, cardiac muscle works as a single, coordinated unit
usually contracts at a steady rate set by the heart’s pacemaker, but neural controls allow for a faster beat for brief periods (i.e. when you perform intense activities)
compared to other muscle types, their speed of contraction is moderate
BIO 102 Muscle Lecture HANDOUT

8. BIO 102 Muscle Lecture HANDOUT
Smooth Muscle
Cells are shorter than skeletal and cardiac muscle cells
Because the cells have fewer contractile proteins they do not have striations (thus their name, smooth muscle)
Cells are spindle shaped (thicker in the middle and tapered at each end); each cell has a centrally located nucleus
Are found in the walls of hollow visceral organs (i.e. stomach, intestines, bladder, blood vessels)
role is to force fluids and other substances through body channels
compared to other muscle types, their speed of contraction is slow and sustained
BIO 102 Muscle Lecture HANDOUT

9. BIO 102 Muscle Lecture HANDOUT
Muscle Contraction
Contractile proteins or myofilaments, called actin and myosin, slide past each other using energy from ATP molecules.
These myofilaments produce alternating light and dark areas called striations
Dark areas are called A-bands
Lighter areas are called I-bands
The Z-line is a thin, dark line where sets of actin myofilaments are woven together
The space between 2 Z-lines is called a sarcomere
A sarcomere is the smallest functional (contractile) unit of a muscle fiber
BIO 102 Muscle Lecture HANDOUT

10. BIO 102 Muscle Lecture HANDOUT
Mechanism of Muscle Contraction: Nerve Activation of Individual Muscle Cells
In order for a muscle to contract, its cells must be stimulated by a nerve
The motor neuron secretes acetylcholine (ACh) at the neuromuscular junction (the space where the motor neuron and muscle cell meet).
ACh is a neurotransmitter--a chemical which can either stimulate or inhibit another “excitable” cell (either a nerve cell or a muscle cell)
The ACh diffuses across the space between the neuron and the muscle cell (called the synaptic cleft) and binds to receptor sites on the muscle cell membrane.
BIO 102 Muscle Lecture HANDOUT

11. BIO 102 Muscle Lecture HANDOUT
The ACh binding causes the muscle cell membrane to generate an electrical impulse which travels along the cell membrane and along the T-tubules (cylindrical extensions of the cell membrane [or sarcolemma] which travel into the interior parts of the cell and activate the sarcoplasmic reticulum);
the function of the T-tubules is to allow the electrical impulse to quickly travel to all cell parts
The electrical impulse triggers the release of calcium from the sarcoplasmic reticulum .
Muscles require calcium in order to contract.
Sliding Filament Mechanism: muscle contracts when the sarcomeres shorten. This occurs when the thick and thin filaments form cross bridges and slide past each other resulting in shortening of the sarcomere.
BIO 102 Muscle Lecture HANDOUT

12. BIO 102 Muscle Lecture HANDOUT
Calcium binds to troponin, a protein molecule, and causes the
Troponin–tropomyosin protein complex to shift position
this exposes the actin-myosin binding sites and allow the myosin heads and actin filaments to make contact, forming cross-bridges.
The actin filaments are pulled toward the center of the sarcomere and the muscle contracts.
In order to stop the contraction, nerve cell stimulation stops
calcium is no longer secreted and the troponin–tropomyosin protein complex shifts position so the myosin heads are no longer exposed. They can no longer make contact with the actin filaments and
the muscle will be unable to contract
BIO 102 Muscle Lecture HANDOUT

13. BIO 102 Muscle Lecture HANDOUT   
GOOD ONE:
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14. BIO 102 Muscle Lecture HANDOUT
Muscle Relaxation
Nerve activation ends, contraction ends
Calcium pumped back into sarcoplasmic reticulum (requires ATP)
Calcium removed from actin filaments
Myosin-binding site covered
No calcium = no cross-bridges
BIO 102 Muscle Lecture HANDOUT

15. Energy Use by Muscle Cells
Muscle contraction requires energy
In the presence of calcium, myosin acts as an enzyme to split ATPADP + inorganic phosphate to release energy.
So, ATP is the muscle’s energy source
Typically muscle cells store enough ATP for approximately 10 seconds of heavy activity
After this, ATP can be replenished by:
Creatine phosphate which makes enough ATP for ~ seconds
After this short amount of time, energy must be obtained from stored glycogen
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16. BIO 102 Muscle Lecture HANDOUT
For long term energy, ATP can also be obtained via aerobic metabolism of glucose, fatty acids, and other high-energy molecules which are constantly supplied by the blood
Glycogen is broken down by a process called glycolysis. (The end products of glycolysis begin the Krebs Cycle [Citric Acid Cycle] and the electron transport chain to generate energy for the organism.)
BIO 102 Muscle Lecture HANDOUT

17. BIO 102 Muscle Lecture HANDOUT
Glucose molecules are removed from the glycogen and the cell uses the glucose to synthesize more ATP.
Part of the glucose breakdown process can be done anaerobically (without oxygen). This is a fast process but only yields 2 ATP molecules per glucose molecule.
Or glucose 2 ATP + lactic acid
It also produces lactic acid as a waste product which can make muscles sore.
BIO 102 Muscle Lecture HANDOUT

18. BIO 102 Muscle Lecture HANDOUT
---The most efficient, but much slower, process for energy production is aerobic metabolism. This yields 36 ATP molecules from 1 molecule of glucose. Carbon dioxide is produced as a waste product.
or glucose + O2  36 ATP + CO2 + H2O
When you perform strenuous activity, it usually takes a few minutes to start breathing heavily. The increase in respirations indicates that aerobic metabolism is now occuring.
During strenuous exercise, typically the blood is unable to carry enough oxygen for complete oxidation of glucose in our muscles. So, the muscles will also contract anaerobically and produce lactic acid.
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The lack of oxygen and subsequent production of lactic acid is called oxygen debt or oxygen deficit.
So, oxygen debt occurs when the muscles have consumed oxygen at a faster rate than it can be replaced. The body must take in an extra amount of oxygen to replace the oxygen in the muscles and return the muscles to their resting state.
Therefore, the reason we breathe heavily after exercise is to erase or pay back the oxygen debt.
BIO 102 Muscle Lecture HANDOUT

20. Gross Anatomy of Skeletal Muscle
Individual muscle fibers are wrapped and held together by several different layers of connective tissue
The individual muscle fibers are surrounded by a fine sheath of connective tissue called the endomysium.
The fibers within the muscle are grouped into fascicles, bundles of muscle fibers with a connective tissue covering and look like a bunch of sticks with an outer wrapping
The perimysium layer is fibrous connective tissue that surrounds the fascicles.
The outermost connective tissue layer is called the epimysium and surrounds the whole muscle.
Portions of the epimysium project inward and divide the muscle into compartments
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21. BIO 102 Muscle Lecture HANDOUT
Skeletal muscles can be made up of hundreds or thousands of muscle fibers bundled together
Myofibrils are contractile units made up of long protein molecules called myofilaments
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22. BIO 102 Muscle Lecture HANDOUT
Fascia, connective tissue outside the epimysium, surrounds the entire muscle
At the ends of the muscle, all of the connective tissues come together and form the tendon that attaches the muscle to bone.
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23. BIO 102 Muscle Lecture HANDOUT
Movememt
If the muscle spans a joint, one bone moves while the other one remains stationary
the muscle’s origin is on the bone which does not move
the muscle’s origin is generally closer to the midline of the body than its insertion
the insertion is on the bone which moves
All muscle cells in a muscle have the same origin and insertion
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24. BIO 102 Muscle Lecture HANDOUT
Muscle Contractions
Isotonic contractions: occur when the muscle shortens and movement occurs
Isometric contractions occur when muscle tension develops but the muscle doesn’t shorten and no bony movement occurs. These contractions help stabilize the skeleton.
Degree of nerve activation influences force generated by the muscle
A single muscle consists of many individual muscle cells typically organized into groups of cells that work together cooperatively.
Each group of cells is controlled by a single nerve cell called a motor neuron.
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25. BIO 102 Muscle Lecture HANDOUT
Terms to know:
Motor unit: the motor neuron and all the muscle fibers it supplies. Is the smallest functional unit of muscle contraction.
Muscle tension: force generated by a contracting muscle upon an object
How much tension is generated by a muscle depends on
The number of muscle cells in each motor unit
The number of motor units active at the time
The frequency of stimulation of individual motor units
All-or-none principle: muscle cells are completely under the control of their motor neuron. Muscle fibers always respond with a complete cycle of contraction and relaxation every time they are stimulated.
Muscle tone: low level of contractile activity in a relaxed muscle.
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26. BIO 102 Muscle Lecture HANDOUT
Muscle Activity
Muscle twitch: a complete cycle of contraction and relaxation
Humans have 2 types of skeletal muscle fibers: slow-twitch and fast-twitch fibers. The difference is based on how quickly the fibers can produce a contraction and whether the muscle contracts aerobically or anaerobically.
Most muscles contain a mixture of both types of fibers
Ratio of fibers depends on the function of the muscle
Abnormal muscle twitches occur when almost all muscle spindles are excited simultaneously
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27. BIO 102 Muscle Lecture HANDOUT
Slow twitch fibers: break down ATP slowly and contract slowly
Tend to make ATP aerobically, as they need it
endurance, long duration contraction, contain myoglobin to store oxygen
Jogging, swimming, biking
Fast twitch: break down ATP quickly, contract more quickly
Store large amounts of glycogen and tend to rely on anaerobic metabolism for quick bursts of high energy
Rapid, powerful contractions but can’t be sustained
strength, short duration contraction:
Sprinting, weight lifting, tennis
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28. BIO 102 Muscle Lecture HANDOUT
Exercise Training
Strength training:
Resistance training:
Short, intense
Builds more fast-twitch myofibrils
Aerobic training:
Builds endurance
Increases blood supply to muscle cells
Target heart rate at least 20 minutes, three times a week
BIO 102 Muscle Lecture HANDOUT

29. Muscle Contraction: Myogram
Latent period-the time
between stimulation and the
start of a contraction
Contraction-time when the muscle actually shortens
Relaxation-muscle returns to its original length
Summation vs. tetanus
Summation: an increase in the frequency with which a muscle is stimulated so that the muscle doesn’t relax completely. This causes a “summation” of the contractile force so total force produced is greater than the force produced by one twitch.
Tetanus: If muscle stimulation is so rapid that the muscle can’t relax at all, it will remain in a state of maximal contraction.
BIO 102 Muscle Lecture HANDOUT
Figure 6.10

30. Diseases and Disorders of the Muscular System
Muscular dystrophy –group of inherited muscle diseases in which muscle fibers are unusually susceptible to damage.
Muscles, primarily voluntary muscles, become progressively weaker. In the late stages of muscular dystrophy, fat and connective tissue often replace muscle fibers OR
loss of muscle fibers due to muscle’s inability to create some proteins it needs to function normally OR
Leak of calcium into the muscle cell which damages muscle proteins and may kill the cell
results in muscle wasting and paralysis; death usually from heart failure or respiratory failure
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31. BIO 102 Muscle Lecture HANDOUT
Tetanus or “lock jaw” –bacterial infection resulting in overstimulation of nerves and therefore muscles resulting in tetanic contractions; death from exhaustion or respiratory failure
Muscle cramps – painful, uncontrollable muscle contractions; caused by dehydration and ion imbalances caused by heavy exercise
Pulled muscles –caused by stretching a muscle too far causing some fibers to tear apart
Fasciitis –inflammation of the connective tissue fascia that surrounds a muscle
BIO 102 Muscle Lecture HANDOUT