1. The Muscular System rev 2-11
Muscle cells are involved in every movement that our bodies perform.
Muscles can:
Shorten or contract to produce movement
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
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2. Muscle tissue is made up of tightly packed cells called muscle fibers
Muscle tissue is made up of tightly packed cells called muscle fibers. The muscle fiber cytoplasm contains proteins which allow the cell to contract
Muscle Lecture BIO 102

3. There are 3 types of muscles:
Skeletal muscles
Cardiac muscles
Smooth muscles
attach to the bones of our skeleton and provide strength and mobility for our body
Cardiac muscle
found in the heart; they pump blood throughout the body
found in most internal organs
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4. Muscles may also be classified as
voluntary (muscles over which we have conscious control)
involuntary (muscles over which we have no conscious control)
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5. cells are arranged in a parallel fashion
Skeletal Muscles
are multinucleated
cells are arranged in a parallel fashion
will not contract unless stimulated by a neuron
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
Muscle Lecture BIO 102

6. Is also called Striated or Voluntary muscle
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
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7. Cells are striated, short, fat, branched and interconnected
Cardiac Muscle
Cells are striated, short, fat, branched and interconnected
Have specialized areas called intercalated discs
because of these connections, cardiac muscle works as a single, coordinated unit
will contract without nervous stimulation
usually contracts at a steady rate set by the heart’s pacemaker, but neural controls allow for a faster beat
compared to other muscle types, their speed of contraction is moderate
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8. Smooth Muscle Cells are shorter than skeletal and cardiac muscle cells
Because the cells have fewer contractile proteins they do not have striations
Cells are spindle shaped; each cell has a centrally located nucleus
Do not require nervous stimulation for contractions
Are found in the walls of hollow visceral organs
role is to force fluids and other substances through body channels
compared to other muscle types, their speed of contraction is slow and sustained
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9. Dark areas are called A-bands Lighter areas are called I-bands
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
many myofilaments bundled together are called myofibrils
The space between 2 Z-lines is called a sarcomere
A sarcomere is the smallest contractile unit of a muscle fiber
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10. 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
ACh is a neurotransmitter--a chemical which can either stimulate or inhibit another “excitable” 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
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11. The T-tubules are in close contact with the sarcoplasmic reticulum
The ACh binding causes the muscle cell membrane to generate an electrical impulse which travels along the cell membrane and along the T-tubules
the function of the T-tubules is to allow the electrical impulse to travel to all cell parts
The T-tubules are in close contact with the sarcoplasmic reticulum
The electrical impulse triggers the release of calcium from the sarcoplasmic reticulum so the muscle can 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
Muscle Lecture BIO 102

12. Calcium binds to troponin and causes the
Troponin–tropomyosin protein complex to shift position
To expose the myosin binding site 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.
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13. So, in order to contract, a muscle must: Be stimulated by a nerve
Cause an electric current along the sarcolemma
Have a rise in intracellular calcium levels
Muscle Relaxation
Nerve cell stimulation ends, contraction ends
In order to stop the contraction, nerve stops
Calcium pumped back into sarcoplasmic reticulum
Calcium removed from actin filaments
Myosin-binding site covered
No calcium = no cross-bridges
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14. 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.
ATP is the muscle’s energy source; typically muscle cells store enough ATP for 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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15. For long term energy, ATP can also be obtained via aerobic metabolism of glucose, fatty acids, and other high-energy molecules
Glycogen is broken down by a process called glycolysis or the Krebs Cycle or the Citric Acid Cycle
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. This is a fast process but only yields 2 ATP molecules per glucose molecule.
It also produces lactic acid as a waste product which can make muscles sore.
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16. This will cause muscle fatigue and cramps.
During vigorous exercise, typically the blood is unable to carry enough oxygen for complete oxidation of glucose in our muscles. So, the muscles will contract anaerobically.
Lactic acid normally leaves the muscles and goes into the blood. But, with continued exercise, the amount of lactic acid in the blood increases so the lactic acid levels will accumulate in the muscles.
This will cause muscle fatigue and cramps.
After exercise, the person rests and must take in enough oxygen to allow the lactic acid to be changed into glucose.
This oxygen is called the oxygen debt.
So, the reason we breathe heavily after exercise is to erase the oxygen debt.
Muscle Lecture

17. 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.
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18. 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.
The perimysium layer is fibrous connective tissue that surrounds the fascicles.
The outermost connective tissue layer is called the epimysium.
At the ends of the muscle, all of the fascia come together and form the tendon.
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19. 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.
Muscle Lecture

20. the muscle’s origin is on the bone which does not move
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 insertion is on the bone which moves
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21. Degree of nerve activation influences force generated by the muscle
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
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22. Muscle tension: force generated by a contracting muscle upon an object
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
All-or-none principle: muscle cells are completely under the control of their motor neuron.
Muscle tone: low level of contractile activity in a relaxed muscle.
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23. Muscle twitch: a complete cycle of contraction and relaxation
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.
Slow twitch fibers: break down ATP slowly and contract slowly
Fast twitch: break down ATP quickly, contract more quickly
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24. Strength training: Aerobic training: Resistance training:
Exercise Training
Strength training:
Resistance training:
Aerobic training:
Builds endurance
Increases blood supply to muscle cells
Muscle Lecture BIO 102

25. Diseases and Disorders of the Muscular System
Muscular dystrophy –inherited; loss of muscle fibers resulting in muscle wasting and paralysis; death usually from heart failure or respiratory failure
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
Muscle Lecture BIO 102