The Muscular System rev 12-12

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

2

• Muscle tissue is made up of tightly packed cells
  called muscle fibers. The muscle fiber cytoplasm
  contains proteins which allow the cell to contract

3

• There are 3 types of muscles
• Skeletal muscles
• Cardiac muscles
• Smooth muscles
• Skeletal 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
• Smooth muscles
• found in most internal organs generally they
  work to push something (fluids or other body
  substances) through a body part

4

• 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

5

• 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

6
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

7
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
  hearts 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

8
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

9
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

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 (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.

11

• 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.

12

• Calcium binds to troponin, a protein molecule,
  and causes the
• Troponintropomyosin 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
  troponintropomyosin 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

13

• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter10/animation__action_potential
  s_and_muscle_contraction.html
•   
• http//www.brookscole.com/chemistry_d/templates/st
  udent_resources/shared_resources/animations/muscle
  s/muscles.html
• http//www.dnatube.com/video/1310/Action-potential
    
• GOOD ONE
• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter10/animation__breakdown_of_atp
  _and_cross-bridge_movement_during_muscle_contracti
  on.html
• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter10/animation__sarcomere_contra
  ction.html 

14
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

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 muscles 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
  20-30 seconds
• After this short amount of time, energy must be
  obtained from stored glycogen

16

• 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.)

17

• 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.

18

• ---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.

19

• 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.

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

21

• 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

22

• 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.

23
Movememt

• If the muscle spans a joint, one bone moves while
  the other one remains stationary
• the muscles origin is on the bone which does not
  move
• the muscles 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

24
Muscle Contractions

• Isotonic contractions occur when the muscle
  shortens and movement occurs
• Isometric contractions occur when muscle tension
  develops but the muscle doesnt 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.

25

• 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.

26
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

27

• 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 cant be
  sustained
• strength, short duration contraction
• Sprinting, weight lifting, tennis

28
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

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
  doesnt 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 cant relax at all, it will remain in
  a state of maximal contraction.

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 muscles 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

31
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