Skeletal muscles are formed when many myoblasts (muscle stem cells) fuse forming one ginormous cells with many nuclei. Some of these cells (we call muscle cells muscle "fibers") are almost a foot long!  The many nuclei per cell permit the transcription/translation of all of the proteins needed by the fiber. 

4. Anaerobic respiration can occurs in the absence of O2. Glucose is broken down via glycolysis into two pyruvates and generates 2 just ATPs. 
The pyruvates are converted to lactic acid which can contribute to muscle fatigue as it accumulates. 

the sliding filament theory of muscle contraction

 Muscle strength is proportional to the # of myofibrils and sarcomeres in a muscle fiber not the number of muscle fibers. 

1. calcium binds to troponin which               shifts tropomyosin exposing the               myosine binding site.
2. the myosin head binds to actin
     forming a cross-bridge. a power               stroke occurs as ADP and P
     (from ATP hydrolysis) is released.
      the myosin head pivots, dragging            the actin toward the sarcomere's
      M line.
3. a new ATP attaches to the myosin             head as the cross-bridge detaches.
     this ATP is hydrolyzed, cocking the         myosin head and preparing the                sarcomere for the next contraction. 

muscular dystrophies
are a group of 30+ genetic diseases which manifest as  progressive weakness and degeneration of the skeletal muscles that control movement. Duchenne MD is the most common form of MD and primarily affects boys since the allele is found on the X chromosome. A mutatged allele prevengts the production of 
dystrophin which is a protein that maintains the integrity of muscle. 

treppe

muscle size

wave summation
(aka temporal or frequency summation)
a stimulus is applied to a muscle before the relaxation period from the previous contraction is complete.
This produces a stronger contraction.

Functions of the Muscular System

Muscle tissue exhibits:

1. contractibility (muscles can                                            contract/shorten when stimulated)
2.  extensibility (muscles can be stretched                      without tearing)
​3.  elasticity (muscles return to their                                resting/normal shape)

muscle tone

What is a twitch?
One stimulation/action potential from one motor neuron causes the fibers of its motor unit to contract. Each single contraction is called a twitch, lasting just a few to 100 milliseconds.
Each twitch can be measured by the amount of tension (force of contraction) produced.

structure of a sarcomere

The Muscular System

 motor units

quick review of muscle contaction...

smooth muscle

 Troponin is a protein attached to another protein called
tropomyosin. When a muscle is relaxed, the troponin/tropomyosin complex  covers the binding site for the myosin head. During contraction, calcium channels open in the sarcoplasmic reticulum and calcium is released. Calcium binds to troponin causing it to change shape. The new shape pulls it away from the binding site, exposing the site to the myosin head and permitting contraction. 

A sarcomere is the functional unit of a muscle fiber responsible for the contraction of muscles.
Each sarcomere is  composed of thick and thin filaments bordered by two z-discs.
Sacrcomeres are lined up end to end parallel
​to the length of a muscle fiber.

skeletal...but also cardiac and smooth muscle

Muscle Types

Rigor mortis 

*cardiac muscle propels blood around the body
*smooth muscle propels food along the alimentary canal and form sphincters to regulate
               entrance/exit of the digestive system
*heat is produced when muscles contract...almost 70% of  body heat is produced by muscles. 
               blood is an essential element in temperature control during exercise, taking heat from the
               body core and working muscles and redirecting it to the skin when the body is overheating.                 When the body's internal temperature is too low, thermoreceptors tell the
               hypothalamus which then stimulates the skeletal muscles to rhythmically contract/relax to                 produce heat = shivering.
*the partnering of skeletal muscles and bones via tendons permits movement
*maintaining posture requires constant tension on many muscle groups.
*muscles also provide protection for our internal organs
and muscle proteins can provide nutrition in times of starvation

​                                                There are three phases to a  twitch:
   1. latent period: an action potential travelling down the sarcolemma causes the sarcoplasmic reticulum to release calcium. 
   2. contraction phase: calcium binds to troponin moving the tropomyosin and exposing the binding sites for the myosin heads. myosin cross-bridges form and the sarcomere shortens as the point of peak tension occurs.
   3. relaxation phase: calcium is pumped back into the SR. the contraction stops/tension decreases. the fiber returns to its resting state. uick review of muscle contaction...

types of muscle fibers

A myosin head binds to actin at a myosin binding site.
​Myosin has a different binding site for ATP where ATP is broken down into ADP and P.
The energy released (exergonic rx) during the hydrolysis of ATP alters the myosin head angle into a “cocked” position.

1. Isotonic contraction:
    the tension in the muscle stays constant as the
    load moves and the muscle changes length.
   a. concentric contractions occur when
       the muscle shortens.
   b. eccentric contractions occur when the
       muscle lenthens. tension diminishes and the muscle lengthens. 
2. Isometric contraction:
     the muscle produces tension without moving a           load even though the muscle shortens.
     the force produced does not overcome the                    resistance provided by the load. isometric                    contractions maintaining posture and joint                  stability.
   most actions are a combination of isotonic and                            isometric contractions.

cardiac muscle

neuromuscular
​ junction

neuromuscular diseases

ex: multiple sclerosis

So, a contraction occurs when the hydrolysis of ATP releases energy causing the myosin head to attach to the actin binding site and complete a working stroke. the actin binding site us available because calcium binds to troponin so that the tropomyosin isn't covering the site. The calcium is released from the sacroplasmic reticulum...sooooo
what causes the calcium to be released??

muscle fibers are innervated by only one motor neuron. a group of muscle fibers innervated by a single motor neuron is called a 
motor unit.
​ all of the motor units in a muscle is called a
motor pool.
the size of a motor unit is depends on the nature of the muscle.
1. small motor units have a single motor neuron supplying a small number of muscle fibers
with the axon branching into several synaptic end bulbs. this permits very fine motor control such as the muscles that move eyeballs and the finger control.
2. large motor units have a single motor neuron supplying a large number of muscle fibers.
 with the axon branching into thousands of synaptic end bulbs. this permits gross motor control such as the muscles that flex the arm or leg.

small motor units in a muscle exhibit smaller, lower-threshold motor neurons which generate a relatively small degree of tension. if more strength is required, larger motor units with bigger, higher-threshold motor neurons activate larger muscle fibers. this increase in muscle tension is called recruitment. some large motor units generate tension 50X that of the smallest motor units in the muscle. think lifting a feather with the same muscles that can lift 10 lb weights.

structure of a skeletal muscle

Tetanus is an infection caused by the Clostridium tetani bacterium.
The toxin produced by this anaerobic pathogen results in prolonged muscle contractions. Sometimes called “lockjaw”, tetanus can cause  neck and jaw muscles to lock. 

3. Aerobic respiration produces >90% of the ATP required for muscle contraction. Glucose is broken down
in the presence of O2 to produce CO2, H2O and ATP. Muscles are able to 
store a small amount of O2 in
myoglobin proteins.  

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2.  Creatine phosphate stores energy in its phosphate bonds. Resting muscles transfer excess ATP energy to creatine forming creatine phosphate and ADP.
This reversible reaction fuels the first few 15 seconds or so of a muscle contraction. 

pathology

fibromylagia

three types of muscle fiber
slow oxidative = SO
 fast oxidative = FO
  fast glycolytic = FG