Skeletal muscles are formed when many myoblasts (muscle stem cells) fuse forming one ginormous cell 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 production of all of the proteins needed by the fiber. 

review question:
what is an intercalated disc? Why does one 
 muscle type have them but the other two do not?

the exact cause of fibromyalgia is unknown.
research suggests it's likely due to a combination of factors:
 genetic predisposition
 environmental triggers
 altered pain processing in the brain and central nervous system. 

slow-twitch muscle fibers have a high concentration of myoglobin.
high myoglobin content gives slow-twitch fibers their reddish appearance and contributes to their ability to efficiently utilize oxygen for energy production, making them ideal for endurance. ​

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 mutated allele prevents the production of 
dystrophin which is a protein that maintains the integrity of muscle. 

the carpal tunnel is the passageway in the wrist where the median nerve and flexor tendons pass through a narrow opening.
Carpal tunnel syndrome
occurs when the tendons become inflamed, causing compression of the median nerve. Symptoms include pain, numbness, and eventual weakness in the hand. Carpal tunnel syndrome can occur for a variety of reasons including hereditary predisposition, repetitive movements, diabetes, or thyroid disorders.

review question:

list four functions of the skeletal system

1. a nerve impulse (action potential) travels down the neuron
     towards the synapse
2. voltage gated calcium channels open
3. calcium enters neuron 
4. calcium causes vesicles containing the neurotransmitter 
     acetylcholine (ACh) to release their contents into the synaptic         cleft = the space between the neuron and muscle cell
5. the neurotransmitter travels across the synapse and binds                 to receptors on the muscle cell's membrane.
6. this causes Na+ on the muscle cell channels to open.
7. Na+ enters cell, starting an action potential down muscle                  cell membrane

​ 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.

ATP (cell energy) fuels these reactions.
If more ATP is added, the contraction lasts longer.
If there's no ATP added, the contraction ends and the filaments slide back to their original positions.
Troponin covers up the binding site again
​and the muscle is at rest. 

review question:

starting with the neuron at the neuromuscular junction, describe how a muscle cell contracts.

the sliding filament theory of muscle contraction

review question:

identify:
sarcolemma
sarcoplasm
sarcomere
T tubule
sarcoplasmic reticulum

*action potential travels down
   T tubules
*voltage gated calcium channels
   on sarcoplasmic reticulum open
* calcium is released
* calcium binds to
   troponin/tropomysosin,                  a molecule on the thin                      filament that blocks a binding                    site on the filament

*this shifts the position of this
   molecule 

*the binding site is now open

​*a piece of the thick filament              called a "head" binds to this            open site forming a
   "cross bridge"

* a "power stroke"  occurs...
      ATP releases energy
      the head pivots and drags the
      thin filament along the thick            filament. 

*the sarcomere shortens as Z lines slide closer to one another.
  as all of the sarcomere's go              through this same process at          once, the whole muscle                          shortens

muscle fibers are connected by intercalated discs which allow electrical signals to quickly pass from one cell to the next = coordinated contraction 

 what are the different

types of contractions?

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

pathology

review question:
smooth and cardiac muscle but not skeletal muscle are controlled by the autonomic nervous system. why?

fibromylagia

here's the sarcoplasmic reticulum

how do muscles get stronger?

 pacemaker cells initiate electrical impulses that spread throughout the heart. 
pacemaker cells have the property of automaticity = they can spontaneously generate electrical impulses 

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

review question:

calcium plays two roles to cause a muscle to contract. what are they?

sarcolemma sarcoplasm sarcomere...

​see a pattern?

the dystrophin protein protects muscle fibers from damage during contraction and relaxation.

muscle fatigue

 different people have different proportions of slow-twitch and fast-twitch muscle fibers which can influence their athletic abilities and overall fitness. most people have a mix of both but some individuals are predominantly slow-twitch or fast-twitch.​

review question:

how do the functions of the 3 types of muscle differ?

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)

what causes the sarcomere's to shorten = contraction?

review question:

what is the sliding filament theory?

review question:

identify:
epimysium
perimysium

endomysium
fascicle

muscle cells are called muscle fibers

review question:
how are small motor untis and large motor units different?

Multiple sclerosis (MS) is a chronic, degenerative disease of the central nervous system characterized by demyelination, or the loss of the myelin sheath that protects nerve fibers. This damage disrupts the flow of nerve impulses, leading to a wide range of neurological symptoms. 
MS is an autoimmune disorder:
the body's immune system mistakenly attacks its own myelin sheath. 

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.

how does a muscle cell relax?

The Muscular System

Fibromyalgia is a chronic condition characterized by widespread pain, fatigue, and other symptoms. While there's no cure, various treatments can help manage symptoms. The first signs of fibromyalgia often include chronic pain, fatigue, and sleep disturbances. 

what's a  motor unit?

we start with the neuromuscular
​ junction

Myogenesis:
​making muscle

so an action potential (nerve impulse) initiated by a neuron travels down the muscle cell membrane 

smooth muscle

structure of a sarcomere

A sarcomereis the functional unit of a muscle fiber responsible for the contraction of muscles.
Each sarcomere is  composed of thick and thin filaments.
Sacrcomeres are lined up end to end
parallel ​to the length of a muscle fiber.
As the sarcomeres shorten, the whole muscle shortens.

enough about skeletal muscle...
what about cardiac and smooth muscle

this is the unique structure of a muscle fiber

and a recap of the muscle
contraction process

review question:

why do skeletal muscle cells have so many nuclei?

and...

Muscle Types

Acetylcholinesterase removes the neurotransmitter acetylcholine from the synaptic cleft. this stops the action potential. the calcium jumps off the troponin and moves back into the sarcoplasmic reticulum.
troponin/tropomyosin once again blocks binding site. filaments slide back.

Z lines spread apart. muscle relaxes.   

rigor mortis 

review question:

the functional unit of a muscle cell is called a ____. This structure exhibits overlapping filaments called
​___ and ___.  When these filaments slide across one another, the _____ lines move closer to one another shortening the ____. 

*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
*the partnering of skeletal muscles and bones via tendons permits movement
         maintains posture via constant tension on many muscle groups,
         provides protection for our internal organs
         provides nutrition in times of starvation

a bit more about how troponin/tropomyosin

​blocks the myosin binding site 

types of muscle fibers

Fibromyalgia is significantly more common in women, with studies showing a female-to-male ratio of approximately 9:1. 

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

type of muscle fiber has to do with myoglobin

neuromuscular disease:MS:

     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.
            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.

    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.