13 September 2011

3. More Pectoral Bones and Muscles/Nerves/Arteries/Veins

Starting with the Sternum
The sternum is the big bone on the front of your chest. Taking it from the top...

Put your hand on your throat.  Now move it down to the top of your chest.  You should feel a little dip of bone, like a cup.  This is called the suprasternal notch or the jugular notch.  It is the top part of the sternum bone. Because of the many important structures running near or behind it, it is a common target in martial arts, for example in jujitsu.  Check out this fight scene from the first Matrix movie, at 1:27 the agent throws a suprasternal finger strike.  Anyway, the manubrium (Latin for 'handle') is the large part beneath it and then it connects to the body of the sternum at the sternal angle.  The sternal angle is a cartilaginous joint with little movement  The manubirum articulates with the clavicles and the first two ribs.  All these are important landmarks because as you learn more about the heart, the various nerves, and other internal organs, you will constantly be seeing references to these, so it is helpful to learn them well now instead of having to keep looking them up later.

A common question on anatomy tests are what I call ice pick questions and they generally go: if you were to stab someone with an icepick at a certain place, what would you hit and in what order?  It requires the student to put together concepts, information, and 3D relationships.  A similar set of questions ask how specific structures are innervated and where those nerves come out of the spinal column, each vertebrae has a letter and a number (Brief Aside: There are 7 cervical vertebrae, called C1-C7, 12 thoracic, T1-T12, 5 lumbar, L1-L5, 5 sacral, S1-S5, and the coccyx, a mass of 5 fused vertebrae).  For example the nerve that innervates the muscle called Serratus Anterior is called the Long Thoracic Nerve, and it is a combination of nerves fibers from C5, C6, and C7, I will always list these after the muscle name.

Great Test Items to Know:

If you ice pick someone at the jugular notch, it will come out the back at T2.
The Manubrium goes from T2 to T4.
The Sternal Angle is at T4 and is about 140 degrees.

Now I am going to cover muscles for the first time.  Generally the format I will use is as follows
Muscle:  Name of the muscle
About: information
Actions: What is does
Origins: where the muscle fibers originate
Insertions: where the muscle fibers insert
Nerve Supply:  Which nerves control the muscle

Origin/Insertion:  A good way to think of this is where a muscle starts and ends.  But which is which?  A good rule to remember is a muscle will generally insert into whatever it is moving. Keep this in mind.

Blood supply will be covered separately because unlike nerves, there is no 1:1 connection, rather networks of blood vessels supply general areas, and even then arteries, but veins especially, tend to highly anomalous, that is, outside of the major vessels many of the smaller ones simply do as they please and travel where they want.  Lets do it...

Muscle: Pectoralis Major
About: The pectoralis major, commonly called the pec major, is the large muscle on the chest of males, and underneath the breast in females.  It is one of the main muscles used when bench pressing or doing push ups.
1. Adduction of the Brachium (aDDuction is moving closer to the body, imagine flapping your arms like you are trying to fly, when your arm is moving downward, it is adducting.  The opposite is aBduction, part where your arms are moving up. And the Brachium is upper arm)
2. Medial Rotation of the Brachium (medial rotation is twisting in toward the body, hold your arm out and give a thumbs down, that is your arm in medial rotation, the opposite way, twist out, is lateral rotation)
3. Flexion of the Brachium (bringing the arm up and forward, benching press is flexion of the arm, or a bowling motion, or how a softball pitcher throws)
Origins: (see photo below as you go through them)
   1.Clavicular- The medial half of the clavicle
   2.Sternocostal- Anterior surface of the sternum and upper 6 costal cartilages, and abdominal portion of the aponeurotic tendon of the external oblique (an aponeurosis is generally where a muscles blends into a tendon, they are white, shiny, have paper like layers and there are a few important ones throughout the body.  Right now just memorize the information, I will go into more depth later)
Insertions: Lateral lip of the intertubercular groove of the humerus (front of the arm)
Nerve Supply:
1. Medial Pectoral Nerve (generally the sternocostal portion, or lower chest) (C8, T1)
2. Lateral Pectoral Nerve (generally the Clavicular portion, or upper chest) (C5, C6, C7)
As explained above, the letter/number referes to where in the spinal cord these nerves originate.  I will later go through all these nerves as an entity, it is called the Brachial Plexus, and in any anatomy class you will need to memorize it completely.  Why is it important?  Say you have a patient with upper shoulder/spinal trauma, if you can figure out which muscle actions do not work, you can figure out which part of the spinal cord might be affected (great test questions).

Now lets think through the origins and insertions (O/Is) and see if we can make sense of how they dictate the actions.  Muscle fibers only contract, they do not do complicated movements.  So if we imagine contraction, it makes sense the the arm will move closer to the body, up, and forward.  Boom, that's the 3 actions.  I prefer understanding the actions, as opposed to simply memorizing them all, because then you don't have to.  If you know O/Is you can figure out actions.

And where there is a major there is usually a minor.  If a structure has a name with major or minor, anterior or posterior, or superior/inferior, it is a good bet that the other also exists.

Muscle: Pectoralis Minor
About: Pec minor is the small little brother of pec major.  It sits behind, or posterior, to the pec major.  It is smaller and plays less of role in movement and big actions and more of stabilizer role for the scapula.  It helps connect the thoracic cage with the scapula.
Actions: Protracts (brings forward, opposite is Retract) the Scapula, and Stabilizes the Scapula
Origin: Ribs 3-5 (though often 2-4)
Insertion: Coracoid process of the Scapula
Nerve Supply: Medial Pectoral, (C8, T1)

Feeling overwhelmed yet?  Don't worry the more time you spend with this material the easier it gets, even though it seems impossible at first. 2 more muscles to go...

Muscle: Serratus Anterior
About: Also called 'The Pull Up Muscle',  The SA is a big muscle going from the ribs to the scapula.  When strongly defined the SA is the main muscle that produce the V shaped torso and is a main muscle used when throwing a punch.
Actions: Connects the ribs to the scapula, thus when it fully contracts it pulls the scapula forward, called protraction of the scapula.  But it is a big muscle and can contract in parts.  The bottom and top halves can contract separately causing the scapula to rotate too, though this is not a major function and thus not commonly listed (for examination purposes). It also helps stabilize the scapula.
Origin: Ribs 1-8
Insertion: Medial border of the Scapula
Nerve Supply: Long Thoracic Nerve (C5, C6, C7)
BIG NOTE and clinical correlation: Winged Scapula.  If the scapula is protruding from the back, giving it a winged appearance, it is commonly the result of a weak serratus anterior muscle or nerve problem.  If someone is really weak, cannot do one push up, and tries, you might see the scapula wing out from simple SA weakness.  Common test question:  A patient comes in from car accident and has sustained lacerations under his arm pit.  You notice his scapulae protruding on the right with a winged appearance.  What nerve has been severed?

Note the winged scapulae

Muscle: Subclavius
About: This is the first of some muscles that I will be blogging about that are simply not very useful. It goes from the ribs to the clavicle.  It is always funny to make fun of someone and call them useless using anatomy. 
      Karen: "Jon why do you never help with are lab group dissections. Stop messing around."
      Alex: "Jon is subclavius of our group."
      Jon: "Wow low blow guys."
Actions: Not much, helps in respiration.
Origin: Border of the 1st rib and its costal cartilage
Insertion: Inferior surface of the middle third of the clavicle
Nerve Supply: Nerve to the Subclavius (C5, C6)

Doesn't bring a lot to the table

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