What do the mesenchymal cells of paraxial mesoderm-derived somites generate?
chondroblasts and osteoblasts that form the vertebral column via endochondral ossification.
What does the lateral plate mesoderm split into?
1) the parietal (somatic) layer
2) the visceral (splanchnic) layer
what do mesenchymal cells of the parietal layer generate into?
chondroblasts and osteoblasts that form the scapula, clavicle, and bones of the upper limb.
Describe the first step of endochondral ossification of long bones.
endochrondral ossification of long bones begins by formation of thin bony collars around the diaphysis or shaft of the cartilaginous replicas laid down by the perichondrium of the cartilage (which is just connective tissue). This perichondrium soon becomes a periosteum.
what occurs after the bony collar periosteum is established?
As soon as the bony collar is established, the internal cartilage matrix begins to calcify and the chondrocytes hypertrophy and die. Osteoblasts, angiogenic capillaries, and macrophages from the periosteum then invade the diaphysis and replace these dead chondrocytes with trabecular bone, forming the primary ossification center in the diaphysis. These periosteal capillaries also force any living chondrocytes to the ends of the bone (growth plates aka epiphyses).
How is lengthening of long bone achieved?
chondrocytes that were forced to the epiphyseal plates (growth plates) at either end of the long bone, organize themselves into longitudinal cellular columns and proliferate away from the diaphysis. The innermost chondrocytes continue to degenerate (hypertophy and accumulate lipids, glycogen, and alkaline phosphatase) until they eventually die and leave behind a calcified cartilage matrix which is then ossified later by osteoblasts.
why is acondroplasia associated with skeletal dysplasia (for example, dwarfism)?
Acondroplasia is the most common form of skeletal dysplasia and it primarily affects long bone. It results in stunted growth of long bones due to a mutated growth factor receptor which usually has an inhibitory affect on growth of long bone. The mutation makes the receptor constitutively active and thus continually inhibits growth of long bone.
what transformation forms compact bone and where is it located?
The transformation of primary osteons into mature osteons forms compact bone which is located on the outer part of the diaphysis.
In neonates, what does residual trabecular bone in the diaphysis form and where can secondary ossification centers be found?
residual trabecular bone in the diaphysis forms the marrow cavity where bone marrow resides and secondary ossification centers can be found in the epiphyses.
What happens to the epiphyses at age 10? ...and at skeletal maturity?
At age 10 the epiphyses mature into cores of trabecular bone which are covered by compact bone. The growth plates still remain active until puberty, and at skeletal maturity the epiphyses and diaphysis converge (growth plate disappears, leaving a continuous connection of ephysiseal and diaphyseal bone).
What makes up the glenohumeral joint (shoulder joint)?
the articulation of the head of the humerus with the glenoid cavity of the scapula.
How much of the humeral head is handled by the glenoid cavity?
The glenoid cavity only handles a third of the humeral head.
what is the purpose of the glenoid labrum?
The glenoid labrum is made up of concentric rings of Type I collagen fibers and its purpose is to attach to the rim of the cavity and slightly deepen it.
what surrounds the glenohuymeral joint and what reinforces it?
the glenohumeral joint is surrounded by a capsule composed of loose connective tissue and it is reinforced by several ligaments (ignore names) that consist of dense connective tissue.
What is essential to holding the humeral head in the glenoid cavity?
The tonus (slight, continuous contraction) of the rotator cuff muscles.
How would loss of rotator cuff muscle tone affect the glenohumeral joint?
The loss of muscle tone in the rotator cuff muscles may increase the risk of dislocating the shoulder.
where is the coracoid process of the scapula in relation to the glenohumeral joint?
the coracoid process of the scapula projects anterolaterally toward the glenohumeral joint.
what can be found on the posteroir surface of the scapula?
There is a thick bony spine that unevenly divides the posterior surface of the scapula and continues laterally toward the acromion. Also recognize that the glenohumeral joint is covered superiorly by the acromioclavicular (AC) joint.
what two components make up the acromioclavicular joint and what does the AC joint allow?
The two components of the AC joint are the acromion and the lateral end of the clavicle. The AC joint allows the clavicle to act as a rigid support suspending the scapula and allowing it to slide along the posterior thoracic wall.
What forms the coraco-acromial arch and what is the arch's purpose?
The coraco-acromial ligament interconnects the coracoid process and forms the coraco-acromial arch. The purpose of this arch is to prevents the humerus from a superior displacement from the glenoid cavity. Its incredibly strong and allows you to hold your body up by your arms (picture standing by a desk and partly supporting your body by leaning on the desk with your arms).
what do the acromioclavicular ligament and the coracoclavicular ligament do in association with the AC joint, respectively?
The acromioclavicular ligament strengthens the AC joint superiorly. The coracoclavicular ligament anchors prevents dislocation of the AC joint so it insures normal strut function of the clavicle.
why odes forceful superior thrusts of the humerus typically fail to dislocate the glenohumeral joint but fracture the humeral shaft or clavicle?
A forceful superior thrust of the humerus usually doesn't dislocate the glenohumeral joint because directly superior to the humeral head is the coraco-acromial ligament which is strong as hell.
why do movements of the medial and lateral fragments of a fratured clavicle result in dropping of the shoulder? and what is a greenstick fracture?
so the clavicle is usually held up by both the trapezius (mainly holds up the lateral end of the clavicle) and the sternocleidomastoid (mainly holds up the medial end of the clavicle). When the clavicle is fractured, the two resulting fragments (medial and lateral) are now incompletely held up by both of these muscles.
The lateral fragment still connects to the upper limb and is therefore considerablty heavier than the medial fragment and the trapezius alone is not strong enough to keep this weight held up without help from the sternocleidomastoid.
The medial fragment has been severed from the part of the clavicle that holds up the upper limb, and therefore it is relatively lighter than usual so the sternocleidomastoid pulls this fragment superiorly with ease.
A greenstick fracture is an incomplete fracture in which one side of the bone is broken and the other side is bent.
When does dislocation of the acromioclavicular joint occur and why does physical examination of the injury lead to its description as a "shoulder separation"?
dislocation of the acromioclavicular joint occurs because the AC joint itself is weak despite its superior reinforcement by the coracoclavicular ligament. It is usually described as a shoulder separation because in severe cases when the both the AC joint and the coracoclavicular ligament are torn it results in the shoulder separating from the clavicle and hanging off.
Why do most dislocatinos of the humeral head occur in an inferior direction, and why are such dislocations commonly described clinically as anterior dislocations?
Most dislocations of the humeral head occur in the inferior direction because of the presence of the coraco-acromial arch which is strong as hell and usually prevents superior dislocations. They are usually described as anterior dislocations because most of the time the dislocation results in the humeral head lying anterior to the glenoid cavity (specifically anterior to the infraglenoid tubercle).
what muscles are considered short intrinsic shoulder muscles, where do they attach, and what do they act on?
The short intrinsic muscles are the rotator cuff, deltoid, and teres major. They attach proximally on the scapula and distally on the humerus. They act on the glenohumeral joint.
where do the three muscles of the deltoid originate and attach?
All three muscles of the deltoid originate at the lateral end of the clavicle and attach distally to the deltoid tuberosity.
What nerve innervates the deltoid?
the deltoid is innervated by the axillary nerve (C5, C6)
How is the axillary nerve commonly injured at the surgical neck of the humerus, and where does loss of sensation occur after the injury?
The axillary nerve is usually injured during a fracture at the surgical neck of the humerus, during a dislocation of the glenohumeral joint, or during compression from the incorrect use of crutches. Loss of sensation occurs over the lateral side of the proximal part of the arm.
what are the roles of the anterior deltoid and the posterior deltoid during walking?
Both of these muscles assist in maintaining balance while walking. The anterior deltoid assists the pectoralis major in flexing the arm while the posterior deltoid assists the latissimus dorsi in extending the arm.
how is abduction of the upper limb to 90 degrees achieved?
abduction of the upper limb to 90 degrees is achieved via contraction of all three parts of the deltoid.
where does the teres major attach proximally and where does it attach distally?
the teres major is attached proximally to the inferior angle of the scapula and distally to the medial surface of the humerus. It serves to medially rotate the arm.
what is characteristic about the posterior surface of the scapula?`
there is a spine that unevenly divides the posterior surface of the scapula into a supraspinous fossae and a infraspinous fossae.
where do the supraspinatus and infraspinatus muscles proximally attach to and where do they distally attach to?
the supraspinatus and infraspinatus muscles proximally attach to the supraspinous and infraspinous fossae, respectively, and they both distally attach to the greater tubercle of the humerus.
what is the function of the subacromial bursa?
the subacromial bursa cusions the supraspinatus tendon as it courses between the humeral head and osseoligamentous coraco-acromial arch toward the greater tubercle.
what is the common innervation of the supraspinatus and infraspinatus muscles? and which of these muscles is responsible for the first 15 degrees of abduction and which is responsible for laterally rotating the arm?
the supraspinatus muscle is innervated by the suprascapular nerve (C4, C5, C6). The infraspinatus muscle is innervated by the suprascapular nerve (C5, C6). So mainly C6 for both. The supraspinatus muscle is responsible for the first 15 degrees of abduction while the infraspinatus muscle laterally rotates the arm.
where does the teres minor muscle attach proximally and where does it attach distally? What innervates this muscle and with what muscle does the teres minor work synergistically with?
the teres minor muscle attaches proximally to the lateral border of the scapula and distally to the greater tubercle. It is innervated by the axillary nerve (C5, C6) and it works synergistically with the infraspinatus during lateral rotation of the arm.
where can the subscapularis muscle be found, where does it distally attach, what innervates it, and what is its function?
the subscapularis muscle covers the anterior surface of the scapula and attaches distally to the lesser tubercle of the humerus. It is innervated by the upper and lower subscapular nerves (C5, C6, C7) and its function is to medially rotate the arm.
Why is the upper limb pulled into medial rotation after an avulsion fracture of the the greater tubercle?
in an avulsion fracture, a small part of the tubercle is avulsed (torn away) from the main humerus. The natural action of the muscles that remain attached to the humerus (usually the subscapularis) rotate the humerus medially with little to no resistance as would normally occur from the muscles that are attached to the now avulsed portion of the bone.
what is the painful arc syndrome?
the painful arc syndrome is a range of motion (50-130 degrees of abduction of the upper limb) in which there is sometimes excrutiating pain that occurs as a result of calcium deposits in the suprasinatus tendon which causes inflammation of the overlying subacromial bursa (subacromial bursitis). This specific range of motion is most affected because during this arc, the supraspinatus tendon is in close contact with the inferior surface of the acromion.
how is degenerative tendinitis of the rotator cuff tested?
to test for degenerative tendinitis of the rotator cuff the patient is asked to lower the fully abducted limb slowly and smoothly. If the rotator cuff is diseased and/or torn the limb will suddenly drop to the side in an uncontrolled manner.
Where can the infraglenoid tubercle of the scapula be found?
the infraglenoid tubercle of the scapula can be found directly under the glenoid cavity of the scapula (in blue).
Where can the radial groove be found and where can the olecranon be found?
The radial groove can be found in the middle of the posterior side of the humerus, while the olecranon can be found at the proximal end of the ulna articulating with the olecranon fossa of the humerus.
where does the triceps brachii muscle attach proximally and distally? What is its role in regards to the humeral head?
the triceps brachii attach proximally to the infraglenoid tubercle and its lateral/medial heads attach to the posterior humerus above and below the radial groove. The common tendon of all three heads attaches distally to the olecranon processes of the ulna (covering the entirety of the posterior humerus). Since this muscle crosses the glenohumeral joint, it has the additional role of helping to resist inferior displacement of the humeral head.
What innervates the three heads of the triceps and what does this nerve travel along?
the triceps are innervated by the radial nerve which travels along the radial groove on the posterior side of the humerus.
which of the three heads of the triceps brachii is the main muscle for forearm extension? Which is the strongest but only recruited to counter resistance in extending the forearm?
The medial head is the main muscle for extending the forearm, while the lateral head is strongest but only recruited when encountering resistance to extension.
Why is forearm extension weakened but not lost after fracture along the radial groove of the humerus?
when the radial nerve is injured in the radial groove, only the medial head is usually affected.
what do the myoblasts derived from mesenchymal cells of paraxial mesoderm-derived somites form? what do myoblasts derived from mesenchymal cells of the parietal layer of lateral plate mesoderm form?
myoblasts from the paraxial mesoderm form the intrinsic back muscles and extrinsic shoulder muscles. Myoblasts from the parietal layer of lateral plate mesoderm form the intrinsic shoulder muscles and the muscles of the upper limb.
how are myotubes formed?
myotubes are formed by the end to end fusion of myoblasts.
what do myotubes transform into?
myotubes transform into thick (myosin) and thin (actin) skeletal muscle fibers called myofilaments. These then bundle together, forming myofibrils.
What are the contents of I bands and A bands, and what is the function of Z-lines?
I bands are light, isotropic myofibrils (actin). A bands are dark, anisotropic myofibrils (myosin). The role of the Z line is to mark the end of the sarcomere (repeating functional units of muscle) and to anchor the thin filaments.
What are T tubules and what is the purpose?
T tubules are a membrane system external to the muscle fiber between A and I bands. They penetrate the muscle fibers and transmit electrical impulses from the sarcolemma to the sarcoplasmic reticulum, ultimately initiating contraction of the muscle fibers.
What is formed externally with every skeletal muscle fiber?
every skeletal muscle fiber forms a synaptic junction with the terminal of a somatic motor axon.
What is the molecule that is released from the terminal of the somatic motor neuron that induces postsynaptic action potentials and intiates muscle contraction? where does it bind?
Acetylcholine is the neurotransmitter that is released, and it binds to the adjacent sarcolemma.
What is the function of the satellite cells that reside along the sarcolemma of skeletal muscle fibers?
satellite cells serve as a population of reserve stem cells (resting myoblasts) either for normal postnatal growth or for repair and regeneration of damaged segments of the skeletal muscle fiber after injury. They are more abundant in slow-twitch muscles than fast-twitch muscles and their mitotic capacity decreases with age.
How does EMG test muscle action?
Electomyography (EMG) is performed by placing electrodes over a muscle and having the patient perform certain movements. The differences in electrical action potentials of various muscles are amplified, recorded, and compared. There is usually a baseline electrical signal always present which comes from resting muscle tone and is only absent during sleep, unconsciousness, or paralysis.
What is the deficiency that causes Duchenne muscular dystrophy and what are the symptoms of this disease?
Duchenne muscular dystrophy is a genetic disorder caused by a deficiency of dystrophin, which is a large cytoskeletal protein that is linked to the side of the sarcolemma of the muscle fiber and maintains mechanical integrity of the cell during contraction by acting as an anchor.
How does the common hereditary disorder myasthenia gravis affect the induction of postsynaptic action potentials?
Myathenia Gravis is an autoimmune disease that is characterized by a reduction in the concentration of acetylcholine receptors at the postsynaptic sarcolemmal membrane of the neuromuscular junction. Antibodies attach themselves to the sarcolemmal membrane of the neuromuscular junction and prevent a regular amount of acetylcholine from binding, resulting in reduced action potentials induced subsequent muscle weakness.
Define the functions of the following muscle fibers:
1) Type I
2) Type IIA
3) Type IIB
1) Type I: highly fatigue resistant, specializes in endurance circumstances such as marathon runnning, maintaining posture, and leisurely walking.
2) Type IIA: moderately fatigue resistant, specializes in endurance circumstances such as marathon running.
3) Type IIB: minimally fatigue resistant, specializes in short high force activites such as sprinting and power lifting.
where can loose connective tissue and dense connective tissue be found within muscle, respectively?
Loose connective tissue can be found in sheaths called endomysium which surround each individual muscle fiber. Dense connective tissue can be found in two types of sheaths: epimysium which surrounds the whole muscle, and perimysium which surrounds the fascicles of muscle fibers.
What are tendons composed of and what is their function?
Tendons are direct continuations of the epimysium which is made of dense connective tissue with closely packed Type I collagen fibers and intervening fibroblasts and capillaries. They are also directly continuous with the fibrous layer of the periosteum at their bony attachment sites.
what characteristic of the proximal ends of the tendons adds to the mechanical stability of the muscle-tendon junction.
at the proximal ends of the tendon, indentations in the sarcolemma can be found which increase the surface area of the sarcolemmal membrane and enhances mechanical stability.
Why are skeletal muscle trained according to their percentages of fiber types?
Muscles such as the biceps, which have a higher percentage of type IIB (minimally fatigue-resistant), require high force training with a low number of repetitions. Whereas the transverse abdominus, which is a postural/stabilizing muscle with a higher percentage of type I fibers (maximally fatigue-resistant), require a low-force high endurance training.
Why are muscle-tendon junctions more susceptible to strain injuries in children?
The muscle-tendon junctions are the most rapidly growing area of the muscle fiber during postnatal life.
Smooth muscle of the gut tube is formed by mesenchymal cells of which layer of what mesoderm?
Smooth muscle of the gut tube is formed by mesenchymal cells of the visceral layer of lateral plate mesoderm.
How are smooth muscle fibers physically arranged? What keeps them in this arrangement?
Smooth muscle fibers are arranged in a staggered fashion to achieve close packing. Actin filaments bind to dense bodies which are attached to the inner surface of the sarcolemma and randomly dispersed throughout the sarcoplasm. Dense bodies act as the "Z bands" of skeletal muscle fibers to anchor the muscle fibers in place.
What is the molecule that is released from post-synaptic sympathetic nerve fibers to cause contraction of smooth muscle? What is present in smooth muscle fibers that enables the rapid spreading of the action potential induced by this molecule?
Varicosities (permanently dilated little blood vessels) along the post-synaptic sympathetic fibers release norepinephrine which binds to the sarcolemma of adjacent smooth muscle fibers. Gap junctions (tiny intercellular pores) between the muscle fibers enable rapid spreading of norepinephrine-induced action potentials, allowing for synchrous contraction of groups of smooth muscle fibers.
How are smooth muscle fiber hyperplasia and hypertrophy distinguished, and how do they contribute to the pathogenesis of hypertension?
Hyperplasia is increased proliferation of cells, so there is a greater number of cells.
Hypertrophy is increased size of cells, so there is a normal amount of abnormally large cells.
Both hyperplasia and hypertophy of smooth muscle cells results in the overall thickening of the tunica media of blood vessels, which increases the pressure within those blood vessels (hypertension).