Musculoskeletal System

MCAT Musculoskeletal System

Types of Muscle

Skeletal Muscle: voluntary movement, innervated by somatic nervous system

·       Pattern of actin and myosin makes a sarcomere (basic functional unit) creates a striated look

·       Multinucleated

·       Red Fiber: Myoglobin → lots of mitochondria→ slow twitch fibers→ use oxidative phosphorylation to make ATP

·       White fibers: less myoglobin → fast twitch fibers

Smooth Muscle: involuntary motion innervated by autonomic nervous system

·       No striations because fibers are disorganized

·       Maintain low level of constant contraction like in blood vessels→tonus; peristalsis, period cramps, arrector pili muscles

Cardiac Muscle: found in heart, Characteristics of smooth and skeletal

·       Uni-nucleated

·       Autonomic but has striations

·       Contain intercalated discs and gap junctions to communicate ions between each other

o    Ga junctions release ions between heart nodes

·       Cardiomyocyte: heart muscle cell

o    Maintain coordinated rhythm

·       Vagus nerve from PNS slows HR

·       NorpEpi (SNS) or Epi (adrenal medulla) → act on adrenergic receptors of heart → ↑ HR by ↑ Ca2+

Muscular Microanatomy

·       Sarcomeres made of thick and thin filaments

o    Thick filaments: bundles of myosin

o    Thin filaments bundle of actin, troponin, and tropomyosin (act-thin)

o    Titin: acts as a spring and anchors actin/myosin to prevent excessive stretch

·       Sarcomere divided into different zones

o    Z: define boundaries of each sarcomere

§  M: line runs down center of sarcomere (through myosin)

o    I: contains only thin filaments

o    H: only thick filaments

o    A: thick filaments in entirety, including some thin filament overlap

·       Contraction of thin filament brings thick filaments together (→)

o    Contraction brings H, I, and distance between Z lines and distance between M lines smaller, but A is constant

·       Muscle fibers are myocytes, those myocytes have cell membrane called sarcolemma (cell membrane of skeletal muscles)

o    Sarcoplasmic Reticulum: modified ER that contains high Ca2+

o    Sarcoplasm: modified cytoplasm just outside of SR

·       Carries its own action potential (AP)

Muscle Fiber Movement

1.)    Nervous system communicates with muscles via motor (efferent) neurons and signal travels down nerve terminal

2.)    Never terminal releases Ach into neuromuscular junction (junction between nerve terminal of neuron and sarcolemma) and binds receptor on sarcolemma

3.)    Sarcolemma depolarizes

4.)    Depolarization carried to T tubules (transverse tubules)

5.)    T tubules transfer AP to sarcoplasmic reticulum → storage site of calcium for muscle contraction

6.)    Calcium released into myocyte

7.)    Calcium binds the calcium binding site of troponin

a.      This causes a conformational change where the tropomyosin spins out of the way of actin.

8.)    Exposes myosin binding groove/domain on actin

a.      Now we can have an actin-myosin interaction

9.)    Myosin (thick filament) pulls on actin (thin filament) and shortens it towards the M line

a.      Myosin has two conformations

                                                i.     Cocked: ADP and Pi sitting on myosin

                                               ii.     Uncocked: ADP and Pi released

10.)  Globular myosin heads bind to actin on myosin binding domains

11.)  Myosin performs a power stroke and shortens sarcomere → known as contraction → leads to ADP and Pi dissociation from myosin (ADP and Pi falls forward and shortens angle of myosin head/uncocks it, shortening sarcomere)

12.)  ADP and Pi are now gone (uncocked myosin)

13.)  New ATP binds myosin and hydrolyzes it, leads to release of myosin from actin and resets tropomyosin (hydrolysis of ATP breaks myosin off the actin and recocks it )

Rigor Mortis: no more new ATP, no ATP to hydrolyze myosin off of the actin → leads to permanent contraction

Relaxation

·       Acetylcholinesterase takes up Ach at the initial synapse and SR takes back up calcium

Stimulation, Summation and Muscle Fatigue

·       Muscle cells experience all or nothing response

·       Latent period: time reaching threshold and onset of contraction (time during which AP spreads along muscle and allows calcium to be release)

·       Frequency Summation: contractions combine and become stronger/more prolonger → leads to tetanus

Oxygen Debt and Muscle Fatigue

·       High amounts of ATP required for muscle contraction

·       Two supplemental energy reserves in muscle  

o    Creatine Phosphate created by transferring phosphate group from ATP and creatinine during times of rest; muscle reverse then when needed

o    Muscle also contains high affinity myoglobin; use myoglobin reserves to keep aerobic metabolism going

o    Fast Twitch White Fivers have fewer mitochondria and rely on glycolysis and fermentation to make ATP

o    However, Red Fibers can quickly get tired and have to switch to anaerobic metabolism and produce lactic acid → muscle fatigue

Oxygen Debt: difference between the amount of oxygen needed by muscles and amount of oxygen that’s present.

Skeletal System

·       Axial Skeletal System: skull, vertebral column, fib cage and hyoid bone

o    Hyoid: floating bone

·       Appendicular Skeletal: limb bone and pectoral girdle (scapula and clavicle) and pelvis

Types of Bone

·       Compact Bone: Dense and strong

·       Spongey (Cancellous) Bone: has cavities filled with bone marrow for erythropoiesis and hematopoiesis

o    Red bone marrow (hematopoiesis)

o    Yellow Bone Marrow (fat)

§  MCAT loves BBLs (connection Bone to Bone is done by Ligaments)

§  Connection of muscle to bone is tendon

Microscopic Bone Structure

o    Bone has organic components: collagen, glycoproteins, and other peptides

o    And inorganic components called hydroxyapatite → calcium, phosphate and hydroxide

o    Blood runs through bones through Volkmann’s canals; has own circulatory system

o    Lacunae houses osteocytes

o    Interconnected y tiny channels called canaliculi for exchange of nutrients and wastes

Bone Remodeling

§  Calcitonin: tones down serum calcium by building bone (activates osteoblasts to build bone)

o    Released by parafollicular cells of thyroid

§  Parathyroid Hormone: increase serum calcium (activates osteoclasts→ polynucleated resident macrophages of the bone that absorb it)

o    Vitamin D also activated by PTH and promotes bone resorption

Cartilage

o   Made up of chondrin by chondrocytes

o    Low vascular tissue; no perfusion

o    Bones are just hardened cartilage → endochondral ossification

Joints

§  Immovable Joints: put together by sutures (skull bones)

§  Synovia fluid: inside of joints

Muscle movement

§  Flexion: decreases angle of joint

§  Extension: increase angle

§  Abductors: move away from body

§  Adductors: bring to body (add to body)

§  Internal rotators: rotate towards midline

§  External rotators: rotates away from midline