MCAT Biology - The Musculoskeletal System

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rhabdomyolysis

Extreme physical trauma to muscles, namely, compression, destroys skeletal muscle tissue; products of skeletal muscle destruction, some of which are toxic, circulate in the blood until they are filtered out

(rhabdo– “striation”, myo– “muscle”, –lysis “breakdown”)

Skeletal muscle

essential for supporting the body and facilitating movement; responsible for voluntary movement and innervated by the somatic nervous system; striated; multinucleated, formed as individual muscle cells fuse into long rods during development

striated

striped

Red fibers / slow-twitch fibers

occur predominantly in muscles of support; high myoglobin content and primarily derive their energy aerobically; contain many mitochondria to carry out oxidative phosphorylation

Myoglobin

oxygen carrier that uses iron in a heme group to bind oxygen, imparting a red color; secondary supplemental energy reserve in muscle

White fibers / fast-twitch fibers

predominantly in muscles for short bursts of motion; contain less myoglobin;

Smooth muscle

responsible for involuntary action, controlled by the autonomic nervous system; respiratory tree, digestive tract, bladder, uterus, blood vessel walls, etc.; single nucleus located in the center of the cell; actin and myosin, but not as well organized; capable of more sustained contractions

tonus

constant state of low-level contraction, as may be seen in the blood vessels

myogenic activity

respond to nervous input, but do not require external signals to undergo contraction; exhibited in smooth and cardiac muscle

Cardiac muscle

involuntary and innervated by the autonomic nervous system; appears striated; primarily uninucleated, but cells may contain two nuclei

intercalated discs

microscopic identifying features of cardiac muscle; major portal for cardiac cell-to-cell communication via gap junctions for rapid and coordinated depolarization of muscle cells and efficient contraction of cardiac muscle

gap junctions

connections between the cytoplasm of adjacent cells, allowing for the flow of ions directly between cells

sinoatrial (SA) node

where depolarisation begins

atrioventricular (AV) node

depolarization spreads using conduction pathways here from the SA node

bundle of His

transmits the electrical impulses from the atrioventricular node to the point of the apex of the fascicular branches

Purkinje fibers

allow the heart's conduction system to create synchronized contractions of its ventricles

vagus nerve

parasympathetic outflow to the heart and slows the heart rate

Norepinephrine / epinephrine

binds to adrenergic receptors in the heart, causing an increased heart rate and greater contractility; increasing intracellular calcium levels within cardiac myocytes

sarcomeres

basic contractile unit of skeletal muscle; thick and thin filaments

thick filaments

organized bundles of myosin

myosin

motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes; ATP-dependent and responsible for actin-based motility

thin filaments

made of actin, troponin and tropoamyosin

actin

globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils

troponin

complex of three regulatory proteins that are integral to muscle contraction in skeletal muscle and cardiac muscle

tropomyosin

works in conjunction with troponin to regulate muscle contraction; present in smooth and striated muscle tissues

titin

acts as a spring and anchors the actin and myosin filaments together, preventing excessive stretching of the muscle

Z-lines

define the boundaries of each sarcomere (end of alphabet/sarcomere); distance between shrinks during contraction

M-line

runs down the center of the sarcomere, through the middle of the myosin filaments; distance between shrinks during contraction

I-band

the region containing exclusively thin filaments; shrinks during contraction

H-zone

region containing only thick filaments; shrinks during contraction

A-band

contains the thick filaments in their entirety, including any overlap with thin filaments; size remains constant during contraction

myofibrils

rod-like organelle of a muscle cell; arrangement of many sarcomeres in series; arranged in parallel in myocyte

sarcoplasmic reticulum (SR)

covering of myofibrils; modified endoplasmic reticulum that contains a high concentration of Ca²⁺ ions; tightly controls intracellular calcium concentrations so that muscles are contracted only when necessary; releases Ca2+ to contract; resorbs to relax

sarcoplasm

modified cytoplasm of myocytes; located just outside the sarcoplasmic reticulum

sarcolemma

cell membrane of a myocyte; capable of propagating an action potential and can distribute the action potential to all sarcomeres in a muscle using a system of transverse tubules

transverse tubules (T-tubules)

oriented perpendicularly to the myofibrils; distribute the action potential to all sarcomeres in a muscle

myocyte / muscle fiber

muscle cell; multinucleated at the periphery; exhibit an all-or-nothing response

neuromuscular junction,

where the nervous system communicates with muscles via motor (efferent) neurons and contraction begins

nerve terminal / synaptic bouton / motor end plate

end of (motor) neuron; releases acetylcholine into the synapse

motor unit

a nerve terminal and the myocytes it affects

Regulation of Contraction with Calcium

Calcium binds to troponin, leading to a conformational change in tropomyosin, which exposes the myosin-binding sites of actin

Shortening of the Sarcomere / sliding filament model

free globular heads of the myosin molecules move toward and bind with the exposed sites on actin and allow myosin to pull on actin, which draws the thin filaments toward the M-line

The Actin–Myosin Cross-Bridge Cycle

1. myosin carrying hydrolyzed ATP (ADP and an inorganic phosphate, Pi) is able to bind with the myosin-binding site.

2. The release of the inorganic phosphate and ADP in rapid succession provides the energy for the powerstroke to slide the actin filament over the myosin filament.

3. ATP binds to the myosin head, releasing it from actin.

4. ATP is hydrolyzed to ADP and Pi, which recocks the myosin head so that it is in position to initiate another cross-bridge cycle

acetylcholinesterase

degrades acetylcholine in the synapse, ending the signal at the neuromuscular junction; allows the sarcolemma to repolarize

rigor mortis

after death. myosin heads cannot detach from actin because ATP production ceases, making it impossible for muscles to relax and lengthen

simple twitch

response of a single muscle fiber to a brief stimulus at or above threshold; latent period, contraction period, and relaxation period

latent period

time between reaching threshold and the onset of contraction; action potential spreads along the muscle and allows for calcium to be released from the sarcoplasmic reticulum

frequency summation

muscle fiber is exposed to frequent and prolonged stimulation with insufficient time to relax; contractions will combine, become stronger and more prolonged

tetanus (symptom)

contractions become so frequent that the muscle is unable to relax at all; will result in muscle fatigue; caused by C. tetani infection but also normal multiple simple twitches

tetanus (disease)

caused by a bacterium called Clostridium tetani that releases the toxin tetanospasmin that blocks the release of GABA from the neurons that inhibit motor neurons, making the motor neurons overexcitable; leads to constant contraction of muscles (tetanus the symptom), which can be so strong as to fracture bones

can usually be prevented after exposure by administration of a tetanus immunoglobulin

Creatine phosphate

created by transferring a phosphate group from ATP to creatine during times of rest; first supplemental energy reserve in muscle

oxygen debt

difference between the amount of oxygen needed by the muscles and the actual amount present; when even red muscle fibers must switch to anaerobic metabolism and produce lactic acid; after exercise; body must metabolize all of the lactic acid it has produced, using oxygen to convert it to pyruvate

skeleton

the structural frame that supports the body of most animals

Exoskeletons

encase whole organisms in structural frame; protect the soft tissue structures; must be shed and regrown to accommodate growth

ex. arthropods, such as crustaceans and insects

endoskeletons

internal structural frame covered by other structures (muscle, connective tissue, and vasculature); not able to protect the soft tissue structures as well as exoskeletons; much better able to accommodate the growth of a larger organism

ex. Vertebrates, including humans

axial skeleton

consists of the skull, vertebral column, rib cage, and hyoid bone; provides the basic central framework for the body

hyoid bone

small bone in the anterior neck used for swallowing

appendicular skeleton

consists of the bones of the limbs, the pectoral girdle (scapula and clavicle); and pelvis

bones of the upper limbs

• humerus

• radius

• ulna

• carpals

• metacarpals

• phalanges

bones of the lower limbs

• femur

• tibia

• fibula

• tarsals

• metatarsals

• phalanges

Bone Composition

connective tissue derived from embryonic mesoderm; much harder than cartilage, but is relatively lightweight; vascular

long bones

characterized by cylindrical shafts

diaphyses

cylindrical shafts of long bones; full of marrow

metaphyses

point of swelling in a long bone; full of marrow

epiphyses

rounded terminal of long bone; spongy cores for more effective dispersion of force and pressure at the joints

compact bone

dense and strong; outermost portion of long bone

spongy / cancellous bone

lattice structure; consists of trabeculae; inner portion of long bone

bone marrow

fills cavities between trabeculae

Red marrow

filled with hematopoietic stem cells, which are responsible for the generation of all the cells in blood

yellow marrow

composed primarily of fat, relatively inactive

epiphyseal (growth) plate

cartilaginous structure at the internal edge of the epiphysis; site of longitudinal growth; filled with mitotic cells that contribute to growth from childhood to puberty

periosteum

fibrous sheath that surrounds the long bone to protect it as well as serve as a site for muscle attachment

Tendons

attach muscle to bone

ligaments

pieces of fibrous tissue that hold bones together at joints; consist of a synovial capsule, which encloses the actual joint cavity

bone matrix

organic matrix of protein, polysaccharides and minerals, secreted by osteoblasts, that becomes bone after mineralization; responsible for strength of compact bone

organic components of bone matrix

collagen, glycoproteins, and other peptides

inorganic components of bone matrix

hydroxyapatite crystals and minerals like sodium, magnesium, and potassium

hydroxyapatite crystals (Ca₁₀(PO₄)₆(OH)₂)

calcium, phosphate, and hydroxide ions, which harden together

osteons / Haversian systems

structural units of bone matrix

lamellae

concentric circles of bony matrix surrounding a central microscopic channel that contains the blood vessels, nerve fibers, and lymph vessels that maintain the health of the bone

Haversian canals

Longitudinal channels with an axis parallel to the bone

Volkmann’s canals

transverse channels with an axis perpendicular to the bone

lacunae

small spaces which house mature bone cells between the lamellar rings; interconnected by canaliculi

canaliculi

tiny channels that connect lacunae allow for the exchange of nutrients and wastes between osteocytes and the Haversian and Volkmann’s canals

Osteoblasts

cells that builds bone using essential ingredients such as calcium and phosphate obtained from the blood

osteoclasts,

polynucleated resident macrophages that resorb bone; release calcium and phosphate back into the bloodstream

Bone remodeling

occurs in response to stress in such a way as to accommodate the repetitive stresses faced by the body

Parathyroid hormone

peptide hormone released by the parathyroid glands in response to low blood calcium, promotes resorption of bone, increasing the concentration of calcium and phosphate in the blood

Vitamin D

activated by parathyroid hormone, also promotes the resorption of bone; encourages the growth of new, stronger bone

calcitonin

peptide hormone released by the parafollicular cells of the thyroid in response to high blood calcium, promotes bone formation, lowering blood calcium levels

Osteoporosis

most common bone disease in the United States; thought to be the result of increased osteoclast resorption and some concomitant slowing of bone formation, leading to loss of bone mass

Estrogen is believed to help prevent osteoporosis by stimulating osteoblast activity

Cartilage

softer and more flexible than bone; consists of chondrin; vascular

chondrin

firm but elastic matrix secrted by chondrocytes that makes up cartilage

chondrocytes

makes chondrin for cartilage

Fetal skeletons

mostly made up of cartilage; advantageous because fetuses must grow and develop in a confined environment and then must traverse the birth canal

Adult cartilage

only in body parts that need a little extra flexibility or cushioning

ex. external ear, nose, walls of the larynx and trachea, intervertebral discs, and joints

endochondral ossification

hardening of cartilage into bone; responsible for the formation of most of the long bones

intramembranous ossification

mesenchymal tissue is transformed into, and replaced by, bone

ex. skull

mesenchymal tissue

undifferentiated embryonic connective tissue

Immovable joints

Fixed joints between bones held together by dense, fibrous tissue

ex. skull