4: Muscular & Musculoskeletal System (+ Imaging Modalities)

muscle tissue

able to contract + produce mechanical force

• made of myocytes (muscle cells/fibers)

  • electrically excitable → conduct propagating electrical signals (action potentials)

  • creates tension force

• 3 types:

  • skeletal

  • cardiac

  • smooth

skeletal muscle

muscles moving skeleton

• long, cylindrical cells arranged in parallel

• multinucleate,

• striated appearance

• innervated by somatic motor neurons

• uses ATP to generate tension force

• has connective tissue packaging

  • provides blood to muscle fibers for O2 + nutrients

cardiac muscle

muscles pumping blood through heart

• short, branched fibers lined up end-to-end via intercalated discs (specialized cell junctions)

• mononucleated

• striated appearance

• innervated by autonomic (visceral) motor neurons

smooth muscle

muscles in blood vessels, digestive tract, etc.

• spindle-shaped cells → thick in middle, tapered at ends

• mononucleated

• no striations

• innervated by autonomic (visceral) motor neurons

epimysium

dense irregular connective tissue

• surrounds entire muscle organ

• connects to tendon

perimysium

dense irregular connective tissue

• surrounds fascicle (bundle of muscle fibers

endomysium

loose/areolar connective tissue

• surrounds each muscle fiber/cell

sarcolemma

plasma membrane of myofiber/myocyte

transverse tubules (T-tubules)

invaginations of sarcolemma that extend into interior of myocyte → creates tunnels filled w/ ECF

• at right angle to cell surface

• connects sarcolemma to myofibril

• brings in sarcolemma AP into vicinity of sarcoplasmic reticulum → triggers Ca²⁺ release

sarcoplasm

cytoplasm of myofiber/myocyte

• contains specialized cytoskeletal + membranous organelles

  • glycogen

    • chains of glucose → broken down to release glucose → used to make ATP

  • myoglobin

    • protein only in myocytes → binds O2 + releases O2 when needed by mitochondria

  • creatine phosphate

    • transfer ADP to reconstitute ATP when additional ATP needed for myocyte contraction

myofibers (myocytes)

muscle fibers/cells

• contains:

  • myofibrils

  • myosatellite cells

  • many superficial nuclei

  • T-tubules

  • sarcoplasmic reticulum

• stretches from muscle origin to insertion → tendon to tendon

• associated w/ support cells + myofibers

• produced by fusion of many myoblasts

myofibrils

specialized organelle found within myofiber

• contractile protein fibers arranged in parallel that extend along length of myocyte → allow muscle contraction

• made of overlapping thin + thick filaments (actin + myosin)

sarcoplasmic reticulum

endoplasmic reticulum of myofibers

• specialized smooth ER

• wraps around myofibril

• stores + releases Ca²⁺

• ends = terminal cisternae (closely associated w/ T-tubule)

• 2 terminal cisternae + T-tubule = triad

triad

2 terminal cisternae + T-tubule

• T-tubule (yellow internal structures)

  • tubes bringing in sarcolemma

• cisternae (blue internal structures)

  • enlargement of sarcoplasmic reticulum

thick myofilament

composed of myosin protein

• double thickness of thin filaments

• myosin heads project to ends of myofilament

• myosin tails attached at middle

• contains titin → tethers filament to Z-line + allows muscle contraction/retraction

myosin

has mobile head + neck that can bind actin

• hinge → moves head

• free head → binds to actin

  • head = ATPases

    • cleave ATP = ADP + P_i energy → used to form cross-bridge b/w myosin head + actin

thin myofilaments

twisted strands of actin, troponin + tropomyosin

• actin contains myosin-binding sites

  • at rest = blocked by tropomyosin

• troponin holds tropomyosin in place → contains Ca²⁺ binding sites

sarcomere

functional, repeating units of myofibrils

• made of overlapping thick + thin myofilaments

• line up end-to-end along length of myofibril

• myofibrils within myocyte arranged so that they align w/ e/o = striated appearance in muscle!

H band

sarcomere region w/ only thick filaments

• decreases in size when sarcomere contracts

A band

overlapping sarcomere region of thick + thin filaments

• darker band

• myosin w/ some overlapping actin

I band

sarcomere region w/ only thin filament

• lighter band

• includes actin of next filament

M line

thick band of thick filaments in center of sarcomere

Z disc

end part of sarcomeres w/ proteins that anchor + hold actin-myofilaments in place

• dark line in middle of I band

• only actin

tropomyosin-troponin complex

wraps around actin helix

• when bound w/ Ca²⁺ reveals myosin-binding site on actin

sliding filament mechanism

• myocyte contraction → myosin heads of thick myofilaments bind thin myofilament

• myosin heads pull thin myofilaments toward M line (sarcomere center)

• ⬆overlap = ⬇ sarcomere length (shortens)

  • all sarcomeres of all myofibrils shorten at same time = entire myocyte shortens

muscle excitation

sarcolemma generates electrical signal that precedes contraction

1. electrochemical signal from somatic motor neuron

   1. motor neurons release NTs into synaptic cleft

2. electrical excitation of NMJ

   1. NTs bind to ligand-gated ion channels on sarcolemma

   2. Na+ flows into myofiber → charges move across sarcolemma = electrical signal

3. propagation of electrical signal

   1. sarcolemma depolarization from Na+ influx opens nearby VGNCs

   2. VGNCs along sarcolemma open as more VGNCs open = AP!

4. calcium release

   1. T-tubules bring sarcolemma AP to sarcoplasmic reticulum

   2. AP triggers release of Ca2+ from sarcoplasmic reticulum

5. Ca2+ presence → binds to troponin → pushes tropomyosin aside = exposes myosin-binding sites on actin = muscle contraction

neuromuscular junction (NMJ)

point of connection b/w motor neuron + myofiber

• made up of:

  • axon terminal (of motor neuron)

  • synaptic cleft (in b/w)

  • motor end plate (of myofiber)

motor end plate

specialized region (midpoint) of myofiber that forms NMJ

• has slightly different proteins + integral membrane proteins

• has extra folds to ⬆ SA for communication

muscle contraction

myofibers + myofibrils becoming shorter = tension

• begins w/ Ca²⁺ release after excitation

• continues as long as Ca²⁺ remains elevated

• myosin + actin move relative to e/o

0. calcium release → active site exposed

1. binding

2. power stroke

3. ADP release

4. ATP binding

5. ATP hydrolysis

contraction cycle step 0 (calcium release)

1. Ca²⁺ released from SR

2. Ca²⁺ binds to tropomyosin-troponin complex → releases from actin

3. myosin-binding site on actin exposed

contraction cycle step 1 (binding)

1. myosin binds to active site on actin

2. cross-bridge formed b/w myosin + actin

contraction cycle step 2 (power stroke)

1. myosin neck goes from straight to pivoted

   1. myosin attached to actin → head pivots = actin filament moved

   2. filaments slide past e/o

contraction cycle step 3 (ADP release)

1. ADP + phosphate group on myosin head released

   1. ATP = energy → release of energy off myosin head

contraction cycle step 4 (ATP binding)

1. new ATP molecule binds to myosin head

2. cross-bridge b/w actin + myosin breaks

contraction cycle step 5 (ATP hydrolysis)

1. ATP hydrolyzes into ADP + phosphate → energy released

2. myosin head goes back into original position → no longer pivoted

radiography (x-ray)

form of electromagnetic radiation that can pass through most objects, ie. body

• x-rays pass through object → contact photographic film/image recording plate = image produced

• images reflect differences in tissue density

  • bones + dense structures absorb x-rays = white

    • x-rays do not pass through

  • hollow/less dense structures pass through x-rays = black

    • x-rays contact photographic film/image recording plate

• superimposed 2D image created

computed tomography (CT)

• collected w/ SPECT data → images co-register together = more accurately localize site of uptake of injected radioactive tracer

• tomography = view anatomical section/slice through body

• x-ray source positioned opposite to detector

  • apparatus rotated around central axis of subject

  • data converted into image sections

• advantages over x-rays:

  • no superimposition of tissues

  • improved contrast differences b/w tissues

• imaging reflect tissue density:

  • ⬆ density = white

    • bone cortex

  • ⬇ density = black

    • fat/air filled structures, ie. lungs

  • gray structures = muscle, bone marrow

magnetic resonance imaging (MRI)

• body tissues have different composition of H2O + fat

• nuclei of hydrogen atoms (protons) in H2O + fat molecules = tiny magnets → precess (spin like top) at specific frequency

• subject in magnetic field → protons align w/ magentic field

  • precess in LOW energy (majority) or HIGH energy configuration

• aligned → precessing protons subjected to radiofrequency (RF) energy pulse

  • protons gain energy → flip to high energy

• RF pulse removed → protons revert to initial configuration + emit energy that can be detected

  • data converted to MR image

• image = energy emitted by protons in different tissues when placed into magnetic field + subjected to RF energy pulses

• T1 - look at structure + anatomy

  • fat = bright/white

  • water = dark/black

• T2 - find disease, fluid, swelling

  • fat = bright/white

  • water = bright/white

single-photon emission computed tomography (SPECT)

• collected w/ CT data → images co-register together = more accurately localize site of uptake of injected radioactive tracer

• radioactive tracer injected → taken up into targeted tissues

  • emission of gamma rays detected

  • data reconstructed to form image

• advantage

  • provides functional info, ex. nutrient + blood flow

• disadvantage

  • difficult to determine exact anatomical location

upper extremity bones

• pectoral (shoulder) girdle

  • clavicle

  • scapula

    • sternal end = medial

    • acromial end = lateral

• arm

  • humerus

• forearm

  • radius

  • ulna

• wrist + hand

  • carpals

  • metacarpals

  • phalanges

coracoid process

(1)

acromion

(2)

supraspinous fossa

(3) above spine + shallow depression

spine of scapula

(4)

infraspinous fossa

(5) below spine + shallow depression

glenoid cavity (glenoid fossa)

(6) articulates w/ head of humerus

subscapular fossa

(7)

glenoid process

(8)

shoulder joints

• sternoclavicular = plane

  • sternum + medial clavicle

• acromioclavicular = plane

  • acromion + lateral clavicle

• glenohumeral = ball + socket

  • glenoid fossa/cavity + humerus

pectoral girdle relational anatomy

• acromion + acromial end of clavicle

  • superior to

    • humerus head

    • glenoid process

    • coracoid process

    • sternal end of clavicle

• coracoid process

  • anterior to

    • humerus head

• humerus head

  • lateral to

    • glenoid process

humerus

(1)

glenoid process

(2)

coracoid process

(3)

clavicle

(4)

body of scapula

(5)

spine of scapula

(6)

coracoid process

(1)

subscapularis

(2)

clavicle

(3)

supraspinatus

(4)

acromion

(5)

infraspinatus

(6)

glenoid fossa

(7)

teres minor

(8)

acromion

(1)

clavicle

(2)

trapezius

(3)

supraspinatus

(4)

glenoid process

(5)

glenohumeral joint

(6)

scapula position

• posterior to ribs II-VII

• anterior surface closely associated w/ ribs

  • subscapular fossa in contact w/ ribs

• shoulder girdle (scapula + clavicle) coordinates shoulder movements

  • humeral head stays in glenoid fossa during arm movement

scapula movements

muscles insert onto scapula + work tgt to adjust scapula position = large range of motion

• elevation + depression = shrug up + down

• protraction (abduction) + retraction (adduction) = arm in + out front

• upward rotation + downward rotation = raise arm up + down at side

scapula muscles

• trapezius

• levator scapulae

• rhomboid minor

• rhomboid major

• pectoralis minor

• serratus anterior

trapezius

muscle acting on scapula

• origin:

  • vertebrae column

• insertion:

  • scapula

• superficial

• movement:

  • elevate → upper fibers

  • retract → middle fibers

  • depress → lower fibers

levator scapulae

muscle acting on scapula

• origin:

  • vertebrae column

• insertion:

  • scapula

• deep

• movement:

  • elevate

rhomboid minor

muscle acting on scapula

• origin:

  • vertebrae column

• insertion:

  • scapula

• deep

• movement:

  • retract

rhomboid major

muscle acting on scapula

• origin:

  • vertebrae column

• insertion:

  • scapula

• deep

• movement:

  • retract

pectoralis minor

muscle acting on scapula

• origin:

  • ribs 3-5

• insertion:

  • coracoid process

• movement:

  • depresses scapula

serratus anterior

muscle acting on scapula

• origin:

  • ribs 1-8

• insertion:

  • medial border of scapula

• movement:

  • protracts

  • keeps scapula against thoracic wall

acromioclavicular joint

(7)

clavicle

(1)

supraspinatus

(2)

acromion

(3)

trapezius

(4)

scapula

(5)

levator scapulae

(6)

lung

(7)

rhomboid minor

(1)

rhomboid major

(2)

infraspinatus

(3)

teres minor

(4)

body of scapula

(5)

humeral shaft

(6)

lung

(7)

rotator cuff muscles

3 originate on posterior scapula + insert on greater tubercle of humerus

• supraspinatus

• infraspinatus

• teres minor

• function:

  • dynamic stabilization of GH joint

    • keep humeral head in glenoid fossa during shoulder motion

    • each rotator cuff has own additional function

1 originates on anterior scapula + insert on less tubercle of humerus

• subscapularis

• function:

  • dynamic stabilization of GH joint

supraspinatus

• origin

  • supraspinous fossa

• insertion

  • greater tubercle of humerus

    • outer, lateral side of upper humerus

infraspinatus

• origin

  • infraspinous fossa

• insertion

  • greater tubercle of humerus

    • outer, lateral side of upper humerus

teres minor

• origin

  • lateral border of scapula

• insertion

  • greater tubercle of humerus

    • outer, lateral side of upper humerus

subscapularis

• origin

  • subscapular fossa

• insertion

  • lesser tubercle of humerus

    • front, inner/medial side of upper humerus

humeral head

(1)