Physiology - MSK Flashcards
Skeletal Muscle Contraction
thin filaments move toward each other
- less distance between Z-lines = sarcomere shortening
- H band disappears (it is a dynamic area)
- A band always remains the same (it is a set length)
1. ATP attaches to myosin head, dislodging it from actin.
2. myosin hydrolyzes ATP to ADP, “cocking” its head into a high energy position.
3. Ca comes in, binds to Troponin C.
4. Ca-Trop C causes change in tropomyosin configuration.
5. Altered tropomyosin reveals actin binding site.
6. myosin-ADP binds to actin.
7. phosphorus leaves, changing the configuration of mysoin head = POWER STROKE. This slides the actin filaments toward each other.
Slow Twitch Type I Skeletal Muscle Fiber
-long contraction-relaxation cycle ~100ms
strong, gross, posture-maintaining, sustained movements
-“Red Muscle” contains high density of Type I fibers “Dark meat” (thigh and leg muscles have sustained action, like turkey walking around), highly oxidative metabolism (more aerobic), more mitochondria/capillaries/myoglobin– oxygen binding pigment found in the respiratory chain enzyme cytochrome c
-erector spinae, gastrocnemius
Fast Twitch Type II Skeletal Muscle Fiber
- quick contraction-relaxation cycle ~8ms
- fine, rapid, precise movements
- “White Muscle” contains high density of Type II fibers, “White meat”, highly glycolytic metabolism (more anaerobic), less mito/capillaries/myoglobin
- extraoocular muscles
2 types of contraction
isometric
- same length
- shortened contractile elements, but no muscle shortening
- “tensing the muscle before motion begins”
isotonic
- same load
- shortened contractile elements leads to shortened muscle
- muscle shortens to do work against a load
concentric movement
- isotonic contraction occuring as muscle shortens
- less force needed = less injury possibilities
- lifting an object against gravity
eccentric movement
- isotonic contraction occuring as a muscle lengthens
- more force needed = more injury possibilities
- trying to stop slipping spread leg hip abduction on ice -> “groin pull”
denervation
- loss of motor nerve input affects muscle function
- due to any nerve injury (trauma, compression, radiculopathy…)
- weakness, typically distal and in the distribution of a nerve
- muscle bx shows atrophy
- normal or slightly increased serum creatinine kinase (CK)
- EMG shows increased muscle excitability as fibrillations (invisible, spontaneous, fine, irregular fiber contractions)
- typically has associated sensory neuropathy: numbness, paresthesias – abnl sensation (pins and needles, walking on marbles), dysesthesias – painful abnl sensation (heat, ice pick), altered proprioception – joint position sense decreased = poor balance
- Ex = hand weakness due to atrophic abductor pollicis brevis: aka Carpal Tunnel Syndrome
myopathy
-pathology is in the muscle itself, and not due to an abnormality in the supplying motor neurons
-due to muscle proper abnormality (typically genetic, inflammatory or autoimmune)
weakness, typically proximal
-possible associated pain (‘myalgia’, typically d/t myositis)
-markedly elevated CK
-muscle bx shows fibrosed and/or degenerative fibers, and variable-sized fibers
-Ex = leg weakness due to Duchenne muscular dystrophy: sex-linked recessive abnormality of the dystrophin protein gene
Cardiac Muscle Morphology
- contains myosin, actin, troponin, tropomyosin
- striated
- extensive folds in the plasma membranes of abutting cardiomyocyctes tie them together. These areas are called intercalated disks, which always occurs at Z lines.
- gap junctions allow for easy spread of excitation
- Length-Tension Relationship: there is a resting length at which the tension developed upon stimulation is maximal, correlates initial length to tension force generated
- Starling’s Law: ventricular pressure is proportionate to the total tension developed, which is determined by diastolic filling and the initial length of the fibers.
Smooth Muscle Morphology
- nonstriated
- no Z lines
- actin and myosin are not arranged in regular arrays, and are of a different isoform than those in skeletal muscle
- troponin absent
- contains tropomysin
- poorly developed sarcoplasmic reticulum
- fewer mitochondria
- will contract independent of nerve impulse: it contracts when stretched, spontaneous activity in the absence of nervous stimulation
Smooth Muscle Types
- visceral: large sheets of muscle functioning as a whole thru many gap junctions, mainly found in hollow visceral walls where gross action is needed: urinary bladder
- multi-unit: no interconnecting gap junctions, doesn’t act “as a whole”, for fine graded contractions: iris
