Skeletal System Physiology Flashcards
Muscle Morphology
- Tendon -> muscle -> tendon
- Muscle fiber = single myocyte surrounded by a membrane (sarcolemma)
- Myofibrils = individual filaments within a fiber (separated by sarcoplasmic reticulum)
- Filament (thick & thin) = collection of contractile proteins
Contractile Proteins
Myosin
Actin
Tropomyosin
Troponin (3 subunits: I, T, and C)
Muscle Filaments
“Striated”- regular position of filaments gives a striped appearance
- Thick filament: myosin proteins = H band
- Thin filament: actin, troponin, & tropomysin = I band
- Thin-thick overlap = A band (constant)
*Sarcomere = functional unit (Z to Z line)
Process of Muscle Contraction
- ATP attaches to myosin head, dislodging it from actin
- Myosin hydrolyzes ATP to ADP, “cocking” its head into a high energy position
- Ca2+ enters 7 binds to troponin C
- Ca2+ & troponin C complex form tropomyosin
- Altered tropomyosin reveals actin binding site
- Myosin-ADP complex binds to actin
- Phosphorus leaves, changing configuration of the myosin head (= power stroke) -> slides actin filaments toward each other
Slow Twitch (Type I) Skeletal Muscle Fibers
- Long contraction-relaxation cycle (100 ms)
- Strong, gross, posture-maintaining, sustained movements
- “Red muscle”-contains high density of type 1 fibers
`“Dark meat” - Highly oxidative metabolism (aerobic), more mitochondria, capillaries, & myoglobin
EX. Erector spinae, gastrocnemius
Fast Twitch (Type II) Skeletal Muscle Fibers
- Quick contraction-relaxation cycle (8 ms)
- Fine, rapid, precise movements
- “White muscle”-contains high density of type 2 fibers
`“White meat” - Highly glycolytic metabolism (anaerobic), less mitochondria, capillaries, & myoglobin
EX. Extraocular muscles
Types of Contraction
- Isometric
2. Isotonic
Isometric Contraction
- No change in muscle length
- Shortened contractile elements, but no muscle shortening
- “Tensing the muscle before motion begins”
Isotonic Contraction
- Same load bilaterally
- Shortened contractile elements leads to shortened muscle
- Shortened muscle works against a load
Concentric Movement
Isotonic contraction occurring as muscle shortens
- less force needed = less injury possibilities
Ex. lifting an object against gravity
Eccentric Movement
Isotonic contraction occurring as a muscle lengthens
- more force needed = more injury possibilities
Ex. trying to stop slipping on ice by spreading legs -> hip abduction -> “groin pull”
Skeletal Muscle Metabolism
- Rest & light exercise: free fatty acids
- Increased exercise intensity: glucose
free glucose in bloodstreamliver & skeletal glycogen stores - Glycolysis
aerobic: citric acid cycleanaerobic: tricarboxylic acid cycle (makes lactic acid)
Muscle Disorder Basics
Denervation: loss of motor nerve input
- d/t nerve injury
- leads to distal weakness & muscle atrophy
- sxs: sensory neuropathy (numbness, paresthesias, dysesthesias)
- Ex. carpal tunnel syndrome
Myopathy: pathology of the muscle, not due to the supplying motor neurons
- leads to proximal weakness & fibrosed fibers
- sxs: myalgias
- Ex. muscular dystrophy
Alpha-Motor Neurons
- Attach to & activate muscles*
- reside in the ventral spinal cord
- send signals via ventral root -> aMNs
Muscle Spindles
~10 muscle fibers enclosed in a connective tissue capsule (= intrafusal fibers)
- 2 types: nuclear bag & nuclear chain fibers
- End spindle capsules are continuous w/ the muscle’s tendon
- Sensing nerves = primary Ia afferents
- Motor neurons = y (gamma) motor efferents
- Function: regulate muscle length
Primary Ia Afferent Nerves
- sensory nerves in muscle spindles*
- transmit afferent activity from the intrafusal fibers -> spinal cord -> a-MN
- a-MN then sends efferent activity down through the neuromuscular junction -> extrafusal fibers (contract)
= monosynaptic reflex arc
Muscle Tone
= resistance of muscle to stretch
Hypotonic–decreased resistance to muscle stretch or flaccid muscle
Hypertonic/spastic–increased resistance to muscle stretch
Inverse Muscle Stretch Reflex
Too much muscle tension -> sudden contraction cessation to prevent injury
- 2 synaptic reflex arc
- negative feedback loop (autogenic inhibition)
- sensing structure = golgi tendon organ
Golgi Tendon Organ
= Network of nerve endings in the tendons
- function: regulate muscle force
- sensing nerves = primary Ib afferents
- synapse w/ interneurons on a-MNs to decrease their activity (stop contraction)
Ex. Sudden stop in contraction while lifting empty gallon of milk presumed to be full
Withdrawal Reflex
- Polysynaptic reflex arc
- Peripheral noxious stimuli (pain or temperature) sensory organs send afferent information to the spinal cord & synapses on a-MNs
activates ipsilateral flexorsinhibits ipsilateral extensors
`activates contralateral extensors
Neuropraxia
Temporary damage to Schwann’s cells (myelin)–“my leg fell asleep”
- demyelination
- best prognosis, probable full recovery
Axonotmesis
Injured axon +/- injury to the myelin, endoneurium, and/or perineurium
Neurotmesis
Injury to all neuronal layers
- Nerve transection
- Worst prognosis, usually partial to no recovery
