CHAPTER 48 ~ Musculoskeletal Systems Flashcards
3 types of vertebrate muscle:
skeletal, cardiac, and smooth
Skeletal muscle
“striated” cells are called muscle fibers; large, multinucleated cells formed by fusion of embrionic myoblasts. Each muscle fiber has several myofibrils.
- for mostly voluntary and some involuntary movement.
Myofibrils
bundles of actin (thin) and myosin (thick) filaments (contractile proteins)
- consist of repeating units called sarcomeres
sarcomeres
overlapping filaments of actin and myosin
motor unit
all the muscle fibers activated by one motor neuron
Sliding filament mechanism of muscle contraction:
ACh is released by the motor neuron at the neuromuscular junction, opens ion channels in the motor end plate, action potential travels into T-tubules, stimulates release of Ca2+ from sarcoplasmic reticulum, Ca2+ binds to troponin on actin filaments, conformational change pulls tropomyosin off of myosin binding sites on actin = myosin heads bind to actin w/ release of Pi = powerstroke, filaments slide past each other and ADP is released. ATP then binds to myosin, causing it to release actin. ATP is hydrolized, and myosin returns to it’s original position.
Cardiac Muscle
also striated, smaller than skeletal, uninucleated; cardiac cells branch and interdigitate = can withstand high pressures.
- arranged in sheets, communicate via gap junctions = allow cytoplasmic continuity.
pacemaker and conducting cells
initiate and coordinate heart contractions
heartbeat
heartbeat is myogenic = generated by the heart muscle itself, no input from nervous system, although autonomic nervous system CAN modify rate of pacemaker cells.
Smooth Muscle
uninucleated, smooth b/c actin and myosin not arranged so regularly; present in most internal organs (i.e. GI tract)
twitch
minimum unit of contraction in skeletal muscle; single action potential generates a single twitch
if action potentials are close together in time:
the twitches are summed, tension increases. Summed because Ca2+ pumps cannot clear Ca2+ from sarcoplasm before next action potential arrives
Tetanus
when action potentials are so frequent that there is a constant presence of Ca2+ in sarcoplasm –> can be maintained as long as there is ATP supply; requires actin and myosin “cycling” to maintain contraction
Muscles have 3 systems for obtaining ATP
immediate system, Glycolytic system, & Oxidative System
Immediate system
uses pre-formed ATP and creatine phosphate - muscles contain CP which stores energy in a phosphate bond that can transfer to ADP. ==> exhausted within seconds
Glycolytic system
metabolizes carbs to pyruvate and lactic acid, ATP available within a few seconds to replace immediate system, but low ATP yield, Anaerobic, and lactic acid accumulates.
Oxidative Phosphorylation
becomes active within about a minute; produces large amounts of ATP.
- Aerobic = takes place in mitochondria, so requires O2
- slower than two systems, but can last much longer
two types of skeletal muscle fibers:
slow-twitch and fast-twitch
Slow-Twitch muscle fibers
contain myoglobin = oxygen binding protein
- oxidative, or ‘red’ muscle
- contain many mitochondria and well supplied with blood vessels
- max tension develops slowly but is highly resistant to fatigue = endurance and aerobic activities
- slow-twitch fibers have fat and glycogen stores, so can produce ATP as long as O2 available
Fast-Twitch muscle fibers
- “glycolytic” or “white” muscle
- no myoglobin, few mitochondria and blood vessels
- develop max tension much faster, and max is higher than slow twitch, but fatigues VERY quickly = for sprinters
Anaerobic Exercise
increases muscle strength, new actin and myosin filaments form, muscle gets larger
Aerobic Activities
increase endurance, oxidative capacity is enhanced by increasing number of mitochondria, blood vessels, myoglobin, and enzymes.
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muchas besitos, morgis
