Week 7 – Muscles Flashcards
Compare and contrast the different types of muscle tissue (i.e., cardiac, skeletal, and
smooth)
Skeletal muscle
- Location: Attached to bones
- Appearance: Striated (Striped)
- Gap junctions?: No gap junctions
- Ca2+ binding: Troponin
- Initiation of contraction: Signal from alpha motor neuron
- Neural control: Somatic motor neuron necessary for contraction
Cardiac muscle
- Location: In heart
- Appearance: Striated
- Gap junctions?: Gap junctions
- Ca2+ binding: Troponin
- Initiation of contraction: Built-in
pacemaker; autorhythmic
- Neural control: Autonomic neurons influence speed & strength but not
necessary for contraction
Smooth muscle
- Location: In hollow internal organs
- Appearance: NOT striated
- Gap junctions?: Gap junctions in most
- Ca2+ binding: Calmodulin
- Initiation of contraction: Can be autorhythmic
- Neural control: Autonomic neuron influence but not necessary
What are graded contractions?
a. Muscles can have graded contractions depending on how many motor units are activated.
- Graded contractions: variation in contraction strength due to the varying number of fibers stimulated (individual muscle fibers are all or none; muscle as a whole is graded.
What are motor units?
ii. Motor unit: somatic motor neuron and all of the muscle fibers it innervates
1. each muscle cell receives input from a single axon terminal from a somatic motor neuron
2. each somatic motor neuron can innervate a number of muscle fibers
3. motor units vary in neuron to muscle fiber ratio
a. fine neural control many small motor units
i. extraocular muscles that position eye has motor units with 1 neuron per 23 muscle fibers; or muscles in hands
b. powerful contractions larger motor units
i. e.g. gastrocnemius muscle of leg has motor units with one neuron per thousands
What is Recruitment?
Increased amount of motor units involved in contraction; More motor units are activated to increase the amount of force
The structure of a Skeletal muscle, both at the cellular and molecular level
Structure of a Skeletal Muscle Cell (muscle fiber):
sarcolemma –plasma membrane of a muscle cell
- motor end plate –specialized region of the sarcolemma at the neuromuscular junction; contains nicotinic ACh receptors
- transverse tubules (T tubules) –narrow membranous tunnels formed from the sarcolemma; action potential travels along T tubules using VG sodium and VG potassium channels; contain voltage-gated Ca2+channels near the sarcoplasmic reticulum
sarcoplasm –cytoplasm of muscle cell
- multinucleated –multiple nuclei per cell. This is because each muscle fiber
originates from fusion of multiple cells during development to make one very long cell that extends the length of the muscle.
o mitochondria: Changes nutrients into ATP molecules & stores them as energy
o sarcoplasmic reticulum (SR)–modified endoplasmic reticulum of muscle cell; stores Ca2+
SR stores Ca2+ when muscle relaxed; has Ca2+ release channels, which are
physically coupled to the VG Ca2+ channels in the T- tubules and release Ca2+when stimulated
- contains numerous myofibrils (myofibrils –protein fibers consisting of thick filaments = myosin and thin filaments = actin), which form sarcomeres
striated (stripes) appearance due to alternating dark A bands and light I bands
What is Muscle anatomy?
Gross Anatomy:
Muscles are composed of bundles of muscle fibers (muscle fiber = muscle cell = myofiber), connective tissue, blood vessels and nerves
What is the Response pathway involved in voluntary control of a Skeletal muscle?
Voluntary control:
Stimulus: sensory input or at will
(Afferent path: sensory neuron if sensory input is involved; none if at will)
Integration:
o various brain parts (e.g. prefrontal cortex for decisions made at will; other functional will also be involved if sensory input was the stimulus)
o then upper motor neurons of primary motor cortex (fire if decision to move is made) and their axons decussate (cross over) in medulla oblongata) and synapse with cell body of lower somatic motor neuron in spinal cord
Efferent path: somatic motor neuron (which fires and synapses with skeletal muscle)
Effector: skeletal muscle
Response: muscle contractions to cause movement
What are the different muscle fiber types?
Slow oxidative:
- Slower contraction
- Fatigue more slowly because: Lots of mitochondria, lots of myoglobin
(appear red), lots of capillaries, so mostly aerobic; thereofer, produce lots of ATP
Fast glycolytic
- Faster contraction
- Fatigue more quickly because: Few mitochondria, no myoglobin, few capillaries, so mostly anaerobic; therefore, produce less ATP
Fast oxidative (intermediate)
Composition of muscles varies from one skeletal muscle to another (.e.g eye and hand = mostly fast twitch and no slow twitch fibers; many back
and calf muscles = mostly slow twitch (maintain posture over long periods); -most muscles mixtures of types; varies from person to person depending in part on genetics and in part on conditioning (ie. exercise/training)
What changes occur in Skeletal muscles with Aerobic Exercise & Endurance Sports (walking, jogging, long-distance running) ?
Changes within muscle:
- Promotes angiogenesis (making new blood vessels) within the active skeletal muscle
- Increase number of mitochondria and myoglobin
- Endurance activities do not typically increase muscle size (e.g. marathon runners are usually toned and lean, but not bulky)
Other changes in the body:
- Increase heart muscle strength and cardiovascular health
What changes occur in Skeletal muscles with Resistance training (weightlifting) ?
Increases muscle size, mass and strength by hypertrophy (increase in size of individual fibers)
DOES NOT increase number of muscle fibers
How do Sarcomeres work?
Sarcomere: functional unit and smallest component of skeletal muscle capable of contraction; arranged end-to-end down the entire length of the muscle and stacked in parallel to each other; made up of the contractile proteins actin (thin filament) and myosin (thick filaments), and various other proteins, such as the regulatory proteins troponin and tropmyosin
A band: length of myosin; dark band due to overlapping of thin and thick filaments; A bands do not shorten during contraction, but move closer together
I band: light band with only thin filaments; I bands (distance between A bands) decrease in length during contraction
Z disc: thin dark line at center of each I band; connects two sarcomere
Contractile proteins
- myosin (thick filaments)
have a head and a tail
myosin tails intertwined
myosin heads project out and can form cross-bridge
Myosin head has Actin binding site
ATPase activity–binds and hydrolyzes ATP into
ADP and inorganic phosphate (Pi); hydrolyzing
ATP causes myosin head to move into high energy, “cocked” position
- Actin (thin filaments) –protein polymer that forms two strands twisted together
has myosin binding site
Regulatory proteins
- tropomyosin: protein that lines in grove of actin spiral and covers myosin binding site on myosin to block interaction of actin-myosin when muscle at rest
- troponin: Ca2+ binding protein; when Ca2+ binds troponin, troponin causes tropomyosin to shift and uncover myosin binding site to allow cross-bridge formation
How does Muscle fiber diameter (thickeness) affect muscle strength ?
Causes stronger muscle contractions due to thicker muscle fibers making more myofibrils/sarcomeres (Actin + Myosin filaments), which increases the entire muscle size also
Describe the Sliding Filament Model of Muscle Contraction
thick and thin filaments DO NOT shorten in length
instead, thick and thin filaments slide past each other , shortening the sarcomere, and thus the myofibril, and thus, the individual muscle fibers
- A bands don’t shorten but move closer together
- I bands (distance between A bands) decrease in length
How does molecular activity at the neuromuscular junction lead to muscle contraction?
Which neurotransmitter is involved? How does it work?
Excitation-Coupled Contraction
1. Action potentials in axon of somatic motor neuron
2. Acetylcholine (ACh) released into neuromuscular junction
3. ACh binds nicotinic ACh receptor on motor end plate of muscle fiber
4. Depolarization; At threshold, AP occurs in muscle fiber
5. VG (voltage-gated) Na+ channels and VG K+ channels open along the sarcolemma, so the
AP travels down sarcolemma into T-tubules
6. Opens VG Ca2+channels in sarcolemma, which are mechanically coupled to the Ca2+
release channels of the SR and Ca 2+ release channels open
7. Ca2+ enters the sarcoplasm and binds to troponin
8. Troponin changes shape and shifts tropomyosin out of the way
9. Myosin binding site on actin is exposed
10. Myosin binds actin forming crossbridge
a. Crossbridge: myosin bound to actin
Muscle shortens & contraction occurs
