KAAP309 Lecture Notes - Lecture 9: Skeletal Muscle, Sarcomere, Cardiac Muscle
18 May 2018
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Chapter 9A: Muscles and Muscle Tissue
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I. Types of Muscle Tissue
A. Skeletal
1. Striated
2. Voluntary
3. Attached to bone (& skin)
B. Cardiac
1. Striated
2. Heart only
3. Involuntary
C. Smooth
1. Involuntary
2. Walls of hollow organs
II. Muscle Characteristics
A. Excitability: ability to receive and respond to stimuli
B. Contractility: ability to shorten when stimulated
C. Elastiit: ailit to streth ad oue ak
III. Muscle Functions
A. Move bones and fluids (e.g., blood)
B. Maintain posture and body position (counteract gravity)
C. Stabilize joints
D. Generate heat
IV. Microscopic Anatomy of Skeletal Muscle Fiber
A. Cylindrical cell 10 to 100 m in diameter, up to 30 cm (!) long
B. Multiple peripheral nuclei; many mitochondria
C. Glycogen granules for energy storage, myoglobin for O2 transport & storage
D. Myofibrils (80% of volume, striated), sarcoplasmic reticulum, T tubules
E. Sarcomere
1. Smallest contractile unit (functional unit) of a muscle fiber
2. The region of a myofibril between two successive Z discs
3. Composed of thick and thin myofilaments made of contractile proteins
F. Sarcoplasmic Reticulum (SR)
1. Network of smooth ER around each myofibril
2. Terminal cisternae at each end
3. Stores & releases Ca2+ to regulate contraction
4. T tubules
5. Continuous with sarcolemma
6. Peetrate ell’s iterior
7. Associate with the paired terminal cisternae to form triads that encircle
each sarcomere
G. Triads
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1. T tubules conduct electrical impulses (action potentials) deep into
muscle fiber
2. Proteins embedded in membranes of T tubules & SR terminal cisternae
connect with one another in precise arrangement
3. T tubule proteins: voltage sensors
4. SR foot proteins: SR Ca2+ release channels
V. Sliding Filament Model
A. In relaxed state, thin and thick filaments overlap only slightly
B. During contraction, myosin heads bind to actin, detach, and bind again, pulling
thin filaments along the thick filaments, increasing the length over which thick &
thin overlap
C. Filaments themselves do not change length significantly
D. Sarcomeres shorten, muscle cells shorten, and whole muscle shortens*
1. Musle otratio auses shorteig iff iard fore eeeds
outward force
VI. Requirements for Skeletal Muscle Contraction
A. Activation: neural stimulation at neuromuscular junction
B. Excitation-contraction coupling:
1. Generation and propagation of an action potential along sarcolemma,
hih auses…
2. Final trigger: a brief rise in intracellular Ca2+
VII. Neuromuscular Junction
A. Situated midway along length of muscle fiber
B. Axon terminal and muscle fiber are separated by the synaptic cleft
C. Synaptic vesicles of axon terminal contain neurotransmitter acetylcholine (ACh)
D. Sarcolemma under the nerve terminal has ACh receptors
E. Events at NMJ
1. Somatic motor neurons (in spinal cord usually*) fire to stimulate
muscles
2. Action potentials travel along axons, in nerves, from spinal cord* to
muscles
a) * Somatic motor neurons for most muscles of the head are in the
brain
3. Each axon forms several branches as it enters a muscle
4. Each axon ending forms a neuromuscular junction with a single muscle
fiber
5. Nerve impulse arrives at axon terminal
6. ACh is released and binds with receptors on sarcolemma
7. If enough ACh is released fast enough, action potential will be
generated
8. ACh effects are temporary because it is destroyed by the enzyme
acetylcholinesterase
VIII. Generating an Action Potential
A. Local depolarization (end plate potential):
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1. Before this occurs, quiet muscle is -70 mV inside. Lot of K inside, Na
outside.
2. ACh binds to ACh-receptor channels in sarcolemma and makes them
open.
3. The open channels are like an electrical short circuit across the
membrane: positive current flows in to the negative cell.
4. Net result: + charge enters cell, so voltage inside cell rises (becomes
less negative), i.e. depolarization (end plate potential) to ~0 mV.
B. Generation and propagation
1. End plate potential spreads to adjacent membrane areas
2. Voltage-regulated Na+ channels open, causing Na+ influx and even more
depolarization
3. If threshold is reached, an action potential is generated: local
depolarization spreads
4. Voltage-regulated Na+ channels open in adjacent patch, causing it to
depolarize to threshold
C. Repolarization
1. Na channels close and voltage-gated K channels open
2. K+ flows out thru the open K channels
3. The outflow (loss) of + charge causes cell to return to a negative voltage
4. Fiber cannot be stimulated and is in a refractory period until
repolarization is complete
5. Ionic conditions of resting state are restored by Na+/K+ pump
IX. Excitation-Contraction Coupling
A. Sequence of events by which an action potential on the sarcolemma causes
contraction (sliding of filaments)
B. Latent period
1. Time when E-C coupling events occur
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Document Summary
Types of muscle tissue: skeletal, striated, voluntary, attached to bone (& skin, cardiac, striated, heart only, involuntary, smooth, involuntary, walls of hollow organs. Muscle characteristics: excitability: ability to receive and respond to stimuli, contractility: ability to shorten when stimulated, elasti(cid:272)it(cid:455): a(cid:271)ilit(cid:455) to stret(cid:272)h a(cid:374)d (cid:862)(cid:271)ou(cid:374)(cid:272)e (cid:271)a(cid:272)k(cid:863) Muscle functions: move bones and fluids (e. g. , blood, maintain posture and body position (counteract gravity, stabilize joints, generate heat. Requirements for skeletal muscle contraction outward force: activation: neural stimulation at neuromuscular junction, excitation-contraction coupling, generation and propagation of an action potential along sarcolemma, (cid:449)hi(cid:272)h (cid:272)auses , final trigger: a brief rise in intracellular ca2+ Generating an action potential acetylcholinesterase: local depolarization (end plate potential), before this occurs, quiet muscle is -70 mv inside. Regulation of muscle force: required for proper control of skeletal movement, responses are graded by, changing the frequency of stimulation, changing the number of motor units activated.
