Exam 2: Ch 12 Skeletal Muscle Flashcards
Fibrous connective tissue from tendons form what
epimysium sheaths that extend around and into skeletal muscles
inside the muscle fibrous connective tissue divides muscle into columns called
fascicles
perimysium
connective tissue around fascicles
muscle fibers are
muscle cells ensheated by thin connective tissue layer called endomysium
plasma membrane is called
sarcolemma
muscle fibers are similar to other cells except are
multinucleate and striated
most distinctive feature of skeletal muscle is its
striations
NMJ
- neuromuscular junction
Includes the single synaptic ending of the motor neuron innervating each muscle fiber & underlying specializations of sarcolemma
motor end plate
place on sarcolemma where NMJ occurs
each motor neuron branches to
innervate a variable # of muscle fibers
a motor unit includes
each motor neuron & all fibers it innervates
when a motor neuron is activated
all muscle fibers in its motor unit contract
Number of muscle fibers in motor unit varies according to
degree of fine control capability of the muscle
Innervation ratio is
motor neurons : : muscle fibers; vary from 1:100 to 1:2000
Fine movements occur when
motor units are small, i.e. 1 motor neuron innervates small # of fibers
Gross movements occur when
motor units are large: 1 motor neuron innervates large # of fibers
Q: Since individual motor units fire “all-or-none,” how do skeletal muscles perform smooth movements?
…
A:Recruitment is used
Brain estimates number of motor units required & stimulates them to contract;
It keeps recruiting more units until desired movement is accomplished in smooth fashion:
• More & larger motor units are activated to produce greater strength
Structure of Muscle Fiber
- Each fiber is packed with
myofibrils
Structure of Muscle Fiber
- Myofibrils are
are 1m in diameter & extend length of fiber
• Packed with myofilaments;
Structure of Muscle Fiber :
myofilaments
composed of thick & thin filaments that give rise to bands which underlie striations
Structure of Muscle Fiber :
A band
is dark, contains thick filaments (mostly myosin);
Structure of Muscle Fiber :
H band
- Light area at center of A band
- area where actin & myosin do not overlap
Structure of Muscle Fiber :
I band
light, contains thin filaments (mostly actin);
Structure of Muscle Fiber :
- Z line/disc where
- At center of I band
- where actins attach
Structure of Muscle Fiber :
Sarcomeres
contractile units of skeletal muscle consisting of components between 2 Z discs
Structure of Muscle Fiber :
M lines
are structural proteins that anchor myosin during contraction
Structure of Muscle Fiber :
Titin
is elastic protein attaching myosin to Z disc that contributes to elastic recoil of muscle
Sliding Filament Theory of Contraction:
- Muscle contracts because
myofibrils get shorter
Muscle contracts because myofibrils get shorter and it occurs because
thin filaments slide over & between thick filaments towards center = shortening distance from Z disc to Z disc
Cross bridges are formed by
heads of myosin that extend toward & interact with actin
Each myosin head contains
ATP-binding site which functions as an ATPase
Myosin can not bind to actin unless
unless it is “cocked” by hydrolyzing ATP;
after binding, myosin undergoes
conformational change (power stroke) which exerts force on actin
Sliding Filament Theory of Contraction
1. Myosin head has hydrolyzed ATP to ADP + Pi
2. Cross bridges are formed by heads of myosin molecules that extend toward & interact with actin
3. Pi is released causing a conformational change in myosin head
4. Powerstroke occurs sliding thin filament over thick and ADP is released
Note: at this point, cross bridges are stuck….ATP is needed for detachment
5. Myosin head binds new ATP and releases actin
6. Myosin head hydrolyzes ATP to ADP + Pi
Control of cross bridge attachment to actin is via
troponin-tropomyosin system = serves as a switch for muscle contraction & relaxation
The filament tropomyosin lies in
grove between double row of G-actins (that make up actin thin filament)
Troponin complex is attached to
tropomyosin at intervals of every 7 actins
In relaxed muscle,
, tropomyosin blocks binding sites on actin so cross bridges can not occur; this occurs when Ca++ levels are low (<10-6 M)
When Ca++ levels rise (>10-6 M),
Ca++ binds to troponin causing conformational change which moves tropomyosin & exposes binding sites Allowing crossbridges & contraction to occur; crossbridge cycles stop when Ca++ levels decrease (<10-6 M)
Ca++ levels decrease because
because it is continually pumped back into the sarcoplasmic reticulum (SR - a calcium reservoir in muscle)
Most Ca++ in SR is in
terminal cisternae;
running along terminal cisternae are
T tubules
Excitation-Contraction Coupling:
- Skeletal muscle sarcolemma is
is excitable
Conducts APs just like axons
Release of ACh at NMJ causes
large depolarizing end-plate potentials & APs in muscle
APs race over
sarcolemma & down into muscle via T tubules
T tubules are extensions of
sarcolemma
Ca++ channels in SR are
mechanically linked to channels in T tubules
APs in T tubules cause release
of Ca++ from cisternae via V-gated Ca++ release channels
Called electromechanical release
Muscle Relaxation
Ca++ from SR diffuses to troponin to initiate crossbridge cycling & contraction
When APs cease, muscle relaxes
- Because Ca++ channels close & Ca++ is pumped back into SR
Twitch
= single rapid contraction & relaxation of muscle fibers
summation)
If 2nd stimulus occurs before muscle relaxes from 1st, the 2nd twitch will be greater
Contractions of varying strength (graded contractions) are obtained by stimulation of
of varying numbers of fibers (motor unit recruitment)
incomplete tetanus
If muscle is stimulated by an increasing frequency of electrical shocks, its tension will increase to a maximum
complete tetanus
If frequency is so fast no relaxation occurs, a smooth sustained contraction results
Treppe or staircase effect
If muscle is repeatedly stimulated with maximum voltage to produce individual twitches, successive twitches get larger
