Skeletal Muscle Flashcards
3 types of muscles
Skeletal, cardiac, smooth
Skeletal Muscle
Striated muscle that generates force;
Contracts only when stimulated
Innervated by SOMATIC MOTOR NEURONS
Cardiac Muscle
Striated; generates force
Contracts rhythmically
Innervated by AUTONOMIC NERVOUS SYSTEM
Smooth Muscle
NON-STRIATED: Used for pressure loads
Maintains Organ Dimensions & Functions
Contracts continuously
Innervated by AUTONOMIC NERVOUS SYSTEM
Endomysium
Connective tissue (?) Surrounds individual muscle fibers
Perimysium
Connective tissue (?) that surrounds muscle fascicles
Muscle fascicle
Functionally discrete bundles of muscle fibers (cells)
Epimysium
Surrounds whole muscle (made up of groups of fascicles)
How do nerves and blood vessels interact with muscle fibers
Through Connective tissue
Describe a muscle fiber…general overview.
Single, multi nucleated cell. Has organelles and contractile structures.
How are muscle cells made in terms of embryology?
1st step
Myogenic precursor cells in somites generate myoblasts and satellite cells
What do myoblasts do embyrologically?
Several myoblasts fuse to form single muscle fiber
What are satellite cells and what do they do?
Include pre and post natal
Stem like cells that survive into adulthood.
Prenatally: divide and fuse with muscle fiber to ensure adequate number of nuclei.
Postnatally: they can replace damaged muscle fibers.
Sarcolemma
cell membrane around a single muscle fiber
T-tubules
Infoldings of sarcolemma
Sarcoplasmic reticulum
Stores and releases Ca ion (2+)…equivalent to endoplasmic reticulum
Regions and function of Sarcoplasmic reticulum
Sarcotubules: store Ca 2+ bound to calsequestrin
Terminal cisternae: contact T-tubules
Triads: T-tubule and 2 terminal cisternae
Function?
Where action potentials trigger release of Ca 2+ from terminal SR
Myofibrils
Organelles of contractile myofilaments segmented into sarcomeres.
What is the structure/ shape of a myofibril?
Cylinders that extend the length of the muscle.
Chain of fused Sarcomeres
Sarcomere: shape and structure
Cytoskeletally distinct unit containing thick and thin myofilaments: INCLUDING ACTIN and MYOSIN
Joined to each other by Z-lines
Surrounded by SR
What makes a muscle striated”?”
Z lines connecting sarcomeres are aligned with each other.
Regularity of sarcomere structures form striations
Titin (connectin) and Nebulin structure and function
Together make flexible filamentous network thatsurrounds myofibrils:
Titin: elastic component in muscle that can stretch under tension
Components of Myofilaments
THIN: Actin; Tropomyosin; Troponin
THICK: Myosin
Actin
Globular protein (G-actin) polymerized into doubled strands (F-actin)
Tropmyosin
Covers myosin binding sites
Troponin
Regulatory protein bound to tropomyosin
Activated by Ca 2+
Myosin structure
Tails form tick portion of myofibril
Head and neck form cross bridges that bind to actin
How is a sarcomere defined structurally?
this may be poorly worded…think letters
Z lines align with each other vertically to form edges
I bands are found on each side of Z band
A bands are found on each side of I band
H zone found in between A bands
M line is in middle of H zone…center of sarcomere
End to end: Z-I-A-H-M-H-A-I-Z
What is a motor neuron?
Cell body in ventral horn that projects its axon out through ventral root to muscle
Contrasts with 2 cell system for sympathetic system (don’t know what this means so didn’t write a good question for it)
What is Neuromuscular Junction?
Synapse formed by motor neuron on skeletal muscle
How is the Neuromuscular junction formed?
Motor neuron axons branch and synapse onto muscle end plates
What lines the Junctional folds on a muscle cell?
Ach receptors
How do the motor neurons communicate with muscle cells?
They release Ach onto neuromuscular junctions
How does Ach facilitate an action potential?
Increases permeability of Na+/K+ channel creating an End Plate Potential
What kind of receptors are Ach receptors?
What can block them?
Ionotropic nicotine receptors
Botullinum toxin (Botox)
What is an end Plate Potential (EPP)
Large depolarization similar to an EPSP except greater in amplitude.
Has sufficient amplitude to exceed threshold for action potential initiation
How many axon action potentials are needed to initiate a muscle action potential?
One (not sure if you can have more than one at times as well)
How is Muscle action potential generated?
End Plate Potentials depolarize neighboring voltage gated Na+ channels that generate muscle action potential
What enzyme breaks down Ach and where is it?
acetylcholinesterase…lining of skeletal muscle cell surface
what does esterase do?
Enhances Neuromuscular junction activity thus enhancing muscle contraction
What ion begins muscle excitation?
Where does it come from?
Ca 2+ is released from sarcoplasmic reticulum
After Ca 2+ is released from SR what happens next?
First couple of steps…
Action potential spreads along T tubule….current activates dihydropyridine (DHP) receptors.
what is a dihydropyridine (DHP) receptor and what does it do?
Does it work the same in cardiac receptors?
L type voltage gated Ca 2+ channel.
When activated, doesn’t allow Ca 2+ flux into cell. Sends voltage to Ryanodine(RCR) receptor.
No…it does permit Ca 2+ into cardiac cells
what is a ryanodine receptor (RCR)?
Where is it and what does it do?
RCR is attached to the DHP in muscle cell.
It absorbs voltage of action potential directed from T-tubule through DHP.
Releases Ca 2+ from the SR terminal cisternae upon voltage activation
Describe Ca 2+ pathway that initiates contraction.
When does it happen in terms of Action Potential?
Ca ++ released from terminal cisternae of SR
Ca++ binds to troponin to initiate contraction.
Happens DURING an action potential
Describe Ca++ pathway during relaxtion.
When does it happen in terms of Action Potential?
Ca++ sequestered into longitudinal sarcotubules of the SR via Ca-ATPase
Ca++ then translocated back to terminal cisernae for subsequent release.
What are the two primary cofactors that facilitate contraction?
How?
Permission:
- Ca++ released from SR & binds to troponin.
- Troponin alters tropomysosin to expose actins binding sites for myosin
Motivation:
- ATP binds to myosin head & hydrolyzes to ADP and P
- ADP & P enable myosin head to bind to actin
Sliding filament mechanism and contraction:
what are the 4 stages
- Crossbridge attachment
- Working stroke
- Detachment
- ATP hydrolysis
Describe the process fully
you are a superstar…tell a friend about how their muscles work :)
Seriously, you got this!
What conditions are required for actin to bind to myosin?
Ca++ needs to be present.
Actin’s binding site is exposed when tropomyosin is altered by Ca++ bound troponin.
What events trigger bending of myosin head?
Myosin head bound to ATP. ATP hydolzes into ADP and inorganic P cocking myosin head.
What conditions are necessary for muscle to relax?
What is meant by rigor?
Ca++ need to return to SR
Rigor is the point at which the myosin head is most tightly bound to the actin after inorganic phosphate is released from myosin.
What causes myosin head to restore to its initial position?
ADP is released from head causing stroke” part of contraction. Then ATP binds to myosin head causing release from actin.”
Discuss difference of 2 headed myosin
Type II vs Type V
Where is each one?
Type II: Skeletal muscle- heads bind individually. 2nd head orients and stabilizes structure of motion generating head…generates maximum displacement.
Type V: intracellular organelles- two heads walk along microtubules- think vesicle toting cartoon.
What happens during sarcomere contraction?
What is moving and where?
net effect of two actin molecules (Z lines) moving toward center of sarcomere…shortens sarcomere.
During continued contraction, come together eventually overlapping.
Can contraction occur without any overlapping of actin and myosin?
NO!
Range of contraction positions?
Overextened to severe overlapping
The sum of what 2 tensions equal total muscle tension?
active and passive
Passive tension
resistance of CT in muscle tissue (elastic titin fibers)
Active tension (isometric)
proportional
Motor Unit
Group of muscle fibers innervated by branches of a single motor neuron
Small Motor Units vs. Large Motor Units
Small Motor Units adapted for fine and precise motor control…small motor neurons
Large Motor Units: adapted for great force…large motor neurons
Twitch
- What is it?
- How is its duration determined?
- Twitches can be summed to generate…?
- Twitch is a movement of a single motor unit caused by a single action potential
- Duration determined by level of myoplasmic Ca++
- Tetanus
What is spasm (cramp) and how is it generated?
Sudden, involuntary contraction generated by continuous CNS activity or muscle dysfunction
Many causes
