Lecture 6 & 7 Muscular System Flashcards
What are the functions of the muscular system?
- Movement of the body
- Maintenance of posture
- Production of body heat
- Constriction of organs and vessels
What are the general properties of muscle? Explain them.
Excitability
- The capacity of muscle to respond to a stimulus
Contractility
- The ability of a muscle to cause movement of the body caused by contractile proteins sliding past one another
Extensibility
- The ability of muscle to be stretched beyond its normal resting length, the contractile proteins decrease their overlap
Elasticity
- The ability of muscle to recoil to its original resting length
How are skeletal muscles composed and structured?
- Composed of muscle cells (fibers), connective tissue, blood vessels, nerves
- Muscle fibers are long cylindrical, and multinucleated. Developed from myoblast
Skeletal muscle fibers are surrounded by a plasma membrane called?
Sarcolemma
Skeletal muscle fibers are surrounded by 2 layers of connective tissue superficial to the sarcolemma called?
- Immediately superficial is the external lamina
- The 2nd layer is the endomysium
Multiple skeletal muscle fibers are grouped together into a?
Fasciculus
Each fasciculus is covered by a connective tissue called?
Perimysium
The entire muscle is covered by a connective tissue called?
Epimysium
What surrounds individual muscles?
Muscular fascia
Explain motor neurons.
- Stimulate muscle fibers to contract
- Axons branch so that each muscle fibers are innervated (supplied by a nerve)
What’s a neurovascular bundle?
An artery, vein, and nerve
The sarcolemma has tube-like invaginations called? Explain them.
Transverse tubules (t-tubules)
- These connect the extracellular environment with the intercellular environment
What is the sarcoplasmic reticulum?
A specialized type of ER that stored Ca2+
What are myofibrils? Explain them.
They are bundles of protein within the sarcoplasm
- Fills up 80% of the volume of a skeletal muscle
- Composed of myofilaments
What are the 2 types of myofilaments? Explain
- Actin myofilaments (thin)
- Myosin myofilaments (think)
- The myofilaments form highly ordered repeating units called sarcomeres
Explain actin myofilaments (thin)
- Each actin myofilament is composed of 2 strands of fibrous actin (F actin).
- The F actin strands are arranged in a double helix and are made of globular actin (G actin) monomers.
- G actin has attachment sites for myosin to bind to during muscle contraction.
- Contains tropomyosin and troponin
Explain tropomyosin and troponin.
Tropomyosin
- is an elongated protein that winds along the groove of the F actin helix
- covers active sites
Troponin
- is spaced between the ends of tropomyosin molecules in the groove between the F actin strands
Explain myosin myofilaments (thick)
- Many elongated myosin molecules shaped like golf clubs (contains a long rod portion and a head portion)
- Myosin heads can bind to active sites on the G actin molecule to form a cross bridge
- Are ATPase enzymes, ATP is broken down.
What are sarcomeres?
- The smallest portion of skeletal muscle capable of contraction
- Arrangement of actin and myosin myofilaments created a striated appearance
What are Z disks?
Serves as a attachment for actin myofilaments
What are I bands?
Consists of only actin and extends either side of Z disk to the ends of myosin filaments
What are A bands?
Length of myosin filaments
What is the H zone?
A region in A band where actin and myosin do not overlap
What is the M line?
Middle of H zone, holds myosin in place
What is the sliding filament theory?
Actin slides over myosin during muscle contraction to shorten sarcomeres
What are Na+/K+ ion pumps? Explain them.
- The inside of the plasma membrane is more negatively charged compared to the outside. The Na+/K+ ion pumps are responsible for maintaining the uneven distribution.
- The pump binds 3 intercellular Na+ ions and ATP, the ATP is broken down and causes a change in shape of the pump. This allows the transport of Na+ ions to the extracellular side.
- The pump then binds 2 extracellular K+ ions and a phosphate is lost causing the pump to revert back to its original shape and transport K+ ions to the intercellular side.
Explain action potentials.
An action potential occurs when an electrically excitable cell is stimulated.
- The inside of the plasma membrane becomes temporarily positive compared to the outside.
- 2 phases of action potential: Depolarization and Repolarization
- An action potential doesn’t happen all at once, it’s a stimulus that causes another action potential to be produced.
Explain depolarization and repolarization.
Depolarization:
- inside the plasma membrane become more positive
- if change reaches threshold an action potential is triggered
Repolarization:
- Returns to resting membrane potential
- Both are due to the opening and closing of voltage-gated ion channels
What is the Neuromuscular Junction?
A specific location where the muscle fiber is innervated by a motor neuron, found in the middle of the muscle fiber. There’s only 1 NMJ per muscle fiber
What is the presynaptic terminal?
The end of the motor neuron where it approaches the muscle fiber
- Also called the synaptic knob
What is the synaptic cleft?
Narrow fluid-filled space between the presynaptic terminal and postsynaptic membrane
What is the postsynaptic membrane?
Specialized region of the sarcolemma
What are neurotransmitters? Explain them.
Stored within the synaptic vesicles, they are chemical messengers
- Acetylcholine (Ach) is the major neurotransmitter in skeletal muscle.
Explain what happens in the Neuromuscular Junction. Each 9 steps.
- An action potential arrives at the presynaptic terminal and causes voltage-gated Ca+ channels in the presynaptic membrane to open.
- Calcium ions enter the presynaptic terminal and initiate the release of the neurotransmitter Ach from synaptic vesicles
- Ach is released into the synaptic cleft by exocytosis
- Ach diffuses across the synaptic cleft and binds to ligand-gated Na+ channels on the postsynaptic membrane
- Ligand-gated channels open and sodium enters the postsynaptic cell causing the postsynaptic membrane to depolarize. If threshold is reached an action potential is made.
- Ach unbinds from ligand-gated Na+ channels which closes them
- The enzyme acetylcholinesterase removes Ach from the synaptic cleft by breaking it down
- Choline is symport with Na+ into the presynaptic terminal where it’s recycled to make Ach
- Ach is reformed and taken by a synaptic vesicle
Explain Excitation Contraction Coupling. Each 4 steps.
Excitation Contraction Coupling is a mechanism where an action potential causes muscle fiber contraction.
- An action potential that was produced at the NMJ is propagated along the sarcolemma. The depolarization also spreads along the membrane of the T tubules. (The action potential travels inside the cell.)
- The depolarization of T tubules causes gated Ca2+ channels in the sarcoplasmic reticulum to open, resulting in a increase of permeability. Calcium ions then diffuse from the sarcoplasmic reticulum to the sarcoplasm.
- Calcium ions then bind to troponin molecules, causing tropomyosin to move and expose the active sites.
- Once active sites on G actin are exposed the heads of myosin binds to them to form cross bridges.
