Skeletal Muscle: Basic Structure and Contractile Unit
Transmission of signals from the brain to the skeletal muscle
1. electrical signals (impulses/AP) from the brain 2. Through motor neurons (nerve) 3. At the junction between the motor neuron and the skeletal muscle (NMJ/end plate) where electrical is being transformed to chemical (neurotransmitter) 4. Chemical (meurotransmitter) is then transformed into electrical in the outer membrane of muscle fibers 5. Electrical is then transformed into mechanical within the mucle fibers (muscle contraction)
Because more than 100 myblasts are fused together during embryonic development, each mature muscle fiber should contain
100 or more nuclei
Each skeletal muscle fibers is a fusion of blank or more blank during embryonic development
100, myoblasts
Each myosin molecule is shaped like
2 golf clubs twisted together with the projection of two globular heads for each molecule called myosin heads
In skeletal muscle, there are about blank molecules of myosin in a single thick filament
250-300
The muscular system composes what percent of total adult body weight?
40-50%
What kind of protein is actin?
A globular protein (G-actin) that represents the major components of the thin filaments within the muscle fiber
Polymerization of multiple G-actin molecules forms
A long filament called F-actin (actin chain)
What binding site does each G-actin molecule have?
A myosin binding site that allows the myosin head to attach
What does titin in skeletal muscle function as?
A spring, which is responsible for passive elasticity of the muscle and stabilizes myosin
ATP is hydrolyzed to form lank as well as blank and energy is released during this process
ADP, inorganic phosphate (Pi)
Myosin heads contain
ATPase
The myosin head has binding sites for
Actin and hydrolyzing ATP (ATP--> ADP + Pi)
What are the contractile proteins of skeletal muscle?
Actin, myosin
The interaction among troponin, tropomyosin, and actin allow
Actin-myosin interaction to be regulated through the changes in sarcoplasmic [Ca2+]
The Fast conduction of muscle action potentials due to the t-tubules ensures that
An AP can excite all parts of the muscle fiber at essentially the same instant
Where are Z disks located in skeletal muscle?
Are zigzag protein structures that serve as the attachment site for thin filaments
Which nervous system modulates the pacemaker activities?
Autonomic nervous system
What system(s) regulate involuntary muscle?
Autonomic nervous system and endocrine system
Where is the sacromere located in skeletal muscle?
Basic unit between skeletal muscle between two Z disks
Why is skeletal muscle multinucelated?
Because each myoblast has its own nucleus
Purpose of troponin C
Binds to Ca2+
Purpose of troponin T
Binds to a single molecule of tropomyosin
What is the H zone?
Center region of the A band, lighter than the outer edges of the A band Occupied only by thick filaments
Each fascicle of skeletal muscle is enclosed by a layer of dense blank (perimysium) with blank to blank more muscle fibers
Connective tissue, 10, 100
The entire skeletal muscle is covered by another layer of blank called blank
Connective tissue, epimysium
Each muscle fiber in skeletal muscle is sheathed in blank (endomysium), are arranged with their blank axes in parallel
Connective tissue, long
3 kinds of important proteins in skeletal muscle
Contractile, regulatory, structural
What does skeletal muscle control?
Control of posture Contraction and relaxation of diaphragm during respiration
Where is tropomyosin located in a relaxed muscle?
Cover the myosin binding sites in the G-actin molecule on actin
Purpose of troponin I
Covers the myosin head binding site on G-actin molecule and inhibits muscle contraction
What is the A band?
Darkest of the sarcomere and occupies the whole length of a thick filament Thick and thin filaments overlap at outer edges Center is occupied by thick filaments only
A group of skeletal muscle fibers bundled together into units are called
Fascicles
What do T tubules allow for?
Fast conduction of the muscle action potentials (electrical signals) into the fibers
Where is myoglobin found in skeletal muscle?
Found in the cytoplasm (sarcoplasm)
Protruding myosin heads are attached to the stuff tail with an elastic hinge region, allowing
Heads to swivel around the point of attachment and to create movement
Purpose of ATPase
Hydrolyzes ATP to release energy which is used by the myosin head to create movement
Where is nebulin located in skeletal muscle?
Lies along the thin filaments, attaches to a Z disk but does not extend to the M line
What is the I band?
Lightest colour of band of the sacromere and only consists of thin filaments with a Z disk in the middle of every I band
Myosin functions as a blank in all 3 types of muscle tissue
Motor protein
Skeletal muscle is a collection of
Muscle fibers
During embryonic development, what fuses to form one skeletal muscle fiber?
Myoblasts
Within muscle fibers (myofibrils) are smaller structures called blank containing 3 kinds of blank
Myofilaments, protein
Contents of skeletal muscle
Nucleus, muscle fiber, muscle fascicle (bundle of fibers), connective tissue, nerve and blood vessels, skeletal muscle, tendon
Myoglobin has the ability to bind
Oxygen molecules
Nuceli of skeletal muscle fibers are located beneath the
Plasma membrane (sarcolemma) of the fiber
What is the role of tropomyosin in a relaxed muscle?
Prevents myosin heads in the thick filaments from binding to actin molecules Regulates binding of myosin head to actin
What does nebulin in skeletal muscle function as?
Regulates the length of thin filament during assembly and helps to align actin
What do striations look like under a microscope?
Repeating light and dark protein bands
What is the M line?
Represents proteins that form the attachment site for thick filaments Each M line divides an A band in half
Release of Ca2+ by the blank triggers blank
Sarcoplasmic reticulum, muscle contraction
A fluid-filled system of membranous sacs called the blank encircles each blank
Sarcoplasmic reticulum, myofibril
What muscle system can also be controlled subconsciously?
Skeletal muscle
Which muscle(s) contain multinucleated cells?
Skeletal muscle
Three types of muscle tissue in humans
Skeletal, cardiac, smooth
Autonomic normally has no influence on contraction of the blank muscle system, with the exception of the blank
Skeletal, diaphragm for respiration
Which muscle system(s) have a pace-making capacity?
Smooth and cardiac
Which muscle(s) contain uninucleated cells?
Smooth and cardiac
Which muscle type(s) have an involuntary function?
Smooth and cardiac muscle
What system(s) regulate voluntary muscle?
Somatic nervous
Where is titin located in skeletal muscle?
Spans the distance from one Z disk to the neighbouring M line
Characteristics of skeletal muscle
Striated Multinucleated Usually under voluntary control Innervated by the somatic NS Acetylcholine as the neural transmitter in the NMJ
Smooth muscle lacks
Striations
Connective tissue layers may extend beyond the muscle fibers to form a
Tendon that attaches to a bone
SR with dilated sacs are called
Terminal cisternae
Muscle fiber and muscle cell are two terms for
The same structure
Skeletal muscle only contracts in response to
The somatic motor neurons
Myosin is the blank filament
Thick
Actin is the blank filament
Thin
Structural proteins of skeletal muscle are
Titin (also known as connectin) and nebulin
Primary functions of the muscular system
Transforms chemical energy into mechanical energy Generates force (muscle shortening) Performs work (movement) Maintain equilibrium (posture/tension generation) Transport substances (circulation and digestion) Heat production (maintain body temp, ex. shivering)
Thousands of tiny invaginations (infolded membrane) of the sarcolemma called the blank tunnel toward the blank
Transverse (T) tubules, center of each fiber
Terminal cisternae sit against the t tubule from both sides and form a
Triad (t tubule sandwiched by 2 terminal cisternae)
Regulatory proteins of skeletal muscle are
Tropomyosin and troponin
Troponin is a heterotrimer consisting of
Troponin T (TnT) Troponin C (TnC) Troponin I (TnI)
What form the helix that creates the thin filament of the myofibril?
Two F-actin polymers twisted together, with the regulatory proteins tropomyosin and troponin
When does myoglobin release oxygen molecules?
When it is needed (ex. during exercise) which enhances the mitochondria's ability to produce ATP
