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