Chapter 12

The human body has 3 types of muscle tissue:

1) Smooth Muscle

2) Cardiac Muscle

3) Skeletal Muscle

Smooth Muscle

Primary muscle of internal organs and tubes, such as the stomach, urinary bladder, and blood vessels. Its primary function is to influence the movement of material into, out of, and within the body. An example is the passage of food through the gastrointestinal tract.

Skeletal Muscle

Often attached to the bones of the skeleton, enabling these muscles to control body movement

Cardiac Muscle

Found only in the heart and moves blood through the circulatory system.

What are considered striated muscles?

Skeletal and cardiac muscles

Skeletal Muscle Fiber Attributes

Fibers are large, multinucleate cells that appear striped or striated under the microscope.

Cardiac Muscle Fiber Attributes

Fibers are striated but they are smaller, branched, and uninucleate. Cells are joined in series by junctions called intercalated disks.

Smooth Muscle Fiber Attributes

Fibers are small and lack striations.

Why aren’t smooth muscle fibers striated?

They have a less organized arrangement of contractile fibers than skeletal/cardiac muscle.

What constitutes voluntary/involuntary muscle groups and why are these imprecise terms?

“Voluntary”: Skeletal Muscle (Can also contract without conscious direction).

“Involuntary”: Smooth/Cardiac Muscle (Although some degree of control can be learned over both)

What is unique about skeletal muscle contraction with regards to innervation?

They contract only in response to a signal from a somatic motor neuron. They cannot initiate their own contraction, and their contraction is not influenced directly by hormones.

Cardiac/Smooth Muscle innervation:

Have multiple levels of control. Their primary extrinsic control arises through autonomic innervation, but some types of smooth and cardiac muscle can contract spontaneously, without signals from the central nervous system. In addition, the activity of cardiac and some smooth muscle is subject to modulation by the endocrine system.

The signal to initiate muscle contraction is an intracellular ______ signal, and movement is created when a motor protein called _____ uses energy from _________ to change its conformation.

calcium, myosin, ATP

_______ muscles make up the bulk of muscle in the body and constitute about 40% of total body weight.

Skeletal

Tendons

Attach skeletal muscle to bone, made of collagen.

Origin

The end of the muscle that is attached closest to the trunk or to the more stationary bone.

Insertion

The end of the muscle that is at the more distal or more mobile end.

Flexor

When the muscle brings the centers of the connected bones closer together during contraction.

Extensor

When the muscle moves the connected bones away from each other during contraction.

Antagonistic Muscle Groups

Flexor/Extensor pairings that exert opposite effects (Muscle contraction can pull on a bone but cannot push a bone away).

_______ moves bones closer together. For example, when doing an arm curl, the radius and ulna move towards the humerus.

Flexion

________ moves bones away from each other. For example, when doing a push-up, the radius and ulna move away from the humerus.

Extension

When one muscle contracts and shortens, the antagonistic muscle must ______ and ________.

relax, lengthen

A skeletal muscle is a collection of muscle cells, or ________, just as a nerve is a collection of neurons

muscle fibers

Satellite Cells

Committed stem cells that lie just outside the muscle fiber membrane. Become active and differentiate into muscle when needed for muscle growth and repair.

Fascicles

Groups of adjacent skeletal muscle fibers bundled together into units.

The fibers in a given muscle are arranged with their long axes in ______.

Parallel

What are found between fascicles? (4 things)

1)Collagen

2) Elastic fibers

3) Nerves

4) Blood vessels

The entire muscle is enclosed in a ______________ that is continuous with the connective tissue around the muscle fibers and fascicles and with the tendons holding the muscle to underlying bones.

connective tissue sheath

Sarcolemma

The cell membrane of a muscle fiber.

Sarcoplasm

Cytoplasm of a muscle cell.

Myofibrils

The main intracellular structures in striated muscle fibers. Highly organized bundles of contractile and elastic proteins that carry out the work of contraction.

Sarcoplasmic Reticulum

A form of modified endoplasmic reticulum that wraps around each myofibril like a piece of lace in skeletal muscle fibers.

Terminal Cisternae

Longitudinal tubules with enlarged end regions that make up the sarcoplasmic reticulum.

The sarcoplasmic reticulum concentrates and sequesters _____ with the help of a _________ in the SR membrane.

Calcium ions, Ca2+-ATPase

______ release from the SR creates signals that play a key role in contraction in all types of muscle.

Calcium

Transverse Tubules (T-tubules)

Run adjacent to the terminal cisternae. The membranes of t-tubules are a continuation of the muscle fiber membrane, which makes the lumen of t-tubules continuous with the ECF.

One t-tubule and its two flanking terminal cisternae are called a ______.

Triad

_______ allow action potentials to move rapidly from the cell surface into the interior of the fiber so that they reach the terminal cisternae nearly simultaneously.

T-tubules

The cytosol between the myofibrils contains many __________ and _________.

glycogen granules, mitochondria

One muscle fiber contains _________ or more myofibrils that occupy most of the intracellular volume, leaving little space for cytosol and organelles.

a thousand

Sarcomeres

Repeating contractile structure unit of a myofibril.

Myosin

A motor protein with the ability to create movement. Forms thick filaments.

One myosin molecule is composed of ________ identical protein chains, each with _____ large heavy chain plus _____ smaller light chains.

two, one, two

Structure of Heavy Chains of Myosin

1. A pair of tadpole-like heads,

2. stiff rodlike sections that intertwine to form a tail, and

3. an elastic neck region that joins the head to the tail

The neck creates a hinge that allows the heads to swivel around their point of attachment.

Function of Light Chains of Myosin

The myosin light chains wrap around the lower neck region of the myosin heads and add rigidity to the hinge. Phosphorylation of myosin light chains in striated muscles enhances the force of muscle contraction.

Motor Domain

Heavy chains of the myosin heads that use energy from the high-energy phosphate bond of ATP.

Because myosin acts as an _______ to hydrolyze ATP, the motor domain is considered a _________

enzyme, myosin ATPase

In skeletal muscle, about ______ myosin molecules join to create a thick filament.

250

How are myosin molecules arranged in a thick filament?

Heads at either end of the filament, tails in the middle.

Actin

Makes up the thin filaments of the muscle fiber. One actin molecule is a G-protein that can be chained together to form “f-actin”.

In skeletal muscle, two _________ twist together like a double strand of beads, creating the thin filaments of the myofibril.

F-actin polymers

Myosin Crossbridges

Connects and spans the space between parallel thick and thin filaments. Form when the myosin heads of thick filaments bind to actin in the thin filaments.

Each myosin head has ____ actin-binding site(s) and each G-actin molecule has _____ myosin-binding site(s).

1, 1

Order of Units in Skeletal Muscle Tissue (Small → Large)

Actin Chain/Myosin → Thin/Thick Filaments → Sarcomere → Myofibril → Muscle Fiber → Muscle Fascicle → Skeletal Muscle (p.415)

Elements of a Sarcomere

1. Z Disks (zigzag protein structures that serve as the attachment site for thin filaments).

2. I Bands (These are the lightest color bands of the sarcomere and represent a region occupied only by thin filaments. Only half of an I band per sarcomere, bisected by Z Disks)

3. A Bands (Encompasses the entire length of a thick filament. At the outer edges of the A band, the thick and thin filaments overlap.)

4. H Zones (Central region of the A band that is lighter than the outer edges of the A band because the H zone is occupied by thick filaments only)

5. M Lines (This band represents proteins that form the attachment site for thick filaments, like Z disks for thin filaments.)

When viewed end-on, each thin filament is surrounded by ______ thick filaments, and ____ thin filaments encircle each thick filament

3, 6

Titin

A huge elastic molecule and the largest known protein, composed of more than 25,000 amino acids. A single titin molecule stretches from one Z disk to the neighboring M line.

Functions of Titin

(1) it stabilizes the position of the contractile filaments and

(2) its elasticity returns stretched muscles to their resting length.

Nebulin

An inelastic giant protein that lies alongside thin filaments and attaches to the Z disk. Nebulin helps align the actin filaments of the sarcomere.

Muscle Tension

The force created by contracting muscle

Load

A weight or force that opposes contraction of a muscle.

Steps of Muscle Contraction

1. Events at the neuromuscular junction convert an acetylcholine signal from a somatic motor neuron into an electrical signal in the muscle fiber.

2. Excitation-contraction (E-C) coupling is the process in which muscle action potentials are translated into calcium signals. The calcium signals in turn initiate a contraction-relaxation cycle.

3. At the molecular level, a contraction-relaxation cycle can be explained by the sliding filament theory of contraction. In intact muscles, one contraction-relaxation cycle is called a muscle twitch.

Sliding Filament Theory of Contraction

Overlapping actin and myosin filaments of fixed length slide past one another in an energy-requiring process, resulting in muscle contraction.

According to the sliding filament theory, tension generated in a muscle fiber is directly proportional to the number of high-force _________ between the thick and thin filaments.

crossbridges

Power Stroke

When myosin crossbridges swivel and push the actin filaments toward the center of the sarcomere.

Tropomyosin

Protein that inactivates myosin-binding sites on actin filaments during muscle relaxation. Controlled by Troponin.

Muscle Contraction Activation by Troponin

1. Contraction begins in response to a Ca²⁺ signal.

2. Troponin-C binds reversibly to Ca²⁺.

3. The calcium-troponin C complex pulls tropomyosin completely away from actin’s myosin-binding sites.

4. This “on” position enables the myosin heads to form strong, high-force crossbridges and carry out their power strokes, moving the actin filament.

For muscle relaxation to occur, Ca2+ concentrations in the cytosol must ________.

decrease

Steps of a Contractile Cycle