Test 5: Anatomy and Physiology

The Muscular System consists of what?

Only of skeletal muscles

Muscle organization and Function

• Muscle organization affects power, range, and speed of muscle movement

• Fascicles - Muscle cells (fibers) are organized in bundles (fascicles)

Classification of Skeletal Muscles

• By the way fascicles are organized

• By relationship of fascicles to tendon

Organization of skeletal muscle fibers

Four patterns of fascicle organization:

1. Parallel 

2. Convergent

3. Pennate

4. Circular

Parallel Muscles

Fibers parallel to the long axis of muscle

Example: Biceps Brachii

Convergent Muscles 

A broad area converges on attachment site (tendon, aponeurosis, or raphe)

Example: Pectoralis Muscles

Pennate Muscles

Form an angle with the tendon 

Example:

Unipennate (Fibers on one side of tendon) - Extensor digitorum 

Bipennate (Fibers on both sides of tendon) - Rectus femoris

Multipennate (Tendon branches within muscle) - Deltoid 

Circular Muscles (Sphincters)

Open and close to guard entrances of body

Example: Orbicularis oris muscle of the mouth

Skeletal motion

• Skeletal muscles attach to skeleton, produce motion

• Type of muscle attachment affects power, range, and speed of muscle movement

Levers

• Mechanically, each bone is a lever (a rigid, moving structure)

-and each joint a fulcrum (a fixed point)

• Muscles provide applied force (AF)

-Required to overcome load (L)

Functions of a lever

To change:

-direction of an AF

-distance and speed of movement produced by an AF

-effective strength of an AF

The Three Classes of Levers

Depend on the relationship between applied force, fulcrum and resistance

1. First-class lever

2. Second-class lever

3. Third-class lever

First-Class lever

• Seesaw or teeter-totter is an example

• Center fulcrum between applied force and load

• force and load are balanced

Second-Class lever

• Wheelbarrow is an example

• Center resistance between applied force and fulcrum

• A small force moves a large weight

Third-Class lever

• Most common levers in the body

• Center applied force between load and fulcrum

• greater force moves smaller load

• maximizes speed and distance traveled

Origins and Insertions

• Muscles have one fixed point of attachment (origin)

-and one moving point of attachment (insertion) 

• Most muscles originate or insert on the skeleton 

• origin is usually proximal to insertion

Actions

• Movements produced by muscle contraction

• Body movements

-Examples: flexion, extension, adduction, etc.

• Described in terms of bone, joint, or region

Muscle Interactions

• Muscles work in groups to maximize efficiency

• Smaller muscles reach maximum tension first, followed by larger, primary muscles

Muscle terminology based on function

• Agonist (or prime mover)

• Antagonist

• Synergist

Agonist (prime mover)

produces a particular movement

Antagonist 

opposes movement of a particular agonist 

Synergist

• A smaller muscle that assists a larger agonist

• Helps start motion or stabilize origin of agonist (fixator)

Muscle opposition

• Agonists and antagonists work in pairs

-When one contracts, the other stretches

-Such as flexors-extensors, abductors-adductors, etc.

Effects of Aging 

• Skeletal muscle fibers become smaller in diameter

• Skeletal muscles become less elastic 

-Develop increasing amounts of fibrous tissue (fibrosis) 

• Decreased tolerance for exercise

• Decreased ability to recover from muscular injuries 

Cardiovascular system

• Delivers oxygen and fuel

• Removes carbon dioxide and wastes

Respiratory system

responds to oxygen demand of muscles

Integumentary system

Disperses heat from muscle activity

Nervous and endocrine systems

Direct responses of all systems

A muscles fiber is the same as

a muscle cell

Muscle tissue

• A primary tissue type, divided into:

-Skeletal muscle tissue

-Cardiac muscle tissue

-Smooth muscle tissue 

Skeletal muscles

• Are attached to the skeletal system

• Allow us to move

• The muscular system

-Includes only skeletal muscles

Remember this about muscles

Shortening = tension = lifting the bone

Six functions of Skeletal Muscle Tissue

“Powerful Muscles Support Great Growth & Stability”

1. produce skeletal movement

2. maintain posture and body position

3. support soft tissues

4. guard entrances and exits

5. maintain body temperature

6. store nutrient reserves 

Skeletal Muscle

• Muscle tissue (muscle cells or fibers)

• Connective tissues

• nerves

• blood vessels

Organization of connective tissues

1. Epimysium

2. Perimysium

3. Endomysium

what does myo mean?

muscle

What does sarco mean?

flesh

plasma membrane of muscle cells

sarcolemma

Epimysium

• Exterior collagen layer

• connected to the deep fascia

• Separates muscles from surrounding tissue

Perimysium 

• Surrounds muscle fiber bundles (fascicles)

• Contains blood vessel and nerve supply to fascicles 

Endomysium

• Surrounds individual muscle cells (muscle fibers)

• Contains capillaries and nerve fibers contacting muscle cells

• Contains myosatellite cells (stem cells) that repair damage

Muscle attachments

• Endomysium, Perimysium, and epimysium come together

-at ends of muscles

-Form connective tissue attachment to bone matrix 

-i.e., tendon (bundle) or aponeurosis (sheet)

Put these in order:

Myofibrils

bundle of muscle cells/fibers

organ

Bundle of fascicles

1. organ

2. bundle of fascicles

3. bundle of muscle cells/fibers

4. myofibrils

The sarcolemma is associated with which one: Bundle of fascicles, bundle of muscle cells/fibers, or myofibrils?

Bundle of muscle cells/fibers

The Sarcoplasmic Reticulum (SR), the Terminal Cisternae (TC), and the T tubules are associated with which step one: Bundle of fascicles, bundle of muscle cells/fibers, or myofibrils?

Myofibrils

Blood vessels and nerves 

• Muscles have extensive vascular systems that

-Supply large amounts of oxygen

-Supply nutrients

-carry away wastes

• Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord)

Action potential

electrical current

Neurotransmitter

Chemical messenger

-Acetylcholine: Neurotransmitter that controls muscles

Skeletal Muscle cells

• very long

• develop through fusion of mesodermal cells (myoblasts) 

• become very large

• contain hundreds of nuclei

The sarcolemma

• the cell membrane of a muscle fiber (cell)

• Surrounds the sarcoplasm (cytoplasm if muscle fibers)

• A change in transmembrane potential begins contractions

The Transverse tubules (T tubules)

• transmit action potential through cells

• allow entire muscle fiber to contract simultaneously

• have same properties as sarcolemma

Myofilaments are associated with

Actin - Troponin/Tropomyosin

Myosin - Thick filament

Myofibrils 

• lengthwise subdivisions within muscle fiber

• made up of bundles of protein filaments (myofilaments)

• Myofilaments are responsible for muscle contraction

Types of filaments

Thin Filaments - made of the protein actin

Thick filaments - made of the protein myosin

The Sarcoplasmic Reticulum (SR)

• A membranous structure surrounding each myofibril

• helps transmit action potential to myofibril 

• similar in structure to smooth endoplasmic reticulum

• Forms chambers (terminal cisternae) attached to T tubules 

The blue part of the image

What does the terminal cisternae store?

Stores calcium

Triad

• formed by one T-tubule and two terminal cisternae

Terminal Cisternae 

• concentrate Ca2+ (via ion pumps)

• Release Ca2+ into sarcomeres to begin muscle contraction

*Sarcomeres

• * The contractile unit of muscle *

• structural units of myofibrils

• form visible patterns within myofibrils

• A striped or striated pattern within myofibrils

-alternating dark, thick filaments (A bands) and light, thin filaments (I bands)

Sarcomeres: The A Band

Made up of the

M line:

-The center of the A band

-At midline of sarcomere

H Band:

-The area around the M line

-Has thick filaments but no thin filaments

Zone of overlap:

-The densest, darkest area on a light micrograph

-Where thick and thin filaments overlap

Sarcomeres: I Band

Made up of the

Z lines:

-The centers of the I bands

-At two ends of sarcomere

Titin:

-Are strands of protein

-Reach from tips of thick filaments to the Z line

-Stabilize the filaments

What does this image represent?

Skeletal muscle

What does this image represent?

Muscle Fascicle

What does this image represent?

Muscle Fiber

What does this image represent?

Myofibril

What does this image represent?

Sarcomere

Thin filaments (F-Actin = filamentous actin)

-is two twisted rows of globular G-actin

-The active sites of G-actin strands bind to myosin

Thin filaments (Nebulin)

-Holds F-actin strands together

Thin Filaments (Tropomyosin)

Trop “rope” omyosin

-is a double strand

-prevents actin-myosin interaction

Thin filaments (Troponin)

-A globular protein

-binds tropomyosin to G-actin

-controlled by Ca2+

Initiating Contraction

• Ca2+ binds to receptor or troponin molecule

• Troponin - Tropomyosin complex changes

• Exposes active site of F-Actin

Thick filaments

The myosin molecule

The tail:

-binds to other myosin molecules

The head:

-made of two globular protein subunits

-reaches the nearest thin filament

Myosin Action

• During contraction, myosin heads:

-interact with actin filaments, forming cross-bridges

-Pivot, producing motion

*Sliding filament theory 

-thin filaments of sarcomere slide toward M line, alongside thick filaments

-the width of A zone stays the same

-Z lines move closer together 

Skeletal muscle contraction

The process of contraction:

• Neural stimulation of sarcolemma

-causes excitation-contraction coupling

• Muscle fiber contraction

-interaction of thick and thin filaments

• Tension production

The control of skeletal muscle activity

• The Neuromuscular junction (NMJ)

-special intercellular connection between the nervous system and skeletal muscle fiber

-controls calcium ion release into the sarcoplasm

Excitation-Contraction coupling

• Action potential reaches a triad

-releasing Ca2+

-Triggering contraction

• Requires myosin heads to be in “cocked” position

-loaded by ATP enegry

The contraction cycle:

1. Contraction cycle begins

2. active-site exposure

3. cross-bridge formation

4. myosin head pivoting

5. cross-bridge detachment

6. myosin reactivation

Fiber shortening

• As sarcomeres shorten, muscle pulls together, producing tension

• Muscle shortening can occur at both ends of the muscle, or at only one end of the muscle

-This depends on the way the muscle is attached at the ends

Relaxation

Contraction duration

• Depends on:

-Duration of neural stimulus

-number of free calcium ions in sarcoplasm

-availability of ATP

• Ca2+ concentrations fall

• Ca2+ detaches from troponin

• Active sites are re-covered by tropomyosin

Rigor Mortis

• A fixed muscular contraction after death 

• Caused when:

-Ion pumps cease to function; ran out of ATP

-Calcium builds up in the sarcoplasm 

Tension production by muscle fibers

Tension Production by Muscle Fibers

• As a whole, a muscle fiber is either contracted or relaxed

• Depends on:

-The number of pivoting cross-bridges

-The fiber’s resting length at the time of stimulation

-The frequency of stimulation

A single muscle contraction

Twitch

Latent period (twitches)

• The action potential moves through sareolemma

• causes Ca2+ release

Contraction phase (twitches)

• Calcium ions bind

• tension builds to peak

Relaxation phase (twitches)

• Ca2+ levels fall

• active sites are covered and tension falls to resting levels

Treppe

• A stair-step increase in twitch tension

• repeated stimulations immediately after relaxation phase

-Stimulus frequency 50 /second

• Causes a series of contractions with increasing tension

  Increase in peak tension

Wave summation

• increasing tension or summation of twitches

• repeated stimulations before the end of relaxation phase

-stimulus frequency 50/seconds

• Causes increasing tension or summation of twitches

Occurs when successive stimuli arrive before the relaxation phase

Incomplete tetanus

• Twitches reach maximum tension

• if rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tension.

Complete tetanus

• If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction.

Tension production by skeletal Muscles (depends on)

• internal tension produced by muscle fibers

•External tension exerted by muscle fibers on elastic extracellular fibers

• Total number of muscle fibers stimulated

Motor units in a skeletal muscle

• Contains hundreds of muscle fibers

• that contract at the same time

• controlled by a single motor neuron

Recruitment (multiple motor unit summation)

In a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated

Maximum tension

• achieved when all motor units reach tetanus

Can be sustained only a very short time

Sustained tension

•Less than maximum tension

•Allows motor units to rest in rotation

Muscle tone

• The normal tension and firmness of a muscle at rest

• Muscle units actively maintain body position, without motion

• Increasing muscle tone increases metabolic energy used, even at rest

Load and speed of contraction

• Are inversely related

• the heavier the load (resistance) on a muscle:

-the longer it takes for shortening to begin

-And the less the muscle will shorten

Muscle relaxation and the return to resting length (elastic forces)

• the pull of elastic elements (tendons and ligaments)

• Expands the sarcomeres to resting length

Muscle relaxation and the return to resting length (opposing muscle contractions)

• Reverse the direction of the original motion

•Are the work of opposing skeletal muscle pairs

Muscle relaxation and the return to resting length (gravity)

Can take the place of opposing muscle contraction to return a muscle to its resting state