Muscular System

6 Functions of Skeletal Muscle

-Producing movement

-Maintaining Posture

-Supports Soft Tissue

-Guards entrances and Exits

-Generates heat

-Stabilizes joints

Producing Movement

-Skeletal muscle contractions pull on tendons and move the bones of the skeleton

-The effects range from simple motions such as extending the arm or breathing to the highly coordinated movements

Maintaining Posture

-Tension in our skeletal muscles maintains body posture

-holding your head in position when you read a book or balancing your body weight above your feet when you walk

-Without constant muscle tension, we would not sit up straight

Supporting Soft Tissues

-The abdominal wall and the floor of the pelvic cavity consist of layers of skeletal muscle

-These muscles support the weight of visceral organs and shield internal tissues from injury

Guarding Exits and Entrances

-The openings of the digestive and urinary tracts are encircled by skeletal muscles

-provide voluntary control over swallowing, defecation, and urination

Generating Heat (Maintaining Homeostasis)

-Muscle contractions require energy; whenever energy is used in the body, some of it is converted to heat

-The heat released by working muscles keeps our body temperature in the range required for normal functioning

Joint Stabilization

Combination of movement & posture to prevent too much movement in a joint, especially in joints where bones don’t articulate well like the shoulder girdle (tendons play a vital role in this function)

Origin

Part of muscle attached to the immovable or less movable bone

Insertion

-part attached to the movable bone

-insertion moves toward the origin

Prime Mover

Muscles that has the major responsibility for causing a particular movement

Antagonist

-muscles that oppose or reverse a movement

-Biceps to the triceps, or triceps to the biceps

Synergists

help prime movers by producing same movements

Fixators

hold a bone still or stabilize the origin of a prime mover so all the tension can be used to move the insertion bone (Postural muscles)

Direction of the muscle fibers

usually a reference to a midline or long axis of a limb (i.e. – rectus = straight; oblique = at a slant to)

Relative size of muscle

maximus, minimus, longus

Location of Muscle

-named for bone associated with the muscle (i.e. – temporalis, tibialis)

-Only gives 1 location

Number of Origins

biceps brachii, triceps brachii

Location of muscle’s origin & insertion

-sternocleidomastoid (originates on sternum & clavicle, inserts on mastoid process of temporal bone)

-Will give 2 or more locations

Action of the muscle

flexor, extensor, adductor, etc.

Skeletal Muscle Contains

-Connective Tissue

-Blood vessels and nerves

-muscle tissue (cells or fibers)

Connective tissues

-Endomysium

-Perimysium

-Epimysium

Endomysium

delicate connective tissue sheath that encloses each muscle fiber

Perimysium

coarser fibrous sheath that surrounds muscle fiber bundles called fascicles

Epimysium

covers bundle of fasciculi (entire muscle); blends into either tendons or aponeurosis

Tendon

cord of dense, fibrous tissue attaching a muscle to a bone

Aponeurosis

fibrous or membranous sheet connecting a muscle and the part it moves (usually found on torso)

Sarcolemma

-plasma membrane of muscle fiber

-under the endomysium

Sarcoplasma

cytoplasm of muscle fiber

Peripheral nuclei

nuclei are pushed aside by myofibrils

Myofibrils

long rope-like organelles made of tiny contractile units called sarcomeres

2 myofilaments in Sarcomeres

-Myosin (Thick)

-Actin (Thin)

-Form a banding pattern (Striations)

Z Line

thin filaments at either end attached to interconnecting proteins, thin filaments extend from Z line

M Line

-zone of overlap of thick and thin filaments

-In the middle of the sarcomere

A Band

area containing thick filaments including some overlap with thin filaments (appears dark)

H Zone

area containing only thick filaments in center of A band

I Band

area containing only thin filaments, including Z line (appears Light)

Transverse Tubules (T Tubules)

-Openings along the sarcolemma that form passageways through the muscle fiber

-Transmit action potential through cell and allow entire muscle fiber to contract simultaneously

Sarcoplasmic reticulum (SR)

-Specialized form of smooth endoplasmic reticulum that forms a tubular network around each myofibril

-Releases Ca2+ into sacromeres to begin muscle contraction and helps transmit action potential to myofibril

Terminal cisternae

are expanded chambers of SR on either side of T-tubule

Triad

A T-tubule sandwiched between 2 terminal cisternae

Actin (Thin) Filaments

-Consists of twisted strand of actin molecules

-Each actin molecule has an active site that interacts with myosin

-Resting muscle, actin molecules covered with the protein tropomyosin which is held in position by troponin

Myosin (Thick) Filaments

-Composed of myosin molecules with a tail and globular head

-Myosin heads attach to actin molecules during contraction (can’t happen until troponin changes position revealing the active sites)

Sliding Filament Theory

-I band gets smaller Z lines move closer together

-H bands decrease Zones of overlap get larger

-Width of the A bands does not change

-Actin filaments slide toward center of the sarcomere aside of stationary myosin filaments

-Cross bridges are created when myosin heads connect to active sites on actin filaments

-Power Stroke- myosin heads drag actin filaments in towards the center

Power Stroke

attach, pivot, detach, and return

Irritability

the ability to receive and respond to a stimulus

Contractility

the ability to shorten (forcibly) when an adequate stimulus is received

Nerve stimulus and action potential

one motor neuron may stimulate a few muscle cells or hundreds of them, depending on the particular muscle and the work it does

Motor unit

one neuron and all the skeletal muscle cells it stimulates

Neuromuscular junction

where the axon (synaptic) terminals form junctions with the sarcolemma

Step 1 Action Potential

-When the nerve impulse reaches the axon terminals, a neurotransmitter travels across the synaptic cleft

-acetylcholine (ACh) – neurotransmitter that stimulates skeletal muscle

Step 2 Action Potential

Ach attaches to receptors which makes the membrane more permeable to Na+

Step 3 Action Potential

Na+ diffuses in and K+ rushes out, generating an action potential (electrical impulse), which travels over the entire surface of the sarcolemma

Step 10 Action Potential

ACh is removed by acetylcholinesterase (AChE) to stop contraction

Step 4 Muscle Contraction

AP travels down T-tubules, which causes Ca2+ to be released from the lateral sacs of the sarcoplasmic reticulum

Step 5 Muscle Contraction

Ca2+ binds to tropinin, causing the troponin-tropomyosin complex to move out of the way – exposing the active site on the actin filament

Step 6 Muscle Contraction

Myosin heads swing back and attach to the active site on actin, forming cross-bridges

Step 7 Muscle Contraction

Myosin heads perform a power stroke – move toward the center of the sarcomere pulling actin filaments towards the center of the sarcomere

Step 8 Muscle Contraction

ATP is broken down to provide energy for the myosin heads to release the active site; leftover energy is stored for the next power stroke

Step 9 Muscle Contraction

Myosin heads grab further & further back each time, whole muscle shortens

Tension

-an active force produced when muscle cells contract, pulling on collagen fibers

-Tension must be greater than resistance in order to move

Resistance

a passive force that opposes movement, needs to be overcome before movement can occur

Compression

-a push applied to an object

-muscle fibers can’t do this, they only contract

All or None Law

-a muscle fiber will contract to its fullest extent when it is stimulated adequately

-it never partially contracts

-is true of muscle cells only (not whole muscle)

-Muscle cells react to stimuli with graded responses or different degrees of shortening

2 Ways graded responses are produced

-By changing number of muscle cells being stimulated

-By changing frequency of muscle stimulation

Changing the number of cells stimulated

-How forcefully a muscle contracts depends largely on the number of muscle cells stimulated

-when only a few cells are stimulated, contractions will be slight

-when all cells are stimulated, contraction is strong

Muscle Twitch

a single, brief contraction (7-100 msec)

Latent Period

-Where there is no tension yet

-AP moves through the Sarcolemma

-Ca2+ released

Contraction Phase

-Maximum tension released

-Cross-bridges interact with active site

Relaxation Phase

-Tension falls to resting

-Ca2+ levels fall

-Active sites covered, number of cross-bridges decline

Nerve Impulses

delivered to the muscle at a very rapid rate, so rapid that muscle does not get a chance to relax completely between stimuli as a result, the effects of the successive contractions are added together (summation)

Incomplete Tetanus

when a muscle produces almost peak tension during rapid cycles of contraction and relaxation

Complete Tetanus

when maximum tension has been reached

Recruitment

the activation of more and more motor units, results in smooth steady increase in tension

Muscle Tone

some motor units are always active, but do not produce enough tension to cause movement

Atrophy

occurs when skeletal muscle is not regularly stimulated, muscle fibers become smaller and weaker

Isotonic Contraction

when myofilaments are successful in sliding movement and muscle shortens

Isometric Contraction

when muscles do not shorten b/c muscles are pitted against some more or less immovable object, but tension keeps building but never exceeds the resistance

Direct phosphorylation of ADP by creatine phosphate

-a phosphate group transfers from CP to ADP, regenerating more ATP regulated by creatine phosphokinase - CPK CP supplies exhaust in about 20 seconds

-Phosphate breaks off of CP and joins with ADP

-P is broken off by the CPK

-One creatine is left, which will go bond with other phosphate

-Lasts for roughly 20 seconds

Slow Twitch Muscle Fibers (Type 1)

-Mostly aerobic

-Smaller diameter

-Take longer to contract

-Can contract for extended periods of time

-Oxygen supply greater due to more extensive capillary network

-Can store oxygen because of red myoglobin pigment that binds with oxygen

-Contain more mitochondria

-Due to capillaries and myoglobin, appear reddish

Fast twitch muscle fibers (Type 2)

-Mostly anaerobic

-Large diameter with densely packed myofibrils and large glycogen reserves

-Quick to contract (good for quick bursts of strength or speed)

-Fatigue quickly, few mitochondria

-Appear pale so termed white muscles or “white meat”

Type 2a Fibers

-Also known as intermediate fast-twitch fibers

-They can use both aerobic and anaerobic metabolism almost equally to create energy

-They are a combination of Type I and Type II muscle fibers.

Type 2b (2x) Fibers

-use anaerobic metabolism to create energy

-the "classic" fast twitch muscle fibers that excel at producing quick, powerful bursts of speed.

Anaerobic glycolysis

-No oxygen required, occurs in cytoplasm

-Glucose is converted to pyruvic acid where energy is captured in ATP bonds

-Get 2 ATP/1 glucose If oxygen, go to aerobic respiration If no oxygen (i.e. – intense muscle activity), go to lactic acid fermentation

-Glycolysis = Breaking down glucose

Aerobic respiration

-provides 95% of ATP at rest and during light exercise

-occurs in mitochondria & involves a series of metabolic pathways that use oxygen – called oxidative phosphorylation

-2 Pyruvic Acids + O2 →Kreb’s Cycle →CO2 + H20 + 34 ATP

-36 ATP from each glucose molecule

-The extra 2 come from Glycolysis

Stored ATP

Lasts 4-6 Seconds

Anaerobic respiration & Lactic acid fermentation

-No oxygen

-When we need quicker (short term) energy

Only lasts a few minutes

-2 Pyruvic Acids + 2 ATP →Lactic Acid + 4 ATP

-Net Gain is 2 ATP for each glucose

- causes muscle soreness and fatigue (muscle fatigue occurs when the muscle can no longer contract despite still being stimulated because ATP is depleted).

-It results from oxygen debt which must be “paid back” (recovery period) by taking deep breaths