A&P 1: module 5

skeletal muscle tissue

3 types of muscle tissue

skeletal, cardiac, smooth

muscle tissue

respond to stimuli as their plasma membrane can change their electrical states from polarized to depolarized; depolarization sends an electrical wave called action potential along the entire length of the membrane; differences include the microscopic organization of the contractile proteins (actin and myosin) - are arranged in the cytoplasm very regularly which creates a pattern, or stripes called striations

muscle tissue characteristics

elasticity, extensibility, contractibility, excitability

elasticity

can return to its original length when relaxed

extensibility

can stretch

contractability

allows muscle tissue to pull on its attachment points and shorten with force when stimulated

excitability

responds to stimuli (also called responsiveness or irritability)

muscle fiber

individual skeletal muscle cell; shape is very long and cylindrical; multinucleated structures that compose the skeletal muscle; skeletal muscle fibers under voluntary control, but relies on electrical stimulation from the nervous system to contract, produce heat when they contract, also function to maintain body posture and making continual small adjustments to hold the body up against gravity - need time to recover after full contractability

cardiac muscle tissue

only found within the heart; each fiber only has one nucleus; cells are cylindrical, branched, and connected via intercalated discs; does not fatigue with contraction

intercalated discs

allow muscle fibers to be physically and electrically connected to each other so that the heart contracts as one unit, called a syncytium; considered involuntary; does not rely on an electrical impulse to contract; function is to continuously pump blood throughout the body

smooth muscle tissue

cytoplasm of the cells have a uniform non striated appearance and is grouped together in sheets; cell only has one nucleus; helps regulate the blood pressure necessary to push blood through the circulatory system; essential for moving all materials through the body and altering the internal volume of body structures; controlled involuntary; does not fatigue with contraction; spindle shaped cells

skeletal muscle function

ability to contract and cause movement by pulling on bones; produce and stop movement; prevent excess movement of the bones and joints, maintaining skeletal stability, and preventing skeletal structure damage or deformation; work to keep joints stable; work to protect internal organs in the abdomen and pelvis by forming the abdominal wall and by supporting the weight of the organs; maintain body temperature homeostasis by generating heat (as muscle contraction breaks down ATP and this energy is “burned”, heat is produced); stores nutrient reserves

skeletal muscle structure

each muscle is an organ that consists of various integrated tissues; each one includes primarily the skeletal muscle tissue, blood vessels, nerves, and connective tissue; each muscle has 3 layers of connective tissue that enclose it and provide structure to the muscle: episysium, perimysium, and endomysium

epimysium

dense, irregular connective tissue made of collagen; wraps each skeletal muscle; connected to the deep fascia; above the muscle; allows a muscle to contract and move powerfully while maintaining its structural integrity; separates muscle from other tissues and organs in the area, allowing the muscle to move independently

fascicle

muscle fibers organized and grouped into bundles; inside each skeletal muscle; covered by a middle layer of connective tissue (perimysium); allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibers within a bundle of the muscle

perimysium

middle layer of connective tissue; wraps around a fascicle; contains collagen fibers, elastic fibers, blood vessels, and nerves

endomysium

thin, connective tissue layer of collagen and reticular fibers; innermost connective tissue layer, wrapping around the actual cells; contains capillary networks which supply nutrients from the blood and extracellular fluids to support the muscle tissue; nerve fibers are also found to innervate muscle fibers to enable movement

aponeurosis

attaches skeletal muscle to bone at each end; forms in a broad sheet; fuses with the periosteum; tension created by contraction of muscle fibers is transferred through the connective tissue to the tendon or aponeurosis and then to the periosteum to pull on the bone for movement of the skeleton (ex: broad sheet of connective tissue in the lower back that latissimus dorsi muscle)

blood and nerve supply - skeletal muscle

every one is richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal, the epimysium and perimysium carry these blood vessels and nerves; every msucle fiber is also supplied by the axon branch of a neuron which signals the fiber to contract - the only way to contract a skeletal muscle is through direct signaling from the CNS; voluntary muscle

muscle fibers

skeletal muscle cells; long, cylindrical and much larger than other cells; formed during early development

myoblasts

embryonic cells; each have their own nucleus; fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers; multinucleated cells allow for multiple copies of genes, permitting the production of large amounts of proteins and enzymes needed for msucle contraction; some remain in the developing muscle fibers and do not fuse - becoming myosatellite cells

myosatellite cells

found in the endomysium; stem cells that repair damage; after injury they can divide to replace damaged muscle fibers

skeletal muscle fibers

known as striated muscle cells - due to the regular arrangement of actin and myosin present when looking at a cell under a microscope

sarcolemma

plasma membrane of muscle fibers

sarcoplasm

cytoplasm of muscle fibers

transverse tubules

or T-tubules; invaginations and branching tubes in the sarcolemma; extend from the surface of the muscle fiber deep into the sarcoplasm; ensure the action potential reaches all parts of the muscle fiber reaches all parts of the muscle fiber by carrying the action potential from the sarcolemma into the center of the cell

myofibrils

divisions within a muscle fiber which are responsible for muscle contraction; surrounded by branches of t-tubules; contain an elastic protein titin

myofilaments

bundles of thick and thin protein filaments

thin protein filaments

composed primarily of actin

thick filaments

composed primarily of myosin; titan extends from thick filaments

sarcoplasmic reticulum (SR)

specialized smooth endoplasmic reticulum which surrounds each myofibril; forms chambers called terminal cisternae; requests and releases calcium ions which must be actively transported into the terminal cisternae

terminal cisternae

enlarged, expanded chambers that attach SR to t-tubules

sarcomere

functional unit of a skeletal muscle fiber; highly organized, repeating unit of the contractile myofilaments actin, myosin, and other support proteins; each one is bundled within the myofibril that runs the entire length of the muscle fiber and attaches to the sarcolemma at its end; bordered by structures called Z-discs/z-lines

z-discs/z-lines

attachment sites for actin myofilaments; sarcomere is defined by the z-line on each side

dark band (A band) vs light band (I band)

contains both thick and thin filaments vs contains only thin filaments

M line

contains proteins for the attachment of the thick filaments at the center of the A band and center of the sarcomere; proteins stabilize the positions of the thick filaments

H zone (H band)

contains only thick filaments on either side of the M line

zone of overlap - sarcomere

dark region where the thick and thin filaments overlap

titin

specialized elastic protein filament that originates at the tip of each thick filament and attaches to the Z line; helps to restore the original sarcomere length at rest and helps to stabilize the myosin filaments

structure of thin filaments

made of F-actin, tropomyosin, troponin

F-actin

filamentous actin; long, twisted strand of two rows of spherical G-action (globular) molecules - contains the active sites which bind to myosin

tropomyosin

long, double twisted rope like protein that runs the length of the F-actin strand; 2 strands cover the active sites on G-actin, preventing actin from binding to myosin

troponin

globular protein with 3 components that attaches to tropomyosin, G-actin, and calcium

troponin-tropomyosin complex

prevents the myosin heads from binding to the active sites on the G-actin molecules

structure of thick filaments

contains about 300 myosin molecules with a core of elastic titin to stabilize the myosin molecules and prevent overstretching of the sarcomere; cross bridge is formed when the myosin heads attach to the thin filaments during a muscle contraction

myosin structure

has 2 parts: the tail and the head; the tail is long and locks into the other myosin molecules of the thick filament - faces toward the M-line; the head portion is made of 2 globular proteins that extend out in the direction of the nearest thin filament where it attaches - the head can pivot toward or away from the M line because of its hinge

sliding filament theory

explanation for how muscles contract; thin filaments are pulled and then slide past the thick filaments within the fibers sarcomeres; the sarcomeres shorten as the area of overlap increases for the myofilaments; the sliding can only occur when myosin binding sites on the actin filaments are exposed by a series of steps that begins with calcium entry into the sarcoplasm

origin of a muscle vs insertion of muscle

end of the muscle that is fixed; insertion: end that is free to move

membrane potentials

electrical gradients; all cells have across their membranes; a cells resting membrane potential: -60 to -90mV

excitable (muscle cells)

can change their membrane potentials and generate electrical signals called action potentials; they do this by controlling the movement of charged particles (ions) across their membranes to create electrical currents

ion channels

specialized proteins in the membrane; very small; form the basis of both neural signaling and muscle contraction

action potential

special type of electrical signal that can travel along a cell membrane as a wave; this allows a signal to be transmitted quickly over long distances

neuromuscular junction (NMJ)

site where a motor neuron’s terminal meets the muscle fiber

synaptic cleft

small space between motor neuron and muscle fiber

motor unit

group of prime movers for a particular action; group of muscle fibers in a muscle innervated by a single motor neuron; size varies depending on task of motor unit - large motor units: concerned with larger or gross motor movements (extending the knee joint)

motor end plate

muscle cell membrane across from the motor neuron; contains folds with a high density of receptors, allowing it to bind with acetlycholine

depolarization

movement of positive ions across a plasma cell membrane, causing a net change in charge

acetylcholinesterase (AChE)

breaks apart acetylcholine into acetic acid and choline

acetylcholine (ACh)

neurotransmitter stored at the end of the axon terminals; transverses the synaptic cleft; once removed, the membrane sodium potassium channels close, allowing the concentration signal to stop

excitation-contraction coupling

phenomenon that helps explain the fast sequence of events seen in a muscle cell; for a muscle to contract the membrane must first be “excited” (stimulated to fire an action potential); this excitation is “coupled” to the actual contraction of the muscle fiber through the release of calcium ions from the SR, once released calcium ions bind to troponin; binding of troponin pulls tropomyosin, exposing the active sites, once they are exposed the contraction cycle begins

cross bridge cycle

sustained by ATP; ordered steps that allow for muscular contraction

ending a muscle contraction

contraction cycle happens over and over, shortening the sarcomeres which shortens the myofibril, as the myofibril get shorter so does the muscle; process begins to end when signaling from the motor neurons ends; ACh is broken down by AChE ending the action potential generation; cross bridge formation ends and without it muscle contraction ends, allowing muscle relaxation to occur

opposing muscle contractions

muscles must work in antagonistic pairs; as one muscle is contracted the other is relaxed; the muscle group that is contracting is also acting to lengthen the opposing muscle group (ex: as biceps contract, the triceps are stretched)

elastic forces

as muscle energy is spent of contracting it is also used to stretch tendons and other elastic components; the elasticity of the muscle fibers and the fibers on which they pull helps to recoil and return the muscle to its original length