Physiology: Chapter 12

thick filaments

A filament composed of staggered arrays of myosin molecules; a component of myofibrils in muscle fibers.

tropomyosin

A helical protein that winds around actin helices in skeletal and cardiac muscle cells to form the thin filament of the sarcomere. In the absence of Ca2+, tropomyosin covers the myosin-binding sites on actin and prevents muscle contraction. When calcium is present, a conformation change in tropomyosin occurs so that the myosin-binding sites are exposed and muscle contraction can occur.

F actin

A fibrous protein made of a long chain of G actin molecules twisted into a helix; main protein of the thin myofilament

creatine phosphate

An energy storage molecule used by muscle tissue. The phosphate from creatine phosphate can be removed and attached to an ADP to generate ATP quickly.

fascicles

Bundle of muscle fibers

relaxation phase

Calcium is being actively transported back into the terminal cisternae causing calcium levels to fall, active sites on Actin are being re-covered by tropomyosin, and tension falls to resting levels: lasts about 25msec

annulospiral ending

In skeletal muscle, the first type of nerve ending of the stretch receptors that wraps itself around the center of a nuclear bag muscle fiber.

transverse tubules

System of tubules that provides channels for ion flow throughout the muscle fibers to facilitate the propagation of an action potential.

size principle

The idea that, as increasing numbers of motor neurons are recruited to produce muscle responses of increasing strength, small, low-threshold neurons are recruited first, followed by large, high-threshold neurons.

Sliding-Filament model

The theory explaining how muscle contracts, based on change within a sarcomere, the basic unit of muscle organization, stating that thin (actin) filaments slide across thick (myosin) filaments, shortening the sarcomere; the shortening of all sarcomeres in a myofibril shortens the entire myofibril

G actin

a globular subunit of F actin with an active site for binding a myosin head

troponin

a regulatory protein that is a component of the thin filament. When calcium ions (Ca2+) bind to troponin, it undergoes a change in shape; this conformational change moves tropomyosin away from myosin-binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin

tetanus

a sustained muscular contraction resulting from a rapid series of nerve impulses; summation peak

isotonic twitch

against a constant load which moves the load. muscle contracts and shortens.

contraction phase

cross bridge actively forms and muscle shortens, tension rises to a peak, Ca2+ bind to troponin, cross-bridges formed

latent period

delay of a few milliseconds that occurs between the action potential in a muscle cell and the start of the contraction

fast myosin

hydrolyzes ATP fast rate = faster fiber contraction (fast twitch fiber)

recruitment

increase in the number of active motor units

myosin heads

interact with actin, form cross bridges, pivot the tail, producing motion, Contain ATP binding sites.

intrafusal fibers

located on the inner part of the muscle, these fibers are the sensory fibers.

antagonistic muscles

muscle pairs arranged to work against eachother to move a joint

twitch

single stimulus-contraction-relaxation sequence in a muscle fiber

slow oxidative fibers

smallest. very fatigue resistant. aerobic generation of ATP. red fibers. high myoglobin content/ mitochondria. slow contractions.

sarcomere

the basic contractile unit of striated muscle; the segment of a myofibril between two adjacent z-lines

cardiac muscle

the muscle tissue of the heart

summation

when a muscle is stimulated repetitively such that additional action potentials arrive before twitches can be completed, the twitches superimpose on one another, yielding a force greater than that of a single twitch; whenever twitches occur at a frequency suh that calcium cannot be removed from the cytosol as rapidly as it is released from the SR

myofibrils

Contractile fibers in muscle cells

sarcoplasmic reticulum (SR)

The smooth ER of a muscle cell, enlarged and specialized to act as a Ca2+ reservoir. The SR winds around each myofibril in the muscle cell.

smooth muscle

a muscle that contracts without conscious control and found in walls of internal organs such as stomach and intestine and bladder and blood vessels (excluding the heart)

oxidative fibers

aerobic, use oxygen during ATP synthesis, fatigue resistant, fibers are well vascularized, lots of capillaries, many mitochondria and myoglobin, and red in color. They are smaller and give out less force, but are fatigue resistant.

glycolytic fibers

fibers that produce ATP by glycolosis. less mitochondria,blood, myoglobin, fatigue easily. generates a lot of strength

pacemaker potentials

initiate the action potentials that spread out through the heart to trigger its rythmic contractions, Na+ enters into cells through special NONGATED Na+ channel. K+ permeability decreases leading to its retention to inner membrane. Some voltage gated Ca+ channel begins to open. Together, these cause the generation of a local potential, also called pacemaker potential. When pacemaker potential reaches the threshold, many voltage gated Ca+ channels open & the rest of the event is same as any cardiac muscle cell. Therefore, unlike other cardiac muscle cells where Na+ ion is responsible for depolarization, in case of pacemaker cells Ca+ ion is primarily responsible for depolarization beyond threshold.

muscle shortening velocity

latent period of shortening increases with increasing load duration of shortening decreases with increasing load velocity of shortening decreases with increasing load

isometric twitch

muscle develops force NO change in length of muscle EX: muscle contracting against something it can't move

sarcolemma

plasma membrane of a muscle fiber

golgi tendon organs

receptors that sense movement of the tendons, which connect muscle to bone

length-tension curve

shows that the active tension is maximal when the muscle is near its rest length, active tension decreases when the muscle is stretched or shortened.

muscle fibers

skeletal and smooth muscles; are elongated; diameter ranges from 10 to 100 um (10 times that of an avergage body cell); multiple nuclei

multi unit smooth muscle

smooth muscle cells that contract individually because they are not coupled together. Each cell must be stimulated independently (i.e. smooth muscle of the uterus).

thin filaments

strand of Actin. Each actin has an active site that can interact with Myosin. Active sites are covered by tropomyosin strands, which are held in place by Troponin.

extrafusal fibers

the skeletal muscle fibers that form the bulk of the muscle (outside the muscle spindle) and generate its force and movement

fast glycolytic fibers

these fibers are biggest in diameter, quick, very powerful but tire easily; uses anaerobic pathways to make atp; contain few mitochondria and capillaries; has little myoglobin; 2 atp

muscle spindles

Fibers that are sensitive to change in length of muscle and rate of that change, major sensory organs of muscle. parallel to muscle fibers. transmit info to cns when stretched. causes muscle to contract to prevent overstretching/ stretching too fast.

crossbridges

Formed by the myosin heads attaching to actin, The force of muscle contraction is most related to the number of?

treppe

If stimuli are repeated at regular time intervals, and the muscle has ample time to recover between stimuli, successive contractions show an increase in contraction force. A staircase phenomenon.

Crossbridge Cycle

-the sequence of events by which a myosin crossbridge binds to actin, generates tension, then detaches from actin -before this begins, the crossbridge holds an ADP and Pi on its ATPase site and is "springloaded" in the high-energy position. -in this position, the crossbridge attempts to bind with actin but tropomyosin prevents it by blocking the myosin-binding sites -however CA ions bind to troponin and cause it to move the tropomyosin off the binding sites, allowing a crossbridge cycle to begin, 1) Myosin head attaches to actin myofilament at myosin binding site 2) ADP and Pi released as myosin head bends and pulls on the actin filament, sliding it toward the M-line 3) A new ATP attaches to the head and myosin detaches 4) ATP split into ADP and Pi, cocking the myosin head back so it is ready to attach to another bit of actin, Step 1: At rest, the situation is the tropomyosin-troponin complexes blocking the myosin binding sites on actin. The globular heads of myosin (in the presence of ATP) have bound ATP and hydrolyzed it using their intrinsic ATPase activity. As such, they have now bound ADP-Pi (into a M-ADP-Pi complex). 2. When Ca2+ is released, it binds to Tn-C and the tropomyosin-troponin complex exposes the myosin binding sites on actin. M-ADP-Pi binds the myosin-binding site on actin. 3. The binding of actin and myosin causes ADP-Pi to be released from myosin, which in turn causes the tilt of the globular head of myosin towards the Z-line (called the power stroke). 4. ATP binds to the globular head, which causes it to dissociate from actin. 1. The intrinsic ATPase activity of the globular head hydrolyzes ATP into ADP-Pi, starting the cycle over again.

skeletal metabolic pathways

1. Creatine phosphate method of regenerating ATP during limited duration (15 sec.) muscle activity: 1CP = 1ATP 2. Anaerobic respiration and glycolysis (wasteful) provides energy for 30-40 sec.: 1 glucose = 2ATP 3. Aerobic respiration (slow to start, but efficient and powerful) kicks in after about 10 min of sustained activity and can last for hours: 1 glucose = 36ATP

excitation-contraction coupling

The mechanism that ensures tehat skeletal muscle contraction does not occur without neural stimulation (excitation). A trest, cytosolic [Calcium] is low, and the troponin, tropomyosin complex covers the myosin-binding sites on actin. When the muscle is stimulated by a neuron, calcium is released from the sarcoplasmic reticulum into teh cytosol of the muscle cell. Calcium binds to troponin, causing a conformation change in the troponin-tropomyosin complex that shifts it away from the myosin-binding sites. This allows mysoin and actin to interact according to the sliding filament theory.