Anatomy&Physiology Ch 9&10

Isotonic contractions

Muscle length changes and moves a load

Terminal cisterns

"End sacs" form larger, perpendicular cross channels at the A band-I junctions and they always occur in pairs.

Four characteristics of muscle tissue

-excitability -contractility -extensibility -elasticity

Glycosomes

Granules of stored glycogen that provide glucose during muscle activity. The cytoplasm of a muscle cell (sarcoplasm) contains large amounts of these

Thin filament

More lateral, contain actin (blue) and extend across the I band and partway into the A band. The Z disc(a coin-shaped sheet composed largely of the protein alpha-actinin) anchors the thin filaments

Generating heat

Muscles generate heat as they contract. This heat is important in maintaining normal body temperature. Because skeletal muscle accounts for at least 40% of body mass, it is the muscle type most responsible for generating heat

Voluntary muscles

Muscles subject to conscious control. Include skeletal muscles

Muscle fibers

the name that designates skeletal and smooth muscle cells because they are elongated

Sarolemma

the plasma membrane of a muscle cell

Rigor mortis

(Death rigor) illustrates the fact that cross bridge detachment is ATP driven. Most muscles begin to stiffen 3 to 4 hours after death. Peak rigidity occurs at 12 hours and then gradually dissipates over the next 48-60 hours. Dying cells are unable to exclude calcium (which is in higher concentration in the extracellular fluid), and the calcium influx into muscle cells promotes formation of myosin cross bridges. Shortly after breathing stops, ATP synthesis ceases, but ATP continues to be consumed and cross bridge detachment is impossible. Actin and myosin become irreversibly cross linked, producing the stiffness of this death rigor, which gradually disappears as muscle proteins break down after death

Principles of muscle mechanics

-the principles governing contraction of a single muscle fiber and of a skeletal muscle consisting of s large number of fibers are pretty much the same -the force exerted by a contracting muscle on an object is called muscle tension. The opposing force exerted on the muscle by the weight of the object to be moved is called the load -a contracting muscle does not always shorten and move the load. If muscle tension develops but the load is not moved, the contraction is called isometrics. If the muscle tension developed overcomes the load and muscle shortening occurs, the contraction is isotonic

How does a motor neuron stimulate a skeletal muscle fiber?

-when a nerve impulse reaches the end of an axon, the axon terminal releases ACh into the synaptic cleft -ACh diffuses across the cleft and attaches to ACh receptors on the Sarcolemma of the muscle fiber -ACh binding triggers electrical events that ultimately generate an action potential

Events that happen in the filament model of contraction

-when the nervous system stimulates muscle fibers, the myosin heads on the thick filaments latch onto myosin-binding sites on actin in the thin filaments, and the sliding begins -these cross bridge attachments form and break several times during a contraction, acting like tiny ratchets to generate tension and propel the thin filaments toward the center of the Sarcomere -as this event occurs simultaneously in Sarcomeres throughout the cell, the muscle cell shortens -notice that as the thin filaments slide centrally, the Z discs to which they attach are pulled toward the M line. Overall, as a muscle cell shortens: 1.)the I bands shorten 2.)the distance between successive Z discs shortens 3.)the H zones disappear 4.)the continuous A bands move closer together but their length does not change

3 steps involved in the generation and propagation of an action potential in a skeletal muscle fiber

1.)generation of an end plate potential 2.)depolarization:generation and propagation of an action potential 3.)repolarization:restoring the Sarcolemma to its initial polarized state

Twitch myogram's three phases

1.)latent period-is the first few milliseconds following stimulation when excitation-contraction coupling is occurring. During this period, cross bridges begin to cycle but muscle tension is not yet measurable and the myogram does not show a response 2.)period of contraction-cross bridges are active, from the onset to the peak of tension development, and the myogram tracing rises to a peak. This period lasts 10-100 ms. If the tension (pull) becomes great enough to overcome the resistance of the load, the muscle shortens 3.)period of relaxation-lasts 10-100 ms, is initiated by re entry of Ca into the SR. Because contractile force is declining, muscle tension decreases to zero and the tracing returns to the baseline. If the muscle shortened during contraction, it now returns to its initial length. Notice that a muscle contracts faster than it relaxes, as revealed by the asymmetric nature of the myogram tracing

What induces a contraction in a skeletal muscle fiber?

1.)the fiber must be activated, that is, stimulated by a nerve ending so that a change in membrane potential occurs 2.)next, it must generate an electrical current, called an action potential, in its Sarcolemma 3.)the action potential is automatically propagated along the Sarcolemma 4.)then, intracellular calcium ion levels must rise briefly, providing the final trigger for contraction

Steps following a constant stimulus strength and a muscle stimulated at an increasingly faster rate

1.)the relaxation time between twitched becomes shorter and shorter 2.)the concentration of Ca in the cytosol rises higher and higher 3.)the degree of wave summation becomes greater and greater and progresses into a unfused or incomplete tetanus

Muscle functions

4 important functions: 1.)producing movement 2.)maintaining posture and body position 3.)stabilizing joints 4.)generating heat

Myasthenia gravis

A disease characterized by dropping upper eyelids, difficulty swallowing and talking, and generalized muscle weakness and involves a shortage of ACh receptors. Is an autoimmune disease

Wave or temporal summation

A phenomenon that shows that the nervous system achieves greater muscular force by increasing the firing rate of motor neurons. Occurs because the second contraction occurs before the muscle has completely relaxed

Myogram

A recording of contractile activity

Myoglobin

A red pigment that stores oxygen. Is similar to hemoglobin, the pigment that transports blood.

Striations

A repeating series of dark and light bands, are evident along the length of each myofibril. The dark A and light I bands are nearly perfectly aligned and give off this banded appearance

Tropomyosin

A rod-shaped protein that spirals about the actin core and help stiffen and stabilize it. Successive tropomyosin molecules are arranged end to end along the actin filaments and in a relaxed muscle fiber, they block myosin-binding sites on actin so that myosin heads on the thick filaments cannot bind to the thin filaments

Sarcomere

A segment of a Myofibrils. Is the contractile unit, composed of Myofilaments made up of contractile proteins

Fused or complete tetanus

A smooth, sustained contraction plateau, known as this

Infused or incomplete tetanus

A sustained but quivering contraction referred to as this

Acetylcholinesterase

After ACh binds to the ACh receptors, its effects are quickly terminated by this enzyme located in the synaptic cleft. This enzyme breaks down ACh to its building blocks, acetic acid and choline. This removal prevents continued and most likely undesirable muscle fiber contraction in the absence of additional nervous system stimulation

Triads

Along its length, each T tubule runs between the paired terminal cisterns of the SR, forming these successive groupings of the three membranous structures (terminal cistern, T tubule, and terminal cistern)

Thick filaments

Are 16 mm in diameter. Contain myosin (red) and extend the entire length of the A band. They are connected in the middle of the Sarcomere at the M line.

More on skeletal muscle fiber

Are huge cells, with a diameter that ranges from 10-100 um-up to ten times that of a average body cell-and their length is up to 30 cm long. The fibers are produced by hundreds of embryonic cells fused together

Tendons

Are mostly tough collagen fibers which can withstand the abrasion of rough bony projections that would tear apart the more delicate muscle tissues. Because of their relatively small size, more tendons than fleshy muscles can pass over a joint-so they also conserve space

More on indirect muscle attachments

Are much more common because of their durability and small size.

Nebulous, myomesin, and c proteins

Are other proteins that bind filaments or Sarcomeres together and maintain their alignment

More on myofibrils

Are rodlike in appearance and 1-2 um in diameter. Are so densely packed in the fiber that mitochondria and other organelles appear to be squeezed between them. They account for about 80% of cellular volume

Skeletal muscle fibers

Are the longest muscle cells and have obvious stripes called striations

Myofibril or fibril

Complex organelle composed of bundles of Myofilaments. Are rodlike contractile elements that occupy most of the muscle cell volume. Appear banded because of sacromeres arranged end to end

Muscle

Consists of hundreds to thousands of muscle cells, plus connective tissue wrappings, blood vessels, and nerve fibers Covered externally by the epimysium

Motor unit

Consists of one motor neuron and all the muscle fibers it innervates, or supplies.

Recruitment or multiple motor unit summation

Controls the force of contraction more precisely. In the laboratory, this would be achieved by delivering shocks of increasing voltage to the muscle, calling more and more muscle fibers into play

Refractory period

During repolarization, a muscle fiber is said to be in this period because the cell canny be stimulated again until repolarization is complete

M line

Each H zone is bisected vertically by a dark line called this, which is formed by molecules of the protein myomesin

Neuromuscular junction or end plate

Each axon ending gives off several short, curling branches that collectively form this elliptical junction with a muscle fiber

H zone

Each dark A band has a lighter region in its midsection called this

Z disc

Each light I band also has a midline interruption, a darker area called this

Titin

Elastic filaments consist of this giant protein. It extends from the Z disc to the thick filament and then runs within the thick filament (forming its core) to attach to the M line. It holds the thick filaments in place, this maintaining the organization of the A band, and helps the muscle cell spring back into shape after stretching (the part of titin that spans the I band is extensible, unfolding when the muscle stretches and recoiling when the tension is released). Does not resist stretching in the ordinary range of extension, but it stiffens as it uncoils, helping the muscle resist excessive stretching which might pull the Sarcomeres apart

Summary of what happens to excite a muscle cell, starting from the nerve ending

Essentially this process activates four sets of ion channels: 1.)the process begins when the nerve impulse reaches the axon terminal and opens voltage-gated calcium channels in the axonial membrane. Calcium entry triggers release of ACh into the synaptic cleft 2.)released ACh binds to ACh receptors in the Sarcolemma, opening chemically gated NaK channels. Greater influx of Na causes a local voltage change (the end plate potential) 3.)local depolarization opens voltage-gated sodium channels in the neighboring region of the Sarcolemma. This allows more sodium to enter, which further depolarizer the Sarcolemma, generating and propagating an AP 4.)transmission of the AP along the T tubules changes the shape of voltage-sensitive proteins in the T tubules, which in turn stimulate SR calcium release channels to release Ca into the cytosol

Stabilizing joints

Even as muscles pull on bones to cause movement, they stabilize and strengthen the joints of the skeleton

Myofilament or filament

Extended marcromolecular structure. Contractile Myofilaments are of two types-thick and thin. Thick filaments contain bundled myosin molecules; thin filaments contain actin molecules (and other proteins). The sliding of the thin filaments past the thick filaments produces muscle shortening. Elastic filaments maintain the organization of the a band and provide elastic recoil when muscle contraction ends

Perimysium

Fibrous connective tissue that surrounds each fascicle ("around the muscle"-fascicles)

Origin(of muscle)

Immovable it less movable bone. In muscles of the limbs, this typically lies proximal to the insertion

Connective tissue sheaths

In an intact muscle, several different connective tissue sheaths wrap individual muscle fibers. Together these connective tissue sheaths support each cell and reinforce and hold together the muscle as a whole, preventing the bulging muscles from bursting during exceptionally strong contractions

Nerve and blood supply

In general, one nerve, one artery, and one or more veins serve each muscle. These structures will enter or exit near the central part of the muscle and branch through its connective tissue sheaths. Unlike cells of cardiac and smooth muscle tissues, which can contract without nerve stimulation, every skeletal muscle fiber is supplied with a nerve ending that controls its activity. Skeletal muscle has a rich blood supply. Muscle cells give off large amounts of metabolic wastes that must be removed through veins if contraction is to remain efficient. Muscle capillaries, are long and winding and have numerous cross links, features that accommodate changes in muscle length. They straighten when the muscle stretches and contort when the muscle contracts

Filaments or f actin

In the thin filaments, g actin subunits are polymerized into these long actin filaments.

Nerve cells that activate skeletal muscle fibers

Include: somatic motor neurons or motor neurons of the somatic (voluntary) nervous system Reside in the brain or spinal chord

Isometric versus isotonic contractions

Increasing muscle tension=isometric contraction Amount of muscle shortening=(distance in mm) is measured for isotonic contractions

Fascicles

Is a discrete bundle of muscle cells, segregated from the rest of the muscle by a connective tissue sheath. Surrounded by Perimysium

Gross anatomy of a skeletal muscle

Is a discrete organ, made up of several kinds of tissues. Skeletal muscle fibers predominate, but blood vessels, nerve fibers, and substantial amounts of connective tissue are also present

Muscle twitch

Is a motor unit's response to a single action potential of its motor neuron

Endomysium

Is a wispy sheath of connective tissue that surrounds each individual muscle fiber. It consists of fine areolar connective tissue. (means "within the muscle")

Epimysium

Is an "overcoat" of dense irregular connective tissue that surrounds the whole muscle. Sometimes it blends with the deep fascia that lies between neighboring muscles or the superficial fascia deep to the skin

Sarcoplasm in reticulum

Is an elaborate smooth endoplasmic reticulum. Its interconnecting tubules surround each Myofibril the way the sleeve of a loosely crocheted sweater surrounds your arm. These tubules run longitudinally along the Myofibril, communicating with each other at the H zone Closely associated with the SE are large numbers of mitochondria and glycogen granules, which are involved in producing the energy used during contraction. The SR regulates intercellular levels of ionic calcium. It stores it and then releases it on demand when the muscle fiber is stimulated to contract

Muscle fiber (cell)

Is an elongated multinucleate cell; that has a banded (striated) appearance Surrounded by endomysium

Smooth muscle

Is found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages. Its role is to force fluids and other substances through internal body channels. Has no striations and is involuntary

Skeletal muscle

Is responsible for overall body mobility. It can contract rapidly, but it tires easily and must rest after short periods of activity. Can exert tremendous power(people lifting cars to save loved ones) and is also adaptable

Elasticity

Is the ability of a muscle cell to recoil and resume its resting length after stretching

Extensibility

Is the ability to extend or stretch. Muscle cells shorten when contracting, but they can stretch, even beyond their resting length, when relaxed

Excitability or responsiveness

Is the ability to receive and respond to a stimulus, that is, any change in the environment either inside or outside the body. The stimulus is usually a chemical. The response (sometimes separated out as an additional characteristic called conductivity) is to generate an electrical impulse that travels along the plasma membrane of the muscle cell and cause the cell to contract

Contractility

Is the ability to shorten forcibly when adequately stimulated. This ability sets muscle apart from all other tissue types

Excitation-contraction coupling

Is the sequence of events by which transmission of an action potential along the Sarcolemma causes Myofilaments to slide

Maximal stimulus

Is the strongest stimulus that increases contractile force. It represent the point at which all the muscle's motor units are recruited

Producing movement

Just about all movements of the human body and its parts result from muscle contraction. Skeletal muscle are responsible for all locomotion(moving from place to place) and manipulation. Blood courses through your body because of the beating cardiac muscle of your heart and the smooth muscle in the walls of your blood vessels, which helps maintain blood pressure. Smooth muscle in organs of the digestive, urinary, and reproductive tracts propels, or squeezes, substances (foodstuffs, urine, semen) through the organs and along the tract

Globular actin or g actin

Kidney shaped polypeptide subunits (part of actin) that bear the active sites to which the myosin heads attach during contraction

Dystrophin

Links the thin filaments to the integral proteins of the Sarcolemma (which in turn are anchored to the extra cellular matrix)

Cardiac muscle

Occurs only in the heart, where it constitutes the bulk of the heart walls. Also striated but involuntary. Contracts at a steady rate provided by the heart's pacemaker, but neural controls allow the heart to speed up for brief periods.

Repolarization

Restores only the electrical conditions of the resting (polarized) state. The ATP-dependent NaK pump restores the ionic conditions of the resting state, but hundreds of action potentials can occur before ionic imbalances interfere with contractile activity

Intercellular tubules

Skeletal muscle fibers contain two sets that help regulate muscle contraction: 1.)the sarcoplasm if reticulum 2.)t tubules

Maintaining posture and body position

Skeletal muscles function almost continuously, making one tiny adjustment after another to counteract the never ending downward pull of gravity, in order to do this

Additional functions of muscle

Skeletal muscles protect the more fragile internal organs (viscera) by enclosing them Smooth muscle forms valves to regulate the passage of substances through internal body openings, dilates and constricts the pupils of your eyes, and forms the attractor pili muscles attached to the hair follicles

The sliding filament model of contraction

States that during contraction the thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree

Subthreshold stimuli

Stimuli that produce no observable contractions

T tubule

T for transverse. At each A band-I junction, the sarolemma of the muscle cell protrudes deep into the cell interior, forming this elongated tube. These tremendously increase the muscle fiber's surface area. Possibly the result of fusing tubelike caveolae (inpocketings of the Sarcolemma), the lumen (cavity) of the tubule is continuous with the extra cellular space

Synaptic cleft

The axon terminal and the muscle fiber are exceedingly close (50-80 mm apart), but they remain separated by this space, which is filled with a gel like extracellular substance rich in glycoproteins and collagen fibers

Direct or Fleshy attachments

The epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage

Insertion(of muscle)

The movable bone, which moves towards the origin

Indirect attachments

The muscle's connective tissue wrappings extend beyond the muscle either as a rope like tendon or as a sheet like aponeurosis. The tendon or aponeurosis anchors the muscle to the connective tissue covering of a skeletal element (bone or cartilage) or to the fascia of other muscles

Troponin

The other major protein in thin filaments, is a globular three-polypeptide complex. One of its poplypeptides (tni) binds to an inhibitory subunit that binds actin. Another (tnt) binds to tropomyosin and helps position it on actin. The third (tnc) binds calcium ions Both troponin and tropomyosin help control the myosin-actin interactions involved in contraction

Principles concerning size in a muscle

The recruitment process is not random, it follows the rules dictated by the size principle. In any muscle: -the motor units with the smallest muscle fibers are activated first because trey are controlled by the smallest, most highly excitable motor neurons -as motor units with larger and larger muscle fibers begin to be excited, contractile strength increase -the largest motor units, containing large, coarse muscle fibers, have as much as 50 times the contractile force of the smallest ones. They are controlled by the largest, least excitable (highest-threshold) neurons and are activated only when the most power contraction is necessary

Sarcomere

The region of s Myofibril between two successive Z discs. Averaging at 2 um long and is the smallest contractile unit of a muscle fiber-the functional unit of skeletal muscle. Contains an A band flanked by half an I band at each end. Within each Myofibril, the sacromeres align end to end (like boxcars in a train)

Threshold stimulus

The stimulus at which the first observable contraction occurs

Junction all folds

The trough-like part of the muscle fiber's Sarcolemma that helps form the neuromuscular junction is highly folded. These folds provide a large surface area for the millions of ACh receptors, which are located there

Graded muscle responses

The variations needed for proper control of skeletal movement are referred to these. Muscle contraction can be graded into two ways: -by changing the frequency of stimulation -by changing the strength of stimulation

A band (outer edge)

Thick and thin filaments overlap

Myosin

Thick filaments are composed primarily of this protein. Each myosin molecule consists of two heavy and four light polypeptide chains, and has a rodlike tail attached by a flexible hinge to two globular heads. The tail consists of two intertwined helical polypeptide heavy chains. The globular heads, each associated with two light chains, are the "business end" of myosin. During contraction, they link the thick and thin filaments together and form cross bridges

H zone

Thick filaments inly

M line

Thick filaments linked by accessory proteins

I band

Thin filaments only

Synaptic vesicles

Within the moundlike axon terminal are these small membranous sacs containing the neurotransmitter acetylcholine or ACh

Skeletal Muscle Tissue

is packaged into the skeletal muscles, organs that attach to and cover the bony skeleton -skeletal -striated -voluntary

Sarcoplasm

muscle cell cytoplasm