A&P Chapter 9: Muscles and Muscle Tissue

Tropomyosin

entwins around the actin. In the unstimulated muscle, the position of the tropomyosin covers the binding sites on the actin subunits & prevents myosin cross bridge binding.

Action potential

an electrical signal causing the depolarization and subsequent repolarization of a nerve or muscle cell membrane, travels along the membrane and functions as a signal to initiate an activity (e.g., a muscle contraction).

Tropomyosin and troponin

are regulatory proteins present in thin filaments. Their function is to allow (or prevent) myosin head from binding to actin.

Glycolysis (anaerobic respiration)

glucose is a major source of energy for synthesizing ATP. It can be obtained directly from blood or by hydrolysis of stored glycogen. Glucose is broken down by glycolysis and 2 ATP molecules and pyruvic acid is produced. In the absence of oxygen, pyruvic acid is converted into lactic acid which is the end product of anaerobic pathway. Excess lactic acid builds up rapidly, bringing about muscle fatigue.

Multiple cross bridge cycles

multiple cross bridge cycling is coordinated sequentially to prevent all cross bridges from either being connected or disconnected at the same time.

The ability of muscles to shorten depends on:

myofilaments

Special features of smooth muscle contraction

smooth muscles: 1.have a smooth muscle tone 2.have prolonged contractile activity 3.have low energy requirements 4.response to stretch - initially contracts when stretched, but contraction is brief, & then the cells relax to accommodate the stretch. The stretch-relaxation response allows hollow organs to fill (expand) slowly to accommodate a greater volume without promoting strong contraction that would expel its contents without giving time for the organ to act upon its contents. (Example - time for the stomach & intestine to digest) 5.length & Tension Change - stretches more & generates more tension when stretched than skeletal muscle. This is possible because of lack of sarcomere & the overlapping of smooth muscle filaments. Contract when it is anywhere from twice to 1⁄2 its resting length - a total length of 150%. This allows organs to tolerate tremendous changes in volume without becoming flabby when they empty. 6.hyperplasia, an increase in cell number through division, is possible in addition to hypertrophy, an increase in individual cell size.

Thick filaments

- composed of bundles of myosin protein - the shape of an individual myosin molecule is similar to a golf club with 2 heads. - contain ATP binding sites and ATPase enzymes, which hydrolyzes ATP into ADP and Pi to generate the power for muscle contraction. - the thick filaments extend the entire length of the dark A band. - the mid-parts of the thick filaments are smooth, but their ends are studded with small projections. - these projections, or myosin heads, are called cross bridges when they link the thick and thin filaments together during contraction. The head (cross bridge) has the ability to move back and forth and it provides the power stroke for muscle contraction

Troponin

to expose the binding sites for binding with myosin, the tropomyosin molecule must be moved aside. This is facilitated by the presence of a third molecular complex called troponin. Troponin is attached & spaced periodically along the tropomyosin strand.

Muscle twitch

- a muscle contraction in response to a single stimulus of adequate strength is called a muscle twitch. In the body the muscle contraction is usually in response to multiple stimulations - not a single stimulus. - a complete twitch is divided into 3 phases: (1) Latent period - Sarcolemma and T-tubules depolarize; Ca+2 are released into the cytosol; cross bridges form but there is no visible shortening of the muscle (Not enough cross bridges to cause shortening) (2) Contraction period - Sarcomeres shorten due to cross bridges formation; speed depends on the weight being lifted & whether the fibers are slow-twitch or fast-twitch (3) Relaxation period - Ca+2 are actively transported back to the terminal cisternae; cross bridge cycling ends; tension is reduced and muscle returns to original state

Muscle attachments

- attachments span joints & cause movement to occur from the movable bone toward the less movable bone. - muscle attachments may be direct or indirect.

Smooth muscle

- in the walls of hollow organs - consists of small, spindle shaped, uninucleated, & non striated cells - under involuntary control.

Cardiac muscle

- occurs only in the heart - consists of uninucleated, short, branched, interconnected, & striated cells with intercalated discs. - under involuntary control.

Contraction of motor units

- the contraction of a skeletal muscle is the result of the activity of groups of muscle cells called motor units. The size and number of motor units being stimulated is an important factor in determining the strength of a contraction. - When a strong contraction is needed, the nervous system may cause more than one motor unit to be stimulated.

Microscopic structure of smooth muscle cells

- the sarcolemma has several gap junctions to transmit action potential from one fiber to another (Skeletal muscle cells are isolated from one another, each stimulated to contract by its own neuromuscular junctions) - some smooth muscle fibers in the stomach and small intestine are pacemaker cells and once excited they act to set the pacemaker of contraction. The rate and intensity of contraction may be modified by neural or chemical stimuli. - lacks neuromuscular junctions. but have diffuse junctions. Nerve fibers of the autonomous nervous system have numerous bulbous swellings called varicosities that release neurotransmitters to a wide synaptic cleft in the general area of the smooth muscle cells. Such junctions are called diffuse junctions. - smooth muscle cells have a less developed sarcoplasmic reticulum, hence stores less Ca++ ions. - the SR touches the sarcolemma at several sites forming half triads. (Half because it is formed only by the SR and sarcolemma) - there are no T-tubules instead the sarcolemma has pouch like infoldings called caveolae that hold large amounts of extracellular fluid containing calcium. - neurotransmitter release causes Ca++ channels to open & Ca++ ions from extracellular space to enter the smooth muscle cell. - contraction ends when Ca++ ions are transported either back to the SR or out of the cell. (excitation-Contraction coupling largely depends on Ca++ ions from extracellular fluid)

Functional characteristics of muscle tissue

1. excitability, or irritability - the ability to receive and respond to a stimulus. 2. contractility - the ability to contract forcibly when stimulated. 3. extensibility - the ability to be stretched. 4. elasticity - the ability to resume the cells' original length once stretched.

The sarcoplasm contains:

1. many mitochondria for ATP production. 2. glycosomes (glycogen granules) in large numbers 3. myoglobin (a red pigment that stores oxygen) 4. sarcoplasmic reticulum 5. myofibrils in large numbers 6. other cell organelles found in eukaryotic cells.

Muscle functions

1. produce movement - by acting on the bones of the skeleton, pumping blood, or propelling substances throughout hollow organ systems. 2. maintaining posture - by adjusting the position of the body with respect to gravity. 3. stabilize joints - by exerting tension around the joint. 4. generate heat - muscles generate heat as a function of their cellular metabolic processes. Skeletal muscles account for 40% of the body mass & are responsible for generating most of the heat.

Gross anatomy of smooth muscle fibers

1. smooth muscle cells are usually arranged into sheets of opposing fibers, forming a longitudinal layer and a circular layer. 3. contraction of the opposing layers of muscle leads to a rhythmic form of contraction, called peristalsis, which propels substances through the organs.

Types of myofilaments

1. thick filaments 2. thin filaments 3. elastic filaments

Role of ATP in muscle contraction

1.energizing the power stroke of the myosin cross bridge 2.disconnecting the myosin cross bridge from the binding site on actin at the conclusion of a power stroke. 3.actively transporting calcium ions into the sarcoplasmic reticulum

Skeletal muscle tissue is made up of:

1.muscle fibers 2.blood vessels 3.nerve fibers 4.connective tissue

Terminology to remember

Whenever you see the prefixes myo- and mys- ("muscle") and sarco- ("flesh"), you will know that muscle is being referred to. For example, in muscle cells the cytoplasm is called sarcoplasm (sarko-plazum).

Structural levels of skeletal muscle

a. endomysium surrounds each muscle fiber (a muscle fiber is a muscle cell) b. perimysium surrounds each fascicle (A group of muscle fibers is called a fascicle) c. epimysium surrounds each muscle (A group of fascicles makes up a muscle)

Nerve and blood supply of skeletal muscle

allows neural control and ensures adequate nutrient delivery and waste removal.

Multiple myofilaments

although the sarcomere shortens, the length of each myofilament does not change. However, the width of the H zone changes.

Single-unit smooth muscle

called visceral muscle, is the most common type of smooth muscle - found in the walls of all hollow organs except heart. All the smooth muscle characteristics described pertain to this type of muscle. It contracts rhythmically as a unit, is electrically coupled by gap junctions, and exhibits spontaneous action potentials.

Temporal summation

increased contractions due to increased availability of calcium ions

Motor unit

one neuron & all the skeletal muscle cells it stimulates are a motor unit

Connective tissue sheaths of skeletal muscle

provide strength and support to the muscle as a whole are found at various structural levels of each muscle

Small motor units

small motor units, containing few muscle cells, are found where precise movements are needed, as in the muscles of the eye.

Treppe

staircase effect, where muscle are warming, no temporal summation occurs.

Fourth step in cross bridge cycle

the binding of ATP to the cross bridge, which results in the cross bridge disconnecting from actin - In order to disconnect the cross bridge from actin, an ATP molecule must bind to its site on the myosin cross bridge

Neuromuscular junction

the place where a motor neuron stimulates a muscle cell. The muscle cell and motor neuron do not actually touch, but are separated by the synaptic cleft.

Third step in cross bridge cycle

the power stroke of the cross bridge that causes the sliding of the thin filaments - The binding of myosin to actin brings about a change in the cross bridge, resulting in the release of ADP and inorganic phosphate. At the same time, the cross bridge flexes, pulling the thin filament inward toward the center of the sarcomere. This movement is called the power stroke.

Axon terminal

the swollen distal end of an axon: contains a neurotransmitter substance called acetylcholine within synaptic vesicles.

Anaerobic threshold

when exercise demands exceed the ability of muscle metabolism to keep up with ATP demand, metabolism converts to anaerobic glycolysis. The point at which this happens is called anaerobic threshold.

Phosphorylation by creatine phosphate

(immediate but limited source) 1 creatine phosphate molecule produces 1 ATP molecule Creatine Phosphate: muscle cells use this phosphorylated form of creatine to store energy. Normal metabolism cannot produce energy as quickly as a muscle cell can use it, so an extra storage source is needed. The phosphate group can be quickly transferred to ADP to regenerate the ATP necessary for muscle contraction. Hydrolysis of creatine phosphate to creatine releases 10.3 kcal/mole.

Striations, sarcomeres, and myofilaments

- a myofibril has a repeating series of dArk A bands and lIght I bands. - each dark A band has a lighter stripe called H zone. - each H zone has a dark stripe called the M line contains tiny protein rods that hold adjacent thick filaments together. - each light I band has a darker stripe called the Z disc (line) - each sarcomere is made up of a region between 2 successive Z lines and contains an A band flanked between 1⁄2 of an I band on each end. - a sarcomere is the smallest contractile unit (functional unit) of a muscle fiber. Several sarcomeres are arranged end to end in a myofibril.

Oxygen debt

- after the exercise period is over, the muscle restores the depleted energy reserves. - oxygen debt is the extra oxygen needed to replenish energy reserves: 1. Glycogen is produced and stored back again. 2. ATP is used to rephosphorylate creatine into creatine phosphate. 3. Lactic acid is converted to pyruvic acid which enters the Kreb's cycle, producing ATP. 4. Additional oxygen re-binds to myoglobin.

Transverse tubules

- at each A band-I band junction, the sarcolemma (plasma membrane) of the muscle cell penetrates into the cell interior, forming an elongated tube called the T tubule. - T tubules are thus infoldings of the sarcolemma & their lumen is continuous with the extracellular space. - each T tubule runs between the paired terminal cisternae of the SR so that triads, successive groupings of the three membranous structures (terminal cisterna, T tubule, & terminal cisterna) are formed. - T tubules conduct electrical impulses from the surface of the cell to the terminal cisternae.

Skeletal muscle

- attached to the bony skeleton - consists of long, multinucleated, & striated cells - controlled voluntarily.

Thin filaments

- composed of strands of the contractile protein actin twisted into a double helical chain. - the actin filaments are anchored to the Z disc. - the light I band includes parts of two adjacent sarcomeres and contains only the thin filaments. - the actin filaments overlap the ends of the thick filaments but they do not extend into the middle of a relaxed sarcomere, and thus the central region (the H zone, which lacks actin filaments and looks a bit lighter) is sometimes called the bare zone. - when contraction occurs, and the actin-containing filaments slide toward each other into the center of the sarcomeres, these H zones disappear because the actin and myosin filaments are completely overlapped.

Elastic filaments

- composed of the giant protein titin - extends from the Z disc to the thick filament, and then runs within the latter to attach to the M line. Acting as a kind of molecular ruler, it has two basic functions: 1. holding the thick filaments in place, thus maintaining the organization of the A band 2. assisting the muscle cell to spring back into shape after being stretched or shortening. - the part of the titin that spans the I bands is extensible, unfolding when the muscle is stretched and recoiling when the tension is released. - titin does not resist stretching in the ordinary range of extension, but it stiffens as it uncoils, helping the muscle to resist excessive stretching, which might pull the sarcomeres apart.

Fast oxidative fibers

- contract rapidly - aerobic (some anaerobic glycolysis) - intermediate endurance and less fatigue resistant than slow oxidative fibers. - muscle fibers are larger because they do not depend on oxygen but depend instead on plentiful glycogen reserves for fuel - since they switch to glycogen reserves after the initial oxygen supply is depleted, they tire more easily due to lactic acid build up - more power than slow oxidative fibers because cells are thicker and have more myofibrils - fewer mitochondria than slow oxidative fibers - lesser myoglobin than slow oxidative fibers (less red) Example: Found in sprinters

Slow oxidative fibers

- contract slowly - aerobic - high endurance and fatigue resistant - muscle fibers are thin for quick diffusion of oxygen - little power because cells are thin and have fewer myofibrils - many mitochondria for energy - good blood supply to bring in oxygen - red due to high myoglobin Example: Found in a marathon runner

Myofibrils

- each muscle cell has many myofibrils. - myofibrils account for roughly 80% of cellular volume. - the myofibrils are composed of individual contractile proteins called myofilaments. (microfilaments of the muscle cell). - there are 2 types of myofilaments - thick & thin filaments. - the myofibrils are so dense that other organelles are squeezed between them. - the skeletal muscles appear striated because of the arrangement of the myofilaments in the myofibrils.

Indirect attachments

- epimysium of the muscle extends beyond the muscle either as a ropelike tendon (Biceps brachii) or as a sheet like aponeurosis (abdominal muscles). 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. more common because: 1.They save space due to their small size 2.They are made up of collagen fibers & are very durable.

Muscle tone

- even in a muscle's relaxed state, random, asynchronous motor unit contractions provide a nearly constant state of low-level tension & resistance to stretch called muscle tone. - these minute contractions are maintained by activities of the spinal cord & result in a firmness of the muscle. - if the motor nerve connecting the spinal cord to a muscle is cut or damaged, the muscle loses all tone and becomes flaccid

Factors affecting muscle tension

- frequency of stimulation (muscle twitch, wave summation, multiple stimuli) - number of motor units stimulated depends on the # of motor neurons stimulated by the central nervous system. - degree of muscle stretch: Length-tension relationship, Ideal condition for a muscle contraction is when a muscle is moderately stretched and there is slight overlap between the myofibrils. An overstretched or under stretched muscle produces weak contractions

Heat production during muscle activity

- heat production during muscle activity is considerable. - 40% of the energy released during muscle contraction is converted to useful work. - excess heat is released through homeostatic mechanisms such as sweating and radiation from the skin.

Multiunit smooth muscle

- is located in large airways to the lungs, large arteries, arrector pili muscles in hair follicles, and internal eye muscles that adjust pupil size & allow the eye to focus. It consists of cells with rare gap junctions & so rarely depolarize synchronously. The muscle fibers are structurally independent of each other, have motor units involving a number of muscle fibers and are capable of graded contractions that involve recruitment - although multiunit smooth muscles have similarities to skeletal muscles, they are innervated by autonomous nerves and respond to hormonal controls. - hormones, lack of O2, histamine, excess of CO2, and low pH alter muscle activity according to the local tissue response

Muscle metabolism

- muscles contain very little stored ATP. - the stored energy in ATP is released when the terminal high energy phosphate bond is broken by hydrolysis. Hydrolysis of ATP results in the production of ADP, inorganic phosphate, and energy. - when the reverse reaction takes place (dehydration synthesis) energy gets stored in the ATP molecule.

3.Kreb's cycle & oxidative phosphorylation (aerobic respiration)

- muscles will function aerobically as long as there is adequate oxygen. Oxygen is available to muscle cells from 2 different sources: a.from the blood b.stored in myoglobin, an oxygen binding protein in muscle cells. - in the presence of oxygen, aerobic respiration takes place. Pyruvic acid is converted to Acetyl CoA which then enters the Kreb's cycle which takes place inside the mitochondria. Kreb's cycle & oxidative phosphorylation will produce water, carbon dioxide, & 36 molecules of ATP from 1 glucose molecule. - aerobic respiration which includes glycolysis, Kreb's cycle, & oxidative phophorylation produces a total of 38 ATP molecules from 1 glucose molecule.

Regulation of contraction

- neural regulation is brought about by hormones or local chemical changes - An action potential is generated by neurotransmitter binding coupled to a rise in Ca++ ions in the cytosol. - different autonomous nerves serving the smooth muscle of visceral organs release different neurotransmitters, each of which may excite or inhibit a particular group of smooth muscle cells. Examples: 1. Acetylcholine --> binds to ACh receptors on smooth muscle cells in the bronchioles --> muscles contract --> narrow bronchioles 2. Norepinephrine --> binds to norepinephrine receptors on the smooth muscle in the bronchioles --> muscle relax --> dilate bronchioles 3. Norepinephrine --> binds to norepinephrine receptors on the smooth muscle in the walls of blood vessels --> muscles contract --> constrict the vessels

Action potential steps

- once begun, the action potential is unstoppable; it travels over the entire surface of the sarcolemma, conducting the electrical impulse from one end of the cell to the other. - the result is contraction of the muscle cell. - while the action potential is occurring, acetylcholine, which began the process, is broken down to acetic acid & choline by enzymes (acetylcholinesterase, or AChE) present on the sarcolemma. - hence, a single nerve impulse produces only one contraction. - this prevents continued contraction of the muscle cell in the absence of additional nerve impulses. - the muscle cell relaxes until stimulated by the next round of acetylcholine release. the events that return the cell to its resting state include: (1) diffusion of potassium ions (K) out of the cell, & (2) operation of the sodium-potassium pump, the active transport mechanism that moves the sodium & potassium ions back to their initial positions.

Isometric contractions

- result in increases in muscle tension but no movement takes place - no lengthening or shortening of the muscle occurs (e.g. when a muscle tries to lift a load like a heavy piano singlehandedly)

Isotonic contractions

- result in movement occurring at the joint - muscles shorten (e.g. picking up a book) or lengthen (e.g. stretching of calf muscles as you climb up a hill)

Nerve stimulus and neuromuscular junction

- skeletal muscle cells are stimulated by motor neurons of the somatic nervous system. - although these motor neurons "reside" in the brain or spinal cord, their long threadlike extensions called axons travel, bundled within nerves, to the muscle cells they serve. - the axon of each motor neuron divides profusely as it enters the muscle, and each axon ending gives off several short, curling branches that form an elliptical neuromuscular junction with a single muscle fiber. - as a rule, each muscle fiber has only one neuromuscular junction, located approximately midway along the fiber's length.

Sarcoplasmic reticulum

- skeletal muscle fibers (cells) contain two sets of intracellular tubules that participate in regulation of muscle contraction: (1) sarcoplasmic reticulum 2) T tubules - the sarcoplasmic reticulum is made up of interconnecting tubules & sacs of the smooth endoplasmic reticulum surrounding each myofibril just as the sleeve of a loosely crocheted sweater surrounds your arm. - most of these tubules run longitudinally along the myofibril. Others form larger, perpendicular cross channels at the A band-I band junctions. These channels are called terminal cisternae (end sacs) and always occur in pairs. - the major role of this elaborate system is to store calcium and to release it on demand when the muscle fiber is stimulated to contract.

Microscopic anatomy of skeletal muscle fibers

- skeletal muscle fibers are long cylindrical cells with multiple nuclei beneath the sarcolemma (plasma membrane of the muscle fiber). - each skeletal muscle cell has a diameter ranging from 10 -100 micrometers & can be up to 30 cm long.

Microscopic anatomy of smooth muscle cells

- small - spindle-shaped - uninucleated - no striations - no sarcomeres - a lower ratio of thick to thin filaments (1:13) when compared to skeletal muscle (1:2) - type of myosin is different (have actin gripping heads along the entire length of the thick filament) - has tropomyosin but no troponin - lack the coarse connective tissue coverings of skeletal muscle. Have a small amount of endomysium found between smooth muscle fibers and secreted by the smooth muscles themselves.

Contraction of smooth muscle

- smooth muscle fibers exhibit slow, synchronized contractions due to electrical coupling by gap junctions. - like skeletal muscle, actin and myosin interact by the sliding filament mechanism. The final trigger for contraction is a rise in intracellular calcium level, and the process is energized by ATP. - during excitation-contraction coupling, calcium ions enter the cell from the extracellular space, bind to calmodulin protein (a replacement for troponin found in skeletal muscles), and activate myosin light chain kinase enzyme, which phosphorylates myosin heads. Since thin filaments lack troponin they are always available to bind with myosin, powering the cross-bridging cycle. - smooth muscle cell relaxation involves Ca++ detachment from calmodulin, & active transport of Ca++ ions into SR or out of cell. - dephosphorylation of the myosin is done by a phosphorylase enzyme. - smooth muscle contracts more slowly and consumes less ATP than skeletal muscle. - smooth muscle takes 30 times longer to contract and relax than does skeletal muscle. - they can maintain the same contractile tension for prolonged periods at less than 1% of the energy cost.

Microscopic fibers of smooth muscle cells

- smooth muscle thick & thin filaments are arranged diagonally within the cell so that they spiral down the long axis of the cell like the stripes on a barber pole. Because of this arrangement, the smooth muscle cells contract in a twisting way so that they look like tiny corkscrews - smooth muscle fibers contain longitudinal bundles of non-contractile intermediate filaments anchored to the sarcolemma & surrounding tissues via dense bodies. The dense bodies: 1. act as anchoring points for thin filaments & so correspond to Z discs of skeletal muscle. During contraction, areas of the sarcolemma between the dense bodies bulge outward, giving the cell a puffy appearance. 2. at the sarcolemma surface they also bind the muscle cell to the connective tissue fibers outside the cell (endomysium) and to adjacent cells, & that partly accounts for the synchronous contraction of most smooth muscles.

Sliding filament theory

- the contraction of a muscle cell occurs as the thin filaments slide past the thick filaments. - during contraction, the sarcomere shortens and the thin and thick filaments overlap. - the sliding filament theory of how a skeletal muscle contracts involves the activities of the following: 1.Myosin 2.ATP 3.Actin 4.Tropomyosin 5.Troponin 6.Calcium ions

Recruitment

- the stimulation of additional motor units for increased strength of contraction is called recruitment. - in addition to the number of motor units firing, the number of cells per motor unit is an important variable affecting the strength and degree of muscle movement.

Velocity and duration of muscle activity

- there are 2 types of muscle fibers based on their speed of contraction: slow & fast fibers - based on their pathway for forming ATP there are 2 types of fibers: oxidative fibers use aerobic pathways & glycolytic fibers use anaerobic pathways - there are three muscle fiber types: slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers. - muscle fiber type is a genetically determined trait, with varying percentages of each fiber type in every muscle, determined by specific function of a given muscle. - as load increases, the slower the velocity and shorter the duration of contraction. - recruitment of additional motor units increases velocity and duration of contraction.

Temporal(wave) summation

- when a second stimulus of the same intensity is applied before the completion of the first relaxation, the second contraction adds to the first one. On a graph the 2nd peak is higher than the 1st due to summation. - for the summation effect to occur, it has to take place quickly before Ca+2 return to the terminal cisternae and cross-bridge cycling stops.

Action potential details

- when the nerve impulse reaches the axon terminals, a chemical called a neurotransmitter is released. The specific neurotransmitter that stimulates skeletal muscle cells is acetylcholine, or ACh. - acetylcholine diffuses across the synaptic cleft & attaches to receptors (membrane proteins) that are part of the sarcolemma. - if enough acetylcholine is released, the sarcolemma at that point becomes temporarily more permeable to sodium ions (Na), which rush into the muscle cell & to potassium ions (K) which diffuse out of the cell. - however, more Na enters than K leaves. - this gives the cell interior an excess of positive ions, which reverses the electrical conditions of the sarcolemma & opens more channels that allow Na entry only. - this "upset" generates an electrical current called an action potential.

Direct(fleshy) attachments

Epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage. Example: Intercostal muscles

Depolarization

a decrease in the negative resting membrane potential (the voltage that exists across the plasma membrane)

A motor unit consists of ____

a motor neuron and all the muscle fibers it innervates. - each muscle is served by at least one motor nerve, which contains axons (fibrous extensions) of up to hundreds of motor neurons. - as an axon enters a muscle, it branches into a number of terminals, each of which forms a neuromuscular junction with a single muscle fiber.

Neuron

a neuron or a nerve cell is a highly specialized cell that carries impulses - electrically signals between body parts. The cell body of the neuron is called the soma & has several structures sticking off in all directions.

Motor neuron

each motor neuron(nerve cell) extends from the brain or the spinal cord to the muscle where it can stimulate several muscle cells.

Multiple stimuli

if a muscle is repeatedly stimulated with stimuli of equal intensity and with the stimuli intervals gradually decreased, the graph would show several aspects of the mechanical activity of muscles.

Aerobic encurance

is the length of time a muscle can continue to contract using aerobic pathways

Muscle fatigue

is the physiological inability to contract even though the muscle still may be receiving stimuli. Muscle fatigue is caused due to the shortage of available ATP

Resting membrane potential

it is the voltage that exists across a cell's plasma membrane when the cell is at rest (not conducting an impulse); cell interior is relatively more negative than the cell exterior. The resting membrane potential is measured in millivolts.

Large motor units

large muscles exhibiting gross movements, such as movements of the thigh, have large motor units in which a single neuron is connected to a large number of muscle cells.

Dendrites

many dendrites extend out of the soma. They bring impulses to the cell.

Synaptic vesicles

membranous organelles containing neurotransmitter substances; found within the axon terminals of neurons

Effect of aerobic or endurance exercises on muscles

swimming, jogging, fast walking, and biking promotes: - increase in capillary penetration - increase in the number of mitochondria - increased synthesis of myoglobin - more efficient metabolism - greater endurance, strength, and resistance to fatigue but no hypertrophy - regular endurance exercise may convert fast glycolytic fibers into fast oxidative fibers.

Calcium pumps

the active transport of calcium involves specialized ion pumps in the membrane of the sarcoplasmic reticulum. These pumps must be energized by ATP.

Second step in cross bridge cycle

the binding of myosin to actin - An energized myosin cross bridge binds to actin.

Axon

the elongated process of a nerve cell that carries impulses away from the nerve cell body. When an axon reaches the muscle, it branches into a number of axon terminals.

Motor end plate

the folded portion of the sarcolemma in close contact with the synaptic ending of the axon terminal.

Fifth step in cross bridge cycle

the hydrolysis of ATP, which leads to the re-energizing and repositioning of the cross bridge - The release of the myosin cross bridge from actin triggers the hydrolysis of the ATP molecule into ADP and Pi Energy is transferred from ATP to the myosin cross bridge, which returns to its high-energy state.

First step in cross bridge cycle

the influx of calcium, triggering the exposure of binding sites on actin - Skeletal muscle cells must be stimulated by nerve impulses to contract. When a muscle cell is stimulated, it results in an action potential. The action potential cause the release of calcium ions from the terminal cisternae of the sarcoplasmic reticulum into the cytosol of the muscle cell. The calcium ions bind to troponin. Troponin moves the tropomyosin away from the binding sites of actin. The actin binding sites are thus exposed.

Excitation-contraction coupling

the sequence of events by which an action potential on the sarcolemma (electrical signal) results in the sliding of myofilaments (contraction). For a skeletal muscle fiber to contract it must be stimulated by a nerve ending & must propagate an electrical current or action potential, along its sarcolemma. This electrical event causes a short-lived rise in intracellular Ca+2 ion levels that is the final trigger for contraction.

Synaptic cleft

the space between the axon terminal and the membrane of the target cell. When reacting with muscle cells it is the space between the axon terminal and the folded region of the muscle cell membrane called the motor end plate.

Sixth step in cross bridge cycle

the transport of calcium ions back into the sarcoplasmic reticulum - Calcium is actively transported from the cytosol into the sarcoplasmic reticulum by ion pumps. As the calcium is removed, troponin-tropomyosin complex again covers the binding sites on actin.

Effect of resistance exercises on muscles

weight lifting or isometric exercise, promotes: - increase in the number of mitochondria - increase in myofilaments & myofibrils - increase in glycogen storage - hypertrophied cells (increase in the size of individual muscle fibers) - no increase in number of muscle fibers. - significant increase in muscle strength & size - fast oxidative fibers can be shifted to fast glycolytic fibers (big muscles due to hypertrophied cells) in response to resistance activities - when exercising it is important to adhere to the overload principle. Doing too much too soon, or ignoring the warning signs of muscle or joint pain, increases the risk of overuse injuries that may prevent future sports activities, or even lead to lifetime disability. - endurance & resistance exercises produce different patterns of muscular response, so it is important to know your exercise goals

Fatigue

with continued rapid stimulation the muscle is no longer able to maintain tension and begins to relax. Fatigue results from inadequate ATP availability and build up of lactic acid

Incomplete tetanus

with increased frequency of stimulation, muscle exhibits shorter contraction-relaxation cycles called incomplete tetanus

Complete tetanus

with rapid multiple stimulations there is an abundance of Ca+2 ions resulting in contractions which fuse into a smooth, continuous, total contraction without relaxation