A&P CH 9 Muscles and Muscle Tissue

Microscopic structure of a SmMC

-small, spindle shaped, uninucleated, no striations, no sarcomeres -a lower ratio of thick to thin fils (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 fil) -has tropomyosin but no troponin -lack the coarse conn tissue coverings of SM -have a small amount of endomysium found between smooth muscles themselves -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 neuromus juncs) -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 contraction may be modified by neural or chemical stimuli

Gross anatomy of smooth muscle fibers

-small, usually arranged into sheets of opposing fibers, forming a longitudinal layer and a circular layer -contraction of the opposing layers of muscle leads to a rhythmic form of contraction, called peristalsis, which propels substances thru the organs

Contraction of Smooth muscle

-smooth muscle fibers exhibit slow, synchronized contractions due to electrical coupling bu\y gap junctions -like skel mus, actin and myosin interact by the sliding fil mechanism, the final trigger for contraction is a rise in intercellular calcium level, and the process is energized by ATP -during excitation-contraction coupling, calcium ions enter the cell from the EC space, bind to calmodulin protein (a replacement for troponin found in skeletal muscles) and activate myosin light chain kinase enzyme which phos's myosin heads. since thin fils lack troponin they are always available to bind with myosin, powering the cross bridging cycle -smooth muscle cell relaxation involves Ca++ detachment from calmodulin, and 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

Treppe

-stage 1 -first few contractions (in animation, first 5) -even though strength of contractions increased, relaxation was complete (temporal summation was not responsible) -increased muscle tension may result from increased muscle warming and efficiency of enzymes

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 fil theory of how a skel mus contracts involves the activities of the following: >myosin, ATP, actin, tropomyosin, troponin, calcium ions

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 (recruitment) -in addition to the # of motor units firing, the # of cells per motor unit is an important variable affecting the strength and degree of muscle movement Small Motor units produce precise movements >containing few muscle cells, are found where precise movements are needed, as in the muscles of the eye 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

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

Threshold

-the minimum stimulus (voltage) that can evoke a response (visible contraction) -subthreshold stimuli are below the threshold, aka are too low in voltage to stimulate the particular muscle

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

Synaptic Cleft

-the space between the axon terminal and the mem 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 mem called the motor end plate

Axon Terminal

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

Velocity and Duration of Muscle Contraction

-there are 2 types of muscle fibers based on their speed of contraction; slow and fast -based on their pathway for forming ATP there are 2 types of fibers; oxidative fibers use aerobic pathways and glucolytic fibers use anaerobic pathways -there are three muscle fiber types: slow oxidative fibers, fast oxi, and fast glycolytic fibers -muscle fiber type is a genetically determined triat, with varying percentages of each 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

Force of Muscle Contraction

1) as the # of muscle fibers stimulated +, the force of the contraction + 2) large muscle fibers generate more force than smaller muscle fibers 3) as the rate of stimulation +, contractions sum up, ultimately producing tetanus and generating more force 4) there is an optimal length tension relationship when the muscle is slightly stretched and there is slight overlap between the myofibrils

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 Ca ions into the sarco ret

aerobic, or endurance, exercising such as swimming, jogging, fast walking, and biking promotes:

>increase in capillary penetration >+ in number of mitochondria >+ 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 ox fibers

Multiple Myofilaments

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

Endo, Peri, and Epimysium

Endo: surrounds each muscle fiber (a muscle fiber is a muscle cell) Peri: surrounds each fascicle (a group of muscle fibers) Epi: surrounds each muscle (group of fascicles makes up a muscle)

Functional Characteristics of Muscle Tissue

Excitability/Irritability - the ability to receive and respond to a stimulus Contractility - ability to contract forcibly when stimulated Extensibility - the ability to be stretched Elasticity - ability to resume the cells original length once stretched

Multiple Cross Bridge Cycles

MCBCs are coordinated sequentially to prevent all cross bridges from either being connected or disconnected at the same time

Muscle Functions

Produce movement - by acting on the bones of the skeleton, pumping blood, or propelling substances throughout hollow organ systems Maintaining posture - by adjusting the position of the body with respect to gravity Stabilize joints - by exerting tension around the joint Generate heat - muscles generate heat as a function of their cellular metabolic processes skeletal muscles account for 40% of body mass and are responsible for generating most of the heat

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 can not produce energy as quickly as a muscle cell can use it, so an extra storage source is needed -the phos group can be quickly transferred to ADP to regenerate the ATP necessary for muscle contraction -hydrolysis of creatine phos to creatine releases 10.3 kcal/mole

Resistance exercise such as weight lifting or isometric exercise promotes:

-+ in # of mitochondria -+ in myofils and myofibrils -+ glycogen storage -hypertrophies cells (+ in the size of individual muscle fibers) -no + in # of muscle fibers -significant increase in muscle strength and size -fast oxidative fibers can be shifted to fast glycolytic fibers in response to resistance activites -when exercising its important to adhere to the overlead 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 and resistance exercises produce different patterns of muscular response, so its important to know your exercise goals

Oxygen Debt

-After the exercise period is over, the muscle restores the depleted energy reserves -oxygen debt is the extra O2 needed to replenish oxygen reserves 1) glycogen is produced and stored back again 2) ATP is used to rephos creatine into creatine phos 3) lactic acid is converted to pyruvic acid, which enters the Kreb's cycle, producing ATP 4) additional oxygen re-binds to myoglobin

Relaxation Period

-Ca++ is actively transported back into terminal cisternae (animation) -cross bridge cycling decreases and ends due to decreasing amounts of Ca++ present -tension is reduced and muscle returns to its original length

Isotonic & Isometric Contraction

-Isotonic >result in movement occurring at the joint >muscles shorten (e.g. picking up a book) or lengthen -isometric >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)

Cell uses 3 sources to get ATP:

-Phosphorylation by creatine phosphate -glycolysis (anaerobic respiration) -Kreb's cycle and oxidative phosphorylation (aerobic respiration)

Nerve Stimulus and Neuromuscular Junction

-SMCs are stimulated by motor neurons of the somatic nervous system -although these motor neurons "reside" in the brain or spinal cord, thier long threadlike extensions called axons travel, bundeled 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 NMJ, located approx midway along the fibers length

Sarcoplasmic reticulum

-SMF (cells) contain 2 sets of intracellular tubules that participate in regulation of muscle contraction: (1) sarcoplasmic reticulum (2) T tubules -the sarcoplasmic is made up of interconnecting tubules and sacs of the smooth endoplasmic reticulum surrounding each myofibril (like a sleeve) -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

Stages of muscle contraction in multiple stimuli (frequency gradually increased between contractions)

-Treppe -temporal summation -incomplete tetanus -complete tetanus -fatigue

Neuron/nerve cell

-a highly specialized cell that carries impulses - electrical signals - between body parts -cell body called the soma, and has several structures sticking off in all directions

Contraction of a skeletal muscle

-a motor unit consists of a motor neuron and all the muscle fibers it innervates, it is smaller in muscles that extend fine control -the muscle twitch is the response of a muscle to a single action potential on its motor neuron -there are three kinds of graded muscle responses: wave summation, multiple motor unit summation (recruitment), and treppe

Striations, sarcomeres, and myofilaments (bands/lines/areas)

-a myofibril has a repeating series of dark A bands and light I bands -each dark band has a lighter stripe called the 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

Energy systems used during sports activites

-aerobic endurance - is the length of time a muscle can continue to contract using aerobic pathways -anaerobic threshold - when exercise demands exceed the ability of muscle metabolism to keep up with ATP demand, metabolism converts to anaerobic gylcolysis, the point at which this happens is called the anaer thres -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

Temporal Summation (Stage)

-after the first few stimuli (5 in animation), frequency increases, causing temporal summation -results in continuous increase in tension as one contraction is added to the previous -increase in tension may result from increased levels of intracellular calcium

Effects of exercise on muscles

-amount of work a muscle does is reflected in in changes in the muscle itself -when used actively or strenuously, muscles may increase in size or strength or become more efficient and fatigue resistant -muscle inactivity, on the other hand, always leads to muscle weakness and wasting

Tropomyosin and Troponin

-are regulatory proteins present in thin fils -function: to allow or prevent myosin heads from binding to actin -tropomyosin entwines around the actin >in the unstimulated muscle, the position of the tropomyosin covers the binding sites on the actin subunits and prevents myosin cross bridge binding -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 and spaced periodically along the tropomyosin strand >when calcium binds to troponin, it moves the tropomyosin and exposes the actin binding sites/heads

T Tubules

-at each A-I band junction, the sarcolemma (plasma mem) 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 and thier 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 cisternae, T tubule, and terminal cisternae) are formed -T tubules conduct electrical impulses from the surface of the cell to the terminal cisternae

Muscle Attachments

-attachments span joints and cause movement to occur from the movable bone towards the less movable bone Direct/Fleshy Attachments: epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage Indirect: epimysium of the muscle extends beyond the muscle either as a ropelike tendon or as a sheet like aponeurosis -the tendon or apo anchors the muscle to the conn tissue covering of a skeletal element (bone or cartilage) or to the fascia of other muscles -indirect are more common bc: >they save space due to their small size >made up of collagen fibers and are very durable

length-tension relationship

-changing the starting length can alter strength of contraction -unstretched - relatively weak contraction, fils are already overlapping -moderately stretched - maximum contraction, actin fils near where myosin cross bridges are (typical position seen in the pics), optimum overlap of thick and thin fils -overstretched- weak contractions, thick and thin fils only overlap slightly, not common in skel muscles, critically important in heart (congestive heart failure)

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 hydrolyze ATP into ADP and Pi (<-- i is a subscript) to generate the power for muscle contraction -the thick fils extend then entire length of the dark A band -the mid-parts of the thick fils 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 fils together during contraction -the head (cross bridge) has the ability to move back and forth and it provides the power stroke for muscle contraction

Thin Filaments

-composed of strands of the contractile protein actin twisted into a double helical chain -actin fils anchored to the Z disc -the light I band includes parts of two adjacent sarcomeres and contains only the thin fils -the actin fils overlap the ends of the thick fils but they do not extend into the middle of a relaxed sarcomere, and thus the central region (the H zone, which lacks actin fils 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 bc actin and myosin fils completely overlap

Elastic Filaments

-composed of the giant protein titin -extends from the Z disc to the thick fils, and then runs within the latter to attach to the M line -acting as a kind of molecular ruler, has 2 basic functions: >holding the thick fils in place, this maintaining the organization of the A band >assisting the muscle cell to spring back into shape after being stretched or shortened ^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, bu it stiffens as it uncoils, helping the muscle to resist excessive stretching, which might pull the sarcomeres apart

Fast Muscle Fibers (fast twitch)

-contract more quickly and powerfully than slow fibers but fatigue much more easily -function best for short bursts of intense activity, like weight lifting or sprinting -most sprinters have 60% fast fibers -they use anaerobic respiration (without oxygen) to produce ATP, make much less ATP per glucose molecule than slow fibers -fast fibers have fewer mitochondria -they have much less myoglobin, making them pale in color -they are thicker than slow ones and that enhances their power -when O2 is not available, they cant completely break down glucose to CO2, instead they produce lactate, a compound that can build up in muslces and make them ache and fatigue -this is why fast muscle fibers are best at supplying short bursts of power -anaerobic ATP production in our muscles is only effective for a minute or so

Fast Ox fibers

-contract rapidly, aerobic (some anaerobic glycolysis) -intermediate endurance and less fatigue resistant than slow ox 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 depeleted, they tire more easily due to lactic acid build up -more power than slow ox fibers because cells are thicker and have more myofibrils -fewer mitochondria than slow oxidative fibers -less myoglobin than slow ox fibers -example: found in sprinters

Fast Glycolytic Fibers

-contract rapidly, anaerobic glycolysis, fatigue easily -muscle fibers are large because they do not depend on oxygen but depend instead on plentiful glycogen reserves for fuel -tire easily due to lactic acid build up -powerful because cells are thick and have many myofibrils -few mitochondria, less myoglobin, low blood supply -example: short intense movements like hitting a baseball

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 -they account for roughly 80% of cellular volume -composed of individual contractile proteins called myofilaments (microfilaments of the muscle cell) -two types of myofils - thick and thin -the myofils are so dense that other organelles are squeezed between them -the skeletal muscles appear striated bc of the arrangement of the myofilaments in the myofibrils

Gross Anatomy of Skeletal Muscle

-each muscle has a nerve and blood supply that allows neural control and ensures adequate nutrient delivery and waste removal -conn. tissue sheaths which provide strength and support to the muscle as a whole are found at various structural levels of each muscle (endo, peri, and epimysium)

Muscle Tone

-even in a muscle's relaxed state, random, asynchronous motor unit contractions provide a nearly constant state of low level tension and resistance to stretch called muscle tone -these minute contractions are maintained by activities of the spinal cord and result in a firmness of the muscle

Excitation contraction coupling

-for a skeletal muscle fiber to contract, it must by stimulated by a nerve ending and must propagate an electrical current, or action potential, along its sarcolemma. This electrical event causes the short lived rise in intracellular calcium ion levels that s the final trigger for contraction. The series of events linking the electrical signal to contraction is callex ex-contr coupling

Factors affecting muscle tension (go on CD)

-frequency of stimulation -# of motor units stimulated -degree of muscle stretch

Strength of muscle contraction determined by...

-frequency of stimulation -number and size of motor units recruited -the number of motor units recruited is determined by the number of motor neurons stimulated by the central nervous system -by varying the number and size of muscle units recruited, the nervous system can control the degree of contraction of a particular muscle

Factors affecting muscle tension

-frequency of stimulation (stages) -number of motor units recruited -degree of muscle stretch

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 pyrivic 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

Special features of smooth muscle contraction

-have a smooth muscle tone -have prolonged contractile activity -have low energy requirements -response to stretch >initially contracts when stretched, but contraction is brief, and then the cells relax to accomodate the stretch >the stretch relaxation response allows hollow organs to fill (expand) slowly to accomodate a greater volume without promoting strong contraction that would expel its contents without giving it time for the organ to act upon its contents -length and tension change >stretches more and generates more tension when stretched than skeletal muscle >this is possible bc of lack of sarcomere and the overlapping of smooth muscle fils >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 -hyperplasio, an increase in cell number thru division, is possible in addition to hypertrophy, an increase in individual cell size

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 thru homeostatic mechanisms such as sweating and radiation from the skin

Temporal (Wave) Summation

-increases muscle tension -when a second stimulus of same intensity is applied -applied before completion of relaxation -second contraction is added to the first -

Depolarization

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

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

Dendrites

many 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

Muscle Twitch (USE CD)

muscle twitch - a muscle contraction in response to a single stimulus of adequate strength -a complete twitch is divided into 3 phases >latent period, contraction period, relaxation period

Calcium Pumps

the active transport of Ca involves specialized ion pumps in the mem of the sarcoplasmic ret, these pumps must be energized by ATP

Motor End Plate

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

Excitation-Contraction coupling

the sequence of events by which an action potential on the sarcolemma results in the sliding of the myofilaments

Recruitment

the stimulation of additional motor units for increased strength of contraction

Micro of SmMC continued

-lacks neuromus juncs, but have diffuse juncs. 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 SmMCs, such junctions are called diffuse juncs -have a less developed sarcoplasmic retic, hence stores less Ca++ ions -the SR touches the sarcolemma at several sites forming half triads -there are no T tubules instead the sarcolemma has pouchlike infoldings called caveolae that hold large amounts of EC fluid containing Ca -neurotransmitter release causes Ca++ channels to open and Ca++ ions from EC 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) -smooth muscle thick and thin fils are arranged diagonally within the cell so that they spiral down the long axis of the cell like stripes down the barber pole, because of this arrangement, the smooth muscle cells contract in a twisting way so that they look like tiny corkscrews -SmM fibers contain longitudinal bundles of non contractile intermediate fils anchored to the sarcolemma and surrounding tissues via dense bodies -dense bodies: >act as anchoring points for thin fils and 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 >at the sarcolemme surface they also bind the muscle cell to the conn tissue fibers outside the cell (endomysium) and to adjacent cells, and that partly accounts for the synchronous contraction of most smooth muscle

Skeletal Muscle Fiber

-long, cylindrical cells with multiple nuclei beneath the sarcolemma (plasma mem of muscle fiber) -each SMF has a diameter ranging from 10 - 100 micrometers and can be up to 30 cm long -the sarcoplasm of a MF is similar to the cytoplasm of other cells but it contains: >many mitochondria for ATP production >glycosomes (glycogen granules) in large numbers >myoglobin (a red pigment that stores oxygen) >sarcoplasmic reticulum >myofibrils in large numbers >other cell organelles found in eukaryotic cells

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 -ATP is utilized very fast and the cells have to rapidly replenish this consumed ATP

Kreb's cycle and oxidative phos (aerobic res)

-muscles will function aerobically as long as there is adequate oxygen, ox is available to muscle cells from 2 different sources: >from the blood >stored in myoglobin, an oxygen binding protein in muscle cells -in the presence of oxygen, aerobic respiration takes place -pyruvic acid is converted to Acytel CoA which then enters the Kreb's cycle and oxidative phos will produce water, C02, and 36 molecules of ATP from 1 glucose molecule -aerobic respiration which includes glycolysis, Kreb's cycle, and ox phos produces 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 neurotrans binding coupled to a rise in Ca++ ions in the cytosol -different autonomous nerves serving the smooth muscle of visceral organs release different neurotrans's, each of which may excite or inhibit a particular group of smooth muscle cells. Exs: >acetylcholine --> binds to Ach receptors on smooth muscle cells in the bronchioles --> muscles contract --> narrow bronchioles >norepinephrine --> binds to norepinephrine receoptors on the smooth muscle in the bronchioles --> muscles relax --> dilate bronchioles >norepinephrine --> binds to norepinephrines receptors on the smooth muscle in the walls of blood vessels --> muscles contract --> constrict the vessels

Motor unit

-one neuron and all the skeletal muscle cells it stimulates -contain axons (fibrous extensions) of up to hundreds of motor neurons

The nerve stimulus and events at the neuromuscular junction

-resting mem potential: the voltage that exists across a cells plasma mem when the cell is at rest (not conducting an impulse) >cells interior is relatively more negative than the exterior >the resting mem potential is measured in millivolts -action potentials: an electrical signal causing the depolarization and subsequent repolarization of a nerve or muscle cell membrane, travels along the mem and functions as a signal to initiate an activity (e.g. a muscle contraction)

Latent Period

-sarcolemma and T tubules depolarize -Ca++ is released into the cytosol -cross bridges begin to cycle but no visible shortening of muscle

Contraction Period

-sarcomeres shorten as a result of myosin cross bridge cycling -speed depends on weight being lifted and fiber typed (slow or fast twitch)

Types of smooth muscle

-single unit SmM, called visceral muscle, most common type of SmM - found in walls of all hollow organs except heart >all the SmM characteristics described pertain to this type >it contracts rythmically as a unit, is electrically coupled by gap junctions, and exhibits spontaneous action potentials -multiunit, located in large airways to the lungs, large arteries, arrector pili, and internal eye muscles that adjust pupil size and allow the eye to focus >consists of cells whith rare gap juncs and 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 SmMs have similarities to skeletal muscles, they are innervated by autonomous nerves and respond to hormonal controls

Action Potential

-when the nerve impulse reaches the axon terminals, a chemical called a neurotransmitter is released, the specific one that stimulates SMCs is acetylcholine, or ACh -ACh diffuses across the synaptic cleft and attaches to receptors (mem proteins) that are part of the sarcolemma -if enough ACh is release, the sarcolemma at that point becomes temporarily more permeable to sodium ions, which rush into the muscle cell, and to potassium ions, which diffuse out of the cell -more Na enters than K leaves -this gives the cell interior an excess of + ions, which reverse the electrical conditions of the sarcolemma and opens more channels that allow Na entry only -this "upset" generates an electrical current called an action potential -once begun, the AP is unstoppable; it travels over the entire surface of the sarcolemma, conducting the electrical impulse from one end of the cell to the other, resulting in contraction -while the AP is occuring, ACh, which began the process, is broken down to acetic acid and choline by enzymes (acetylcholinesterase, or AChE) present on the sarcolemma -hence, the 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 ACh 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 Na - K pump, the active transport mechanism that moves the Na and K ions back to their initial positions

Fatigue

-with continued rapid stimulation, muscle is no longer able to sustain its level of tension and gradually elongates -fatigue due to build up of acidic compounds which affect protein functioning, relative lack of ATP, and ionic imbalances resulting from membrane activities -with rest and adequate blood supply, fatigue conditions are corrected and muscle will again be ready to respond to stimulation

Incomplete (unfused) tetanus

-with increased frequency of stimulation, muscle exhibits shorter contraction-relaxation cycles -some degree of relaxation is visable after each contraction

Complete (fused) tetanus

-with rapid multiple stimulation , contractions fuse into one smooth, continuous, total contraction without any evidence of cyclical relaxation -abundant intracellular calcium provides continual availability of binding sites on actin for cross bridge cycling

Steps of Cross Bridge Cycling

1) the influx of calcium, triggering the exposure of binding sites on actin >SMCs must be stimulated by nerve impulses to contract >when a MC is stimulated, it results in an action potential, which 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, which moves the tropomysin away from the binding sites of actin. The actin binding sites are thus exposed 2) The bindng of myosin to actin - an energized myosin cross bridge binds to it 3) the power stroke of the cross bridge causes the sliding of the thin fils - 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 fil inward toward the center of the sarcomere, this movement is called the power stroke 4) the binding of ATP to the cross bridge, which results in the cross bridge disconnecting from actin - in order to disconnect to cross bridge from actin, an ATP molecule must bind to its site on the myosin cross bridge 5)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 6) the transport of calcium ions back into the sarcoplasm. retic - Ca is actively transported from the cytosol into the sarcopl retic by ion pumps >as the Ca is removed, troponin-tropomysin complex again covers the binding sites on actin