Characteristics of Muscular Tissue

Types and Characteristics of Muscular Tissue

• Muscle cells in animals are specialized for movement
• Skeletal muscles hold the body erect against gravity and produce visible movement
• Five universal characteritics of muscle cells are excitability, conductivity, contractility, extensibility, and elasticity

Five Universal Characteristics of Muscle Cells

• Excitability or responsiveness is highly developed in muscles and nerve cells
• Conductivity is when stimulation produces excitation that travels and initiates muscle contraction
• Contractility is the ability to shorten when stimulated and allows them to pull on bones and organs
• Extensibility is the ability to stretch between contractions; skeletal muscle fibers can stretch up to three times their contracted length
• Elasticity is the ability to recoil to original resting length after being stretched and the tension released

Defining Skeletal Muscle

• Skeletal muscles are voluntary striated muscles usually attached to one or more bones
• Skeletal muscle has light and dark transverse bands called striations
• Subject to conscious control that is referred to as voluntary where other types of muscles are involuntary, and are never attached to bones
• A typical skeletal muscle cell measures 100 μm in diameter and 3 cm long, but some are as thick as 500 μm and 30 cm long
• Skeletal muscle cells can also be called muscle fibers or myofibers due to their length
• Skeletal muscle comprises muscular and fibrous connective tissue

Connective Tissue Layers

• The endomysium surrounds each muscle fiber
• The perimysium bundles muscle fibers into fascicles
• The epimysium encloses the entire muscle
• Connective tissues converge with collagen fibers of tendons, which are continuous with with the bone matrix
• Collagen resists excessive stretching and protects the muscle from injuries, but is not excitable nor contractile, but extensible and elastic
• Some believe the recoil of tendons contributes to the power output and efficiency of a muscle while others believe elasticity in humans is negligible.

Skeletal Muscle Cells

• Muscle fiber has a tightly organized internal structure related to its contractile function
• The plasma membrane is called the sarcolemma, and the cytoplasm is the sarcoplasm
• The sarcoplasm is occupied mainly by protein cords called myofibrils with a diameter of about 1 μm
• The sarcoplasm contains glycogen, that is a starchlike carbohydrate which stores energy, and myoglobin, a red pigment, which binds oxygen.
• Muscle fibers have multiple flattened or sausage-shaped nuclei pressed against the inside of the sarcolemma.
• Myoblasts (stem cells) fuse to produce each muscle fiber during embryonic development where each contributes a nucleus
• Some myoblasts remain as unspecialized satellite cells between the muscle fiber and the endomysium
• The unspecialized satellite cells play an important role in the regeneration of damaged skeletal muscles
• Most other organelles are packed into the spaces between the myofibrils

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic reticulum (SR) is a smooth endoplasmic reticulum and forms a network around each myofibril, periodically has dilated end-sacs called terminal cisternae
• Sarcolemma has tubular infoldings called transverse (T) tubules that penetrate through the cell and emerge on the other side
• Each T tubule is closely associated with two terminal cisternae to constitute a triad
• The sarcoplasmic reticulum stores calcium ions and has gated channels in its membranes that allows a flood of calcium into the cytosol to activate muscle contraction
• The calcium bursts are triggered by the T tubule signals to the SR

Myofibrils and Myofilaments

• Myofibrils are bundles of parallel protein microfilaments called myofilaments
• There are three kinds of myofilaments
• Thick filaments measure about 15 nm in diameter and each is made of several hundred myosin molecules
• A myosin molecule is shaped like a golf club consisting of two chains intertwined that form a shaftlike tail and a double globular head projecting from it at an angle
• A thick filament can be seen as a bundle of golf clubs, with their heads directed outward in a helical array around the bundle
• The heads angle to the left on one half of the thick filament, and to the right on the other half, with a bare zone in the middle
• Thin filaments measure 7 nm in diameter, and consist of two intertwined strands of fibrous (F) actin

Actin, Tropomyosin and Troponin

• Each F actin strand is string consisting of subunits called globular (G) actin
• Each G actin has an active site that can bind to the head of a myosin molecule
• A thin filament consists of 40-60 molecules of of the protein tropomyosin which blocks active sites of some G actins when a muscle fiber is relaxed
• A calcium-binding protein, called troponin, is bound to every tropomyosin molecule
• Elastic filaments are 1 nm in diameter and consist of a large protein called titin

Titin

• Elastic filaments run though the core of each thick filament and anchor it to a structure called the Z disc at one end and the M line at the other, helping stabilize the thick filament
• Myosin and actin are contractile proteins due to their ability to shorten the muscle fiber
• The regulatory proteins tropomyosin and troponin act as a switch that determine when the fiber can contract and when it canno
• Action of the regulatory proteins depends on the availability of calcium ions, which bind to troponin
• At least seven other accessory proteins occur in the thick and thin filaments or associated with them, with the most clinically important one being dystrophin

Dystrophin

• Dystrophin is an enormous protein between the sarcolemma and the outermost myofilaments; it links actin filaments to a peripheral protein on the inner face of the sarcolemma
• Dystrophin leads ultimately to the fibrous endomysium surrounding the muscle fiber through a series of links
• The thin filaments then move and pull on the dystrophin, then pulls the extracellular connective tissues leading to the tendon
• Muscular dystrophy is caused by genetic defects in dystrophin

Arrangement of Myosin and Actin

• Myosin and actin are arranged in a precise array that accounts for the striations in skeletal and cardiac muscles
• Striated muscle has dark A (anisotropic) bands alternating with lighter I (isotropic) bands
• A bands are made of thick filaments lying side by side
• Part of the A band is especially dark where each thick filament is surrounded by a hexagonal array of thin filaments
• The middle of the A band has a light region called the H band, where thin filaments do not reach

Z Disc and Sarcomere

• Thick filaments are linked to each other through the M line, a dark transverse protein complex in the middle of the H band
• Each light I band is bisected by the Z disc (Z line), which anchors the thin and elastic filaments
• Sarcomere refers to each segment of a myofibril from one Z disc to the next and it is the functional contractile unit of the muscle fiber
• A muscle shortens as its individual sarcomeres shorten and pull the Z discs closer together
• Overall cell shortening is achieved as the Z discs pull on the sarcolemma
• Dystrophin and the linking proteins pull on the extracellular proteins of the muscle.
• The structural hierarchy of a skeletal muscle is shown in Table

Nerve-Muscle Relationship

• Muscle contraction necessitates nerve stimulation, either naturally or artificially
• Somatic motor neurons stimulate muscle fibers via somatic motor fibers (axons)
• Motor unit - single motor fiber and all the muscle fibers it innervates
• Muscle fibers are dispersed throughout the muscle to cause weak contraction over a wide area instead of clustered together.
• 200 muscle fibers can be innervated by a average motor neuron, but can be smaller or larger to serve different purposes.

Size of Motor Units

• For fine control, a small motor is needed, as in the muscles of the eye (3 to 6 muscle fibers per neuron)
• A large motor is needed when strength is more important than control, as in the gastrocnemius (1,000 muscle fibers per neuron)
• Muscle fibers can work in shits because of multiple motor units, so when some unit becomes fatigued, other units can take over and sustain long term contraction
• The neuromuscular junction (NMJ) refers to the synapse where a nerve fiber meets the target cells that is a muscle fiber, also known as a motor end plate
• Each terminal branch of the nerve fiber has its own synapse with the muscle fiber, and stimulates it at different points within the NMJ
• Nerve fiber terminates in a bulbous swelling called synpatic knob at each synapse
• A synaptic clef is the space that is 60 to 100 nm wide that separates the synaptic knob from the muscle fiber

Synaptic Knobs

• Schwann, a third cell envelops the junction
• The synaptic knob contains synaptic vesicles, which are spheroid organelles filled with with the neurotransmitter acetylcholine (ACh)
• The muscle fibers contains around 50 million Ach receptors incorporated into its sarcolemma

ACh

• Junctional folds are numerous infoldings in the sarcolemma increasing area of the ACh-sensitive membrane
• Deficiency of ACh receptors leads to muscle paralysis of myasthenia gravis
• A basal lamina surrounds the muscle fiber and the Schwann cell of the NMJ and separates it from the surrounding connective tissue
• The basal lamina contains acetycholinesterase as well as sarcolemma, which break down ACh

Electrically Excitable Cells

• Muscle and nerve cells are electrically excitable due to plasma membrane voltage change in response to stimulation.
• Electrophysiology studies the electrical activity of the cell
• Membrane is polarized in an unstimulated (resting) cell, where more negative ions are on the inside of the plasma membrane compared to the outside
• In a resting muscle cell, sodium (Na+) is excess in the extracellular fluid (ECF) and potassium (K+) is in excess in the intracellular fluid (ICF)

Electrical Charge

• Different in electrical charge between two points is called electrical potential, or voltage
• Resting membrane potential (RMP) is voltage across the sarcolemma of a muscle cell and only measures approximately -90 mV
• The negative sign in the RMP indicates the relatively negative charge on the intracellular side of the membrane
• The sodium-potassium pump maintains the resting membrane potential
• When the nerve/muscle cell is stimulated, ion channels in the plasma membrane open and sodium (Na+) flows into the cell down its electrochemical gradient

Depolarization and Action Potential

• Depolarization of the membrane occurs as positive sodium (Na+) ions neutralize the negative charge inside the membrane, and the inside of the membrane briefly becomes positive
• Repolarization is when sodium (Na+) channels then close and potassium (K+) channels open; potassium (K+) rushes out of the cell (down its electrochemical gradient) turning the inside of the membrane negative again
• The voltage shift of depolarization followed by repolarization results in an action potential.
• Action potentials moving long a nerve fiber is called a nerve impulse or nerve signal

Skeletal Muscle Fiber Behavior

• Muscle contraction and relaxation consist of excitation, excitation-contraction coupling, contraction, and relaxation
• Excitation is when action potentials in the nerve fiber lead to action potentials in muscle fiber, which consists of steps:
• A nerve signal arrives at a synaptic knob and voltage regulated Ca2+ gates to open and calcium enters
• Ca2+ induces exocytosis to release ACh into the synaptic cleft

Excitation Steps

• ACh diffuses across the synaptic cleft and binds to receptors on the sarcolemma.
• Two ACh molecules binds to open receptors of ligand-gated ion channels
• If the channel opens, Na flows into the cell and exits K. The polarity across the sarcolemma reverses from −90 mV to +75 mV and goes back to normal since K exits
• The rapid fluctuation in membrane voltage that happens at the end-plate junction results in end-plate potential

Sarcolemma

• Areas of the sarcolemma that adjoin the NMJ are voltage gated which opens the flow of the sodium Na+ that goes in, and the flow of the potassium K+ that goes out. This process generates an action potential and fiber then becomes stimulated.
• The way that action is spread on the sarcolemma activating the myofilaments is called excitation-contraction
• The wave starts from the end plate traveling in several directions and continues down to the cell interior entering the T-tubules

T Tubules

• The voltage-gated openings in the T tubules releases calcium that have SR links
• Calcium diffuses from the SR to the cytosol down concentration gradient when the chains open in the SR
• This calcium binds to troponin of thin filaments
• Lastly the complex shapes exposing all sites that make them free and available

Sliding filament theory

• Muscular contraction involves development of tension where the muscles shorten
• This is divided into the following steps where ATP becomes hydrolyzed to ADP and P1
• The head of the released engird is activated and "cocks" into position that gives high energy
• The myosin head binds to the filament now exposed making a cross-bridge with bound ADP and phosphate

Power Stroke

• Myosin releases ADP and Pi to low position in order to drag the thin filament called power stroke
• In order to begin this again, another ATP must bind to myosin filament releasing the filaments
• Many heads are going in sequence so thin filaments will not revert

Myofilaments

• Myofilaments do not shorten, rather the thin filaments are sliding over thick parts
• Cycles are repeated using molecules of ATP to repeat power strokes for at least 5 times an action
• Muscles relax and return when nerve stimulation is stopped

Relaxation Steps

• Once nerve is stopped synpatic terminals hold releasing ACh
• Choline separates the receptors of acetylcholineserase while the terminals recapture molecules that are broken
• Excitation from SR releases and then recapture calcium ions so nerve stops firing in order to reabsorb

Troponin & Tropomyosin

• Calcium in SR binds with a calsequestrin containing stimulating signal
• Troponin causes less calcium that is free to move and does not need to be replaced
• To prevent binding to mysoin, tropomyosin is needed
• This generates enough force to return a muscle aid

Resting length

• Contraction aids in stretching the biceps brachii and triceps brachii
• Tension generated from the muscle is the before or after the stimulus being related for length
  • Shorten muscles result in weak contractions since thick filaments come to disc
  • Stretching causes myosin and thin filaments to have less overlap so contractions are weak

Muscle Tone

• Muscles have ideal length so they react with the highest level of force
• In the natural position the muscle stays at the best state of force
• Length in muscles is monitored so muscles have constant contraction

Behavior of Whole Muscles and Myogram

• Chart contraction for muscle called myogram
• The voltages that increase can cause potential issues and react as threshold happens
• threshold - the minimum voltage necessary to generate an action potential
• At threshold stimulus creates the quick contraction and relaxation of twitch called
• Excitation and contraction is what happens during elastic period

Internal vs External Tension

• Force generated is know as internal, elastic period that does not make muscle any shorter and it causes force during twitch
• However, the muscles may begin to produce external tension and the contraction is that of a twitch
• Short contraction due the SR absorbing the remaining tension, while calcium causes filaments and decline to relax
• Muscle contracts quicker, but relaxation needs 7 to 100 ms

Stimulus

• No muscle contraction with a lower level than threshold, but once above the threshold muscle can still not be that strong
• The strength in the twitch is based on the stretch
• Muscles that have stretched may cause tension in length
• Fatigue and hydration gives contraction and allows the filaments to contract together

Stimulus Intensity and Frequency

• Muscle need to contract and variable strength to get different tasks
• But contractions can affect strength for both

Strength Contractions

• Muscle can either increase or decrease nerve fiber to gain motor skills
• This results in recruitment, or motor unit or MMU summation
• For more needed power smaller nerves are activated for easier tasks while motors need more frequency
• With lower frequency stimulation from muscle contraction the muscle contracts the same each time
• Contractions and frequency stimulate muscle to get higher and more tension called summantion

Tetanus

• Contraction sustained at 40 to 50 stimulus per sec
• Incomplete tetanus creates the fluctuating state that is sustained
• Complete or fused tetanus creates prolonged contraction
• This is not to be confused by the the toxins during the state when fused can never happen
• Contraction is when it only has internal tension

Contraction

• Contraction known as isotonic vs isometric and and known as eccentic and concentric
• Contraction with same length or isometric
• Antagonistic with muscles joint for isomatic reasons
• posture has muscles that support

Isotonic Contration

• Contraction changing length and stays in same tension
• Contraction moves if only a load is moving in a shorts amout of time for example
• Concentric contractions can remain same for a short time for example the weights
• Eccentic is when its tension can be held or lowered.

Muscle Metabolism in ATP

• ATP is involved during all levels
• Supply is dependent on the energy and fats avalible
• Synthesis ATP can be made 2 ways from anaraobic and aerobic
• Dependednt on duration, various ways can be used

Pathways

• Anaerobic creates ATP without the O
• Limited at yield for the acid to fatigue
• Aerobic creates with higher output and no acid
• Constant 0 is needed

Aerobic Respiration

• Fats are primarily used during resotration
• Immediate burst with dash and only needed for oxygen that is held
• Energy from molecuels from phosphaste that is transferred with creatine kinase

Phosphgen

• System of phosphagen creats bursts system needed for sports and high bursts as they are shortly intense
• System depletes so muscle shifts to short term until function catches up. Muscle is using its glycogen as well

Fatigue

• Lactic acid pathways are needed at 30 or second burst while over 40 all cardiovascular needs at its peak to meet O2 demand

Fatigue

• Weakness or loss prolonged
  • Potasium accumulate and make excitable
  • Hydrolysis decreases mechanism of contraction
  • Depletion of fuel declines, ATP synthesize as well and changes fluid Exercising can absorb ammonioa and produces low signal out

Oxygen

• Max needs reaches over a time period where plateau reaches over where intense does not get maintained
• 20 has peak where size of male vs females is different and the endurance in training is better
• Exercise in oxygen needs to create the creatine/and regenerat. The liver also need exercise do diminish
  • Temperature raises and consumes

Fiber

• Fibers characterized as physiological or functional
• Slow twitch that holds posture and endurance to fatigue using I fibers
  • This fiber resist as it uses metabloism to make ATP and it helps make muscle in dense and high concentration
  • Myosin that slow in release and reabsorb results in colors
  • Red fibers used for high concentration

Quick Responses

• quick, small responses such as in hands and eyes requires an extensive release of calcium in the synapse
• glysolysis uses quick anareobic fermenation to create glycogen and high ATP
• mitocondria is smaller due to pale results for fiber

Histology

• fiber types and enzymes needed
• Muscles in red/white is needed where activity must be the same for each muscle

Strength: Tension

• Anatomy strength that helps the human generate bones/tendons
• size determined as size -Pennante are muscles parallel while circular can be as small

Motor

• Motors must have large units or tension in potentials to to make stronger than fatigue
• Weight stimulate and create growth minute times for a slow week -Enlargement results in cell division -More density in fiber Skeletol enhances for endurance

Performance

• Cross train has both endurance levels Muscle cells needs to be shortened for the cell Cardio functions a lot for cells
• The heart must rhythmically function for the blood

Functions

• Resist fatigue
• Chambers contract the unison for pressure of what is held
• Cardinocytes are shorter but stronger
• Linked as disks with dark thick lines

Excitation

• Excitation can happen to those who don't know how to have control Cardiac is aerobic and lacks 0 as well for a long cell Muscle fibers can be autonomic as well as fibers are found under the skin
• Smooth can be found and create layers for different organs

Cell Shape

• Cell is shaped as fusion and only has nucleus
• sarcoplasmic reduces but there are t tubules
• thick filaments do not create striations
• dense proteins throughout all

Movement

• Actin move and transfer as filaments to the body
• functional is one to not has many and other with Many passes and controls eyes as innervation to the nerve fibers synapse has motor unit

Innervention

Varicosites are associated as independently others contract better for viseral muscles Muscles stimulation helps the muscles Muscle receives help

Action of Smooth Muscle Cells

Stimulation makes receptors of AC stimulate and make oxygen to low cells

Energy and Stimulation

Pacemaker needs stimulation Calcium makes atp muscle needs a quick break to contract, for 2 seconds Latent needs at least a time range Low atp with smooth muscle and 2nd power to make a strong contraction that results in vessels Tone that has stretch and tone will help maintain a vessel that makes contraction to help maintain blood

Smooth Muscle

• Lack of disc is why filaments don't stop at the same time
• Tension adjustment causes degree of stretch and helps the bladder