muscle physiology in orthodontics / orthodontic courses by indian dental academy

MUSCLE PHYSIOLOGY INDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.com

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INTRODUCTION• Muscles as a whole helps to orient the basic

structure of human body. For every movement to take place an association has to be there between the bones and muscles.A thorough knowledge of muscles is a must for proper delivery of orthodontic and orthopedic treatment .

• It consists of 50% of total body weight.apart from regulating the body position,it helps to maintain proper organ volume ,generates heat and propels fluid.

• Study of muscles is called as myologywww.indiandentalacademy.com

TYPES OF MUSCLES

• Skeletal muscle:so named because it moves the bones of the skeleton.Actions are mainly voluntary but can act involuntarily to an extent and are striated.e.g.: stretch reflexes.

• Cardiac muscle:It forms the heart walls and has auto rythmicity.

• Smooth muscles:Found in the hollow internal structures like blood vessels and non striated and mostly always involuntary.


Functions of muscle tissues.

• Produces total and localized body movements.• Skeletal muscle contractions stabilizes joints and

helps maintain body positions.• Helps regulate organ volume through smooth

muscles called as sphincters.• Helps moving substances within the body like

blood.• Helps produce heat.


Properties of muscle tissues.• Four functions enable homeostasis:• Electrical excitability or irritability:is the ability to respond to

certain stimuli by producing signals,examples are autorythmic electrical signals or chemical stimuli such as neurotransmitters.

• Contractility is the ability of muscle tissue to contract forcefully when stimulated by an action potential.isometric and isotonic contractions.

• Extensibility is the ability to be stretched without being damaged.

• Elasticity is the ability to return to original length and shape after contraction and extension.


Microscopic anatomy of a skeletal muscle fiber

• In embryonic development each skeletal muscle fiber arises from the fusion of a million mesodermal cells called as myoblasts.of which few cells called satellite cells which persists later in life fuse and cause regeneration.

• Connective tissue components are of three layers extending from the deep fascia to further protect and strengthen the skeletal muscle fiber.

• The outermost layer encircling the whole muscle is the epimysium . Perimysium surrounds groups of 10-100 individual muscle fibers separating them into bundles called as fascicles which are large enough to be seen with the naked eye.


• Each individual muscle fiber is separated from one another by the endomysium.

• Sarcolemma is the fiber’s plasma membrane and tiny invaginations tunnel from the surface to the center of each muscle fiber called as ‘T tubules or transverse’ tubules.

• They are filled with ECF thus when an action potential propagates ,it ensures that all parts of the muscle is stimulated simultaneously.


Cell structure • The cytoplasm called as sarcoplasm contains

glycogen which can be split into glucose used for ATP synthesis.It also contains myoglobin, an oxygen binding protein found exclusively in muscle fibers.

• Myofibrils-Little thread like small contractile structures about 2micrometer diameter run along the whole length making the muscle fiber striated.

• Sarcoplasmic reticulum are fluid filled sacs encircling each myofibril and store calcium and its release triggers contraction.


Filaments:• Inside each myofibril there are

even more smaller structures called as filaments which are of two types thick and thin filaments.

• They are arranged in compartments called as sarcomeres which are the basic functional unit of a myofibril.

• Filaments overlap causing zones and bands .The darker middle portion extending along the entire length of the thick filaments is A band.

I bands are less dense and are the Thin filaments.Z discs pass throughCenter of each I band.


Muscle proteins:• Contractile proteins consists of actin and myosin.• Myosin functions as a motor protein,push and pull

their cargo to achieve movement by converting chemical energy in ATP to mechanical.each molecule has projections called cross bridges .

• Actin forms the thin filaments,individual molecules form a helix and each has a myosin binding site where the head of the myosin can attach to.


What keeps the myosin and actin filaments in place?

• The regulatory proteins are tropomysin and troponin which are part of thin filaments.In relaxed muscle,myosin blocked from binding to actin because tropomysin covers the myosin binding site on actin . Tropomysin instead holds troponin in place.It helps switch contraction on and off.

• Titin - filamentous structural proteins accounts for the elasticity and extensibility of myofibrils and helps sarcomere return to resting length after contraction.


General mechanism of muscle contraction.

• An action potential travels along a motor nerve to its endings on muscle fibers.at each ending ,the nerve secretes a small amount of neurotransmitters called acetylcholine.

• It acts on local areas of the muscle fiber membrane to open multiple,Acetylcholine gated channels through protein molecules floating in the membrane.

• Opening of Acetylcholine gated channels allows large quantities of sodium ions to flow to interior of muscle fiber membrane.This initiates an action potential in the muscle fiber.


Contd…• This action potential travels along the muscle fiber

membrane the same way it travels in nerves.Action potential depolarizes the muscle membrane and travels deeply within the muscle fiber causing the SR to release large amount of Ca++ stored.

• Ca++ initiates attractive forces between actin and myosin filaments causing them to slide alongside with each other,which is the contractile process.

• After a fraction of a second ,the Ca++ are pumped back into SR by a Ca++ membrane pump and remain stored until new muscle potential comes along;This removal of Ca++ causes muscle contraction to cease.


EFFECTS OF EXCITATION ON A MUSCLE FIBER.


The contraction cycle.• At the onset of contraction the SR releases Ca++which

binds to troponin and causes the troponin –tropomyosin complexes to move away from the myosin binding sites on actin.Once the sites are free, the contraction cycle – the repeating sequence of events that causes the filaments to slide begins. The contraction cycle - fours steps:

• 1.ATP hydrolysis:the myosin head includes an ATP binding pocket and an ATPase,an enzyme that hydrolyzes ATP into ADP and a phosphate group.this hydrolysis reaction energizes the myosin head.


Contd…

• 2.attachment of myosin to actin to form cross bridges.the energized myosin head attaches to the myosin binding site on actin and then releases the previously hydrolyzed phosphate group.

• 3.Power stroke.The release of the phosphate group triggers the power stroke of contraction.the pocket on the myosin head where ADP is still bound opens,which rotates the myosin head and releases the ADP. The myosin head generates force as it rotates towards the center of the sarcomere,sliding the thin filament past the thick towards the middle.


Contraction cycle contd….• 4.detachment of the myosin

from the actin. At the end of the power stroke,the myosin head remains firmly attached to actin until it binds another molecule of ATP.As ATP binds to the ATP BINDING POCKET on the myosin head ,the myosin head detaches from actin.

• The contraction cycle repeats over and over,as long as ATP is available and Ca++level near the thin filament is sufficient.


Sliding mechanism of muscle contraction

• In a relaxed state ends of actin filaments barely begin to overlap one another and lie close to the myosin filaments.

• Contracted state these actin filaments are pulled inward among the myosin so that their ends overlap.Also Z discs are pulled by actin up to the end of myosin filaments.This muscle contraction is called sliding filament mech.


What causes the actin filaments to slide inward among the

myosin filaments?• The mechanical forces generated by the

interaction of cross bridges of myosin with actin filaments is the reason.when action potential travels it triggers release of Ca++ in turn it activates the forces between myosin and actin filaments and contraction begins.Energy required is derived from ATP being degraded to ADP.


Inhibition of the actin filaments by troponin tropomysin complex,

activation by Ca++• When this complex is present in actin,

binding does not take place.• Role of Ca++ in inhibition is that when it

combines with the troponin complex,each molecule can bind with up to 4 Ca++ and pushes the complex to deeper grooves within the actin strand.Thus uncovers active actin site allowing myosin head to attach and cause contraction.www.indiandentalacademy.com

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• When a muscle contracts ,work is performed and energy is required.large amounts of ATP are cleaved to form ADP during contraction process,the greater the amount of work performed by the muscle ,the greater the amount of ATP is cleaved which is called as the Fenn effect.

• Muscle fibers have three main sources of ATP:1)creatinine phosphate 2)Anaerobic cellular respiration 3)aerobic cellular respiration.

Energy sources for muscle contraction


Creatinine phosphate:

• While at rest muscle fibers produce more ATP than they need for resting metabolism.some of the excess ATP is used to synthesize Creatinine Phosphate,an energy rich molecule unique to the muscle fibers.The enzyme creatine kinase catalyses the transfer of one of the high energy PO4 groups of ATP to creatine,forming creatine phosphate and ADP.Together CP and ATP provide enough energy for muscles to contract maximally for about 15 seconds.


Anaerobic cellular respiration:

• It is a series of ATP producing reactions that do not require O2.when muscle activity continues and the supply of CP is depleted,glucose is catabolised to generate ATP.

• Glucose easily passes from the blood into contracting muscle fibers via facilitated diffusion.


Aerobic cellular respiration• Muscle activity that lasts longer than half a min depends

increasingly on aerobic cellular respiration,a series of oxygen requiring mitochondrial reactions that produce ATP.

• Muscle tissue has two sources of O2: 1)oxygen that diffuses into muscle fibers from the blood. 2)oxygen released by myoglobin an oxygen binding protein present in the muscle fibers.

• Aerobic cellular respiration provides enough ATP for prolonged activity so long as sufficient oxygen and nutrients are available

• Glycogen/glucose +ADP+P1+O2H2O+CO2+ATP


TWITCH CONTRACTION• In myography i.e. muscle

graphing a single brief threshold stimulus produces a quick jerk of a muscle called as twitch contraction.

• A muscle doesn’t begin contraction at the instant of stimulation but rather a fraction of a sec.there are 3 phases respectively the latent period,contraction phase and relaxation phase.entire twitch lasts for 1/10th of a sec.

Twitch rarely happens in the body ,can occur when there's over activity of the nervous system to stimulate all motor neuronsin a muscle fiber.


TREPPE:THE STAIR CASE PHENOMENON

• Treppe is the gradual step like increase in the strength of contraction that can be observed in a series of twitch contractions that occur about one second apart.

• A muscle contracts more forcefully after it has contracted a few times than when it first contracts – a principle used by athletes to warm up.Possibly it relates partly to the rise in temp of the active muscle and partly to their accumulation of metabolic products.

• After first few stimuli muscle responds to a considerable number of successive stimuli with


Treppe contd… maximal contractions.

Eventually it will respond with less and less strong contractions i.e. relaxation phase shortens and disappears and muscle stays in a state of abnormal contraction- contracture.

• Repeated stimuli lessens excitability and contractility resulting in muscle fatigue such that the muscle wont respond even to the strongest stimuli.


MUSCLE TONE• A tonic contraction (tonus)is a continual partial

contraction.At any one moment a small number of the total fibers in a muscle contract producing tautness and firmness of the muscle rather than a recognizable contraction.It’s a characteristic of a muscle fiber when they are awake.muscles with less tone are flaccid and more spastic.

• Its maintained by a negative feed back mechanism centered in the spinal cord.Stretch sensors detect the info for degree of stretch and send them to the integrator mechanism in spinal cord.When actual stretch deviates from set point ,signals are sent via somatic motor neurons and adjust the strength of contraction.this subconscious reflex is called spinal reflex.


ISOTONIC AND ISOMETRIC CONTRACTIONS • Isotonic contraction is one in which the tone or tension

within a muscle remains the same but the length of the muscle changes,i.e.,same tension.Since there’s no /little tension placed on a muscle in this contraction,all of the energy is used to pull on the thin filaments and decrease the length of a fiber’s sarcomere.

• Two types:concentric and eccentric isotonic contraction.• In concentric ,a muscle shortens and pulls on another

structure like a tendon .in eccentric is when the overall length of a muscle increases during contraction.Repeated eccentric


Contd…

contractions are more damaging than concentric and causes more delayed onset muscle soreness.

• Isometric contractions: muscle length remains same but in which muscle tension increases,i.e.,same length.the tension produced by the power stroke of myosin cross bridges cant overcome the load placed on the muscle.


Graded Strength Principle• Skeletal muscles contract with varying degrees of strength at different times-Graded strength principle.

• Improved metabolic conditions that produce the treppe effect allow a muscle to increase its contraction strength.

• Another factor influencing GSP is number of fibers contracting simultaneously.The more the fibers contracting at the same time the more stronger the contraction.


Length Tension Relation• The maximal strength that a muscle can develop is

directly related to the initial length of its fiber- this is the length tension relationship.A muscle that starts contraction from a short initial tension cannot develop much tension since its already compressed.

• Conversely overstretched muscle cant develop much tension because the thick myofilament are too far away from the thin to effectively pull them and compress the sarcomere.The strongest maximal contraction is possible only when the muscle organ has been stretched to an optimal length.


TYPES OF SKELETAL MUSCLE FIBERS.

• Depending on the color of the fibers they are called red and white muscle fibers.Red muscle fibers are called so because they contain myoglobin and more blood capillaries and ATP.

• Also based on enzyme histochemistry, ATPase,speed of contraction and susceptibilty to fatigue they are divided into three main types:1)slow oxidative glycolytic fibers 2)fast oxidative glycolytic fibers 3)fast glycolytic fibers.


Slow Oxidative Fibers:• They are the smallest in diameter and thus the least

powerful type.they are dark red because they contain large amounts of myoglobin and many blood capillaries.They also have a lot of mitochondria ,SO fibers generate ATP mainly by aerobic cellular respiration,which is why they are called oxidative fibers.

• Has low contraction velocity ,very resistant to fatigue and capable of prolonged,sustained contraction for many hours.These slow twitch ,fatigue resistant fibers help maintain posture of the mandible and mastication of soft diet.Temporalis,Masseter,ant Medial pterygoid and Lateral pterygoid muscle all have SO fibers.


Fast Oxidative Glycolytic Fibers• Intermediate in diameter they contain large

amounts of myoglobin and blood capillaries but lesser than that of SO fibers.FOG fibers can generate considerable ATP by aerobic cellular respiration, which give them a moderately high resistance to fatigue and also generate ATP by anaerobic glycolysis.Their contraction velocity is higher than SO fibers.FOG fibers reach peak tension quickly than those of SO fibers but are briefer in duration.Used in walking and sprinting.


Fast Glycolytic Fibers:• Has the largest diameter and contains the highest

number of myofibrils and this generates the most powerful contractions.They have low myoglobin and relatively few blood capillaries and few mitochondria and appear white in color.

• ATP produced by glycolysis.Contract rapidly and strongly.These fast twitch fibers are adapted for intense anaerobic movements of short duration, but they fatigue quickly.

• More in subjects with high biting forces.Found in jaw muscles 45% in superior temporalis muscle,post medial pterygoid and ant digastric muscle.


STRETCH REFLEX• Another factor that influences strength of a skeletal

muscle contraction is amount of load imposed on the muscle.With in certain limits the heavier the load the stronger the contraction.

• An example:Lift your left hand with your palm up in front of you and then put a book in your palm.you can feel the muscle contract more strongly.This occurs because of a stretch reflex,a response in which the body tries to maintain a constancy of muscle length.

• An increase in load threatens to stretch the muscle beyond the set point length that you are trying to maintain.Your body exhibits a –ve feed back when


Contd.. it detects the increased stretch

caused by an increased load,feeds the information back to the nervous system and in turns increases stimulation of the muscle to counteract the stretch.

• This reflex maintains constant muscle length.When load increases body abandons this reflex as it can damage the muscle and forces the muscle to relax and drop the load.


MYOTACTIC RELFEX• The stretch reflex when elicited causes contraction of the

stretched muscle.Muscle stretch receptors are proprioceptive nerve endings called muscle spindles.It consists of a bundle of thin intrafusal muscle fibers within the spindle.

• The long slender ends of the intrafusal fibers are striated and contractile whereas the nuclear bag region is noncontractile.The impulses arising from here are conducted by the group 1A sensory fibers.These sensory fibers synapse with the motor neuron called alpha efferent that supply the extrafusal fibers responsible for the contraction of the stretched muscle.Therefore this is a monosynaptic reflex.


Contd….• The functional significance is

that, it serves as a mechanism for maintaining posture.The stretch of the muscle acts as a stimulus to elicit the stretch reflex causing a sustained contraction of the stretched muscle maintaining it upright against the action of gravity.

• The same reflex acts in the mandibular musculature to maintain the postural rest position of the mandible in relation to the maxilla. In an activator the clinician induces a

A myotactic reflex in the muscles of mastication by using a bite registration,thus maintaing the main force providedthrough active tension in the stretched muscles of mastication.


CLASP KNIFE REFLEX• Also called as autogenic inhibition.if one tries to flex the

the spastic limb of a patient forcibly,resistance is encountered as soon as the muscle is stretched.This is a resistance due to the hyperactive reflex contraction of the muscle in response to stretch.If flexion is carried further forcibly a point is reached where all resistance melts and the rigid limb collapses readily.Because the resistance of the limb resembles that of a spring loaded folding knife blade its called clasp knife reaction i.e. the muscle first resists ,then relaxes.

• The stimulus necessary to elicit such a reflex is excessive stretch and when elicited it inhibits muscular contraction,causing the muscle to relax.


Contd……• The receptors for this are the

golgi tendon organs,impulses carried by the group 1B sensory fibers.impulses act on the motor neuron supplying the stretched muscle.It’s a disynaptic reflex arc because an interneuron is interposed in between the sensory and motor neuron.The output of the motor neuron pool depends upon the balance between the two antagonistic inputs .

Functional significance is to protectthe overload by preventing damaging contraction against strong stretching forces.www.indiandentalacademy.com

ACTIVE AND PASSIVE TENSION.• All the mandibular levator muscles posses spindles,thus

stretch reflexes can be elicited .The tension resulting from contraction of muscles is called as ACTIVE TENSION.

• Facial muscles don’t posses spindles they don’t elicit a stretch reflex.The tension in these muscles will increase with elongation because of the elastic properties of the muscle and its investing tissues.Tension which results from the viscoelastic properties of the tissue is called as PASSIVE TENSION.In many muscles,elongation will result in an increase in both active and passive tensions.the sum of both tension is called TOTAL TENSION.


Contd….

• Initially there’s no passive tension,so the total tension is equal to active tension when the muscle contracts.As stretching increases the muscle begins to behave elastically.So passive tension now begins to add to total tension.As it elongates further active tension is inhibited while passive tension continues to increase.On further stretching active tension suppressed and passive tension increases exponentially.At this length total tension equals passive tension.


Contd…• The total tension in a muscle

is the result of active tension from the myotactic reflex and passive tension from viscoelastic properties of the tissues.Moderate bite registrations in activator treatment attempt to use active tension to achieve correction of malocclusion.More extreme vertical openings in which the mandible is opened 8mm beyond the rest position uses passive tension.


Contd…..• Many treatment procedures in orthodontics results in

the elongation of jaw and facial muscles.Expansion of the dental arches stretches the cheek/lips and increases tension in the buccinator and orbicularis oris muscle.Increasing the occlusal vertical dimension in closed bite malocclusion will stretch the levator muscles.Bite planes and activators which increase vertical dimension and advance the mandible increase tension in both levator and retractor muscles.

• Following immobilization in a shortened muscle extensibility is decreased i.e its less elastic.During sleep in jaw muscles stretch reflex is abolished passive tension generated by appliances.


ELECTROMYOGRAPHY.• Its used to evaluate the activity of an orofacial muscle

on an electromyogram.The electrical activity is the easiest to record.Electrodes are inserted into the skin surface or into the body of a muscle.They record the membrane potential from several fibers in a single motor unit which arrives at different times.At high force levels there’s interaction of negatively and positively charged action potentials leading to a leveling of in the electromyogram causing an interference pattern.

• This gives useful information about when a muscle is active and defines when activity begins and ends but its impossible to know how much activity is being missed.


conclusion• The functions of oral and facial muscles is

complex in nature.• Studies have been conducted to know the nature

of these muscles and relationship to various facial types.

• The knowledge about the relationship between muscles and facial types is important in understanding different facial forms and formulating ideal treatment for the patient.


• Removable orthodontic appliances-Graber and Neumann.

• Handbook of orthodontics-Robert.E.Moyers.• Principles of anatomy and physiology-Graboski

and Tortora.• Review of medical physiology-Ganong.• Anthony’s textbook of anatomy and physiology-

Gary.A.Thibodeau and Kevin .T.Patton.

REFERENCES


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muscle physiology in orthodontics / orthodontic courses by indian dental academy

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