Skeletal and smooth muscles

Skeletal muscles make about 40% of our body and almost another 10% is added by smooth and cardiac muscles.It shows fifty percent of the human body is muscles.Skeletal muscles vary in size from the very small stapedius muscle in the middle ear to the very large quadriceps muscle.Human body has over 600 muscles.Muscle is an organ containing two basic tissues,the muscular tissue and connective tissue.All the muscles bear four important characteristics that are:excitability,contractility,extensibility and elasticity.Each muscle fiber is a structural unit of muscle formed by synctium.The functional unit of a muscle is ''motor unit'' thats defined as the number of muscle fibers innervated by a single motor neuron.The commenest proteins resposible for the contraction of muscles are the actin and myosin.

All skeletal muscles are composed of muscle fibers,each fiber is innervated by one nerve ending located near the middle of the fiber.Structurally,each muscle fiber is covered by an outer layer,sarcolemma and each fiber contains several myofibrils which in turn are composed of actin and myosin filaments responsible for muscle contraction.

The myofibrils are suspended inside an intracellular matrix called sarcoplasm.The fluid of the sarcoplasm contains large quantities of potassium,magnesium,phosphate,protein enzymes, mitochondria and extensive endoplasmic reticulum called here as sarcoplasmic reticulum.Energy is required when the work(W=L * D) is done.An actin filament is composed of 3 protein components:actin,troponin and tropomyosin.A myosin molecule contains two heads and a long tail and is composed of multiple myosin molecules,each one composed of 6 polypeptide chains;two heavy chains and four light chains. Contracting myofibrils fullfil the energy need by taking ATP formed by the mitochondria.The energy required to perform the work comes from the ongoing chemical reactions in the muscle cells or myocytes during contraction.Most of this energy is used for pulling the actin filaments,also energy is required in small amounts for:(1)pumping 'Ca' from the sarcoplasm into the sarcoplasmic reticulum after the contraction is over and pumping 'Na' and 'K' ions through the cell membrane of fibers to provide suitable ionic environment for the propagation of action potential.Action potential is an electrical change spreading along the excitable tissue (muscles,glands) causing depolarization.

Its calculated as 4 millimolar of ATP is only sufficient for the full contraction of muscle lasting 1-2 second at most.After this ATP splits into 'ADP' and 'P'(phosphate).The released ADP is rephosphoralyed to form new Adenosine TriPhosphate molecules.There are several sources of energy required for the rephosphorylations.Three energy sources are:

Phosphocreatine is a substance having high energy phosphate bond similar to the bond of ATP,its cleared and the released energy is used for the binding of phosphate ion to ADP to reform ATP.

Glycogen is the second important source of energy already stored in the muscles that is used to reconstitute both ATP and Phoshocreatine.This energy source is sufficient for about 1 minute muscle contraction because of accumulation of many end product of glycolysis in the muscle cells. Oxidative metabolism(combination of oxygen with carbohydrates,fats,proteins to liberate ATP) is the final source of energy.95 percent of energy is derived in long term muscle contraction from this source.

In short, the process of muscle contraction is the contraction of myofibrils by taking ATP.Actin filaments are pulled inward among the myosin filaments and muscles go contraction,it happens so when an action potential (AP) i.e., contractile signals travels alng the muscle fiber membrane causing the sarcoplasmic reticulum to release large quantities of 'Ca' ions that penetrate the myofibrils and activate actin and myosin filaments to slide along side eachother. If the myofibrils are observed under electron microscope,they appear as light(I bands) and dark bands.The light bands contain only actin filaments while dark bands(A bands) contain myosin filaments as well as the ends of actin filaments.A large number of filamentous molecules of a protein called Titin keeps the actin and myosin filaments in place.

Rigor mortis is a process occuring after death where all the muscles of the body go into a state of contracture and become rigid even without action potential.The muscles remain rigor or rigid as all the ATP are lost which are required for the separation of the cross bridges during the relaxation process after contraction.They remain so, until the destruction of muscle proteins by enzymes released from lysosomes some 15-25hrs after death.

Smooth muscles are involuntary in nature and generally divided into two types:
1)Unitary smooth muscles(visceral or synctical):Mostly found in body organs especially in the walls of the gut ,bile ducts,ureters,uterus and so forth.They show spontaneous contraction and react to non-nervous stimuli.Visceral or unitary smooth muscles are arranged in sheets or bundles where cell membrane of the fibers contact eachother at multiple points to form many gap junctions usually contracting as a single unit so called as single unit or unitary smooth muscles.
2)Multiunit smooth muscles are found in ciliary muscles of the eye,iris of the eye,pilorector muscles & smooth muscles of large blood vessels.
They react to nervous stimuli (i.e. the contraction is controlled by nerve signals) and show non spontaneous contraction.They rarely exhibit spontaneous contraction.Multiunit smooth muscles are composed of discrete smooth muscle fibers,each fiber operates independantly resembling skeletal muscle fibers.

How smooth muscles contract: Smooth muscles contain both actin and myosin filaments that are chemically similar to skeletal muscle fibers.But these fibers are far smaller in size and lack Troponin complex.As contraction in skeletal muscles is controlled by Troponin complex,hence smooth muscles have a different method of contraction apart from the basic mechanism of contraction similar to that of skeletal muscles.The regulation of smooth muscle activity by calcium is mediated by Calmodulin. The initating process for the smooth muscles contraction is an increase in intracellular 'Ca' ions,similar to as in skeletal muscles but this increase canbe caused by nerve stimulation,hormonal stimulation,stretch of the fiber or local tissue factors and hormones(like scrotinin,histamine,acetylcholine).
Some differences between skeletal & smooth muscles regarding to contraction process: 1)Most of skeletal muscles contract and relax rapidly and are activated by only the nervous stimuli,smooth muscles canbe stimulated by other factors(stretch,hormones,nerve signals).
Smooth muscle relax slowly in comparison with skeletal muscles as Ca pumps are slow acting while sarcoplasmic reticulim pump in skeletal muscles act fastly.These pumps are responsible for the removal of Ca ions from the intracellular fluids of actin and myosin filaments to cause relaxation of a contracted muscle.
2)The contractile process in both type of muscles is activated by 'Ca' ions but the source of supply is different.In skeletal muscles all the Ca ions are derived from sarcoplasmic reticulum.In the skeletal muscles Ca ions mostly enter muscle cells from the extracellular fluid at the time of action potential or any other stimulus(hormones,stretch etc)
3)The specific filamentous arrangement of smooth muscles allow them to contract as much as 80% of their length instead of 30% in skeletal muscles.
Smooth muscles contraction is almost entirely dependant on 'Ca' ions flow to the enterior of the fiber while Na' ions in addition to Ca ions also play an important role in the skeletal muscle comtraction.Its because of far more voltage-gated 'Ca' channels but few voltage-gated 'Na' channels in the cell membrane of smooth muscles.

Contraction of a muscle against a constant load is called isotonic(same tension) contraction.Contraction of a muscle without decrease in its length is termed as isometric(same length) contraction.