This month we continue our discus-sion about muscle. In this installment we’llcover some basic concepts about muscleanatomy and physiology. This will lead toan understanding of the adaptive responseof muscle to exercise, so you can betterdesign an effective program to ahieveyour training goals.First off, you should realize that thereare three basic types of muscle tissue inthe human body. Skeletal muscle, alsocalled striated muscle, is attached via ten-dons to the skeleton. Usually the two endsof a skeletal muscle are attached to twodifferent bones across a joint. The onlything a muscle can do is contract, andwhen it contracts (shortens) it brings thetwo bones closer together.
This causesmovement of the skeleton at the particu-lar joint spanned by the muscle. Thus thefunction of skeletal muscle is to move theskeleton. Other skeletal muscles, suchas those between vertebrae and betweenthe ribs, play more of a structural role,helping the skeleton to maintain its propershape. Notably, skeletal muscle is undervoluntary control, which means that youcontrol when a skeletal muscle contracts.Another type of muscle tissue issmooth muscle, called that because itlacks striations. Probably the best ex-ample of smooth muscle is the stomachand intestines. Your digestive tract is ahuge muscular tube which propels foodfrom your mouth to the other end. Yourintestines are ringed by circular bands ofmuscular tissue which squeeze the foodthrough the tube when they contract.This is called a peristaltic contraction, andis the same kind of contraction that pro-pels food down your esophagus when youswallow.
Smooth muscle is not undervoluntary control, but instead is controlledby the autonomic nervous system. Theautonomic nervous system receives inputfrom the brain, but at the subconsciouslevel. The autonomic nervous system hastwo divisions: the parasympathetic divi-sion which speeds up movement of foodthrough the digestive tract, and the sym-pathetic division which slows down move-ment. Of interest, the intestines have theirown nervous system built right into theintestinal wall, between muscle layers. It’scalled the myenteric nerve plexus, andintegrates information from the intestineswith input from the autonomic nervoussystem to precisely control digestion andfood movement. Most people don’t real-ize that there are nearly as many nervecells in their intestines as in their brains.The complexity of this system has onlybecome understood in the last couple ofyears. It’s what gives us the freedom tothink about other things instead of havingto constantly be worried about con-sciously controlling the movement of foodthrough the gut.
The third type of muscle tissue is car-diac muscle, found exclusively in theheart. It’s somewhere between striatedmuscle and smooth muscle. It does havestriations like skeletal muscle but the fi-bers are arranged more like smoothmuscle, so it has elements of both. Car-diac muscle is not strictly under consciouscontrol (your heart beats without youthinking about it), but is influenced byconscious thought. For example, beingnervous or scared will speed up your heartrate, while relaxing will slow down yourheart rate. This too is mediated by theautonomic nervous system. Here, thesympathetic division speeds up heart ratewhile the parasympathetic division slowsit down. Heart muscle has some veryspecial properties which allow it to do itsjob. For one thing, it has a built in pace-maker which causes the heart to sponta-neously contract all by itself. You cancut all the nerves going to the heart and itwill still beat just fine. (This is what makesheart transplants possible.)The rest of this series is devoted to astudy of skeletal muscle, especially howit works and what to do to make it biggerand stronger. Each muscle is made ofmuscle cells, also called muscle fibers ormyofibers (1,2,3). Muscle cells are verylong, sometimes spanning the length ofthe entire muscle, and are about 50-100microns in diameter (about the size of ahuman hair). Each muscle cell containsmany nuclei which are located around theoutside, just beneath the muscle cell mem-brane, or sarcolemma.
When you look ata muscle fiber with a microscope, youcan make out cross striations runningperpendicular to the length of the fiber.Each muscle is contained in a bag of toughconnective tissue for protection. Thisconnective tissue is called epimysium,because it’s around the muscle. This isthe stuff you’re stretching when you doyour stretches and fascial planing. It’sreally tough, kind of like those nylon-re-inforced mailing envelopes, except thin-ner. The idea is if you stretch the con-nective tissue covering the muscle you willmake it easier for the underlying muscletissue to expand, but that’s a story foranother day.Just under the epimysium the musclecells are grouped into bundles calledfasiculi, which contain as many as 150fibers (cells) each (1). Inside the musclecell is the sarcoplasma, the name for thecytoplasm of a muscle cell (1). Therewe find the contractile proteins, storedglycogen and fat granules, and mitochon-dria. The contractile proteins are groupedinto bundles called myofibrils, which areabout 1 micron in diameter, or about 1/100 the width of a human hair (1). Themyofibrils are made of the contractile pro-teins themselves, which are called myo-filaments.
The two main proteins in themyofilaments are actin and myosin. Sowe have myofibers, myofibrils, and myo-filaments. (Don’t blame me – I didn’tmake this up.)Actin and myosin, the two main pro-teins responsible for muscle contraction,are organized into structural and functionalsubunits called sarcomeres. When youbuild more muscle tissue, mostly whatyou’re doing is increasing the amount of actin and myosin proteins in your musclecells, thereby making the muscle biggerand stronger. As you know, proteins aremade from building blocks called aminoacids. What happens is when you eatsome protein your stomach and intestinesdigest it and break it down into the indi-vidual amino acids. These then enter thebloodstream and are transported to all thecells of the body. The cells absorb theamino acids from the bloodstream andthen use them as building blocks to as-semble whatever proteins the cell needs.Muscle cells absorb the amino acids anduse them to build more actin and myosin.Each protein has a unique sequence ofamino acids linked together in a chain.You need a different ratio of amino acidsto build different proteins, just like youneed different letters to spell differentwords. The proteins in muscle contain alot of the branched chain amino acids(BCAAs), along with a specific ratio ofthe other amino acids as well.
This is thereasoning behind our Muscle Amino For-mula™ and our Hi-Protein Powder.™These products are designed to supplyyour body with the perfect balance ofamino acid building blocks it needs to buildmore muscle protein.Like all cells of the body, muscle cellsare surrounded by a plasma membrane.The membrane of a muscle cell is calledthe sarcolemma. The sarcolemma re-ceives electrical impulses from motornerves at specialized structures calledmotor end plates. At the ends of themuscle, the sarcolemma fuses with thetendon, which inserts into bone (2).When the muscle contracts force is trans-mitted to the tendon, and in turn to theskeleton, which brings about movement.Inside the sarcolemma is the cytoplasmof the muscle cell – the sarcoplasma (2).The sarcoplasma is a gel-like substancethat bathes the myofibrils – thecontractile elements. The sar-coplasma contains minerals,ions, glycogen, fat, and or-ganelles called mitochondria(2). Mitochondria are pow-erhouses inside the cell wherecarbohydrates and fat are con-verted into ATP, which is thesource of chemical energy directly usedby the myofibrils when they contract.The sarcoplasma also contains a proteincalled myoglobin, which functions to bindoxygen much like hemoglobin binds oxy-gen in red blood cells.Enclosures of the sarcolemma calledtransverse tubules, or T tubules, pass tothe interior of the muscle cell. When anerve impulse, or action potential, arrivesat a muscle cell it releases a neurotrans-mitter called acetylcholine (ACh) from thenerve terminal at the motor end plate.
TheACh diffuses across a space called a syn-apse and binds to receptors on the sarco-lemma. This initiates an electrical poten-tial, called a wave of depolarization, alongthe surface of the sarcolemma. The Ttubules function to bring this electricalimpulse inside the muscle cell to the myo-fibrils. The sarcoplasmic reticulum is aseries of membranous channels which runparallel to the myofibrils. The sarcoplas-mic reticulum serves as a storage site forcalcium, which is essential for muscularcontraction (2). When the electrical im-pulse is conducted inside the cell by the Ttubules, it reaches the sarcoplasmic reticu-lum where it triggers the release of cal-cium. Calcium then binds to a regulatoryprotein called troponin C. This initiates aseries of molecular events which allowsATP to bind to myosin. Myosin breaksdown the ATP to release energy which isused to power muscle contraction.This is starting to get technical. Tosum up so far: when you decide to con-tract a muscle an electrical impulse is gen-erated by your brain. This travels downthe spinal cord and out a peripheral nervecalled a motor neuron. This nerve car-ries the electrical signal to the muscle. Thenerve releases ACh which diffuses acrossthe synapse at the motor end plate. Thisin turn initiates another electrical signal,or wave of depolarization, along the sar-colemma of the surface of the muscle cell.
The T tubule system carries this infor-mation to the interior of the muscle cell,where it triggers the release of calciumfrom the sarcoplasmic reticulum. Calciumbinds to a protein on the surface of themyofibrils which normally prevents ATPbinding. After calcium binds, these pro-teins shift positions which subsequentlyallows ATP to bind to the myofibrils. ATPis then broken down to release energy.This energy causes the actin and myosinproteins in the myofibrils to slide past eachother, thus making the muscle contract.This contractile force is transmitted to thetendons and then to the bones, causingmovement of the skeleton and the bodyas a whole (1,2,3). Simple eh?Next month we’ll talk about exactlyhow ATP binding to the myofilamentscauses the actin and myosin proteins tomove, resulting in muscle contraction.That will conclude the anatomy and bio-chemistry part of our muscle super-fea-ture. Then we’ll move into exercisephysiology to discuss how exercise train-ing affects muscle performance. Finallywe’ll tie everything together and explainhow exercise training elicits an adaptiveresponse in muscle – cellular and molecu-lar changes which make the muscle big-ger and stronger. Once you understandhow all of this works, you will be betterable to design a training program specifi-cally to achieve your individual traininggoals.
1. Baechle TR. Essentials of StrengthTraining and Conditioning. Human Kinet-ics, Champaign, IL, 1994.
2. Wilmore JH and Costill DL. Physi-ology of Sport and Exercise. HumanKinetics, Champaign, IL, 1994.
3. McArdle WD, Katch FI, andKatch VL. Exercise Physiology -Energy, Nutrition, and HumanPerformance. Lea & Febiger,Malvern, PA, 1991.