Locomotion of Muscles in Human Body

In this article we will discuss about the Locomotion of Muscles in Human Body.

Types of Muscles:

In humans muscles constitute about 40 to 50 percent of the total body weight.

These muscles are broadly classified into three categories:

1. Skeletal or striped or striated or voluntary muscles

2. Smooth or un-striped or non-striated or visceral or involuntary muscles

3. Cardiac muscles

1. Skeletal Muscles:

These muscles are found in the limbs, body wall, tongue, phar­ynx and beginning of oesophagus. These muscles are under the control of animal’s will. These muscles are normally attached to the skeleton. The major component of muscles is water.

Potassium is the most abundant mineral element in muscles. Other minerals such as sodium, calcium, phosphorus and magnesium are present only in traces. Muscles store glycogen. They have oxygen carrying pigment myoglobin or “muscle haemoglobin”. Muscles also contain ATP, phosphocreatine, creatine, urea, etc.

2. Visceral Muscles (Smooth Muscles):

These are found in the posterior part of oe­sophagus, stomach, intestine, lungs, urinogenital tract, urinary bladder, blood vessels, iris of eyes, dermis of skin and arrector pili muscle of hair. Smooth muscles never connect with skeleton. These muscle fibres or cells are elongated and spindle shaped. Each fibre contains a single oval nucleus surrounded by the cytoplasm (sarcoplasm).

In the cytoplasm the myo­fibrils are arranged longitudinally. There is no sarcolemma, however, the fibre is enclosed by plasma membrane. Action of these muscles is controlled by autonomic nervous system and hence they are not under the control of the animal’s will.

3. Cardiac Muscles:

The cardiac muscles are found in the wall of the heart and in the wall of large veins (e.g., pulmonary veins and superior vena cava) where these veins enter the heart. These fibres show the characters of both un-striped and striped muscle fibres.

Each fibre is a long and cylindrical structure which lacks a definite sarcolemma. The fibres are uninucleate and the nuclei lie near the centre. The fibres have some lateral branches, known as oblique bridges to form a contractile network.

The myofibrils have transverse faint dark and light bands, which alternate with each other. In this way cardiac muscle fibres are also striped, but having dark intercalated discs at intervals.

The intercalated discs are specialized regions of cell membrane of two adjacent fibres. The intercalated discs function as boosters of contraction wave and permit the wave of muscle contraction to be transmitted from one cardiac fibre to another.

Detailed Structure of Skeletal Muscle:

To understand the structure and mechanism of contraction, each skeletal muscle is made of a number of muscle bundles or fascicles. Each muscle bundle contains a number of muscle fibres (muscle cells). Each muscle fibre is surrounded by the plasma membrane called sarcolemma enclosing the sarcoplasm.

Muscle fibre is a syncitium as the sarcoplasm contains many nuclei. The endoplasmic reticulum of the muscle fibre is called sarcoplasmic reticulum which is store house of calcium ions. There are present large number of parallelly arranged filaments called myofilaments or myofibrils.

A myofibril has dark and light bands. The dark bands are also called A-bands (Anisotropic bands). The light bands are also called I-bands (Isotropic bands). At the centre of A- band, a comparatively less dark zone called H-Zone (= Hensen zone, named after Hensen who first described) is present.

In the centre of the H-zone is the M-line: The letter ‘M’ is from the German word Mittleschiebe (mittle = middle). Each I-band has at its centre a dark membrane called Z-line. The letter ‘Z’ is from the German word Zwischenschiebe (zwischen = between, schiebe = disc).

The Z-line is also called Z-disc, or Krause’s membrane or Dobie’s line. The part of the myofibril between two successive Z-lines is called sarcom­ere. Therefore, the sarcomere com­prises А-band and half of each adjacent I-band.

The sarcomere is the functional unit of myofibril. In fact each sarcomere is a bundle of thick and thin myofilaments. The thick myofilaments have diameters of about 150A, whereas the thin myofilaments have diameters of about 70A.

Structure of Contractile Proteins:

The thick myofilaments are formed by myosin protein. The thin myofilaments are formed by three types of proteins called actin, tropomyosin and troponin. These four proteins are called contractile proteins.

(i) Thick Myofilaments:

The thick filament consists mainly of myosin protein. Myosin contributes 55% of muscle protein by weight. The myosin molecule is composed of six polypeptide chains, two identical heavy chains and four light chains.

The two heavy chains wrap spirally around each other to form a double helix. However, one end of each of these chains is folded into a globular protein mass called myosin head. Thus, there are two free heads lying side by side at one end of the double helix myosin molecule.

The elongated part of the coiled helix is called the tail. The four light chains are also parts of the myosin heads, two to each head. These light chains help control the function of the head during the process of muscle contraction. Myosin is split by enzyme trypsin into two fragments, called light meromyosin (LMM) and heavy meromyosin (HMM).

LMM lacks adenosine triphos­phatase (ATPase) activity and does not combine with actin. HMM consists of two globular sub-fragments and one rod shaped fibrous sub fragment. Each globular sub fragment contains an ATP binding site and actin binding site (Fig. 20.2B). It can form a cross bridge with the active site present on the actin.

(ii) Thin Myofilaments:

The thin filament is composed of three different proteins— actin, tropomyosin and troponin (Fig. 20.3).

(a) Actin:

Actin is a globulin protein and has low molecular weight. It occurs in two forms, the monomeric G-actin and the polymeric F-actin. G-actin (G = globular) polymer­izes to the fibrous form F-actin (F = fibrous) in the presence of Mg⁺⁺.

(b) Tropomyosin:

Tropomyosin is a double stranded a-helical rod. It is fibrous mol­ecule that attaches to F-actin in the groove between its filaments. In the resting state, the tropomyosin molecules are believed to lie on top of the active sites of the actin strands so that attraction cannot occur between the actin and myosin to cause contraction.

(c) Troponin:

Troponin is a complex of 3 polypeptides. Troponin T (TpT) binds to tropomyosin as well as to the other two troponin components. Troponin I (Tpl) inhibits the F-actin-myosin interaction and also binds to other components of troponin. Troponin С (TpC) is a calcium-binding polypeptide. The strong affinity of the troponin for calcium ions is believed to initiate the contraction process.

i. Tendons:

A muscle may be attached to a single bone or different bones by one end or by both the ends, either directly by epimysium or by way of inelastic connective tissue cords, the tendons.

ii. Fascia:

The term fascia is applied to a sheet or broad band of fibrous connective tissue beneath the skin or around muscles and other organs of the body.

iii. Motor Unit:

A neuron that transmits a stimulus to muscle tissue is called motor neuron. A motor unit consists of a single motor neuron (nerve cell) and the muscle fibres it innervates.

The portion of the muscle plasma membrane (sarcolemma) that lies beneath the nerve endings (axon terminals) is called the motor end plate. The axon terminals and the motor end plate together constitute the neuro­muscular junction or neuromotor junction.

iv. Single Muscle Twitch:

A muscle fibre contracts only once if it is stimulated by a single nerve impulse or by a single electric shock of adequate strength. This single isolated contrac­tion of the muscle fibre is called single muscle twitch. Immediately after a twitch, the muscle fibre relaxes.

v. Threshold Stimulus:

For contraction, muscle fibre always requires a specific minimum strength or intensity of the stimulus or nerve impulse. This is called threshold stimulus. If the stimulus or the nerve impulse is below this intensity, the muscle fails to contract.

vi. AII-or-None Principle (= Bowditch’s Law):

According to the ‘all-or-none law’, when a fibre contracts, it contracts maximally.

vii. Tetanus:

The continued state of contraction is called tetanus.

viii. Muscle Tonus (= Muscle Tone):

The state of sustained partial contraction is called muscle tonus or muscle tone. It is a sort of mild tetanus. It is essential to maintain posture and form of the body.

ix. Proteins of Muscles:

Myosin actin tropomyosin, troponin, myoglobin.The major component of muscle is water. Potassium is the most abundant mineral element in muscle. Other minerals such as sodium, calcium, phosphorus and magnesium are present only in traces. Muscles store glycogen. They have oxygen carrying pigment myoglobin or ‘muscles haemoglobin’. Muscles also contain ATP, phosphocreatine, creatine, and urea. Etc.

Functions:

Striated muscles are under the control of animal s will. Calcium is an essen­tial element for the contraction of muscles. In the presence of calcium ions and energy from ATP, actin and myosin interact forming actomyosin which causes contraction of muscles. During muscle contraction conversion of pyruvic acid to lactic acid proceeds anaero­bically.

Lactic acid is transported by blood to liver where it is converted to glycogen. Chemical energy is changed into mechanical energy during muscle contraction. The contrac­tion of muscles of shortest duration is seen in eye lids. Shivering in cold is a method for production of heat by muscle contraction.

Functional Classification of Body Muscles:

According to the type of movement they bring about, the skeletal muscles are of the following types:

1. Flexor:

This muscle bends one part of a limb on another at a joint, e.g., biceps. It brings the fore arm towards the upper arm.

2. Extensor:

This muscle extends or straightens a limb, e.g., triceps. It extends the fore arm.

3. Adductor:

This muscle brings a limb towards the mid line of the body, e.g., latissimus dorsi. It presses the entire arm against the side.

4. Abductor:

This muscle pulls a limb away from the mid-line of the body, e.g., deltoideus. It draws the entire arm to the side.

5. Pronator:

This muscle turns the palm downward or to the posterior, e.g., pronatorteres.

6. Supinator:

This muscle turns the palm upward or to the anterior, e.g., supinator.

7. Elevator:

This muscle raises a part of the body, e.g., masseter. It lifts up the lower jaw to close the mouth.

8. Depressor:

This muscle lowers a part of the body, e.g., depressor mandibulae. It lowers down the lower jaw to open the mouth.

9. Rotator:

This muscle rotates a part of the body, e.g., pyriformis. It raises and rotates the thigh.

10. Sphincter:

This muscle decreases the size of an opening, e.g., pyloric sphincter between stomach and duodenum.

11. Dilator:

This muscle enlarges the size of an opening.

Antagonistic Muscles:

Muscles which act in opposition to other muscles are called the antagonistic muscles. The biceps, for example bends or flexes the arm and is called a flexor. Its antagonist, the triceps straightens or extends the arm and is termed as exten­sor. Similar pairs of opposing flexors and extensors are found at the wrist, knee, ankle and other joints. When a flexor con­tracts, the opposing extensor must relax to permit the bone to move.

This requires proper coordination of nerve impulses go­ing to the two sets of muscles. Other antagonistic pairs of muscles are adduc­tors and abductors, which move parts of the body toward or away from the central axis of the body; elevators and depres­sors, which raise and lower parts of the body; pronators which rotate body parts downward and backward and supinators, which rotate them upward and forward; and sphincters and dilators which decrease and enlarge the size of an opening.

Mechanism of Muscle Contraction

Sliding Filament Theory (Fig. 20.6):

Two groups of workers proposed the sliding filament theory.

The essential features of this theory are as follows:

1. During muscle contraction, the thin myofilaments slide inward towards the H-zone.

2. The sarcomere shortens, but the lengths of thin and thick myofilaments do not change.

3. The cross bridges of the thick myofilaments connect with portions of actin of the thin myofilaments. The myosin cross bridges move on the surface of the thin myofilaments and the thin and thick myofilaments slide past each other.

4. As the thin myofilaments move past the thick myofilaments, the H zone narrows and even disappears when the thin myofilaments meet at the centre of the sarcomere. Thus the length of the sarcomere decreases during contraction. Size of I band also decreases.

5. The lengths of the thick and thin myofilaments do not change during muscle contrac­tion.

In a resting muscle fibre, the outside of sarcolemma is positively charged with respect to the inside. This potential difference across a membrane is called resting potential. A membrane with a resting potential is said to be polarised. It is maintained by sodium and potassium ions.

Sodium ions predominate on the outside of the sarcolemma and potassium ions predominate on the inside. Sodium ions are pumped out and potassium ions enter inside, both by active transport. The process of moving ions against concentration is called sodium pump (= sodium-potassium exchange pump).

Electrical and Biochemical Events in Muscle Contraction:

These events have been worked out by Albert Szent Gyorgyi and others.

These events are summarized as follows:

1. As a nerve impulse reaches the terminal end of the axon, small sacs called synaptic vesicles fuse with the axon membrane and release a chemical transmitter, acetylcholine.

Acetylcholine diffuses across the synaptic cleft (the space between the axon membrane and the motor end plate) and binds to receptor sites of the motor end plate. When depolarization of the motor end plate reaches a certain level, it creates an action potential.

After this, an enzyme cholinesterase present along with receptor sites for acetylcholine, breaks down acetylcholine into acetate and choline. A portion of the choline diffuses back to the axon and is reused to synthesize more acetylcholine for transmission of subsequent impulses.

2. An action potential (impulse) passes from the motor end plate over the sarcolemma (muscle plasma membrane) and then into the T-tubules and sarcoplasmic reticulum and stimulates the sarcoplasmic reticulum to release calcium ions into the sarcoplasm (cytoplasm of the muscle fibre).

3. Calcium plays a key regulatory role in muscle contraction. The calcium ions bind to troponin causing a change in its shape and position. This in turn alters shape and the position of tropomyosin, to which troponin binds. This shift exposes the active sites on the F-actin molecules. Myosin cross-bridges are then able to bind to these active sites.

4. The heads of myosin molecules project laterally from thick myofilaments towards the surrounding thin myofilaments. These heads of myosin are called cross bridges. The head of each myosin molecule contains an enzyme myosin ATPase. In the presence of myosin ATPase, Ca⁺⁺ and Mg⁺⁺ ions, ATP breaks down into ADP and inorganic phosphate, releasing energy in the head.

ATP – Myosin ATPase/Ca⁺⁺, Mg⁺⁺ → ADP + Pi + Energy

5. Energy from ATP causes energized myosin cross bridges to bind to actin.

6. The energized cross-bridges move, causing thin myofilaments to slide along the thick myofilaments. This movement is like the movement of the oars of a boat. Rowing with oars pushes a boat across the water, or the water along the sides of the boat in somewhat the same way that thick and thin myofilaments slide along one another.

As stated in the sliding filament theory, there is no shortening of thin and thick myofila­ments. However, the sarcomere shortens because of the sliding of the thin myofilaments produced by cross-bridge movements. The H-zones and I-bands shorten, but the width of the А-band remains constant.

Cori’s Cycle:

It was proposed by Cori and Cori, who got Nobel prize with Houssay in 1947. This cycle occurs in the muscles and liver. During glycolysis lactic acid is produced from pyruvic acid in the muscles.

This lactic acid is carried in the blood to the liver where 1/5th of lactic acid is oxidised to water and carbon dioxide and 4/5th of lactic acid is converted into glycogen. The glycogen releases glucose into the blood which is reconverted to glycogen in the muscles. The cycle is repeated.

Oxygen Debt:

During strenuous exercise, the muscle does not get sufficient oxygen to meet its energy needs immediately. So it contracts anaerobically and accumulates lactic acid produced by anaerobic glycolysis. During recovery, the oxygen consumption of muscle exceeds. The extra oxygen consumed during recovery is called oxygen debt of the muscle.

It is used in oxidising the accumulated lactic acid aerobically and in restoring the depleted creatine phos­phate and ATP in the muscle fibre. A small part of oxygen debt also goes to myoglobin which binds and stores oxygen for future use. For extra oxygen, deep and rapid breathing occurs carrying more oxygen into the lungs and eventually to the tissues.

Role of Biomolecules in Muscle Contraction:

A number of biomolecules are involved in the muscular contraction.

1. Muscle proteins such as myosin, actin, tropomyosin and troponin play a significant role during muscle contraction as described above. Myoglobin is similar to that of haemo­globin, that is, to carry oxygen.

2. Carbohydrates (e.g., glycogen) and lipids (e.g., neutral fats) are stored as food and supply energy.

3. High energy phosphates such as ATP and phosphocreatine provide energy.

4. Inorganic substances.

(i) Cations:

Potassium is the principal mineral of the muscle. The other cations are sodium, calcium and magnesium,

(ii) Anions:

Chloride and phosphate are the anions present in the muscle. Role of potassium, sodium, calcium, magnesium, chloride and phosphate have already been described in the mechanism of muscle contraction.

5. Enzymes catalyse all biological reactions that are involved in muscular contraction. Myosin ATPase is the important enzyme taking part in the muscle contraction.

Muscle Relaxation:

After muscle contraction, the calcium ions are quickly returned to the sarcoplasmic reticulum by active transport, a process that requires ATP. Troponin and tropomysin mol­ecules move to their previous position and block the active sites on the thin myofilaments. The myosin cross-bridges separate from actin. When myosin cannot attach to actin, the muscle relaxes.

Isotonic and Isometric Contraction:

The force produced by a whole muscle when it contracts is termed muscle tension and the force exerted on a muscle by a weight is called the load.

For example, when we pick up a book, the book is the load and the force produced by the muscles in our arm is the tension. Thus load and tension are opposing forces. When the tension remains the same whereas the change occurs in the length of the muscle fibres, it is called isotonic (same tension) contraction.

The muscle shortens during this type of contraction. Example of isotonic contraction is the simple bending of arm. When the length of muscle fibres remains the same and the tension is increased, it is termed as isometric (same length) contraction. The muscle does not shorten during this type of contraction. Example of isometric contrac­tion is pulling any heavy object.

Differences between Isotonic Contraction and Isometric Contraction

i. Summation:

If a second stimulus is given before complete relaxation of muscle’s response to the firs stimulus, the force produced by the second contraction will be stronger than the first; similarly, the third will be stronger than the second. This phenomenon is called summation.

ii. Rigor mortis:

The rigidity of muscles that occurs after death is called rigor mortis. Cellular metabolism comes to halt. Rigor mortis disappears some fifteen to twenty five hours after death as proteins are degraded.

iii. Energy:

Muscle fibres contain two organic phosphates, namely adenosine triphosphate (ATP) and phosphocreatine. They store energy in the muscles.

Like other cells of the body, muscle cells synthsize ATP as follows:

ADP + P + E (energy) →ATP

Whenever required ATP gives energy

Creatine is produced in the liver. In a resting muscle some of the ATP produced reacts with creatine to form phosphocreatine and ADP. When a muscle is active some of the energy from phosphocreatine is transferred back to ATP where the energy can be used to power contraction.

Muscle Fatigue:

The reduction in the force of contraction of a muscle after prolonged stimulation is called muscle fatigue.

Cause:

A muscle is able to contract for a short time in the absence of oxygen. But it gets fatigued sooner because in the absence of oxygen, the metabolic products of glycolysis mainly lactic acid accumulate.

The accumulation of lactic acid leads to muscle fatigue. Pain is experienced in the fatigued muscle. The site of fatigue is the junction between nerve and muscle. A muscle gets fatigued sooner after a strenuous exercise than after a mild exercise.

Remedy:

Fatigued muscle needs extra oxygen to dispose off excess lactic acid. After a strenuous exercise, faster breathing should be continued for some time to supply extra oxygen for oxidizing excess lactic acid. This results in the disappearance of fatigue.

a. Hypertrophy:

Increase in the size of muscle cells is called hypertrophy.

b. Atrophy:

Reduction in the size of individual muscle cells is called atrophy.

Red and White Muscle Fibres:

Birds and mammals have in their skeletal muscles two kinds of striated muscle fibres, red or slow muscle fibres and white or fast muscle fibres.