Muscles: Meaning and Groups | Biology

In this article we will discuss about the meaning and groups of muscles.

Meaning of Muscles:

Muscles are essential to the body to provide stability and mobility. They are present in almost all the parts of the body and the type of muscle that is present in a particular part of the body is functionally very well suited for that region. Like neurons, even muscles are excitable tissues.

The classification of muscle can be done based on various criteria:

i. Functionally, they can be classified into voluntary and involuntary.

ii. Histologically into striated and non-striated.

Groups of Muscles:

The three groups of muscles present in the body are:

a. Skeletal

b. Smooth

a. Skeletal Muscles:

i. They are striated.

ii. Voluntary in function.

iii. Attached to the skeleton through tendon.

iv. Composed of many muscle fibers, which are parallel to each other in arrangement.

v. Each of the muscle fiber in turn is made of many sarcomeres, which are also arranged serially.

vi. For the muscle to act, the impulse has to come from CNS (brain or spinal cord).

Sarcomere (Fig. 2.16):

i. Band A is present in the center is made up of myosin proteins. In the center of the A band, there is narrow gap that is known as H band. In the center of the H band is the M line. A band (anisotropic band) is so- called because light cannot pass through this band.

ii. Extending from the Z line on either side of the A band is I band. This is called I band (isotropic band) because light can pass through this band.

iii. I band is composed of actin proteins. In addition to this, in I band there will be presence of troponin and tropomyosin proteins.

iv. Under resting conditions though there is some amount of overlap between bands A and I, there is some gap in the center and this is H band.

v. During contraction, I band slides over the A band and bring about the shortening of the muscle fiber.

vi. Myosin and actin proteins are the contractile proteins in a sarcomere.

vii. In addition to these proteins, the other proteins present are troponin and tropomyosin. These are called as regulatory proteins.

viii. The protoplasmic membrane namely the sarcolemmal membrane covers the muscle fiber. This membrane shows dipping in at specific parts that is at the junction of bands A and I. This part of the sarcolemmal membrane is known as T tubule (transverse tubule).

ix. Sarcoplasmic reticulum is placed horizontally and between the two consecutive T tubules. These are known as L tubules (longitudinal tubules). The ends of the L tubules are dilated and are known as lateral cisterns.

x. The function of the T tubule is to conduct the impulse through the muscle fiber and that of L tubules (terminal cisterns of this) is to store and release of calcium ions during the process of contraction.

xi. Two T tubules and one L tubule together constitute sarcoplasmic triad.

Microscopic examination also shows two other important structures in the muscle. The sarcolemmal sheath covering the muscle dips into the muscle forming the T tubules. The action potentials produced at the neuromuscular junction travel along the sarcolemmal membrane enter the interior of the muscle fiber along the T tubules.

The other structure is the longitudinal tubules (L tubules) with their expanded ends as the lateral cisterns. They closely surround the myofibrils. These form the sarcoplasmic reticulum. The expanded portion (terminal cistern) stores ionic calcium, which plays important role in excitation-contraction coupling of muscle.

Excitation-Contraction Coupling:

Events during Excitation-Contraction Coupling:

A muscle action potential reaches the T tubule by traveling along the sarcolemmal membrane. This triggers the release of the calcium ions from the cisterns of the longitudinal tubules. The calcium ions occupy the C part of the troponin molecule. This in turn brings about a conformational change in the tropomyosin molecule.

This change is responsible for exposing the active site on the actin filaments. The head of the myosin filament gets attached to the active sites and stepwise it gets attached and detached to the active sites. In this process, the actin filament is drawn inwards towards the center of the sarcomere. This requires energy and it is supplied by the breakdown of ATP.

Myosin head itself acts as ATPase (actin myosin ATPase) and ATP is broken down to ADP and high energy PO₄ is released. The number of cross bridges occupied depends on the amount of ionic calcium available. Greater the amount of ionic calcium, greater will be the number of binding between actin and myosin and, therefore, greater will be the force or tension developed.

During this process, the width of the H band decreases and hence the width of the sarcomere decreases (Figs 2.17a to c). The width of the A band remains unchanged. There may be overlapping of the actin filaments at the center of the sarcomere. This is known as the sliding filament theory of muscle contraction.

Immediately following this, the calcium ions are actively pumped back into the L tubules by means of calcium pump. The pumping of calcium into cisterns also requires expenditure of energy.

Thus ATP has two important roles in the muscle:

(1) it is necessary for muscle contraction and

(2) Also necessary for muscle relaxation (Figs 2.18 and 2.19). This action of ATP is known as the plasticizer action of ATP.

Four important changes occur in the muscle when it is stimulated to contract:

1. Electrical—in the form of muscle action potential

2. Mechanical—in the form of muscle contraction.

3. Chemical—in the form of breakdown of ATP and creatnine phosphate

4. Thermal—in the form of heat production.

Mechanical changes:

When a muscle is made to contract, two types of contractions may be noted.

1. Isotonic contraction

2. Isometric contraction

During an isotonic type of muscle contraction, the length of the muscle fiber decreases but the tension in the muscle fiber remains the same. When a weight is lifted leads to certain amount of external work is done, and this is an example for isotonic contaction.

In isometric type of muscle contraction, the length of the muscle fiber remains the same but the tension developed in the muscle is increased. Example for isometric contraction is pushing against a wall. Walking is a good example for both isometric and isotonic type of contraction. The muscles of the limb which is on the ground contract isometrically to support the body weight and the muscles in limb which is lifted up to move contract isotonically.

Chemical changes that occur in the muscle during muscular contraction:

The immediate source of energy supply for muscle contraction is by the breakdown of adenosine triphosphate (ATP). During muscular contraction, it is found out that the ATP content of the muscle is not markedly decreased. This shows that ATP is not only broken down but it is also getting synthesized.

The PO₄ (high energy phosphate), required for the resynthesis of ATP from ADP, is obtained from the breakdown of creatine phosphate. During repeated contraction of the muscle, the required energy can also come from the breakdown of glucose or glycogen. Free fatty acids can also provide energy for muscular contraction.

Thermal changes:

A certain amount of heat is released even when the muscle is at rest. This is known as the resting heat. When the muscle is made to contract, some amount of heat is generated known as the heat of shortening. During relaxation of the muscle, the heat that is produced is known as the heat of relaxation. These can be measured by using thermocouples.

A gastrocnemius-sciatic (GS) nerve preparation is used to study the properties of skeletal muscle contraction. When a threshold stimulus is applied to the sciatic nerve, the muscle responds by contraction, which can be recorded on a moving drum. The recording is known as a simple muscle twitch. There is a short time lag between the application of the stimulus and the onset of contraction. This duration is known as the latent period.

Causes for the latent period are:

1. The time taken for the nerve action potential to reach the neuromuscular junction.

2. The time taken for the release of ACh.

3. Time taken for the production of the muscle action potential, etc.

Following this, the muscle starts contracting and the contraction reaches its maximum. This duration from the onset of contraction until the peak of contraction is known as the contraction period. After the contraction peak, the muscle fibers start relaxing.

The duration from the peak of contraction until the complete relaxation is called relaxation period. The total time required for the twitch period (from the moment of application of stimulus to the relaxation of muscle is complete), will be approximately about 100 milliseconds.

The latent period is approximately 10 milliseconds. The first half of the latent period is absolute refractory period. After the first stimulus, whatever is the intensity of the second stimulus applied during this period, it will not have any effect on the muscle.

Following the absolute refractory period, during the contraction phase, if a second stimulus is applied, a bigger contraction is obtained. This effect is known as wave summation. The effect of two stimuli is added together resulting in a bigger contraction.

If a second stimulus is applied during the relaxation period of the first response, a second contraction is obtained; the second stimulus will not allow the muscle to relax completely before another contraction starts. The response is termed as superposition.

If a second stimulus of the same strength is applied after the complete relaxation period for the first response, the curve obtained is bigger than the first response. This is known as the beneficial effect.

The beneficial effect is due to:

i. Increase in the ionic calcium available at the actin and myosin level.

ii. Slight decrease in the viscosity of the muscle proteins.

iii. Slight increase in the temperature due to the previous contraction.

iv. Slight fall in pH in muscle.

Instead of a second stimulus after the relaxation has started, if a number of stimuli are applied one following the other at very short intervals during the contractile phase, the responses for the different stimuli get added up.

This results in a sustained contraction called as tetanus (tetanic type of a response). This type of a response can be produced in the skeletal muscle fibers. Since the cardiac muscle fiber has a long absolute refractory period, it cannot be tetanized.

Starling’s law:

It is applicable to all the types of muscle fibers. The law states that force of contraction in the muscle is directly proportionate to the initial length of the muscle fiber within physiological limits. Greater the initial length, greater will be the force of contraction. This can be demonstrated by performing experiments on a gastrocnemius-sciatic preparation.

Muscle contractions are recorded when the muscle is preloaded or when it is after-loaded state. It is observed that the height of the contraction obtained is much larger when the muscle is preloaded than when it is after-loaded.

Preloading a muscle will increase its initial length unlike when it is after-loaded wherein the load starts acting on the muscle only after muscle starts contracting. This will not alter the initial length of the muscle fiber.

Muscle fatigue:

When a muscle is repeatedly stimulated, the amplitude of the response gradually gets decreased. The work done by the muscle gradually decreases and a stage is reached when the muscle fails to respond. The relaxation becomes incomplete. When this happens, it shows that the muscle has undergone fatigue.

Which is the seat of fatigue?

In an isolated GS preparation, the seat of fatigue is the neuromuscular junction. It is due to exhaustion of acetylcholine. This can be proved by directly stimulating the muscle after the GS preparation has failed to produce a response when stimulated through the nerve. When the muscle is stimulated directly, the muscle responds again.

The motor nerve is not the seat of fatigue. Recording action potentials from the nerve of a GS preparation which has demonstrated fatigue can prove this. The muscle might have undergone complete fatigue but action potentials can still be recorded from the nerve fiber. This shows that the nerve is not the seat of fatigue.

In the case of human beings, the seat of fatigue is muscle itself. This can be proved by finger ergography. Blood pressure cuff is tied over the upper arm and the sling of the ergometer is hooked to the index finger. The person is asked to repeatedly lift the weight attached to the instrument till the muscles get tired.

This performance can be recorded and the duration for which the exercise was carried out can be noted. During the exercise, the blood flow to the exercising muscle gets increased and this washes away the metabolic products that are produced. Next the whole procedure is repeated by inflating the blood pressure cuff so that the venous return is prevented.

Inflation of cuff prevents the metabolic waste getting washed away. Hence they get accumulated at the muscle itself. The duration at the end of which fatigue sets in is noted and compared. The fatigue sets in early when the cuff is in inflated state suggests that the seat of fatigue is the muscle itself and it is due to accumulation of the metabolic waste.

Muscular contraction is associated with production of lactic acid. As more lactic acid accumulates at the actin and myosin site, it prevents the sliding mechanism. The site of fatigue in the CNS is the synapse.

Fatigue can be postponed in the human body. During exercise, adrenaline is secreted. This in turn increases the blood glucose and free fatty acid levels in the circulation. These supply the necessary fuel for muscular contraction. It also increases the blood flow to the muscle tissue by bringing about vasodilatation. Thus adrenaline postpones fatigue and this action of adrenaline is called Orbelli’s effect.

b. Smooth Muscle:

i. This is another type of muscle present in the body.

ii. It is non-striated that is there are no definite cross- striations in the muscle fibers.

iii. Thick and thin filaments are present with no regular arrangement of the filaments.

iv. It is supplied by nerve fiber belonging to autonomic nervous system.

v. Hence the function of these muscles is not under voluntary control.

Types of Smooth Muscle (Table 2.6):

There are two types of smooth muscle namely visceral/single unit/unitary smooth muscle and multi-unit smooth muscle.

Visceral Smooth Muscle:

i. In this type of muscle, the propagation of action potential is from cell to cell. That is the whole of the muscle acts as a single unit (structural syncitium).

ii. It shows spontaneous development of action potential.

iii. Present in the walls of GI tract, uterus, urinary bladder, etc.

Multi-unit Smooth Muscle:

i. Each fiber is almost similar to the skeletal muscle fiber but there are no definite cross-striations.

ii. There is no cell to cell propagation of impulse just like what is seen in skeletal muscle fiber.

iii. There is no spontaneous activity in the muscle fiber.

iv. Present in iris, and other examples are ciliaris muscle, erector pilorum muscle, etc.

The activity of the visceral smooth muscle is influenced by:

i. Impulses coming along the autonomic nervous system.

ii. Stretch of the smooth muscle.

iii. Hormones adrenaline, thyroxine, etc. acting on it.

iv. Local factors like hypoxia, hypercapnia, acidosis, and other inorganic ions, like potassium, sodium, etc.

v. Apart from, ICF calcium, even ECF calcium has role to play in the process of contractions.