Preview:

• This chapter mainly revolves around the structures involved in locomotion. Any movement that results in change of place of the body is called, locomotion.
• Based on the structure formed by different cells in the human body for locomotion, it is of three types: Amoeboid, Ciliary and Muscular movements.
• Muscles are specialised tissue of mesodermal origin which exhibit several properties to facilitate movement.
• Contractile Proteins are proteins that generate force for muscular contraction. Actin and Myosin are examples of contractile proteins.
• The Sliding Filament theory explains the mechanism by which muscles contract.
• The skeletal system comprises of the axial and the appendicular skeleton; each of which is made of a combination of bones.
• Joints are points of contact between bones, or between bones and cartilages. There are three types: Fibrous, Cartilaginous and Synovial.
• Some of the disorders of Muscular and Skeletal System are: Muscular dystrophy, Myasthenia gravis, Tetany, Arthritis, Osteoporosis and Gout.

20.0 Introduction:

• Animals and plants exhibit a wide range of movements. Unicellular organisms like Amoeba move by streaming of protoplasm. Other organisms move by their movement of cilia, flagella or tentacles.
• Human beings can move limbs, jaws, eyelids and tongue. As we know, some of these movements result in a change of place or location. Such voluntary movements are called locomotion. Walking, running, climbing, flying, swimming are all some forms of locomotory movements too.
• Locomotory structures need not be different for different kinds of movement. For example, in Paramoecium, cilia helps in the movement of food through cytopharynx and in locomotion as well; Hydra can use its tentacles for capturing its prey and also use them for locomotion. Similarly, we use limbs for changes in body postures and locomotion as well.
• The above observations suggest that movements and locomotion cannot be studied separately. The two may be linked by stating that all locomotions are movements but all movements are not locomotions!
• Methods of locomotion performed by animals vary with their habitats and the demand of the situation. However, locomotion is generally for search of food, shelter, mate, suitable breeding grounds, and favourable climatic conditions or to escape from enemies and predators.

20.1 Types of Movement

Cells of the human body exhibit three main types of movements: Amoeboid, ciliary and muscular.

Amoeboid movement: It is a type of crawling-like movement accomplished by protrusion of cytoplasm of the cell called, pseudopodia. Some specialised cells in our body like macrophages and leucocytes in blood and, cytoskeletal elements like microfilaments are also involved in amoeboid movement. The pseudopodia move by streaming of protoplasm (as in Amoeba).

Ciliary movement: Ciliary movement occurs in most of our internal tubular organs which are lined by ciliated epithelium. For example, the coordinated movements of cilia in the trachea help us in removing dust particles and some of the foreign substances inhaled along with the atmospheric air. Passage of ova through the female reproductive tract is also facilitated by the ciliary movement.

Muscular Movement: The contractile property of muscles facilitated by contraction of the myofibrils in muscle cells are effectively used for locomotion and other movements by human beings and majority of multicellular organisms. Movement of our limbs, jaws and tongue require muscular movement. This type of locomotion requires a perfect coordinated activity of muscular, skeletal and neural systems.

Questions from section 20.1:

1. Describe the three types of movements exhibited by cells in humans. 

2. State the properties of muscles.

3. What are the criteria used for classifying muscles?

4. Explain the three types of muscles based on their location.

20.2 Muscle

Muscle is a specialised tissue of mesodermal origin. About 40-50 per cent of the body weight of a human adult is contributed by muscles.

Properties of mucles:

a) excitability,

b) contractility,

c) extensibility and,

d) elasticity.

Different criteria used for classifying muscles:

a) location,

b) appearance and

c) nature of regulation of their activities.

Concept box: Syncitium is a single cell or a cytoplasmic mass containing several nuclei, formed by fusion of cells or by division of nuclei.

Based on their location in the body, three types of muscles are identified:

(i) Skeletal: Skeletal muscles are closely associated with the skeletal components of the body. They have a striped appearance under the microscope and hence are called striated muscles. As their activities are under the voluntary control of the nervous system, they are voluntary muscles. They are primarily involved in locomotory actions and changes of body postures.

• Let us examine a skeletal muscle in detail to understand the structure and mechanism of contraction.
• Each organised skeletal muscle in our body is made of a number of muscle bundles or fascicles held together by a common collagenous connective tissue layer called fascia.
• Each muscle bundle contains a number of muscle fibres as shown in Figure 20.1 below:

• Each muscle fibre is lined by the plasma membrane called sarcolemma enclosing the sarcoplasm.
• Muscle fibre is a syncitium as the sarcoplasm contains many nuclei.
• The endoplasmic reticulum here sarcoplasmic reticulum of the muscle fibres is the store house of calcium ions.
• A characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils. Each myofibril has alternate dark and light bands on it. A detailed study of the myofibril has established that the striated appearance is due to the distribution pattern of two important proteins Actin and Myosin. The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called A or Anisotropic band contains myosin. Both these proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils. Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively.
• In the centre of each I band is an elastic fibre called Z line which bisects it. The thin filaments are firmly attached to the Z line. The thick filaments in the A band are also held together in the middle of this band by a thin fibrous membrane called M line. The A and I bands are arranged alternately throughout the length of the myofibrils.
• The portion of the myofibril between two successive Z lines is considered as the functional unit of contraction and is called a sarcomere (Figure 20.2).

• In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part of thick filament, not overlapped by thin filaments is called the H zone.

(ii) Visceral: Visceral muscles are located in the inner walls of hollow visceral organs of the body like the alimentary canal and reproductive tract. They do not exhibit any striation and are smooth in appearance. Hence, they are called smooth muscles (non-striated muscle). Their activities are not under the voluntary control of the nervous system and are therefore involuntary muscles. They assist, in the transportation of food through the digestive tract and gametes through the genital tract.

(iii) Cardiac: As the name suggests, Cardiac muscles are the muscles of heart. Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle. Based on appearance, cardiac muscles are striated and involuntary in nature as the nervous system does not control their activities directly.

20.2.1 Structure of Contractile Proteins

Concept box: Contractile Proteins are proteins that generate force for muscular contraction. Examples include: Actin and Myosin which we just studied about under skeletal muscle.

Structure of Actin:

• Each actin (thin) filament is made of two F (filamentous) actins helically wound to each other. Each F actin is a polymer of monomeric G (Globular) actins.
• Two filaments of another protein, tropomyosin also runs close to the F actins throughout its length. A complex protein, Troponin is distributed at regular intervals on the tropomyosin. In the resting state, a subunit of troponin masks the active binding sites for myosin on the actin filaments as shown in Figure 20.3a.

Structure of Myosin:

• Each myosin (thick) filament is also a polymerised protein.
• Many monomeric proteins called Meromyosins (Figure 20.3b) together form one thick filament. Each meromyosin has two important parts, a globular head called, Heavy meromyosin (HMM) with a short arm and a tail called, the Light meromyosin (LMM).
• The head and short arm projects outwards at regular distance and at an angle from each other from the surface of a polymerised myosin filament and is known as cross arm.
• The globular head is an active ATPase enzyme which has binding sites for ATP and active sites for actin.

20.2.2 Mechanism of Muscle Contraction

Sliding Filament Theory:

• Mechanism of muscle contraction is best explained by the sliding filament theory which states that, contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments .
• Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron. A motor neuron along with the muscle fibres connected to it constitutes for a motor unit.
• The junction between a motor neuron and the sarcolemma of the muscle fibre is called the neuromuscular junction or motor-end plate. A neural signal reaching this junction releases a neurotransmitter Acetyl choline which generates an action potential in the sarcolemma.
• The action potential spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm. Increase in $Ca^{++}$ level leads to the binding of calcium with a subunit of troponin on actin filaments. This causes removal of masking on active sites for myosin to bind. Utilising the energy from ATP hydrolysis, the myosin head now bind to the exposed active sites on actin to form a cross bridge as shown in Figure 20.4 below. This pulls the attached actin filaments towards the centre of A band.

• The Z line attached to the actin are also pulled inwards thereby causing a shortening of the sarcomere, which is nothing but, contraction.
• It is clear from the above steps that, during shortening of the muscle or contraction, the I bands get reduced, whereas the A bands retain their length (as shown in Figure 20.5 below). The myosin, releasing the ADP and $P_1$ goes back to its relaxed state. A new ATP binds and the cross-bridge is broken. The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding. The process continues till the $Ca^{++}$ ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments. This causes the return of Z lines back to their original position, that is, relaxation.

• The reaction time of the fibres can vary in different muscles.
• Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue.

Red fibres:

Muscles contain a red coloured oxygen storing pigment called, myoglobin. Myoglobin content is high in some of the muscles which gives it a reddish appearance and are hence called, Red fibres. These muscles also contain plenty of mitochondria which to utilise the large amount of oxygen stored in them for ATP production. These muscles are therefore also called, aerobic muscles.

White fibres:

Some of the muscles possess very less quantity of myoglobin and therefore, appear pale or whitish. These are hence called, White fibres. Number of mitochondria in them is few as they depend on anaerobic process for energy; but the amount of sarcoplasmic reticulum is high.

Questions from section 20.2:

1.Describe the structures of actin and myosin.

2. Explain the sliding filament theory with relevant diagrams.

3. Write a note on:

a) Red Fibres

b) White Fibres

20.3 Skeletal System

Characteristics:

• The skeletal system is comprised of a framework of bones and a few cartilages.
• This system has a significant role in causing movement in the body. For example, chewing food is facilitated by jaw bones and the act of walking around by the limb bones.
• The Bone and Cartilage are specialised connective tissues. Bones have a very hard matrix due to calcium salts in them and the cartilage has slightly pliable matrix due to chondroitin salts.
• In human beings, the skeletal system is made up of 206 bones and a few cartilages.
• The skeletal system is grouped into two principal divisions the axial and the appendicular skeleton.

Axial Skeleton: The Axial skeleton comprises of 80 bones distributed along the main axis of the body. The skull, vertebral column, sternum and ribs together constitute the axial skeleton.

a) The skull

• The skull (Figure 20.6) is composed of two sets of bones cranial and facial, that totals to 22 bones. Cranial bones are 8 in number and form the hard protective outer covering called, cranium for the brain. The facial region is made up of 14 skeletal elements which form the front part of the skull.

• A single U-shaped bone called, hyoid is present at the base of the buccal cavity. The hyoid is also a part of the skull.
• The skull also houses a pair of middle ears which contains three tiny bones Malleus, Incus and Stapes, collectively called, Ear Ossicles.
• There are two rounded knobs at the base of the skull which continue as the first vertebra and are called, occipital condyles or dicondylic skull.
• Check above two points for accuracy

b) The Vertebral Column

• The vertebral column (refer Figure 20.7 above) is formed by 26 serially arranged units called, vertebrae and is dorsally placed. Each vertebra has a central hollow portion called, neural canal through which the spinal cord passes. The first vertebra acts like the atlas which articulates with the occipital condyles.
• The vertebral column extends from the base of the skull and constitutes the main framework of the trunk.
• The vertebral column is differentiated into cervical (7), thoracic (12), lumbar (5), sacral (1-fused) and coccygeal (1-fused) regions starting from the skull.
• The vertebral column protects the spinal cord, supports the head and serves as the point of attachment for the ribs and musculature of the back.

c) The sternum

• Sternum is a flat bone on the ventral midline of thorax. It is connected to the ribs, via the cartilage. The sternum has two surfaces meant for communication on its dorsal end and is hence said to be, bicephalic (two heads).
• There are 12 pairs of ribs. Each rib is a thin flat bone connected dorsally to the vertebral column and ventrally to the sternum.
• First seven pairs of ribs are called true ribs. Dorsally, they are attached to the thoracic vertebrae and ventrally connected to the sternum with the help of hyaline cartilage.
• The $8^{th}, 9^{th}$ and $10^{th}$ pairs of ribs do not articulate directly with the sternum but join the seventh rib with the help of hyaline cartilage. These are called vertebrochondral (false) ribs.
• Last 2 pairs ($11^{th}$ and $12^{th}$) of ribs are not connected ventrally and are therefore, called floating ribs.
• Thoracic vertebrae, ribs and sternum together form the rib cage (refer Figure 20.8 below).

The appendicular skeleton:

• The bones of the limbs along with their girdles constitute the appendicular skeleton.
• Each limb is made of 30 bones the bones of the hand (fore limb) are humerus, radius and ulna, carpals (wrist bones 8 in number), metacarpals (palm bones 5 in number) and phalanges (digits 14 in number).

While, Femur (thigh bone the longest bone), tibia and fibula, tarsals (ankle bones 7 in number), metatarsals (5 in number) and phalanges (digits 14 in number) are the bones of the legs (hind limb). A cup shaped bone called patella cover the knee ventrally (knee cap).

• Pectoral bones help in movement of the upper limbs and Pelvic girdle bones help in the movement of lower limbs with the axial skeleton. Each girdle is formed of two halves each half of pectoral girdle consists of a clavicle and a scapula as shown in figure 20.9.
• Scapula is a large triangular flat bone situated in the dorsal part of the thorax between the second and the seventh ribs. The dorsal, flat, triangular body of scapula has a slightly elevated ridge called the spine which projects as a flat, expanded process called the acromion. The clavicle articulates through the acromion. Each clavicle is a long slender bone with two curvatures. This bone is commonly called the collar bone.
• Below the acromion is a depression called the glenoid cavity which connects to the head of the humerus to form the shoulder joint.
• Further, the pelvic girdle consists of two coxal bones (Figure 20.10). Each coxal bone is formed by the fusion of three bones ilium, ischium and pubis. At the point of fusion of the above bones is a cavity called acetabulum to which the thigh bone connects.
• The two halves of the pelvic girdle meet ventrally to form the pubic symphysis containing fibrous cartilage.

Questions from section 20.3:

1. List the characteristics of skeletal system.

2. Explain the axial skeletal system and its constituents in detail (with figures).

3. Explain the appendicular skeletal system and its constituents in detail (with figures).

20.4 Joints

Significance:

• Joints form the points of contact between bones, or between bones and cartilages.
• Joints are essential for all types of movements including movements that bring about locomotion.
• Joints act as fulcrum that causes movement and the required force is generated by the muscles. The movability at these joints varies depending on different factors based on which joints have been classified into three major structural forms: fibrous, cartilaginous and synovial.

a) Fibrous joints: Fibrous joints do not allow any movement. This type of joint is shown by the flat skull bones which fuse end-to-end with the help of dense fibrous connective tissues in the form of sutures, to form the cranium.

b) Cartilaginous joints: Cartilaginous joints permit limited movements. The bones forming this type of joint are joined together with the help of cartilages. The joint between the adjacent vertebrae in the vertebral column is of this type.

c) Synovial joints: The synovial joints help in locomotion and many other movements, such as bending or lifting a hand. Synovial joints are characterised by the presence of a fluid filled synovial cavity between the articulating surfaces of the two bones. Such an arrangement allows considerable movement. Ball and socket joint (between humerus and pectoral girdle), hinge joint (knee joint), pivot joint (between atlas and axis), gliding joint (between the carpals) and saddle joint (between carpal and metacarpal of thumb) are some examples.

Questions from section 20.4:

1. Explain the significance of joints. Also explain its types.

20.5 Disorders of Muscular and Skeletal System

a) Myasthenia gravis: Auto immune disorder affecting neuromuscular junction leading to fatigue, weakening and paralysis of skeletal muscle.

b) Muscular dystrophy: Progressive degeneration of skeletal muscle caused due to mutation of genes that code for proteins which form muscle fibres. 

c) Tetany: Rapid spasms (wild contractions) in muscle due to low $Ca^{++}$ in body fluid. 

d) Arthritis: Inflammation of joints. 

e) Osteoporosis: Age-related disorder characterised by decreased bone mass and increased chances of fractures. Decreased level of estrogen is a common cause. 

f) Gout: Inflammation of joints due to accumulation of uric acid crystals.

Questions from section 20.5:

1. List and explain 6 disorders of the muscular and skeletal system.