Unit 12: Human Skeletal and Muscular Systems — Short Questions

Ans. Types of Bone Cells

There are three types of cells i.e., osteoblasts, osteocytes, and osteoclasts involved in the development, growth and remodelling of bones.

1. Osteoblasts are bone forming cells that synthesize and secrete unmineralized ground substance. Once the osteoblasts are surrounded by matrix, they become the osteocytes.

2. Osteocytes are mature bone cells which maintain healthy bone tissue by secreting enzymes and bone mineral content. They also regulate the calcium release from bone tissue to blood.

3. Osteoclasts develop from macrophages and are involved in bone resorption, i.e., they break down bone and release calcium and phosphate in blood.

Ans. Eight cranial bones form cranium also called brain box.

i. Paired bones: Two paired bones, parietal and temporal.

ii. Unpaired bones: Four unpaired bones, Frontal, occipital, ethmoid and sphenoid.

Ans. Vertebral column It consists of 33 vertebrae. The vertebrae make five groups:

i. Seven cervical vertebrae: These are the vertebrae of the neck. The first one is called atlas and the second one is called axis.

ii. Twelve thoracic vertebrae: These are rib-carrying vertebrae and are found in chest region.

iii. Five lumbar vertebrae: These are present in abdominal region.

iv. Five sacral vertebrae: These are five fused vertebrae forming the sacrum. The sacrum articulates with the iliac bones of the hip to form the back of the pelvis.

v. Four coccygeal vertebrae or coccyx: These vertebrae are fused in the adults. Sacral and coccygeal vertebrae are together called pelvic vertebrae.

Ans. The rib cage consists of 24 bones (12 pairs) called ribs and a sternum. The sternum (chest bone) is a long flat bone located in the central part of the chest. The ribs articulate posteriorly with the thoracic vertebrae.

True ribs On anterior side, first 7 pairs of ribs attach directly with the sternum by means of separate costal cartilages. These are called true ribs.

False ribs The 8th, 9th and 10th pairs attach to the sternum by means of a common costal cartilage and are called false ribs.

Floating ribs The last 2 pairs of ribs (11th and 12th) are known as floating ribs, because they do not attach to the sternum.

Ans. Pectoral girdle It consists of 2 pairs i.e., a pair of clavicles (collar bones) and a pair of scapulae (shoulder bones). One end of each clavicle articulates with the sternum. The other end articulates with the scapula.

Pelvic girdle It is made up of two hip bones. Each hip bone contains 3 fused bones i.e., ilium, ischium and pubis. In each hip bone, there is a bony socket, called acetabulum that is formed by the fusion of three bones. The two hip bones are joined at the front by the pubic symphysis (a cartilaginous joint that connects the pubic bones at the midline of the body).

Ans.

Fore Limb

• One humerus
• One ulna and one radius
• Eight carpels
• Five meta carpels
• Fourteen phalanges

Hindlimb

• One femur
• One tibia and one fibula, one patella
• Seven tarsals
• Five metatarsals
• Fourteen phalanges

Ans. Fibrous Joints In fibrous joints, the bones are directly connected to each other by fibrous connective tissue consisting mainly of collagen. These joints permit no movement of bones. Examples of fibrous joints include:

• Sutures that occur only between the immovable bones of the skull.
• Joints between the tibia and fibula bones in the lower leg.
• Joints between teeth and their sockets in the jawbone (mandible).

Ans.

i. Hematoma formation

ii. Soft callus formation

iii. Bony callus formation

iv. Remodelling

Ans. Osteoarthritis is the most common chronic arthritic. It occurs when the articular cartilage at the ends of bones in joints gradually softens and disintegrates. It affects knee, hip and intervertebral joints.

Ans. Each skeletal muscle cell i.e., muscle fibre is a cylindrical multinucleated cell, enclosed by a plasma membrane called sarcolemma. Its cytoplasm is called sarcoplasm and it contains sarcoplas mic reticulum (SR). The sarcolemma penetrates deep into the cell to form hollow elongated tubes, the transverse tubules (T-tubules). The T-tubules reach the ends of SR.

Ans.

I-Band The thin filaments of myofibril in a muscle cell create light bands called I-band (isotropic: they can't polarize light). These thin filaments are attached to protein discs called Z-lines (Zwishen: in between) They are composed of actin.

A-Band The thick filaments of myofibril in muscle fibre create dark bands called A-Band (Anisotropic: they can polarize light). They are composed of myosin.

H-zone The region in A-Band which appear lighter (where no overlap between thick and thin filaments occur) is called H-zone (hele = bright).

Ans. For the movement of bones at a joint in two directions muscles work in pairs. They produce opposing actions when they contract. Such arrangement of muscles is called antagonistic arrangement. In such arrangement, when one muscle, called flexor, contracts it bends the bone at joint. When the opposing muscle, called extensor, contracts it straightens the bone at joints.

During antagonistic action, when a muscle e.g., flexors contracts, the other muscle i.e., extensor is relaxed and vice versa.

Ans.

• Ligaments can stretch slightly under stress and retain to their original shape due to higher amount of elastin.

• Tendons are stiffer and resist stretching to efficiently transfer muscle forces due to higher collagen and relatively low elastin.

Ans.

Figure Sliding filament model of muscle contraction

Ans. Compact and spongy bone: Compact bone is the outer thick layer, made of hard material, provides attachment and surface for muscles. Spongy bone is the inner thin layer made up of latticework structure of bony spikes, light and strong, contains bone marrow for blood cells production.

Axial skeleton and appendicular skeleton: Axial skeleton forms the axis of the body, supports and protects organs of head, neck and chest (skull, ribs, spine, sternum). Appendicular skeleton includes bones in appendages - arms and legs (pectoral girdle, pelvic girdle, forelimbs, hind limbs).

True ribs, false ribs and floating ribs: True ribs are first seven pairs articulating directly with sternum via separate costal cartilages. False ribs are 8th, 9th and 10th pairs attached to sternum via common costal cartilage. Floating ribs are last 2 pairs (11th, 12th) not attached to sternum.

Rheumatoid arthritis and osteoarthritis Rheumatoid arthritis results from autoimmune disorder where synovial membrane becomes inflamed. Osteoarthritis is when articular cartilage at ends of bones gradually softens and disintegrates.

Fibrous and cartilaginous joints Fibrous joints have bones directly connected by fibrous connective tissue (mainly collagen), permit no movement, examples are sutures. Cartilaginous joints have bones connected by layer of cartilage, allow little movement, include primary (synchondrosis) and secondary (symphysis) types.

Cartilaginous and synovial joint Cartilaginous joints have bones connected by cartilage layer allowing little movement. Synovial joints are most common, allow wide range of movement, have articular cartilage covering bone ends, fibrous capsule surrounding joint with synovial membrane secreting synovial fluid for lubrication.

Osteoblasts and osteocytes Osteoblasts are bone forming cells synthesizing and secreting unmineralized ground substance; once surrounded by matrix become osteocytes. Osteocytes are mature bone cells maintaining healthy bone tissue by secreting enzymes and bone mineral content, regulating calcium release from bone to blood.

Tropomyosin and troponin Tropomyosin strands wrap around actin core, stiffen it, block myosin binding sites on actin in relaxed muscle. Troponin is component of thin myofilaments with three polypeptides: TnI (inhibitory), TnT (binds to tropomyosin), TnC (binds calcium ions).

Ligament and tendon Ligaments attach bone to bone, slightly elastic, example lateral and medial ligaments of foot. Tendons attach muscles to bones, non-elastic/inelastic, example biceps tendon.

Tetany and tetanus Tetany is involuntary muscle contractions due to increased muscle tone and nerve hyperexcitability, commonly from hypocalcaemia, affects various body parts, less serious. Tetanus is severe bacterial infection from Clostridium tetani producing toxin causing muscle stiffness and spasms, mainly affects jaw and neck, more serious and life-threatening.

Ans. Haversian canal the concentric layers of lamellae surround a central canal called the Haversian canal.

Components It contains blood vessels, nerves and lymphatic vessels.

In addition to these structures, there are small channels that run perpendicular to the Haversian canals and connect them with each other and with the periosteum. Collagen fibres anchor the periosteum to the underlying bone tissue, providing additional strength and stability to the bone.

Ans.

Figure Structure of bone

Ans. Many bones contain a soft tissue called bone marrow, which can be either red or yellow.

• Red bone marrow is found in spongy bone, the ends of long bones, ribs, vertebrae, the sternum, and the pelvis. It produces red blood cells, platelets, and white blood cells.

• Yellow bone marrow fills the shafts of long bones. It consists of fat cells and serves as an energy reserve. It can also be converted to red bone marrow and produce blood cells when severe blood loss occurs.

Ans. Definition

• The process of bone formation is also called osteogenesis.

Formation

• It begins during embryonic development and continues throughout life, playing a vital role in growth, maintenance, and repair of bones.

• There are two primary pathways of osteogenesis.

• The formation of long bones e.g., femur and humerus, involves the transition of cartilage into bone. In this process, the center of cartilage begins to harden (calcify), and the chondrocytes (cartilage cells) in this area die, leaving behind cavities. Blood vessels penetrate these cavities and introduce osteoblasts and osteoclasts.

Ans.

• A few bones, e.g., some bones of the skull, develop directly into hard bone without forming cartilage first.

• In these cases, the osteocytes are initially scattered randomly throughout the embryonic connective tissue but soon fuse into layers and become flat plates of bones.

Ans. The step in the formation of bone by which cartilage is replaced by bone by the deposition of minerals is called ossification.

Ans.

• Periosteum: The bone's surface is covered by a tough layer of connective tissue called periosteum.

• Parts of bone: The broad ends of a bone are called epiphysis while the middle part along the length of bone is called diaphysis or shaft.

Figure Structure of bone

Ans.

• A layer of connective tissue called perichondrium surrounds the cartilage.

• It contains blood vessels, lymphatic vessels, and nerves that supply the cartilage tissue.

• Inside perichondrium is the cartilage matrix which is composed of collagen elastin, proteoglycans, and other fibres. It gives the tissue its strength, flexibility, and resistance to compression.

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• The cartilage cells, called chondrocytes, are present within small spaces called lacunae, which are embedded in cartilage matrix. Chondrocytes are responsible for synthesizing and maintaining the matrix of cartilage.

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Comparison between Bone and Cartilage

Feature | Bone | Cartilage
External covering: Periosteum | Perichondrium
Cell types: Osteoblast, osteocytes and osteoclasts | Chondrocytes
Extracellular matrix: Contains calcium crystals and collagen fibres | Contains collagen and other fibres
Blood vessels: Present | Absent
Growth & repair: Have the ability to grow and repair themselves throughout life | Has limited ability to repair itself, as it has no direct blood supply

Ans.

• Fourteen facial bones are attached to the cranium.

• The 6 paired bones are lacrimal, zygomatic, nasal bones, inferior nasal concha, maxilla and palatine.

• The 2 unpaired bones are mandible (lower jaw bone) and vomer.

Ans.

• Middle ear: There are 6 bones (3 pairs) in middle ears. These are called ossicles and include malleus, incus and stapes.

• Neck bone: Hyoid bone is a small single bone which lies at the base of skull below the tongue. It does not articulate with any other bone of head.

Ans. A fibrous capsule surrounds the synovial joint and helps to hold the bones together. The fibrous capsule is composed of an outer layer of ligaments and an inner lining of synovial membrane, which secretes synovial fluid, that lubricates the joint.

Ans. There are six main types of synovial joints based on the range of motion.

• Ball-and-socket joints allow movement in all directions e.g., shoulder and hip joints.

• Hinge joints allow movement in only one plane, like a door hinge e.g., elbow and knee joints.

• Pivot joints allow rotational movement around a single axis e.g., joint between the first and second vertebrae of the neck.

• Ellipsoidal joints allow movement in two planes, but not rotation e.g., joint of wrist with radius.

• Saddle joints allow movement in two planes because one bone has a concave surface and the other has a convex surface e.g., thumb joint.

• Gliding joints allow gliding movements between bones e.g., joints between the vertebrae and the joints between the bones in wrist and ankle.

Ans. Joint Transplantation

It is a surgical procedure in which a damaged joint is replaced with a healthy natural joint (from a donor) or an artificial joint. The most common types of joint transplantation are:

Total joint replacement In this procedure, the entire damaged joint is replaced with an artificial joint made of metal, plastic, or ceramic.

Partial joint replacement In this procedure, only the damaged part of the joint is replaced with an artificial component. This is often used in the knee joint.

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Allograft

• It is a type of joint transplantation in which a healthy joint from a donor is transplanted to replace the damaged joint.
• This technique is often used in the ankle and knee joints.

Chondrocyte Implantation

• It is a surgical procedure, in which chondrocytes from patient's own joint are implanted into the damaged joint.
• This technique is often used in the knee joint.

Ans. Problems due to Improper Posture: Improper posture can negatively affect bones and joints, causing:

Vertebral misalignment This can lead to back and neck pain, and herniated discs by putting pressure on vertebrae and nerves.

Joint strain Poor posture can strain neck, shoulders, hips, and knees, leading to pain, inflammation, and potentially arthritis.

Muscle imbalances Overused and underused muscles from poor posture can pull bones and joints out of alignment

Ans. The intervertebral discs between vertebrae act as shock absorbers and help in movement.

Causes

• A herniated or slipped disc occurs when the outer layer of the intervertebral disc tears or ruptures, causing the inner gel-like substance to leak out and press against nearby nerves or spinal cord. It may be due to a trauma, degenerative changes due to aging, or repetitive strain on vertebral column.

Symptoms Symptoms of slipped disc include pain, numbness, and tingling in the affected area, weakness or loss of muscle function, and in severe cases, bowel or bladder dysfunction.

Ans.

• Spondylosis means degeneration of vertebrae, intervertebral discs, ligaments or cartilage of vertebral column.

Causes The most common cause is the natural degeneration of intervertebral discs. It occurs with aging, genetic factors, trauma, and prolonged periods of poor posture and obesity.

Symptoms Symptoms include back or neck pain, stiffness, and reduced range of motion. Spondylosis is most common in the lower back (lumbar vertebrae) and neck (cervical vertebrae).

Ans. The sciatic nerve is a spinal nerve which starts from the lower lumber region of spinal cord in the lower back and descends through the buttocks innervating the muscles of each leg.

Causes

• Herniated disc or bulging disc, can put pressure on the sciatic nerve, trauma, infection, inflammation, and spondylosis resulting into a disease called sciatica.

Symptoms Symptoms include pain or discomfort in the lower back, buttocks, legs or feet, tingling or numbness in the legs or feet, weakness or difficulty moving the legs or feet.

Ans. Arthritis is the inflammation of joints and includes different inflammatory conditions that affect joints.

Symptoms Symptoms of include joint pain and stiffness. Other symptoms that include redness, warmth, swelling in affected joints.

Ans. Gouty arthritis (or gout) occurs when there is a build-up of uric acid in the blood, which can form crystals in the joints and cause inflammation.

• The most common joint affected is the joint of the big toe.

• Other joints (knees, wrists and fingers) may also be affected.

Ans.

• It is most probably the case of osteoporosis. Osteoporosis is a condition characterized by weakened bones that are more prone to fractures and breaks. It occurs when bone density decreases, making the bones fragile and porous. Its causes include:

• As people age, bone mass naturally decreases. But it can be more pronounced in some individuals.

• In women, a drop in oestrogen levels after menopause accelerates bone loss. Lack of calcium and vitamin D in the diet can impair bone health.

• Long-term use of corticosteroids, can contribute to bone loss.

• Smoking and alcohol consumption can also increase the risk of osteoporosis.

• Lack of weight-bearing exercise can lead to weakened bones.

Ans. Bone dislocation is a medical term, which refers to a condition in which the bones in a joint are forced out of their normal positions.

Causes

• This can happen as a result of a sudden impact or trauma. A severe dislocation can cause tearing of the muscles, ligaments and tendons.

• Rheumatoid arthritis can also cause joint dislocation.

Symptoms Symptoms include swelling, intense pain, and immobility of the affected joint.

• A dislocated joint can only be successfully corrected by a physiotherapist. Surgery may be needed to repair or tighten the stretched ligaments.

Ans. A sprain is an injury to the ligaments that connect bones in a joint. Commonly injured ligaments are in the ankle, knee and wrist.

Causes

• This can happen when the joint is forced beyond its normal range of motion, causing the ligaments to over stretch or tear.

Sprains are usually treated with physical therapy. Dressings is done to immobilize the sprain and provide support.

Ans. First aid treatment for dislocation and sprain includes the following steps.

Immobilize the affected area Keep the affected area immobile and do not attempt to re-align the dislocated joint. Use a sling or splint to support the limb.

Apply ice Apply an ice pack or cold compress to the affected area to reduce swelling and pain.

Elevate the affected limb In the case of dislocation, if possible, elevate the affected limb above to help reduce swelling.

Seek medical attention Dislocations and sprain require medical attention, so call for emergency medical services 1122 or take the person to the hospital for further evaluation and treatment.

Ans. Muscles' ability to contract and relax not only enables the body to move, but also provides the force that pushes substances, such as blood and food, through the body.

• When a mosquito flies, its wings are move rapidly through the air by quickly contacting flight muscles.

• When an earthworm burrows through the soil, its movement is driven by strong muscles pushing its body past the surrounding soil.

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Smooth Muscles

• Smooth muscle fibres are spindle-shaped, have a single nucleus and lack striations.
• Smooth muscles are present in the walls of the stomach, intestines, blood vessels, and other internal organs. Because most of their movements cannot be consciously controlled, smooth muscle is referred to as involuntary muscle.

Cardiac Muscles

• The muscle fibres of cardiac muscles are striated like skeletal muscle, but each cell usually contains one nucleus located near the centre.
• These are found only in the walls (myocardium) of the heart. Their fibres branch extensively.
• Its cytoplasm is called sarcoplasm. It has organelles enzymes and glycogen.
• Sarcoplasmic reticulum (SR), SER of skeletal muscles have stored Ca++ and no ribosomes.
• The sarcolemma penetrates deep into the cell to form hollow elongated tubes, the transverse tubules (T-tubules). The T-tubules reach the ends of SR.

Ans. Sarcolemma

• Each skeletal muscle cell i.e., muscle fibre is a cylindrical multinucleated cell, enclosed by a plasma membrane called sarcolemma.

Sarcoplasm

• Its cytoplasm is called sarcoplasm and it contains sarcoplas mic reticulum (SR). The sarcolemma penetrates deep into the cell to form hollow elongated tubes, the transverse tubules (T-tubules). The T-tubules reach the ends of SR.

Ans. When a nerve impulse reaches sarcolemma, a neurotransmitter (acetylcholine) is released by motor neuron at the synapse. It stimulates the sarcolemma to produce its own electrochemical impulses which are carried into the muscle fibre to the T tubules.

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Figure Cardiac muscles

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Figure Structure of myofilament

Ans. Cross bridge When binding sites on actin are exposed, the myosin heads bind to them and form cross-bridges.

Power stroke Once the cross-bridges are formed, the myosin heads release the ADP and Pi, and undergo conformational change. They bend towards the centre of sarcomere, pulling actin filaments with them. This pulling action is called a power stroke.

Ans. A skeletal muscle is made of groups of cells called muscle fibres. Each muscle fibre contains bundles of myofibrils in its cytoplasm. Each myofibril is made of 2 types of myofilaments (myosin and actin).

Ans. The sliding filament model explains how a muscle contracts. According to this model, a muscle contracts when its thin myofilaments slide past the thick ones so that they overlap to a greater degree. It occurs in the following steps.

• Sarcomere at relaxed state
• Arrival of nerve impulse
• Cross-bridges and power stroke
• Separation of myosin heads from actin

Ans. It is a state of stiffness or rigidity of muscles after death due to ATP deficit, the cells can no longer produce ATP and therefore the cross-bridges cannot be broken. It causes the muscle stiffness of death, or rigor mortis. A living cell, however, always has enough ATP to allow the myosin head to detach from actin.

Ans. Typically, a muscle has two attachment points on different bones.

Origin The end attached to the stationary bone during contraction is called the origin.

Quadriceps The quadriceps originate at the ilium (part of the pelvic girdle and femur).

Insertion The end attached to the bone that moves is the insertion. The middle part of the muscle is known as the belly.

Example Insertions at the patella (knee cap) and tibia.

Ans. Flexor The muscles which bends the bone at joint upon contraction is called flexor e.g., biceps brachii' hamstrings.

Extensor The muscle which straightens the bone at joint upon contraction is called extensor e.g., triceps brachii, quadriceps.

During such antagonistic action, when a muscle e.g., flexors contracts, the other muscle i.e., extensor is relaxed and vice versa.

Ans. Movement at Knee Joint

The knee joint is located between the femur (thigh bone) and the tibia and fibula (lower leg bones).

Flexion or bending, of the lower leg is done by the hamstrings. It is a group of three muscles at the back of the thigh.

Extension or straightening, of the lower leg is done by the quadriceps. It is a group of four muscles at the form of the thigh.

• When the hamstrings contract, the lower leg bends and the quadriceps relax.
• When the quadriceps contract, the lower leg straightens and the hamstrings relax.

Ans. Muscle cramps are sudden, involuntary, and often painful contractions of a muscle or group of muscles. They usually last from a few seconds to several minutes and most commonly occur in the legs and feet.

• Common causes include dehydration, an imbalance of salts, overuse or injury of the muscle, certain medications (like diuretics), and medical conditions such as diabetes, liver disease, and nerve damage.

To relieve muscle cramps, gently stretch and massage the affected muscle. Applying heat or cold to the area and using pain-relieving medications can also help.

Ans. Tetany is a condition characterized by involuntary muscle contractions or spasms due to increased muscle tone and hyperexcitability of the nerves. These contractions can occur in various parts of the body such as hands, feet, face, or larynx.

Causes

• The most common cause of tetany is hypocalcaemia (low level of calcium in blood) which may be due to vitamin D deficiency, renal failure, or thyroid disorders.

• Tetany may also be due to other salts imbalances, such as a low level of magnesium in blood.

Treatment for tetany depends on the underlying cause. If tetany is caused by salts imbalances, treatment may involve calcium or magnesium supplements or intravenous fluids to restore electrolyte balance.

Ans. The skeleton works like a system of rods and levers. Bones act as the rods, giving structure and support to the body and protecting internal organs.

In this system, joints serve as fulcrums (pivot points) for the levers, allowing movement. Muscles generate the effort or force, while the weight or resistance being moved is the load.

For example, when lifting a weight, the bicep muscle in the upper arm acts as a lever. The elbow joint is the fulcrum, the bicep provides the effort, and the weight is the load.

Ans. Muscles can only pull, not push. This is because muscle fibres are designed to contract and shorten, pulling on tendons and thus moving bones. When a muscle contracts, it pulls on the bone via the tendon, and when it relaxes, the bone moves back to its original position.

Muscles cannot push because they only generate force by pulling. If a muscle were to push, it would need to be attached to bones at both ends and make both ends move closer together, which is not possible in the body. Muscles are usually attached to bone at only one end.

Ans. See Q1 of theory.

Ans. See Q3 of theory.

Ans. See Q11 of theory.

Ans. See Q12 of theory.

Ans. See Q15 of theory.

Ans. See Q16 of theory.

Ans. See Q18 of theory.

Ans. See Q19 of theory.

Ans. See Q16 of theory.

Ans. See Q20 of theory.

Ans. See Q21 of theory.

Ans. See Q22 of theory.

Ans. Role of Ca++ in bones

Calcium is the major component of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), the mineralized matrix that gives bones, hardness and strength, without adequate calcium, bones become weak and porous, leading to increased risk of fractures.

Calcium is also critical in muscle contraction, by binding to TnC of Troponin of thin filaments to initiate muscle contraction by excitation contraction coupling process. Low levels of calcium in blood (hypocalcaemia) leads to tetany (a muscular disorder).

Ans. The rigid bones provide the body with strength, support and protection while the joints are compulsory in the sense that primarily they are involved in mobility flexibility, shock absorbance, reducing energy cost while moving. If our skeleton were a single solid structure, we couldn't have movements alongside higher risks for fractures.

Ans. Muscles can't push, they can only pull producing contraction, therefore they must have to work in opposing pairs to enable controlled movements like extension and flexion. If an arm needs to bend at elbow joint, biceps will contract, to straighten the arm there must be a muscle (in this case triceps brachii) to pull in opposite direction. So that's why most of our muscles work in pair with the exception of orbicularis on present around the mouth, which contracts alone rather than in pairs.

Ans.

• Actually our muscles require mechanical load or stress to maintain its protein synthesis if there is lack of regular stress and reduced nervous stimulation, muscles start losing their mass due to prolonged inactivity also in aspace there is almost zero gravity, our muscles do not work in the absence of gravity which leads to atrophy.

• Likewise bones also remodel and grow in response to stress so during longer periods of inactivity leads to loss of Ca++ by kidneys which inturn weakens the bones and in space travel under the effect of microgravity, there will be no stress on bones, body starts its demineralization leading to weak bones.