MUSCLES
Muscles are responsible for all types of movements and changes in body posture. They are developed from mesoderm. Excitability, contractility and relaxation are the main properties of muscles. Muscles are of three types namely, striated (skeletal) muscles, unstriated (smooth) muscles and cardiac muscles.
I. Structure of a skeletal muscle:
Skeletal muscle is formed by a number of bundles of muscle fibres called fascicles. The fascicles of a muscle are held together by collagenous connective tissue layer called fascia.
a. Muscle fibre: Muscle fibre of a skeletal muscle is a syncitium because it is formed by the fusion of many uninucleate myocytes of embryo. It is bound by a sarcolemma (plasmalemma) and the cytoplasm is called sarcoplasm. It contains many, peripheral nuclei. Endoplasmic reticulum of muscle fibre is known as sarcoplasmic reticulum (it is the store house of Ca++ ions). Sarcoplasm contains many parallel filaments called myofibrils.
b. Myofibril: A myofibril of striated muscle fibre has alternate dark (A - band / Anisotropic band) and light (I - band/ Isotropic band) bands. A - band contains thick filaments formed by a protein called myosin. I - band contains thin filaments formed by a protein called actin besides two regulatory proteins, troponin and tropomyosin. The thin filaments extend into A - band among thick filaments and therefore, overlap in certain parts of A-band giving more darker appearance. Hence, the part of A - band which contains only thick filaments is little bit lighter and is known as H - zone or Hensen's disc. The thick filaments in A - band are held together in the middle by a thin membrane called M - line. In the centre of I - band, there is an elastic fibre called Z - line (Krause's membrane or Dobie's line). The portion of myofibril between two successive Z - lines is known as sarcomere. It is the functional unit of contraction.
c. Contractile proteins:
i. Thin filament: Each thin filament is formed by two filamentous actin (F - actin) molecules, which are wound around each other. Each F - actin formed by many globular actin (G - actin) molecules. Running close to the F - actin molecules, there are two filaments of protein molecules known as tropomyosin. Another protein, namely, troponin is distributed on the tropomyosin at regular intervals. Troponin has three units namely,
Troponin T(Tn - T), Troponin I (Tn - I) and Troponin C (Tn - C). Tn - T is attached to the tropomyosin. Tn - I inhibits the attachment of myosin and actin and Tn - C binds with Ca++ ions. In the absence of Ca++, Tn - C, stabilizes tropomyosin in its blocking position over the active sites of actin.
ii. Thick filament: Thick filaments are formed by motor proteins called myosin. It is able to convert the chemical energy (ATP) into mechanical energy. Myosin is a polymer formed by many monomers called meromyosins. Each meromyosin has a head, a neck and a tail. Head and neck are formed by heavy meromyosin and tail by light meromyosin. A thick filament contains 200 - 300 myosin molecules. The heads are directed towards Z - lines and tails towards M - line. The head and neck project outwards at regular intervals and angles from each other from the surface of a thick filament is known as cross arm. Each head has two binding sites - one for ATP and the other for an active site of thin filament.
Troponin and tropomyosin are often called regulatory proteins because they play major role in masking and unmasking the active sites of actin.
d. Triad system: In a striated muscle, sarcolemma invaginates into the muscle fibre at A - I junctions forming the T - tubule. T - tubule along with terminal cisternae of sarcoplasmic reticulum on either side together forms the triad system of the muscle.
II. Mechanism of muscle contraction:
Muscle contraction is a physico chemical phenomenon. Sliding filament Hypothesis explains the physical changes. It was proposed by Hugh Huxley and Hanson (1965). Sliding filament hypothesis states that during muscle contraction, thin filaments slide over the thick filaments resulting in shortening of sarcomere.
1. Stimulation of muscle: When a nerve impulse reaches the neuromuscular junction, sarcolemma of muscle fibre gets depolarised. It extends into the T - tubule. Then cisternae of sarcoplasmic reticulum releases Ca++ ions into sarcoplasm.
2. Contraction of muscle:
a. Formation of cross bridges:
i. The Ca++ ions bind to the TnC of troponin of thin filament.
ii. Thus troponin - tropomyosin complex is shifted and hence, active sites are exposed in thin filament.
iii. Using energy released from ATP, the head of myosin now binds to the exposed active site to form a crossbridge (actin - myosin complex). It releases inorganic phosphorus.
b. Power stroke:
i. The cross bridge swings towards H - zone. This is called power stroke.
ii. The Z - lines attached to thin filaments are pulled inwards over the thick filaments on both sides causing shortening of sarcomere, that is contraction.
During contration
* A - Band remains same.
* I - Band shortens.
* H - Zone also shortens or some times disappear.
c. Recovery stroke:
i. The power stroke is followed by the release of ADP.
ii. A new ATP is binds to the head of myosin and thus cross bridge is broken.
iii. The new ATP is again hydrolysed into ADP and inorganic phosphorus.
Thus head of myosin swings to its original position and then attached to a new active site and the process repeats.
d. Relaxation of muscle:
i. When motor impulses stop, the Ca++ ions detach from the thin filament and diffuse into the cisternae of sarcoplasmic reticulum.
ii. Troponin permits the tropomyosin to cover the active sites of thin filaments. Thus heads of myosin are unable to attach the active sites.
iii. These changes cause the return of Z - lines to their original places. Then sarcomere is said to be relaxed.
The swinging movements and attachment and detachment of heads of myosin
from thin filaments is known as rachet mechanism (Walk along mechanism),
which forms the basis of sliding filament hypothesis.
III. Fatigue:
Repeated contractions of skeletal muscle lead to accumulation of lactic acid in it due to breakdown of glycogen in it, causing fatigue. During fatigue, skeletal muscle temporarily fails to contract.
IV. Cori cycle:
Lactic acid formed in the skeletal muscle reaches the liver through blood circulation. In liver during rest, 80% of lactic acid is utilised in the resynthesis of glycogen, which is transported back to muscle. All these cyclical events together constitutes the cori cycle. The remaining 20% of lactic acid is oxidised as CO2 and H2O.
V. Aerobic and anaerobic muscles:
Myoglobin is a reddish pigment found in sarcoplasm and it stores Oxygen. Based on the quantity of myoglobin, muscles are divided into two types.
SKELETON
Skeleton of man consists of 206 bones besides some cartilages. Skeleton can be divided to axial skeleton (skull, vertebral column and sternum) and appendicular skeleton (girdles and limb bones).
I. Axial Skeleton
1. Skull: All bones present in the head constitute the skull. It is composed of cranial bones (8) and facial bones (14).
A) Cranium is formed by a frontal, a pair each of parietals and temporals, an occipital, a sphenoid and an ethmoid.
* Parietals are joined with frontal by a coronal suture.
* Posteriorly parietals are articulated to occiput by lamboid suture.
* Due to presence of two occipital condyles, human skull is described as dicondylic skull.
* Sphenoid is present in the middle of base of skull an articulates with all other cranial bones. Hence spheroid is called Keystone bone.
B) Facial part is formed by a pair each of nasals, maxillae, zygomatic bones, lacrimals, palatines, inferior nasal conchae, a mandible and an vomer.
* Middle ear contains three bones (ear ossicles), namely malleus (modified articular), incus (modified quadrate) and stapes (modified hyomandibular).
* U - shaped hyoid bone is present at base of buccal cavity. It keeps the laryux open.
2. Vertebral column
Human vertebral column is formed by 26 vertebrae. Among vertebrae, inter vertebral discs are present. Vertebrae are classified into cervical (seven), thoracic (twelve), lumbar (five), sacral (one) and coccygeal (one) vertebrae.
* First vertebra is called atlas and second one is called axis.
* Five sacral vertebrae are fused to form a triangular bone called sacrum.
* Four coccygeal vertebrae are fused to form a coccyx.
3. Sternum: It is a flat bone present along the mid ventral line of thorax. It consists of anterior manubrium middle body and posterior xiphoid process. Sternum is also called breast bone.
4. Ribs: Connecting the vertebral column and sternum, there are 12 pairs of ribs in man. Dorsally, each rib has two articular surfaces (bicephalic).
* First seven pairs of ribs are called true ribs (Directly attached to vertebral column and sternum).
* Next five pairs of ribs are called false ribs (Ventrally they are articulated with 7th pair of ribs). Last two pairs of ribs are called floating ribs because they will not join sternum or other ribs.
II. Appendicular Skeleton
1. Girdles
A) Pectoral Girdle: Pectoral girdle is formed by two identical halves and each half of pectoral girdle is formed by a clavicle and a scapula. The body of scapula has a ridge called spine, which projects as a flat acromion process. Below the acromion process is a concavity called glenoid cavity.
B) Pelvic girdle: Pelvic girdle is formed by two identical halves called coxal bones. Each coxa consists of three fused bones, namely, ilium, ischium and pubis. At the junction of these bones, is a concavity called acetabulum. The two coxal bones of pelvic girdle are joined by a pubic symphysis.
2. Limb bones
A) Fore limb bones: Each fore bone limb of man is formed by thirty bones
a. Upper arm - Humerus (1)
b. Fore arm - Radius and ulna (2)
c. Wrist - Carpals (8)
d. Palm - Metacarpals (5)
e. Fingers - Phalanges (2 + 3 + 3 + 3 + 3 = 14)
B) Hind limb bones: Each hind limb of man is formed by thirty bones (including patella)
a. Thigh - Femur (1)
b. Shank - Tibia and Fibula (2)
c. Ankle - Tarsals (7)
d. Instep - Metatarsals (5)
e. Toes - Phalanges (2 + 3 + 3 + 3 + 3 = 14)
f. Knee cap - Patella (1)
JOINTS
A joint is an articulation between two bones. Flexible connective tissue form the joints between bones. Study of joints is known as arthrology.
Types of joints: Joints are of three types, namely fibrous joints, cartilaginous joints and synovial joints.
1. Fibrous Joints: These joints does not allow any movement fibrous joints are classified into 3 types.
a) Sutures: These joints are found in cranial bones, in which thin layer of dense fibrous tissue connects two flat bones.
e.g.: Coronal suture between parietal and frontal.
Lamboid suture between perietal and occiput.
b) Syndesmoses: In these joints, the space between the articulating bones is filled with fibrous connective tissue which is arranged like a bundle (ligament) or layer.
e.g.: Interosseus membrane between boarders of libia and fibula.
c) Gomphoses: In this type a cone shaped peg of one bone is articulated with socket of another bone by fibrous connective tissue.
e.g.: Dentalveolar joint.
2. Cartilaginous joints: These joints allow little or no movement. Articulating bones are connected by cartilage (fibrous or hyaline). These joints are of two types.
a) Synchondroses: In this type hyaline cartilage connects the articulating bones.
e.g.: Epiphyseal plate connecting epiphysis and diaphysis.
b) Symphysis: Ends of articulating bones are covered by hyaline cartilage. Broad disc of fibrous cartilage connects the two articulating bones.
e.g.: Pubic symphysis of pelvic girdle.
3. Synovial joints: These joints allow free movement and all of them are diarthroses.
a) Structure: The articulating surfaces of the bones are covered by hyaline cartilage. The entire joint is enclosed by a synovial capsule which is formed by irregular connective tissue. It also contains ligaments. The inner lining of synovial capsule is formed by a synovial membrane. The cavity enclosed by the synovial capsule is known as synovial cavity. It is filled with synovial fluid (Phagocytes + hyaluronic acid + interstitial fluid) which is secreted by the synovial membrane. It acts as a lubricant and allows free movement of bones.
b. Types of synovial joints:
DISORDERS OF MUSCULO SKELETAL SYSTEM
1. Arthritis: Inflammation of joints.
2. Tetany: Repeated and prolonged contraction of muscles. It may be due to low Ca++ levels in blood, hypo parathyroidism or due to deficiency of vitamin D.
3. Osteoporosis: Decreased bone mass due to reabsorption of calcium. A decrease of oestrogen is one of the reasons in post menopause women.
4. Gout: Inflammation of joints due to accumulation of uric acid crystals.
5. Muscular dystrophy: Degeneration muscle due to genetical disorders (DMD) or due to nutritional disorders.
6. Myasthenia gravis: It is an autoimmune disorder affecting neuromuscular junctions leading to fatigue, weakness, paralysis etc.,
