The skeletal system consists of bones and cartilage which provide structure, protect organs, allow movement, store minerals, produce blood cells, and store fats. The skeleton is divided into the axial skeleton which includes the skull, vertebral column, ribs, and sternum, and the appendicular skeleton including the pectoral girdle, upper limbs, pelvic girdle, and lower limbs. Bones are made of organic and inorganic components and contain osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts. Joints connect bones and allow movement, with synovial joints providing the most mobility.

1. SKELETAL SYSTEM
Subject: Human anatomy and physiology-I
UNIT-II
Prepared by: Fulchand V. Kajale
(M.Pharm Pharmacology)
Assistant Professor at
Shivai Charitable trust’s College of Pharmacy.

2. Functions of Bone and the Skeletal System
1. Support. The skeleton serves as the structural framework for the body by supporting
soft tissues and providing attachment points for the tendons of most skeletal muscles.
2. Protection. The skeleton protects the most important internal organs from injury. For
example, cranial bones protect the brain, and the rib cage protects the heart and lungs.
3. Assistance in movement. Most skeletal muscles attach to bones; when they contract,
they pull on bones to produce movement.
4. Mineral homeostasis (storage and release). Bone tissue makes up about 18% of the
weight of the human body. It stores minerals, especially calcium and phosphorus, which
contribute to the strength of bone. Bone tissue stores about 99% of the body’s calcium.
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3. 5. Blood cell production. In bones, a connective tissue called red bone marrow produces
red blood cells and white blood cells, and platelets, this process is called as hemopoiesis
Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and
macrophages within a network of reticular fibers. It is present in developing bones of the
fetus and in some adult bones, such as the hip (pelvic) bones, ribs, sternum (breastbone),
vertebrae (backbones), skull, and ends of the bones of the humerus (arm bone) and femur
(thigh bone).
6. Triglyceride storage. Yellow bone marrow consists mainly of adipose cells, which
store triglycerides. The stored triglycerides are a potential chemical energy reserve.
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4. Structure of Bone
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The study of bone structure and the treatment of bone
disorders is known to as Osteology
The bone is a rigid body tissue that makes up our body
skeleton.

5. • The diaphysis is the bone’s shaft or body the long, cylindrical, main portion of the
bone.
• The epiphyses are the proximal and distal ends of the bone.
• The metaphyses are the regions between the diaphysis and the epiphyses.
• The articular cartilage is a thin layer of hyaline cartilage covering the part of the
epiphysis where the bone forms an articulation (joint) with another bone.
• The periosteum is a tough connective tissue sheath and its associated blood supply
that surrounds the bone surface wherever periosteum is attached to the underlying bone
by perforating fibers or Sharpey’s fibers, thick bundles of collagen that extend from
the periosteum into the bone extracellular matrix.
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6. • The medullary cavity or marrow cavity, is a hollow, cylindrical space within the
diaphysis that contains fatty yellow bone marrow
• The endosteum is a thin membrane that lines the medullary cavity
Types of cells in bone tissue
1. Osteoprogenitor cells are unspecialized bone stem cells derived from mesenchyme,
the tissue from which almost all connective tissues are formed. They are the only bone
cells to undergo cell division; the resulting cells develop into osteoblasts.
Osteoprogenitor cells are found along the inner portion of the periosteum, in the
endosteum, and in the canals within bone that contain blood vessels.
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7. Types of cells in bone tissue
2. Osteoblasts are bone-building cells. They synthesize and secrete collagen fibers and
other organic components needed to build the extracellular matrix of bone tissue, and they
initiate calcification. As osteoblasts surround themselves with extracellular matrix, they
become trapped in their secretions and become osteocytes.
(Note: The ending –blast means that the cell secretes extracellular matrix.)
3. Osteocytes, are the main cells in bone tissue and maintain its daily metabolism, such as
the exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do not
undergo cell division.
• (Note: The ending -cyte means that the cell maintains and monitors the tissue.)
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8. 4. Osteoclasts are huge cells derived from the fusion of as many as 50 monocytes (a type
of white blood cell) and are concentrated in the endosteum. These cell releases powerful
lysosomal enzymes and acids that digest the protein and mineral components of the
underlying extracellular bone matrix. This breakdown of bone extracellular matrix,
termed bone resorption, is part of the normal development, maintenance, and repair of
bone.
(Note: The ending -clast means that the cell breaks down extracellular matrix.)
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Divisions of skeletal system

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11. Types of Bones
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1. Short bone: cube-shaped and are nearly equal
in length and width.
2. Long bones: have greater length than width,
consist of a shaft and a variable number of
epiphyses (ends), and are slightly curved for
strength.
3. Flat bones: are generally thin and composed of
two nearly parallel plates of compact bone tissue
enclosing a layer of spongy bone tissue.
4. Irregular bones: have complex shapes
5. Sesamoid bones ( sesame seed like shape).
6. Sutural bones: are small bones located in
sutures

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The Axial skeleton
Skull

13. • The skull is the bony framework of the head.
It contains 22 bones (not counting the bones of the middle ears)
The bones of the skull are divided into two categories: Cranial bones(8) and facial
bones(14) = 22
Cranial Bones
Frontal bone, two parietal bones, two temporal bones, occipital bone, the sphenoid
bone, and the ethmoid bone.
Facial Bones
Two nasal bones, two maxillae (or maxillas), two zygomatic bones, the mandible, two
lacrimal bones, two palatine bones, two inferior nasal conchae, and the vomer.
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14. Hyoid Bone(U shape)
• It is a unique component of the axial skeleton because it does not articulate with any
other bone.
Vertebral Column
7 cervical vertebrae in the neck region. (C1–C7)
12 thoracic vertebrae posterior to the thoracic cavity. (T1–T12)
5 lumbar vertebrae supporting the lower back. (L1–L5)
1 sacrum consisting of five fused sacral vertebrae. (S1–S5)
1 coccyx usually consisting of four fused coccygeal vertebrae. (Co1–Co4)
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The vertebral column also called the spine, backbone, or
spinal column, it is composed of a series of bones called
vertebrae. it about 71 cm (28 in.) in an average adult male
and about 61 cm (24 in.) in an average adult female. it
shows four slight bends called normal curves. The curves
of the vertebral column increase its strength, help maintain
balance in the upright position, absorb shocks
during walking, and help protect the vertebrae
from fracture.

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17. Thorax
• The thoracic cage, is a bony enclosure formed by the sternum, ribs and their costal
cartilages, and the bodies of the thoracic vertebrae.
1. Sternum
The sternum, or breastbone, is a flat, narrow bone located in the center of the anterior
thoracic wall that measures about 15 cm (6 in.) in length and consists of three parts
Manubrium, Body, xiphoid process
2. Ribs
Twelve pairs of ribs, numbered 1–12 from superior to inferior, give structural support to
the sides of the thoracic cavity. The first through seventh pairs of ribs have a direct
anterior attachment to the sternum by a strip of hyaline cartilage called costal cartilage.
The ribs that have costal cartilages and attach directly to the sternum are called true ribs.
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19. The Appendicular Skeleton
A. Pectoral Girdle (Shoulder)
Clavicle (S-shaped),Scapula
Upper Limb
Humerus, Ulna and Radius,
Carpals, Metacarpals, and Phalanges
1. Scaphoid (boat like)
2. Lunate (moon-shaped)
3. Triquetrum (three-cornered)
4. Pisiform (pea-shaped).
5. Trapezium (four-sided figure with no
two sides parallel)
6. Trapezoid (four-sided figure with two
sides parallel)
7. Capitate (head-shaped)
8. Hamate (hooked)
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20. B. Pelvic Girdle (Hip)
• The complete ring composed of the hip bones, pubic symphysis, sacrum, and coccyx
forms a deep, basin like structure called the bony pelvis.
• The portion of the bony pelvis superior to the pelvic brim is called as the false (greater)
pelvis
• The portion of the bony pelvis inferior to the pelvic brim is the true (lesser) pelvis.
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21.  Lower Limb
• Femur
• Patella
• Tibia
• Fibula
Skeleton of the Foot:
• Tarsals
• Metatarsals
• Phalanges
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1. Talus
2. Navicular
3. Calcaneus
4. Third cuneiform
5. Second cuneiform
6. First cuneiform
7. Cuboid
Foot:

23.  Disorders of Skeleton system :
1. Herniated Disc
2. Abnormal Curves of the Vertebral Column (Scoliosis, Kyphosis, Lordosis)
3. Spina Bifida
4. Fractures of the Vertebral Column
5. Hip Fracture
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24. Organisation of skeletal muscle
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25. Microscopic Anatomy of a Skeletal Muscle Fiber
The diameter of a mature skeletal muscle fiber ranges from 10 to 100 μm.
• Sarcolemma, Transverse Tubules, and Sarcoplasm
• Myofibrils and Sarcoplasmic Reticulum
• Filaments and the Sarcomere
Thin filaments are 8 nm in diameter and 1–2 μm long and composed of the protein actin,
while thick filaments are 16 nm in diameter and 1–2 μm long and composed of the
protein myosin. In skeletal muscle, about 300 molecules of myosin form a single thick
filament.
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26. • Smaller amounts of two regulatory proteins tropomyosin and troponin are also part of
the thin filament.
• In relaxed muscle, myosin is blocked from binding to actin because strands of
tropomyosin cover the myosin- binding sites on actin. The tropomyosin strands in turn
are held in place by troponin molecules. when calcium ions (Ca2+) bind to troponin,
troponin undergoes a conformational change this change moves tropomyosin away from
myosin-binding sites on actin, and muscle contraction subsequently begins as myosin
binds to actin.
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29. Contraction and Relaxation of Skeletal Muscle Fibers
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31. The Neuromuscular Junction
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33.  Joints
A joint, also called an articulation or arthrosis is a point of contact between two bones,
between bone and cartilage.
oThe scientific study of joints is called Arthrology.
oThe study of motion of the human body is called Kinesiology.
A. Fibrous joints
B. Cartilaginous joints
C. Synovial joints
The functional classification of joints
Synarthrosis :An immovable joint.
Amphiarthrosis: A slightly movable joint.
Diarthrosis :A freely movable joint. All diarthroses are synovial joints.
Selected Joints of the Body
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34. A. Fibrous joints
1. Suture joint
2. Syndesmoses joint
3. Interosseous Membranes (radius and ulna)
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35. B. Cartilaginous joints
1. Synchondroses joint (joint between the first rib and the manubrium of the sternum)
2. Symphyses joint
3. Epiphyseal Cartilages joint
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36. C. Synovial joints
1. Plane Joints
2. Hinge Joints
3. Pivot Joints
4. Condyloid Joints
5. Saddle Joints
6. Ball-and-Socket Joints
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37.  Selected Joints of the Body
1. Temporomandibular Joint
2. Knee Joint,
3. Hip Joint
4. Elbow Joint
5. shoulder joint
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38. Reference
1. Anatomy and Physiology in Health and Illness by Kathleen J.W. Wilson,
Churchill Livingstone, New York
2. Principles of Anatomy and Physiology by Tortora Grabowski. Palmetto, GA,
U.S.A.
3. Essentials of Medical Physiology by K. Sembulingam and P. Sembulingam.
Jaypee brothers medical publishers, New Delhi.
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