Skeletal system Divisions of skeletal system, types of bone, salient features and functions of bones of axial and appendicular skeletal system Organization of skeletal muscle, physiology of muscle contraction, neuromuscular junction.

1. Skeletal System
By
Yogeshwary M. Bhongade
Assistant Professor
Kamla Neharu College of Pharmacy, Butibori, Nagpur

2. Introduction
 The branch of science that deals with the study of skeletal system, their
structure and functions is known as OSTEOLOGY.
 Skeletal system includes bone and joints
 Bone tissue make up about 18% of the total body weight
 The skeletal system responsible for supports and protects the body
while giving it shape and form.
 Skeletal system mainly composed of Bone, Cartilage, Joints and
Ligaments
 Total 206 bones are present in body

3. Functions of Skeletal System
 Support-
 Protection
 Movement
 Storage
 Blood cell formation

4. Classification
 The bones of the body come in a variety of sizes and
shapes.
 The four principal types of bones are long, short, flat and
irregular.
Long Bones-
 Bones that are longer than they are wide are called long
bones.
 They consist of a long shaft with two bulky ends or
extremities.
 They are primarily compact bone but may have a large
amount of spongy bone at the ends or extremities.
 Long bones include bones of the thigh, leg, arm, and
forearm.

5. Short Bones
 Short bones are roughly cube shaped with vertical and horizontal
dimensions approximately equal.
 They consist primarily of spongy bone, which is covered by a
thin layer of compact bone.
 Short bones include the bones of the wrist and ankle.
Flat Bones
 Flat bones are thin, flattened, and usually curved. Most of the
bones of the cranium are flat bones.

6. Irregular Bones
 Bones that are not in any of the above three categories are classified as
irregular bones.
 They are primarily spongy bone that is covered with a thin layer of
compact bone.
 The vertebrae and some of the bones in the skull are irregular bones.
 All bones have surface markings and characteristics that make a specific
bone unique.
 There are holes, depressions, smooth facets, lines, projections and other
markings.
 These usually represent passageways for vessels and nerves, points of
articulation with other bones or points of attachment for tendons and
ligaments

7. Division of Skeletal System

9. Axial Skeletal
 Skull
 Vertebral Column (Spinal Column)
 Sternum
 Ribs

10. Skull
Cranial Bone
Cranial Bones
Parietal (2)
Temporal (2)
Frontal (1)
Occipital (1)
Ethmoid (1)
Sphenoid (1)

11. Facial Bones
 Maxilla (2)
 Zygomatic (2)
 Mandible (1)
 Nasal (2)
 Platine (2)
 Inferior nasal concha (2)
 Lacrimal (2)
 Vomer (1)

12. Auditory Ossicles/ Ear Ossicle
• Malleus (2)
• Incus (2)
• Stapes (2)

13. Hyoid Bone(1)

14. Vertebral Column
• Cervical
vertebrae (7)
• Thoracic
vertebrae (12)
• Lumbar
vertebrae (5)
• Sacrum (1)
• Coccyx (1)

15. Sternum

16. Ribs

17. Appendicular Skeletal
Hand
• Humerus (2)
• Radius (2)
• Ulna (2)
• Carpals (16)
• Metacarpals
(10)
• Phalanges
(28)

18. Legs 30/Legs
• Femur (2)
• Tibia (2)
• Fibula (2)
• Patella (2)
• Tarsals (14)
• Metatarsals (10)
• Phalanges (28)

19. Girdle Bone
Pectorial Girdle

20. Pelvic Girdle

21. Organisation of Skeletal Muscle
 All activities that involve movement depend on muscles
 650 muscles in the human body
 Various purposes for muscles for:
ü Locomotion
ü Upright posture
ü Balancing on two legs
ü Support of internal organs
ü Controlling valves and body openings
ü Production of heat
ü Movement of materials along internal tubes
ü Three types of muscles in the human body
üSkeletal
üCardiac
üSmooth

22.  Skeletal muscles are muscles which are attached to the skeleton.
 40% of human body mass
 Skeletal muscles are mainly responsible for locomotion, and voluntary
contraction and relaxation.

23. Organisation of Skeletal Muscle
1. Muscle (whole organ)
2. Fascicle (portion of muscle)
3. Muscle Fiber (single muscle cell)
4. Myofibril (muscle cell organelle)
5. Sarcomere (portion of myofibril)
6. Myofilament (part of sarcomere)

25. Structure of Skeletal Muscle

26.  Each skeletal muscle fiber is a single cylindrical muscle cell.
 An individual skeletal muscle may be made up of hundreds, or even
thousands, of muscle fibers bundled together and wrapped in a connective
tissue covering. Each muscle is surrounded by a connective tissue sheath
called the epimysium.
 Fascia, connective tissue outside the epimysium, surrounds and separates
the muscles. Portions of the epimysium project inward to divide the muscle
into compartments. Each compartment contains a bundle of muscle fibers.
Each bundle of muscle fiber is called a fasciculus and is surrounded by a
layer of connective tissue called the perimysium.
 Within the fasciculus, each individual muscle cell, called a muscle fiber, is
surrounded by connective tissue called the endomysium.

27.  Skeletal muscle cells (fibers), like other body cells, are soft and fragile. The
connective tissue covering furnish support and protection for the delicate cells
and allow them to withstand the forces of contraction. The coverings also
provide pathways for the passage of blood vessels and nerves.
 Commonly, the epimysium, perimysium, and endomysium extend beyond the
fleshy part of the muscle, the belly or gaster, to form a thick ropelike tendon or a
broad, flat sheet-like aponeurosis.
 The tendon and aponeurosis form indirect attachments from muscles to the
periosteum of bones or to the connective tissue of other muscles. Typically a
muscle spans a joint and is attached to bones by tendons at both ends. One of
the bones remains relatively fixed or stable while the other end moves as a result
of muscle contraction.Skeletal muscles have an abundant supply of blood
vessels and nerves. This is directly related to the primary function of skeletal
muscle, contraction. Before a skeletal muscle fiber can contract, it has to receive
an impulse from a nerve cell.

28.  Generally, an artery and at least one vein accompany each
nerve that penetrates the epimysium of a skeletal muscle.
 Branches of the nerve and blood vessels follow the connective
tissue components of the muscle of a nerve cell and with one
or more minute blood vessels called capillaries.

29. Muscle Types
 In the body, there are three types of muscle: skeletal (striated), smooth, and
cardiac.
Skeletal Muscle
 Skeletal muscle, attached to bones, is responsible for skeletal movements. The
peripheral portion of the central nervous system (CNS) controls the skeletal
muscles. Thus, these muscles are under conscious, or voluntary, control. The
basic unit is the muscle fiber with many nuclei. These muscle fibers are striated
(having transverse streaks) and each acts independently of neighboring muscle
fibers.
Smooth Muscle
 Smooth muscle, found in the walls of the hollow internal organs such as blood
vessels, the gastrointestinal tract, bladder, and uterus, is under control of the
autonomic nervous system.

30.  Smooth muscle cannot be controlled consciously and thus acts
involuntarily. The non-striated (smooth) muscle cell is spindle-shaped
and has one central nucleus. Smooth muscle contracts slowly and
rhythmically.
Cardiac Muscle
 Cardiac muscle, found in the walls of the heart, is also under control of
the autonomic nervous system. The cardiac muscle cell has one central
nucleus, like smooth muscle, but it also is striated, like skeletal muscle.
The cardiac muscle cell is rectangular in shape. The contraction of
cardiac muscle is involuntary, strong, and rhythmical.

31. Muscle Groups
 There are more than 600 muscles in the body, which together account for about
40 percent of a person's weight.
 Most skeletal muscles have names that describe some feature of the muscle.
Often several criteria are combined into one name. Associating the muscle's
characteristics with its name will help you learn and remember them. The
following are some terms relating to muscle features that are used in naming
muscles.
 Size: vastus (huge); maximus (large); longus (long); minimus (small); brevis
(short).
 Shape: deltoid (triangular); rhomboid (like a rhombus with equal and parallel
sides); latissimus (wide); teres (round); trapezius (like a trapezoid, a four-sided
figure with two sides parallel).

32.  Direction of fibers: rectus (straight); transverse (across); oblique
(diagonally); orbicularis (circular).
 Location: pectoralis (chest); gluteus (buttock or rump); brachii (arm); supra-
(above); infra- (below); sub- (under or beneath); lateralis (lateral).
 Number of origins: biceps (two heads); triceps (three heads); quadriceps
(four heads).
 Origin and insertion: sternocleidomastoideus (origin on the sternum and
clavicle, insertion on the mastoid process); brachioradialis (origin on the
brachium or arm, insertion on the radius).
 Action: abductor (to abduct a structure); adductor (to adduct a structure);
flexor (to flex a structure); extensor (to extend a structure); levator (to lift or
elevate a structure); masseter (a chewer).

33. Muscles of the Head and Neck
 Humans have well-developed muscles in the
face that permit a large variety of facial
expressions.
 Because the muscles are used to show
surprise, disgust, anger, fear, and other
emotions, they are an important means of
nonverbal communication. Muscles of facial
expression include frontalis, orbicularis oris,
laris oculi, buccinator, and zygomaticus.
 These muscles of facial expressions are
identified in the illustration below.

34.  There are four pairs of muscles that are responsible for chewing
movements or mastication. All of these muscles connect to the mandible
and they are some of the strongest muscles in the body.
 Two of the muscles, temporalis and masseter, are identified in the
illustration above.
 There are numerous muscles associated with the throat, the hyoid bone
and the vertebral column; only two of the more obvious and superficial
neck muscles are identified in the illustration: sternocleidomastoid and
trapezius.

35. Muscles of the Trunk
 The muscles of the trunk include those that move the vertebral column,
the muscles that form the thoracic and abdominal walls, and those that
cover the pelvic outlet.

36.  The erector spinae group of muscles on each side of the vertebral
column is a large muscle mass that extends from the sacrum to the skull.
These muscles are primarily responsible for extending the vertebral
column to maintain erect posture.
 The deep back muscles occupy the space between the spinous and
transverse processes of adjacent vertebrae.
 The muscles of the thoracic wall are involved primarily in the process of
breathing. The intercostal muscles are located in spaces between the ribs.
They contract during forced expiration. External intercostal muscles
contract to elevate the ribs during the inspiration phase of breathing. The
diaphragm is a dome-shaped muscle that forms a partition between the
thorax and the abdomen.
 It has three openings in it for structures that have to pass from the thorax
to the abdomen.

37.  The abdomen, unlike the thorax and pelvis, has no bony reinforcements or
protection. The wall consists entirely of four muscle pairs, arranged in layers,
and the fascia that envelops them. The abdominal wall muscles are identified
in the illustration below.
 The pelvic outlet is formed by two muscular sheets and their associated fascia.

38. Muscles of the Upper Extremity
 The muscles of the upper extremity include
those that attach the scapula to the thorax and
generally move the scapula, those that attach the
humerus to the scapula and generally move the
arm, and those that are located in the arm or
forearm that move the forearm, wrist, and hand.
 The illustration below shows some of the
muscles of the upper extremity.
 Muscles that move the shoulder and arm include
the trapezius and serratus anterior. The
pectoralis major, latissimus dorsi, deltoid, and
rotator cuff muscles connect to the humerus and
move the arm.

39.  The muscles that move the forearm are located along the humerus,
which include the triceps brachii, biceps brachii, brachialis, and
brachioradialis.
 The 20 or more muscles that cause most wrist, hand, and finger
movements are located along the forearm.

40. Muscles of the Lower Extremity
 The muscles that move the thigh have their origins on some part
of the pelvic girdle and their insertions on the femur. The largest
muscle mass belongs to the posterior group, the gluteal muscles,
which, as a group, adduct the thigh. The iliopsoas, an anterior
muscle, flexes the thigh.
 The muscles in the medial compartment adduct the thigh. The
illustration below shows some of the muscles of the lower
extremity.
 Muscles that move the leg are located in the thigh region. The
quadriceps femoris muscle group straightens the leg at the knee.
The hamstrings are antagonists to the quadriceps femoris muscle
group, which are used to flex the leg at the knee.

41.  The muscles located in the leg that move the ankle and foot are
divided into anterior, posterior, and lateral compartments.
 The tibialis anterior, which dorsiflexes the foot, is antagonistic to
the gastrocnemius and soleus muscles, which plantar flex the foot.

42. Neuromuscular Junction
 Junction between neuron and muscle
 The neuromuscular junction (NMJ) is a synaptic connection
between the terminal end of a motor nerve and a muscle (skeletal/
smooth/ cardiac).
 It is the site for the transmission of action potential from nerve to
the muscle.
 It is also a site for many diseases and a site of action for many
pharmacological drugs.
 Diseases of the NMJ produce muscle weakness through different
mechanisms that may affect presynaptic, synaptic, or postsynaptic
portions of the NMJ.

44. Physiological Anatomy of Neuromuscular Junction
 For convenience and understanding, the structure of NMJ can be
divided into three main parts:
1. a presynaptic part (nerve terminal),
2. the postsynaptic part (motor endplate), and
3. an area between the nerve terminal and motor endplate (synaptic cleft).

45. Nerve Terminal
 A myelinated motor neuron, on reaching the target muscle, loses its myelin
sheath to form a complex of 100-200 branching nerve endings.
 These nerve endings are called nerve terminals or terminal boutons.
 The nerve terminal membrane has areas of membrane thickening called active
zones. A
 ctive zones have a family of SNAP proteins (syntaxins and synaptosomal-
associated protein 25) and rows of voltage-gated calcium (Ca) channels.
 A nerve terminal also has potassium channels on its membrane and contains
mitochondria, endoplasmic reticulum, and synaptic vesicles (SVs). Each SV
stores around 5000-10000 molecules of acetylcholine (ACh), the
neurotransmitter at NMJ. The SVs are concentrated around the active zone.
 The membrane of SVs has synaptotagmin and synaptobrevin proteins. These
proteins are essential for fusion and docking of SVs at active zones.

46.  On arrival of an action potential at the nerve terminal, Ca channels
open to cause influx.
 Increased Ca inside the nerve terminal causes a series of events
leading to docking of SVs at active zones and exocytosis of the ACh
from the synaptic vesicles into the synaptic cleft.

47. Synaptic Cleft / Junctional Cleft:
 The space between the nerve terminal and the plasma membrane of
muscle is called synaptic/junctional cleft and measures ∼ 50 nm.
 It is the site where presynaptic neurotransmitters, ACh is released
before it interacts with nicotinic ACh receptors on the motor
endplate.
 Synaptic cleft of NMJ contains acetylcholinesterase enzyme,
responsible for the catabolism of released ACh so that its effect on
the post-synaptic receptors is not prolonged.

48. Motor End Plate
 Motor End Plate forms the postsynaptic part of NMJ.
 It is the thickened portion of the muscle plasma membrane (sarcolemma)
that is folded to form depressions called junctional folds.
 The terminal nerve endings do not penetrate the motor endplate but fit into
the junctional folds. Junctional folds have nicotinic ACh receptors
concentrated at the top.
 These receptors are ACh gated ion channels. Binding of ACh to these
receptors opens the channels allowing the influx of sodium ions from the
extracellular fluid into the muscle membrane.
 This creates endplate potential and generates and transmits AP to the muscle
membrane.

49. Physiology of neuromuscular junction
Mechanism
 ACh is synthesized in the pre-synaptic terminal using choline and acetyl-CoA
and the enzyme choline acetyltransferase.
 It subsequently goes through a series of modifications before being packaged in
vesicles.
 Upon depolarization, an action potential travels down the axon, causing voltage-
gated calcium channels to open, resulting in an influx of calcium ions into the
nerve terminal.
 This causes the vesicles to migrate towards the nerve terminal membrane and
fuse with the active zones.
 Different vesicular (SNAP-25, syntaxin) and nerve terminal membrane proteins
(synaptobrevin and synaptotagmin) play a role in the fusion of SVs to active
zones and exocytosis of ACh into the synaptic cleft.

50.  The released ACh subsequently binds to nicotinic ACh receptors on the
junctional folds of the motor endplate.
 The binding of ACh to receptors triggers the opening of ACh gated ion
channels that allow the influx of sodium ions into the muscle.
 The sodium influx changes the postsynaptic membrane potential from -90 mV
to -45 mV.
 This decrease in membrane potential is called endplate potential.
 In the NMJ, endplate potential is strong enough to propagate action potential
over the surface of the skeletal muscle membrane that ultimately results in
muscle contraction.
 To prevent sustained depolarization and muscle contraction, as well as to allow
for repolarization, ACh is metabolized by acetylcholinesterase into its subunits,
choline, and acetate.
 Choline can then be re-used for the synthesis of ACh.

51.  Three main diseases that involve NMJ are-
1. Myasthenia Gravis (MG),
2. Lambert-Eaton syndrome (LES), and
3. Botulism.

52. Myasthenia Gravis
 Myasthenia Gravis is an auto-immune condition (type II hypersensitivity
reaction), resulting in the production of auto-antibodies against ACh
receptors, at the neuromuscular junction.
 The antibodies against ACh receptors decrease the availability of ACh
receptors to endogenous ACh.
 This prevents endplate potential and muscle contraction.
 As a result, symptoms such as muscle weakness come as no surprise,
especially in the extra-ocular muscle, as these muscles are in constant use
and have a lower density of ACh receptors.
 Patients may also experience difficulty chewing and limb weakness. These
symptoms worsen with use and progress throughout the day as the ACh in
the pre-synaptic cleft gets depleted and insufficient to compete with the ACh
receptor antibodies.

53.  MG is diagnosed by looking for the ACh receptor antibodies
(80% to 90% sensitivity), or in cases where clinical suspicion is
high but the ACh receptor antibodies are negative (seronegative
MG), the anti-muscle specific kinase (MuSK) can help make the
diagnosis.

54. Thank you