Unit 12: Human Skeletal and Muscular Systems — Interactive Notes
Skeleton: Bones, cartilage and other connective tissues make an internal framework called skeleton that provides structural support, protects vital organs, and produces movement and locomotion.
Bones are made of connective tissue reinforced with calcium and specialized bone cells. The bone's surface is covered by a tough membrane called periosteum. The broad ends of a bone are called epiphysis while the middle part along the length of bone is called diaphysis or shaft.
The thick layer under periosteum is made of hard material and is called compact bone. The basic structural units of compact bone are called Haversian systems, made of three parts:
(i) Lamellae: Concentric layers of mineralized extracellular matrix containing collagen fibres and needle-shaped crystals of calcium phosphate. The crystals are brittle but rigid, giving bone great strength; collagen is flexible but weak — together bone is both strong and flexible.
(ii) Lacunae and Osteocytes: The lamellae are separated by small spaces called lacunae. Osteocytes (mature bone cells) are located in the lacunae, connected to each other and to the Haversian canal by small channels called canaliculi.
(iii) Haversian canal: The concentric layers of lamellae surround a central canal called the Haversian canal, which contains blood vessels, nerves, and lymphatic vessels.
3. Spongy Bone: Beneath the compact bone there is spongy bone. It has a latticework structure consisting of bony spikes that make it light and strong.
4. Bone Marrow: Many bones contain a soft tissue called bone marrow, which can be red or yellow. Red bone marrow (spongy bone, long bone ends, ribs, vertebrae, sternum, pelvis) produces blood cells. Yellow bone marrow (fills shafts of long bones) consists of fat cells and serves as an energy reserve.
5. Types of Bone Cells: Osteoblasts (bone-forming cells, become osteocytes once surrounded by matrix), Osteocytes (maintain healthy bone tissue, regulate calcium release), Osteoclasts (develop from macrophages, resorb bone — break it down and release calcium/phosphate into blood).
Definition: "The process of bone formation is called osteogenesis." It begins during embryonic development and continues throughout life, playing a vital role in growth, maintenance, and repair of bones.
Pathways: There are two primary pathways of osteogenesis.
1. Endochondral Ossification (transition of cartilage into bone): The formation of long bones e.g., femur and humerus, involves the cartilage center hardening (calcifying); chondrocytes in this area die, leaving cavities. Blood vessels penetrate these cavities and introduce osteoblasts and osteoclasts. Osteoblasts build bone tissue, replacing cartilage with new bone; osteoclasts break down calcified cartilage, making room for more bone. As the bone matures, some osteoblasts become trapped and transform into osteocytes. This continues until all cartilage is changed to bone except some remaining at articular (joint) surfaces.
Ossification: The step by which cartilage is replaced by bone via mineral deposition.
2. Intramembranous ossification: Short bones develop directly into hard bone without forming cartilage first e.g., some bones of the skull. The osteocytes are initially scattered randomly throughout embryonic connective tissue but soon fuse into layers and become flat plates of bone.
Definition: Cartilage is a type of connective tissue, made up of perichondrium, cartilage matrix and chondrocytes.
1. Perichondrium: A layer of connective tissue surrounding cartilage, containing blood vessels, lymphatic vessels, and nerves.
2. Cartilage matrix: Inside perichondrium, composed of collagen, elastin, proteoglycans, and other fibres — gives strength, flexibility, and resistance to compression. Unlike other connective tissues, there are no blood vessels inside; cells are supplied by diffusion, so it heals very slowly.
3. Chondrocytes: Cartilage cells present within small spaces called lacunae, embedded in cartilage matrix; responsible for synthesizing and maintaining the matrix.
Types:
• Hyaline cartilage — most common, found in nose, trachea, and articulating surfaces of bones in joints.
• Fibrocartilage — found in high-stress/tension areas, such as intervertebral discs and pubic symphysis.
• Elastic cartilage — found in external ear and epiglottis.
The axial skeleton forms the axis of the body. It is composed of 80 bones. Its bones support and protect the organs of the head, neck, and chest. It consists of skull, ribs, spine, and sternum.
a. Skull (22 bones + 6 middle ear ossicles): Cranial Bones (8) — 2 paired (parietal, temporal), 4 unpaired (frontal, occipital, ethmoid, sphenoid). Facial Bones (14) — 6 paired (lacrimal, zygomatic, nasal, inferior nasal concha, maxilla, palatine), 2 unpaired (mandible, vomer).
b. Middle ear: 6 bones (3 pairs) called ossicles — malleus, incus, stapes (smallest human bone).
c. Neck bone: Hyoid bone — a small U-shaped single bone at the base of skull below the tongue, doesn't articulate with any other bone.
d. Vertebral column (33 bones, 26 in adults after fusion):
(i) Cervical (C1-C7) — neck vertebrae; C1=atlas, C2=axis (facilitates rotation).
(ii) Thoracic (T1-T12) — rib-carrying, chest region.
(iii) Lumbar (L1-L5) — abdominal region.
(iv) Sacral (S1-S5) — fused, form sacrum, articulates with iliac bones.
(v) Coccygeal (Co1-Co4) — fused, form coccyx (tailbone). Sacral+coccygeal = pelvic vertebrae.
e. Rib Cage & Chest bone: 24 ribs (12 pairs) + sternum. True ribs (1st-7th, direct costal cartilage), False ribs (8th-10th, common costal cartilage), Floating ribs (11th-12th, no sternum attachment).
Appendicular skeleton includes the bones present in appendages (arms and legs). It is composed of 126 bones — pectoral girdle, pelvic girdle, forelimbs and hindlimbs.
a. Pectoral girdle: 2 pairs i.e., clavicles (collar bones) and scapulae (shoulder bones). Clavicle articulates with sternum on one end and scapula on the other.
b. Forelimbs (30 bones each): One humerus (fits into glenoid cavity, forms shoulder/glenohumeral joint); one ulna (inner) and one radius (outer, thumb side) — form elbow joint with humerus; eight carpels (wrist, radiocarpal joint); five metacarpals (palm); fourteen phalanges (3 per finger, 2 for thumb).
c. Pelvic girdle: A pair of hip bones (coxal bones), each made of 3 fused bones — ilium, ischium, pubis — forming the acetabulum socket. The two hip bones join at the front by the pubic symphysis.
d. Hindlimbs (30 bones each): One femur (largest human bone, fits into acetabulum); one patella/knee cap (sesamoid bone in quadriceps tendon); one tibia (shin) and one fibula (thin outer bone); seven tarsals (ankle); five metatarsals (sole); fourteen phalanges (3 per toe, 2 for big toe).
Joint: A place where two bones or bone and cartilage come together.
1. Fibrous Joints: Bones directly connected by fibrous connective tissue (mainly collagen). Permit no movement. Examples: sutures (skull), tibia-fibula joint, teeth-socket joints.
2. Cartilaginous Joints: Bones connected by a layer of cartilage, allowing little movement.
i. Primary (synchondrosis): connected by hyaline cartilage e.g., sternocostal joint (1st rib-sternum).
ii. Secondary (symphysis): connected by fibrocartilage e.g., pubic symphysis, intervertebral discs.
3. Synovial joints: Characterized by a fluid-filled cavity surrounded by a fibrous capsule — the most common and most mobile joint type.
Components: Articular cartilage (smooth frictionless surface), Fibrous capsule (outer ligaments + inner synovial membrane), Synovial fluid (lubricant, secreted by synovial membrane), Ligaments (connect the bones).
Definition: A type of joint characterized by a fluid-filled cavity surrounded by a fibrous capsule.
Classification (6 types by range of motion):
i. Ball-and-socket: motion in all directions e.g., shoulder, hip.
ii. Hinge: movement in one plane like a door hinge e.g., elbow, knee.
iii. Pivot: rotational movement around a single axis e.g., Atlas(C1)-Axis(C2) joint.
iv. Ellipsoidal: movement in two planes, no rotation e.g., wrist-radius joint.
v. Saddle: movement in two planes (concave+convex surfaces) e.g., thumb, sternoclavicular joint.
vi. Gliding (Plane): gliding movements e.g., inter-carpal, inter-tarsal joints.
Joint Transplantation: A surgical procedure in which a damaged joint is replaced with a healthy natural joint (from donor) or an artificial joint.
Types:
1. Total arthroplasty: entire damaged joint replaced with artificial joint (metal, plastic, or ceramic).
2. Partial arthroplasty: only the damaged part is replaced — often used in the knee.
3. Allograft transplantation: healthy joint from a donor — often used in ankle and knee.
4. Chondrocyte implantation: patient's own chondrocytes implanted into damaged joint — often used in the knee.
(a) Bipedal posture adaptations:
• S-Shaped Vertebral Column: distributes weight evenly, maintains balance while standing/walking.
• Broad and Short Pelvis: stabilizes the torso and supports body weight on two legs.
• Angled Femur: angled inward towards knee, keeping body's center of mass over the feet.
• Gluteal Muscles (gluteus maximus/medius/minimus): much larger in humans, stabilize torso and propel body forward.
• Calf Muscles (gastrocnemius, soleus): well-developed, provide power for walking/running.
• Longitudinal Arches of Foot (medial/lateral): absorb shock, distribute weight evenly.
• Short, less prehensile toes: allow the foot to function as a lever during walking/running.
(b) Problems due to Improper Posture:
i. Vertebral Misalignment: back/neck pain, herniated discs.
ii. Joint Strain: strains neck/shoulders/hips/knees, potential arthritis.
iii. Muscle Imbalance: pulls bones/joints out of alignment.
Disc Slip: The intervertebral discs between vertebrae act as shock absorbers and help in movement. 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.
Causes: Trauma, degenerative changes due to aging, repetitive strain on vertebral column.
Symptoms: Pain, numbness, or tingling in the affected area, weakness or loss of muscle function, and in severe cases, bowel or bladder dysfunction.
Spondylosis: Degeneration of vertebrae, intervertebral discs, ligaments or cartilage of vertebral column. May result in narrowing/fusion of intervertebral disc and development of bone outgrowths, putting pressure on nerves or spinal cord. Most common in lumbar and cervical regions.
Causes: Natural degeneration of intervertebral discs, aging, genetic factors, trauma, prolonged poor posture and obesity.
Symptoms: Back or neck pain, stiffness, and reduced range of motion.
Sciatica: Compression or irritation of the sciatic nerve, which starts from the lower back and goes down through the buttocks into each leg.
Causes: Often a herniated disc or bulging disc; also trauma, infection, inflammation, and spondylosis.
Symptoms: Pain or discomfort in the lower back, buttocks, legs, or feet, tingling or numbness, weakness or difficulty moving the legs or feet.
Definition: The inflammation of joints is called arthritis. It includes different inflammatory conditions that affect the joints.
Symptoms: Pain, stiffness, redness, warmth and swelling in affected joints.
1. Osteoarthritis: Most common type. Occurs when articular cartilage at ends of bones gradually softens and disintegrates. Affects knee, hip and intervertebral joints.
2. Rheumatoid arthritis: Result of an autoimmune disorder in which synovial membrane becomes inflamed. Most commonly involves wrist and hands.
3. Gouty Arthritis (Gout): Occurs when there is a build-up of uric acid in blood, forming crystals in joints and causing inflammation. Most common joint affected: big toe; also knees, wrists, fingers.
Osteoporosis: Literally means "porous bones" — a condition characterized by decreased density, amount and thickness of bone tissue, leading to weak, fragile and thin bones.
Causes:
• Aging: bone mass naturally decreases (more pronounced in some individuals).
• Drop in estrogen levels: accelerates bone loss after menopause.
• Lack of calcium and vitamin D: impairs bone health.
• Lack of weight-bearing exercise: leads to weakened bones, especially in post-menopausal women.
• Long-term use of corticosteroids: contributes to bone loss.
• Smoking and alcohol consumption: increase risk.
Treatment: Treated according to cause, e.g., Estrogen replacement therapy (ERT) for drop in estrogen levels in postmenopausal women.
Dislocation (luxation): Abnormal separation or disruption of the normal position of bones at a joint.
Causes: Sudden impact or trauma (severe dislocation can tear muscles, ligaments and tendons); rheumatoid arthritis can also cause joint dislocation.
Symptoms: Swelling, intense pain, and immobility of the affected joint.
Treatment: Surgery may be needed to repair or tighten stretched ligaments. A dislocated joint can only be successfully corrected by a physiotherapist.
Sprain: An injury to the ligaments that connect bones in a joint. Commonly injured ligaments are in the ankle, knee and wrist.
Causes: Joint forced beyond its normal range of motion, causing ligaments to over-stretch or tear.
Treatment: Usually treated with physical therapy; dressing done to immobilize the sprain and provide support.
First aid treatment for dislocation and sprain:
i. Immobilize the affected area — use a sling or splint, don't re-align the joint.
ii. Apply ice — reduces swelling and pain.
iii. Elevate the affected limb — helps reduce swelling.
iv. Seek medical attention — call emergency services (1122) or go to hospital.
Definition: Muscle is the tissue that can contract in a coordinated way to produce movements of body parts or the whole body. Individual cells of muscle are called muscle fibres, myofibres, or myocytes.
| Property | Skeletal Muscle | Smooth Muscle | Cardiac Muscle |
|---|---|---|---|
| Appearance | Regular striped | Un-striped | Irregular striped |
| Cell shape | Spindle/cylindrical | Spindle | Branched |
| Number of nuclei | Many per cell | One per cell | One per cell |
| Voluntary control | Voluntary | Usually no voluntary control | No voluntary control |
| Function | Move skeleton | Move substances | Pump blood through hollow organs |
i. Skeletal Muscles: Move body parts (limbs, trunk, face). Elongated cells with striations, many nuclei per cell. Voluntary muscles.
ii. Smooth Muscles: Present in walls of stomach, intestines, blood vessels, other organs. Spindle-shaped, single nucleus, lack striations. Involuntary.
iii. Cardiac Muscles: Found only in heart walls (myocardium). Fibres branch extensively. Striated like skeletal muscle, but one nucleus per cell located near center. Involuntary.
Muscle fibres (myofibres) are bundles enclosed by collagen fibres and connective tissue. At the ends of a skeletal muscle, collagen and connective tissue form tendons which attach the muscle to bones.
Each skeletal muscle cell (muscle fibre) is a cylindrical multinucleated cell, enclosed by a plasma membrane called sarcolemma. Its cytoplasm is sarcoplasm (organelles, enzymes, glycogen), containing sarcoplasmic reticulum (SR, stores Ca²⁺, no ribosomes). The sarcolemma penetrates deep into the cell to form hollow elongated T-tubules, reaching the ends of SR.
Myofibrils: Each muscle fibre contains 4 to 20 elongated threadlike myofibrils, made of two types of filaments — thick (myosin, creates dark A-bands) and thin (actin, creates light I-bands). These alternating bands give skeletal muscle its striated appearance.
Z-lines: Thin actin filaments attach to protein discs called Z-lines. The section between two Z-lines is a sarcomere, the smallest unit of muscle contraction. Within a sarcomere, thin filaments overlap with thick filaments creating the A-band, with a lighter central H-zone where no overlap occurs. The center of the H-band may have a dark M-line stabilizing the thick filaments.
Thick myofilaments: About 16 nm in diameter, made of many myosin proteins. Each myosin protein has two intertwined polypeptide chains, ending in a globular "head" that extends and connects to actin during contraction.
Thin myofilaments: 7-8 nm in diameter, made of three proteins:
(i) Core of two twisted strands of actin.
(ii) Two strands of tropomyosin wrap about the actin core and stiffen it — in a relaxed fibre, they block myosin binding sites.
(iii) Troponin, present at regular intervals, made of three polypeptides: TnI (inhibitory, binds actin), TnT (binds tropomyosin), TnC (binds calcium ions).
The sliding filament model explains how a muscle contracts — thin myofilaments slide past thick ones so they overlap to a greater degree.
1. Sarcomere at relaxed state: Myosin heads not bound to actin (binding sites blocked by tropomyosin). Myosin heads have hydrolysed ATP into ADP and Pi.
2. Arrival of Nerve Impulse: Impulse travels along sarcolemma to T-tubules then to SR, which releases calcium ions. Ca²⁺ binds troponin, shifting tropomyosin away from myosin-binding sites.
3. Cross-bridges and Power-stroke: Myosin heads bind exposed sites, forming cross-bridges. They release ADP and Pi, undergo conformational change, bend towards sarcomere center, pulling actin filaments — the power stroke. Sarcomere shortens, Z-lines come closer, H-zone disappears.
4. Separation of Myosin Heads from Actin: Myosin head receives a new ATP molecule, detaches from actin. Splitting this ATP resets the head, allowing the cycle to begin again.
Skeletal muscles are attached to bones by tendons. A muscle has two attachment points: origin (attached to stationary bone) and insertion (attached to bone that moves); the middle part is the belly.
Antagonism: For movement of bones at a joint in two directions, muscles work in pairs, producing opposing actions. Flexor: bends the bone at joint. Extensor: straightens the bone at joint. When flexor contracts, extensor relaxes, and vice versa.
Movement at Knee Joint (femur + tibia/fibula):
Flexion (bending) is done by the hamstrings (3 muscles at back of thigh). Origin: pelvic girdle and top of femur; Insertion: upper parts of fibula and tibia.
Extension (straightening) is done by the quadriceps (4 muscles at front of thigh). Origin: ilium and femur; Insertion: patella and tibia. When hamstrings contract, leg bends and quadriceps relax; when quadriceps contract, leg straightens and hamstrings relax.
1. Muscle Fatigue: A decline in muscle performance after prolonged/intense physical activity or disease.
Causes: Low ATP; metabolic waste (lactate, hydrogen ions, reactive oxygen); Ca²⁺ ion handling impairment; damage to muscle fibres from intense exercise.
Symptoms: Pain, decreased muscle strength, reduced endurance. Improves with rest; severe cases need medical attention.
2. Muscle Cramps: Sudden, involuntary, often painful contractions, lasting seconds to minutes, most common in legs/feet.
Causes: Dehydration, salt imbalance, overuse/injury, medications (diuretics), medical conditions (diabetes, liver disease, nerve damage).
Treatment: Gently stretch/massage the muscle; apply heat or cold; pain-relieving medications.
3. Tetany: Involuntary muscle contractions/spasms due to increased muscle tone and nerve hyperexcitability, affecting hands, feet, face or larynx.
Causes: Hypocalcaemia (most common, from vitamin D deficiency), renal failure, thyroid disorders, low magnesium.
Treatment: Depends on cause; calcium/magnesium supplements or IV fluids for salt imbalances.
Rods and levers: 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. 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 acts as the effort, the elbow joint is the fulcrum, and the weight is the load.
Muscles can only pull: 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; 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.
Tap an option to check your answer instantly.
Tap a card to flip it.
Click a topic to explore its key facts from this chapter.