muscles

• Surrounded by a connective tissue called fascia

• Contains muscle bundles (fascicles)

• Muscle (whole)

• A bunch of muscle fibers (cells)

• Fascicle (muscle bundle)

• Long, cylindrical, multi-nucleated cell

• Surrounded by sarcolemma (plasma membrane)

• Inside is sarcoplasm (cytoplasm)

• Contains myofibrils

• Muscle Fiber (Muscle Cell)

• Made up of repeating units called sarcomeres

• Contain actin (thin) and myosin (thick) filaments

1. Myofibrils

• Light band → has only actin (thin) filaments
  Center has Z-line

• I-band (Isotropic Band):
  

• Dark band → has myosin (thick) filaments
  May overlap with actin

• A-band (Anisotropic Band):
  

• Zigzag line in the center of the I-band
  Anchors actin filaments
  Two Z-lines form a sarcomere

• Z-line:
  

• Central part of A-band with only myosin, no overlap with actin
  Gets smaller during contraction

• H-zone:
  

• Runs through the center of H-zone
  Holds myosin filaments together

• M-line:
  

Actin is the thin filament in the sarcomere. It looks like 2 pearl chains twisted together:

🧩 What is it made of?

• G-actin (Globular actin) = Single round protein unit (like a pearl)

• F-actin (Filamentous actin) = A long chain of G-actins

• Two F-actin strands twist together to form the thin filament

🌊 What else is attached?

• Tropomyosin – A ribbon-like protein that lies along the F-actin chain

• Troponin – A complex of 3 proteins attached to tropomyosin at intervals

⛔ At rest, one subunit of troponin blocks the binding sites on actin so myosin can't bind. This is a safety lock until contraction is needed!

Actin (Thin Filament)

• Meromyosin = Building block of myosin

  • Heavy Meromyosin (HMM) = Head + short arm

  • Light Meromyosin (LMM) = Long tail

🔧 Special features of the myosin head:

• ATPase activity – Breaks down ATP to release energy ⚡

• Has binding sites for ATP and actin

These heads are called cross-arms, and they reach out to the actin like little grabbing hands during contraction.

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• Meromyosin

how it works

• A motor neuron sends an electrical signal to the neuromuscular junction.

• It releases a chemical messenger: Acetylcholine (ACh).

• This creates an action potential in the sarcolemma (muscle membrane).

The Signal Arrives (Nervous system → Muscle)

• The signal spreads and triggers the sarcoplasmic reticulum (Ca²⁺ storehouse) to release Calcium ions into the muscle cell (sarcoplasm)

Calcium Floods In 🚿

• Calcium binds to troponin (on the actin filament).

• This moves tropomyosin away from the binding sites on actin — now the myosin can attach.

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Roadblocks Removed 🚧

• Myosin (engine) uses ATP to get "cocked" and attaches to actin → cross-bridge is formed.

4. Cross-Bridge Formation 🪝

• The myosin head pulls the actin filament towards the center of the sarcomere → contraction!

• ADP + Pi (used-up ATP) are released.

5. Power Stroke 🚂

• A new ATP molecule binds to myosin → cross-bridge breaks.

• Myosin resets, hydrolyzes the new ATP, and reattaches to actin → the cycle continues.

Reset and Repeat 🔁

• When the signal stops, Ca²⁺ is pumped back into the sarcoplasmic reticulum.

• Troponin and tropomyosin block actin again → no binding → muscle relaxes.

• Z-lines return to their original position → sarcomere lengthens.

7. Relaxation 😴

• A band = myosin (thick filament) — doesn't change during contraction.

• I band = actin (thin filament) — shortens during contraction.

• H zone = only myosin (no overlap) — becomes smaller or disappears.

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• Bones: Hard due to calcium salts

• Cartilage: Softer, slightly bendy due to chondroitin salts

🧍‍♀ Humans have 206 bones, divided into:

The skeletal system is like your body’s scaffolding—it gives shape, protection, support, and helps with movement. It is made of:

This is the central axis of the body—head to tail.

a. Skull (22 bones)

• 8 Cranial bones: Protect the brain (cranium)

• 14 Facial bones: Make up your face

• Hyoid: U-shaped bone at the base of the mouth (helps in speech)

• Ear ossicles (3 per ear): Malleus, Incus, Stapes → smallest bones in your body

1. Axial Skeleton (80 Bones)

⚠ The skull joins the spine at the occipital condyles, so it’s a dicondylic skull

• Protect the brain (cranium)

• 8 Cranial bones:

• Make up your face

• 14 Facial bones:

• U-shaped bone at the base of the mouth (helps in speech)

• Hyoid:

• Ear ossicles (3 per ear):

• Ear ossicles (3 per ear): Malleus, Incus, Stapes → smallest bones in your body

Has a neural canal through which the spinal cord passes

Vertebral Column (26 vertebrae)

Sternum: Flat bone at the center of the chest Ribs: 12 pairs

🛡 Together, thoracic vertebrae + ribs + sternum = rib cage

Sternum and Ribs (1 + 24 = 25 bones)

these are your limbs and girdles—the parts that move.

Appendicular Skeleton (126 Bones)

• Humerus (upper arm)

• Radius & Ulna (forearm)

• Carpals (8) – Wrist bones

• Metacarpals (5) – Palm bones

• Phalanges (14) – Fingers

Forelimbs (Arms - 30 bones each)

• Femur – Thigh bone (longest bone in body)

• Tibia & Fibula – Lower leg

• Tarsals (7) – Ankle bones

• Metatarsals (5) – Foot bones

• Phalanges (14) – Toes

• Patella – Knee cap

Hindlimbs (Legs - 30 bones each)

Link Limbs to Axial Skeleton 1. Pectoral Girdle (Shoulder)

Each half has:

• Clavicle (collar bone)

• Scapula (shoulder blade): Has

  • Acromion (ridge for clavicle)

  • Glenoid cavity (socket for humerus)

🪶 Girdles –

Each half is one coxal bone formed by:

• Ilium

• Ischium

• Pubis

All 3 fuse at a socket called acetabulum (for thigh bone).
The two halves join at the front to form pubic symphysis.

2. Pelvic Girdle (Hip)