Muscle and Bone Tissue

Description of Muscle and Bone Tissue (Include Sarcomere and Osteon)

Muscle tissue and bone tissue are essential components of the human body that work together to provide structure, movement, and support. Muscle tissue is specialized for contraction, allowing movement of the body and its parts. Bone tissue provides support, protection, and a framework for the body.

• Muscle Tissue Types:

  • Skeletal muscle: Voluntary and striated, attached to bones by tendons, responsible for body movement.

  • Cardiac muscle: Found only in the heart, striated and involuntary; cells are branched and interconnected.

  • Smooth muscle: Involuntary, non-striated, lines walls of internal organs.

• Functional Unit of Skeletal Muscle: Sarcomere

  • The sarcomere is the basic contractile unit of muscle fibers, defined by Z-lines. It contains thick filaments (myosin) and thin filaments (actin, troponin, tropomyosin).

  • During contraction, the sarcomere shortens as actin filaments slide over myosin filaments.

• Bone Tissue:

  • Osteon (Haversian system): The basic structural unit of compact bone, consisting of a central canal surrounded by concentric rings (lamellae) of calcified matrix.

  • Contains osteocytes housed in lacunae, connected by canaliculi for nutrient exchange.

Example: The femur contains many osteons for strength and support, while skeletal muscle fibers contain thousands of sarcomeres for contraction.

Bone Growth and Development

Compare and Contrast Bone Growth (Fetal vs. Adult)

Bone growth occurs through ossification, where cartilage is gradually replaced by bone tissue. In childhood and adolescence, growth occurs at the epiphyseal plate (growth plate) until maturity.

• Fetal Bone Growth: Begins in the primary ossification center (middle of bone diaphysis), then secondary centers develop in the epiphyses.

• Adult Bone Growth: Focuses on bone remodeling and repair, not length increase. Remodeling involves osteoblasts (build bone) and osteoclasts (break down bone).

Example: Growth plate fractures in children can affect future bone length, while adults may experience osteoporosis due to imbalanced remodeling.

Muscle Contraction

Sliding Filament Theory & Neuronal Signaling

Muscle contraction begins with a signal from the nervous system, which leads to the shortening of sarcomeres inside muscle fibers. The Sliding Filament Theory explains contraction as the interaction between actin and myosin filaments.

• Sliding Filament Theory:

  • Myosin heads bind to actin, forming cross-bridges.

  • ATP hydrolysis powers the movement, pulling actin filaments inward.

  • Calcium ions (Ca2+) released from the sarcoplasmic reticulum enable binding by exposing active sites on actin.

• Neuronal Signaling:

  • Motor neuron releases acetylcholine (ACh) at the neuromuscular junction.

  • ACh binds to receptors on the muscle membrane, causing depolarization and action potential propagation.

  • Action potential triggers Ca2+ release, initiating contraction.

Example: Lifting a weight involves motor neuron activation, sarcomere shortening, and ATP-driven myosin-actin interaction.

Bundle Within Bundle: Muscle Organization

Muscle Fiber Organization

Muscles are organized in layers known as a "bundle within a bundle." The entire muscle is surrounded by the epimysium, which encloses bundles of fascicles. Each fascicle is wrapped in perimysium and contains multiple muscle fibers (cells). Each muscle fiber is further wrapped in endomysium and contains myofibrils, which are made of myofilaments (actin and myosin) that perform contraction.

Anatomy Topics

Muscled and Bones of the Face

• Parts of a Vertebra:

  • Body

  • Vertebral foramen

  • Spinous process

  • Transverse process

  • Lamina

  • Pedicle

• Ends and Parts of Long Bones (6 total):

  1. Proximal epiphysis

  2. Distal epiphysis

  3. Metaphysis (growth plate area)

  4. Diaphysis (shaft)

  5. Articular cartilage (joint surface)

  6. Medullary cavity (center containing marrow)

Bone Types

Bone Coverings

• Periosteum: Outer covering with osteoblasts for growth and repair.

• Epimysium: Surrounds entire muscle for protection and structure.

Thick vs. Thin Filaments

Osteon and Its Parts

• Central (Haversian) canal: Contains blood vessels and nerves

• Lamellae: Rings of calcified matrix

• Lacunae: Small spaces housing osteocytes

• Canaliculi: Channels connecting osteocytes for nutrient exchange

• Perforating (Volkmann's) canals: Connect central canals horizontally

Muscle Physiology

Sarcomere Parts (for drawing practice)

• Z-line

• I-band

• A-band

• H-zone

• M-line

• Actin (thin filament)

• Myosin (thick filament)

Sliding Filament Theory – Steps Summary (7–12 Steps)

1. Nerve impulse reaches neuromuscular junction.

2. ACh released and binds to muscle membrane.

3. Na+ enters → depolarization.

4. SR releases Ca2+.

5. Action potential travels down T-tubules.

6. Ca2+ binds to troponin → tropomyosin shifts.

7. Myosin binds actin → cross-bridge forms.

8. Power stroke uses ATP to pull actin inward.

9. ATP binds to myosin → detachment.

10. ATP splits → myosin re-cocks.

11. Ca2+ pumped back into SR.

12. Muscle relaxes.

Neuronal Signaling – Steps (1–6 Steps)

1. Nerve impulse travels down motor neuron.

2. ACh released into synaptic cleft.

3. ACh binds to receptors on muscle fiber.

4. Na+ influx causes depolarization.

5. Action potential spreads along sarcolemma and T-tubules.

6. SR releases Ca2+.

7. Muscle contraction begins.

Muscle Twitch and Fiber Types

• Type I (slow-twitch): High endurance, more mitochondria, used in posture.

• Type IIa (fast oxidative): Moderate endurance, quick response.

• Type IIb (fast glycolytic): Powerful, fatigues quickly.

How Muscles Get Stronger

Muscles do not divide but grow larger through hypertrophy. This means each fiber increases the number of myofibrils, mitochondria, and enzymes. Consistent exercise and tension lead to stronger contractions and greater force production.

Additional info: The notes cover topics from Ch. 6 (Bones and Bone Structure), Ch. 10 (Muscle Tissue), and Ch. 11 (The Muscular System), with supporting details from Ch. 4 (Tissue Level of Organization).