Muscles of the Body

Main Chewing Muscle

The primary muscle responsible for chewing is the masseter. It is one of the strongest muscles in the human body and plays a crucial role in mastication.

• Masseter: Elevates the mandible to close the jaw.

• Buccinator: Assists in chewing by keeping food between the teeth but is not the main chewing muscle.

• Lateral pterygoid: Assists in opening the jaw and moving it side-to-side.

• Hyoglossus: Involved in tongue movement, not chewing.

Example: The masseter can be felt when clenching the jaw.

Boxer's Muscle

The serratus anterior is known as the "boxer's muscle" because it is responsible for the protraction of the scapula, which is essential for punching movements.

• Serratus anterior: Protracts and stabilizes the scapula.

• Rectus abdominis, biceps brachii, flexor digitorum longus: Not primarily involved in boxing movements.

Muscle Contraction and Structure

Muscle contraction is influenced by several factors, including the number and size of muscle fibers stimulated, the load on the fiber, and muscle length.

• Number of muscle fibers stimulated: More fibers result in stronger contractions.

• Size of muscle fibers: Larger fibers produce more force.

• Load on the fiber: Heavier loads require greater force.

• Muscle length: Optimal length allows maximal force generation.

Muscle Cell Structure

Each muscle cell (fiber) is surrounded by a connective tissue layer called the endomysium.

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Surrounds bundles of fibers (fascicles).

• Epimysium: Surrounds the entire muscle.

Muscle Groups and Actions

• Quadriceps femoris: Composed of three vastus muscles and the rectus femoris.

• External oblique: Pulls ribs downward, compresses abdominal wall.

• Thigh adductors: Involved in "pulled groin" injuries.

• Hamstrings: Involved in crossing legs (cross-legged position).

Bones and Skeletal System

Bone Structure and Periosteum

Bones are covered by a protective layer called the periosteum. The inner (osteogenic) layer consists primarily of osteogenic cells, which are responsible for bone growth and repair.

• Periosteum: Secured to underlying bone by perforating (Sharpey's) fibers.

• Osteogenic cells: Stem cells that differentiate into osteoblasts.

Bone Growth and Ossification

Bone growth occurs at the epiphyseal plate during childhood and adolescence. Premature closure can be caused by elevated sex hormones or other factors.

• Epiphyseal plate: Region of growing tissue near the ends of long bones.

• Growth hormone: Most important for bone growth during infancy and childhood.

• Ossification: Endochondral ossification uses cartilage as a model for bone formation.

Bone Markings and Joints

Bones have various markings that serve as attachment points or passageways for nerves and blood vessels.

• Foramen: Round or oval opening through a bone.

• Meatus: Canal-like passageway.

• Sutures: Immobile joints between skull bones (e.g., squamous suture).

Axial vs. Appendicular Skeleton

The skeleton is divided into axial and appendicular components.

• Axial skeleton: Skull, vertebral column, rib cage.

• Appendicular skeleton: Limbs and girdles; the skull is not part of this division.

Special Bones and Features

• Hyoid bone: Only bone not articulating with any other bone.

• Sphenoid bone: "Key stone" of the skull, articulates with all other cranial bones.

• Zygomatic bone: Commonly referred to as the cheekbone.

• Temporal bones: House special sense organs.

Bone Tissue Types

Bones are composed of different types of tissue adapted for various functions.

• Compact bone: Dense, strong, supports weight.

• Spongy bone: Porous, found at ends of long bones.

• Hyaline cartilage: Forerunner of long bones in embryo.

Physiology of Muscle Contraction

Sliding Filament Model

Muscle contraction occurs via the sliding filament model, where actin and myosin filaments slide past each other.

• Actin and myosin: Slide past each other and partially overlap during contraction.

Equation:

Muscle Contraction Periods

• Refractory period: Time when muscle cannot respond to a new stimulus.

• Fatigue period: Muscle loses ability to contract efficiently.

Myoglobin

Myoglobin is a protein in muscle cells that stores oxygen, facilitating aerobic respiration.

Clinical and Applied Anatomy

Osteoporosis

Osteoporosis in older women is often due to estrogen deficiency after menopause, leading to decreased bone density.

Rigor Mortis

Rigor mortis occurs after death because no ATP is available to release attached actin and myosin molecules, causing muscles to stiffen.

Muscle Injuries

• Pulled groin: Involves thigh adductors.

Summary Table: Key Bone and Muscle Features

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Key terms and concepts have been defined and grouped logically for study purposes.