anstomy 9/22 Microanatomy (Part 1)

Muscle tissue is one of the four basic types of tissue in the body, alongside epithelial, connective, and nervous tissues.
This section focuses on the microanatomy of muscle tissue, similar to how bone tissue has both micro and macroanatomy.
There are three primary types of muscle tissue:
- Skeletal Muscle: Primarily responsible for voluntary movement and attached to bones.
- Smooth Muscle: Found in the walls of internal organs and blood vessels, responsible for involuntary movements.
- Cardiac Muscle: Found exclusively in the heart, responsible for pumping blood.
All muscle types share the fundamental ability to contract and relax, leading to movement.

Skeletal muscle possesses several distinct properties that enable its function:

Extensibility: The ability of muscle tissue to be stretched or extended beyond its resting length.
Elasticity: The ability of muscle tissue to recoil and return to its original resting length after being stretched.
- Analogy: Similar to a rubber band that can be extended but then recoils to its original shape.
- Clarification: It's important not to confuse extensibility with elasticity. While a muscle can be extended, its elasticity is specifically its capacity to return to normal. The phrase "I'm an elastic person" is anatomically incorrect because elasticity implies returning to normal after stretching, not just stretching itself.
Contractility: The ability of muscle cells to shorten forcibly when stimulated.
Excitability: The ability to respond to a stimulus (e.g., from a nerve impulse).

When a biological tissue, such as a joint, ligament, or tendon, is stretched beyond its elastic limit or extensibility, it can undergo plastic deformation.
This refers to permanent damage or change in shape, where the tissue gets thinner or whitish, similar to how a rubber band loses its recoil property if stretched too far.
This term is commonly used in biomechanics and engineering to describe irreversible structural changes.

Skeletal muscles perform vital roles beyond just movement:

Movement: Facilitate gross bodily movements, such as walking, standing, and manipulating objects. They provide controlled movement, for example, when lowering oneself to the floor.
Guard Entrances and Exits: Skeletal muscles are strategically located at body openings (e.g., orifices of the digestive and urinary tracts) to provide voluntary control over the passage of substances.
Maintain Body Temperature: Muscle contraction generates heat, which is crucial for maintaining core body temperature.
Maintain Posture: Constant, subtle contractions of skeletal muscles stabilize joints and help maintain proper body posture against gravity.

Muscle names often provide clues about their characteristics or functions:

Shape: For example, muscles named 'orbicularis' (e.g., orbicularis oculi, orbicularis oris) indicate a round or circular shape.
Action/Function: Names like 'flexor' (bends a joint), 'extensor' (straightens a joint), 'levator' (elevates), or 'depressor' (depresses) indicate the muscle's primary action.
- Examples:
  - Flexor pollicis brevis: A short muscle that flexes the thumb (pollicis).
  - Extensor pollicis longus: A long muscle that extends the thumb. If there's a 'longus,' there's usually a 'brevis' (short).
  - Orbicularis oculi: Round muscle surrounding the eye.
  - Orbicularis oris: Round muscle surrounding the mouth.
  - Levator labii superioris: Elevates the upper lip (labii indicates lip, superioris indicates upward).
  - Depressor labii inferioris: Depresses the lower lip.
Corrugator Supracilii: A small muscle located above the eyebrow. It draws the eyebrows inferiorly and medially, creating a 'mean' or 'mad' expression. It significantly impacts non-verbal communication, altering the perception of smiles (can appear creepy) or sadness (can appear angry).

Skeletal muscle exhibits a highly organized, hierarchical structure that can be conceptualized as