MUSCLE TISSUE

OBJECTIVES

Students should be able to:
- Specify and interpret light microscopic sections of stained muscle (e.g. muscle type, structure, fibre type).
- Identify structural features, similarities and differences between various types of muscle fibres.

All cells are capable of some movement; however, several specialized cell types generate force through contraction.
Movement in these specialized contractile cells results from the interaction between two contractile proteins:
- Actin
- Myosin
There are three types of muscle:
- Skeletal Muscle
- Smooth Muscle
- Cardiac Muscle

Function: Responsible for the movement of the skeleton and organs such as the globe of the eye and the tongue.
Characteristics:
- Voluntary muscle (capable of conscious control).
- Appears striated due to the arrangement of contractile proteins.
- Composed of elongated, multinucleate contractile cells, often called muscle fibres.
- Muscle fibres are bound by collagenous supporting tissue.

Motor Control:
- Skeletal muscle contraction is regulated by large motor nerves.
- Individual nerve fibres branch within muscle supplying groups of muscle fibres (motor unit).
Fasciculi:
- Individual muscle fibres are grouped into elongated bundles referred to as fasciculi.
- Endomysium: Delicate supporting tissue between individual muscle fibres.
- Each fascicle is encircled by loose collagenous tissue known as perimysium.
- Whole muscle mass is enveloped in epimysium (dense collagenous sheath).
Blood Supply:
- Large blood vessels and nerves enter through the epimysium and spread through perimysium and endomysium.
- Muscle fibres are anchored to connective tissue for effective force transmission.

Transverse Section of Skeletal Muscle:
- Pink-stained muscle cells appear polygonal with peripherally located nuclei.
- Endomysium contains reticulin fibers, collagen, blood vessels, lymphatics, and nerves.
- Perimysium makes up a collagen structure surrounding individual fasciculi.
Striated Skeletal Muscle:
- Visualization of bundles of muscle fibres; visible components include epimysium, perimysium, endomysium.
Longitudinal Section:
- H&E-stained sections reveal striations in muscle fibers when cut longitudinally.

Characteristics of Skeletal Muscle:
- Large, elongated multinucleated cells (up to 1m in length).
- Nuclei are located circumferentially.
- Voluntary muscle, innervated by alpha motor neurons.
- Arranged in fascicles with endomysium, perimysium, and epimysium.
- Cross-striations are due to the organization of myofibrils.

Function:
- Specialized for continuous, low-force contractions, producing diffuse movements.
- Contractility is an inherent property, often rhythmic and wave-like.
- Modulated by the autonomic nervous system, hormones, and local metabolites in response to functional demands.
- Part of visceral structures (e.g. blood vessels, gastrointestinal tract, uterus, urinary bladder).

Smooth muscle fibres are:
- Elongated, spindle-shaped, shorter than skeletal muscle fibres, and contain a single, centrally located nucleus.
Fibres are arranged in irregular branching fasciculi which represent functional contractile units.
Contractile proteins are not arranged in myofibrils, hence smooth muscle is non-striated.

Longitudinal Section:
- Nuclei are elongated and described as ‘cigar-shaped’ with blunt ends.
- Cytoplasm appears eosinophilic, lacking striations present in skeletal and cardiac muscles.
Transverse Section:
- Cells appear spindle-shaped due to varying sectioning points along their length, inducing discrepancies in apparent diameter.

Duodenum:
- Smooth muscle layers defined as inner circular and outer longitudinal layers of the tunica muscularis.
- Their arrangement aids in propulsive movements (peristalsis) within the digestive tract.
Arteries:
- Smooth muscle arranged in a near-circular helical structure is crucial in controlling blood flow to various organs.

Characteristic features of smooth muscle:
- Small, fusiform shape; single central nucleus.
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