Chapter 9 part 3 Study Guide

Muscle Tissue: Comparative Anatomy and Physiology

Gross and Microscopic Anatomy of Smooth vs. Skeletal Muscle Cells

Muscle tissue is classified into three main types: skeletal, cardiac, and smooth. Here, we focus on the structural differences between smooth muscle and skeletal muscle at both the gross and microscopic levels.

• Skeletal Muscle: Composed of long, cylindrical, multinucleated cells (muscle fibers) with obvious striations due to the organized arrangement of actin and myosin filaments. These muscles are usually attached to bones and are under voluntary control.

• Smooth Muscle: Consists of spindle-shaped, uninucleate cells lacking visible striations. The filaments are arranged less regularly, resulting in a smooth appearance. Smooth muscle is found in the walls of hollow organs (e.g., intestines, blood vessels) and is under involuntary control.

• Microscopic Features:

  • Skeletal muscle: Striated, multiple peripheral nuclei, organized sarcomeres.

  • Smooth muscle: Non-striated, single central nucleus, no sarcomeres, dense bodies anchor actin filaments.

• Gross Features:

  • Skeletal muscle: Forms large, discrete muscles attached to the skeleton.

  • Smooth muscle: Forms sheets or layers within organ walls.

Example: The biceps brachii (skeletal muscle) vs. the muscular wall of the intestine (smooth muscle).

Contractile Mechanisms and Activation: Skeletal vs. Smooth Muscle

The mechanisms by which muscle cells contract and are activated differ significantly between skeletal and smooth muscle.

• Skeletal Muscle:

  • Contraction is initiated by action potentials from motor neurons (neuromuscular junction).

  • Calcium ions bind to troponin, causing a conformational change that allows actin-myosin interaction.

  • Contraction is rapid and can be precisely controlled (voluntary).

• Smooth Muscle:

  • Contraction can be initiated by neural, hormonal, or local chemical signals.

  • Calcium ions bind to calmodulin (not troponin), which activates myosin light chain kinase (MLCK), leading to phosphorylation of myosin and subsequent contraction.

  • Contraction is slower, sustained, and involuntary.

Equation (Skeletal Muscle Contraction):

$\text{Ca}^{2+} + \text{Troponin} \rightarrow \text{Conformational Change} \rightarrow \text{Actin-Myosin Binding} \rightarrow \text{Contraction}$

Equation (Smooth Muscle Contraction):

$\text{Ca}^{2+} + \text{Calmodulin} \rightarrow \text{MLCK Activation} \rightarrow \text{Myosin Phosphorylation} \rightarrow \text{Contraction}$

Example: Skeletal muscle contracts to move limbs; smooth muscle contracts to move food through the digestive tract.

Unitary vs. Multi-Unit Smooth Muscle: Structure and Function

Smooth muscle is further classified based on how its cells are organized and functionally connected.

• Unitary (Visceral) Smooth Muscle:

  • Cells are connected by gap junctions, allowing coordinated contraction as a single unit.

  • Found in the walls of hollow organs (e.g., intestines, uterus).

  • Exhibits spontaneous rhythmic contractions (peristalsis).

• Multi-Unit Smooth Muscle:

  • Cells function independently, with little or no gap junctions.

  • Each cell is innervated by a nerve ending, allowing fine control.

  • Found in the iris of the eye, arrector pili muscles of the skin, and large airways.

Table: Comparison of Unitary and Multi-Unit Smooth Muscle

Feature	Unitary Smooth Muscle	Multi-Unit Smooth Muscle
Cell Coupling	Extensive gap junctions	Few or no gap junctions
Contraction	Coordinated, as a single unit	Independent, precise control
Location	Viscera (e.g., GI tract, uterus)	Iris, large airways, piloerector muscles
Innervation	Autonomic, few fibers per cell	Each cell innervated

Example: Peristalsis in the intestines (unitary) vs. pupil dilation (multi-unit).