Histology of Muscle Notes

Histology of Muscle

Introduction

• Subject: Histology of Muscle

• Presented by: Dr. Samson Dowland

• Institution: University of Sydney

Functional Considerations of Muscle

Role of muscles in bodily functions:

• Movement: Muscles are essential for actions such as changing body position, maintaining posture, chewing, and swallowing. They enable locomotion and the ability to interact with the environment.

• Organ function: Muscles are involved in the size and shape changes of various organs through processes like peristalsis in the digestive system, urination, and childbirth.

• Heart function: The muscles of the heart regulate heartbeat, ensuring proper blood circulation throughout the body, contributing to overall cardiovascular health.

Action initiated by muscle contraction:

• When muscle cells contract, they shorten in length, allowing groups of cells to work together to generate force and facilitate movement, which is crucial for numerous physiological processes.

Types of Muscle

Classification Types

• Muscles are classified based on the histological appearance and structural characteristics of contractile cells.

Striated Muscles:

1. Skeletal Muscle:

   • Primarily responsible for voluntary movements of the body.

   • Attached to bones via tendons and controlled by the somatic nervous system.

2. Cardiac Muscle:

   • Found exclusively in the heart, responsible for involuntary rhythmic contractions.

   • Maintains blood circulation without conscious thought.

Non-Striated Muscles:

1. Smooth Muscle:

   • Responsible for involuntary movements in organs and blood vessels, aiding in processes such as digestion and blood flow regulation.

Skeletal Muscle Structure

• Composition: Skeletal muscle is made up of groups called fascicles, which are further composed of muscle cells, or fibers, held together by connective tissue.

• Muscle fibers: These can vary in length, with some fibers measuring up to 1 meter long, indicating their potential for extensive contraction.

Characteristic Features:

• Peripheral nuclei: Muscle fibers have multiple nuclei located at the periphery, enabling efficient protein synthesis and repair.

• Multinucleate cells: The presence of multiple nuclei is a unique feature that assists with the metabolic demands of skeletal muscles.

• Presence of striations: Striations are visible under a microscope due to the arrangement of myofibrils and the repeating contractile units called sarcomeres.

• Non-branching cells: Unlike cardiac muscle, skeletal muscle cells do not branch.

• Myofibrils: These structures consist of sarcomeres, the functional units of muscle contraction, composed of actin and myosin filaments.

Skeletal Muscle Contraction

Mechanism:

• Neuromuscular junction: The site where motor neurons connect with skeletal muscle fibers.

• Action potentials: When a nerve impulse travels down the motor neuron, it triggers an action potential in the muscle fiber's membrane, called the sarcolemma.

• Calcium Release: This action potential prompts the release of Ca2+ ions from the sarcoplasmic reticulum, which is critical for the contraction of sarcomeres.

Cardiac Muscle Structure

• Composition: Composed of interconnected branching fibers that form a complex network optimized for function.

• Multiple cardiac muscle cells: Cells are joined together, facilitating coordinated contractions necessary for effective heart function.

Characteristics of Cardiac Muscle:

• Myofibrils: Present and organized in a striated pattern, similar to skeletal muscle.

• Centrally located nucleus: Typically, each cell has a single nucleus located in the center, contributing to efficient function.

• Intercalated disks: Specialized junctions that allow for mechanical attachment and ionic continuity, ensuring synchronized contractions and effective heartbeats.

Smooth Muscle Structure

• Composition: Comprised of bundles or sheets of smooth muscle cells arranged in a way that allows for coordinated activity.

Key Features:

• Fusiform-shaped: Smooth muscle cells are spindle-shaped and lack striations.

• Single nucleus: Each cell has one centrally located nucleus, contributing to their simpler structure compared to striated muscles.

• Non-branching structure: Smooth muscle cells appear as a continuous sheet, facilitating peristalsis and propulsion of substances through body systems.

• Gap junctions: Connections between cells that allow ionic continuity and coordinated contractions, important for involuntary movements such as those in the gastrointestinal tract.

Smooth Muscle Contraction

• Types of contraction:

  • Wave-like: Such as in the gastrointestinal tract, facilitating the movement of food through digestion.

  • Whole muscle contraction: Generating extrusive movements, as seen in the urinary bladder and uterus during childbirth.

Muscle in Specific Locations

• Muscle in the tongue: Involved in complex movements essential for speech and swallowing.

• Muscle in the oesophagus: Structures in the upper and lower oesophagus are specialized for transporting food, with distinct muscle types for voluntary and involuntary control.

Key Resources

• Textbook Reference: Histology: A Text and Atlas, 8th Edition, 2018. Authors: Ross & Pawlina.