Muscle Types and Features

Overview

This lecture covers the three major types of muscles in the human body, focusing on their structure, function, control, location, and microscopic appearance. Movement is the key concept to associate with muscles, as each type is responsible for different kinds of movement in the body.

Types of Muscles and Locations

• Three major muscle types:
  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle
• Skeletal muscle:
  • Usually attached to bones by tendons.
  • Some are attached to flat, sheet-like tendons called aponeuroses (not always to bones).
  • These are the muscles visible on the body and are responsible for most voluntary movements.
• Cardiac muscle:
  • Found only in the heart.
  • Specialized cells unique to the heart.
• Smooth muscle:
  • Located in the walls of hollow organs (e.g., stomach, bowels) and blood vessels (e.g., aorta).
  • Present wherever there is a hollow space that needs to move substances, such as food or blood.

Muscle Function and Control

• Muscles enable movement throughout the body.
• Skeletal muscle:
  • Responsible for voluntary movements (e.g., walking, running, giving a high five).
  • Movements are under conscious control.
• Cardiac muscle:
  • Controls the heartbeat.
  • Involuntary—operates automatically without conscious thought.
• Smooth muscle:
  • Moves substances through hollow organs and blood vessels (e.g., food in the stomach, blood flow).
  • Involuntary—functions automatically.

Speed of Muscle Action

• Skeletal muscle:
  • Fastest acting muscle type.
  • Enables quick, voluntary movements.
• Cardiac muscle:
  • Intermediate speed—faster than smooth muscle, slower than skeletal muscle.
• Smooth muscle:
  • Slowest to contract and act.
  • Suited for gradual, sustained movements (e.g., moving food through the digestive tract, regulating blood vessel diameter).

Microscopic Structure and Features

• Smooth muscle cells:
  • Spindle-shaped (tapered at both ends, like an almond or eye).
  • Single, central nucleus.
  • Not striated (no visible stripes under a microscope).
• Cardiac muscle cells:
  • Branched structure.
  • One or two central nuclei per cell.
  • Striated (have visible stripes under a microscope).
• Skeletal muscle cells:
  • Long, straight fibers.
  • Many nuclei located at the periphery (edges) of the cell.
  • Striated (striped appearance under a microscope).
• Striated muscle:
  • Only skeletal and cardiac muscle are striated.
  • Smooth muscle is not striated.

Key Terms & Definitions

• Skeletal Muscle: Muscle attached to bones (sometimes via aponeuroses), under voluntary control, responsible for body movement.
• Cardiac Muscle: Muscle found only in the heart, involuntary, branched cells, striated.
• Smooth Muscle: Muscle in hollow organs and blood vessels, involuntary, spindle-shaped cells, non-striated.
• Tendon: Fibrous tissue connecting muscle to bone.
• Aponeurosis: Flat, sheet-like tendon connecting muscles, especially where there is no direct bone attachment.
• Striated Muscle: Muscle with a striped appearance under a microscope (includes skeletal and cardiac muscle).
• Involuntary Muscle: Muscle that contracts automatically, not under conscious control (cardiac and smooth muscle).
• Voluntary Muscle: Muscle moved by conscious decision (skeletal muscle).

Important Points to Remember

• Not all skeletal muscles are attached to bones by tendons; some use aponeuroses.
• Cardiac muscle is unique to the heart and cannot be found elsewhere.
• Smooth muscle is found in the walls of hollow organs and blood vessels, not attached to bones.
• Skeletal and cardiac muscles are striated; smooth muscle is not.
• Skeletal muscle is under voluntary control; cardiac and smooth muscles are involuntary.
• Speed of contraction: Skeletal (fastest), cardiac (intermediate), smooth (slowest).
• Cell structure:
  • Skeletal: long, straight, many peripheral nuclei
  • Cardiac: branched, one or two central nuclei
  • Smooth: spindle-shaped, one central nucleus

Action Items / Next Steps

• Review and memorize the differences between skeletal, cardiac, and smooth muscle for identification.
• Focus on the structural and functional characteristics of each muscle type, especially their control (voluntary/involuntary), location, speed, and microscopic features.
• Pay special attention to highlighted features for quick recall and identification.