## Muscles This section outlines the structure, function, and types of muscle tissue in the body. Muscles are specialized tissues that contract and relax to produce movement, maintain posture, move body fluids, and generate heat. The study of muscles is known as *myology*. ### General Characteristics of Muscle Tissue - **Contractility:** The ability to shorten forcefully. - **Relaxation:** The ability to return to a resting state. - **Functions:** - Movement of body parts and locomotion (whole-body movement). - Maintaining posture and holding body parts in position. - Movement of body fluids (e.g., blood, contents of the digestive tract). - Heat production. ### Types of Muscle Tissues There are three distinct types of muscle tissue: *smooth*, *cardiac*, and *skeletal*. #### Smooth Muscle - **Appearance:** Non-striated (lacks a striped pattern) because its myofilaments are not organized into sarcomeres. - **Cell Structure:** Spindle-shaped cells, each with a single, centrally located nucleus. Contains non-contractile intermediate filaments. - **Control:** *Involuntary* (not under conscious control). - **Location:** Found in the walls of hollow organs such as the digestive tract, reproductive and urinary tracts, and blood vessels. - **Function:** Controls the movement of substances through these hollow organs. #### Cardiac Muscle - **Appearance:** Striated (striped), similar to skeletal muscle. - **Cell Structure:** Branched cells that join together. Each cell typically contains one centrally located nucleus. - **Control:** *Involuntary*. - **Location:** Found exclusively in the walls of the heart. - **Function:** To pump blood throughout the body. - **Special Features:** Adjacent cells are connected by ***intercalated discs***, which are specialized junctions containing gap junctions. These allow action potentials (electrical signals) to pass rapidly from cell to cell, enabling the heart to contract as a coordinated unit. #### Skeletal Muscle - **Appearance:** Striated with regular, parallel stripes. - **Cell Structure:** Long, cylindrical cells (fibers) that are *multinucleated* (contain many nuclei per cell). - **Control:** *Voluntary* (under conscious control). - **Location:** Attached to bones of the skeleton. - **Function:** Responsible for the movement of body parts and locomotion. For more details on how skeletal muscles work together, refer to . For information on common skeletal muscle disorders, see . ### Comparison of Muscle Tissues | Property | Smooth Muscles | Cardiac Muscles | Skeletal Muscles | | :--- | :--- | :--- | :--- | | **Muscle Appearance** | Unstriped (non-striated) | Irregularly striped (striated) | Regularly striped (striated) | | **Cell Shape** | Spindle | Branched | Spindle or cylindrical | | **Number of Nuclei** | One per cell | One per cell | Many per cell | | **Speed of Contraction**| Slow | Intermediate | Slow to rapid | | **Contraction Cause** | Nervous system, hormones | Spontaneous (intrinsic pacemaker) | Nervous system | | **Function** | Controls movement of substances through hollow organs | Pumps blood | Moves the skeleton | | **Voluntary Control** | No (involuntary) | No (involuntary) | Yes (voluntary) | ### Possible Questions/Answers - **Q:** What are intercalated discs and why are they important? **A:** Intercalated discs are specialized junctions that connect adjacent cardiac muscle cells. They contain gap junctions which allow electrical signals to pass quickly between cells, ensuring the heart muscle contracts in a coordinated, wave-like manner to pump blood effectively. - **Q:** Why is smooth muscle not striated? **A:** Smooth muscle lacks striations because its contractile filaments (myofilaments) are not arranged in the highly organized, repeating units called sarcomeres that are found in skeletal and cardiac muscle. - **Q:** What is the key difference between voluntary and involuntary muscle control? **A:** *Voluntary* control means the muscle can be consciously controlled (e.g., deciding to lift your arm using skeletal muscles). *Involuntary* control means the muscle functions automatically without conscious thought (e.g., the beating of the heart or the movement of food through the intestines).