Muscular System Overview PDF

**Overview of Muscle Tissue** The body has three primary types of muscle tissue, each responsible for specific functions: 1. **Skeletal Muscle**: Attached to bones, responsible for voluntary skeletal movements. 2. **Cardiac Muscle**: Found in the heart, responsible for pumping blood thr...

**Overview of Muscle Tissue** The body has three primary types of muscle tissue, each responsible for specific functions: 1. **Skeletal Muscle**: Attached to bones, responsible for voluntary skeletal movements. 2. **Cardiac Muscle**: Found in the heart, responsible for pumping blood throughout the body. 3. **Smooth Muscle**: Found in the walls of hollow organs (e.g., blood vessels, bladder, uterus), responsible for involuntary movements like changing the shape of organs. All muscle types share certain properties: - **Extensibility**: Ability to stretch or extend. - **Contractility**: Ability to contract and generate force. - **Elasticity**: Ability to stretch and return to its original shape. - **Excitability**: Ability to respond to stimuli, generating an electrical signal (action potential). **Structural Differences** - **Skeletal Muscle**: Striated appearance, multinucleate cells. - **Cardiac Muscle**: Striated appearance, single nucleus per cell. - **Smooth Muscle**: Non-striated appearance, single nucleus per cell. **Functions of Muscles** 1. **Movement**: Muscles generate force that moves bones and body parts. 2. **Posture**: Muscles resist gravity to maintain body position. 3. **Stabilizing Joints**: Muscles prevent excessive movement, maintaining joint integrity. 4. **Homeostasis**: Skeletal muscles help generate heat (e.g., shivering during cold conditions). 5. **Other Functions**: - Control movement of substances (e.g., swallowing, urination, defecation). - Protect internal organs (e.g., abdominal muscles protect vital organs). **Skeletal Muscle Anatomy** Skeletal muscles are organs composed of muscle fibers (cells), blood vessels, nerves, and connective tissue. They are organized into layers: 1. **Endomysium**: A thin layer of connective tissue that surrounds individual muscle fibers. 2. **Perimysium**: A connective tissue layer that groups muscle fibers into bundles called fascicles. 3. **Epimysium**: Dense connective tissue that surrounds the entire muscle, providing structural integrity and separation from other tissues. **Muscle Contraction and Relaxation** The functional unit of muscle contraction is the **sarcomere**, which is made of two main proteins: - **Myosin**: Thick filaments. - **Actin**: Thin filaments. When a muscle is stimulated by an action potential, calcium ions are released from the sarcoplasmic reticulum (endoplasmic reticulum of muscle cells). This allows **myosin** to bind to **actin**, using ATP to pull the actin filaments toward the center of the sarcomere, causing the muscle fiber to shorten (contract). **Muscle Fiber Interactions** - **Tendons**: The connective tissue layers (endomysium, perimysium, epimysium) merge with tendon collagen, transferring muscle tension to bones. - **Attachments**: Skeletal muscles attach to bones via tendons at **two points**: - **Insertion**: The muscle attachment to the bone that moves. - **Origin**: The attachment to the stable, fixed bone. **Roles of Muscles in Movement** 1. **Prime Mover (Agonist)**: The main muscle responsible for a particular movement (e.g., biceps brachii for elbow flexion). 2. **Synergist**: Muscles that assist the prime mover in producing the desired movement (e.g., brachialis and brachioradialis assist the biceps brachii). 3. **Fixator**: Muscles that stabilize the origin of the prime mover (e.g., deltoid stabilizes the shoulder during arm movement). 4. **Antagonist**: Muscles that oppose the action of the prime mover (e.g., triceps brachii is the antagonist to the biceps brachii during elbow flexion). **Special Muscles** - **Facial Muscles**: Some skeletal muscles (e.g., those controlling facial expressions) are attached to skin rather than bones. - **Sphincter Muscles**: Skeletal muscles like those in the anal and urinary sphincters regulate voluntary functions such as urination and defecation. - **Diaphragm**: Skeletal muscle involved in respiration, changing the volume of the thoracic cavity without moving bones. **Summary of Key Points** - **Muscle Types**: Skeletal, cardiac, and smooth muscle each have unique structures and functions. - **Muscle Function**: Movement, posture, joint stabilization, heat production, and protection of organs. - **Skeletal Muscle Structure**: Comprised of fibers organized into fascicles, wrapped in connective tissues (endomysium, perimysium, epimysium). - **Contraction Mechanism**: Involves the sliding of myosin and actin filaments within sarcomeres, triggered by electrical signals and calcium ion release. - **Movement Mechanics**: Involves coordinated action of agonists, synergists, antagonists, and fixators to produce smooth, controlled motion. By understanding the structure, function, and coordination of muscles, we gain insight into how the body generates movement and maintains homeostasis. **Cardiac Muscle** Cardiac muscle is specialized tissue found only in the heart. It plays a crucial role in pumping blood throughout the body. Key characteristics of cardiac muscle include: 1. **Structure**: - Like skeletal muscle, cardiac muscle has **striations** and **sarcomeres**. - **Shorter fibers**: Cardiac muscle fibers are shorter than skeletal muscle fibers. - **Single nucleus**: Typically, each cardiac muscle cell contains one nucleus, located centrally. - **Branched fibers**: Cardiac muscle fibers are branched and interconnect at their ends by **intercalated discs**. 2. **Intercalated Discs**: - These are specialized structures in the cardiac muscle cell membrane that facilitate coordinated contractions. - **Gap junctions** within the intercalated discs allow the direct transmission of **action potentials** from one cardiac muscle cell to the next, ensuring synchronized contraction. - **Desmosomes** are also found in intercalated discs and help **anchor** the ends of muscle fibers, preventing them from pulling apart during contraction. 3. **Contraction Control**: - Cardiac muscle contractions are **involuntary** and controlled by specialized cardiac muscle cells called **pacemaker cells**. - Pacemaker cells are **self-excitable**, meaning they can generate an action potential independently, setting the pace for the heartbeat. - **Autonomic nervous system** and **hormones** can influence the activity of pacemaker cells, adjusting the heart rate as needed for maintaining **homeostasis** (e.g., to regulate **blood pressure**). **Smooth Muscle** Smooth muscle is located in the walls of various **hollow organs**, such as the digestive, cardiovascular, respiratory, urinary, and reproductive systems. It is also found in other tissues like the iris of the eye and the skin. Key features of smooth muscle include: 1. **Structure**: - **Spindle-shaped fibers**: Smooth muscle cells are wider in the middle and taper at both ends. - **Single nucleus**: Each smooth muscle fiber has a single nucleus. - **Non-striated appearance**: Unlike skeletal and cardiac muscle, smooth muscle lacks striations because it does not have sarcomeres. 2. **Contractile Proteins**: - Smooth muscle fibers contain **actin** and **myosin**, which are the primary proteins responsible for contraction. - **Dense bodies**: Actin and myosin filaments are anchored to structures called **dense bodies**, which are attached to the cell membrane. - During contraction, myosin pulls on actin, causing the **dense bodies** to be pulled inward. This action also pulls on **intermediate filaments**, resulting in a **corkscrew-like contraction** that shortens the muscle cell and causes the cell to bulge in the middle. 3. **Contraction Control**: - Similar to cardiac muscle, smooth muscle contractions are **involuntary**. - **Hormones** and **autonomic nervous system stimulation** are the main triggers for smooth muscle contraction. The contraction can be initiated in response to changes in the environment, such as the need to constrict blood vessels or move food through the digestive tract. **Summary of Key Differences** - **Cardiac Muscle**: - Found only in the heart. - Striated with intercalated discs. - Self-excitable pacemaker cells control contraction. - Involuntary control. - **Smooth Muscle**: - Found in walls of hollow organs and other tissues (e.g., eyes, skin). - Non-striated. - Contraction is regulated by hormones and autonomic nervous system. - Involuntary control. Both muscle types (cardiac and smooth) are crucial for essential involuntary functions, such as pumping blood and moving substances through organs, but they differ in structure and the mechanisms by which their contractions are controlled. **KEY TERMS** **actin** protein found in muscle fibres **action potential** change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal unique to neurons and muscle fibres **agonist** (also, prime mover) principle muscle involved in an action **antagonist** muscle that opposes the action of an agonist **cardiac muscle** striated, involuntary muscle found in the heart, joined to one another by intercalated discs and, under the regulation of pacemaker cells, contract as one unit to pump blood around the body. **contractility** ability to shorten (contract) forcibly **dense body** structure attached to the cell membrane of smooth muscles that are connected to myosin, actin and intermedia filaments **desmosome** cell structure that anchors the ends of cardiac muscle fibres together **elasticity** ability to stretch and rebound **endomysium** loose connective tissue covering each muscle fibre in a skeletal muscle **epimysium** outer layer of connective tissue around a skeletal muscle **excitability** ability to undergo neural stimulation **extensibility** ability to lengthen (extend) **fascicle** bundle of muscle fibres within a skeletal muscle **fixator** muscle that assists an agonist by preventing or reducing movement at another joint, thereby stabilising the origin of the agonist **gap junction** structure that allows communication to occur between cells **insertion** end of a skeletal muscle that is attached to the structure (usually a bone) that is moved when the muscle contracts **intercalated disc** part of the cell membrane that connects cardiac muscle cells and contains gap junctions and desmosomes **intermediate filament** type of cytoskeletal filament made of keratin, characterised by an intermediate thickness that plays a role in resisting cellular tension **myofibril** long, cylindrical organelle composed of sarcomeres that run parallel within muscle fibres **myosin** protein found in muscle fibres **origin** end of a skeletal muscle that is attached to another structure (usually a bone) in a fixed position **perimysium** connective tissue that bundles skeletal muscle fibres into fascicles within a skeletal muscle **prime mover** (also, agonist) principle muscle involved in an action **sarcomere** longitudinally, repeating functional unit of skeletal muscle, with all the associated proteins involved in muscle contraction **skeletal muscle** striated, multinucleated, voluntary muscle that requires signalling from the nervous system to trigger contraction and move bones to produce movement **smooth muscle** nonstriated, mononucleated muscle located in the skin and walls of internal organs, blood vessels, and internal passageways **striations** banded patterns formed by the alignment of parallel contractile proteins, actin and myosin, in cardiac and skeletal muscles. **synergist** muscle whose contraction helps a prime mover in an action