4- Muscle Microanatomy.docx

- **Muscle Tissue: General Info** - Muscles are specialized cells capable of contraction to produce movement which is utilized for locomotion, propulsion, and pressure regulation. - Muscle cells can be referred to as myocytes or myofibers or muscle fibers....

- **Muscle Tissue: General Info** - Muscles are specialized cells capable of contraction to produce movement which is utilized for locomotion, propulsion, and pressure regulation. - Muscle cells can be referred to as myocytes or myofibers or muscle fibers. - Muscle cells are spindle shaped and originate from the mesoderm (from myoblasts). - The **sarcoplasm** is the cytoplasm of muscle cells that contains glycogen and myoglobin. - The **sarcolemma** is the plasma membrane of the muscle cell. - The **sarcoplasmic reticulum** is the highly specialized smooth endoplasmic reticulum of a muscle cells that regulates calcium flow. - The sarcoplasmic reticulum stores and releases calcium ions to initiate muscle contractions. - **Skeletal Muscle: General Info** - The diameter of skeletal muscle cells varies from 10-110micrometers, and the length varies from 40mm-50cm. - Muscle cells are cylindrical in shape and are multinucleated. - Skeletal muscle makes up 50% of the body weight, with most skeletal muscles inserting on bones of the skeleton. - The tongue and masseter muscles are some of the strongest muscles of the body. - Skeletal myocytes contain multiple, peripherally (hypolemmal) located nuclei. - Hypolemmal refers to being located underneath the sarcomere, rather than being perfectly centered. - Striations can be seen when skeletal myocytes are cut into longitudinal sections. - Skeletal muscle will contract fast and is under voluntary control. - Voluntary sphincters in the GI tract, urinary system, as well as the muscles in the esophagus and tongue, are all composed of skeletal muscle. - The **terminal cisterna** expands the ends of the sarcoplasmic reticulum. - The (**transverse) T-tubule** is a deep tubular, finger-like invagination of the sarcolemma (plasma membrane) of skeletal muscle that creates an anastomosing network of tubules. - These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell, allowing calcium release from the sarcoplasmic reticulum. - **Skeletal Muscle: Development** - **Myotubes** are multinucleated tubes that are formed due to mesenchymal cells (myoblasts) aligning and fusing together. - Myotubes differentiate forming functional myofilaments and the nuclei are displaced against the plasma membrane. - Some cells do not differentiate and remain as mesenchymal stem cells which are called "**satellite cells**" that function in muscle repair. - Satellite cells are positioned between the basal lamina and sarcolemma of the muscle cell. - They retain mitotic potential which allows them to accomplish some repair. - **Fibroblasts** also form connective tissue (scars) as part of the repair process. - **Skeletal Muscle: Organization** - The **epimysium (AKA: fascia)** are a layer of dense irregular connective tissue that surround the muscles. - Multiple bundles of fascicles make up a whole muscle. - **Perimysium** is a dense connective tissue that surrounds each fascicle (or a group of cells, in other words). - Each **fascicle** is made up of many muscle cells. - **Myofibers** contain cylindrical bundles of myofibrils. - Myofibrils contain many smaller bundles of myofilaments. - **Myofibrils** are the contractile element within skeletal muscle cells, as they are composed of actin (thin) and myosin (thick) myofilaments. - The thick and thin myofilaments give skeletal myocytes the appearance of having light and dark banding (striations). - The **A band** references the dark bands. - The A bands are composed of actin and myosin. - The **I band** references the light bands. - The I bands are composed of just actin. - The I bands are bisected by a protein disc known as the **Z-line** which actin filaments are anchored to. - 1 sarcomere is located from 1 Z-line to the next Z-line. - Actin and myosin are contractile proteins, while tropomyosin is a regulatory protein. - Myofilaments can only be seen with an electron microscope. - Individual myocytes are surrounded by reticular fibers forming the **endomysium**. - The sarcomere is the contractile or functional unit of the myocyte. - The sarcomere includes all elements from one Z-line to the next Z-line. - **Skeletal Muscle: Contraction** - Motion is mediated by muscle cells and is based on the conversion of chemical energy (ATP) into mechanical energy. - Myofibrils are composed of repeating assemblies of thick and thin filaments constituting dark A and pale I segments. - During muscle contractions, the actin filament slides over the myosin filament, resulting in a shortening of the I band. - Actin and myosin are held in position in the myofibril by other proteins such as desmin, tropomyosin, and troponin. - The summation of all sarcomeres shortening produces contractions of a muscle cell. - **Sliding Filament Model** - Step 1: Each sarcomere shortens, while the myofilament length is constant. - Step 2: The I band shortens and almost disappears. - Step 3: The thin (actin) filaments slides past the thick (myosin) filaments. - **Skeletal Muscle: Fiber Types** - **Type 1 (slow)** - Type 1 fibers compose red muscle which have a slow twitch, uses aerobic metabolism, and are fatigue resistant. - **Type 2** - Type 2 fibers compose white muscle that uses anaerobic metabolism and can be further divided into type 2A and 2B. - **Type 2A (intermediate)** uses mixed oxidative-glycolytic metabolism and is slow fatiguing. - **Type 2B (fast)** is fast contracting, fast fatiguing, and uses glycolytic metabolism (meaning that it depends on glycogen for energy). - **Cardiac Muscle: General Info** - Cardiac muscle cells are arranged into fibers, with each cell only containing one central nucleus. - Cardiac cells are cross striated and has intercalated discs that contain gap junctions and desmosomes. - **Intercalated discs** attach cardiac muscle cells to each other, providing strength and the ability to function as a syncytium. - A **true syncytium** is a multinucleated cell, such as a skeletal muscle cell. - Intercalated discs consists of a transverse element and a longitudinal element. - **Transverse elements** act as an anchor, such as desmosomes, and serve for strong attachment. - **Desmosomes** are fascia adherens and/or macula adherens. - **Longitudinal elements** aid in communication, such as gap junctions (nexus) with ion channels, and propagate electric impulses. - Cardiac muscle cells have a sarcoplasmic reticulum, and mitochondria, with the mitochondria making up 20% of the cellular volume. - The high quantity of mitochondria means that the cell requires a lot of oxygen. - Each cardiac muscle cell is 100micrometers long, with a diameter of 15micrometers. - Cardiac cells can be referred to as cardiomyocytes or myocardiocytes or cardiac myocytes. - Cardiomyocytes can be either contractile or conductile. - **Contractile** cardiac muscle cells are red. - **Conductile** cardiac muscle cells (Purkinje fibers) are pale pink. - **Purkinje cells** are neuronal cell bodies in the middle layer of the cerebellar cortex and are characterized by a large, globose body and massive branching dendrites but a single slender axon. - **Purkinje fibers** are modified cardiac muscle fibers in the subendothelial tissue, concerned with conducting impulses through the heart. - Modified cardiac muscle cells form the cardiac nodes and fibers involved in impulse conduction. - **Smooth Muscle: General Info** - Smooth muscle cells have a fusiform spindle shape, and they are surrounded by a basal lamina and reticular fibers - Smooth muscle cells have a single central nucleus. - Smooth have parasympathetic and sympathetic innervation, causing it to have involuntary contractions. - Smooth muscle cells are 6-8micrometers in diameter, and are typically 20-40micrometers in length. - However, in the uterus, smooth muscle cells can get up to 1mm in length. - Smooth muscle cells can act as a single unit or multi-unit. - **Single (unitary) smooth muscle** - They are found in visceral organs and have sparse innervation, yet cells communicate via multiple gap junctions. - In single-unit smooth muscle cells, the cells behave like a syncytium where they are contracting in a network. - **Multi-unit smooth muscle** - They are found in the iris of eyes and have precise contractions. - Multi-unit smooth muscle has individual innervations of each monocyte, and they lack gap junctions, causing them to function individually. - Smooth muscle functions include: peristalsis, vascular dynamics, propulsion, and secretion. - Peristalsis describes a wave-like contraction that is present within the GI tract. - Vascular dynamics describes contractions altering blood flow which is important for blood pressure. - Smooth muscle cells practice propulsion in the urinary bladder and uterus. - Smooth muscle cells play a minor role in secretion. - The walls of luminal organs contain "**tunica mucosa**" (mucous membrane) and "**tunica submucosa**". - Smooth muscle cells utilize dense bodies. - Dense bodies are the equivalent of Z discs within skeletal and cardiac muscle. - Dense bodies have an oval (cigar-shaped) nucleus. - While in a contracted state, the oblique arrangement of actin/myosin filaments and intermediate filament arrangement, allows the entire cell to shorten by 80% (which is more sustained than striated cells)- resulting in a corkscrew appearance of the nucleus. - **Contractile: Non-muscle Cells** - **Myoepithelial cells** - Myoepithelial cells have an ectodermal origin and contain actin and myosin. - These cells are similar to smooth muscle. - They can be stimulated by hormones such as those produced by the mammary gland. - They are basket-like shaped and are known as "basket cells". - Myoepithelial cells are located in salivary, mammary, and lacrimal glands. - They are enclosed in clusters of glandular cells. - **Myofibroblasts** - Myofibroblasts are from the mesoderm and are involved in wound contraction. - **Muscle Regeneration** - Cardiac muscle cells lack the ability to regenerate. - Smooth muscle regeneration is limited. - In addition to some mitotic activity, new smooth muscle cells may be derived from pericapillary mesenchymal cells. - Skeletal muscle has limited regeneration capacity - In all muscle types, repair is completed by scar tissue formation and requires an intact basal lamina.