Microscopic Anatomy of Muscular System Notes PDF

# Microscopic anatomy of the muscular system ## Exercise #12: The Microscopic Anatomy, Organization, and Classification of Skeletal Muscle ### There are three kinds of muscle: 1. Smooth 2. Cardiac 3. Skeletal ### Characteristics of skeletal muscles: 1. Voluntary control 2. Striated 3. Multi-nucle...

# Microscopic anatomy of the muscular system ## Exercise #12: The Microscopic Anatomy, Organization, and Classification of Skeletal Muscle ### There are three kinds of muscle: 1. Smooth 2. Cardiac 3. Skeletal ### Characteristics of skeletal muscles: 1. Voluntary control 2. Striated 3. Multi-nucleated 4. Cylindrical 5. Found in Bundles The cell membrane of the skeletal muscle cell or Fiber is called the **Sarcolemma**, the nuclei of the muscle cells are found under the sarcolemma. The sarcolemma surrounds **Myofibrils**, which are bundles of filaments. Each myofibril is made up of thick **Myosin** and thin **Actin**, which are arranged into subunits called **Sarcomeres**, the functional contractile unit of the muscle cell. The sarcomere is divided into different regions, zones, or bands. * The **A band** is the zone of overlap of actin and myosin. * The **H zone** is an area that contains only myosin. * The **I band** contains only actin. Bundles of myofibrils and the sarcolemma constitute a muscle fiber or cell. This muscle fiber is surrounded by areolar connective tissue, the **Endomysium**. Bundles of muscle fibers are called **Fascicles** or **Fasciculus** and are surrounded by **Perimysium**. Bundles of fascicles which are surrounded by **Epimysium** are called **Muscles**. Epimysium of many muscles combine into denser **Deep Fascia**, which then combine into **Tendons** or **Aponueroses** which bind muscles to bones. Tendons are strong and provide a path for nerves and blood vessels. The **Nueromuscular Junction** is the junction of the muscle and the motor neuron. The **Axon Terminal** is the end of the motor neuron that approaches the muscle fiber. A **Motor Unit** is a motor neuron, and all the muscle fibers it innervates. The **Synaptic Cleft** is a space between the axon terminal and the sarcolemma of the muscle fiber. ## The Muscle Contraction An **Action Potential/ Nerve Impulse** runs down the axon of a motor neuron and reaches the axon terminal, whose end contains vesicles filled with **Acetylcholine**, a neurotransmitter. This neurotransmitter is released into the cleft and binds receptors on the sarcolemma. The membrane becomes permeable to sodium causing a depolarization in the membrane that runs down the **T-tubules** (continuous with the sarcolemma), which is in very close proximity to the **Sarcoplasmic Reticulum** which is filled with calcium. The calcium enters the sarcomere causing a cascade of events leading to a contraction (more about the sequence of events inside the sarcomere, later.) ## Naming Skeletal Muscles 1. Muscles may be named by their direction: Transverse, Oblique, and Rectus. 2. Muscles may be named according to their size: Maximus, Minimus, Longus, and Brevis. 3. Muscles may be named for their location: Extensor carpi ulnaris, named after the bone it is associated with. 4. Muscles may be named be the # of origins: Bicep (2 origins), triceps (3 origins). 5. Muscles may be named by the location of their origin and insertion: Sternocleidomastoid, origin in the sternum and insertion in the mastoid process. 6. Muscles may be named according to their shape: Deltoid (triangular). 7. Muscles may be named according to their action: Adductors. ## Types of muscles * **Prime Movers/Agonists** are muscles primarily responsible for a particular movement. * **Antagonists** are muscles that oppose or reverse the movement of a prime mover, may also be a prime mover. * **Synergists** aid actions of agonists by reducing undesirable or unnecessary movements. * **Fixators** are specialized synergists which immobilize the origin of a prime mover, to put extra tension on the insertion. ## Muscle activity ### Exercise 14: Skeletal Muscle Activity The **Resting Potential** of a muscle cell is between -60 and -90 mV, this is due to the potassium concentration in and out of the cell, which diffuses easily, and the sodium concentration in and out of the cell regulated by pumps. An **Action Potential** occurs when sodium gates open, causing a depolarization after the threshold of -60 to -55 is reached, the polarity then increases to +35, at which time sodium gates close and potassium gates open leading to **Repolarization**. After **Depolarization** the cell gets less negative inside. The **Absolute Refractory Period** is the time when the sodium gates are open but are inactivated and no other action potential can occur. The **Relative Refractory Period** is the time when the sodium gates close and potassium gates open, another action potential may be possible. ## Muscle Contraction After calcium enters the sarcomere it binds to **Troponin**, which is in contact with **Tropomyosin** which blocks the actin filament active site. The calcium and troponin move the tropomyosin away exposing the active site and allowing the myosin head to attach causing a **Power stroke** (similar to a spring being allowed to unwind, releasing its potential energy) of the myosin head, moving the actin towards the center of the sarcomere shortening it. If ATP is available the myosin head will detach (winding the spring), and another contraction may then be possible. ### All or none of muscle fiber contraction A muscle fiber either contracts or it doesn't, not partially. ### Graded Response Is the differing number of muscle cells that can be activated within a muscle according to contractile need. ### Muscle Twitch Is a single contraction of a skeletal muscle fiber. It has three phases, Latent, Contraction, and relaxation. ### Threshold/Subthreshold stimulus Either you have threshold stimulus that leads to a contraction or subthreshold which does not. ### Maximal Stimulus Is the weakest stimulus that will cause all muscle cells in a muscle too contract. ### Multiple Motor Unit Summation or Recruitment Is the increase in contractile strength due to an increase in the number of muscle cells stimulated. It is caused by an increased intensity of stimulation. This is an example of a graded response. ### Treppe Is the staircase effect of progressive muscle twitches increasing in contractile strength, with the same stimulus strength due to heat and calcium. ### Wave Summation Muscles are stimulated with stimulus of the same intensity, but at a greater frequency causing an increase in contractile strength, this is due to the muscle fiber still being contracted. ### Tetanus Is stimulation at such a rate/frequency that no relaxation occurs, a sustained contraction. This is an extreme example of wave summation. ### Muscle Fatigue Is the loss of ability for the muscle cell to contract. This results from depletion of ATP and oxygen debt which leads to lactic acid buildup. ### Isometric contractions Are contractions in which muscle length doesn't change, although the force generated can. ### Isotonic contractions Are contractions in which muscle length changes, but forces generated stay the same. ### Tonus Is the constant state of slight tension muscles maintain for readiness. ### Electromyography Is the detection, amplification, and recording of skin voltage due to skeletal muscle contraction. ### Electromyograph/electromyogram Is the recording obtained. ### Work Is the application of muscle force generated by muscle contraction needed to move an object. ### Dynamometry Is the study of power. A Dynogram is the recording obtained.