Introduction To The Muscular System
Human Physiology
There are 3 types of muscle tissue found in the human body:1. Smooth
2. Cardiac
3. SkeletalWith over 600 skeletal muscles in the body, there are many shapes, fiber arrangements, and fiber types to consider. However, there are four basic properties common to all skeletal muscle:1. Excitability
2. Contractility
3. Extensibility
4. ElasticityBefore going any further, though, it's important to understand the basic structures which make up the muscle tissue.MicrostructureThe functional unit of muscle tissue is the muscle fiber. You could also think of muscle fibers as the "cells" of our muscle. These "cells" are made up of the following:I. MyofibrilsThese thread-like structures extend the full length of the fiber and are responsible for the "striated" appearance of skeletal muscle; multiple myofibrils make up an individual muscle fiber. Within each myofibril are the myofilaments, arranged into tiny compartments along the myofibril, called sarcomeres. We'll come back to these.II. Intracellular ComponentsSo if the muscle fiber is the cell, the intracellular fluid is called sarcoplasm and its cellular membrane is called the sarcolemma. Within each muscle fiber are thousands of tunnel-like structures extending from the sarcolemma to the center of each fiber called transverse tubules. These are meant for propagating an action potential along the whole fiber as quick as possible.

In the sarcoplasm, we also find large amounts of glycogen and myoglobin. The mitochondria of the muscle fibers are situated in rows, close to the myofilaments which use ATP during contraction.

If you understand the basic components of the muscle fiber, it's also important to understand where these fit in, on a larger scale, with the entire muscle.MacrostructureSo, we know the muscle fiber is the fundamental functional component of muscle tissue, and we've covered some of the microstructures, but what about the macro-level? Here, multiple fibers team up to form what's known as a fascicle. These are distinguished by the type of fibers that make it up, as well as their direction. Fascicles are then grouped together to form the whole muscle tissue.

Muscle tissue is supported by special sheets or bands of connective tissue called fascia. These structures hold together the muscular components that have similar functions, allowing for independent movement as well as carrying nerves, blood vessels, and lymphatic vessels. There are three distinct layers of connective tissue in the muscle:a. Endomysium
b. Perimysium
c. EpimysiumAs a whole, the number of muscle fibers you have, as well as the ratio of type I to type II fibers, generally stays the same throughout your life. When a muscle gets bigger or stronger, it is due to growth in size of fibers rather than number. Likewise, when a muscle weakens or loses size, it is due to a reduction in size of the muscle fibers. This will happen anytime muscle tissue is inactive for an extended period.

Now that we have a basic idea of the structures, small and large, we need to look at the functional aspects of the muscle.MyofilamentsAs previously mentioned, protein filaments are found within each myofibril and organized into distinct sarcomeres. There are two types of filaments in the sarcomere, one "thick" and one "thin":a. Myosin
b. ActinThese filaments overlap each other to a greater or lesser extent. The pattern of this overlap at rest is described by a number of specific zones and bands.

Neuromuscular JunctionThe neuromuscular junction is where the synaptic end bulb of a motor neuron meets the motor end plate on the sarcolemma of a muscle fiber.

Each motor end plate is fitted with millions of acetylcholine (ACh) receptors, which is convenient since nerve impulses from the brain cause the motor neuron to release ACh into the synaptic cleft.
Once ACh binds onto receptors in the motor end plate, a special ion channel opens up, allowing small cations to flow across the sarcolemma into the muscle fiber. The inflow of cations then creates a net positive charge in the proximal portion of the muscle fiber, which is distributed along the fiber by the t-tubules mentioned previously, thereby stimulating the contraction process.

With the general structures understood, and a brief overview of the neuromuscular components, we can now begin to understand specifically how muscle contracts.Sliding Filament TheoryThe sliding filament theory is a model of explaining how muscle contraction actually occurs.Note:the filaments themselves do not change in basic structure during a contraction, but the amount of overlap between them does.

This whole process can be broken down into five phases:a. Resting Phase
b. Excitation-Contraction Coupling Phase
c. Contraction Phase
d. Recharge Phase
e. Relaxation PhaseNeuromuscular CharacteristicsSo you have some basic understanding of how the muscle communicates with the nervous system and what that translates into with respect to functioning, but there are a few miscellaneous things to keep in mind:I. Motor UnitsAlthough each muscle fiber has only one neuromuscular junction, the axon of a motor neuron can branch off many times and form junctions with many different fibers. Therefore, the single motor neuron branch and all the different fibers it innervates are collectively known as the motor unit.

This is an important principle of neuromuscular functioning for a few reasons, which are summarized below:a. Fine vs. Gross Motor Skills
b. "All Or None" Principle
c. Factors of Muscle Tension DevelopmentII. Muscle ToneEven during rest, skeletal muscle exhibits a small amount of tension due to weak, alternating action potentials from a small group of motor units. Although these impulses do stimulate contractions, they happen on a very small scale and do not generate the required force that leads to noticeable movement.

This characteristic of muscle is loosely related to the following principles:a. Length-Tension Relationship
b. Force-Velocity RelationshipIII. Muscle ContractionsThere are basically 3 types of contractions that can be made:a. Isometric
b. Concentric
c. EccentricFurthermore, there are 3 types of muscle fibers which may be called on, depending on the time and number of contractions.a. Type I
b. Type IIa
c. Type IIbAll else the same, the maximum force produced during a contraction is determined by motor recruitment patterns and the cross-sectional area of the muscle itself.IV. ProprioceptionMore or less, proprioception is the muscle's awareness of its own position in space. Proprioceptors are important when it comes to unconscious reflexes. The two important reflexes to remember are:1. Muscle Spindles
2. Golgi Tendon Organs
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