Muscle Series Part 1: Skeletal, Smooth, and Cardiac Muscle
In this first episode of a three-part series we’ll be reviewing the fundamental muscle types in the human body
By Amirali Banani
Movements of the body and body parts result from the contraction of muscle tissue, the only tissue in the body that is specialized for contraction (shortening)
Skeletal
contractions can be voluntary
Skeletal muscles are composed of skeletal muscle cells and connective tissue. These muscles extend from one bone across a joint to another bone, and consists of two ends: the origin and the insertion. The origin is the immovable attachment, and the insertion is the movable attachment. The skeletal muscles are attached to bone by tendons (fibrous connective tissue). Bursae — synovial-lined sacs containing synovial fluid — are situated between some tendons and underlying bones and assist in movement by lubricating the joints to reduce friction between soft tissue as they pass over skeletal prominences.
2 Fibre Types
When we are born, we are genetically endowed with certain ratios of 2 muscle fibre types:
Slow-twitch (S-T): Skeletal muscle fibres that have a steadier “tug” or “power” and more endurance despite having a fewer number of muscle units with a smaller number of muscle fibres. These muscle fibres are useful in endurance sports such as running, biking, swimming or jogging. Most of their energy is developed aerobically and usually only tire when they use up their fuel or oxygen supply. S-T are characterized as having many mitochondria and contain the blood pigment myoglobin and are consequently “red” in colour. S-T fibers have a low maximum tension but are highly resistant to fatigue.
Fast-twitch (F-T): Skeletal muscle fibres that tend to be anaerobic and seem designed for strength and power because their motor units contain many muscle fibres. They are used for sports such as weightlifting, sprinting, swinging a golf club or shot put. These fibers are characterized as being “white” in colour because they lack mitochondria and myoglobin. Consequently, these muscles can “tire” easily because they build up lactic acid more quickly. Remember that lactic acid occurs when glucose is burned by the body without oxygen to obtain its energy.
Location: Attached to bones by tendons (hence the name “skeletal”). Skeletal muscles are found across the body in the tongue, diaphragm, eye socket, limbs, upper esophagus, etc.
Control: Voluntary
Distinguishing characteristics: Cylindrical, multi-nucleated, and striated
Smooth
comprises the bulk of hollow visceral structures in the body
2 Fibre Types
Thick filaments: Myofilaments that are composed mainly of myosin. This can be seen through dark bands on a sarcomere.
Thin filaments: Myofilaments that are composed mainly of actin, along with troponin and tropomyosin. These can be seen primarily through the light bands on a sarcomere. By interacting with myosin, actin helps generate force and ultimately produce muscle contraction.
Location: In the walls of the stomach, intestines, bladder, uterus, blood vessels, lymphatic vessels, etc.
Control: Involuntary
Distinguishing Characteristics: Spindle-shaped, single nucleus, lack striations, centrally located nucleus
Cardiac
composes the bulk of the heart
2 Types of Cells
Myocardial Contractile Cells: comprise the bulk of the heart (99% of cells in the atria and ventricles) and, as its name suggests, are responsible for the muscular contractions of cardiac fibres in the heart that pump blood throughout the body.
Myocardial Conduction Cells: make up the remaining 1% of cells in the heart to form the cardiac conduction system. Specifically, they’re responsible for initiating and propagating the action potentials that travel through the nodes of the heart and their accompanying conduction pathways, triggering the contractions that propel blood.
Location: Heart
Control: Involuntary
Distinguishing Characteristics: Striated, branched, many mitochondria
Thank you for reading part 1 of this three-part series on muscles! Feel free to check out parts 2 and 3 below:
