Introduction
Muscles are contractile tissues of the body derived from the mesodermal layer of embryonic germ cells. They function to produce force and cause motion, either locomotion or movement within internal organs. Muscle is classified as:
- Skeletal
- Cardiac
- Smooth
Cardiac and skeletal muscle cells contain sarcomeres, which give the cells a striated appearance under a microscope. Skeletal muscle cells have long, multinucleated fibers, while cardiac muscles have only one nucleus per cell. The molecular events underlying muscle actions lie in the sliding filament model of muscle contraction. This model is applicable to all contractile tissues.
Skeletal Muscle Cell
Biochemistry Of Muscle Action
- An average adult male is made up of 40-50% skeletal muscle.
- An average adult female is made up of 30-40% skeletal muscle.
- Formed by long, multinucleated, cylindrical cells (fibers).
- Usually attaches to tendons.
- Used to create movement by applying force to bones and joints via contraction.
- Generally contract voluntarily (via somatic nerve stimulation), although they can contract involuntarily through reflexes.
- The whole muscle is wrapped in a special type of connective tissue called epimysium.
Skeletal muscle fibers are grossly divided into two types:
- Slow-twitch (Type I)
- Fast-twitch (Type II): further divided into Type IIa and Type IIb
Slow Twitch (Type I) Muscle Fibres
- Also known as oxidative fibers.
- Primarily utilize fatty acid oxidation.
- Contain a high concentration of mitochondria.
- Store appreciable amounts of oxygen as oxymyoglobin.
- Red in color.
- Capable of continuous, extended contractions.
- Do not fatigue quickly.
Fast Twitch (Type II) Muscle Fibres
- Also known as glycolytic fibers.
- Contain less mitochondria and myoglobin than slow-twitch fibers.
- White muscle fibers.
- Used for short, rapid bursts of energy.
- Fatigue more quickly than slow-twitch fibers.
Type IIa fibers: Intermediate fast-twitch fibers. Utilize both aerobic and anaerobic metabolism for ATP production.
Type IIb fibers: Classic fast-twitch fibers.
Muscle Contraction
Skeletal muscle cells are stimulated by acetylcholine, which is released at neuromuscular junctions by motor neurons. Once the cells are "excited," their sarcoplasmic reticulum releases ionic calcium (Ca2+), which interacts with the myofibrils to induce muscular contraction via the sliding filament mechanism. This process also requires ATP. The ATP is produced by:
- Metabolizing creatinine phosphate and glucose (stored as glycogen or absorbed from blood) within the muscle cells by mitochondria.
- Metabolizing fatty acids obtained from the blood and within the cell.
Each motor neuron activates a group of muscle cells, and collectively the neurons and muscle cells are known as motor units.
Smooth Muscle
Found within the walls of organs and structures such as:
-
Oesophagus
-
Stomach
-
Intestines
-
Uterus
-
Bladder
-
Blood vessels
-
It is not under conscious control.
-
Functions to maintain organ dimensions against forces.
-
Cells are fastened to one another by adherens junctions.
-
Cells are mechanically coupled to one another such that contraction of one cell invokes some degree of contraction in an adjoining cell.
In the intestine, smooth muscles form two distinct layers:
- The circular layer runs around the intestine – its contraction causes segmentation.
- The longitudinal layer runs along the intestine – it causes wave-like contractions.
Cardiac Muscle
- A specialized kind of muscle found only within the heart.
- Cardiac muscle cells are faintly striated, branching, mononucleated cells, which connect by means of intercalated disks to form a functional framework.
- Intercalated discs are anchoring structures containing gap junctions.
- The action potential travels through all cells connected together, forming a functional syncytium in which cells function as a unit.
- It is an involuntary muscle.
- Similar in some fundamental ways to smooth and skeletal muscles.
- Has dual functions of contracting and carriage of action potential (conduction of electricity).
- Some of the cells have the ability to generate action potential (cardiac automaticity).
- As the muscle contracts, it propels blood into the heart and through the blood vessels of the circulatory system.
- The cardiac muscles can contract without external stimulation from hormones or nerves, and it does not fatigue or stop contracting if supplied with sufficient oxygen and nutrients.
