We have learned about the basic classification of muscles several times from the school days. But still, there is lots of chaos while understanding the correct classification. So, I’ve come up with a small description of the same to come out of the bubble of the chaos.
What is muscle?
A muscle is a group of connective tissues bounded together, which contract to produce a force. Muscles make up most of the fleshy parts of the body and an average of 43% of the body's weight. They produce body movements, maintain posture, and generate heat. They based on several properties that are classified into three major types:
Skeletal muscles,
Cardiac muscles, and
Smooth Muscles.
The body comprises 40% of skeletal muscle and 10% of cardiac as well as smooth muscles.
SKELETAL MUSCLES:
Skeletal muscle is made up of many long thin cells called muscle cells or fibers. They are multi-nucleated, cylindrical, 1-40 mm long 50-100 μm in diameter, and surrounded by the sarcolemma.
As by the name suggests, skeletal muscle is attached to the skeleton and form 'somatic' musculature. They have well-developed cross-striations and are generally under voluntary control. They are contractile i.e. it contracts when excited.
For muscle contraction, they continuously produce ATP. They are categorized into slow-twitch and fast-twitch muscle fibers, which are further, subdivided into Type I, Type IIA, and Type IIB, based on the differences in their contraction speed and ATP production; the physical differences related to these factors also play an important role.
Type IC, Type IIAC, and Type IIAB are also identified by scientists recently.
They are supplied by somatic nerves and by special nerve endings.
Muscle Protein
Myofibrils are built from three kinds of protein:
· Contractile protein:
1. Myosin.
2. Actin
· Regulatory protein:
1. Tropomyosin.
2. Troponin.
· Structural protein:
1. Titin.
2. α - Actinin.
3. Myomesin.
4. Nebulin.
5. Dystrophin.
Sarcotubular system
It is made up of T-system (transverse tubular system) and a longitudinal sarcoplasmic reticulum.
There is the contiguity of the terminal cisternae and the T-system. Transverse tubule with two terminal cisterns is called the Triad structure which is found in the A-I junction and hence, there are two triads per sarcomere.
Properties
They are well classified in three categories discussed below:
Electrical properties
The duration of the action potential is 30-40 msec, initial depolarization of 30 mV produces rapid depolarization and re-polarization.
Mechanical properties
The skeletal muscles perform two types of contraction: isometric and isotonic.
Metabolic properties
Energy utilization is under the basal state is 20% by fats, >60% from carbohydrates, and 20% by proteins. The blood supply is 840 ml/min, with moderate oxygen consumption.
CARDIAC MUSCLES:
Cardiac muscle is an involuntary muscle, present only in the heart. They have well-developed cross-striations and are functionally syncytial. The individual muscle cell is 100 μm long and 15 μm broad. The fibers are branched and interlock freely with each other, but each is a complete unit surrounded by sarcolemma. The muscle fibers are surrounded by a very rich capillary network. They show well-developed sarcoplasmic reticulum with plenty of mitochondria, cytoplasm, and are rich in glycogen.
The muscle fibers are made up of myofibrils, each of which is 1-2 μm in diameter, which lie parallel to each other. It contracts rhythmically and spontaneously in the absence of external innervation due to the presence of pacemaker tissue.
They are supplied via two branches of ANS with ganglia and free nerve terminals.
Muscle Protein
They are as same as in skeletal muscles.
Sarcotubular system
Like the skeletal muscle, the sarcotubular system is well developed in cardiac muscle but 'T-system' penetrates the sarcomere at the Z-line. Therefore, there is only one triad per sarcomere in cardiac muscles.
Properties
Electrical properties
Electrical properties include excitability, autorhythmicity, and conductivity.
Mechanical properties
It includes contractility, all or none law, refractory period, and staircase effect (Treppe).
Metabolic properties
Energy utilization is 60% by fats, 35% by carbohydrates, and 5% by ketones and amino acids. The blood supply is abundant, 250 mL/ min, with high oxygen consumption.
SMOOTH MUSCLES
Smooth muscles are mostly found in hollow viscera. They lack cross-striations, therefore also called plain muscles or unstriated muscles. They are of two types: a single-unit which is functionally syncytial, multi-unit which are functionally non-syncytial. Vascular smooth muscle has properties of both single-unit and multi-unit smooth muscle. These cells are smaller, uni-nucleated, and spindle-shaped with varying dimensions. In general, it contains few mitochondria and depends largely on glycolysis for its metabolic need. They have longer actin and myosin filaments with low myosin ATPase activity than skeletal muscles do.
The sarcotubular system is present, but not so characteristic. The nerve supply is the same as in the cardiac muscles. The rhythmicity is of two types: regular and irregularly discharging pacemaker (asynchronous).
Properties
Electrical properties
The resting membrane potential is unstable, super-imposed with slow sine wave-like fluctuations and pacemaker potential. There are a rapid rise and fall in the action potential.
Mechanical properties
The muscle starts to contract approx. 200 msec. after the start of the spike. The peak contraction is reached approx. 500 msec after the spike. The duration of muscle twitch is approx. 1000 msec.
It shows the property of plasticity.
Metabolic properties
Energy utilization is low, mainly provided by the fats. The blood supply is 350mL/ min, with less oxygen consumption.
References:
Textbook of Medical Physiology, 12/E Guyton & Hall
Ganong Review of Medical Physiology, 23/E
Essential of Medical Physiology, 5/E Sembulingam
APC Dr. A.K. Jain Textbook of Physiology Volume I, 7th ed.
Vanshita Gupta
BPT 1st Year
SGT University, Gurugram, Haryana
Very informative content