What is the Difference Between Skeletal and Smooth Muscle Contraction

The main difference between skeletal and smooth muscle contraction is that skeletal muscle contraction occurs through the binding of calcium to troponin, whereas smooth muscle contraction occurs through the binding of calcium to calmodulin. Furthermore, the source of calcium for skeletal muscle contraction is intracellular calcium in the sarcoplasmic reticulum while the sources of calcium for the smooth muscle contraction are intracellular calcium in the sarcoplasmic reticulum and extracellular calcium entered into the cell through calcium channels. Moreover, during the skeletal muscle contraction, the release of calcium is initiated by the action potentials generated by neurotransmitters while the release of calcium is initiated by both spontaneous action potential and the action potentials generated by the binding of neurotransmitters to the receptors on the cell membrane during smooth muscle contraction. 

Skeletal and smooth muscle contraction are two of the three types of muscle contraction mechanisms occurring in the body of animals. Generally, they are responsible for the external and internal movements of the body. 

Key Areas Covered 

1. What is Skeletal Muscle Contraction
     – Definition, Mechanism, Importance
2. What is Smooth Muscle Contraction
     – Definition, Mechanism, Importance
3. What are the Similarities Between Skeletal and Smooth Muscle Contraction
     – Outline of Common Features
4. What is the Difference Between Skeletal and Smooth Muscle Contraction
     – Comparison of Key Differences

Key Terms 

Calcium, Calmodulin, Skeletal Muscle Contraction, Smooth Muscle Contraction, Troponin

What is Skeletal Muscle Contraction 

Skeletal muscle contraction is one of the three types of muscle contractions that occur in the animal body. Moreover, these muscles are attached to the bones through tendons. Skeletal muscles are striated muscles whose contraction is regulated by the somatic nervous system. Therefore, the synaptic input of motor neurons which innervate skeletal muscles is responsible for the skeletal muscle contraction. Skeletal muscle contraction is responsible for the external movement of the body parts. Furthermore, the tension-generating sites of the skeletal muscles are muscle fibres. However, a single motor neuron innervates multiple muscle fibres, contracting them at the same time. 

Figure 1: Skeletal Muscle Contraction

Furthermore, in response to an action potential, motor neurons release acetylcholine into the synaptic cleft of the neuromuscular junction. Notably, this depolarizes muscle fibres through the activation of sodium/potassium channels. After that, this depolarization activates the release of calcium ions in the sarcoplasmic reticulum into the cytosol, allowing the binding to troponin C, unmasking the active sites of actin previously blocked by tropomyosin. Ultimately, the binding of myosin to the actin causes skeletal muscle contraction. 

What is Smooth Muscle Contraction 

Smooth muscle contraction is the second type of muscle contractions. It is basically myogenic, same as the contraction of cardiac muscles, which is the third type. This means the contraction of both smooth and cardiac muscles is involuntary and is modulated by the autonomic nervous system. However, their basic mechanism of muscle contraction is similar to that of skeletal muscles. Thus, the depolarization of smooth muscle cells causes the release of intracellular calcium ions into the cytosol from the sarcoplasmic reticulum. However, intracellular calcium ions are not the only source of cytosolic calcium in smooth muscles.

Figure 2: Smooth Muscle Contraction 

Moreover, smooth muscle cells generate their own action potentials by the influx of calcium ions. Therefore, the second source of cytosolic calcium ions is this extracellular calcium. Smooth muscle cells do not contain troponin and tropomyosin. However, cytosolic calcium binds to calmodulin. Also, the elevated levels of calcium-calmodulin complex result in the activation of myosin light-chain complex, which is an enzyme that phosphorylates the light chain in myosin head. After that, the phosphorylated head increases the affinity or the active sites on the thin filaments and binds to them to initiate contraction.  

Similarities Between Skeletal and Smooth Muscle Contraction  

• Skeletal and smooth muscle contraction are the two of three types of muscle contraction responsible for the different types of movements of the body.  
• Generally,  the nerve endings release neurotransmitters in response to an incoming action potential. These neurotransmitters induce the process of calcium release, initiating the contractile process in which the actin-myosin complex slides along with each other.   
• To cease the contraction, calcium pumps back into the sarcoplasmic reticulum. 

Difference Between Skeletal and Smooth Muscle Contraction 

Definition 

Skeletal muscle contraction refers to the activation of the tension-generating sites within skeletal muscle fibres, while smooth muscle contraction refers to the activation of the tension-generating sites within smooth muscles. 

Occurrence 

Moreover, skeletal muscles occur attached to the bones and tendons. In contrast, smooth muscles occur in the stomach, intestines, urinary bladder, uterus, walls of the blood vessels, etc. 

Source of Calcium 

The source of calcium for the skeletal muscle contraction is intracellular calcium in the sarcoplasmic reticulum. However, the sources of calcium for the smooth muscle contraction are the intracellular calcium in the sarcoplasmic reticulum and extracellular calcium entered into the cell through calcium channels. 

Control Mechanism 

During the skeletal muscle contraction, the release of calcium is initiated by the action potentials generated by neurotransmitters while the release of calcium is initiated by both spontaneous action potential and the action potentials generated by the binding of neurotransmitters to the receptors on the cell membrane during smooth muscle contraction.  

Contraction Mechanism 

Moreover, skeletal muscle contraction occurs through the binding of calcium to troponin, while smooth muscle contraction occurs through the binding of calcium to calmodulin. 

Type of Contraction 

Skeletal Muscle contraction is a type of voluntary muscle contraction, while smooth muscle contraction is a type of involuntary muscle contraction. 

Innervation 

The somatic nervous system is responsible for the regulation of skeletal muscle contraction autonomic nervous system is responsible for the regulation of smooth muscle contraction. 

Importance 

Skeletal muscle contraction is important for the external movement of body parts, while smooth muscle contraction is important for the internal movement, which occurs in organs. 

Conclusion 

Skeletal muscle contraction is the type of voluntary muscle contraction regulated by the somatic nervous system. Furthermore, skeletal muscle cells use intracellular calcium to bind with troponin in response to an action potential to contract. In contrast, smooth muscle contraction is a type of involuntary muscle contraction regulated by the autonomic nervous system. However, smooth muscle cells use both intracellular and extracellular calcium to bind to calmodulin to contract. Therefore, the main difference between skeletal and smooth muscle contraction is the mechanism and importance. 

References:

1. “70 10.8 Smooth Muscle.” Anatomy and Physiology, OpenStax, 6 Mar. 2013, Available Here.
2. “Muscle Contraction.” Wikipedia, Wikimedia Foundation, 19 Sept. 2019, Available Here.

Image Courtesy:

1. “1010a Contraction new” By OpenStax (CC BY 4.0) via Commons Wikimedia   
2. “1028 Smooth Muscle Contraction” By OpenStax (CC BY 4.0) via Commons Wikimedia