Difference Between Skeletal and Smooth Muscle Contraction

Key Difference – Skeletal vs Smooth Muscle Contraction
 

Muscles provide a shape to the body and involve in the movement and various other functions of the body. They involve in different activities of the body that are controlled by both voluntary and involuntary controls. There are three main types of muscles namely skeletal muscle, cardiac muscle, and smooth muscle. Skeletal muscles are attached to the skeletal system and smooth muscles are found in the walls of the hollow organs such as stomach, bladder, uterus, etc. During skeletal muscle contraction, a special type of protein called troponin plays an integral part while troponin is not involved with smooth muscle contraction. This is the key difference between skeletal muscle and smooth muscle contraction.

CONTENTS

1. Overview and Key Difference
2. What is Skeletal Muscle Contraction
3. What is Smooth Muscle Contraction
4. Similarities Between Skeletal and Smooth Muscle Contraction
5. Side by Side Comparison – Skeletal vs Smooth Muscle Contraction in Tabular Form
6. Summary

What is Skeletal Muscle Contraction?

In the context of skeletal muscle contraction, all skeletal muscles contract through a series of electrochemical signals that are originated in the brain. These signals pass through the nervous system into the motor neuron that is located in the skeletal muscle fibers. The signal will initiate the muscle contraction process. When describing the structure of the skeletal muscle fiber at its basic level, it is made up of smaller fiber unit that is referred to as myofibrils. Within the myofibrils, special types of contractile proteins are present. These contractile proteins are actin and myosin. They are the most important components of the skeletal muscle when it comes to contraction.

Actin and myosin filaments slide in and out across each other that initiate the muscle contraction process. Therefore, this process is known as ‘sliding filament theory’ due to the sliding of these contractile proteins across each other. There are few important structures that come under the spotlight when describing skeletal muscle contraction. They are myofibril, sarcomere (which is the functional unit of myofibril), actin and myosin, tropomyosin (a protein that binds to actin in the regulation of muscle contraction) and troponin (which is a three-protein complex that is present in the tropomyosin unit).

Initially, a nervous impulse generated by the brain travels through the nervous system into a place referred to as neuromuscular junction. This causes the release of acetylcholine, which is a neurotransmitter. This leads to a state of depolarization. This results in the release of Calcium ions (Ca²⁺) from the sarcoplasmic reticulum. Ca²⁺ binds to troponin which alters its shape and causes the movement of tropomyosin from the actin protein (active site of actin). This phenomenon initiates the binding of myosin (myosin heads) to actin. This forms a cross-bridge between these two contractile proteins. Conversion of ATP to ADP + Pi, releases energy and enables the pulling of actin filaments inwards by myosin. This pulling shortens the muscle.

When an ATP molecule binds to the myosin, it detaches from actin filament and breaks the formed cross bridge. This process takes place continuously until the nervous stimulus stops and adequate amount of ATP and Ca²⁺ present. When the impulse ceases, Ca²⁺ is returned back to the sarcoplasmic reticulum and the actin filament moves to its resting position. This lengthens the muscle to its normal position.

What is Smooth Muscle Contraction?

Smooth muscle contraction occurs as a nervous stimulation and also by humoral stimulation. The whole contraction process could be controlled by means of extrinsic and intrinsic control. Under extrinsic, it is composed of neuronal control and humoral control.  Neuronal control takes place with the presence of sympathetic fibers that control both constriction and relaxation. Relaxation is primarily caused by β adrenergic receptors and contraction is caused by α adrenergic receptors. Under humoral control component, different compounds such as angiotensin II, epinephrine, vasopressin induce the contraction and relaxation.

Local humoral control and myogenic autoregulation take place under the intrinsic control. During myogenic autoregulation, it takes place as a response to spontaneous depolarization and contraction that takes place in the smooth muscle. This regulation system is not present in every smooth muscle of the body, but it is primarily found in blood vessels such as afferent glomerular arteriole. During local humoral control, compounds that are secreted by cells that mimic autocrine and paracrine cells lead to the contraction and relaxation of smooth muscle fibers. These compounds include bradykinin, prostaglandins, thromboxane, endothelin, adenosine, and histamine. Endothelin is considered as the most potent constrictor whilst adenosine is considered as the most abundant vasodilator.

During the smooth muscle contraction, the action potential generated in the sympathetic motor neuron travels and reaches synaptic terminal and causes the induction of Ca²⁺ influx inside the cytoplasm. The increment in Ca²⁺ concentration within the cell leads to the development of conformational changes in the microtubules of the neural cytoskeleton. This causes the release of norepinephrine, which is a neurotransmitter into the interstitial space.

Norepinephrine moves into the smooth muscle cell and binds to a channel receptor that is coupled with a G protein.  This results in the formation of a transmitter receptor complex and the activation of the G protein.  Also, the accumulated Ca²⁺ within the cell leads to the binding with calmodulin and forms the Ca²⁺-calmodulin complex. This complex binds and activates Myosin Light Chain Kinase (MLCK). MLCK involves a phosphorylation reaction which phosphorylates myosin light chain and enables the binding of myosin cross bridge to the actin filaments. This initiates contraction. This process is terminated by the dephosphorylation of the myosin light chain and through the involvement of the enzyme Myosin Light Chain Phosphatase (MLCP).

What are the Similarities Between Skeletal and Smooth Muscle Contraction?

• Both skeleton and smooth muscle contractions depend on the Ca²⁺ concentration.
• Both skeleton and smooth muscle contractions are very important for maintaining body movement and shape.

What is the Difference Between Skeletal and Smooth Muscle Contraction?

Summary – Skeletal vs Smooth Muscle Contraction 

All skeletal muscles contract through a series of electrochemical signals that are originated in the brain. When describing the structure of the skeletal muscle fiber at its basic level, it is made up of smaller fiber units that are referred to as myofibrils. Within the myofibrils, special types of contractile proteins are present. These contractile proteins are actin and myosin. Skeletal muscle contraction is based on the Sliding Filament Theory. During the smooth muscle contraction, an action potential is generated in the sympathetic motor neuron. The whole smooth muscle contraction process could be controlled through means of extrinsic and intrinsic control. Under extrinsic, it is composed of neuronal control and humoral control. Local humoral control and myogenic autoregulation take place under the intrinsic control.

Reference:

1.“Mechanism of Skeletal Muscle Contraction.” MEDCHROME. Available here
2.MEFANET, Czech and Slovak medical faculties network. “Physiology of the smooth muscle contraction.” Physiology of the smooth muscle contraction – WikiLectures. Available here

Image Courtesy:

1.’1008 Skeletal Muscle Contraction’By OpenStax , (CC BY 4.0) via Commons Wikimedia 
2.’Smooth muscle contraction1’By Boumphreyfr – Own work, (CC BY-SA 3.0) via Commons Wikimedia