Written by Kylemaxinne Panzo
Reviewed by Dr. Reuben J C. Los Baños, Ph.D.
Muscles are the tissues all around the body that allow you to move because of their ability to contract. They can be generally categorized into different types based on morphology and function. Smooth muscles, also called non-striated muscles, involve slow and involuntary movement.
Imagine yourself in the gym, you may only be thinking about the skeletal muscles in your arms or legs. But there are also the muscles working hard in your lungs and blood vessels when you breathe. Smooth muscles are essential in hollow organs for such activities.
In the digestive tract, the wave-like motion called peristalsis occurs to propel food. Smooth muscles in the esophagus, stomach, and intestines are responsible for peristalsis. They also mix food and digestive juices into smaller particles to absorb nutrients.
Smooth muscle forms the middle layer of blood vessel walls. Constricting (vasoconstriction) or dilating (vasodilation) these muscles can change their diameter. In this way, regulation of blood pressure is possible.
Smooth muscles in the bronchi and bronchioles also control airflow. Their constriction (bronchoconstriction) triggers coughing and shortness of breath. Relaxing these muscles (bronchodilation) leads to easier breathing in asthma and COPD.
In the urinary bladder, smooth muscles expand as it is being filled. They also excrete toxins and regulate the balance of electrolytes. Smooth muscles are also used in the uterus during pregnancy and the movement of the sperm.
Smooth muscles are also in the eyes and the skin. In your eyes, they function to change the size of the iris and the shape of the lens. In your skin, they cause hair to stand straight when exposed to cold or when in fear causing goosebumps.
All things considered, smooth muscles serve vital purposes all over your body. They help to maintain homeostasis, transport chemicals, and regulate organ function.
What is the difference between smooth and skeletal muscle?
The main difference between smooth and skeletal muscles is in whether we can control them or not.
Smooth muscles are involuntary muscles found within the body, especially in the organs. This helps in proper digestion, nutrition, and the balance of waste and toxins in the body.
At the same time, skeletal muscles are voluntary muscles found in the bones. This aids humans to move, this also helps in posture, balance, and protecting organs in the body.
Smooth muscles often contract slower allowing them to sustain more fatigue. This is due to its pacesetter cells, latch-bridge, and cross-bridges that need low ATP. So, skeletal muscles contract longer due to the force produced by their motor units.
The cells in smooth muscles are non-striated, while skeletal muscles have striations. In the microscope, smooth muscles are not striated since they do not have sarcomeres. The striation of the skeletal muscles is due to its actin and myosin filaments.
Under the microscope, smooth muscles have one nucleus and skeletal muscles have many. Many nuclei allow the skeletal muscles to gain more oxygen to be able to do work and contractions. While single nuclei aid the metabolic demands of the smooth muscle within the body.
The central nervous system controls the skeletal muscle hence allowing voluntary movements. Smooth muscle is involuntary and controlled by the autonomic nervous system. This allows the muscles to perform both movement and metabolism for the body.
A good example of this voluntary and involuntary movement is through running. When we run, the movement of our feet and their pace is a voluntary movement caused by the skeletal muscles. The respiration that we do is an involuntary movement caused by smooth muscles in the lungs.
In conclusion, smooth muscles are non-striated, involuntary muscles found in our organs. While skeletal muscles have striations and we can find them in our bones. Both aid the body with primary functions that are essential for our daily survival.
How does smooth muscle contract?
The contraction of smooth muscles controls the heart, lungs, and other organs. This happens through reactions in our body controlled by electrical and chemical signals. Without this, we would not be able to breathe or metabolize things in our bodies.
Calcium controls the contraction of the smooth muscles. This reaction happens when calcium binds with calmodulin and activates enzymes for contraction. Calcium channel regulation allows our muscles to relax when calcium is not present.
To start contraction, depolarization needs to happen since this opens the calcium channel. When the channels open, calcium ions can enter the system, bind, and activate enzymes. The enzyme is from the activated myosin light chain kinase when a high amount of calcium enters.
The myosin light chain adds a phosphate group to the myosin that allows it to bind with actin. This action will result in the cross-bridge formation of actin and myosin and will turn into a cycle. Hydrolysis of ATP also happens which activates the contraction of smooth muscles.
Contractions are also affected by factors such as tension and length of the muscles. Length-tension relationship allows smooth muscles to contract longer. Since there is less tension because of their loose arrangement within our body.
When our organs are empty smooth muscles maintain it through muscle tone. Powered by the myosin light chain kinase, slow contractions aid muscle toning. This is vital so that organs would not fail their function and return to their normal state when not used.
Smooth muscle contraction happens due to the myosin light chain kinase reaction. Without this, natural reactions of the body such as metabolism will not happen. Thus, if there is no contraction in our smooth muscles, this will affect the state of our body and organs.
What happens when smooth muscles relax?
Our smooth muscles also need to return to their normal state through relaxation. This happens when myosin removes a phosphate group in the myosin light chain kinase. This is essential for regulating blood pressure and metabolism in our body.
To dig deeper, relaxation starts with the reduction of calcium in the gated channels. When there are lower calcium ions binding for conformational changes is not possible. Hence this would not activate calmodulin and the other enzymes of contraction.
To remove the phosphate group, myosin undergoes the myosin light chain phosphatase. This regulates the reduction of actomyosin-based contractility. This allows the contraction to be undone and the smooth muscles of the body to relax
During relaxation, muscle tone also happens and it’s maintained when phosphorylation is low. This relaxation is important for organ systems such as the urinary system. This allows the bladder to relax which is vital in storing urine in the body.
Smooth muscle relaxation is also triggered by hormones and neuron signals. Neurotransmitters such as Nitric oxide diffuse in our muscle cells for it to relax. While hormones like progesterone affect nitric oxide levels for muscle relaxation.
Smooth muscle relaxation is important for the following actions:
- Smooth muscle flexibility: our body needs to adapt to the changes. Continuous contraction of muscles stops actions like breathing and blood pressure control.
- Stimuli Response: Relaxed smooth muscles help in quick cell communication. This allows our body to respond to stimuli immediately.
Relaxation of the smooth muscles has benefits for the body’s health. These health benefits revolve around proper organ function and cell response. Which ensures that our body is well for everyday functions.
Indeed, smooth muscle relaxation is also an important function of the muscles in our body. This happens through various processes from low levels of calcium, to neurotransmitter signals. But, this can also only happen with the aid of contraction in the muscles of the body.
Are smooth muscles multinucleated?
No, smooth muscles are not multinucleated.
When observed under the microscope, you will see a single nucleus within each cell. In comparison, when we observe skeletal muscles we can see that they have many nuclei in their cells.
The smooth muscles also have a cigar-like shape for their nucleus, as seen with the tapered ends of the shape. This shape of the nucleus influences the contraction that happens in the muscle of an organ. Since it helps in the diffusion of ions in the cells and the quick response to chemical triggers.
Besides the shape, the nucleus location of smooth muscle is vital for cell integrity. The presence of the nucleus at the center allows it to adjust to the high metabolic demands of the body. A centered nucleus opens space for more protein production, especially for muscle tissue.
Smooth muscles are homogenous when observed under the microscope because of their nucleus. It is also important that these cells are homogenous as they perform the same function in organs. An example of this is when smooth muscles are in the lining of the lungs which act for respiration.
Additionally, the smooth muscle also has many myosin and actin proteins. This is possible due to the single nucleus of the cells which gives more space for proteins in the cell. Hence these proteins that are key for contraction help the cells to maintain muscle tone.
These smooth muscles can’t have many nuclei since it will permit more fatigue. During contraction and relaxation processes, muscles need a lot of protein. If the cells have many nuclei it won’t be able to produce the amount of protein to sustain the muscle demands.
The single nucleus is also essential for the regulation and adaptation of cells. This allows the efficient regulation of gene expression in the muscles. This is vital for specific functions like digestion, blood pressure changes, and breathing.
Generally, the single nucleus of smooth muscles allows for its cells to communicate. It’s needed for protein production, and gene expression for muscle to handle fatigue. Without it, functions such as breathing and digestion are not possible in the body.
Can we control smooth muscle?
As we know, we can observe the functions of smooth muscles from digestion to breathing. These actions are often involuntary since we cannot control smooth muscles. The autonomic nervous system controls the smooth muscles hence actions are unconscious decisions.
The autonomic nervous system releases hormones and neurotransmitters for smooth muscles. These things serve as a stimulus that the smooth muscle receives. An example is norepinephrine which allows muscle spasms in the vascular walls.
Going deeper, two divisions help the smooth muscle:
- The Sympathetic System: This releases neurotransmitters which are vital for organ wall function.
- The Parasympathetic System: This system controls the muscles needed for organ support.
We cannot control smooth muscles because of neurotransmitters. These act as signals where smooth muscle response for contraction and relaxation. Examples of these are acetylcholine and norepinephrine.
Most of the time organs in the body use these signals to start certain reactions or functions. For example, glandular secretion is only possible when there is a neurotransmitter. Since this sends a signal for the release of these substances.
There are also two types of smooth muscle present in the body. These are the single-unit and the multi-unit smooth muscle. Both have specific and independent functions in the body.
- Single-unit smooth muscles have connexins that allow the connection of many cells. This single connection also allows the one synaptic input of these cells. These are often found in the walls of hollow organs and reaction is through gap junctions.
- Multi-unit smooth muscles do not have a single series for one synaptic input. Each cell of this muscle unit receives the synapse for a more controlled reaction. These are often in the muscles of the eye, arteries, and airways of the lungs.
Besides internal stimuli, there are also external factors that affect smooth muscle function. Things such as temperature, pressure, and chemical composition affect the body. This affects how smooth muscle adapts to maintain homeostasis in the body.
For example, during a drop in temperature, the contraction of smooth muscles slows. This is because of the lack of tension since force is lower when there is a lower temperature. This forces the release of transmitters to balance the change in the environment.
The autonomous nervous system and external factors influence the action of muscles. Since smooth muscles are involuntary, they rely on the triggers in the environment. These triggers allow response so the body and organs remain functional.
What happens when smooth muscles don’t work?
Imagine if one day our smooth muscle stops functioning, what do you think will happen in our body? It is important to understand that the dysfunction of smooth muscles is a threat to our body. It puts our health and our safety at risk since it affects major organs like the heart and the lungs.
Smooth muscles allow respiration in our body, it cannot be dysfunctional. These cases will impair the lungs and hence will lead to respiratory problems. Major examples are asthma-like symptoms since the bronchi and alveolar will be close.
Problems in smooth muscles affect the gastrointestinal tract. Since it handles the lining of the organ that allows muscle toning for digestion. Without it, propelling bolus from what we intake would not be possible.
This can also lead to discomfort and digestive problems. Smooth muscles control the intestine to prevent situations like diarrhea. Smooth muscle problems in the arteries and vessels can also impair the function of the heart.
Smooth muscle dysfunction also affects the urinary and reproductive systems. It can cause urinary incontinence or the unintentional release of urine. It can compromise uterine contraction and fetal delivery.
In conclusion, smooth muscles are crucial for organ function and balance in the body. Compromised muscles lead to complications and it affects many parts of the body. Hence this puts us at risk, especially our health and functionality for every day.
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