Dominic Stone
There are three types of muscle tissue in the human body: skeletal, smooth, and cardiac. Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. Under a microscope, sarcomeres are visible along muscle fibers, giving a striated appearance to the tissue. The two types of striated muscle are skeletal muscle and cardiac muscle. Smooth muscle, therefore, is the type of muscle that is not striated.
| Characteristics | Values |
|---|---|
| Types | Smooth muscle, Cardiac muscle |
| Appearance | Smooth muscles are fusiform, or spindle-shaped |
| Location | Smooth muscles are located in the walls of visceral organs like the liver, pancreas, and intestines. Cardiac muscles are located in the walls of the heart. |
| Control | Smooth and cardiac muscles contract and are controlled involuntarily. Skeletal muscles are under voluntary control. |
| Function | Smooth muscles help with digestion and nutrient collection, and play a role in the urinary system and the regulation of blood pressure and tissue oxygenation. Cardiac muscles pump blood throughout the body. |
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What You'll Learn
Smooth muscle fibres are located in the walls of visceral organs
Smooth muscles are non-striated muscles that consist of thick and thin filaments that are not arranged into sarcomeres. They are located in the walls of hollow visceral organs, such as the liver, pancreas, intestines, and urinary bladder, and are also found in the skin and eyes. These muscles are spindle-shaped and have a single nucleus, with individual cells ranging in size from 30 to 200 μm. Smooth muscle fibres often form sheets of tissue and function in a coordinated manner due to the presence of gap junctions between the cells. This type of muscle is termed unitary smooth muscle or visceral muscle and is the most common type observed in the human body, forming the walls of hollow organs.
The function of smooth muscle in organ systems is incredibly broad, and smooth muscle contributes to many different tissues throughout the body. One important function is the regulation of blood flow and pressure via vascular resistance. Smooth muscle helps control the diameter of blood vessels, which can have a significant impact on pathology if affected by vascular pathologies. For example, continuous activation of vascular smooth muscle has been linked to the formation of pulmonary issues.
Smooth muscle can be stimulated by pacesetter cells, the autonomic nervous system, hormones, or by stretching. The stimulation of smooth muscle by neurons differs from skeletal muscle in that it does not form highly organized neuromuscular junctions. Instead, there is a series of neurotransmitter-filled bulges called varicosities that release neurotransmitters over a wide synaptic cleft. Smooth muscle can also undergo hypertrophy to increase in size, and it can divide to produce more cells through a process called hyperplasia.
Smooth muscle has greater elastic properties than striated muscle, which is important in organ systems like the urinary bladder, where contractile tone must be preserved. Single-unit smooth muscle, found in the walls of the viscera, has a stress-relaxation response that allows the muscle to stretch, contract, and relax as the organ expands. This type of smooth muscle can maintain low-level contractions for long periods due to latch-bridge cross-bridges that cycle slowly without requiring ATP.
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Cardiac muscles are striated and found in the heart
There are three types of muscle tissue in the human body: cardiac, smooth, and skeletal. Cardiac muscles, also known as myocardium, are striated and found in the walls of the heart. They are responsible for the contractility of the heart, which leads to the pumping action that propels blood throughout the body.
Cardiac muscle cells, or cardiomyocytes, are striated, branched, and contain many mitochondria. Under a microscope, the repeating functional units called sarcomeres are visible along the muscle fibres, giving striated muscle tissue its distinctive appearance. Sarcomeres are also present in skeletal muscle, enabling contraction in both types of muscle tissue. However, unlike skeletal muscle, cardiac muscle cells are under involuntary control, responding to signals from the autonomic nervous system to adjust the heart rate.
The contractile functions of the heart require ATP, which can be obtained from various substrates, including fatty acids, carbohydrates, proteins, and ketones. Cardiac muscle contractions are due to a myogenic response of the heart's pacemaker cells. These cells have autorhythmicity, and their set intervals for depolarizing and firing action potentials determine the heart rate. The pacemaker cells transfer depolarization to other cardiac muscle fibres through gap junctions, allowing them to contract in unison and generate sufficient force to pump blood into circulation.
While cardiac muscle is essential for the heart's function, it has a limited ability to regenerate compared to skeletal muscle. This is due to the absence of satellite cells, which play a crucial role in the regeneration process of skeletal muscle tissue. Any structural changes or defects during the heart's development can lead to congenital heart disorders.
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Skeletal muscles are attached to the bones
There are over 600 skeletal muscles in the human body, and they vary in size, shape, and arrangement of fibres. These fibres are flexible and range from less than half an inch to just over 3 inches in diameter. Each muscle may contain thousands of fibres, and they usually span the entire length of the muscle. The fibres contract, allowing the muscles to move bones and enabling a wide range of movements and functions.
The contractile part of a skeletal muscle consists of its fibres, and it also contains a non-contractile part of dense fibrous connective tissue that forms the tendons at each end. Skeletal muscles include skeletal muscle fibres, blood vessels, nerve fibres, and connective tissue. Each muscle is surrounded by a layer of connective tissue called fascia, and the outermost layer of tissue surrounding the entire muscle is called the epimysium.
The strength of a skeletal muscle is determined by its cross-sectional area, with the strongest muscles typically being those with the largest cross-sectional area. The strongest muscle in the body is usually considered to be the quadriceps femoris or the gluteus maximus.
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Smooth muscles have greater elastic properties than striated muscles
Smooth muscles are one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. Smooth muscles are non-striated, meaning they do not have sarcomeres and therefore do not exhibit striations or stripes under a microscope. They are found in the walls of hollow visceral organs, such as the liver, pancreas, intestines, stomach, bladder, and uterus, as well as in the walls of blood and lymph vessels. Smooth muscles are involuntary muscles, meaning their contractions are not under conscious control.
In contrast, skeletal muscles, also known as striated muscles, are the most common type of muscle in the human body. They are attached to the skeleton and are responsible for various functions, including breathing, eating, and movement. Skeletal muscles are voluntary, allowing conscious control over their contractions.
Cardiac muscles, found in the walls of the heart, are also striated and involuntary. They play a crucial role in pumping blood throughout the body through their contractions.
The unique property of smooth muscles that sets them apart from striated muscles is their greater elastic properties. Smooth muscle cells secrete an extracellular matrix containing collagen, elastin, glycoproteins, and proteoglycans, contributing to the viscoelasticity of the surrounding tissues. This elasticity is vital for certain organ systems, such as the urinary bladder, where maintaining contractile tone is essential. Additionally, in the cardiovascular system, smooth muscles help maintain blood pressure and flow, and in the lungs, they open and close airways.
The higher elastic properties of smooth muscles compared to striated muscles can be attributed to the absence of sarcomeres in smooth muscles. Sarcomeres are functional units composed of actin and myosin myofilaments that create the striated appearance in skeletal and cardiac muscles. Smooth muscles, on the other hand, consist of thick and thin filaments that do not form sarcomeres, resulting in a non-striated pattern and greater elasticity.
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Skeletal muscles are controlled by the peripheral portion of the CNS
The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system. The CNS includes the brain and spinal cord, while the peripheral nervous system consists of everything else. The somatic nervous system is a component of the peripheral nervous system and is associated with the voluntary control of body movements via skeletal muscles. It is responsible for functions that we can consciously influence, such as moving our arms and legs. The somatic nervous system consists of both afferent (sensory) and efferent (motor) nerves. The peripheral nervous system delivers command signals from the brain to the muscles, allowing us to move and perform tasks.
The skeletal muscle is the most common type of muscle in the body and is under voluntary control. Skeletal muscles are striated muscles, which means they have a striped appearance due to repeating functional units called sarcomeres. These sarcomeres are visible along muscle fibres under a microscope. Skeletal muscles are attached to the skeleton and consist of flexible muscle fibres that contract, enabling movement, breathing, and posture maintenance. Each muscle can contain thousands of fibres, and these fibres usually span the length of the muscle.
The basic motor pathway involves upper motor neurons in the precentral gyrus (primary motor cortex), which sends signals through the spinal cord to lower motor neurons. These signals travel through the ventral horn of the spinal cord and synapse with the lower motor neurons, sending signals through peripheral axons to the neuromuscular junction (NMJ) of skeletal muscle. The axons of motor neurons are found in skeletal muscles and regulate voluntary and involuntary reflexes.
Therefore, skeletal muscles are controlled by the peripheral portion of the CNS, specifically through the somatic nervous system and its efferent nerves, which carry signals from the CNS to the skeletal muscles, allowing for voluntary movement.
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Frequently asked questions
Striated muscles are muscles that feature repeating functional units called sarcomeres. Under a microscope, sarcomeres are visible along muscle fibres, giving a striated appearance to the tissue.
Smooth muscles are not striated.
Smooth muscles are located in the walls of hollow visceral organs such as the liver, pancreas, and intestines. They are also found in the stomach, urinary system, and throughout arteries and veins.
Smooth muscles are described as fusiform, which means they are round in the centre and taper at each end.
Smooth muscle contraction is dependent on calcium influx. Calcium enters the smooth muscle cell through depolarisation, hormones, or neurotransmitters. Once calcium has entered the cell, it binds with calmodulin, activating the enzyme myosin light chain kinase (MLCK).
