I wanted to research the topic of smooth muscle function because I was sure it could shed some light on the digestive and urinary problems of many an MSer. However, the anatomy and function of smooth muscles is a very complex subject and I can only hope to provide a potted summary of this biological exploration.
Had I been to university and acquired an honours degree in biology, I may have been able to provide greater clarity. But, I think that at the tender age of 62 with over twenty years of MS under my belt, I may have left university life a little too late.
So, what is smooth muscle?
I started this biology investigation with the knowledge that the walls of the bladder are made from smooth muscle.
Smooth Muscle Function
Before we can get into the smooth muscle function specifics, we must consider human musculature in general.
In the human body, there are roughly 850 muscles which make up 50% of the body weight. These muscles are grouped into three muscle types: skeletal muscles, cardiac muscles and smooth muscles.
These are the muscles you can see in your arms and legs. They provide movement, maintain posture, stabilise joints and produce heat.
When most people speak of muscles or think about muscles they mean skeletal muscles. These are the muscles we work out when we are at the gym.
As well as helping you to walk and run. It is skeletal muscles that hold your body shape and posture. They also maintain your joint integrity.
Exist only in the heart, as the name would suggest.
Unlike skeletal muscle, cardiac muscles never tire and work constantly, under the control of the autonomic nervous system.
This muscle type is found in hollow organs and the vascular system. And are the main subject of this post.
Smooth Muscle Cell
At a cellular level, smooth muscle can be described as an involuntary non-striated muscle. Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres, giving it a non-striated pattern.
The shape of smooth muscle is described as fusiform, which is described as being round in the centre and tapering at each end. Smooth muscle can tense and relax but has greater elasticity than striated muscle.
Smooth Muscle Location
Where in the human body, can we find smooth muscle? The short answer is, almost everywhere.
Smooth muscle can be found in all of the organ systems below:
- Gastrointestinal tract
- Cardiovascular – blood vessel and lymphatic vessels
- Renal – urinary bladder
- Genital – uterus, male and female reproductive tracts
- Respiratory tract
- Integument – erector pili of the skin
- Sensory – ciliary muscle and iris of the eye
The main function of smooth muscle is contraction.
Smooth muscle consists of two types: single-unit and multi-unit. Single-unit smooth muscle consists of multiple cells connected through connections that can be stimulated in a synchronous pattern from only one synaptic input.
The function of smooth muscle can be expanded on a much larger scale to the organ systems it helps to regulate.
The basic functions of smooth muscle in the organ systems are listed below.
- Gastrointestinal tract – propulsion of chyme
- Cardiovascular – regulation of blood flow and pressure via vascular resistance
- Renal – regulation of urine flow
- Genital – contractions during pregnancy, propulsion of sperm
- Respiratory tract – regulation of bronchiole diameter
- Integument – raises hair with erector pili muscle
- Sensory – dilation and constriction of the pupil as well as changing lens shape
Smooth Muscle Contractions
You will find that smooth muscle (so-named because the cells do not have striations) is present in the walls of hollow organs like the urinary bladder, uterus, stomach, intestines, and in the walls of passageways, such as the arteries and veins of the circulatory system, and the tracts of the respiratory, urinary, and reproductive systems.
Smooth muscle fibres are spindle-shaped (fusiform) and have a single nucleus; they range from about 30 to 200 μm (thousands of times shorter than skeletal muscle fibres), and they produce their own connective tissue, endomysium.
While smooth muscle contraction relies on the presence of Ca++ ions, smooth muscle fibres have a much smaller diameter than skeletal muscle cells.
Muscle contraction continues until ATP-dependent calcium pumps actively transport Ca++ ions back into the SR and out of the cell. However, a low concentration of calcium remains in the sarcoplasm to maintain muscle tone.
Because most smooth muscles must function for long periods without rest, their power output is relatively low, but contractions can continue without using large amounts of energy.
Smooth muscle is not under conscious control; thus, it is called involuntary muscle.
The triggers for smooth muscle contraction include hormones, neural stimulation by the ANS (Autonomic Nervous System), and local factors.
Smooth muscle tissue, unlike skeletal or cardiac tissues, does not have clearly defined striations visible on the cells. This is because smooth muscle cells are organized in a different way than other muscle cells.
As seen in the image below, the actin and myosin filaments in smooth muscle are arranged in a stacked pattern across the cell. This “staircase” arrangement of actin and myosin is much different from the structure in skeletal and cardiac muscle.
The actin filaments (red lines) in smooth muscle run from one side of the cell to the other, connecting at dense bodies and at the cell membrane. In skeletal and cardiac muscle, the actin filaments are attached to Z plates, which hold many actin filaments and show up as dark bands under the microscope.
Neuromuscular Junction (NMJ)
The neuromuscular junction (NMJ) is a synaptic connection between the terminal end of a motor nerve and a muscle (skeletal/ smooth/ cardiac). It is the site for the transmission of action potential from the nerve to the muscle.
This is muscle tissue that is marked by transverse dark and light bands, is made up of elongated usually multinucleated fibres, and includes skeletal muscle, cardiac muscle, and most muscle of arthropods.
Vascular Nervous System
Vascular nerves are nerves that innervate arteries and veins. The vascular nerves control vasodilation and vasoconstriction, which in turn lead to the control and regulation of temperature and homeostasis.
These nerve functions fall into the category of autonomic. The autonomic nervous system comprises two branches: the sympathetic and parasympathetic nervous systems.