Because these muscles are not involved in locomotion, structural support, or fast action these muscles are quite different from skeletal and cardiac muscles. Here are some of the major ways in which smooth muscle differs:
- It is not striated muscle
- Smooth muscle lacks sarcomere structures
- It is involuntary muscle (though cardiac muscle is as well)
- Smooth muscle has a poorly developed sarcoplasmic reticulum.
- Smooth muscle does not have T-tubules.
- Gap junctions are present throughout the smooth muscle tissue.
Smooth Muscle Locations
Smooth muscles are found within hollow internal organs, the skin, the eyes, and within blood vessels. Here is a list of many locations where you can find smooth muscle (the list is not exhaustive).
- Small Intestine
- Large Intestine
- Airways to the lungs
- Arrector pili of the skin
- Gallbladder and Bile Duct
- Urine Bladder
- Veins, Arteries, arterioles
- The eye
- Reproductive systems (both genders)
Smooth muscle comprises roughly 10% of the mass of the human body. Smooth muscle is essential for the proper function of many vital organs, eyesight, blood pressure regulation, and a great many other innate bodily functions.
Smooth Muscle Functions
Smooth muscle is responsible for many bodily functions. We will discuss many of these functions to give you a better appreciation for how important these muscles are to your normal daily activities. This discussion does not cover every smooth muscle function.
In the skin, smooth muscle is responsible for causing your hair to stand erect when you are cold or experience fear. A small arrector pili muscle surrounds several hair follicles. When activated the muscle causes the associated hairs to stand more perpendicular to the skin surface in a process called piloerection. One potential benefit of this action is to trap more air near the surface of the skin thereby increasing the insulating effect of hair in mammals. In humans this largely has the effect of producing what are commonly called goosepimples, gooseflesh, or goosebumps (or any of a great many other colloquial expressions for the sensation).
Within blood vessels smooth muscle can control the size of a blood vessel. This controls the amount of blood that flows through the vessel. This allows both regulation of blood pressure and regulation of blood flow. The body can use this mechanism to direct blood to areas of the body that need it the most (such as during digestion or during heavy exercise).
In the eye the iris opens and closes in response to light intensity. This is a smooth muscle function. The ciliary muscle is also responsible for involuntarily changing the shape of the eye lens to allow the eye to focus on objects at different distances.
The digestive system uses smooth muscle to produce the rhythmic waves that continually push digestive material through the system. Similar wave patterns exist in other places such as the esophagus as well. Various sphincter muscles in the digestive system are also composed of smooth muscle tissues and are involuntary muscle.
Smooth muscle is found in various hollow organs to support squeezing of the organ to force contents out of the organ. The gallbladder, urine bladder, uterus, and stomach use this mechanism.
Within the respiratory system smooth muscle helps control the rate of airflow into and out of the lungs. During exercise smooth muscles relax to dilate the bronchi and bronchioles in the respiratory system allowing increased airflow. These muscles contract when the body is at rest to reduce airflow and prevent hyperventilation (excessive removal of CO2 from the body).
Smooth Muscle Classifications
Anatomists classify smooth muscle according to its characteristics. Here two common smooth muscle classifications:
- Phasic or Tonic muscle
- Multi-unit or Single-unit muscle
Phasic (also called rhythmic) smooth muscle refers to those muscles that are normally relaxed but contract when performing their intended function. The muscles of the digestive tract are phasic muscles since they relax and contract when needed to produce the required wave actions needed by these organs. The bladder also has phasic muscle since it is normally relaxed but contracts when it is necessary to empty the bladder.
Tonic muscles represent the opposite case. These muscles are normally contracted and only relax when needed. Some sphincter muscles are tonic muscles since they normally remain closed except for brief periods when they need to open. Blood vessels have tonic muscle that is normally in various stages of contraction to direct blood flow and to control blood pressure. Tonic muscles can hold a contraction level for very long periods of time.
Smooth muscles are also classified by whether they contract as one single unit (i.e. the entire muscle contracts when required) or if the cells in a muscle contract generally independent of one another (multi-unit muscle). Phasic muscles are generally single-unit muscles. Tonic muscles are usually multi-unit muscles. So the muscles in the digestive tract and the bladder are single-unit muscles, while those in the airways of the lung and blood vessels are multi-unit muscles.
A major difference between single-unit and multi-unit smooth muscle is that single-unit muscle is myogenic. It does not require stimulation from a nervous system neuron to initiate contraction. Pacemaker cells can initiate contractions independent of the nervous system. Multi-unit smooth muscle is neurogenic and requires that an autonomous nervous system neuron initiates contractions. The autonomic nervous system may connect to individual muscle cells in multi-unit muscle.
It is important to realize that smooth muscle normally has some mixture of multi-unit and single-unit behavior. A muscle is therefore largely multi-unit or largely single-unit in its behavior. There is usually some small component of multi-unit behavior in single unit muscle and vice versa.
Smooth Muscle Structure
Most smooth muscle has a single-unit structure. The structure of smooth muscle is quite different from that of skeletal and cardiac muscle.
Myocyte cells form smooth muscle tissue. These are relatively small cells with a long elliptical (cigar) shape and a central nucleus. They are not composed of sarcomere structures as is found in striated muscles. In addition, the cells do not have T-tubules.A myocyte appears in the bottom left corner. It has a single nucleus and multiple stacked myosin and actin filaments. This represents a different arrangement than that found in sarcomere. In smooth muscle multiple filaments alternate in a stacked or staircase fashion. The ends of each stack adheres to the cell wall. So when contraction occurs the cell shape changes, primarily by shortening.
Smooth Muscle Contraction
Smooth muscles can start a contraction (or relaxation) due to the following events:
- Mechanical stretching or movement
- Neuron Activation
- Hormonal Activation
- Chemical Activation
- Pacemaker Cell Activation
Some muscle tissue will contract if the tissue stretches. Stretching the tissue allows calcium to enter individual cells which initiates a cascading contraction cycle. Arteries use this mechanism to help control blood flow. If blood flow increases then the artery wall stretches. The muscle will then further contract to restore established blood flow requirements.
Contractions can also be initiated via neurons. Contractions for both single-unit and multi-unit muscle may start due to signals from the autonomic nervous system. But multi-unit muscle has far more neurons which allows more localized activation of muscle tissues. In single-unit muscle a few neurons may initiate a contraction throughout the entire muscle tissue.
Hormones can cause release of calcium across a muscle surface causing contractions. This is often found during childbirth where hormones initiate contractions of the uterus.
Chemical activation can occur if certain waste byproducts build up in an area of the body. The chemicals may then cause the muscle in nearly blood vessels to change so that more blood can travel to the affected part of the body.
A few specialized pacemaker cells reside in phasic muscle tissues. These pacemaker cells slowly build up an internal voltage which controls calcium and potassium movement into the cell. This voltage slowly increases over time until it reaches a trip or threshold level. At that point calcium can enter the cell, initiating a cascading release of calcium throughout the muscle tissue. Now potassium enters the cell which causes the contraction to end and the voltage within the pacemaker cells to drop below the threshold level. The voltage level now begins to slowly increase toward the threshold level again as the cycle repeats. This is a periodic and rhythmic method for initiating muscle contractions.The diagram at right depicts contraction of a smooth muscle cell. When the chains of myosin and actin filaments contract they pull on the cell walls and cause the muscle cell to deform, primarily by shortening. In the case of single-unit muscle all the affected muscle cells contract in unison to contract the entire muscle. A smaller subset of the muscle is normally affected in multi-unit muscle.