Most of the time we think of a sculpted physique and a six pack when we hear the word muscles. However, there is much more to these biological ‘motors’. Even when you are sleeping there are several muscle groups working throughout your body. From the muscles that enable you to breathe to your heart muscles, bowel wall muscles and even the eyeball muscles which are responsible for the rapid eye movements during sleep. These biological ‘motors’ not only allow us to move our body or parts of it, the muscle cells keep us alive.
What are muscles?
Whether your aim is to build, tone or maintain your muscles, it is important to first understand what these structures are. Muscles are a type of soft tissue which contract and relax to facilitate movement. It is composed of muscle cells or myocytes which facilitate movement of the entire muscle. The cells are relatively unique to some degree. It can contract and relax, thereby changing its size as neeed.
These myocytes (also known as myofibers) have the ability to contract when stimulated by nerve impulses. Large scale muscular movements are a result of the coordinated contraction of hundreds to thousands of these tiny muscle cells. The ability of myocytes to contract is due to the presence of actin and myosin proteins in these cells. Mycocytes are also abundant in mitochondria, the cell’s energy ‘factories’, because of its higher energy demands.
Muscle cells, despite having the ability to elongate and shorten to a large degree, are similar to other cells. It not only moves parts of the skeleton by contracting and relaxing but can also narrow or widen cavities and move the contents within a cavity. Therefore almost all movement within the human body is due to the activity of muscles and the tiny myocytes that comprise the muscles.
Read more on how muscles work.
Types of Muscles
When we think of muscles, we typically think of the muscles that allow us to move our body. These muscles are known as the skeletal muscles. It is responsible for moving parts of the body and maintaining our posture, whether sitting or standing. However, skeletal muscle is not the only type of muscle in the body. Two other types of muscles, known as smooth muscle and cardiac muscle, are responsible for tissue movements within the body.
Despite the differences among these muscle cells, they all function in a similar manner and have similar characteristics. Smooth muscles are located in various organs and structures and are not under voluntary control. Some examples include the tiny muscles in the wall of blood vessels and bowels. Cardiac muscles are found in the heart. It makes up most of the heart wall and are responsible for the pumping of blood from the heart.
Almost 40% of the body mass is made up of skeletal muscles. Also known as “voluntary muscles”, we can contract most skeletal muscles when we want to bring about a specific body movement. Skeletal muscles are therefore responsible for voluntary movement. These also play a role in the largely involuntary maintenance of body posture. Skeletal muscles connect to the bones via tendons.
When a nerve impulse reaches the muscles, the myocyctes (muscle cells) contract. This contraction causes the muscle to pull on the bone it is attached to and this causes the movement that we see. Once the nerve impulse ceases then the muscle relaxes. The impulse causes the release of neurotransmitters at the end of the nerve. It is these chemicals (neurotransmitters) that are causes the muscle fibers to contract.
Red and White Skeletal Muscles
The human body contains two kinds of skeletal muscle cells: slow twitch muscle fibres and fast twitch muscle fibres.
Muscles that are predominantly composed of slow twitch muscle fibres are also known as red muscle or type I skeletal muscle.These slow twitch muscle fibres are rich in mitochondria and myoglobin. This gives them the ability to sustain aerobic activity for long periods of time. It is more commonly associated with endurance rather than strength. However, both types of muscle fibers are used in all physical activity.
Muscles that are predominantly composed of fast twitch muscle fibres are known as white muscle or type II skeletal muscle. Fast twitch muscle fibres have a relatively lower concentration of mitochondria and depend largely on anaerobic energy generation processes to fuel their contractions. It can contract with more force than the slow twitch muscle fibres and also has a higher potential for mass gain than slow twitch muscle fibres.
Smooth muscles are found in the walls of internal structures such as blood vessels, oesophagus (food pipe), stomach, bowels, bladder and parts of the airways. Tiny smooth muscles are even found attached to the small hairs on our skin surface which stands when we are cold. Unlike skeletal muscles, smooth muscles are not under voluntary control. Their contraction is controlled by the autonomic nervous system.
Based on various factors and stimuli, the body decides when to stimulate these muscles to contract or relax and to what degree. Some of these factors may be indirectly under voluntary control but to a limited extent. Structurally, smooth muscles are very different from skeletal muscles with respect to the arrangement of actin and myosin proteins within myocytes.
Cardiac muscles are also involuntary muscles. These muscle cells are found exclusively in the heart. Also known as cardiomyocytes, the cardiac muscle cells are responsible for the rhythmic and involuntary contraction of the heart. This beating of the heart drives the circulation of blood throughout the body. Structurally, cardiac muscles are similar to skeletal muscles. Both have a similar arrangement of the actin and myosin bundles within their myocytes.
Read more on fluttering heart.
The cardiac muscles are stimulated internally by the SA (sinoatrial) node, which is the heart’s natural pacemaker. It generates impulses and this is distributed throughout the heart wall to the cardiac muscles. The electrical conduction system of the heart ensures that the atria contract first, followed by the ventricles a short while later. The SA node may also be influenced hormones and nerve impulses from beyond the heart.