| The muscular system
provides the mechanism for movement of the body, and for
physical control over internal functions. The systems involving smooth muscle and cardiac muscle are described in their relevant sections. |
Muscle Tissue
| Muscle tissue has
the property of being able to contract when stimulated
via the nervous system. Contraction occurs through a
complex biochemical reaction, which requires
‘food’ and Oxygen. There are three basic types of muscle tissue: smooth, cardiac, and striated. |
Smooth Muscle Tissue
| Smooth muscle is
also known as ‘visceral’ or
‘involuntary’ muscle. It provides control over internal organs and vessels, and is not under the control of the conscious will. Smooth muscle cells each have one central nucleus, and have a ‘double cone’ shape. The cells are formed into sheets of muscle, without additional fibrous tissue. [Figure 4 - 1] |
Figure 4 - 1 smooth muscle tissue
The construction of smooth muscle tissue
Cardiac Muscle Tissue
| Cardiac muscle is
found only in the myocardium of the heart. Cardiac muscle cells each have one nucleus, and one or more branches. The ends of the cells and branches link to give a contiguous sheet of muscle. [Figure 4 - 2] Cardiac muscle has the special property that a stimulus of contraction may be passed from one cell to the next, without the need for external nerve connections. |
Figure 4 - 2 cardiac muscle tissue
The construction of cardiac muscle tissue
Striated Muscle Tissue
| Striated muscle is
also known as ‘voluntary’ or
‘skeletal’ muscle. It provides for the movement
of the body and limbs, and is under the control of the
conscious will. Striated muscle cells are very long (up to 350mm) and thin (0.01 - 0.1mm). They have several nuclei found just under the outer membrane. The fibres have transverse stripes or striations (hence their name). [Figure 4 - 3] |
Figure 4 - 3 striated muscle tissue
The construction of striated muscle tissue
| A muscle is made up from a large bundle of parallel fibres enclosed in fibrous tissue. The overall muscle is then encased in further fibrous tissue. |
Skeletal Muscular System
| The skeletal muscles
operate together with the bones of the skeleton to
provide the power of bodily movement. There are approximately 600 muscles in the skeletal muscular system. [Figure 4 - 4 and Figure 4 - 5] They are generally arranged in pairs, such that when one contracts, its partner is stretched. This allows movements to be reversed. |
Muscle Attachments
| All skeletal muscles are
attached at their ends, most commonly to bones, but in
some cases to the skin, and to other organs. The most
common attachment is an extension of the outer fibrous
sheath, formed into a cord, or tendon. Tendons connecting to bones are attached through the periosteum into the actual bone tissue, providing a very strong linkage. |
Classes of Muscle
| Skeletal muscles are
often classified according to the movements they perform.
[Table 4 - 1] Most muscular movements involve complex interactions between several muscles or groups of muscles. When interacting, muscles act in one of four ways: Agonists. These muscles are prime movement producers. Antagonists. These muscles oppose the movement of agonists, but only such as to provide for smooth movement. Synergists. These muscles assist the movement of agonists by restricting the movement of intermediate joints. Fixators or postural muscles. These muscles provide stability for agonists by fixing otherwise moveable joints. Which of these actions a muscle performs will depend on the nature of the movement being carried out. |
Table 4 - 1 muscle classifications
Main muscle classifications and the types of movement performed by them
| Muscle class | Movement |
| Abductor | Movement away from the midline of the body or structure |
| Adductor | Movement towards the midline of the body or structure |
| Depressor | Lowering movement |
| Extensor | Bending to increase the angle between two bones |
| Flexor | Bending to reduce the angle between two bones |
| Elevator | Lifting movement |
| Protractor | Forward movement |
| Retractor | Backward movement |
| Rotator | Rotational movement around a longitudinal axis |
| Sphincter | Reduction of the size of a tube or orifice |
| Tensor | Increase in tension |
Figure 4 - 4 skeletal muscles - anterior
Anterior view, showing main skeletal muscles
Figure 4 - 5 skeletal muscles - posterior
Posterior view, showing main skeletal muscles
Muscle Contractions
| Muscle contraction
is initiated by stimulation via motor nerve end plates. The exact biochemical process is
complex (and still open to controversy), but is known to
rely on an energy producing reaction involving calcium
ions and the substance adenosine triphosphate. All-or-none principle Whenever a muscle fibre is stimulated to contract, it contracts to its maximum extent. The actual strength of a muscular movement is based on the number of individual fibres which are stimulated to contract simultaneously, rather than on an overall magnitude of contraction. Types of muscular contraction Muscle contractions are identified into five groups: Twitch. This is a momentary contraction in response to a single nervous stimulus. Isotonic contraction.
This occurs when a muscle contracts by becoming shorter
and thicker. Isometric contraction.
This occurs when a muscle develops tension, but remains
the same length. Staircase phenomenon. (Sometimes referred to as treppe) This occurs when a rested muscle is given repeated stimuli; the contractions increase in strength until a steady tension is reached. Tetanus. This occurs when repeated stimuli occur too fast for intermediate relaxation. At the limit, the muscle is in a complete state of contraction, with no relaxation between stimuli. Muscle tone Even when skeletal muscles are ‘at rest’, some fibres are partially contracted. This contraction is known as muscle tone, and is caused by a continuous nervous stimulation via reflex paths. Fatigue There is a limit to the rate at which the circulation can supply Oxygen for the creation of adenosine triphosphate from glucose. If adenosine triphosphate usage for muscular contractions exceeds this rate, incomplete breakdown of glucose leads to a build-up of lactic acid in muscle tissue. This, plus a resultant deficit of adenosine triphosphate and an excess of carbon dioxide, leads to the sensation of fatigue. As fatigue develops, further muscular action becomes increasingly difficult and uncomfortable. In the extreme, it will become impossible. |
Cramp
| Cramp is the term associated with the painful sensation caused by either prolonged or sudden and extreme contraction of a muscle. The condition may be connected with electrolyte imbalance, and is often associated with fatigue, stress, and poor posture. |
Muscular Dystrophy
| The term muscular
dystrophy refers to a group of inherited diseases
where muscle fibres degenerate and are replaced by fatty
tissue. Increasing weakness and loss of ability result. The severity of the condition varies from slight weakness through to damage to the intercostal muscles or myocardium, with eventual fatal consequences. |
Poliomyelitis
| Although poliomyelitis is actually an infectious viral disease of the central nervous system, it is often attributed to the muscular system as it can lead to muscular stiffness, weakness, and eventual paralysis. |
Strain
| A strain occurs
when there is excessive use or stretching of muscle
tissue. This may damage the muscle fibres or tendons by
tearing them and generally leads to pain and swelling. In an extreme case, the tissue is completely torn apart, with extensive haemorrhage, swelling, pain, and loss of function. |
Tendinitis
| Tendinitis occurs when excessive use of a particular muscle leads to a painful inflammation of the associated tendon sheath. The condition may also be caused by bacterial infection or rheumatic disease. |
Tension Headache
| A tension headache
is probably the most common form of headache. It occurs
when muscular contractions in the back of the neck,
caused by emotional stress, fatigue, or other factors,
lead to pain. Muscular spasms may then constrict blood
vessels - increasing the problem, which can be compounded
by a build-up of toxic waste substances in the scalp. Migraine A migraine is a condition similar to the tension headache, but with much more severe symptoms. Often a migraine is initiated by a trigger factor, and may occasionally be preceded by an ‘aura’, or warning symptoms. Blood vessels supplying the brain constrict. At some point later, they dilate, causing an intense headache, and perhaps inducing other problems such as visual disturbances, dizziness, vertigo, and vomiting. |