VOL: 102, ISSUE: 48, PAGE NO: 26
BD, RN, CertEd, DipNurs, RNT, is senior lecturer, University of Hertfordshire.
The body contains three distinct types of muscles: skeletal, cardiac and smooth. Part one of this series gave an overview of muscle physiology and explored the characteristics of the three types of muscles.
Skeletal muscles are the largest muscle group in the body and in this article, and the remaining two parts of the series, the structure and function of these muscles will be covered in detail.
There are almost 700 skeletal muscles in the body, all of which are associated with the bones of the skeletal system. Muscles involved with movement and posture work across joints to produce skeletal movement, those that support soft tissue form single sheets between relatively stable bony parts and those that guard entrances or exits encircle the opening (Martini, 2005).
There are some general principles that apply to all skeletal muscles.
- Muscle fibres are as long as the muscle they are part of - in the sartorius muscles of the thigh, fibres can be up to 30cm in length (McLaren, 2005) - and the length of the fibres determines the range of movement at a joint.
- The strength of a muscle contraction is determined by the number and size of the fibres within the muscle.
- Three layers of connective and collagen tissue support and strengthen muscle fibres. Around each individual muscle fibre is the endomysium. Groups of fibres are bound together in bundles known as fasciculi and each bundle is surrounded by a collagen perimysium. Finally, the entire muscle is surrounded by the connective tissue layer of the epimysium (Fig 1).
- The sarcolemma is the plasma membrane of the muscle fibre and is selectively permeable - that is, there can be different concentrations of ions inside and outside the cell, which is filled with sarcoplasm.
- Each muscle fibre contains hundreds of nuclei just below the sarcolemma. The nuclei direct the production of enzymes and structural proteins that are needed for movement; having many nuclei in a cell makes this a speedier process.
- It is important that such a large cell functions as a single unit. To aid this, transverse (or T) tubules filled with extracellular fluid lead from the sarcolemma at right angles into the sarcoplasm like a series of tunnels.
- Inside the sarcoplasm are hundreds of thousands of fine fibres that run longitudinally. These myofibrils are as long as the muscle itself and are composed of bundles of thick and thin protein filaments containing, predominantly, actin and myosin. Myofibrils can actively shorten and are responsible for muscle contraction. This activity will be discussed in the next article of this series.
- Scattered among the myofibrils are mitochondria, which generate the energy for the cell through the production of adenosine triphosphate (ATP). Glycogen granules (the storage form of glucose) are also scattered among the myofibrils.
- A membrane complex, known as the sarcoplasmic reticulum, surrounds each myofibril. The T tubules are in close contact with this membrane and form expanded chambers known as terminal cisternae. Calcium is actively pumped out of the sarcoplasm and into the terminal cisternae. The concentration of calcium in the cisternae can be 40,000 times as great as in the sarcoplasm (Fig 2).
Thus a very long muscle cell has all the components it needs to enable it to contract as a single unit. But in order to function, muscles need both a good blood supply and a functioning nerve supply (Marieb, 2006).
Skeletal muscles receive approximately 20% of the cardiac output at rest, which is approximately 1l of blood every minute. During exercise, this increases dramatically to around 15-20l per minute and in trained athletes may reach as much as 30l every minute. This, in part, is due to the increase in cardiac output during exercise but it is also because the sympathetic nervous system redistributes the body’s blood so a greater proportion goes to the skeletal muscle.
Every cubic millimetre of muscle contains 300-400 capillaries (McLaren, 2005) that run, along with the nerves, in the connective tissue of the epimysium and perimysium.
This rich supply of blood is essential for delivering oxygen and nutrients to generate the enormous energy supply required by muscle cells. The blood also carries away the waste products of muscle cell metabolism and the excess heat that is produced.
Nerves enter the muscle cells and branch, along with the blood vessels, into the epimysium and perimysium. The axons of the neurons then branch through the perimysium and enter the endomysium to innervate individual nerve fibres.
One nerve innervates up to 3,000 fibres so there are only 420,000 motor nerves for 250 million muscle fibres. The gastrocnemius muscle in the calf, for example, has 580 motor neurons controlling 41,000 fibres.
The neurons can all fire together if a burst of strength is required (for example in weightlifting), or they can fire asynchronously (some neurons fire while others rest) if endurance is necessary. The neurotransmitter acetylcholine is released at the synaptic terminals where nerves meet muscle fibres, which alters the permeability of the sarcolemma.
Muscles and their tendons also contain sensory nerves with receptors that are sensitive to stretch, tension and pressure. These nerves relay information to both the conscious and unconscious areas of the central nervous system about muscle dynamics and limb position and movement (proprioception). This constantly monitored information enables people to maintain their body posture and to coordinate their movements.
Constant partial contractions of the muscles help people to remain in one position for a length of time. Other sensory nerves in the muscles are responsible for signalling painful stimuli such as ischaemia, necrosis and inflammation (McLaren, 2005).
Contraction of skeletal muscle is voluntarily controlled in the cerebral cortex, cerebellum, basal ganglia and brainstem nuclei (McLaren, 2005).
Motor nerves descend from these areas, exit the spinal cord and travel to the muscles, terminating in a motor endplate or neuromuscular junction.
Stimulation of these nerves causes a series of reactions resulting in the shortening of the muscle fibres and the contraction of the muscle.
Inside the muscle, the neuron branches to supply an individual fibre at the neuromuscular junction or motor endplate. An electrical signal travels along the neuron and arrives at the neuromuscular junction. Acetylcholine is released from the end of the neuron and attaches to receptors on the sarcolemma. This process fires an electrical stimulus in the muscle cell that spreads along the length of the cell and conducts into the T-tubule system. Contraction of the muscle cell follows within a fraction of a second. Interactions within the smallest functional units of muscle tissue - the sarcomeres - enables contraction to happen and this will be outlined in the next article of this series.
McLaren, S. (2005) Skeletal muscles. In: Montague, S. et al (eds). Physiology for Nursing Practice (3rd edn). Ballière Tindall: London.
Marieb, E. (2006) Essentials of Human Anatomy and Physiology. San Francisco, CA: Pearson Benjamin Cummings.
Martini, F.H. (2005) Fundamentals of Anatomy and Physiology. San Francisco, CA: Pearson Education.