By Chandan Sekhon - Medicine Student @ Peterhouse, Cambridge
Skeletal muscle makes up every voluntary muscle in the body, however it can also have involuntary roles. For example, running, jumping and other voluntary movements are made possible by skeletal muscle contraction. The involuntary functions of skeletal muscle include those actions in reflex arcs, for example in response to sharp pain. This involuntary function of skeletal muscle plays a protective role to ensure the body does not become injured. Skeletal muscle is often attached to bones via tendons across joints which conduct these movements across the joints by moving bones closer together. Skeletal muscle can also have a stabilising role, for example the rotator cuff muscles in the shoulder stabilise the head of the humerus. Contraction results following the propagation of action potentials to neuromuscular junctions in muscle fibres.
Skeletal muscle is arranged in a series of long multinucleated cells called myocytes which lie parallel to each other in fascicles. The entire muscle is supported in a sheath of epimysium. Myofibrils comprise of multiple repeat contractile units called sarcomeres giving skeletal muscle a striated appearance, as shown below:
Figure 1: Structure of a sarcomere
All contractile cells in the body use the actin/myosin method of contraction, which is demonstrated by the striations seen in skeletal muscle and the length-tension relationship in both skeletal and smooth muscle. Contraction occurs by stabilising the contractile proteins troponin and tropomyosin which are located in the isotropic (I) bands. When an action potential reaches the muscle, a wave of depolarisation is conducted throughout the cells via T-tubules (invaginations of the muscle membrane). During contraction, myosin heads, comprising meromyosin globular proteins, join to myosin binding sites on actin filaments, which pull the actin filaments inwards, shortening the I-band and H-zone as a tight cross-bridge forms. This process continues, resulting in contraction. Relaxation occurs when Ca2+ ions are actively transported back to the sarcoplasmic reticulum (SR).
Skeletal muscle is extremely well adapted to perform its role. In the muscle, myofibrils are bundled together into a single muscle cell, which means it is extremely unlikely to initiate action potentials in neighbouring cells. As well as this, Skeletal muscle is highly ordered and can be divided into motor units where each cell in the unit is innervated by a single motor nerve. Because of this, and because a single muscle is made up of many motor units, the force of contraction can be tightly controlled, and minute adjustments can be made to contraction patterns and rhythms. This can be done by changing the number of motor units recruited during contraction, for example.
The extreme control allowed by the structure of skeletal muscle is required to allow skeletal muscle to perform the vast array of movements it needs to be able to. As a result, individual motor units can sometimes act independently from one another – such as in the shoulder muscle deltoid, where contraction of the posterior aspect results in extension, while the anterior aspect causes flexion at the shoulder joint. As well as this, the multinucleated structure of skeletal muscle allows for much faster and more efficient proteins synthesis and other nucleus-related processes.
This article gives a great introduction to the basics of skeletal muscle. Quite lengthy, but definitely informative and goes beyond the specification for biology at school: https://link.springer.com/article/10.1007/s00223-014-9915-y
This book goes into extreme depth about skeletal muscle and is definitely a lot to read, but choosing specific areas of interest to read may be useful: https://books.google.co.uk/books?hl=en&lr=&id=-EfxEhSFaMYC&oi=fnd&pg=PP9&dq=skeletal+muscle&ots=t5-C4p_Hib&sig=j2bMXb4I70THypwSux3R0uzqI8c&redir_esc=y#v=onepage&q=skeletal%20muscle&f=false
A slightly different article as this one talks about how skeletal muscle develops embryologically. A very interesting read, especially if you would like to learn more about how muscles are developed in an embryo: https://onlinelibrary.wiley.com/doi/full/10.1046/j.1469-7580.2003.00139.x
This article talks more about how skeletal muscle deteriorates and becomes fatigued. With various factors, skeletal muscle can wear down, so this article is an interesting way to link the anatomy and physiology of muscle to a clinical situation: https://journals.physiology.org/doi/full/10.1152/physrev.00015.2007?url_ver=&
Again, this article is more to do with muscle fatigue, more specifically about the processes occurring as a person ages. Shorter than the article above, this is a very worthwhile read: https://onlinelibrary.wiley.com/doi/full/10.1002/jcsm.12238