Podcast- Teach me Biology by Ria Corbett and Sarah Matthews.

On the 7^{th of} September 2022, Ria and Sarah recorded this podcast to help teach A-level biology students about skeletal muscles, what it is made from and how it works. This podcast made it extremely easy for students to follow along and fully understand the full concept of the structure of the skeletal muscles. Ria is a biology teacher, while her younger sister Sarah is acting as the student, Ria can ask Sarah questions about the basic knowledge of the subject, then they will explain what the different questions and statements mean in greater details. Ria and Sarah split the podcast into 5 different sections, the first section is called new knowledge. Throughout New Knowledge the 2 women talk in grave details about how muscles act in antagonist paints against an incomprehensible skeleton, gross and microscopic structure of skeletal muscle, the ultra structure of a myofibril, location, structure, and general principle of slow and fast skeletal muscle fibres. For the duration of the first section of the podcast, Ria and Sarah also provide a resource for this podcast, which provides a microscopic picture of the skeletal muscles so their listeners can look at the different muscles and fibres the women are referring to throughout this section of the podcast, which is extremely helpful for visual learners. In section 2 and 3 it is all about exam questions, they take the listeners through a question that Sarah had for Ria on sarcomeres, Ria goes through in detail what information needs to be covered to get the full 3 marks on the question. In section 4, they call it Ria’s roundup, where Ria will explain everything she covered in the lesson in simpler terms. She explains how moving the human body requires both muscle and a rigid Skeleton as muscles will only allow movement if they pull on a structure that is incompressible. She further explains how muscles work in antagonist pairs e.g. Bicep and tricep muscles. Ria then explained what a skeletal muscle is, which is that skeletal muscles make up the muscles in the body that are attached to the skeleton, it is made up of bundles of muscle fibres which are the long highly specialised muscles, each muscle fibre is surrounded by a cell surface membrane called the sarcolemma. This has many folded projections, that fold in from its outer surface known as the transverse tubules. She further explained how each muscle fibre contains sarcoplasm which is multi-nuclei. In section 5, they call it Sarahs takeaways, Sarah explains how each muscle fibre has myofibrils in them, and how each myofibril has sarcomeres. Moreover, Ria and Sarah explain what wider reading the listers can do to understand the topic in further detail. In conclusion, this podcast is immensely useful to anatomy students to get a better understanding of the anatomy of the skeletal muscle structure, it supplies tremendous detail, which is easy to understand, as well as a picture to help visual learners follow along without any confusion or difficulty.

The video starts at the epimysium, a tough, irregular connective tissue that surrounds the entire muscle acting as a protective sheath or layer.Then the video goes on to explain that this outer layer protects the muscle from friction against other muscles and bones which could result in injury. It continues at the end of the muscle to form along with other connective tissues the tendon which attaches the muscles to the bone.

Next, it shows the fasciculi which are made up from a number of bundles of muscle fibres. The presenter shows how the fasciculi make up another connective tissue called the perimysium which similar to the epimysium is a dense irregular connective tissue. She then states that “each muscle fibre contains anywhere from ten and one hundred individual fibres.” We are then shown a diagram of an athlete running and explained that in a large strong group in the quadriceps would have a large number of fibres within each bundle. To then compare we are shown a smaller muscle used for precision movement in the hand would contain far fewer muscle fibres in each bundle.

Following this, the video delves into the endomysium, a loose connective tissue, which envelopes each muscle fibre in a fibrous connective tissue layer that insulates the fibers, allowing for efficient electrical signal transmission during muscle contraction.

Beneath the endomysium the video shows us the sarcolemma, which is the muscle fibre cell membrane and under the sarcolemma is the sarcoplasm. The sarcoplasm is explained to us as a gelatinous fluid which contains glycogen and fats that provide the cells energy along with mitochondria inside which the cell’s energy is produced.

We go on to see the myofibrils next, it is explained to us that each muscle fibre itself contains this cylindrical organelle and each muscle fibre contains hundreds or thousands of myofibrils. The myofibrils contain proteins called actin and myosin these run the length of the muscle fibre and are crucial in the contraction of muscles.

Surrounding the myofibril we are shown a network of tubules and channels called the sarcoplasmic reticulum. This is where calcium is stored and is also important for the contraction of muscles.

The video finishes with an explanation of the sarcomere, the fundamental unit of muscle contraction. Each myofibril is divided into these repeating segments, which are crucial for understanding muscle contraction mechanisms. She then discusses the sliding filament theory, which describes how the sliding motion of actin and myosin filaments within the sarcomere leads to muscle contraction, emphasising its significance in the overall process of muscle action.

Skeletal muscle is made up of skeletal muscle fibres bound together by various layers of connective tissue that support

extensive network of nerves and blood vessels and form tendons. Skeletal muscle is one of the three distinct muscle tissues in the human body and is also an important structure responsible for producing movement, storing nutrients, maintaining posture, and providing stability across joints. Skeletal muscle attaches to bone via tendons to enable movement. The fibres of skeletal muscle are striated because the fibre composition creates a pattern on microscopy 

Figure 1 shows the structure of the skeletal muscle with the three layers of connective tissue. Endomysium, perimysium, and epimysium     

A skeletal muscle contains thousands of muscle fibres that are grouped together in fascicles and surrounded by three layers of connective tissue. The three layers of connective tissue are known as endomysium, perimysium, and epimysium.

The Endomysium layer surrounds muscle fibres  and contains vessels and nerves to supply the individual fibres. Perimysium surrounds muscle fascicles and the Epimysium surrounds muscle bundles, and this layer gives the muscle its structural integrity during powerful contractions. Actin and myosin are the main proteins responsible for muscle contraction, together they make a contractile unit termed a sarcomere, which aligns longitudinally to form myofibrils. Myofibrils are formed together to form muscle fibres which contain cytoplasm called the sarcoplasm and a plasma membrane called the sarcolemma. Within sarcoplasm, there is a membranous sac known as the sarcoplasmic reticulum and acts as a storage site for calcium. The sarcoplasmic reticulum is a longitudinal network of tubules, and its volume gives it its ability to pump calcium. Transverse tubules, otherwise known as T -tubules, are invaginations within the sarcolemma and is the setting for ion exchange during action potentials, which allows nerve impulses to be transmitted to myofibrils. The membrane components of the sarcolemma making up the T tubule system are located at specified sites along the surface  

Figure 2 shows a cross section of a muscle fibre . The sarcolemma being the membrane surrounding the muscle fibre and as shown the myofibrils giving the cell its striated appearance. 

A sarcomere is the basic functional unit of a myofibril and the basic contractile unit of the muscle. As shown in the figure, the sarcolemma is a membrane which surrounds a muscle fibre. The myofibrils give the cell its stripped appearance. The sarcomere exhibits several lines and bands, and actin and myosin are the primary proteins. The Z disc is where actin is anchored, it represents the two ends of the sarcomere. The I bands is the area which contains thin filaments of actin connected at Z discs, there is no myosin in this region and muscle reduces in the length during contraction known as concentric. A – Bands contain thick filaments of myosin, spanning the centre of the sarcomere and they remain the same length during contraction known as isometric. The H- bands are situated in the middle of the A-bands, containing myosin but no actin, these reduce in length during contraction known as concentric. Finally, the M-lines are the middle portion of the sarcomere where thick filaments are anchored through the protein myomesin 

References  

Allen, J., Ramsden, M. and Nisar, S. (2024) Skeletal muscle structure, function, and pathology. Orthopaedics and Trauma, 38(3), 137-144.