Trifocus Fitness Academy - skeletal muscle

What is the structure of skeletal muscle?

Skeletal muscle is a specialised contractile tissue connected to the skeleton which enables you to move your body. The skeletal muscle consists of muscle tissue, connective tissue, nerve tissue and vascular tissue. These muscle fibres are surrounded by protective membranes.

The combination of muscle fibres and membranes allows the skeletal muscle to contract and release quickly.

The layers of connective tissue covering muscle fibres support, protect and allow the cells to withstand contraction. They also create pathways for blood vessels and nerves.

What skeletal muscle is made up of

Epimysium

Epimysium is a connective tissue sheath that surrounds the hundreds, sometimes thousands, of muscle fibres that are bundled together to make the skeletal muscle. It protects the skeletal muscle from friction against other muscles and bones.

Fascia

Fascia is connective tissue outside the epimysium that surrounds and separates the muscles. Fascia contains closely packed bundles of collagen fibres in a wavy pattern parallel to the direction the muscle pulls. It is flexible and able to resist great tension forces from different directions until the wavy pattern of the collagen has straightened out.

Fascicles

Portions of epimysium project inward to divide the muscle into compartments. Each of these compartments contains a bundle of fibres. This is called a fascicle (fasciculus).

Perimysium

Perimysium is the layer of connective tissue that surrounds the fasciculus with resistance to traction.

It consists of three different layers of collagen fibres:

  1. Superficial: straight fibres that have a smaller diameter and spread out without a definite direction.
  2. Intermediate: larger-diameter fibres, flattened and curved that intersect.
  3. Deep: soft lamina that is in direct contact with endomysium.

Endomysium

In the fasciculus, each muscle fibre is surrounded by connective tissue called the endomysium. The endomysium contains minute blood vessels (capillaries) and nerves.

It is the deepest and smallest component of muscle connectivity tissue. It provides an appropriate chemical environment for calcium, sodium and potassium to be exchanged. This makes the muscles contract.

Aponeurosis and tendons

When epimysium, perimysium and endomysium extend beyond the muscle fibres it creates a tough band of connective tissue that connects muscle to bone to withstand tension (tendons) or a sheet of tissue that takes the place of a tendon in flat muscles that have a wide area of attachment (aponeurosis.

The tendon and aponeurosis form indirect attachments from muscles to a dense layer of vascular connective tissue that envelopes the bones (periosteum). Alternatively, this indirect attachment is formed to the connective tissue of other muscles.

Blood vessels and nerves

Skeletal muscles have lots of blood vessels and nerves. This is because, before the muscle can contract, the nerve has to send out an impulse.

An artery and at least one vein accompany each nerve that penetrates the epimysium of the skeletal muscle. These nerves and blood vessels follow the connective tissue of the muscle, along with a few capillaries.

The skeletal muscle is a special type of contractile tissue that allows you to move the skeleton. It is made up of various muscle components, including epimysium, fascia, fascicles, perimysium, endomysium, aponeurosis, tendons, blood vessels and nerves.

What are contractile elements of skeletal muscle fibres?

A single muscle cell is known as a muscle fibre. Under a microscope a distinct series of light and dark bands can be seen. Muscle fibres are enclosed by a membrane known as the sarcolemma. It contains typical cell components such as plasma that is called sarcoplasm which is a composed of glycogen, fats, minerals and oxygen-binding myoglobin.

Nuclei mitochondria transform energy into food. They are unlike other cells because they have structures made up of myofibrils. These contain myofilaments which are the contractile components of muscle tissue. Myofilaments are also known as actin and myosin which are thin and think filaments which form repeating sections within a myofibril.

Each one of these sections are known as a sarcomere. A sarcomere is the functional unit of the muscle that produces muscular contraction and consists of repeating sections of actin and myosin.

Tropomyosin and troponin are the two other protein structures that are important for the muscle contraction. Tropomyosin is found in the actin filament and blocks the myosin-binding sites located on the actin filament so prohibiting myosin from attaching to actin while the muscle is in a relaxed state. Troponin, also located on the actin filament, plays a role in muscle contraction by providing binding sites for both calcium and tropomyosin when the muscle needs to contract.

How to generate muscle force?

A muscle generates force in a variety of different methods such as neural activation.

Neural activation

Neural activation is made possible by the communication between the nervous system and the muscular system. It makes muscle contraction and stabilisation possible. Where a connection is made with a motor neuron and the muscle fibres this is called the motor unit. The point where a single neuron meets a single fibre is called the neuromuscular junction. Impulses travel down from the central nervous system into the axon on the neuron. When the impulses reach the end of the axon, chemicals called neurotransmitters are released.

Neurotransmitters send messages between the neurons, nerves and muscle fibres. They fall into receptor sites on the muscle fibre. The neurotransmitter that is required by the neuromuscular system is called acetylcholine (Ach). Ach stimulates the muscle fibres to go through the necessary steps to produce a muscle contraction.

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