Good day everyone, Today we are going to discuss fully on the skeletal muscle;
After studying this section, you should be able to:
- identify the main characteristics of skeletal muscle
- relate the structure of skeletal muscle fibres to their contractile activity
- describe the nature of muscle tone and fatigue
- discuss the factors that affect the performance of skeletal muscle
- name the main muscles of the body regions described in this section
- outline the functions of the main muscles described in this section.
Muscle cells are specialised contractile cells, also called fibres. The three types of muscle tissue, smooth, cardiac and skeletal, each differ in structure, location and physiological function. Smooth muscle and cardiac muscle are not under voluntary control and are discussed elsewhere (smooth muscle and cardiac muscle. Skeletal muscles, which are under voluntary control, are attached to bones via their tendons and move the skeleton. Like cardiac (but not smooth) muscle, skeletal muscle is striated (striped), and the stripes are seen in a characteristic banded pattern when the cells are viewed under the microscope.
Also read:axial skeleton
Organisation of skeletal muscle
A skeletal muscle may sometimes contain hundreds of thousands of muscle fibres as well as blood vessels and nerves. Throughout the muscle, providing internal structure and scaffolding, is an extensive network of connective tissue. The entire muscle is covered in a connective tissue sheath called the epinysium. Within the muscle, the cells are collected into separate bundles called fascicles, and each fascicle is covered in its own connective tissue sheath called the perimysium. Within the fascicles, the individual muscle cells are each wrapped in a fine con nective tissue layer called the endomysium. Each of these connective tissue layers runs the length of the muscle They bind the fibres into a highly organised structure and blend together at each end of the muscle to form the tendon, which secures the muscle to bone. Often the tendon is rope-like, but sometimes it forms a broad sheet called an aponeurosis, e.g. the occipitofrontalis muscle. The multiple connective tissue layers throughout the muscle are important for transmitting the force of contraction from each individual muscle cell to its points of attachment to the skeleton.The fleshy part of the muscle is called the belly.
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Skeletal muscle cells (fibres)
Contraction of a whole skeletal muscle occurs because of coordinated contraction of its individual fibres.
Under the microscope, skeletal muscle cells are seen to be roughly cylindrical in shape, lying parallel to one another, with a distinctive banded appearance consisting of alternate dark and light stripes. Individual fibres may be very long, up to 35 cm in the longest muscles. Each cell has several nuclei (because the cells are so large), found just under the cell membrane (the sarcolemma). The cytoplasm of muscle cells, also called sarcoplasm, is packed with tiny filaments running longitudinally along the length of the muscle; these are the contractile filaments. There are also many mitochon- dria, essential for producing adenosine tri- phosphate (ATP) from glucose and oxygen to power the contractile mechanism. Also present is a specialised oxygen-binding substance called myoglobin, which is similar to the haemoglobin of red blood cells and stores oxygen within the muscle. In addition, there are extensive intracellular stores of calcium, which is released into the sarcoplasm by nervous stimulation of muscle and is essential for the contractile activity of the myofilaments.
Actin, myosin and sarcomeres. There are two types of contractile myofilament within the muscle fibre, called thick and thin, arranged in repeating units called sarcomeres . The thick filaments, which are made of the protein myosin, correspond to the dark bands seen under the microscope. The thin filaments are made of the protein actin. Where only these are present, the bands are lighter in appearance.
Each sarcomere is bounded at each end by a dense stripe, the Z line, to which the actin fibres are attached, and lying in the middle of the sarcomere are the myosin filaments, overlapping with the actin.
The skeletal muscle cell contracts in response to stimulation from a nerve fibre, which supplies the muscle cell usually about halfway along its length. The name given to a synapse between a motor nerve and a skeletal muscle fibre is the neuromuscular junction. When the action poten- tial spreads from the nerve along the sarcolemma, it is conducted deep into the muscle cell through a special network of channels that run through the sarcoplasm, and releases calcium from the intracellular stores. Calcium triggers the binding of myosin to the actin filament next to it, forming so-called cross-bridges. ATP then provides the energy for the two filaments to slide over each other, pulling the Z lines at each end of the sarcomere closer to one another, shortening the sarcomere. This is called the sliding filament theory. If enough fibres are stimulated to do this at the same time, the whole muscle will shorten (contract).
The muscle relaxes when nerve stimulation stops. Calcium is pumped back into its intracellular storage areas, which breaks the cross-bridges between the actin and myosin filaments. They then slide back into their starting positions, lengthening the sarcomeres and return- ing the muscle to its original length.
The neuromuscular junction
The axons of motor neurones, carrying impulses to skeletal muscle to produce contraction, divide into a number of fine filaments terminating in minute pads called synaptic knobs. The space between the synaptic knob and the muscle cell is called the synaptic cleft. Stimulation of the motor neurone releases the neurotransmitter acetylcholine (ACh), which diffuses across the synaptic cleft and binds to acetylcholine receptors on the postsynaptic membrane on the motor end plate (the area of the muscle membrane directly across the synaptic cleft, . Acetylcholine causes contraction of the muscle cell.