The sarcolemma, also called the myolemma, is the cell membrane of a skeletal, cardiac, or smooth muscle cell.1 It consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber, and the tendon fibers in turn collect into bundles to form the muscle tendons that then insert into bones. The membrane is configured to receive and conduct stimuli.
Alterations in the sarcolemma membrane stability and repair system can lead to muscular dystrophy. The mechanism of one type of muscular dystrophy, for example, is lack of functional dystrophin. This means that the sarcolemma is not attached to the cytoskeleton. Therefore during muscle contraction, the sarcolemma is not synchronized with the interior of the cell. The looseness of the sarcolemma permits membrane calcium channels to open. The rise in internal calcium ions causes activation of the proteolytic enzyme calpain, which digests proteins including contractile proteins, rendering the muscle much weaker.
The sarcolemma invaginates into the cytoplasm of the muscle cell, forming membranous tubules called transverse tubules (T-tubules). Terminal cisternae is the enlargement of smooth endoplasmic reticulum found in muscle cells on either side of the transverse tubules. The triad of transverse tubules surrounded by two smooth ER cisternae transmit altered membrane permeability down the tubules. The nuclei lie just adjacent to the sarcolemma, at the periphery of the fiber.
Inside the Sarcolemma
A skeletal muscle fiber is a single cell that contracts in response to stimulation and then relaxes when the stimulation ends. Each skeletal muscle fiber is a thin, elongated cylinder with rounded ends, which may extend the full length of the muscle. The sarcolemma contains the cytoplasm called sarcoplasm. Within the sarcoplasm are many perpendicular myofibrils composed of smaller filaments called myofilaments. These myofilaments are actually two types of filaments, a thicker filament composed of the protein myosin and a thinner filament made mostly of the protein actin. The dark stripes are called A bands and the light bands are called I bands. A sarcomere is defined as a unit extending from one Z line to the next, which is the center of the I band. T tubules are open transverse tubules that are located in the surface invaginations of the sarcolemma. They are associated with the sarcoplasmic reticulum, which contains the calcium ions which is necessary for muscular contraction. All muscular contraction happens inside the sarcolemma.
Muscular Contraction
Muscular contraction is defined through the sliding filament theory, which states that as myosin filaments attach to the binding sites on actin, it pulls itself past that binding site to the next, until they slide past each other and bind, thus forming a muscular contraction. During a contraction, the motor neuron releases a neurotransmitter called acetylcholine from its synaptic vesicles into the synaptic cleft to initiate the contraction. The minimum amount of stimulus needed in order for a muscle to contract is called the threshold stimulus. After the threshold stimulus is reached, protein receptors in the motor end plate detect the neurotransmitter and an impulse spreads over the sarcolemma and into the T-tubules, where it goes to the sarcoplasmic reticulum. The sarcoplasmic reticulum releases its calcium ions to the sarcoplasm where it reacts with troponin and tropomyosin molecules, which are associated with actin. The troponin and tropomyosin molecules move aside, which exposes the sites on actin where myosin can attach. The myosin heads then bind and pull on the actin filaments, causing the sarcomeres to shorten. After the nerve impulse has been received, the enzyme acetylcholinesterase, which is found in the synapse between the motor neuron and the muscle membrane, decomposes and removes the actylcholine. The calcium is then returned to the sarcoplasmic reticulum and the linkage between the myosin and actin filaments is broken. Muscle contraction causes a lot of heat, because much of the energy produced by cellular respiration is lost in the form of heat. This is why during exercise, many people feel hot and get sweaty. When a muscle fiber contracts, it contracts to its full extent, it doesn’t have the ability to contract partially. This is called having an all-or-none response. A muscle fiber that receives a series of stimuli of increasing frequency reaches a point when it’s not able to relax all the way, and the force of the individual twitches combine in a process called summation. In cases such as this, the sustained contraction lacks any relaxation, and it is called a tetanic contraction. A muscle twitch occurs when a single, short contraction involving only a few motor units happens. This is not considered a useful contraction, and does nothing for building muscle tone. An increase in the number of activated motor units within a muscle at higher intensities of stimulation is called recruitment. When muscles are in a continuous state of sustained contraction of motor units within a muscle, it is called having muscle tone, which is useful in maintaining good posture.
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