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Skeletal muscle fibers are long, multinucleate, and striated. Skeletal muscle is the sole muscle that is voluntary. Each muscle fiber has a nerve ending that gets impulses from brain and stimulates the muscles
Skeletal muscle has a striated appearance because of myofibrils, which are composed of actin and myosin filaments.
Voluntary contraction is only possible in the skeletal muscle. You can manage your body with your mind, which means you can decide whether to relax or tighten certain muscles. These muscles are related to bones and make it possible to walk, run, and lift things
Sarcomeres are the smallest contractile units in muscle fibers, made up of actin and myosin filaments
Calcium ions, which are necessary for muscle contraction, are stored and released by the sarcoplasmic reticulum.
Muscle contraction depends on the protein called myosin, which creates thick filaments in muscle fibers.
EPIMYSIUM --> PERIMYSIUM --> ENDOMYSIUM
Muscle contraction generates heat, which helps in maintaining body temperature.
Calcium ions stored in the sarcoplasmic reticulum are released during excitation-contraction coupling, enabling actin-myosin interaction
Recruiting more motor units increases the number of fibers contracting, enhancing overall muscle tension.
Skeletal muscles are voluntary muscles and require stimulation from a motor neuron to initiate contraction.
The motor end plate is a specialized region of the sarcolemma at the neuromuscular junction. It contains acetylcholine receptors that bind ACh released from the motor neuron, initiating muscle fiber depolarization and contraction
The sarcomere and the H zone are the structures that shorten during contraction. The sarcomere shortens as Z lines move closer, and the H zone and I band narrow or disappear when the muscle is maximally contracted.
Muscle contraction occurs because myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments towards the M line. As a result, the thin filaments slide inward and meet at the center of a sarcomere. The A-band (length of thick filaments) stays the same, while the I-band and H-zone narrow as actin overlaps more with myosin.
Release of acetylcholine into the synaptic cleft is the immediate response to presynaptic depolarization and Ca+ entry. Vesicle’s dock and fuse, exocytosing ACh that then activates nicotinic receptors on the motor end plate to generate an end-plate potential. This event is tightly coupled to the arrival of the nerve action potential and depends on presynaptic voltage-gated Ca+ channels
ATP binding to myosin is required for detachment of the cross-bridge from actin. If ATP were not available, the cross bridge would not be able to detach from actin. The muscle would remain in a state of rigidity, as occur in rigor mortis
Onset of contraction, sarcoplasmic reticulum releases Ca²⁺ into sarcoplasm. They will bind with troponin. Troponin moves tropomyosin away from myosin binding site on actin.
Thus, Cross-bridge formation occurs; myosin performs the power stroke.
ATP binds myosin, allowing detachment from actin. ATP-driven Ca²⁺ pumps return calcium to the SR for relaxation.
The myocardium is the middle layer and contains cardiac muscle responsible for contraction.
Cardiac muscle contracts automatically (involuntary) and follows a regular rhythmic pattern controlled by the heart’s pacemaker.
The myocardium is the middle and thickest layer of the heart and is composed mainly of cardiac muscle cells responsible for contraction.
Gap junctions permit ions to pass directly between cells, allowing rapid spread of electrical impulses
Sheets or layers of smooth muscle cells are contained in the walls of various organs and tubes in the body, including the blood vessels, stomach, intestines, bladder, airways, uterus, and the penile and clitoral cavernosal sinuses.
Although smooth muscle cells do not have striations, smooth muscle fibers do have actin and myosin contractile proteins which interact to generate tension.
What are three functions of ATP in muscle contraction
-Its hydrolysis by an ATPase activates the myosin head so it can bind to actin and rotate.
-Its binding to myosin causes detachment from actin after power stroke
-It powers the pumps that transport Ca²⁺ from the cytosol back into sarcoplasmic reticulum
Cardiac muscle fibers are branched and connected end-to-end by intercalated discs, forming a functional network.
Smooth muscle cells are non-striated, spindle-shaped, and contain a single centrally located nucleus.
Smooth muscle is found in the walls of hollow organs like the gastrointestinal tract, blood vessels, and urinary bladder
Smooth muscle contraction is involuntary and regulated by the autonomic nervous system and hormones.
Smooth muscle uses calmodulin to bind calcium, which activates myosin light-chain kinase for contraction.
Smooth muscle can maintain prolonged contractions (tone) with minimal energy, making it efficient for functions like regulating blood pressure.
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