An action potential is a special type of electrical signal that can travel along a cell membrane as a wave. This allows a signal to be transmitted quickly and faithfully over long distances. The myosin then pulls the actin filaments toward the center, shortening the muscle fiber. In skeletal muscle, this sequence begins with signals from the somatic motor division of the nervous system. The motor neurons that tell the skeletal muscle fibers to contract originate in the spinal cord, with a smaller number located in the brainstem for activation of skeletal muscles of the face, head, and neck.
These neurons have long processes, called axons, which are specialized to transmit action potentials long distances— in this case, all the way from the spinal cord to the muscle itself which may be up to three feet away. The axons of multiple neurons bundle together to form nerves, like wires bundled together in a cable. Signaling begins when a neuronal action potential travels along the axon of a motor neuron, and then along the individual branches to terminate at the NMJ.
At the NMJ, the axon terminal releases a chemical messenger, or neurotransmitter , called acetylcholine ACh. The ACh molecules diffuse across a minute space called the synaptic cleft and bind to ACh receptors located within the motor end-plate of the sarcolemma on the other side of the synapse.
Once ACh binds, a channel in the ACh receptor opens and positively charged ions can pass through into the muscle fiber, causing it to depolarize , meaning that the membrane potential of the muscle fiber becomes less negative closer to zero. As the membrane depolarizes, another set of ion channels called voltage-gated sodium channels are triggered to open.
Things happen very quickly in the world of excitable membranes just think about how quickly you can snap your fingers as soon as you decide to do it. Immediately following depolarization of the membrane, it repolarizes, re-establishing the negative membrane potential. Meanwhile, the ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase AChE so that the ACh cannot rebind to a receptor and reopen its channel, which would cause unwanted extended muscle excitation and contraction.
Propagation of an action potential along the sarcolemma is the excitation portion of excitation-contraction coupling. The arrangement of a T-tubule with the membranes of SR on either side is called a triad Figure 7. The triad surrounds the cylindrical structure called a myofibril , which contains actin and myosin. Skip to main content. Ankle sprain is a common sports injuries caused by overstretching and tearing the supporting ligaments. Content on this website is provided for information purposes only.
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There are about muscles in the human body. Muscles have a range of functions from pumping blood and supporting movement to lifting heavy weights or giving birth. Muscles work by either contracting or relaxing to cause movement. This movement may be voluntary meaning the movement is made consciously or done without our conscious awareness involuntary. An individual skeletal muscle may be made up of hundreds, or even thousands, of muscle fibers bundled together and wrapped in a connective tissue covering.
Each muscle is surrounded by a connective tissue sheath called the epimysium. Fascia , connective tissue outside the epimysium, surrounds and separates the muscles.
Portions of the epimysium project inward to divide the muscle into compartments. Each compartment contains a bundle of muscle fibers. Each bundle of muscle fiber is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium. The cells do not fuse during development, leaving each cell with a single nucleus. One commonality between skeletal and cardiac muscle is the presence of striations due to the arrangement of actin and myosin into regular myofibrils.
The presence of myofibrils and many mitochondria in cardiac muscle cells provides them with great strength and endurance to pump blood throughout an entire lifetime. Visceral muscle cells are found in the organs, blood vessels, and bronchioles of the body to move substances throughout the body. Visceral muscles are also commonly known as smooth muscle due to their lack of striations. Four characteristics define smooth muscle tissue cells: they are involuntarily controlled, not striated, not branched, and singly nucleated.
The unconscious regions of the brain control visceral muscle through the autonomic and enteric nervous systems. Thus, visceral muscle is involuntarily controlled. This is evidenced by our inability to consciously control many physiological processes such as our blood pressure or digestion.
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