Innervated muscle fibers

Regeneration of the Skeletal Neuromuscular Junction and the Innervated Muscle Fibers... 

Fig. 3. Effects of 5 days of intensive oral dosing with vitamin E (200 IU/day) plus selenium (50 pg/day) on early degeneration of cat soleus motor nerve terminals at 48 hours after motor axon section. On the left is a comparison of the nerve-evoked contractile tension in the 48-hour degenerating soleus nerve-muscle preparations to that observed in the contralateral normal preparations. In the untreated animals, about 50% of the motor nerve fibers have ceased to excite their innervated muscle fibers. In contrast, antioxidant pretreatment resulted in a significant preservation of neuromuscular function. Similarly, on the right, a comparison of the ability of the motor nerves to maintain repetitive transmission at a high frequency (100 Hz) for a 10-second period also shows a better preservation of this neuromuscular parameter in the antioxidant pretreated animals.

Spencer RF, McNeer KW. Botulinum toxin paralysis of adult monkey extraocular muscle. Structural alterations in orbital, singly innervated muscle fibers. Arch Ophthalmol 1987 105(12) 1703-11. 

Alpha motor neurons innervate skeletal muscle fibers to cause contraction. 

Gamma motor neurons innervate intrafusal fibers of the muscle spindle, which monitors muscle length. 

Axon divides each axon terminal innervates single muscle fiber directly... 

Muscle fibers are incapable of mitosis. In fact, the number of muscle fibers per muscle is likely determined by the second trimester of fetal development. Therefore, enlargement of a whole muscle is not due to an increase in the number of fibers in the muscle, but rather to the hypertrophy of existing fibers. Because muscle fibers have no gap junctions between them, electrical activity cannot spread from one cell to the next. Therefore, each muscle fiber is innervated by a branch of an alpha motor neuron. A motor unit is defined as an alpha motor neuron and all of the muscle fibers that it innervates. 

Each muscle fiber is innervated by a branch of an alpha motor neuron. The synapse between the somatic motor neuron and the muscle fiber is referred to as the neuromuscular junction. Action potentials in the motor neuron cause release of the neurotransmitter acetylcholine. Binding of acetylcholine to its receptors on the muscle fiber causes an increase in the permeability to Na+ and K+ ions. The ensuing depolarization generates an action potential that travels along the surface of the muscle fiber in either direction that is referred to as a propagated action potential. This action potential elicits the intracellular events that lead to muscle contraction. 

A motor unit is defined as an alpha motor neuron and all of the skeletal muscle fibers it innervates. The number of muscle fibers innervated by an alpha motor neuron varies considerably, depending upon the function of the muscle. For example, the muscles of the eyes and hands have very small motor units. In other words, each alpha motor neuron associated with these muscles synapses with only a few muscle fibers. As a result, each of these muscles is innervated by a comparatively large number of alpha motor neurons. Densely innervated muscles are capable of carrying out more precise, complex motor activities. On the other hand, antigravity muscles have very large motor units. For example, the gastrocnemius muscle of the calf has about 2000 muscle fibers in each motor unit. Muscles with large motor units tend to be more powerful and more coarsely controlled. 

Skeletal muscle is neurogenic and requires stimulation from the somatic nervous system to initiate contraction. Because no electrical communication takes place between these cells, each muscle fiber is innervated by a branch of an alpha motor neuron. Cardiac muscle, however, is myogenic, or self-excitatory this muscle spontaneously depolarizes to threshold and generates action potentials without external stimulation. The region of the heart with the fastest rate of inherent depolarization initiates the heart beat and determines the heart rhythm. In normal hearts, this "pacemaker region is the sinoatrial node. 

Because most skeletal muscle fibers are innervated only by a single endplate, activation of such fibers, with lengths up to 30 cm, entails propagation of the Lullmann, Color Atlas of Pharmacology 2000 Thieme All rights reserved. Usage subject to terms and conditions of license. 

Hall-Craggs, E.C., and Seyan, H.S., 1975, Histochemical changes in innervated and denervated skeletal muscle fibers following treatment with bupivacaine (marcain), Exp Neurol, 46, pp 345—354. 

Degenerating axons in the peripheral nerves can also be examined as described below, and electrophysiological estimates of motor unit number could be informative. Motor units are defined as a single motor neuron (axon) and the muscle fibers its terminals innervate. In motor neuron diseases, the number of motor units is anticipated to decrease as motor neurons die, but the size of motor units may increase with compensatory sprouting and reinnervation (e.g., (1)). Interpretation can be further confounded by factors such as a change in muscle fiber number or innervation of muscle fibers by multiple motor axons. Therefore, the best interpretation results from corroborating evidence from the spinal cord, nerve, synapse, and the muscle. 

Fig. 20.8. Neuromuscular junctions analyzed by transmission electron microscopy. (A) In wild-type mice, the motor nerve terminal (MN) is depressed into the muscle fiber surface. The terminal is polarized, with small clear vesicles near the presynaptic membrane and mitochondria in the more proximal portion of the terminal. The postsynaptic membrane has deep convolutions (junctional folds, JF) and the membrane near the tops of these folds is very electron dense because of the high density of acetylcholine receptors (arrowheads). (B) In some myasthenias where the nerve sprouts but remains in contact with the muscle, terminals with mitochondria and vesicles are observed in the absence of any postsynaptic specialization. Presumably these are sprouting terminals that have not established a functional connection. (C) Partial innervation of postsynaptic sites is evident as elaborate junctional folds in the muscle membrane with no overlying nerve terminal. In these examples, the interpretations were aided by light microscopy examination of other samples as described in Fig. 20.8 in parallel with electron microscopy. The mutation shown in (B, C) is an unpublished ENU-induced allele of agrin.

Figure 7.7. Response of the pupil to activation of autonomous innervation. The movements are caused by the radial and the concentric muscle fibers of the pupil dilatator and sphincter muscles, which have sympathetic/adreneigic and parasympathet-ic/cholineigic innervation and receptors, respectively.

Because sympathetically innervated smooth muscle fibers also exist in the lower eyelid, oculosympathetic paresis can produce elevation of the lower lid (so-called upside-down ptosis). This condition is often subtle. However, this sign, along with ptosis of the upper lid, contributes to a narrowing of the palpebral fissure, giving the appearance of enophthalmos. 

Cardiac muscle cells are surrounded by an endomysi-um like the skeletal muscle cells. But innervation of autonomic nerves to the heart do not form any special junction like that found in skeletal muscle. Instead, the branching structure and extensive interconnectedness of cardiac muscle fibers allows for stimulation of the heart to spread into neighboring myocardial cells this does not require the individual fibers to be stimulated. Although external nervous stimuli can enhance or diminish cardiac muscle contraction, heart muscles can also contract spontaneously making them myogenic. Like skeletal muscle cells, cardiac muscle fibers can increase in size with physical conditioning, but they rarely increase in number. 

Smooth muscle is unstriated with innervations from 2 both sympathetic (flight or fight) and parasympathetic (more relaxed) nerves of the autonomic nervous system. E. Smooth muscle appears unstriated under a polarized light microscope, because the myofilaments inside are less or-ganized. Smooth muscle fibers contain actin and myosin myofilaments which are more haphazardly arranged than they are in skeletal muscles. The sympathetic neurotransmitter, Ach, and parasympathetic neurotransmitter, norepinephrine, activate this type of muscle tissue. 

The concentric arrangement of some smooth muscle fibers enables them to control dilation and constriction in the intestines, blood vessels, and other areas. While innervation of these cells is not individual, excitation from one cell can spread to adjacent cells through nexuses which join neighbor cells. Smooth muscle cells have a small diameter of about 5-15 micrometers and are long, typically 15-500 micrometers. They are also wider in the center than at their ends. Gap junctions connect small bundles of cells which are, in turn, arranged in sheets. 

The size of the pupil is determined by the balance of forces exerted by the dilator muscles fibers (sympathetically innervated and radially arranged) and the constrictor muscle fibers (parasympathetically innervated and circularly arranged) of the iris. Normally both sets of muscle fibers have a constant degree of tonus and act reciprocally to dilate or constrict the pupil. Any substance that paralyzes the constrictor muscle fibers (parasympathol3d ic) allows the unopposed tone of dilator muscle fibers to widen the pupil. 

Acetylcholine is the transmitter between the constrictor muscle fibers and the parasympathetic nerve that innervates them. Therefore, acetylcholine and its congeners stimulate the constrictor muscle fibers of the iris and constrict the pupil. Atropine and related compounds paralyze the constrictor muscle fibers and cause widening or dilatation of the pupil. 

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