A-motor neuron

FIGURE 1-4 A motor neuron from the spinal cord of an adult rat... 

FIGURE 1-8 A dendrite (D) emerging from a motor neuron in the anterior horn of a rat spinal cord is contacted by four axonal terminals terminal 1 contains clear, spherical synaptic vesicles terminals 2 and 3 contain both clear, spherical and dense-core vesicles (arrow) and terminal 4 contains many clear, flattened (inhibitory) synaptic vesicles. Note also the synaptic thickenings and, within the dendrite, the mitochondria, neurofilaments and neurotubules. x33,000. 

FIGURE 1-11 An electro tonic synapse is seen at the surface of a motor neuron from the spinal cord of a toadfish. Between the neuronal soma (left) and the axonal termination (right), a gap junction flanked by desmosomes (arrows) is visible. (Photograph courtesy of Drs G. D. Pappas and J. S. Keeter.) X80,000. 

Muscle spindles are composed of nuclear bag (dynamic) and chain (static) fibres known as intrafusal fibres and these are innervated by y motor neurones. Extrafusal fibres make up the muscle bulk and are innervated by a motor neurones. Stimulation of the muscle spindle leads to increased skeletal muscle contraction, which opposes the initial stretch and maintains the length of the fibre. This feedback loop oscillates at 10 Hz, which is the frequency of a physiological tremor. 

Figure 1.14 (a) Basic structure of a neurone. A motor neurone is shown, but the basic structure of all the neurones is the same. Dendrites transfer information from other nerves to the neurone, while the axon transfers information from the neurone to other neurones or tissues. The axon is particularly long in motor neurones (Chapter 14). (b) Structures of unipolar, bipolar and multipolar neurones. Unipolar neurones transfer information from tissues or organs to the brain. Multipolar neurones are the most abundant in the nervous system. 

The cell body of a mammalian motor neuron has an extended process termed an axon that branches at its tip to make multiple contacts (synapses) with a muscle cell (fig. S1.1). Some axons actually reach lengths of several meters. Much of the axon may be encased in myelin sheaths, which are multilayered membranes formed by other cells that wrap themselves around the neuron. Shorter processes (dendrites) extend from the cell body to make contacts with other neurons. If a motor neuron is stimulated electrically or is triggered by its connecting neurons, an electric signal called an action potential sweeps down the axon and is transmitted to the muscle cell, which then proceeds to contract. Other... 

A number of researchers have tried training neural networks to achieve color constancy. A neural network basically consists of a set of nodes connected by weights (McClelland and Rumelhart 1986 Rumelhart and McClelland 1986 Zell 1994). Artificial neural networks are an abstraction from biological neural networks. Figure 8.2 shows a motor neuron in (a) and a network of eight artificial neurons on the right. A neuron may be in one of... 

Figure 8.2 A motor neuron (a) and small artificial neural network (b). A neuron collects signals from other neurons via its dendrites. If the neuron is sufficiently activated, it sends a signal to other neurons via its axon. Artificial neural network are often grouped into layers. Data is entered through the input layer. It is processed by the neurons of the hidden layer and then fed to the neurons of the output layer. (Illustration of motor neuron from Life ART Collection Images 1989-2001 by Lippincott Williams Wilkins used by permission from SmartDraw.com.)...

Neurons assume a vast array of forms in accordance with the functions they serve. In most neurons the cell body and dendrites are separated from the axonal terminal by a very long tube, the axon. This creates problems unique to nerve cells. In the motor neurons that innervate hands and feet, for example, more than 90 percent of the mass of the neuron is in the cell processes. An often given example of this relationship is that if the cell body of a motor neuron were enlarged to the size of a baseball, the corresponding axon would be about 1 mile long, and the dendrites and their branches would arborize throughout a large amphitheater. 

Stimulation of the peripheral nerve trunk of intact animals leads to generation of muscle action potentials of three types. According to the duration of latent periods, they fall into the following order M-response (the result of the direct stimulation of a-motor neuron axons), Fl-response (the monosynaptic response), and polysynaptic responses with the variable latent period from 8-12 up to about 40 ms. In test animals of the III group, the changes of temporal parameters refer mainly to the latent period and duration of M-response (Table 7.4). Polysynaptic responses occur at all intensities of excitation and have a more pronounced character than in intact rats. A marked level and more distinct differentiation of the peaks of the complex action potential were noted. 

It is clear that these criteria are fulfilled in the cases of ACh and NE acting as neurotransmitters at different synapses. Let us consider, for example, the effects of ACh at the synapse made by a motor neuron with fast skeletal muscle ( neuromuscular junction ). 

Figure 14.4. Drawing of a motor neuron (figure used witli ijermission from Biology Mad http //www.biologymad.com/Nei vousSystem/ nei voussystemintro.htm).

The motor unit has four components a motor neuron in the brain or spinal cord, its axon and related axons that comprise the peripheral nerve, the neuromuscular junction, and all the muscle fibers activated by the neuron. Like other cells, nerve and muscle cells have an external membrane that separates the inner fluids from those on the outside. The fluid on the inside is rich in potassium (K), magnesium (Mg), and phosphorus (P), whereas the fluid on the outside contains sodium (Na), calcium (Ca), and chloride (Cl). When all is quiet, the internal chemical composition of both nerve and muscle cells is remarkably constant and is called resting membrane potential. A primary reason for this constancy lies in the cells ability to regulate the flow of sodium— thanks to an enzyme in the membrane called Na+/K+ ATP-ase. Because the inside of the cell has less sodium than the outside, there is a negative potential (like a microscopic battery) of 70-90 mV. Under ordinary circumstances, the interior of the cell is 30 times richer in potassium than the extracellular fluid and the sodium concentration is 10-12 times greater on the outside of the cell. At rest, sodium tends to flow into cells and potassium oozes out. 

Efferent neuron— A motor neuron that carries an impulse from the central nervous system to muscles or glands. 

Neurotransmitters affect receptors in two basic ways. Some bind to receptors which are said to have ionic effects. These receptors, when activated, operate to open tiny pores (ion-channels), allowing electrically charged particles (ions) to enter the nerve cell. When numerous ionic receptors are activated, this can result in either an excitation of the nerve cell (action potential) or, conversely, a calming of the nerve cell (hyperpolarization, which makes it less likely that the cell will fire). Excitation or inhibition depends on which specific type of channel is activated. This phenomenon is responsible for eliciting immediate and transient changes in neuronal excitability (for example, this occurs when a motor neuron is activated and there is corresponding activation of a muscle, or when sensory events are perceived). 

Initiation of muscle contraction by action potential from an a motor neuron depolarizes the muscle-cell membrane and causes an increase in the myoplasmic [Ca " ] (Chapter 21). The increase in calcium activates phosphorylase kinase and Ca +/calmoduIin-dependent kinase, which inactivate glycogen synthase and active glycogen phosphorylase (Figure 15-11). This step coordinates muscle contraction with glycogenolysis. These steps are reversed by one or more phosphatases at the end of contraction. 

As we have seen, action potentials can move down an axon without diminution at speeds up to 1 meter per second. But even such fast speeds are insufficient to permit the complex movements typical of animals. In humans, for instance, the cell bodies of motor neurons innervating leg muscles are located in the spinal cord, and the axons are about a meter in length. The coordinated muscle contractions required for walking, running, and similar movements would be impossible if it took one second for an action potential to move from the spinal cord down the axon of a motor neuron to a leg muscle. The presence of a myelin sheath around an axon increases the velocity of Impulse conduction to 10-100 meters... 

Careful monitoring of the membrane potential of the muscle membrane at a synapse with a cholinergic motor neuron has demonstrated spontaneous, intermittent, and random 2-ms depolarizations of about 0.5-1.0 mV in the absence of stimulation of the motor neuron. Each of these depolarizations is caused by the spontaneous release of acetylcholine from a single synaptic vesicle. Indeed, demonstration of such spontaneous small depolarizations led to the notion of the quanta release of acetylcholine (later applied to other neurotransmitters) and thereby led to the hypothesis of vesicle exocytosis at synapses. The release of one acetyl-choline-containing synaptic vesicle results In the opening of about 3000 ion channels In the postsynaptic membrane, far short of the number needed to reach the threshold depolarization that induces an action potential. Clearly, stimulation of muscle contraction by a motor neuron requires the nearly simultaneous release of acetylcholine from numerous synaptic vesicles. 

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