Axon, nerve cell

Saltiness is sensed by taste receptor cells that respond primarily to sodinm chloride. The proteins in the cell membranes involved in transforming the presence of salt into nervons signals are epithelial sodium channels that allow the sodium ions to enter the cells, initiate the release of chemical neurotransmitters, and stimulate adjacent gustatory afferent axons (nerve cells that carry taste information to the brain). Sourness (hydrochloric acid, citric acid, or acetic acid) is fikewise sensed by taste receptor cells in ways that directly affect ion channels. Protons either enter via the epithelial sodinm channels or block epithelial potassinm channels to initiate the cellular response. The bitterness of quinine and calcium is also sensed by blocking potassium channels in taste receptor cell membranes. 

FIGURE 17.8 (a) Rapid axonal transport along microtnbnles permits the exchange of material between the synaptic terminal and the body of the nerve cell, (b) Vesicles, mnltivesicn-lar bodies, and mitochondria are carried throngh the axon by this mechanism. 

Component of the myelin sheath surrounding the axons of nerve cells. Additional compounds of the myelin sheath are phospholipids, cholesterol, cerebrosides, and specific keratins. The myelin sheath constitutes an isolating barrier during electrophysiological axonal signaling. 

Neuroanatomists have taken advantage of the phenomenon of fast retrograde transport to locate remote nerve cell bodies in the CNS of an experimental animal that are connected to an identified axonal fiber tract whose origin is uncertain. The tracer material [purified horseradish peroxidase (HRP) enzyme] is injected in the region of the axon terminals, where it is taken up by endocytosis and then is carried by retrograde axonal transport over a period of several hours to days back to the nerve cell body. The animal is sacrificed, and the enzyme tracer is localized by staining thin sections of the brain for peroxidase activity. 

Intracellular motility is also of vital importance in the lives of cells and the organisms they form. Material and organelles are transported within cells along microtubules and microfilaments an extreme example of this are the axons of nerve cells which transport materials to the synapses where they are secreted—another motile event. Other examples of intracellular motility include phagocytosis, pino-cytosis, the separating of chromosomes and cells in cell division, and maintenance of cell polarity. 

The idea that signals are transmitted along the nerve channels as an electric current had arisen as early as the middle of the nineteenth century. Yet even the first measurements performed by H. Helmholtz showed that the transmission speed is about lOm/s (i.e., much slower than electric current flow in conductors). It is known today that the propagation of nerve impulses along the axons of nerve cells (which in humans are as long as 1.5m) is associated with an excitation of the axon s outer membrane. 

The structural design of nerve cells is a striking example of dendritic architecture, which acts as a signal transduction system. Neurons are known to send out a series of long specialized processes that will either receive electrical signals (dendrites) or transmit these electrical signals (axons) to their target cells (Fig. 5.40). 

The transport of information from sensors to the central nervous system and of instructions from the central nervous system to the various organs occurs through electric impulses transported by nerve cells (see Fig. 6.17). These cells consist of a body with star-like projections and a long fibrous tail called an axon. While in some molluscs the whole membrane is in contact with the intercellular liquid, in other animals it is covered with a multiple myeline layer which is interrupted in definite segments (nodes of Ranvier). The Na+,K+-ATPase located in the membrane maintains marked ionic concentration differences in the nerve cell and in the intercellular liquid. For example, the squid axon contains 0.05 MNa+, 0.4 mK+, 0.04-0.1 m Cl-, 0.27 m isethionate anion and 0.075 m aspartic acid anion, while the intercellular liquid contains 0.46 m Na+, 0.01 m K+ and 0.054 m Cl-. 

Raff It also seems to be true in nerve cells, in the formation of dendrites and axons. This seems to be a stochastic process. 

Figure 7.1 Cross-sectional view of the spinal cord. In contrast to the brain, the gray matter of the spinal cord is located internally, surrounded by the white matter. The gray matter consists of nerve cell bodies and unmyelinated intemeuron fibers. This component of the spinal cord is divided into three regions the dorsal, lateral, and ventral horns. The white matter consists of bundles of myelinated axons of neurons, or tracts. Each segment of the spinal cord gives rise to a pair of spinal nerves containing afferent and efferent neurons. Afferent neurons enter the spinal cord through the dorsal root and efferent neurons exit it through the ventral root.

Tau is a microtubule-binding protein that is believed to be important for the assembly and stabilization of microtubules [ 19]. In nerve cells, tau is normally found in axons,... 

Fundamentally, the eel is simply a living battery. The tips of its head and tail represent the poles of the eel s battery . As much as 80 per cent of its body is an electric organ, made up of many thousands of small platelets, which are alternately super-abundant in potassium or sodium ions, in a similar manner to the potentials formed across axon membranes in nerve cells (see p. 339). In effect, the voltage comprises thousands of concentration cells, each cell contributing a potential of about 160 mV. It is probable that the overall eel potential is augmented with junction potentials between the mini-cells. 

Figure 1.1 Neurons (nerve cells) transmit information throughout the brain and the body. A typical neuron is shown here. Electrical impulses are received by the dendrites and transmitted to the next neuron via the axon. The myelin sheath insulates the axon and increases the speed at which electrical impulses can travel.

The nervous system consists of two main units the central nervous system (CNS), which includes the brain and the spinal cord and the peripheral nervous system (PNS), which includes the body s system of nerves that control the muscles (motor function), the senses (the sensory nerves), and which are involved in other critical control functions. The individual units of the nervous system are the nerve cells, called neurons. Nenrons are a nniqne type of cell becanse they have the capacity to transmit electrical messages aronnd the body. Messages pass from one nenron to the next in a strnctnre called a synapse. Electric impnlses moving along a branch of the nenron called the axon reach the synapse (a space between nenrons) and canse the release of certain chemicals called neurotransmitters, one of which, acetylcholine, we described earlier in the chapter. These chemicals migrate to a nnit of the next nenron called the dendrites, where their presence canses the bnild-np of an electrical impnlse in the second nenron. 

The cell body gathers the incoming action potentials from the dendrites and sends along a single action potential to the axon. The action potential travels the length of the axon until reaching the axon terminals. At this point, the nerve cell must pass the impulse to its neighboring cells. This communication from one neuron to another is accomplished by neurotransmission. 

Neurotransmitter Production. Neurotransmitters are relatively simple chemicals, and our bodies make most of the ones that we use. The nerve cell receives precursor substances such as amino acids from proteins in the diet and chemically processes these precursors to form neurotransmitter chemicals. The neurotransmitter is then stored in small sacs inside the neuron called storage vesicles. These storage vesicles reside inside the axon terminals. 

Neurotransmitter Release. When the nerve cell is stimulated, an action potential is generated that travels the length of the cell from dendrite to cell body to axon. Once the action potential reaches the axon terminal, it causes the storage vesicles... 

As we noted earlier, when the neurotransmitter is released from the axon terminal into the synapse, it is free to diffuse across the synapse to bind the receptors on the neighboring nerve cell. However, other fates may await the neurotransmitter once it s released into the synapse. In general, these other processes act to terminate neurotransmission by preventing the neurotransmitter from reaching the receptor on the adjacent nerve cell. There are, in fact, five distinct mechanisms for terminating the neurotransmitter signal once it has been released into the synapse. 

Negative Feedback. Some of the neurotransmitter diffuses back to the surface of the nerve cell that released it. There are also receptors that tit the neurotransmitter here. When a neurotransmitter binds a receptor (called an autoreceptor) at the axon terminal of the nerve cell that released it, it tells the nerve cell that there s plenty of neurotransmitter already in the synapse. So don t release anymore This process is called negative feedback and is analogous to the way a thermostat works in your home to control room temperature. 

Reuptake. The nerve cell that released the neurotransmitter also has what are called reuptake sites on its surface. These reuptake sites are actually transporter proteins that are specific to each type of neurotransmitter. They act like miniature vacuum cleaners to retrieve the neurotransmitter from the synapse. The neurotransmitter is removed from the synapse at the reuptake site and returned to the inside of the nerve cell s axon terminal. Although the reuptake process recycles the neurotransmitter molecules for future use, the process does, in fact, serve to terminate the current neurotransmitter signal. 

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