Nodes, nerve cell

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-. 

Nerve cells Nerve cells, or neurons, consist of a cell body from which the dendrites and axon extend. The dendrites receive information from other cells the axon passes this information on to another cell, the post-synaptic cell. The axon is covered in a myelin membranous sheath except at the nodes of Ranvier. The axon ends at the nerve terminal where chemical neurotransmitters are stored in synaptic vesicles for release into the synaptic cleft. 

The structure of a nerve is not simple. In the following account, the stress is upon a single aspect of the mechanism of the action of a nerve, the origin of the spike potential in sections of the nerve called nodes in which the axon is in contact on the outside with the extracellular fluids. The relevant properties of a nerve cell free of a myelin sheath can be seen in Table 14.1. 

Figure 1. Nerve cell structure 1. neuron body, 2. dendrite, 3. axon, 4. myelin sheath, 5. node of Ranvier.

Myelination in the PNS is preceded by invasion of the nerve bundle by Schwann cells, rapid multiplication of these cells and segregation of the individual axons by Schwann cell processes. Smaller axons (<1 pm), which will remain unmyelinated, are segregated several may be surrounded by one Schwann cell, each within its own pocket, similarly to the single axon shown in Figure 4-10A. Large axons (>1 pm) destined for myelination are enclosed singly, one cell per axon per internode. These cells line up along the axons with intervals between them the intervals become the nodes of Ranvier. 

In myelinated peripheral nerves, the sheath is intenupted by small gaps, the nodes of Ranvier. It is these nodes that increase markedly the velocity of the action potential as it travels along the axon (see below). Myelinated and nonmyelinated axons, together with their Schwann cells, are shown in Figure 14.1. 

The cytoskeleton is found near the axonal membrane and consists of microfilaments linked internally to microtubules and the plasma membrane by a network of filamentous protein that includes the brain-specific protein fodrin. This protein forms attachment sites for integral membrane proteins either by means of the neuronal cell adhesion molecule (N-CAM) or indirectly by means of a specific protein called ankyrin in the case of the sodium channels. This may provide a means whereby the sodium channels are concentrated in the region of the nodes of Ranvier. Thus the cortical cytoskeleton plays a vital role in neuronal function by acting as an attachment site for various receptors and ion channels, but also for s)maptic vesicles at nerve terminals, thereby providing a mechanism for concentrating the vesicles prior to the release of the neurotransmitter. 

Transmembrane action potential of a sinoatrial node cell. In contrast to other cardiac cells, there is no phase 2 or plateau. The threshold potential (TP) is -40 mV. The maximum diastolic potential (MDP) is achieved as a result of a gradual decline in the potassium conductance (gK+). Spontaneous phase 4 or diastolic depolarization permits the cell to achieve the TR thereby initiating an action potential (g = transmembrane ion conductance). Stimulation of pacemaker cells within the sinoatrial node decreases the time required to achieve the TR whereas vagal stimulation and the release of acetylcholine decrease the slope of diastolic depolarization. Thus, the positive and negative chronotropic actions of sympathetic and parasympathetic nerve stimulation can be attributed to the effects of the respective neurotransmitters on ion conductance in pacemaker cells of the sinuatrial node. gNa+ = Na+ conductance. 

The white matter is a covering for the nerve fiber known as the myelin sheath. Myelin is a sphingolipid produced by oligodendrocytes (Schwann cells). It acts as a special type of insulation which allows for a much more rapid transmission of the nerve signal. The myelin cover is interrupted periodically. This "bare" area is called a node of Ranvier. It is very important to rapid nerve conduction. See Figure 21. 

It should be noted that the myelin sheath helps to propagate the AP even faster down the neuron. The signal will "jump" from one node of Ranvier to the next (saltatory motion). The distribution of sodium and potassium channels is uneven in the myelinated regions, appropriate to the node and covered areas of the nerve. Should the myelin become stripped from a normally myelinated cell, then the electrical signal cannot... 

The sinoatrial (SA) node is innervated by both the sympathetic (beta and parasympathetic (vagus) nervous systems. Sympathetic activation increases the discharge rate of the SA pacemaker cells, and thereby increases heart rate (a positive chronotropic effect). Sympathetic nerves also innervate adrenergic receptors (betaj) on cardiac ventricular cells leading to an increase in stroke volume (a positive inotropic effect). Vagal activation, on the other hand, has the opposite effect and decreases heart rate and conduction velocity. In normal adults, cardiac vagal innervation is functionally predominant, so abolition of vagal activity results in a pronounced tachycardia (increased heart rate). 

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