The Nerve Cell

The nerve cell membrane, which is about 5 nm thick, consists primarily of lipids and proteins. When at rest it is permeable basically to potassium ions (although its resistance in this state is rather high, ca. 10 fl cm ), and therefore the electric potential difference between the inner and outer solutions (this difference is called the membrane potential or simply the potential) is negative at rest and amounts to a few tens of mV (about -60 mV on the giant axon). The membrane capacitance is of the order of 1 /iF/cm. Thus a neuron membrane is already polarized when at rest. If some external action shifts the potential from its value at rest to more negative values (its absolute value increasing), the resultant situation is usually called hyperpolarization. Potential shift in the positive direction is called depolarization. If the potential reverses its sign and becomes positive the term is overshoot.  

---

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. 

While these functions can be a carried out by a single transporter isoform (e.g., the serotonin transporter, SERT) they may be split into separate processes carried out by distinct transporter subtypes, or in the case of acetylcholine, by a degrading enzyme. Termination of cholinergic neurotransmission is due to acetylcholinesterase which hydrolyses the ester bond to release choline and acetic acid. Reuptake of choline into the nerve cell is afforded by a high affinity transporter (CHT of the SLC5 gene family). 

Reuptake transporters are structures within the cell membranes of the presynaptic nerve terminal that serve to transport biogenic amines released from vesicles back into the nerve cell. These structures are targets for antidepressants, which block the transporter, thus increasing the bioavailability of neurotransmitters at postsynaptic receptors. 

Herpes zoster (shingles) is caused by the varicella (chickenpox) virus. It is highly contagious. The virus causes chickenpox in the child and is easily spread via the respiratory system. Recovery from childhood chickenpox results in the infection lying dormant in the nerve cells. The virus may become reactivated later in life as the older adult s immune system... 

When the nerve cell is polarized positive, ions (Q) are on the outside of the cell membrane and the negative ions ( )) are on the inside of the cell membrane. 

Nitrous oxide dissolves in the fats that sheath the nerve cells, and produces numbing and mild intoxication. It is the laughing gas dentists use to make patients less aware of pain. 

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. 

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-. 

According to Fig. 6.17 the nerve cell is linked to other excitable, both nerve and muscle, cells by structures called, in the case of other nerve cells, as partners, synapses, and in the case of striated muscle cells, motor end-plates neuromuscular junctions). The impulse, which is originally electric, is transformed into a chemical stimulus and again into an electrical impulse. The opening and closing of ion-selective channels present in these junctions depend on either electric or chemical actions. The substances that are active in the latter case are called neurotransmitters. A very important member of this family is acetylcholine which is transferred to the cell that receives the signal across the postsynaptic membrane or motor endplate through a... 

Fig. 6.24 A hypothetic scheme of the time behaviour of the spike linked to the opening and closing of sodium and potassium channels. After longer time intervals a temporary hyperpolarization of the membrane is induced by reversed transport of potassium ions inside the nerve cell. Nernst potentials for Na+ and K+ are also indicated in the figure. 

Hydroxy tryptamine, or serotonin, is a neurotransmitter in the central nervous system (CNS). The nerve-cell bodies of the major serotoninergic neurones are in the midline raphe nuclei of the rostral pons, and ascending fibers innervate the basal ganglia, hypothalamus, thalamus, hippocampus, limbic forebrain, and areas of the cerebral cortex. The serotoninergic system plays an important role in the control of mood and behavior, motor activity, hunger, thermoregulation, sleep, certain hallucinatory states, and some neuro-endocrine mechanisms. 

Neurotoxin that produces a massive release of transmitters from cholinergic and adrenergic nerve endings resulting in continuous stimulation of muscles. It also induces formation of an ion channel allowing the inward flow of calcium ions into the nerve cell. It is a white powder obtained from the venom of the black widow spider. 

From a historical standpoint, no other cell type has attracted as much attention or caused as much controversy as the nerve cell. It is impossible in a single chapter to delineate comprehensively the extensive structural, topographical and functional variation achieved by this cell type. Consequently, despite an enormous literature, the neuron still defies precise definition, particularly with regard to function. It is known that the neuronal population usually is established shortly after birth, that mature neurons do not divide and that in humans there is a daily dropout of neurons amounting to approximately 20,000 cells. These facts alone make the neuron unique. 

Lane NJ, Swales LS. Interrelationships between Golgi, GERL and synaptic vesicles in the nerve cells of insect and gastropod ganglia. J Cell Sci 1976 22 435 153. 

What is a synapse In the brain, the nerve cells or neurons are connected at special functional junctions called synapses, which depend on many proteins, including large complexes. They participate in basic functions with important roles in coordinating every characteristic of the nervous system, including physiology, emotions, learning, sleep, memory, and pain signal transmission. 

Similar considerations would apply at the synapse between the preganglionic fibre and the nerve cell of the postganglionic fibre of the sympathetic and parasympathetic systems and curare is the blocking agent (fig. 7 A). 

The nerve cell has proved to be an extremely valuable source of ideas about networks of automata. A fundamentally different approach to computation... 

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. 

---