Electrical impulses, nervous system

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-. 

Another clinically important effect I would like to mention is the inhibition of salivary secretion by clonidine. Both the sympathetic nervous system and the parasympathetic nervous system are involved in the physiological regulation of salivation. HOEFKE (53) as well as RAND and coworkers (54) found that parasympathetic salivary secretion stimulated by electrical impulses on the chorda tympani and by carbachol could not be blocked by clonidine in anaesthetised animals. In our own experiments in rats with clonidine and the 2,6-diethyl derivative St 91 which does not penetrate to the CNS, secretion of saliva was blocked only after clonidine, (HOEFKE (55)) indicating a central mode of action. 

Signaling in the nervous system is accomplished by networks of neurons, specialized cells that carry an electrical impulse (action potential) from one end of the cell (the cell body) through an elongated cytoplasmic ex-... 

Classically, the central nervous system has been envisioned as a series of hard-wired synaptic connections between neurons, not unlike millions of telephone wires within thousands upon thousands of cables (Fig. 1—4). This idea has been referred to as the anatomically addressed nervous system. The anatomically addressed brain is thus a complex wiring diagram, ferrying electrical impulses to wherever the wire is plugged in (i.e., at a synapse). There are an estimated 100 billion neurons, which make over 100 trillion synapses, in a single human brain. 

The central point within our consciousness, our "spirit" in the hermetic sense, can now be seen as an entity that can work to control quantum probabilities. To our "spirits" our brain is a quantum sea providing a rich realm in which it can incarnate and manifest patterns down into the electrical/chemical impulses of the nervous system. (It has been calculated that the number of interconnections existing in our brains far exceeds the number of atoms in the whole universe - so in this sense the microcosm truly mirrors the macrocosm ). Our "spirit" can through quantum borrowing momentarily press a certain order into this sea and this manifests as a thought, emotion, etc. Such an ordered state can only exist momentarily, before our spirit or point of consciousness is forced to jump and move to other regions of the brain, where at that moment the pattern of probability waves for the particles in these nerve cells, can reflect the form that our spirit is trying to work with. 

There is not space here to explain the process of nerve impulse transmission. It is an electrical process and involves pumping of Na+ and K+ ions across barriers. It should be noted that there are cells other than neurons in the nervous system, of which the most abundant are glial cells. 

Typically in the nervous system and in a synapse, the movement of muscle is controlled by a nerve. An electrical signal or nerve impulse is controlled by ACh... 

It is now well ascertained that dendrites are capable of propagating action potentials not only in distal to proximal direction, but also in the reverse direction by back-propagation after initiation at the cell body (Ludwig and Pittman, 2003). The so-called law of dynamic polarization enunciated by Cajal (see Berlucchi, 1999) was aimed at stating the unidirectional propagation of excitations within the nervous system, and assumed that nerve impulses are conducted from the dendrite or soma to axon terminals. This dogma is now being reconsidered, not only in view of the evidence of dendrodendritic synapses, but also in view of the existence of electrical synapses in which the flow of information can be bidirectional. 

Gap junctions provide in the nervous system the structural correlate of one class of electrical synapses, characterized by very close apposition between the presynaptic and postsynaptic membranes. It should be noted, in this respect, that different junctional specializations can mediate different forms of electrical transmission between neurons (Bennett, 1997). Electrical synapses transmit preferentially, but not exclusively, low-frequency stimuli, that allow the rapid transfer of a presynaptic impulse into an electrical excitatory potential in the postjunctional cells. Electrical transmission, via the intercellular channels, can be bidirectional. The widely held opinion that electrical transmission is characteristic of lower vertebrates probably derives from the large cell systems in which electrical synapses were identified in the initial period of intracellular recording (reviewed by Bennett, 1997). Contradicting this view, electrotonic coupling between neurons has now been demonstrated in many areas of the mammalian central nervous system and has been implicated in neuronal synchronization. Gap junctional intercellular communication can occur between glial cells, glia and neurons, as well as between neurons. 

A nervous system is essential for the passage of information through the body by means of electrical signals. As shown in Figure 7.1, in insects reflex is initiated when a sensory receptor detects a stimulus. A sensory neuron conveys the electrical impulse to the effector organs via an interneuron involving two synapses. 

Observations of metabolic cross-correction provided the rationale for cellular replacement, achieved primarily through allogeneic hematopoietic stem cell or bone transplantation (HSCT) (Prasad and Kurtzberg, 2008). More recently, the use of neural stem cells (NSC) implanted in the brain of patients with late-infantile neuronal ceroid lipofuscinosis has been contemplated (Pierret et al., 2008) but there are no reports as yet of its potential efficacy. Within the central nervous system there must be proper integration of donor cells, and differentiation into appropriate cell types. As specialized cell types within the nervous system elaborate neurotransmitters and are involved with conducting electrical impulses, functional differentiation may be a major hurdle for the neurodegenerative LSDs. 

In contrast to the endocrine system that achieves long-term control via chemical (hormonal) mechanisms, the nervous system relies on more rapid mechanisms of chemical and electrical transmission to propagate signals and commands. The rapid conduction of impulses is essential in allowing the nervous system to mediate shortterm and near immediate communication and control between various body systems. 

The nervous system conducts electrical impulses throughout the body to regulate and control physiological processes of the other organ systems. Organs of the nervous system include the brain, spinal cord, and nerves. 

The nervous system consists of sensory and motor compo-nenLs. The. sensory compiment responds to various external stimulations, which it transmits in the form of a nerve impulse to the CNS for interpretation. The motor component of the nervous. system carries a signal from the CNS to the appropriate part of the bixly to elicit the rasponse to the stimulation. One of thc.se rcspoascs is the sensation known as pain. Nerve impulses arc now known to take the form of an electrical impulse. Experimental evidence suggests that both stimulation and the transmission of a nerve impulse may be bUx ked by the action of local anesthetic agents. Consequently, understanding this action requires a knowledge of the structure and action of the nervous system. 

There are three distinct types of muscle tissue in vertebrates striated, smooth, and cardiac. Striated, or skeletal, muscle is attached, at least at one end, to the skeleton via tendons. This muscle type is often referred to as the voluntary muscle, as it can be consciously controlled. Smooth muscle is usually arranged in sheets or layers in tubular systems, such as arteries and veins (see Blood Vessels), the gastrointestinal and respiratory tracts, and the genitourinary tracts. The activities of the smooth muscles are not under conscious control rather they are coordinated by the autonomic (involuntary) nervous system. The cardiac muscle comprises the bulk of the heart wall proper and small amounts are found in the superior vena cava and pulmonary vein. The cardiac muscle is not under conscious control it has an automaticity center which responds to the autonomic nervous system when needed (see section Impulse Conduction). In the heart, cardiac muscle cells are joined in a network of fibers and are connected by gap junctions, which facilitate the conduction of electrical impulses through the cardiac muscle network. In addition to the typical cardiac myocytes, there are other cardiac muscle cells that are specialized to initiate, attenuate, or accelerate the electrical impulses for coordinated contraction of the cardiac network. 

Electrical impulses are generated first by the sinoatrial (SA) node and then moves to the atrioventricular (AV) node at 60 to 80 contractions per minute. Ventricles can contract independently at 30 to 40 contractions per minute. Contractions are influenced by the autonomic nervous system (see chapter 15) and medication. 

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