Neuron wiring

The wrong neuronal wiring of the anatomically addressed nervous system could thus be quite problematic for proper brain functioning. 

To understand the chemical-imbalance theory, it will be helpful to first review some basic aspects of how the brain functions. The human brain contains about ioo billion nerve cells called neurons. Each neuron is like an electrical wire with many branches. When a neuron fires, electrical impulses travel along its length from one end to the other. When an impulse reaches the end of a branch, it may stimulate the next neuron, influencing whether or not it fires. 

Communication between neurons involves neurotransmitters. Up until the beginning of the last century, synaptic transmission was regarded as probably electrical. It was suggested that the close apposition of two neurons allowed the current to jump the synaptic cleft, rather like an electrical spark between two closely positioned wires. There is indeed evidence for electrical synapses in animal species where the synaptic cleft is particularly narrow (2 nm, or nanometres), as well as in the myocardium where the close coupling of cells allows electrical current to flow from one cell to the next,... 

The tissue-compatible microprobes represent an advance over the typical aluminum wire electrodes used in studies of the cortex and other brain structures. Researchers accumulate much data using traditional electrodes, but there is a question of how much damage they cause to the nervous system. Microprobes, which are about as thin as a human hair, cause minimal damage and disruption of neurons when inserted into the brain. 

A new method has been developed for stimulation of the brain trans-cranial magnetic stimulation [TMS]. Magnetic stimulation of the brain may offer a means to examine the importance of the convulsion for ECT effects. TMS is a novel noninvasive method for the stimulation of neurons (for review, see Barker 1991]. High electrical current flow in a spiral of wire induces a magnetic field. The magnetic field produces an electric field that initiates ion flow and consequent membrane depolarization directly in brain tissues. Therefore, magnetic stimulation of deep structures may be achieved with relatively little induced current in the skin or skull and without convulsions [Barker 1991]. 

Under normal circumstances, neurons (nerve cells in the brain) carry electrical signals along wire-like nerve fibers... 

Activation-synthesis model. (A) Systems model. (B) Wiring diagram. As a result of disinhibition caused by cessation of aminergic neuronal firing, brainstem reticular systems autoactivate. Their outputs have effects including depolarization of afferent terminals causing phasic presynaptic inhibition and blockade of external stimuli, especially during the bursts of REM, and postsynaptic hyperpolarization... 

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 arrival of an electrical signal (4) then initiates a series of further steps. This electrical signal is called an action potential. It is a very small electrical disturbance that moves very quickly along the axon away from the cell body. The axon is a long, straight extension of the neuron that carries the action potential and allows one neuron to communicate with other neurons. Axons are rather like electrical wires that connect the different parts of the brain. The arrival of the action potential at the end of the axon induces the entry of calcium ions, which initiate the next step in the communication of one neuron with... 

Patch-clamping was developed in 1970 by Neher135 and Sakmann136 to study the electrochemistry inside cells, single-ion channels, and neurons A thin metal wire is inserted into a glass capillary, which is heated to shrink it around the wire, thus producing electrodes as small as 1 pm in diameter. 

There are approximately 100 billion neurons in the human brain. Neurons have wire-like processes called axons, which transmit information away from the cell, and processes called dendrites, which receive information from other cells. A single neuron s axon can transmit information to other cells over long distances (even four feet), while other neurons may transmit information short distances to neighboring cells. Axon endings make contacts on the cell bodies or dendrites of other neurons. The cell body is the part of the cell containing the nucleus. Neurons can have highly branched dendrites, on which axons from other cells can terminate, forming a synapse, where information can be communicated from one cell to the other. 

Actually, unipolar neurons have two axons rather than an axon and dendrite (Figure 14.3). Mostintemeurons, which form all the neural wiring within the CNS, are of this type. Examples include spinal ganglia, most cranial nerve sensory ganglia, and the trigeminal mesencephalic nucleus. Dorsal root ganglia cells extend one axon centrally toward the spinal cord, and the other axon toward the skin or muscle. 

---