Neurons and Nerves

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Figure 01a Nervous System

The human nervous system has two main divisions (Figure 01a): the central nervous system (CNS), and the peripheral nervous system (PNS), which includes the somatic motor nervous system, and the sensory nervous system. The CNS consists of the brain and spinal cord. It acts as the central control region of the human nervous system, processing information and issuing commands. The autonomic nervous system (ANS) is the command network the CNS uses to maintain the body's homeostasis. It automatically regulates heartbeat and controls muscle contractions in the walls of blood vessels, digestive, urinary, and reproductive tracts. It also carries messages that help stimulate glands to secrete tears, mucus, and digestive enzymes.

Figure 01b Neuron and Nerve

The nerves (Figure 01b) that are easily visible to the unaided eye are not single cells. Rather, they are bundles of nerve fibers (neurons) each of which is itself a portion of a cell. The fibers are all traveling in the same direction and are bound together for the sake of convenience, though the individual fibers of the bundle may have widely differing functions. There are no cell bodies in nerves; cell bodies are found only in the CNS or in the ganglia. Ganglia are collections of cell bodies within the PNS.

The main portion of the neuron, the cell body, is not too different from other cells. It contains a nucleus and cytoplasm. Where it is most distinct from cells of other types is that out of the cell body, long threadlike projections emerge. Over most of the cell there are numerous projections that branch out into still finer extensions. These branching threads are called dendrites ("tree" in Greek). At one point of the cell, however, there is a particularly long extension that usually does not branch throughout most of its sometimes enormous length. This is the axon (the axis).

Figure 01b shows the three parts of the neurons: dentrite(s), cell body, and axon. A dendrites conducts nerve impulses toward the cell body, the part of a neuron that contains the nucleus and other organelles. An axon conducts nerve implses away from the cell body. There are three types of neurons: sensory neuron,motor neuron, and interneuron. A sensory neuron takes a message from the recptors in the sense organ to the CNS. A motor neuron sends a message away from
the CNS to an effector, a muscle fiber or a gland. An interneuron is always found completely within the CNS and conveys messages between parts of the system. In addition to neurons, nervous tissue contains glial cells such as the Schwann cells covering the neurons with sheath. These cells maintain the tissue by supporting and protecting the neurons. They also provide nutrients to neurons and help to keep the tissue free of debris. The neurons require a great deal of energy for the maintenance of the ionic imbalance between themselves and their surrounding fluids, which is constantly in flux as a result of the opening and closing of channels through the neuronal membranes. Thus while the brain is only 2% of our body weight, it consumes 20% of our energy and moreover 80% of this energy consumption is devoted to maintain the imbalance.

Figure 01c Neurons

Neurons are dynamically polarized, so that information flows from the fine dendrites into the main dendrites and then to the cell body, where it is converted into all-or-none signals, the action potentials, which are relayed to other neurons by the axon, a long wirelike structure. The neuron is actually a very poor conductor; the signal drops to 37% of its original strength in only about 0.15 mm. Thus it needs amplification all along its length in the form of sodium-potassium pumps and gates (see Figure 01d). The amplification is initiated by detection of small changes in voltage across the membrane with the opening of voltage-sensitive sodium channels in the membrane of the neuron. Sodium ions rush into the neurons from the extracellular fluid, resulting in a transient change in the voltage difference between the neuron and the surrounding environment. The action potential travels like a wave from the cell body down the neuron via the repeating amplifications. Thus, the action potential enables the neuron to communicate rapidly with other neurons over sizable distances, sometime more than a meter away with a speed from 20 -200 m/sec. When the action potential reaches an axon terminal (the synapse), it causes the terminals to secrete a chemical messenger (neurotransmitter), generally an amino acid or its derivative, which binds to receptors in the post-synaptic neurons on the far side of the synaptic cleft. When the postsynaptic potential has reached a specific value an action potential is triggered and the signal is passed to the next neuron.

Figure 01d Action Potential

The Human Connectome Project (HCP) was launched in 2009. Its aim is to trace the brain's long-range communication network using two main techniques, both of which rely on magnetic resonance imaging (MRI) to obtain data from living people. The "connectome" is a web of nerve-fibre bundles that criss-cross the brain in their thousands and form the bulk of the brain's white matter. It relays singals between specialized regions devoted to functions such as sight, hearing, motion and memory, and ties them together into a system that perceives, decides and acts as a unified whole. If a standard map of the connections can be produced from the average, it can be used to shed light on what the variations might mean for qualities such as intelligence or sociability, and possibly reveal what happens if the network goes awry. It is believed that brain disorders - from schizophrenia to depression to post-traumatic stress disorder - are disorders of connectivity. One of the two techniques is Diffusion-Spectrum Imaging (DSI), which traces the direction of water molecules moving in the brain (Figure 01e). This method maps out the structure of the brain. The jumbling mess of the neuron connection is similar to the entangled cables in the average home computer room - it takes a while to figure out what are the cables for? Thus the second method of resting-state functional MRI (re-fMRI) is required to look for activated brain regions without the participants to carry out a specific cognitive task such as in fMRI. It is assumed that the increase in blood flow is related to brain activity. In re-fMRI, there is no task, it is instead assumed that correlations among the activity levels in different areas are linked. This method looks for specific function preformed by different area of the brain (Figure 01e). The two methods together would allow construction of standard connectome to identify the wiring with functions.
 

Figure 01e Connectome