Neuronal axons

A cognate property of this continuous production throughout life is the repair of damaged parts of the neurone axonal and or dendritic re-growth readily occurs, an ability retained even by higher primates (Graziadei et al., 1980). Such a regenerational capacity is probably due to the peripheral site of the cell body, since those within the CNS mostly lack this property. 

At this point, it is important to note that a nerve is defined as a bundle of neuronal axons some are afferent and some are efferent. A nerve does not consist of entire neurons, only their axons. Furthermore, nerves are found only in the peripheral nervous system. Bundles of neurons with similar functions located within the CNS are referred to as tracts. Therefore, technically speaking, no nerves are within the brain or the spinal cord. 

Figure 7.2 Ascending and descending tracts in white matter of the spinal cord. Tracts are formed of bundles of neuronal axons that transmit similar types of information.

The corticospinal tracts originate in the cerebral cortex. Neurons of the primary motor cortex are referred to as pyramidal cells. Most of these neurons axons descend directly to the alpha motor neurons in the spinal cord. In... 

As discussed previously, the neurohypophysis has a direct anatomical connection to the hypothalamus. Therefore, the hypothalamus regulates the release of hormones from the neurohypophysis by way of neuronal signals. Action potentials generated by the neurosecretory cells originating in the hypothalamus are transmitted down the neuronal axons to the nerve terminals in the neurohypophysis and stimulate the release of the hormones into the blood. The tracts formed by these axons are referred to as hypothalamic-hypophyseal tracts (see Figure 10.2). The action potentials are initiated by various forms of sensory input to the hypothalamus. Specific forms of sensory input that regulate the release of ADH and oxytocin are described in subsequent sections in this chapter. 

Median forebrain bundle A bundle of monoaminergic neuronal axons travelling from the brainstem through the diencephalon to various limbic structures. 

FIGURE 50-1 A schematic diagram of the olfactory epithelium. The initial events in odor perception occur in the olfactory epithelium of the nasal cavity. Odorants interact with specific odorant receptors on the lumenal cilia of olfactory sensory neurons. The signals generated by the initial binding events are transmitted along olfactory neuron axons to the olfactory bulb of the brain. 

Pyrrolidation of proteins appears to be a necessary step in -hexane neurotoxicity, and the targets relevant to toxicity are thought to be neuronal axon proteins (Graham et al. 1995). However, w-hexane metabolites can pyrrolidate a variety of proteins at lysine residues, which upon oxidation can become crosslinked. Pyrrolidated proteins in rat hair have been measured after intraperitoneal administration of... 

Aggregates of ubiquinated a-synuclein are found in so-called Lewy bodies, intracellular inclusions present within dopaminergic neurones, axons and synapses of the substantia nigra, and are a characteristic feature of PD. 

Tohda C. 2003. Comprehensive identifying method for localized mRNAs in single neuronal axons. J Biochem Biophys Meth 57 57-63. 

Once you get an action potential generated, it will flow down the neuronal axon. Remember that an action potential is a sequence of membrane depolarization and repolarization events mediated by voltage-gated sodium ion and potassium ion channels. The basic idea is provided in figure 21.3. The initial depolarization will spread to adjacent voltage-gated ion channels, which will open, ions will flow, and so forth. Thus, the wave of depolarization moves down the axon. It is the means by... 

Neuromuscular junction a chemical synapse between a spinal motor neuron axon and a skeletal muscle fiber. 

At the end of each neuron are stores of chemicals called neurotransmitters that can be released to stimulate adjacent neurons (Figure 2.2). There are many different neurotransmitters, dependent on location and specific function in the nervous system. Generally, once a neuron is stimulated, the stimulus travels along the neuronal axon until it reaches the end of the neuron from which a neurotransmitter is released. The neurotrans-... 

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... 

As previously described, one of the major neuropathological hallmarks of AD are NFTs. NFTs are composed of paired helical filaments, with the principal protein subunit of paired helical filaments being abnormally hyper-phosphorylated tau (p-tau). Physiologically, tau protein is located in the neuronal axons and in the cytoskeleton. There are six different tau isoforms. Total tau (t-tau) and truncated forms of monomeric and phosphorylated tau are released and can be found in the CSF [71, 80]. 

Degenerating axons in the peripheral nerves can also be examined as described below, and electrophysiological estimates of motor unit number could be informative. Motor units are defined as a single motor neuron (axon) and the muscle fibers its terminals innervate. In motor neuron diseases, the number of motor units is anticipated to decrease as motor neurons die, but the size of motor units may increase with compensatory sprouting and reinnervation (e.g., (1)). Interpretation can be further confounded by factors such as a change in muscle fiber number or innervation of muscle fibers by multiple motor axons. Therefore, the best interpretation results from corroborating evidence from the spinal cord, nerve, synapse, and the muscle. 

Olfactory perception translates abstract chemical features of odorants into meaningful neural information to elicit appropriate behavioral responses (Shepherd, 1994 Buck, 1996). Specialized bipolar olfactory sensory neurons (OSNs) are responsible for the initial events in odor recognition. These have ciliated dendrites exposed to the environment, and a single axon that extends into the brain and forms synapses with second order projection neurons (PNs) (Shepherd, 1994 Buck, 1996). In arthropods and mammals, the first olfactory synapse is organized into glomeruli, spherical structures in which afferent olfactory neuron axons synapse with projection neuron dendrites (Hildebrand and Shepherd, 1997). 

MS is an autoimmune disease that attacks the myelin sheath of oligodendrocytes around the neuronal axons. This allows the axonal cytoskeleton to be damaged, bringing about secondary axonal loss and persisting neurological dysfunction. The characteristic pathology is of a lesion or plaque in the CNS white matter, formed by inflammation and demyelination and these can be classified into active, chronic active, or chronic silent plaques [86]. 

Konopaske GT, Sweet RA, Wu Q, Sampson A, Lewis DA. 2006. Regional specificity of chandelier neuron axon terminal alterations in schizophrenia. Neuroscience 138(1) 189-196. 

Pierri JN, Chaudry AS, Woo TU, Lewis DA. 1999. Alterations in chandelier neuron axon terminals in the prefrontal cortex of schizophrenic subjects. Am J Psychiatry 156 1709-1719. 

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