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22-407: A pseudoganglion looks like a ganglion, but only has nerve fibers and has no nerve cell bodies. Ganglia are primarily made up of somata and dendritic structures , which are bundled or connected. Ganglia often interconnect with other ganglia to form a complex system of ganglia known as a plexus . Ganglia provide relay points and intermediary connections between different neurological structures in

44-495: A neuron is polarized, with its inside at about −70 mV relative to its surroundings. When an excitatory neurotransmitter is released by the presynaptic neuron and binds to the postsynaptic dendritic spines, ligand-gated ion channels open, allowing sodium ions to enter the cell. This may make the postsynaptic membrane depolarized (less negative). This depolarization will travel towards the axon hillock, diminishing exponentially with time and distance. If several such events occur in

66-480: A positive feedback loop (rising phase). At around +40 mV, the voltage-gated sodium channels begin to close (peak phase) and the voltage-gated potassium channels begin to open, moving potassium down its electrochemical gradient and out of the cell (falling phase). The potassium channels exhibit a delayed reaction to the membrane repolarisation, and, even after the resting potential is achieved, some potassium continues to flow out, resulting in an intracellular fluid that

88-462: A short time, the axon hillock may become sufficiently depolarized for the voltage-gated sodium channels to open. This initiates an action potential that then propagates down the axon. As sodium enters the cell, the cell membrane potential becomes more positive, which activates even more sodium channels in the membrane. The sodium influx eventually overtakes the potassium efflux (via the two-pore-domain potassium channels or leak channels , initiating

110-567: A signal that must be transported up the length of the axon to the nucleus. A current theory of how such survival signals are sent from axon endings to the soma includes the idea that NGF receptors are endocytosed from the surface of axon tips and that such endocytotic vesicles are transported up the axon. Intermediate filaments are abundant in both perikarya and axonal and dendritic processes and are called neurofilaments . The neurofilaments become cross linked with certain fixatives and when impregnated with silver, they form neuro fibrils visible with

132-487: A specialized plasma membrane that contains large numbers of voltage-gated ion channels, since this is most often the site of action potential initiation and triggering. The survival of some sensory neurons depends on axon terminals making contact with sources of survival factors that prevent apoptosis . The survival factors are neurotrophic factors , including molecules such as nerve growth factor (NGF). NGF interacts with receptors at axon terminals, and this produces

154-405: Is a localized thickening of the main part or trunk of a nerve that has the appearance of a ganglion but has only nerve fibers and no nerve cell bodies. Pseudoganglia are found in the teres minor muscle and radial nerve . Soma (biology) In cellular neuroscience , the soma ( pl. : somata or somas ; from Greek σῶμα (sôma)  'body'), neurocyton , or cell body

176-441: Is more negative than the resting potential, and during which no action potential can begin (undershoot phase/ refractory period ). This undershoot phase ensures that the action potential propagates down the axon and not back up it. Once this initial action potential is initiated, principally at the axon hillock, it propagates down the length of the axon. Under normal conditions, the action potential would attenuate very quickly due to

198-507: Is the bulbous, non-process portion of a neuron or other brain cell type, containing the cell nucleus . Although it is often used to refer to neurons, it can also refer to other cell types as well, including astrocytes , oligodendrocytes , and microglia . The part of the soma without the nucleus is called perikaryon ( pl. : perikarya ). There are many different specialized types of neurons, and their sizes vary from as small as about 5 micrometres to over 10 millimetres for some of

220-513: The RNA that is produced in neurons. In general, most proteins are produced from mRNAs that do not travel far from the cell nucleus. This creates a challenge for supplying new proteins to axon endings that can be a meter or more away from the soma. Axons contain microtubule -associated motor proteins that transport protein-containing vesicles between the soma and the synapses at the axon terminals . Such transport of molecules towards and away from

242-424: The axon and a much higher density of voltage-gated ion channels than is found in the rest of the cell body. In dorsal root ganglion cells, the cell body is thought to have approximately 1 voltage-gated sodium channel per square micrometre, while the axon hillock and initial segment of the axon have about ~100–200 voltage-gated sodium channels per square micrometre; in comparison, the nodes of Ranvier along

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264-541: The axon are thought to have ~1000–2000 such channels per square micrometre. This clustering of voltage-gated ion channels is a consequence of plasma-membrane and cytoskeletal associating proteins such as ankyrin . In electrophysiological models, the axon hillock is included with the initial segment of the axon where membrane potentials propagated from synaptic inputs to the dendrites or cell body are summed . Both inhibitory postsynaptic potentials ( IPSPs ) and excitatory postsynaptic potentials ( EPSPs ) are summed in

286-431: The axon hillock and once a triggering threshold is exceeded, an action potential propagates through the rest of the axon (and "backwards" towards the dendrites as seen in neural backpropagation ). The triggering is due to positive feedback between highly crowded voltage-gated sodium channels , which are present at the critical density at the axon hillock (and nodes of ranvier) but not in the soma. In its resting state,

308-457: The axon originates. A high amount of protein synthesis occurs in this region, as it contains many Nissl granules (which are ribosomes wrapped in RER ) and polyribosomes. Within the axon hillock, materials are sorted as either items that will enter the axon (like the components of the cytoskeletal architecture of the axon, mitochondria, etc.) or will remain in the soma. In addition, the axon hillock also has

330-485: The axon. For many years, it was believed that the axon hillock was the usual site of initiation of action potentials —the trigger zone . It is now thought that the earliest site of action potential initiation is at the axonal initial segment : just between the peak of the axon hillock and the initial (unmyelinated) segment of the axon . However, the positive point, at which the action potential starts, varies between cells. It can also be altered by hormonal stimulation of

352-420: The body, such as the peripheral and central nervous systems. Among vertebrates there are three major groups of ganglia: In the autonomic nervous system, fibers from the central nervous system to the ganglia are known as preganglionic fibers , while those from the ganglia to the effector organ are called postganglionic fibers . The term "ganglion" refers to the peripheral nervous system . However, in

374-521: The brain (part of the central nervous system ), the basal ganglia are a group of nuclei interconnected with the cerebral cortex , thalamus , and brainstem , associated with a variety of functions: motor control, cognition, emotions, and learning. Partly due to this ambiguity, the Terminologia Anatomica recommends using the term 'basal nuclei' instead of 'basal ganglia'; however, this usage has not been generally adopted. A pseudoganglion

396-460: The light microscope. Axon hillock The axon hillock is a specialized part of the cell body (or soma ) of a neuron that connects to the axon . It can be identified using light microscopy from its appearance and location in a neuron and from its sparse distribution of Nissl substance . The axon hillock is the last site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to

418-418: The neuron, or by second messenger effects of neurotransmitters. The axon hillock also delineates separate membrane domains between the cell body and axon. This allows for localization of membrane proteins to either the axonal or somal side of the cell. The axon hillock and initial segment have a number of specialized properties that make them capable of action potential generation, including adjacency to

440-412: The porous nature of the cell membrane. To ensure faster and more efficient propagation of action potentials, the axon is myelinated . Myelin, a derivative of cholesterol, acts as an insulating sheath and ensures that the signal cannot escape through the ion or leak channels. There are, nevertheless, gaps in the insulation ( nodes of Ranvier ), which boost the signal strength. As the action potential reaches

462-408: The smallest and largest neurons of invertebrates , respectively. The soma of a neuron (i.e., the main part of the neuron in which the dendrites branch off of) contains many organelles , including granules called Nissl granules , which are composed largely of rough endoplasmic reticulum and free polyribosomes . The cell nucleus is a key feature of the soma. The nucleus is the source of most of

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484-405: The soma maintains critical cell functions. In case of neurons, the soma receives a large number of inhibitory synapses, which can regulate the activity of these cells. It has also been shown that microglial processes constantly monitor neuronal functions through somatic junctions, and exert neuroprotection when needed. The axon hillock is a specialized domain of the neuronal cell body from which

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