Synapse junction

The neuromuscular junction is the synapse (junction) of an axon terminal of a motorneuron, which terminates in a depression of the sarcolemma (the cell membrane of a muscle cell), the motor end plate. It is here that the initiation of action potentials across the muscle surface ultimately leads to muscle contraction. In vertebrates, the neurotransmitter is acetylcholine. 

The ability to connect electronic parts is crucial to the construction of circuit-like molecular devices. However, it is quite difficult to connect components solely through covalent linkage. Therefore, it would be better to connect the electronic parts through supramolecular interactions. In fact, biological systems use a similar concept. Information is transmitted as electrical signals through nerve systems, but nerve cells are not connected to each other covalently. There is a synapse junction between the nerve cells where signals are transmitted via a chemical mediator. 

Synapse—junction between cells where the exchange of electrical or chemical information takes place. 

These are nerves which carry messages from the CNS to the skeletal muscles. There are no synapses (junctions) en route and the neurotransmitter at the neuromuscular junction is acetylcholine. The final result of such messages is contraction of skeletal muscle. 

SXRIE synchrotron radiation-induced X-ray emission spectrometry (see SRXRF) synapse junction (site of excitation transfer) betv een tivo nerves or between a nerve and a muscle... 

Synaptic transmission depends on neurotransmitters, chemicals that are released at the synapse (junction) between two nerve cells as a result of the action potential, which has traveled down the axon. 

Medical response, treatment and prevention (prophylaxis) to nerve agent exposure are ever expanding fields of research [6,12,13]. Atropine, which rapidly reverses cholinergic over expression at synapse junctions, is the most commonly used anticholinergic. Mark I kits, which contain 2 mg atropine with an autoinjector for intramuscular use, are issued to US military personnel [2]. Recommended initial field doses for atropine are 2, 4 or 6 mg with re-treatment every 5-10 min depending on the symptoms. In addition, oximes reactivate cholinesterase enzymes, ensuring normal function. Mark I kits also include 600 mg autoinjectors of 2-pralidoxime chloride (2-PAM Cl) with initial field doses of 600, 1200 or... 

Two specialties of the nervous system are speed and localization, accompHshed using highly developed electrical signaling and close cellular apposition. At specialized points of communication, such as the synapse and the neuromuscular junction, the cells are separated by a nanometer or less. 

Tree like networks of nerve fiber called dendrites protrude outward from the neuron s cell body, or soma. Extending outward from the soma is also a long fiber called the axon that itself eventually branches out into a set of strands and sub strands. At the ends of these strands are the transmitting ends of communication junctions between nerve fibers called synapses. The receiving ends of these junctions exist both on dendrites and on the somas themselves. Each neuron is typically connected to several thousand other neurons. 

Cholinesterases (ChEs), polymorphic carboxyles-terases of broad substrate specificity, terminate neurotransmission at cholinergic synapses and neuromuscular junctions (NMJs). Being sensitive to inhibition by organophosphate (OP) poisons, ChEs belong to the serine hydrolases (B type). ChEs share 65% amino acid sequence homology and have similar molecular forms and active centre structures [1]. Substrate and inhibitor specificities classify ChEs into two subtypes ... 

Populations of receptors that are excluded from synaptic junctions. These may be distributed over neuronal cell bodies or located around but not directly beneath synapses (perisynaptic). Some receptors have become specialised to setve an extrasynaptic function producing a tonic level of activity in response to ambient levels of neurotransmitter. This tonic current can be used to maintain homeostatic control over neuronal excitation. 

Neuromuscular junction (NMJ) is the synapse or junction of the axon terminal of motoneurons with the highly excitable region of the muscle fibre s plasma membrane. Neuronal signals pass through the NMJ via the neurotransmitter ACh. Consequent initiation of action potentials across the muscle s cell surface ultimately causes the muscle contraction. 

The PNS has two neurohormones (neurotransmitters) acetylcholine (ACh) and acetylcholinesterase (ACliE). ACh is a neurotransmitter responsible for die transmission of nerve impulses to effector cells of die parasympathetic nervous system. ACh plays an important role in die transmission of nerve impulses at synapses and myoneural junctions. ACh is quickly... 

Neurohumoral transmitters are chemicals that facilitate the transmission of nerve impulses across nerve synapses and neuroeffector junctions. Acetylcholine is a neurohumoral transmitter that is present in the peripheral autonomic nervous system, in the somatic motor nervous system, and in some portions of the central nervous system. 

There are regional asymmetries in membranes. Some, such as occur at the villous borders of mucosal cells, are almost macroscopicaUy visible. Others, such as those at gap junctions, tight junctions, and synapses, occupy much smaller regions of the membrane and generate correspondingly smaller local asymmetries. 

A substance which is released at the end of a nerve fibre by the arrival of a nerve impulse and by diffusing across the synapse or junction effects the transfer of the impulse to another nerve fibre (or muscle fibre or some receptor). 

In the periphery at the mammalian neuromuscular junction each muscle fibre is generally influenced by only one nerve terminal and the one NT acts on one type of receptor localised to a specific (end-plate) area of the muscle. The system is fitted for the induction of the rapid short postsynaptie event of skeletal muscle fibre contraction and while the study of this synapse has been of immense value in elucidating some basic concepts of neurochemical transmission it would be unwise to use it as a universal template of synaptic transmission since it is atypical in many respects. 

Fig. 5.12 (a) Synaptic types along dendritic spines of M/T and GC units uni-, and bi-directional junctions, (b) Transmitter systems at a reciprocal synapse, Mitral-Granule cell junction. [Glu, glutamate (R, receptor) GABA, y-aminobutyric acid (R, receptor) E, intracellular effector and aAR, alpha-adrenergic receptor.]. (From Hayashi et al., 1993.)... 

According to Fig. 6.17 the nerve cell is linked to other excitable, both nerve and muscle, cells by structures called, in the case of other nerve cells, as partners, synapses, and in the case of striated muscle cells, motor end-plates neuromuscular junctions). The impulse, which is originally electric, is transformed into a chemical stimulus and again into an electrical impulse. The opening and closing of ion-selective channels present in these junctions depend on either electric or chemical actions. The substances that are active in the latter case are called neurotransmitters. A very important member of this family is acetylcholine which is transferred to the cell that receives the signal across the postsynaptic membrane or motor endplate through a... 

Describe how the neuroeffector junction in the autonomic nervous system differs from that of a neuron-to-neuron synapse... 

Synapses between the autonomic postganglionic neuron and effector tissue — the neuroeffector junction — differ greatly from the neuron-to-neuron synapses discussed previously in Chapter 5 (see Table 9.1). The postganglionic fibers in the ANS do not terminate in a single swelling like the synaptic knob, nor do they synapse directly with the cells of a tissue. Instead, the axon terminals branch and contain multiple swellings called varicosities that lie across the surface of the tissue. When the neuron is stimulated, these varicosities release neurotransmitter over a large surface area of the effector tissue. This diffuse release of the neurotransmitter affects many tissue cells simultaneously. Furthermore, cardiac muscle and most smooth muscle have gap junctions between cells. These specialized intercellular communications... 

For any substance to serve effectively as a neurotransmitter, it must be rapidly removed or inactivated from the synapse or, in this case, the neuroeffector junction. This is necessary in order to allow new signals to get through and influence effector tissue function. Neurotransmitter activity may be terminated by three mechanisms ... 

The primary mechanism used by cholinergic synapses is enzymatic degradation. Acetylcholinesterase hydrolyzes acetylcholine to its components choline and acetate it is one of the fastest acting enzymes in the body and acetylcholine removal occurs in less than 1 msec. The most important mechanism for removal of norepinephrine from the neuroeffector junction is the reuptake of this neurotransmitter into the sympathetic neuron that released it. Norepinephrine may then be metabolized intraneuronally by monoamine oxidase (MAO). The circulating catecholamines — epinephrine and norepinephrine — are inactivated by catechol-O-methyltransferase (COMT) in the liver. 

Each muscle fiber is innervated by a branch of an alpha motor neuron. The synapse between the somatic motor neuron and the muscle fiber is referred to as the neuromuscular junction. Action potentials in the motor neuron cause release of the neurotransmitter acetylcholine. Binding of acetylcholine to its receptors on the muscle fiber causes an increase in the permeability to Na+ and K+ ions. The ensuing depolarization generates an action potential that travels along the surface of the muscle fiber in either direction that is referred to as a propagated action potential. This action potential elicits the intracellular events that lead to muscle contraction. 

Acetylcholine (ACh) The first neurotransmitter to be discovered. It is located at numerous synapses and neuroeffector junctions, in both the central and peripheral nervous systems. 

Acetylcholinesterase The catabolic enzyme that rapidly terminates the physiological action of acetylcholine at synapses and neuroeffector junctions. 

The synapse is a specialized junctional complex by which axons and dendrites emerging from different... 

FIGURE 1-11 An electro tonic synapse is seen at the surface of a motor neuron from the spinal cord of a toadfish. Between the neuronal soma (left) and the axonal termination (right), a gap junction flanked by desmosomes (arrows) is visible. (Photograph courtesy of Drs G. D. Pappas and J. S. Keeter.) X80,000. 

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