Neuronal synapses

The axon terminals of one neuron synapse with other neurons either on the dendrites (axo-dendritic synapse) or soma (axo-somatic synapse). Synapses on another axon... 

Garcia RA, Vasudevan K, Buonanno A 2000 The neuregulin receptor ErbB-4 interacts with PDZ-containing proteins at neuronal synapses. Proc Natl Acad Sci USA 97 3596-3601... 

Fig. 45. GABAergic neurone synapse. GAD, glutamic acid decarboxylase M, mito-chodria G, GABA. 

As discussed, the first-order neuron is the afferent neuron that transmits impulses from a peripheral receptor toward the CNS. Its cell body is located in the dorsal root ganglion. This neuron synapses with the second-order neuron whose cell body is located in the dorsal horn of the spinal cord or in the medulla of the brainstem. The second-order neuron travels upward and synapses with the third-order neuron, whose cell body is located in the thalamus. Limited processing of sensory information takes place in the thalamus. Finally, the third-order neuron travels upward and terminates in the somatosensory cortex where more complex, cortical processing begins. 

A reflex is initiated by stimulation of a sensory receptor located at the peripheral ending of an afferent or first-order sensory neuron. This afferent neuron transmits impulses to the spinal cord. Within the gray matter of the spinal cord, the afferent neuron synapses with other neurons. As such, the spinal cord serves as an integrating center for the sensory input. The afferent neuron must ultimately synapse with an efferent or motor neuron. When the afferent neuron synapses directly with the motor neuron, it forms a monosynaptic reflex. An example of this type of reflex is the stretch reflex. When the afferent neuron synapses with an intemeuron that then synapses with the motor neuron, it forms a polysynaptic reflex, e.g., the withdrawal reflex. Most reflexes are polysynaptic. The motor neuron then exits the spinal cord to innervate an effector tissue, which carries out the reflex response. 

An example of the mechanism of the withdrawal reflex is illustrated in Figure 7.4. When a painful stimulus activates a sensory receptor on the right foot, action potentials are transmitted along the afferent neuron to the spinal cord. By way of divergence, this neuron synapses with several other neurons within the gray matter of the spinal cord ... 

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... 

Sensory afferent neuron Synapse Synapse Motor efferent neuron... 

The monoamine oxidase inhibitors (MAOIs) phenelzine and tranylcypromine increase the concentrations of NE, 5-HT, and DA within the neuronal synapse through inhibition of the monoamine oxidase (MAO) enzyme system. Both drugs are nonselective inhibitors of MAO-A and MAO-B. Selegiline is available as a transdermal patch for treatment of major depression. It inhibits MAO-A and MAO-B in the brain, but has reduced effects on MAO-A in the gut. 

Another example of molecular communication is found in a neuronal synapse, which is a communication junction between two neurons as shown in Fig.2. The presynaptic membrane releases the neurotransmitter molecule that is recognized and captured by the receptor located on the surface of the postsynaptic membrane. 

Nerve Agent Substances that interfere with the central nervous system. Organic esters of phosphoric acid used as a chemical warfare agent because of their extreme toxicity (tabun-GA, sarin-GB, soman-GD, GF, and VX). All are potent inhibitors of the enzyme, acetylcholinesterase, which is responsible for the degradation of the neurotransmitter, acetylcholine in neuronal synapses or myoneural junctions. Nerve agents are readily absorbed by inhalation and/or through intact skin. 

Moreover, an UCH (Ap-uch) that interacts with the proteasome was found to be induced by 5-HT, the neurotransmitter that induced long-term facilitation. Ap-uch was found to be critical for the induction of longterm facilitation. Subsequently, Chain et al showed that at sensory-motor neuron synapses, injection of lactacystin, a specific proteasome inhibitor blocked induction of long-term facilitation. Since R subunit inhibits the activity of C subunits of PKA, the results were interpreted to suggest that the ubiquitin-proteasome pathway operates to remove inhibitory constraints on the formation of long-term memory. This has been... 

Inactivation of pilocarpine is thought to occur at neuronal synapses and probably in plasma. Pilocarpine and its minimally active or inactive degradation products, including pilocarpic acid, are excreted in the urine. 

Absorption decreased if faken wifh a high-faf meal. Inactivafion of pilocarpine fhoughf fo occur af neuronal synapses and probably in plasma. Excrefed in urine. Half-life 4-12 hr. 

Highly evolved four-dimensional creatures would have extraordinarily developed nervous systems due in part to the increased number of possible neuronal synapses. Therefore the beings would be super-intelligent. I would expect new and higher senses to be present. For example, propioception, or pressure sensing, would probably not occur perpendicular to a 2-D creature s plane of existence. Similarly, four-dimensional creatures would have senses and sensory receptors that would make little sense to us. 

The anatomy of autonomic synapses and junctions determines the localization of transmitter effects around nerve endings. Classic synapses such as the mammalian neuromuscular junction and most neuron-neuron synapses are relatively "tight" in that the nerve terminates in small boutons very close to the tissue innervated, so that the diffusion path from nerve terminal to postsynaptic receptors is very short. The effects are thus relatively rapid and localized. In contrast, junctions between autonomic neuron terminals and effector cells (smooth muscle, cardiac muscle, glands) differ from classic synapses in that transmitter is released from a chain of varicosities in the postganglionic nerve fiber in the region of the smooth muscle cells rather than boutons, and autonomic junctional clefts are wider than somatic synaptic clefts. Effects are thus slower in onset and often involve many effector cells. 

As in the sympathetic division, parasympathetic preganglionic neurons synapse in the periphery with a postganglionic fiber. This synapse usually takes place in a terminal ganglion that is located directly in the organ or tissue supplied by the postganglionic neuron. Consequently, the parasympathetic ganglia are usually embedded directly in the innervated organ or tissue. 

Pathways in the central nervous system. A shows two relay neurons and two types of inhibitory pathways, recurrent and feed-forward. The inhibitory neurons are shown in black. B shows the pathway responsible for presynaptic inhibition in which the axon of an inhibitory neuron synapses on the axon terminal of an excitatory fiber. C Diagram illustrating that dendrites may be both pre-and postsynaptic to each other, forming reciprocal synapses, two of which are shown between the same dendrite pair. In triads, an axon synapses on two dendrites, and one of these dendrites synapses on the second. In serial synapses, a dendrite may be postsynaptic to one dendrite and presynaptic to another, thus connecting a series of dendrites. Dendrites also interact through low-resistance electrotonic ("gap") junctions (two of which are shown). Except for one axon, all... 

Zsiros V, Rojik I, Kovacs T, et al. 1997. Comparative morphological and physiological aspects of aluminum actions on central neurons and neuronal synapses of invertebrate and vertebrate animals. Neurotoxicology 18 1092. 

Flores-Hernandez J, Galarraga E, Bargas J (1997) Dopamine selects glutamatergic inputs to neos-triatal neurons. Synapse 25(2) 185-95... 

Trettel J, Levine ES (2003) Endocannabinoids mediate rapid retrograde signaling at intemeuron-pyramidal neuron synapses of the neocortex. J Neurophysiol 89 2334-8 Trettel J, Fortin DA, Levine ES (2004) Endocannabinoid signalling selectively targets perisomatic inhibitory inputs to pyramidal neurones in juvenile mouse neocortex. J Physiol 556(Pt 1) 95-107... 

Gray R, Rajan AS, Radcliffe KA, Yakehiro M, Dani JA (1996) Hippocampal synaptic transmission enhanced by low concentrations of nicotine. Nature 383 713-16 Gu JG, MacDermott AB (1997) Activation of ATP P2X receptors elicits glutamate release from sensory neuron synapses. Nature 389 749-53... 

Dopaminergic neurons synapse upon the parenchymal cells of the intermediate lobe (IL) of the rat pituitary gland. Dopamine decreases the capacity of the IL cells to synthesize cyclic AMP and inhibits the release of otMSH and other peptides from this tissue. The presence of a D-2 receptor accounts for both of these phenomena. This D-2 dopamine receptor can be studied in a binding assay using [3H]-spiroperidol, a dopamine... 

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