Autonomic Figs and Muscle Figs(7 pages)
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Autonomic Figs and Muscle Figs
- Lecture number:
- Lecture Note
- University of Southern California
- Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 11 Current Lecture Transmitters and Receptors (Traditional View) The basic idea in anatomical pathways, whether you are in the parasympathetic or sympathetic system, follows the same basic plan: a preganglionic neuron whose cell body is in the CNS sends an axon out to synapse onto a postganglionic neuron in the periphery, which sends an axon to synapse onto a target cell. These synapses between pre and postganglionic neurons are located in ganglia, which can be separate/discrete ganglia, or ganglia that are scattered and embedded in the target tissue. Shown above is the old, inaccurate view of how the Autonomic Nervous System works. In the PS subdivision, the preganglion uses Ach(Acetylcholine), which binds to nAChRs (nicotinic Ach receptors) on the postsynaptic membrane. This causes a permeability change, so this ion-gated receptor channel opens and allows Na and K through – this generates a strong inward current, and depolarizes the postganglionic neuron, bringing it to threshold. This mechanism results in a fast EPSP. The PS postganglionic neuron releases Ach when excited, but this Ach’s target cells have muscarinic AChRs, which are metabotropic, not inotropic receptors. So they initiate secondary messenger cascades and cause cellular response. In the sympathetic system, Ach is released and binds to nAChRs, just like in the PS system, but the postganglionic neuron has a different transmitter, norepinephrine, which binds to adrenergic receptors on target tissues of 2 basic types and several subtypes - 2 alphas, 3 betas. Transmitters and Receptors (Current View) Evidence has been accumulating for this more current and accurate view. Just like in the traditional view, the first half of the process involving the preganglionic neuron is the same in both the sympathetic and parasympathetic subdivision. ACh is the main neurotransmitter, and there are nAChRs as well as mAChRs(muscarinic) in the postganglionic neuron – this new idea is that you can have multiple receptors at the same site in the same membrane. The mAChRs are metabotropic receptors, and create a slowly developing EPSP or IPSP by modulating the K channel. For example, if it closes a previously opened K channel, it would slowly cause depolarization/EPSP. Or if it were to open a K channel, it would create a slowly developing IPSP. Another new addition to this current view is that Ach is not the only neurotransmitter, there are various peptides acting in the synapse that act as cotransmitters with Ach. So in the PS subdivision, the postganglionic neuron is excited and releases Ach, which binds to mAChRs. But many also release a peptide called VIP(vasoactive intestinal peptide). VIP binds to a VIP receptor, and functions as a potent vasodilator. It causes the relaxation of smooth muscle in blood vessels, allowing blood pressure to inflate the vessel, lowering vascular resistance and increasing blood flow. The actions of these two transmitters (ACh and VIP) are synergistic. For example, in the salivary gland, the PS nerves co-release ACh and VIP. ACh triggers salivary secretion, while VIP triggers vasodilation in the blood vessels going ...
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