this lecture introduces neurotransmitters and neuropeptides, the receptors that bind them, the actions they produce, and what role they play in the autonomic nervous system. the first section introduces neurotransmitters and synapse terminology, indicating that the object of focus in this lecture is the specific type of neurotransmitter and receptor that is involved in the synapse.
the second section gives an overview of the nervous system, specifically the peripheral nervous system and the distribution of neurons throughout the body. the differences between the parasympathetic and sympathetic nervous systems are summarized, mainly being that they produce different results in the organs that they innervate (stimulation vs. relaxation), originate in different areas of the spinal cord, and also have ganglia in different locations (autonomic ganglia or adrenal medulla for sympathetic and closer to the organs for parasympathetic).
the third section introduces the different types of ANS neurotransmitters that are dealt with in this lecture, with the two broad categories being cholinergic and catecholamine. the main cholinergic neurotransmitter is acetylcholine and it is associated with nicotinic and muscarinic receptors, which are differentiated by their location; neurons / skeletal muscle for nicotinic, and body tissue / CNS neurons for muscarinic. the other broad category of neurotransmitter is catecholamines, which include dopamine, norepinephrine, and epinephrine. a later slide describes the chain of synthesis of these three NT's from tyrosine: tyrosine to L-dopa to dopamine to norepinephrine to epinephrine.
nicotinic receptors are looked at in greater detail, first starting with the phenomenon of excitatory post synaptic potential's, also called end-plate potentials in these neuromuscular junctions between the post synaptic neuron and the skeletal muscle cell. this is caused when the release of acetyl choline causes an increase in the permeability of the ionotropic Na+ and K+ channels, causing a net influx of positive ions and therefore a depolarization of the membrane. the location is on the dendrite, and an example includes that of glucose.
muscarinic receptors, on the other hand, are not covered in much detail in terms of their physiology. they are found in the parasympathetic postganglionic synapses and associated with acetylcholine. two examples are given of muscarinic receptor action: the vagus nerve innervating the heart's SA node, the pacemaker, and hyperpolarizing the membrane by opening the K+ channels directly. the other example is stimulating smooth muscle contraction via IP3.
the next few slides deal with norepinephrine and epinephrine and their receptors, alpha and beta. not much physiology is covered here either, except to say that alpha receptors regulate Ca++ and K+ channels and beta receptors regulate smooth muscle, cardiac, and metabolism. beta 1 and 2 are also responsive to drugs, such as ephedra (Ma Huang), Propranolol, and amphetamines (ie: cocaine).
neuropeptides are then introduced as a different type of neurotransmittter, one that is synthesized and packaged into larger vescicles in the cell body, transported to the axon, and are released only with higher frequency stimulation (translating into higher concentration gradients of intracellular calcium), at which point they are co-released with other neuropeptides and create a longer lasting effect than neurotransmitters. neuropeptides act at much lower concentrations than neuotransmitters.
the last two slides are a huge chart of the entire peripheral nervous system, showing the somatic, parasympathetic and sympathetic divisions, the neurotransmitters secreted and receptors used in each synapse, and the end target of innervation. the somatic nervous system shows a single axon (motorneuron) going to the skeletal muscle, releasing ACh into the neuromuscular junction (NMJ) and picked up by a nicotinic receptor. the sympathetic nervous system shows one preganglionic cell synapsing in the autonomic ganglia with ACh and nicotinic receptors, with one of the post ganglionic branches going to smooth muscle, using norepinephrine and alpha/beta receptors, and the other branch going to the sweat glands with a muscarinic receptor and ACh neurotransmitter. the adrenal medulla is also part of the sympathetic, being innervated using ACh and releasing hormones (mainly epinephrine) directly into the bloodstream. the last branch is the parasympathetic, which uses muscarinic receptors and acetyl choline and goes to smooth muscle, cardiac muscle, and glands.
some larger thoughts about this chart: ACh and nicotinic receptors are always used in the synapses between pre and postganglionic neurons. alpha and beta receptors are only used in the sympathetic nervous system for smooth muscle and glands. muscarinic receptors are only used in the parasympathetic postganglionic synapses and the sweat glands of the sympathetic.
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