Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 154 of 367 13.11 Neuromodulators Synaptic knobs may release a mixture of active compounds. These compounds may have a variety of functions. Those that alter the rate of neurotransmitter release by the presynaptic neuron or change the postsynaptic cell’s response to neurotransmitters are called neuromodulators . Neuromodulators, called opioids, have results similar to those of the drugs opium and morphine because they attach to the same group of postsynaptic receptors. Four classes of opioids are identified in the CNS: endorphins, enkephalins, endomorphins, and dynorphins. The primary function of opioids is probably the relief of pain—they inhibit the release of the neurotransmitter substance P at synapses that relay pain sensations. Dynorphins have far more powerful analgesic (pain-relieving) effects than morphine or the other opioids. 13.10 Neurotransmitters The nervous system relies on a complex form of chemical communication. Some usually have excitatory effects; others have inhibitory effects. Major categories of neurotransmitters include biogenic amines, amino acids, neuropeptides, dissolved gases, and a variety of other compounds. Some of the most prominent neurotransmitters are: • Acetylcholine (ACh) is a neurotransmitter included in both the peripheral nervous system (PNS) and central nervous system (CNS) in many organisms including humans. Acetylcholine is one of many neurotransmitters in the autonomic nervous system (ANS) and the only neurotransmitter used in the motor division of the somatic nervous system. Acetylcholine is also the principal neurotransmitter in all autonomic ganglia. • Norepinephrine, or NE, is a neurotransmitter that is widely distributed in the brain and portions of the ANS. Norepinephrine is also called noradrenaline, and synapses that release NE are known as adrenergic synapses. Norepinephrine typically has an excitatory, depolarizing effect on the postsynaptic membrane, but the mechanism is quite distinct from that of ACh. • Dopamine, a CNS neurotransmitter released in many sections of the brain, may have either inhibitory or excitatory effects. Inhibitory effects play a valuable role in our precise control of movements. Cocaine inhibits the removal of dopamine from synapses in specific areas of the brain. The resulting rise in dopamine concentrations at these synapses is responsible for the “high” experienced by cocaine users. • Serotonin is another valuable CNS neurotransmitter. Inefficient serotonin production can have extensive effects on a person’s attention and emotional states and may be accountable for many cases of severe chronic depression. Interactions among serotonin, norepinephrine, and other neurotransmitters are thought to be involved in the regulation of sleep and wake cycles. • Gamma-aminobutyric acid, or GABA, generally has an inhibitory effect. Although roughly 20 percent of the synapses in the brain release GABA, its functions remain incompletely understood. •