Autonomic Neurotransmitters

Acetylcholine is a neurotransmitter at the neuromuscular junction in autonomic ganglia and at postgangHonic parasympathetic nerve endings (see Neuroregulators). In the CNS, the motor-neuron collaterals to the Renshaw cells are cholinergic (43). In the rat brain, acetylcholine occurs in high concentrations in the interpeduncular and caudate nuclei (44). The LD q (subcutaneous) of the chloride in rats is 250 mg/kg. 

Parasympathetic nervous system. That portion of the autonomic nervous system that utilizes acetylcholine as the neurotransmitter at the neuro-effector junctions. 

Acetylcholine (Ach) is an ester of acetic acid and choline with the chemical formula CH3COOCH2CH2N+ (CH3)3. ACh functions as a chemical transmitter in both the peripheral nervous system (PNS) and central nervous system (CNS) in a wide range of organisms, humans included. Neurotransmitter involved in behavioral state control, postural tone, cognition and memory, and autonomous parasympathetic (and preganglionic sympathetic) nervous system. 

Neurotransmitter and biogenic amine derived from the amino acid histidine synthesized in hypothalamic tuber-omamillary neurons (TMN) to maintain wakefulness, feeding rhythms, energy balance, neuroendocrine autonomic control, and memory functions prominent immu-nomodulator and proinflammatory signal released from mast cells in response to allergic reactions or tissue damage. 

Nonadrenergic noncholinergic inhibitory responses to autonomic nerve stimulation are mainly mediated through NO synthesized by nNOS NO plays a crucial role as a neurotransmitter from the peripheral efferent nerves, thus being called nitrergic. This provides a... 

Action potentials, self-propagating. Action potentials of smooth muscle differ from the typical nerve action potential in at least three ways. First, the depolarization phases of nearly all smooth muscle action potentials are due to an increase in calcium rather than sodium conductance. Consequently, the rates of rise of smooth action potentials are slow, and the durations are long relative to most neural action potentials. Second, smooth muscle action potentials arise from membrane that is autonomously active and tonically modulated by autonomic neurotransmitters. Therefore, conduction velocities and action potential shapes are labile. Finally, smooth muscle action potentials spread along bundles of myocytes which are interconnected in three dimensions. Therefore the actual spatial patterns of spreading of the action potential vary. 

Neurotransmitter in the brain and peripheral autonomic nervous system... 

Figure 1. A depiction of the several different ionic currents necessary for the acute function of neuromuscular transmission in the skeletal motor and the efferent autonomic nervous system. The boxed current designations are associated, by the arrows, with those cellular regions where their physiological role is most evident, although these currents often exist in other regions of the cell. = neurotransmitter-activated current ...

Dopamine is a catecholamine neurotransmitter in the CNS and at some ganglia in the autonomic nervous system. To date, three main types of receptors have been found D1( D2, and D3. The main dopaminergic systems in the brain are the nigro-neostriatal... 

For each neurotransmitter in the autonomic nervous system, list the neurons that release it and the type and location of receptors that bind with it... 

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... 

Table 9.3 Distinguishing Features of Neurotransmitters of Autonomic Nervous System...

Adrenal medulla. Derived from neural crest tissue, the adrenal medulla forms the inner portion of the adrenal gland. It is the site of production of the catecholamines, epinephrine and norepinephrine, which serve as a circulating counterpart to the sympathetic neurotransmitter, norepinephrine, released directly from sympathetic neurons to the tissues. As such, the adrenal medulla and its hormonal products play an important role in the activity of the sympathetic nervous system. This is fully discussed in Chapter 9, which deals with the autonomic nervous system. 

The autonomic nervous system (ANS) modifies contractile activity of both types of smooth muscle. As discussed in Chapter 9, the ANS innervates the smooth muscle layer in a very diffuse manner, so neurotransmitter is released over a wide area of muscle. Typically, the effects of sympathetic and parasympathetic stimulation in a given tissue oppose each other one system enhances contractile activity while the other inhibits it. The specific effects (excitatory or inhibitory) that the two divisions of the ANS have on a given smooth muscle depend upon its location. 

The concept of chemical neurotransmission originated in the 1920s with the classic experiments of Otto Loewi (which were themselves inspired by a dream), who demonstrated that by transferring the ventricular fluid of a stimulated frog heart onto an unstimulated frog heart he could reproduce the effects of a (parasympathetic) nerve stimulus on the unstimulated heart (Loewi Navratil, 1926). Subsequently, it was found that acetylcholine was the neurotransmitter released from these parasympathetic nerve fibers. As well as playing a critical role in synaptic transmission in the autonomic nervous system and at vertebrate neuromuscular junctions (Dale, 1935), acetylcholine plays a central role in the control of wakefulness and REM sleep. Some have even gone as far as to call acetylcholine a neurotransmitter correlate of consciousness (Perry et al., 1999). 

Interaction with autonomic neurotransmitters in vitro or in vivo Drug dependency... 

Somatic nerves originate in the CNS and terminate at the neuromuscular junction where acetylcholine is the transmitter. Nerves of the autonomic system also use acetylcholine as the neurotransmitter at the end of the preganglionic fibres within the ganglia. With few exceptions, the postganglionic sympathetic fibres secrete noradrenaline (norepinephrine) whilst postganglionic parasympathetic fibres secrete acetylcholine. 

ACh is necessary for control of skeletal muscle in verterbrates, acting as the neurotransmitter at the neuromuscular junction. It is also involved in transmission in the autonomic nervous system (see below, under "Neuroanatomy"). Central ACh is produced in two general areas in the brain incuding the basal forebrain (medial septal nuclei, diagonal band... 

The autonomic nervous system is itself divided into two parts the sympathetic and parasympathetic nervous systems. The sympathetic nervous system serves several glands and involuntary muscles. The primary neurotransmitter of the sympathetic nervous system is norepinephrine, which acts through a and p adrenergic receptors. 

The concept of chemical transmission in the nervous system arose in the early years of the century when it was discovered that the functioning of the autonomic nervous system was largely dependent on the secretion of acetylcholine and noradrenaline from the parasympathetic and sympathetic nerves respectively. The physiologist Sherrington proposed that nerve cells communicated with one another, and with any other type of adjacent cell, by liberating the neurotransmitter into the space, or synapse, in the immediate vicinity of the nerve ending. He believed that transmission across the synaptic cleft was unidirectional and, unlike conduction down the nerve fibre, was delayed by some milliseconds because of the time it took the transmitter to diffuse across the synapse and activate a specific neurotransmitter receptor on the cell membrane. 

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