The Primary Neurotransmitters of the ANS Are Acetylcholine and Norepinephrine

In the somatic nervous system, neurotransmitter is released from specialized nerve endings that make intimate contact with the target structure. The mammalian motor endplate, with one nerve terminal to one skeletal muscle fiber, illustrates this principle. This arrangement contrasts with the ANS, where postganglionic axons terminate in varicosities, swellings enriched in synaptic vesicles, which release the transmitter into the extracellular space surrounding the effector cells (see Fig. 6.1). The response to the ANS output originates in some of the effector cells and then propagates to the remainder via gap junctions.

Acetylcholine. Acetylcholine (ACh) is the transmitter released by the preganglionic nerve terminals of both the sympathetic and the parasympathetic divisions (Fig 6.2). The synapse at those sites utilizes a nicotinic receptor similar in structure to the receptor at the neuromuscular junction. Parasympathetic postganglionic neurons release ACh at the synapse with the effectors. The postganglionic sympathetic neurons to the sweat glands and to some blood

Autonomic nervous system

Somatic motor system

Ganglion-

Preganglionic axon

Postganglionic-axon

Anterior Horn Cell

( ^Anterior horn cell

Neuromuscular junction

Effectors

IntermediolateraK ro / varicosities horn cell

Skeletal muscle fibers

The efferent path of the ANS as contrasted with the somatic motor system. The

'ANS uses a two-neuron pathway. Note the structural differences between the synapses at autonomic effectors and skeletal muscle cells.

vessels in skeletal muscle also use ACh as the neurotransmitter. The synapse between the postganglionic neuron and the target tissues utilizes a muscarinic receptor. This receptor classification scheme is based on the response of the synapses to the alkaloids nicotine and muscarine, which act as agonists at their respective type of synapse. The nicotinic receptor of the ANS is blocked by the antagonist hexamethonium, in contrast to the neuromuscular junction receptor, which is blocked by curare. The muscarinic receptor is blocked by atropine.

The nicotinic receptor is of the direct ligand-gated type, meaning that the receptor and the ion channel are contained in the same structure. The muscarinic receptor is of the indirect ligand-gated type and uses a G protein to link receptor and effector functions (see Chapter 3). The action of ACh is terminated by the enzyme acetylcholinesterase. Choline released by the enzyme action is taken back into the nerve terminal and resynthesized into ACh.

Norepinephrine. The catecholamine norepinephrine

(NE) is the neurotransmitter for postganglionic synapses of the sympathetic division (see Fig. 6.2). The synapses that utilize NE receptors can also be activated by the closely related compound epinephrine (adrenaline), which is released into the general circulation by the adrenal medulla— hence, the original designation of these type receptors as adrenergic. Adrenergic receptors are classified as either a or P, based on their responses to pharmacological agents that mimic or block the actions of NE and related compounds. Alpha receptors respond best to epinephrine, less well to NE, and least well to the synthetic compound isoproterenol. Beta receptors respond best to isoproterenol, less well to epinephrine, and least well to NE. Propranolol is a drug that acts as an antagonist at P receptors but has no action on a receptors. Each class of receptors is further classified as at or a2, and P), p2, or p3 on the basis of responses to additional pharmacological agents.

The adrenergic receptors are of the indirect, ligand-gated, G protein-linked type. They share a general structural similarity with the muscarinic type of ACh receptor. The at receptors activate phospholipase C and increase the intracellular concentrations of diacylglycerol and inos-itol trisphosphate. The a2 receptors inhibit adenylyl cy-clase, while the P types stimulate it. The action of NE and epinephrine at a synapse is terminated by diffusion of the molecule away from the synapse and reuptake into the nerve terminal.

Other Neurotransmitters. Neurally active peptides are often colocalized with small molecule transmitters and are released simultaneously during nerve stimulation in the CNS. This is the same in the ANS, especially in the intrinsic plexuses of the gut, where amines, amino acid transmitters, and neurally active peptides are widely distributed. In the ANS, examples of a colocalized amine and peptide are seen in the sympathetic division, where NE and neuropeptide Y are coreleased by vasoconstrictor nerves. Vasoactive intestinal polypeptide (VIP) and calcitonin-gene-related peptide (CGRP) are released along with ACh from nerve terminals innervating the sweat glands.

Nitric oxide is another type of neurotransmitter produced by some autonomic nerve endings. The term non-adrenergic noncholinergic (NANC) has been applied to such nerves. Nitric oxide is a highly diffusible substance important in the regulation of smooth muscle contraction, (see Chapter )).

Thoracic spinal cord

Autonomic nervous system Parasympathetic division

Autonomic nervous system Parasympathetic division

Thoracic spinal cord

Autonomic Neural Pathway

The neurochemistry of the autonomic paths. The structures of the neurotransmitters and the agonists for which the synapses were originally named are shown.

Dorsal root gangN°n Ventral nerve root

( ^--Sympathetic chain

Dorsal root gangN°n Ventral nerve root

( ^--Sympathetic chain

Spinal nerve

Paravertebral Nerve Block Anatomy

Spinal nerve

Paravertebral sympathetic ganglion

^Peripheral sympathetic anatomy. The preganglionic axons course through the spinal nerve and white ramus to the paravertebral ganglion. Synapse with the postganglionic neuron may occur at the same spinal level, or at levels above or below. Postganglionic axons rejoin the spinal nerve through the gray ramus to innervate structures in the limbs or proceed to organs, such as the lungs or heart, in discrete nerves. Preganglionic axons may also pass to a prevertebral ganglion without synapsing in a paravertebral ganglion.

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  • kinfe
    What neurotransmitters are at a sympathetic effector synapse?
    8 years ago
  • furuta
    What neurotransmitters are released by the ans?
    8 years ago
  • nina bruce
    What is the primary neurotransmitter for action of skeletal/smooth muscles?
    8 years ago
  • bisrat
    What is the primary neurotransmitter of the sympathetic nervous system?
    8 years ago
  • elizabeth
    What are the two primary neurotransmitters of the ANS?
    8 years ago
  • Maik
    What neurotransmitter are used in each of the divisions of the ANS?
    7 years ago
  • jessika
    What are the two most important neurotransmitters released by nerves innervating smooth muscles?
    7 years ago
  • Eglantine
    What neurontransmitter released from the somatic nervou systerm to skeletal muscle is?
    6 years ago
  • natsnet
    What are the primary neurotransmitters for the sympathetic system?
    6 years ago
  • ERNEST
    Which neurotransmitter is released at the sympathetic chain ganglia?
    6 years ago
  • gebre
    What is the primary neurotransmitter in the skeletal muscle?
    6 years ago
  • Lorena Ferrari
    Which neurotransmitter is released at the synapse in the structure of the ANS pathway?
    6 years ago
  • Ernest
    Is norepinephrine a primary parasympathetic neurotransmitter?
    6 years ago
  • bisrat demsas
    What neurotransmitter is released at the autonomic ganglion in the sympathetic pathway?
    6 years ago
  • maik
    Which neurotransmitters are used in each ANS division?
    6 years ago
  • Harrison
    What are the primary transmitters for adrenergic agents?
    5 years ago
  • nicola
    What are the primary neurotransmitters for adrenergic receptors?
    5 years ago
  • Karri
    What neurotransmitter is released by the nerve endings innervating sweat glands?
    4 years ago
  • matthias
    What is the primary neurotransmitter used in motor system?
    4 years ago
  • emily wallace
    What ANS system releases an adrenergic neurotransmitter?
    4 years ago
  • William
    What neurotransmitters are released with postganglionic endings?
    4 years ago
  • arvi
    What is the neurotransmitter that a motor neuron uses to communicate with a skeletal muscle?
    4 years ago
  • Carol
    What structure releases neurotransmitter in a paravertebral ganglion?
    4 years ago
  • isla
    Which of following structures release neurotransmitter in a paravertebal ganglion?
    4 years ago
  • Camelia Brownlock
    Which of the following structures releases neurotransmitter in a paravertebral ganglion?
    3 years ago
  • Anette
    Which structural protein of skeletal muscle also acts as an enzyme?
    3 years ago
  • kyle
    What is the primary neurotransmitter of each division of the ANS?
    3 years ago
  • Dale
    What are the major sympathetic nervous system neurotransmitters?
    2 years ago
  • matilda
    What is the primary neurotransmitter of the parasympathetic nervous system?
    2 years ago
  • Nazzareno
    Which of the following structures releases neurotransmitter molecules in a paravertebral ganglion?
    2 years ago
  • David
    Why does the sweat gland use acetylcholine as its neurotransmitters instead of adrenaline?
    2 years ago
  • anu tuikka
    What are the primary autonomic neurotransmitters?
    2 years ago
  • aleardo
    Why do sweat gland uses acetylcholine intead of adrenaline as their neurotransmitter?
    2 years ago
  • Judy
    Why does sweat gland uses noradrenaline as neurotransmitters?
    2 years ago
  • Awet
    What are the primary neurotransmitters for sympathetic nervous system?
    1 year ago
  • bungo
    What type of neuro transmitter recptor is used in skeletal muscles?
    6 months ago
  • claudio
    What neurotransmitter family is activated with the sympathetic nervous system?
    2 months ago

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