Overview As discussed previously, nerves of the ANS extend from the CNS to smooth, or cardiac muscle, organs, and glands via a two neuron system-; namely a preganglionic neuron and a postganglionic neuron. In this system, there are 2 synapses-; one separating preganglionic and postganglionic neurons, and the other between the postganglionic neuron and effector (see figure below). The preganglionic neuron releases neurotransmitters stored in synaptic vesicles of axon terminals. An action potential reaching the axon terminal causes release of these stored neurotransmitters into the synaptic cleft. After crossing the synapse, neurotransmitters bind to receptors imbedded in postganglionic cell membranes (see figure below). This binding depolarizes the postsynaptic cell membrane (EPSP) and results in action potentials. Action potentials arriving at the axon terminal lead to release of neurotransmitters into the synaptic cleft, separating the postganglionic neuron and effector (neuro-effector junction). Finally, binding of neurotransmitter to the receptor expressed on effector cells can result in excitation or inhibition of the effector. Sympathetic and parasympathetic divisions differ in the types of neurotransmitters they release and the receptors and second messenger systems they express.  Image by Nate Shoemaker Spring 2016 ANS preganglionic and postganglionic neurons and locations of ANS receptors. Nicotinic receptors are located on the postganglionic neurons of the sympathetic and parasympathetic cell bodies. Nicotinic receptors respond to the binding of acetylcholine (ACH), which causes an excitatory effect. Muscarinic receptors are located on all parasympathetic effector cells and some (generalized sweat glands) sympathetic effector cells. Muscarinic receptors respond to the binding of ACH, and may have an excitatory or inhibitory effect. Adrenergic receptors are located on most sympathetic effector cells. Adrenergic receptors respond to the binding of norepinephrine (NE), which may have an excitatory or inhibitory effect. Neurotransmitters are chemicals that travel across the synapse connecting two neurons, or between a neuron and an effector. For example, when we discussed the neuromuscular junction we were talking about a neuron-effector synapse and the neurotransmitter used was acetylcholine (ACH). ACH is also one of the neurotransmitters used by the ANS. Cholinergic neurons produce ACH and store ACH in their synaptic terminals. The preganglionic neuron for both parasympathetic and sympathetic nervous systems is cholinergic. The postganglionic neuron of the parasympathetic division is also cholinergic. The postganglionic neuron for the sympathetic division is usually an adrenergic neuron which means that it produces norepinephrine (NE) as its neurotransmitter. Sympathetic postganglionic neurons innervating sweat glands and some blood vessels are the exception-; they are cholinergic and release ACH. Usually a particular receptor subtype for each division of the ANS will dominate in a certain gland or organ. In general, activation of some receptor subtypes lead to stimulation of the effector and activation of others to inhibition of the effector. Even numbered subtypes are usually inhibitory and odd numbered subtypes are usually excitatory, but there are no hard and fast rules. Ultimately, the relative amounts of each receptor subtype expressed in the tissue will determine the overall effect (stimulation or inhibition) on the particular gland or organ. Cholinergic Receptors As mentioned, preganglionic neurons of both sympathetic and parasympathetic divisions produce and release ACH. The receptors for ACH are known as cholinergic receptors. There are two main subtypes of cholinergic receptors-; nicotinic and muscarinic. They are named after alkaloids found in tobacco and certain mushrooms respectively. The alkaloid nicotine specifically activates nicotinic cholinergic receptors, while muscarine activates muscarinic cholinergic receptors, and ACH activates both types. The cell bodies of postganglionic neurons for both sympathetic and parasympathetic nervous systems express nicotinic receptors (see figure above). To distinguish nicotinic receptors in neurons from nicotinic receptors found in the neuromuscular junction, we use the terms nicotinic neural (NN or N2) cholinergic receptors and nicotinic muscle (NM or N1) cholinergic receptors respectively. Muscarinic receptors (M) are located on cells of all parasympathetic effectors and on cells of some sweat glands innervated by the sympathetic nervous system. There are several subtypes of muscarinic receptors (M1-M5) which may be stimulatory or inhibitory. Adrenergic Receptors As mentioned, neurons that produce and release the neurotransmitter NE are known as adrenergic neurons. NE is secreted by postganglionic neurons of the sympathetic nervous system and binds to adrenergic receptors expressed on effector cells. Epinephrine (EPI) released by the adrenal gland also binds to adrenergic receptors expressed on effectors (see figure above). There are two main types of adrenergic receptors, namely, alpha and beta which have several subtypes. For our purposes, we will focus on the following four subtypes: alpha 1, 2, and beta 1, 2, and 3. Activation of adrenergic receptors expressed on effectors by NE or EPI may result in stimulation or inhibition of the effector depending on the tissue involved. Odd subtypes of adrenergic receptors (alpha 1, and beta 1, and 3) generally have stimulatory effects and even subtypes (alpha 2 and beta 2) have inhibitory effects. The next section explores the effects of a ligand binding to its receptor in a particular tissue. **You may use the buttons below to go to the next or previous reading in this Module**
autonomic nervous systemEncyclopædia Britannica, Inc. The autonomic nervous system is the part of the peripheral nervous system that regulates the basic visceral processes needed for the maintenance of normal bodily functions. It operates independently of voluntary control, although certain events, such as stress, fear, sexual excitement, and alterations in the sleep-wake cycle, change the level of autonomic activity. The autonomic system usually is defined as a motor system that innervates three major types of tissue: cardiac muscle, smooth muscle, and glands. However, it also relays visceral sensory information to the central nervous system and processes it so that alterations can be made in the activity of specific autonomic motor outflows, such as those that control the heart, blood vessels, and other visceral organs. It also stimulates the release of certain hormones involved in energy metabolism (e.g., insulin, glucagon, and epinephrine [also called adrenaline]) or cardiovascular functions (e.g., renin and vasopressin). These integrated responses maintain the normal internal environment of the body in an equilibrium state called homeostasis. The autonomic system consists of two major divisions: the sympathetic nervous system and the parasympathetic nervous system. These often function in antagonistic ways. The motor outflow of both systems is formed by two serially connected sets of neurons. The first set, called preganglionic neurons, originates in the brainstem or the spinal cord, and the second set, called ganglion cells or postganglionic neurons, lies outside the central nervous system in collections of nerve cells called autonomic ganglia. Parasympathetic ganglia tend to lie close to or within the organs or tissues that their neurons innervate, whereas sympathetic ganglia are located at more distant sites from their target organs. Both systems have associated sensory fibres that send feedback into the central nervous system regarding the functional condition of target tissues. A third division of the autonomic system, the enteric nervous system, consists of a collection of neurons embedded within the wall of the gastrointestinal tract and its derivatives. This system controls gastrointestinal motility and secretion. The sympathetic nervous system normally functions to produce localized adjustments (such as sweating as a response to an increase in temperature) and reflex adjustments of the cardiovascular system. Under conditions of stress, however, the entire sympathetic nervous system is activated, producing an immediate, widespread response called the fight-or-flight response. This response is characterized by the release of large quantities of epinephrine from the adrenal gland, an increase in heart rate, an increase in cardiac output, skeletal muscle vasodilation, cutaneous and gastrointestinal vasoconstriction, pupillary dilation, bronchial dilation, and piloerection. The overall effect is to prepare the individual for imminent danger. Sympathetic preganglionic neurons originate in the lateral horns of the 12 thoracic and the first 2 or 3 lumbar segments of the spinal cord. (For this reason the sympathetic system is sometimes referred to as the thoracolumbar outflow.) The axons of these neurons exit the spinal cord in the ventral roots and then synapse on either sympathetic ganglion cells or specialized cells in the adrenal gland called chromaffin cells. Sympathetic ganglia can be divided into two major groups, paravertebral and prevertebral (or preaortic), on the basis of their location within the body. Paravertebral ganglia generally are located on each side of the vertebrae and are connected to form the sympathetic chain, or trunk. There are usually 21 or 22 pairs of these ganglia—3 in the cervical region, 10 or 11 in the thoracic region, 4 in the lumbar region, and 4 in the sacral region—and a single unpaired ganglion lying in front of the coccyx, called the ganglion impar. The three cervical sympathetic ganglia are the superior cervical ganglion, the middle cervical ganglion, and the cervicothoracic ganglion (also called the stellate ganglion). The superior ganglion innervates viscera of the head, and the middle and stellate ganglia innervate viscera of the neck, thorax (i.e., the bronchi and heart), and upper limbs. The thoracic sympathetic ganglia innervate the trunk region, and the lumbar and sacral sympathetic ganglia innervate the pelvic floor and lower limbs. All the paravertebral ganglia provide sympathetic innervation to blood vessels in muscle and skin, arrector pili muscles attached to hairs, and sweat glands. The three preaortic ganglia are the celiac, superior mesenteric, and inferior mesenteric. Lying on the anterior surface of the aorta, preaortic ganglia provide axons that are distributed with the three major gastrointestinal arteries arising from the aorta. Thus, the celiac ganglion innervates the stomach, liver, pancreas, and the duodenum, the first part of the small intestine; the superior mesenteric ganglion innervates the small intestine; and the inferior mesenteric ganglion innervates the descending colon, sigmoid colon, rectum, urinary bladder, and sexual organs. Upon reaching their target organs by traveling with the blood vessels that supply them, sympathetic fibres terminate as a series of swellings close to the end organ. Because of this anatomical arrangement, autonomic transmission takes place across a junction rather than a synapse. “Presynaptic” sites can be identified because they contain aggregations of synaptic vesicles and membrane thickenings; postjunctional membranes, on the other hand, rarely possess morphological specializations, but they do contain specific receptors for various neurotransmitters. The distance between pre- and postsynaptic elements can be quite large compared with typical synapses. For instance, the gap between cell membranes of a typical chemical synapse is 30–50 nanometres, while in blood vessels the distance is often greater than 100 nanometres or, in some cases, 1–2 micrometres (1,000–2,000 nanometres). Owing to these relatively large gaps between autonomic nerve terminals and their effector cells, neurotransmitters tend to act slowly; they become inactivated rather slowly as well. To compensate for this inefficiency, many effector cells, such as those in smooth and cardiac muscle, are connected by low-resistance pathways that allow for electrotonic coupling of the cells. In this way, if only one cell is activated, multiple cells will respond and work as a group. nicotinic receptorEncyclopædia Britannica, Inc. At a first approximation, chemical transmission in the sympathetic system appears simple: preganglionic neurons use acetylcholine as a neurotransmitter, whereas most postganglionic neurons utilize norepinephrine (noradrenaline)—with the major exception that postganglionic neurons innervating sweat glands use acetylcholine. On closer inspection, however, neurotransmission is seen to be more complex, because multiple chemicals are released, and each functions as a specific chemical code affecting different receptors on the target cell. In addition, these chemical codes are self-regulatory, in that they act on presynaptic receptors located on their own axon terminals. autonomic nervous systemEncyclopædia Britannica, Inc. The chemical codes are specific to certain tissues. For example, most sympathetic neurons that innervate blood vessels secrete both norepinephrine and neuropeptide Y; sympathetic neurons that innervate the submucosal neural plexus of the gut contain both norepinephrine and somatostatin; and sympathetic neurons that innervate sweat glands contain calcitonin gene-related peptide, vasoactive intestinal polypeptide, and acetylcholine. In addition, other chemicals besides the neuropeptides mentioned above are released from autonomic neurons along with the so-called classical neurotransmitters, norepinephrine and acetylcholine. For instance, some neurons synthesize a gas, nitric oxide, that functions as a neuronal messenger molecule. Thus, neural transmission in the autonomic nervous system involves the release of combinations of different neuroactive agents that affect both pre- and postsynaptic receptors. Neurotransmitters released from nerve terminals bind to specific receptors, which are specialized macromolecules embedded in the cell membrane. The binding action initiates a series of specific biochemical reactions in the target cell that produce a physiological response. In the sympathetic nervous system, for example, there are five types of adrenergic receptors (receptors binding epinephrine): α1, α2, β1, β2, and β3. These adrenoceptors are found in different combinations in various cells throughout the body. Activation of α1- adrenoceptors in arterioles causes blood-vessel constriction, whereas stimulation of α2 autoreceptors (receptors located in sympathetic presynaptic nerve endings) functions to inhibit the release of norepinephrine. Other types of tissue have unique adrenoceptors. Heart rate and myocardial contractility, for example, are controlled by β1-adrenoceptors; bronchial smooth muscle relaxation is mediated by β2-adrenoceptors; and the breakdown of fat (lipolysis) is controlled by β3-adrenoceptors. Cholinergic receptors (receptors binding acetylcholine) also are found in the sympathetic system (as well as the parasympathetic system). Nicotinic cholinergic receptors stimulate sympathetic postganglionic neurons, adrenal chromaffin cells, and parasympathetic postganglionic neurons to release their chemicals. Muscarinic receptors are associated mainly with parasympathetic functions and are located in peripheral tissues (e.g., glands and smooth muscle). Peptidergic receptors exist in target cells as well. The length of time that each type of chemical acts on its target cell is variable. As a rule, peptides cause slowly developing, long-lasting effects (one or more minutes), whereas the classical transmitters produce short-term effects (about 25 milliseconds). |
What is the neurotransmitter used by both sympathetic and parasympathetic preganglionic neurons?
Related Posts
Copyright © 2024 SignalDuo Inc.
