Where synapses of preganglionic sympathetic neurons
The autonomic nervous system - structure, function and pathology
Table of Contents
Image: “Sympathetic Innervation” by BruceBlausen. License: CC BY 3.0
Functional organization of the vegetative / autonomic nervous system
The vegetative or autonomic nervous system (Latin: cars = self; nomos = law) uses both sensory and efferent neurons, which primarily control the activity of the internal organs.
Image: "Diagram showing the divisions of the nervous system." By Fuzzform. License: CC BY-SA 3.0
Most of the main input to the ANS comes from autonomic sensory (viscerosensory) neurons, mostly with Interoceptors associate and are located as sensory receptors in blood vessels, visceral organs and muscles. These neurons are located in the stomach and lungs in particular and have the task of transmitting information to the CNS.
In contrast to signals that trigger a nice smelling fragrance or delicious-looking food, inner sensory signals are usually not perceived consciously, although the activation of interoceptors can penetrate into consciousness. Two typical examples would be pain from damaged intestines or angina pectoris (brood pain) with inadequate perfusion of the myocardium.
The efferent neurons, on the other hand, conduct the nerve impulses from the CNS to the target tissue (smooth muscles, heart muscle or glands) and regulate visceral activities by increasing them (Excitation) or reduction (Inhibiting).
Nerves from the Sympathetic and Parasympathetic nervous system ensure these opposing effects.
Sympathetic neurons e.g. accelerate the heartbeat and support physical processes or exertion and ensure a "fight or flight" reaction. It serves to increase performance and thus it is stimulated during arousal, activity and stress.
Parasympathetic Neurons slow down the heartbeat and ensure a "rest-and-digest" (resting and digesting) activity. These neurons are used for relaxation, recovery and regeneration and ensure that vital energy reserves are built up.
The efferent responses are not under conscious control, so the activity of the autonomic nervous system (ANS) is involuntary. Some autonomous reactions are used as the basis for so-called “lie detector tests”, since one cannot consciously bring the pulse to half the normal value. However, with appropriate relaxation techniques, some people can modify some autonomous activities.
Structure of the VNS
As preganglionic neuron we call the first of the two motor neurons in each motor signal pathway. Its soma lies in the brain or spinal cord, while its axon leaves the CNS as part of a cranial or spinal nerve.
Usually the preganglionic neuron reaches one autonomous ganglionwhere it connects to the second neuron of the signal pathway, the postganglionic neuron, forms a synapse. The soma and the dendrites of the postganglionic neuron are located in an autonomic ganglion by forming synapses with one or more preganglionic neurons.
The autonomic nervous system conducts preganglionic neurons nerve impulses from the CNS to autonomous ganglia, where the signals are switched to postganglionic neurons and passed on from the autonomous ganglia to the target tissue.
The following example is given:
Spinal cord (CNS) → preganglionic neuron→ autonomous ganglion→ postganglionic neuron→ Heart (target tissue / effector)
The sympathetic part of the ANS is also called Pars thoracolumbalis, because the somata of the preganglionic neurons are located in the lateral horns of the twelve thoracic segments and the first two to three lumbar segments of the spinal cord.
In contrast, the parasympathetic part of the ANS is also called Pars craniosacralis, because the somata of the parasympathetic preganglionic neurons are located in the nuclei of four cranial nerves in the area of the brain stem and in the lateral horns of the second to fourth sacral segments in the spinal cord.
Autonomous ganglia are divided into three groups:
- Sympathetic trunk ganglia
- Sympathetic prevebral ganglia
- Parasympathetic ganglia
Sympathetic ganglia are where the synapses between preganglionic and postganglionic sympathetic neurons lie.
Sympathetic trunk ganglia
Sympathetic trunk ganglia (paravertebral ganglia) lie in a vertical row on each side of the spine and extend from the base of the skull down to the coccygeum. For the most part, organs above the diaphragm are innervated by postganglionic axons of the sympathetic trunk ganglia.
Sympathetic prevertebral ganglia
Sympathetic prevertebral ganglia lie anterior to the spine and near the large abdominal arteries. In contrast, postganglionic axons of the paravertebral ganglia innervate organs below the diaphragm. The 3 largest prevertebral ganglia are:
- Ganglia coelica (located just below the diaphragm)
- Superior mesenteric ganglion (lies in the upper abdomen)
- Inferior mesenteric ganglion (located in the middle abdomen)
Image: "Parasympathetic Ganglion" by Ed Uthman. License: CC BY 2.0
Preganglionic axons of the parasympathetic nervous system form synapses with postganglionic neurons in terminal or intramural ganglia. For the most part, the ganglia are near or in the wall of an organ.
The axons preganglionic parasympathetic neurons are usually longer than most axons of preganglionic sympathetic neurons because they extend from the CNS to an intramural ganglion of the innervated organ.
Axons of preganglionic sympathetic neurons can be connected to postganglionic neurons after being drawn to sympathetic trunk ganglia in three of the following ways:
- An axon can form a synapse with a postganglionic neuron directly in the first ganglion reached.
- An axon can ascend or descend to a higher or lower ganglion before it is connected to the postganglionic neuron. Axons of incoming sympathetic preganglionic neurons, which run vertically along the sympathetic trunk, form the trunk.
- An axon can pass through the sympathetic ganglion without forming a synapse and end in a prevertebral ganglion, where it is switched to the postganglionic neuron.
Image: “Sympathetic Innervation” by BruceBlausen. License: CC BY 3.0
A single preganglionic sympathetic fiber has many branches and therefore can be synaptically connected to 20 or more postganglionic neurons. The postganglionic axons then typically terminate in various target tissues.
Image: “Parasympathetic Innervation” by BruceBlausen. License: CC BY 3.0
In these ganglia, the presynaptic neurons are only switched to four to five postsynaptic neurons. They all individually supply a visceral target tissue and a target tissue can thus be controlled separately parasympathetically.
Axons, sympathetic and parasympathetic neurons form networks that are called autonomic plexus designated. They run along the major arteries and are found in the thorax, abdomen, and pelvis. The great Cardiac plexus in the thorax is for the supply of the heart and the Pulmonary plexus responsible for the bronchial tree.
The largest autonomic plexus is the Celiac plexus (solaris), which draws to the liver, gallbladder, stomach, pancreas, spleen, kidneys, adrenal cortex, testes and ovaries.
Neurotransmitters and receptors of the VNS
For neurotransmitters, receptors are integral membrane proteins that are located in the plasma membrane of the postsynaptic neuron or a cell of the target tissue.
One distinguishes between cholinergic and adrenergic neurons.
Cholinergic neurons and receptors
The ANS includes the following cholinergic neurons:
- All sympathetic and parasympathetic preganglionic neurons
- Sympathetic postganglionic neurons for most sweat glands
- All postganglionic parasympathetic neurons
The cholinergic neurons put the neurotransmitter Acetylcholine (ACh), which is stored in synaptic vesicles and released by exocytosis. It then diffuses through the synpathetic gap and binds to the specific ones cholinergic receptors.
Cholinergic receptors are once again in nicotinergic and muscarinic receptors distinguished, both of which bind ACh.
Nicotinic receptors are embedded in sympathetic and parasympathetic postganglionic neurons as well as in the neuromuscular endplate. They are named because nicotine simulates the action of ACh after binding to the receptors. This substance cannot be detected in non-smokers, since nicotine is not a physiologically occurring substance in the human organism.
In contrast, the plasma membranes of all target tissues (smooth muscles, myocardium, glands) contain the muscarinic receptorswhich are supplied by parasympathetic postganglionic axons. At some receptors arises Inhibition and with others Excitation. The sweat glands also have muscarinic receptors, which leads to increased sweating.
Acetylcholine is quickly deactivated by the enzyme acetylcholinesterase, so the effects triggered by cholinergic neurons are short.
Adrenergic neurons and receptors
Norepinephrine (NAdr) is carried out in the ANS adrenergic neurons released. Many of the postganglionic sympathetic neurons are adrenergic. The NAdr, like the ACh, is stored in synaptic vesicles, released by exocytosis, diffuses through the synaptic cleft and binds to specific ones adrenergic receptors in the postsynaptic membrane. The consequence is one Excitation or one Inhibition the effector cell.
Norepinephrine as adrenaline are bound by adrenergic receptors. The NAdr can be released into the blood as a neurotransmitter by sympathetic postganglionic neurons or as a hormone from the adrenal medulla. Adrenaline is only released as a hormone.
Also the adrenergic receptors can be further subdivided into two subtypes that are innervated by most postganglionic sympathetic neurons. One speaks of Alpha (α) receptors and Beta (β) receptorswhich are further subdivided according to specific responses and corresponding binding properties (α1, α2, β1, β2, etc.).
In general, it can be said that activation of the α1 and β1 receptors causes excitation, while α2-β2 receptors inhibit the target tissue.
Thus, the effects triggered by adrenergic neurons work longer than the actions caused by cholinergic neurons.
The responses arising from nerve impulses in an autonomic reflex arc are referred to as autonomous reflexes. They play a key role in the following processes:
- With blood pressure (e.g. by adjusting the heart rate)
- During digestion (adjusting motility and muscle tone in the GI tract)
- During defecation
- During urination (regulation of the sphincter openings or closures)
The main control and integration center of the ANS is the Hypothalamuscontaining sensory information about visceral functions (smell, taste, temperature, etc.). Signals from the limbic system that are related to emotions are also included. The signals emanating from the hypothalamus influence autonomous centers in the brain stem (Truncus cerebri) as well as in the spinal cord (medulla spinalis).
The following components belong to an autonomous reflex arc:
- The distal end of a sensory neuron is the receptor of the autonomic reflex arch, which is located on a stimulus reacts and can trigger a nerve impulse. Mostly autonomous sensory receptors associate with Interoceptors.
- The sensory neuron forwards nerve impulses from the receptor to the CNS.
- The main integration centers for autonomic reflexes are in the hypothalamus and brain stem. Few autonomous reflexes are located in the integration centers in the spinal cord, which are primarily responsible for urine excretion and defecation.
- Interconnection: Interneurons in the CNS interconnect signals from sensory neurons to Motor neurons.
- Signals triggered by integration centers leave the CNS via motor neurons to the target tissue. Two Motor neurons connect the CNS in one autonomic reflex arc with the Effector. The impulse is conducted from the preganglionic neuron to an autonomous ganglion and from there to the postganglionic neuron to the target tissue.
Target tissue (effector)
- The effector in the autonomic reflex arc are the smooth muscles, the heart muscle and the glands.
Diseases of the ANS / VNS
In contrast to the somatic nervous system, the tissues innervated by the ANS continue to work even if the neural supply is damaged. The heart, for example, keeps beating when it is separated from its autonomic nerve connections. Smooth muscle cells in the GI tract contract independently and rhythmically, and glands produce some substances even without control of the ANS. For this reason, most autonomous responses cannot be decisively changed or suppressed at will.
Nevertheless, pathological damage also occurs in the autonomic / vegetative nervous system.
In Horner's syndrome, the sympathetic supply of one side of the face is damaged, which affects the sympathetic exit from the superior cervical ganglion. The causes can be an injury, illness or a hereditary mutation. The Horner syndrome is characterized by the following symptoms:
- Ptosis (hanging upper eyelid)
- Miosis (constricted pupil)
- Enophthalmus (Receding the eye)
Further diseases of the autonomic nervous system
- Reynaud's Syndrome
- Autonomic dysreflexia
- Autonomous neuropathy
- Sudeck's disease
Popular exam questions about the autonomic nervous system
The answers are below the sources.
1. What is the correct order of essential components of the autonomic reflex arc?
- Receptor, sensory neuron, integration center, preganglionic neuron, autonomous ganglion, postganglionic neuron, effector
- Effector, sensory neuron, integration center, preganglionic neuron, autonomous ganglion, postganglionic neuron, receptor
- Effector, postganglionic neuron, autonomous ganglion, preganglionic neuron, sensory neuron, integration center, receptor
- Receptor, sensory neuron, integration center, autonomous ganglion, postganglionic neuron, preganglionic neuron, effector
- Receptor, sensory neuron, integration center, autonomous ganglion, preganglionic neuron, postganglionic neuron, effector
2. Which statement is wrong?
- ACh can activate both types of cholinergic receptors.
- Nicotine cannot activate muscarinic receptors.
- Muscarin cannot activate nicotinergic receptors.
- Acetylcholinesterase slowly deactivates acetylcholine and thus the effects triggered by cholinergic neurons are long.
- The cholinergic neurons release the neurotransmitter acetylcholine (ACh).
3. What is the name of the largest autonomic plexus?
- Renal plexus
- Cardiac plexus
- Pulmonary plexus
- Celiac plexus (solaris)
- Hypogastric plexus
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G.J. Tortora and B.H. Derrickson: Anatomy and Physiology, Wiley-VCH Verlag
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