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Structure and Functions of Sympathetic and Parasympathetic Nervous System

The autonomic nervous system (ANS) provides motor functions for some cranial and spinal nerves, which makes it part of the peripheral nervous system.
The autonomic nervous system (ANS) provides motor functions for some cranial and spinal nerves, which makes it part of the peripheral nervous system. An autonomic nervous system is comprised of visceral motor neurons that send signals to smooth muscle, cardiac muscle, and glands plus, Muscles and glands possess these visceral effectors, with muscles either contracting or relaxing, and glands either secreting more or less secretion. A sympathetic division and a parasympathetic division make up the autonomic nervous system although you will see, they often function in opposition to each other. The sympathetic nervous system predominates in stressful situations and the parasympathetic nervous system tends to be active when resting and in addition it, the hypothalamus integrates both divisions, which results in the visceral effectors being able to respond appropriately in responding to the situation at hand.

A part of the nervous system that is automated or involuntary controls the functions are done automatically by the body, i.e., under the cerebrum exists those that arise in the brain. A person may be aware of the effects of stimulation, such as an increase in heart rate, even when stimulation does not occur voluntarily.

Autonomic pathways
Two motor neurons within a ganglion in the extracranial nervous system form an autonomic nerve pathway from the CNS to a visceral effector. Neurons in the CNS that lead to the ganglion are called preganglionic neurons and an area in the body known as the visceral effector is the third neuron, known as the postganglionic neuron. During postganglionic development, the ganglia form from the cells of postganglionic neurons.

Sympathetic nervous system
Therefore, the thoracolumbar outflow is a more appropriate alternative name considering that the preganglionic neurons originate in both thoracic and lumbar spinal cords.

The Preganglionic Neuron
Located between thoracic vertebrae 1 and 2 or 3 in the spinal cord, this cell body is found between these two vertebrae. As shown in the lateral chain or passing through one of the prevertebral ganglia, the nerve fibers in this cell leave the cord via the anterior root and terminate in one of the sympathetic ganglia. Sympathetic ganglia release Acetylcholine as a neurotransmitter.

A postganglionic neuron
Typically, these cells have their cell bodies in ganglions and terminate in the organ or tissue they supply. Nerve cells in sympathetic effector organs usually use noradrenaline (norepinephrine) as a neurotransmitter and among the major exceptions are that sweat glands, skin, and blood vessels in skeletal muscles lack parasympathetic supply. Axons supplying these structures are primarily sympathetic postganglionic neurons, which are called sympathetic cholinergic nerves because they normally contain acetylcholine in their neurotransmitters.

The sympathetic ganglia
i) A lateral chain of the sympathetic ganglia
Chains are located on each side of the vertebral bodies, beginning at the upper cervical level and ending at the sacrum. Nerve fibers are connecting one ganglion to another. Synaptic connections can be formed by neurons that emerge from the cord at the same level, bypassing up or down the chain, passing through several ganglia, before they make synaptic connections with their postganglionic counterparts at another level. In the chain of the optic nerves, one stimulates the superior cervical ganglion, which triggers the pupil dilation by synaptic attaching with the postsynaptic neuron cell body at the level of the 1st thoracic vertebra. The postganglionic neuron passes through the eyes plus the location of the ganglia causes an increase in sympathetic nerve activity on multiple levels, which provides for rapid and widespread releases of sympathetic hormones.

ii) The prevertebral ganglia
In the abdominal cavity are three prevertebral ganglia, close to the origins of the same-named arteries: the coeliac ganglion, superior mesenteric ganglion, and inferior mesenteric ganglion. Neuronal bodies are diffusely distributed within dense networks of nerve fibers arranged into plexuses within the ganglia. The lateral chain carries sympathetic fibers to these ganglia via the preganglionic nervous system.

Parasympathetic nervous system
During transmission from the source to the effector organ, two types of neurons are responsible (preganglionic and postganglionic) and both synapses are controlled by acetylcholine.

Preganglionic neurons
A cortical dopaminergic cell has either a brain or a spinal cord cell body and is usually much longer than its sympathetic counterpart. It is thought that five cranial nerves originate from the brain: the III, VII, IX, and X, which arise from central nervous system nuclei and terminate at and near effector organs. In the lateral columns of the spinal cord, some cells contain sacral outflows where Segment 2 and segment 3 contain fibers that leave the cord, and segment 4 passes onto postganglionic neurons lining the pelvic organs.

Postganglionic neurons
A ganglion cell body is usually very short and is found either in the ganglion to be supplied or in the organ wall.

Functions
An autonomic nervous system operates several involuntary reflexes and is controlled by sensory input and motor output. The reflex action in this case is a rapid contraction or inhibition of involuntary (cardiovascular or smooth muscle) contraction or glandular secretion. This activity is coordinated subconsciously in the brain and becomes temporarily inhibited when some sensory input reaches the cortex. For instance, reflex micturition becomes temporarily inhibited when certain sensory input reaches the cortex. Stress and danger can strengthen the body's defenses when sympathetic stimulation is applied. For instance, sympathetic stimulation can strengthen the body's defenses in extreme environmental temperatures. Stress, embarrassment, and anger also stimulate the sympathetic nervous system.

In addition, testosterone (adrenaline) and noradrenaline (norepinephrine) are released by the adrenal glands. These hormones increase the effects of sympathetic stimulation, which was believed to mobilize the body to fight or flee. By stimulating the heart, the blood vessels, and the lungs, the body can react by preparing itself for physical activity. Moreover, increased glycogen to glucose conversion and increased metabolic rate can also be noted. If the skeletal muscles require more energy and oxygen to respond to increased demands during exercise, e.g., fighting or running away, these changes enhance the body's ability to react quickly to the increased demands.

In general, parasympathetic stimulation slows down body processes, except digestion and absorption of food and the functioning of the genitourinary system. Generally speaking, its effect is that of a 'peacemaker,' which allows for restoration processes to continue quietly and peacefully. When these two systems are operating normally, the heart pulses regularly, the body temperature is normal, and the internal environment is compatible with the immediate external environment.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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