Autonomic nervous system

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The autonomic nervous system is an efferent branch of the peripheral nervous system. The peripheral nervous system can be broken down into efferent and afferent branches. The efferent branch can be further divided into the autonomic nervous system and the somatic nervous system. The main role of the autonomic nervous system is to innervate the main effector organs and tissues. It is under subconscious control so is therefore not noticed by an individual.

Contents

Innervation

There are two branches in the autonomic nervous system and both branches innervate the major organs making the autonomic nervous system a dual innervation system. The two branches have opposite functions in most cases and they are active in different conditions. The two branches are called either the sympathetic nervous system or the parasympathetic nervous system . The sympathetic nervous system is mostly active when the body is excited or physically active as it can increase the heart rate and decrease the digestive tract. It also switches the main energy supply to the skeletal and cardiac muscles in preparation for fight or flight. This is the body's response to stress. The parasympathetic nervous system has the opposite effect and is mainly active during periods of rest.

Anatomy of the Autonomic Nervous System

The pathways of the autonomic nervous system are efferent and contain two neurons which are arranged in series and this is the communication between the central nervous system and the effector organ. In order to communicate between the two neurone, synapses must be present and these are called autonomic ganglia. The preganglionic neurons travel from the central nervous system to the ganglia and the postganglionic neurons travel from the ganglia to the effector neurons. Therefore, the axon and the dendrites are found within the ganglion. One preganglionic neuron will synapse with several postganglionic neurons. There are also other neurons present called the intrinsic neurons and these regulate the flow of information to the effector organs.

The Sympathetic Nervous System

The preganglionic neurons in the SNS emerge from the thoracic and lumbar region of the spinal cord and they originate from in the grey matter called the lateral horn. The preganglionic and the postganglionic neurons are typically arranged in three different arrangements. In the most common pattern, the preganglionic neurons have short axons that extend from the lateral horn and out of the ventral root. These form a spinal nerve and it leaves via the white ramus and enters sympathetic nerves outside of the spinal cord where they synapse with the postganglionic nerves which then return to the spinal cord via the grey ramus before reaching the effector organ. The sympathetic ganglia are linked together to form sympathetic chains that run parallel to the spinal cord.

The second arrangement of the SNS differs because instead of innervating the effector organs, the preganglionic neurons innervate endocrine tissue such as the adrenal glands. Stimulation through these neurons causes the release of hormones from the adrenal gland and these use the endocrine system to secrete hormones throughout the body to their various target organs.

The third arrangement in the SNS is where the preganglionic neurons synapse with the collateral ganglia which can be found between the CNS and the effector organ. This allows the SNS to target specific tissues in order to exert effects.

The Parasympathetic Nervous System

The preganglionic nervous system originate in the brain stem or the sacral spinal cord. They are relatively long and terminate close to the effector organ in the ganglia. One important nerve of note is a cranial nerve called the vagus nerve which originates in the medulla oblongata and innervates the lung, heart, liver, stomach and small intestines.

Autonomic Neurotransmitters

There are two main types of neurotransmitter which are used in this part of the nervous system- acetylcholine and norepinephrine. Acetylcholine is released in both the sympathetic and parasympathetic nervous systems . Most sympathetic postganglionic neurons also release norepinephrine. In order for these two transmitters to have an effect, they must come into contact with a receptor. The receptor is specific to the type of neurotransmitter and there are two different types of cholinergic receptor - the nicotinic and the muscarinic. The nicotinic receptors are found in the postganglionic neurons and the muscarinic are present in the effector organs of the parasympathetic nervous system. When acetylcholine binds to these receptors, sodium diffuses into the cell and potassium diffuses out at different ratios which leads to depolarisation.

There are also two types of adrenergeic receptors, alpha and beta which can be found in the effector organs of the sympathetic nervous system. These receptors act through a G protein based mechanism whereby binding of the neurotransmitter to the receptor causes a G protein to become activated within the cell so that it can then activate phospholipase C in order to have a variety of effects that lead to a phosphorylated protein and a response.

Neuroeffector Junctions

The neuroeffector junction is the synapse between the efferent neuron and the organ. Neurotransmitters are released across the synapse in swellings called variscosities. It is within these swellings that the neurotransmitter is produced and stored. Once an action potential reaches the synapse, calcium enters and causes the release of the neurotransmitter via exocytosis where it is either taken up by the receptors on the postsynaptic neuron or recycled for use at a later date. The recycling of the neurotransmitter is of importance for is terminates the end of the signal so that there is not prolonged stimulation.

Autonomic Function Regulation

In order to maintain homeostasis, the two systems of the autonomic nervous system must balance each other out. When the body is at rest, the majority of the energy supply is targeted to the digestive system and the parasympathetic nervous system exerts control. However, upon activity, the demands of the body rise and therefore the energy demands of certain tissues increase and this is taken away from the digestive tract and to the skeletal and cardiac muscle. The body needs to detect the switch in demands and the brain can do so by visceral reflexes.

There will be a change in the organ function upon movement and this is detected by the brain. For instance, when a person stands up, there is a drop in blood pressure as the blood is pooling in the legs which is detected by receptors which send a message up to the brain. The brain then stimulates the sympathetic nervous system to act on the heart and blood vessels to move the blood out of the legs and regain the average pressure of the blood.

There are three major areas of the brain that are responsible for this form of regulation which include the hypothalamus, the pons and the medulla oblongata. The hypothalamus is responsible for the fight or flight mechanism displayed when an individual is in a stressful situation. It is also able to control the body temperature, food intake and water balance. The medulla oblongata and the pons can control the heart, blood vessels and the smooth muscle found in the respiratory system so they can therefore control breathing.

The systems mentioned can also control and regulate swallowing and vomiting. Emotions can also be controlled by the autonomic nervous system such as blushing, fainting or a racing heartbeat.

Motion Sickness

It is the autonomic nervous system that can cause an individual to suffer from motion sickness due to an incorrect matching of the inputting systems. For example if the signals from the vestibular apparatus in the ear which detects motion, the visual system and the proprioceptors are incorrectly matched then the person will experience symptoms such as nausea and sweating. Treatment for motion sickness has focussed on targeting the autonomic nervous system with a drug called scopolamine which is a muscarinic cholinergic antagonist.

Diabetic Neuropathy

A condition called diabetic neuropathy can greatly affect the functioning of the autonomic nervous system and is quite serious for there are a wide range of efffects. The autonomic nervous system is linked to all the major organs in the body and therefore the condition can affect any of the organs causing different effects. Patients usually suffer damage to the autonomic nerves which can affect the function of the heart. If it affects the nerves of the digestion system then an individual suffers from vomiting and constipation. It also makes the diabetes difficult to deal with. Hypoglycaemia can occur much easier as it is not detected as the autonomic nervous system is impaired.

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