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The Nervous System

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The Nervous System

Noradrenaline - Noradrenaline is a hormone that is produced naturally by the body. It is synthesized and released by the central nervous system, and also by a division of the autonomic nervous system called the sympathetic nervous system. Norepinephrine acts on target cells by binding to and activating noradrenergic receptors located on the cell surface. It is the main neurotransmitter of the sympathetic nerves in the cardiovascular system and is released as a response to stress.

Function:
The sympathetic nervous system triggers a response that is commonly referred to as our 'fight or flight response.' When faced with a dangerous situation, our body needs to decide whether to stay and face the problem, or run away. So, In order to make our body work as efficiently as possible, norepinephrine causes several changes in our body function. These include the following: 1. An increase in the amount of oxygen going to our brain - this helps us think clearer and faster. 2. An increase in our heart rate - this pumps more blood around our body, helping our muscles work faster and more efficiently. 3. An increase in glucose (or sugar) release - this additional sugar gives our muscles something to 'feed on,' which helps them work better and faster. 4. An increase in breathing rate - when we breathe faster, we are delivering more oxygen to the body and brain. This helps our entire body work better. 5. A shutting down of metabolic processes - shutting down processes, like digestion and growth, allows blood and energy that would normally go to these functions to be shunted to our muscles and brain.

Receptors
Like many biologically active substances, norepinephrine exerts its effects by binding to and activating receptors located on the surface of cells. Two norepinephrine receptors have been identified, known as alpha and beta adrenergic receptors. Alpha receptors are divided into subtypes α1 and α2; beta receptors into subtypes β1, β2, and β3. Alpha-2 receptors usually have inhibitory effects, but many are located pre-synaptically (i.e., on the surface of the cells that release norepinephrine), so the net effect of alpha-2 activation is often a decrease in the amount of norepinephrine released. Alpha-1 receptors and all three types of beta receptors usually have excitatory effects.
Alpha blockers
Alpha 1 receptors are located on: * Vascular smooth muscle * Urinary sphincters * Dilator muscle of the iris * Muscle of hair follicles
Alpha-1 receptors have excitatory effects and consequently, blockage of alpha-1 receptors usually results in blocking some of the effects of norepinephrine. Drugs such as phentolamine that act on both types of receptors (Alpha 1 and Alpha 2) can produce a complex combination of both effects.
Alpha-2
Alpha-2 receptors are inhibitory and many are located presynaptically on norepinephrine-releasing cells so the net effect of these drugs is usually to reduce the amount of norepinephrine released. Drugs in this group that are capable of entering the brain often have strong sedating effects, due to their inhibitory effects. Clonidine, for example, is used for the treatment of anxiety disorders and insomnia, and also as a sedative premedication for patients about to undergo surgery.

Beta Blockers
Beta 1 receptors are located on:
 Cardiac pacemaker
 Myocardium
 Salivary gland ducts
 Sweat glands
The cardiac pacemaker responds by increasing the heart rate. Simultaneously, the myocardium contracts more forcefully. The physiology of the response of salivary duct cells to beta-1 activation is unclear; it appears that beta 1 activation of certain duct cells reabsorbs some water in the slowly passing saliva making it more viscous.
Most tissues express multiple receptors. However, the dominant beta receptor in the normal heart is the beta1 receptor while the beta2 receptor is the dominant regulatory receptor in vascular and non-vascular smooth muscle. Noradrenaline activates only the beta 1 receptor.

Cardio-selective beta blockers
Beta blockers, also called beta-adrenergic receptor antagonists, are prescription medications used primarily in the treatment and prevention of cardiovascular disease, including heart attacks, high blood pressure and arrhythmia.
There are two predominant types of beta-adrenergic receptors: beta-1 and beta-2. Beta-1 receptors are found primarily in the heart, while beta-2 receptors are found primarily in tissues other than the heart, such as the airway, muscle and blood vessels.
Drugs that mostly target beta-1 receptors are called cardioselective beta blockers. Cardioselective beta blockers slow heart rate, reduce electrical conduction speed in the heart and decrease the force of heart contraction.

How they work
Beta blockers work by changing how certain sensors in the body, called beta receptors, respond to signals from the central nervous system. These medications are used to treat heart and blood pressure problems because both the heart and blood vessels have very high numbers of beta receptors.
The cardioselective beta blockers include: * Acebutolol * Atenolol * Bisoprolol * Esmolol * Metoprolol
Therefore, an asthmatic heart patient would be grateful for a cardioselective beta blocker because it targets the heart and would slow down their heart rate and decrease the force of heart contraction which would make it easier for them to breathe.

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