How do the anatomical differences between the sympathetic and parasympathetic divisions of the autonomic nervous system compare and contrast? What are the types of receptors and neurotransmitters each division has?
The autonomic nervous system (ANS) is composed of the sympathetic and the parasympathetic systems. The sympathetic system stimulates the fight-or-flight response, which refers to our responses when we are afraid or otherwise upset. The sympathetic system makes our pupils dilate, makes our mouths feel dry by reducing saliva production, and increases our heart rate, as well as induces many other symptoms of our fight-or-flight response. In contrast, the parasympathetic system stimulates calming symptoms to allow us to rest and digest. The system constricts our pupils, increases saliva production, decreases heart rate, and increases secretion of gastric juices, as well as many other functions needed to promote rest and digestion.
In the sympathetic system, most neurons use the norepinephrine neurotransmitter. Norepinephrine is formed from dopamine with the addition of a dopamine enzyme. In the adrenal glands, norepinephrine can be converted into adrenaline, which induces symptoms of the fight-or-flight response. There are, however, some neurons in the sympathetic system that do not use norepinephrine as the neurotransmitter. Neurons in ganglia, which are clusters of nerve cells that form connections between the body's different neurological parts, use acetylcholine (ACh) as their neurotransmitter. In addition, neurons in the adrenal medulla and the sweat glands also use ACh.
In the parasympathetic system, the main neurotransmitter is ACh, which is produced by the nerve cells called cholinergic neurons. ACh influences movement of the skeleton and muscles, including the cardiac muscle. ACh also influences our ability to learn, our memories, and our emotions.
In the sympathetic system, the three receptors are alpha, beta 1, and beta 2. Alpha receptors are found on the walls of the arteries. Alpha receptors make arteries constrict when neurons release the neurotransmitters epinephrine or norepinephrine. Beta 1 receptors are found in the heart, and neurotransmitters make Beta 1 receptors increase heart rate. Beta 2 receptors are found in the air passageways of the lungs called bronchioles. Beta 2 receptors are also found on the walls of the arteries that run through the skeletal muscles. These receptors allow for more air to travel in and out of the lungs and for an increased blood flow through the skeletal muscles.
The two types of parasympathetic receptors are muscarinic receptors and nicotinic receptors. Nicotinic receptors cause muscle contraction. Muscarinic receptors lower the heart rate.
Neurotransmitters are released from the neuron and bind with receptors to produce a chemical reaction. Neurotransmitters are activated by either agonists or antagonists. Agonists are chemicals that bind to receptors and make nerve responses likely to happen by triggering the receptors. An agonist can be considered a drug or a natural molecule such as an antibody or hormone. Antagonists are chemicals that block receptors thereby, sometimes, blocking neurotransmitters from being released. Antagonists can include drugs or hormones.