Where does nicotine act on the nervous system?

In order to properly answer this question, it's good to have a brief overview of the nervous system (Figure 4). The nervous system can be subdivided into two broad categories, the central nervous system (CNS) and the peripheral nervous system (PNS). Nicotine receptors are found in both the CNS and PNS. The CNS includes the brain and the spinal column, both of which are protected by bone; the PNS includes all the nerves that branch out from the CNS and end at the organs of the body. Additionally, the PNS can be further subdivided into the somatic (or voluntary nervous system) and the autonomic (or involuntary nervous system). We first discuss the PNS and the location of nicotine receptors there. Next, we'll talk about the CNS, most notably the brain, and what role nicotine receptors play there.

Figure 4 The central Nervous system

From D. Chiras, Human Biology, 5th Edition. © 2005, Jones and Bartlett LLC.

Peripheral Nervous System (PNS)

Somatic Nervous System

The somatic nervous system is that part of the PNS that plays a role in the voluntary movement of skeletal muscle (as opposed to the autonomic or involuntary nervous system, which will be discussed later). Electrochemical signals begin at the motor cortex of the CNS, are transmitted down the spinal column, and ultimately through the PNS, terminating at their respective muscles via the neuromuscular junction. The neuromuscular junction^[1] is the point at which the nerves end and their electrochemical communication is converted into a release of the neurotransmitter acetylcholine^[2], which then prompts the muscles to move. This occurs by acetylcholine binding to the nicotinic acetylcholine receptors.

Autonomic Nervous system

Sympathetic Nervous System

The fight or flight response causes a sudden change in bodily functions as a result of a perceived threat in the environment. It diverts blood away from the digestive organs and skin to the muscles, raises the heart rate and blood pressure, dilates the airways of the lungs, and promotes alertness. In other words, it readies the body to react to the perceived threat so that it can either fight or flee from the threat.

These actions occur with the aid of the hormone epinephrine^[3] (also known as adrenalin because it comes from the adrenal glands) and the neurotransmitter norepinephrine in the sympathetic nervous system. (Finally, the neurotransmitter dopamine is also activated but in a more limited manner.) All of these chemicals are collectively known as catecholamines^[4]. Norepinephrine and epinephrine bind to adrenergic receptors (also known as adrenoreceptors after adrenalin). There are two broad subtypes of adrenoreceptors known as alpha- and beta-adrenergic receptors, and many of the blood pressure medications prescribed today act at these sites by blocking them in order to lower blood pressure.

The sympathetic nervous system is also activated by the neurotransmitter acetylcholine. However acetylcholine's role is more complicated and indirect in that it stimulates receptors that are located between nerve bundles known as ganglion^[5] (Figure 5). The ganglion are located parallel between the

Figure 5 Autonomic Nervous system

spinal cord and the organs in the illustration, which are junctions between the nerves coming from the CNS (also known as preganglionic^[6]) to the PNS, with peripheral nerves going to their respective organs (also known as postganglionic^[7]). As stated earlier, at the end organ sites, instead of acetylcholine, epinephrine and norepinephrine act as the neurotransmitters. The only exceptions to that are the adrenal medulla^[8], which can be considered like a giant ganglion, and the sweat glands, which also respond to acetylcholine. As a reminder, while not directly part of the sympathetic nervous system, acetylcholine is also the neurotransmitter of the somatic nervous system as mentioned previously. Acetylcholine controls voluntary muscles, causing them to react.

Parasympathetic Nervous System

The rest and restoration response of the parasympathetic nervous system has the opposite effect of the sympathetic nervous system's fight or flight response. This response diverts blood from the muscles to the digestive organs and skin, lowers the heart rate and blood pressure, constricts the airways of the lungs, and promotes sleep. Unlike the sympathetic nervous system, all of these actions are mediated by only one neurotransmitter, acetylcholine, both at the ganglion and at the terminal endings of the organ systems. Moving blood from the muscles to the other organs aids in digestion and promotes rest so that the body can rejuvenate and prepare for the next threat. However, the ganglionic acetylcholine receptors differ from the terminal end organ acetylcholine receptors.

• [1] The junction of the axon terminal of a motor neuron with the muscle fiber responsible for ultimately causing the muscle to contract.
• [2] A neurotransmitter found in both the peripheral nervous system (PNS) and the central nervous system (CNS). In the PNS, it is involved in both muscle contraction as well as that part of the involuntary nervous system involved with "rest and restoration." In the CNS, it is involved with memory function.
• [3] (Also known as the hormone adrenaline.) A catecholamine derived from tyrosine, an amino acid, which is produced in the adrenal medulla and released into the bloodstream to activate the "fight or flight" response via the sympathetic nervous system.
• [4] Chemicals used as neurotransmitters and produced from the amino acid tyrosine. They include epinephrine, norepinephrine, and dopamine, all of which are produced in the brain as well as in the adrenal medulla, which is part of the sympathetic nervous system.
• [5] A mass of tissue, generally nervous, which provides relay points and intermediary connections between different neurological structures in the body, such as the peripheral and central nervous systems.
• [6] The end of the central nervous system as it is communicating with the autonomic nervous system, which is part of the peripheral nervous system.
• [7] The beginning of the autonomic nervous system, which transmits from the central nervous system to the various organs. Nicotinic receptors are located here.
• [8] The central part of the adrenal gland surrounded by the adrenal cortex. It produces adrenalin (also known as epinephrine), norepinephrine, and dopamine.