Question

Neurotransmitters within the ANS can either be excitatory or inhibitory depending on where they are located as well as what receptor binds them. Give an example of how the parasympathetic sympathetic divisions can sometimes be inhibitory or excitatory. Include the effectors and receptors.

Answer 1

The autonomic nervous system is regulated by integrated reflexes through the brainstem to the spinal cord and organs. Autonomic functions include control of respiration, cardiac regulation (the cardiac control center), vasomotor activity (the vasomotor center), and certain reflex actions such as coughing, sneezing, swallowing and vomiting. Those are then subdivided into other areas and are also linked to autonomic subsystems and the peripheral nervous system. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving autonomic regulatory input from the limbic system.

The autonomic nervous system has three branches: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system.Some textbooks do not include the enteric nervous system as part of this system.The sympathetic nervous system is often considered the "fight or flight" system, while the parasympathetic nervous system is often considered the "rest and digest" or "feed and breed" system. In many cases, both of these systems have "opposite" actions where one system activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitatory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a "quick response mobilizing system" and the parasympathetic is a "more slowly activated dampening system", but even this has exceptions, such as in sexual arousal and orgasm, wherein both play a role.

There are inhibitory and excitatory synapses between neurons. Relatively recently, a third subsystem of neurons that have been named non-noradrenergic, non-cholinergic transmitters (because they use nitric oxide as a neurotransmitter) have been described and found to be integral in autonomic function, in particular in the gut and the lungs.

Although the ANS is also known as the visceral nervous system, the ANS is only connected with the motor side. Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control.

Excitatory neurotransmitters have excitatory effects on the neuron. This means they increase the likelihood that the neuron will fire an action potential. Inhibitory neurotransmitters have inhibitory effects on the neuron. This means they decrease the likelihood that the neuron will fire an action.

There are two major types of neurotransmitter receptors: ionotropic and metabotropic. Ionotropic means that ions can pass through the receptor, whereas metabotropic means that a second messenger inside the cell relays the message (i.e. metabotropic receptors do not have channels).

In the sympathetic nervous system, catecholamines (norephinephrine, epinephrine) act on specific receptors located on the cell surface of the target organs. These receptors are called adrenergic receptors.

• Alpha 1 receptors exert their effect on smooth muscle, mainly by constriction. Effects may include constriction of arteries and veins, decreased motility within the GI (gastrointestinal) tract, and constriction of the pupil. Alpa1 receptors are usually located postsynaptically.
• Alpha 2 receptors bind both epinephrine and norepinephrine, thus reducing the effect of alpha 1 receptors to a certain extent. However, alpha 2 receptors have several specific effects of their own, including vasoconstriction. Effects may include coronary artery constriction, constriction of smooth muscle, constriction of veins, decreased intestinal motility and inhibition of insulin release.
• Beta 1 receptors exert their effect mostly on the heart, causing an increase in cardiac output, increased contractility and increased cardiac conduction, leading to an increase in heart rate. There is also stimulation of the salivary glands.
• Beta 2 receptors exert their effect mostly on the skeletal and cardiac muscles. Increased contraction speed and mass of muscles, as well as dilation of blood vessels occurs. Receptors are stimulated by circulating neurotransmitters (catecholamines).