Motor Systems | Autonomic Motor
Objective 2
Describe the circuitry of the autonomic motor pathways.
General Features
Motor systems require an effector. The output of the nervous system is either contraction of a muscle or changing the secretion of a gland.
The somatic motor system (Objective 1) contracts skeletal muscle. The autonomic motor system (visceral motor system) contracts smooth muscle, contracts cardiac muscle, or changes the secretion of glands.
Remember from Unit 11 Objective 2 that the sympathetic and parasympathetic nervous systems have different anatomy. We will learn in this objective that they have different signaling molecules as well. Both pathways consist of two neurons: a preganglionic neuron, with its cell body in the central nervous system and its axon terminals in an autonomic ganglion; and a postganglionic neuron, with its cell body in an autonomic ganglion and its axon terminals sending a chemical signal to the effector organ (cardiac muscle, smooth muscle, or a gland).
Sympathetic Nervous System
The sympathetic nervous system, sometimes called the “fight-flight-freeze” system, is tuned to activate the entire body at once. The preganglionic cell bodies are located in the lateral part of the gray matter at the first thoracic level (T1) through the second lumbar level (L2) of the spinal cord. These neurons send out short axons which enter the nearby sympathetic chain ganglia where they release acetylcholine onto the postganglionic neurons. The axon of the postganglionic neurons travels a long distance to the organs where it releases norepinephrine (noradrenaline) onto smooth or cardiac muscle.
Let’s say a tiger entered the room where you were reading this. What would you need to do? Your pupils would dilate, your mouth would go dry, your heart rate would increase, the air passages of the lungs would dilate to take in more air, and you would sweat. All these functions are controlled by norepinephrine released from the postganglionic neurons of the sympathetic nervous system. At the same time, the digestive organs would shut down and as much blood as possible would be shunted to the skeletal muscles so you could run if needed.
The response of the nervous system generally is too quick and transient to run for very far or for very long. The sympathetic nervous system also stimulates a gland on top of the kidney, the adrenal medulla, to release epinephrine (adrenaline) which travels in the bloodstream and activates the same receptor as norepinephrine.
Noradrenaline or norepinephrine? Adrenaline or epinephrine?
You might have noticed, and been confused by, the interchangability of the terms “adrenaline” and “epinephrine” and the similar terms “noradrenaline” and “norepinephrine“.
The “nor–” prefix is the easiest one to deal with. Adrenaline is made by adding a methyl (–CH3) group to noradrenaline.
Both noradrenaline and adrenaline bind to the same receptor, called the adrenergic receptor.
In general, noradrenaline is used as a n