Nice Reflexes
The CNS is organized into separate control centers for sensory information and motor output. The motor side is divided into the somatic portion (control for skeletal muscles) and the autonomic (subconscious control of smooth and cardiac muscle as well as glands). This division is further subdivided based on anatomy, neurotransmitters used, and physiological effects that result.
Autonomic Reflex
Unlike the knee-jerk reflex, which consists of one sensory nerve and one motor nerve for an immediate and subconscious reaction, the autonomic reflex involves a series of two motor neurons that deliver motor instructions to target tissues. The first motor neuron leaves the ventral horn of the spinal cord as all other nerves do; however, before reaching a target, it forms a synapse with a second neuron in the periphery of the body, typically in a collection of nerve cell bodies (ganglia). These two nerve cells are named relative to the ganglion: the preganglionic nerve and the postganglionic nerve. The postganglionic nerve innervates the target tissue and elicits the response.
Sympathetic System
Also known as the fight-or-flight nervous system, the sympathetic system instantly prepares the body for intense physical activity. The effects of this system are facilitated and initiated by neurotransmitters and a related hormone, epinephrine (adrenaline). Via the blood stream, this hormone can quickly spread throughout the body and cause the entire body to prepare for action.
Preganglionic sympathetic nerves leave the spinal cord in the middle of the spinal cord from the first thoracic vertebrae (T1) to the second lumbar vertebrae (L2). These fibers are said to form in the thoracolumbar region.
As soon as the preganglionic nerves leave the spinal cord and become part of spinal nerves, they exit the spinal nerve via a pathway called a white ramus. This is not unlike a car exiting an interstate highway to gain access to a side road. In this case, the nerve exits the spinal nerve to gain access to one of the ganglia that form an interconnected chain that mirrors the spinal cord on the right and left side. This sympathetic chain of ganglia allows dispersal of nervous system information along a wide stretch of the thoracic and abdominal region, and controls activities such as shutting down much of the digestive system during a sympathetic response.
From the synapse in the sympathetic ganglia, the postganglionic nerves enter the spinal nerve via a gray ramus (the on ramp) and run to and innervate the target tissue. However, some nerves bypass the chain of ganglia and form two large nerve bundles, the greater and lesser splanchnic nerves. These course through the body and form synapses with postganglionic nerves in collateral ganglia, such as the celiac, superior mesenteric, and inferior mesenteric ganglia, before extending to the target tissue.
Effects
The physiological effects during a sympathetic response rev up the body in preparation for intense activity. These include:
· increased heart and respiratory rate
· dilation of the pupils
· increased blood pressure
· direction of more of the body’s blood supply to the skeletal muscles
These changes occur partly from dilation of blood vessels to the muscles; however, much of this shift is because of the constriction of blood to the digestive and urinary system. Those systems deemed nonessential for immediate survival are restricted in blood supply (to a minimal level) to divert those resources to the organs and tissue needed for short-term survival.
Parasympathetic System
The opposite of the sympathetic system, the parasympathetic system is in control during those periods of rest where the body can perform nonactive functions such as digestion. Thus, this system is often referred to as the rest-and-digest system. Glandular secretions of the GI tract, peristaltic movement, and absorption in the lower alimentary canal occur. Heart and respiratory rates decrease, as does the amount of blood going to skeletal muscles.
The preganglionic fibers exit the spinal cord in the areas not used by the sympathetic system: the cervical and the sacral regions. Sympathetic nerves are in the middle, and the parasympathetic nerves are at the top and bottom of the spinal cord. In the head region, cranial nerves serve as preganglionic neurons for the parasympathetic system. These run practically the entire distance to the target before forming the synapse with postganglionic neurons in what are termed terminal ganglia. Other nerves terminate within the organ itself.
Neurotransmitters and nerves
Nerves use different neurotransmitters to transduce their signals:
· Since the preganglionic nerve always excites the postganglionic nerve, the neurotransmitter used by all preganglionic nerves (to signal the postganglionic nerve) is acetylcholine (in both sympathetic and parasympathetic systems).
· Norepinephrine is the neurotransmitter used by sympathetic postganglionic nerves to stimulate their target tissue into action (or inaction).
· A few postganglionic neurons use acetylcholine, such as the nerves going to sweat glands to stimulate perspiration.
· The single neurotransmitter used in the parasympathetic nervous system is acetylcholine. Its effect on the body is the result of signal transduction from muscarinic acetylcholine receptors and their downstream targets.