2.1 Autonomic Nervous System
- Controls body activities automatically.
- It is composed of a specialized group of neurons that regulate cardiac muscle, smooth muscles and glands.
Figure 2- Anatomy of the Autonomic Nervous System
The autonomic nervous system, however, has a chain of two motor neurons.
The first motor neuron of each pair, the preganglionic neuron, is in the brain or spinal cord.
Its axon, the preganglionic axon, leaves the CNS to synapse with the second motor neuron in a ganglion outside the CNS.
The axon of this ganglionic neuron, the postganglionic axon (postganglionic neuron), then extends to the organ it serves
Both serve the same organs but cause essentially opposite effects, counterbalancing each other’s activities to keep body systems running smoothly. The sympathetic division mobilizes the body during extreme situations (such as fear, exercise, or rage), whereas the parasympathetic division allows us to “unwind” and conserve energy.
2.1.1 Anatomy of the Parasympathetic Division
- The parasympathetic division is also called the craniosacral division (the preganglionic neurons of the parasympathetic division are located in brain nuclei of several cranial nerves—III, VII, IX, and X (the vagus being the most important of these) and in the S2 through S4 levels of the spinal cord).
- The neurons of the cranial region send their axons out in cranial nerves to serve the head and neck organs.
- There they synapse with the ganglionic motor neuron in a terminal ganglion.
- From the terminal ganglion, the postganglionic axon extends a short distance to the organ it serves.
- In the sacral region, the preganglionic axons leave the spinal cord and form the pelvic splanchnic nerves, also called the pelvic nerves, which travel to the pelvic cavity.
- In the pelvic cavity, the preganglionic axons synapse with the second motor neurons in terminal ganglia on, or close to, the organs they serve.
- The parasympathetic division is most active when the body is at rest.
2.1.2 Anatomy of the Sympathetic Division
- The sympathetic division is also called the thoracolumbar division (because its preganglionic neurons are in the gray matter of the spinal cord from T1 through L2).
- The preganglionic fibers originate from neurons in the gray matter of the spinal cord.
- Their axons leave the cord through the ventral roots of spinal nerves in the first thoracic through the second lumbar segments.
- After traveling a short distance, these fi bers leave the spinal nerves, and each enters a member of a chain of sympathetic ganglia (paravertebral ganglia).
- One of these sympathetic chains extends longitudinally along each side of the vertebral column.
- Within paravertebral ganglia, preganglionic fi bers form synapses with second neurons.
- The axons of these neurons, the postganglionic fibers, typically return to spinal nerves and extend to visceral effectors.
Figure 3- Motor pathways. (a) Autonomic pathways include two neurons between the CNS and an effector. (b) Somatic pathways usually have a single neuron between the CNS and an effector. Note that in both cases the motor fi bers pass through the ventral root of the spinal cord.
|Explanation is available on
Anatomy of the Autonomic Nervous System
Table 1- Functions of Sympathetic & Parasympathetic Nervous System
|Target organ/system||Parasympathetic effects||Sympathetic effects|
|Digestive system||Increases smooth muscle mobility (peristalsis) and amount of secretion by digestive system glands; relaxes sphincters||Decreases activity of digestive system and constricts digestive system sphincters (for example, anal sphincter)|
|Liver||No effect||Causes glucose to be released to blood|
|Lungs||Constricts bronchioles||Dilates bronchioles|
|Urinary bladder||Relaxes sphincters (allows voiding)||Constricts sphincters (prevents voiding)|
|Kidneys||No effect||Decreases urine output|
|Heart||Decreases rate; slows and steadies||Increases rate and force of heartbeat|
|Blood vessels||No effect on most blood vessels||Constricts blood vessels in viscera and skin (dilates those in skeletal muscle and heart); increases blood pressure|
|Glands—salivary, lacrimal, gastric||Stimulates; increases production of saliva, tears, and gastric juice||Inhibits; result is dry mouth and dry eyes|
|Eye (iris)||Stimulates constrictor muscles; constricts pupils||Stimulates dilator muscles; dilates pupils|
|Eye (ciliary muscle)||Stimulates to increase bulging of lens for close vision||Inhibits; decreases bulging of lens; prepares for distant vision|
|Adrenal medulla||No effect||Stimulates medulla cells to secrete epinephrine and norepinephrine|
|Sweat glands of skin||No effect||Stimulates to produce perspiration|
|Penis||Causes erection due to vasodilation||Causes ejaculation (emission of semen)|
|Cellular metabolism||No effect||Increases metabolic rate; increases blood sugar levels; stimulates fat breakdown|
|Adipose tissue||No effect||Stimulates fat breakdown|
|Explanation is available on
Functions of the Autonomic Nervous System