Short & Sweet: Some notes to try to clear up fuzziness about Nerves, Ganglia, and Nuclei, and what the PNS is doing:
Peripheral Nervous System
Nerves are bundles of neurons; either long dendrites and/or long axons.
There are no cell bodies in nerves. The cell bodies are in the ganglia (PNS) or nuclei (in gray matter of the CNS).
Most nerves contain both kinds of neurons (sensory and motor). The sensory neurons conduct information to the CNS (“afferent”), the motor neurons conduct away from the CNS (“efferent” -- “E” for “Exit”).
All of the neurons in some nerves conduct in the same direction. These nerves contain either sensory or motor neurons.
Cranial Nerves and Spinal Nerves
Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.
Cranial nerves are sensory, motor, or mixed, and all but the vagus are involved with the head and neck region; the vagus nerve manages the internal organs.
Spinal nerves are all mixed nerves. Their regular arrangement reflects the segmentation of the human body.
Spinal nerves are connected to the spinal cord by two branches called roots.
The dorsal root contains sensory neurons. The dorsal root ganglion contains the cell bodies of sensory neurons. Sensory neurons therefore have long dendrites.
The ventral root contains motor neurons. Motor neurons have short dendrites and long axons.
Somatic Nervous System
The somatic nervous system provides conscious, voluntary control.
It includes all of the nerves that serve the skeletal muscles and the exterior sense organs.
Reflexes are simple, stereotyped and repeatable motor actions (example: movements) brought about by a specific sensory stimulus. The reflex is involuntary but may involve the use of voluntary (skeletal) muscle and nerves.
Reflexes are quick and produce behaviors that are typically beneficial. For example, when you fall, reflex arcs immediately act to extend your arm so that your arm prevents your head and body from hitting the ground.
Some reflexes involve the brain, others do not.
A whole series of responses may occur since some sensory neurons stimulate several interneurons which, in turn send impulses to other parts of the CNS. If you were to fall forward, interneurons would use information from the ears to determine the direction of the fall and extend the arms in a forward direction. If you were to fall toward the left side, interneurons would select neurons that activate muscles to extend your arm to the left side.
Example: The stretch reflex
The stretch reflex is involved in helping the body maintain its position without having to consciously think about it.
When a muscle is stretched, stretch-sensitive receptors are stimulated. An action potential is conducted to the spinal cord. The axon terminals synapse with motor neurons leading right back to the muscles. This causes the muscle to contract to its original position.
Autonomic Nervous System
This part of the nervous system sends signals to the heart, smooth muscle, glands, and all internal organs.
It is generally without conscious control.
The autonomic nervous system uses two or more motor neurons:
The cell body of one of the motor neurons is in the CNS. The cell body of the other one is in a ganglion.
The sympathetic nervous system prepares the body to deal with emergency situations. This is often called the "fight or flight" response.
Stimulation from sympathetic nerves dilates the pupils, accelerates the heartbeat, increases the breathing rate, and inhibits the digestive tract.
The neurotransmitter is norepinephrine (similar to epinephrine [adrenaline], the heart stimulant).
Sympathetic nerves arise from the middle (thoracic-lumbar) portion of the spinal cord.
When there is little stress, the parasympathetic system tends to slow down the overall activity of the body.
It causes the pupils to contract, it promotes digestion, and it slows the rate of heartbeat.
The neurotransmitter is acetylcholine.
The actual rate of stimulus to each organ is determined by the sum of opposing signals from the sympathetic and parasympathetic systems.
Parasympathetic nerves arise from the brain and sacral (near the legs) portion of the cord.