General visceral afferent, general somatic afferent, general visceral efferent, general somatic efferent, special visceral afferent, special somatic afferent, special visceral efferent and special somatic efferent. These confusing terms that you often encounter in Neuroanatomy, Neurophysiology, and Neuroembryology are explained conceptually using Dr. Najeeb style diagrams in this visual lecture by Dr. Aizaz from MedicoVisual.
The nervous system can be divided into Central Nervous System that includes brain and spinal cord and the peripheral nervous system that takes information from the CNS, or towards the CNS.
Afferent vs. Efferent:
A peripheral nerve fiber that takes the information towards the CNS (from the sensory receptors), is called Afferent nerve fiber. While, if a peripheral nerve fiber takes information away from CNS, towards the effector organs (so that some action may be taken as per the orders of CNS), such peripheral nerve fiber is termed as efferent nerve.
Naturally, the afferent nerve fibers are sensory (they sense a change, for example, temperature changes in skin). while the efferent are motor (the bring about an action, such as moving your hand).
Note that a peripheral nerve may contain multiple types of fibers, including afferent or efferent.
Visceral vs. Somatic:
Visceral nerve fibers, as the name indicates, carry information to or from the internal viscera (internal organs of the body).
Visceral organs include GI tube, Respiratory system, Urogenital System. These fibers are supplied to internal mucosa of these organs and can sense distension of these organs. For example, if your stomach is full and mucosa is distended, it will be sensed by visceral fibers. In other words, visceral fibers are concerned with sensing or manipulating the changes in internal chemical environment of the body .
Besides mucosa of these organs, they also supply the smooth muscles of these organs, and the blood vessels of these organs. Since Heart is also a visceral organ, they also supply the cardiac muscles.
Visceral nerve fibers mainly supply the nonskeletal (nonstriated) muscles but do also supply the skeletal muscles that are NOT derived from the somites. While the skeletal muscles that are derived from the somites are supplied by somatic nerve fibers.
Embryologically speaking, most of the visceral organs (but not all), arise from the endoderm plastered with the splanchnopleuric mesoderm. That’s why the visceral nerve fibers are also termed as splanchnic nerve fibers.
The walls of the body typically constitute the “soma”. So somatic nerve fibers typically supply the body wall, that consists of skin, muscles that are derived from the somites, and structures that are not supplied by the visceral nerve fibers. Again, other than a few exceptions, most of the somatic fibers supply the tissues that arise from the embryonic somatopleura (Ectoderm + Somatopleuric mesoderm).
So, in summary we can say that:
Visceral fibers are concerned mainly with internal visceral organs of the body while somatic fibers are concerned with the body wall (skin, muscles and the limbs that arise from the body wall)
Or we can say that visceral fibers are concerned with changes in internal chemical environment of the body and somatic fibers are concerned with sensing the external environment.
For example, If you externally touch my skin, touch receptors on my skin will be stimulated and sensation generated externally will be carried via Somatic Afferent Fibers. Opposite to that, my stomach becomes distended due to any reason, this change in internal chemistry of my body will be taken by visceral afferent fibers.
Other than a few exceptions, most of the visceral fibers supply the tissues that arise from embryonic splanchnopleure, while most of the somatic fibers supply the tissues that arise from embryonic somatopleure (Ectoderm + Somatopleuric mesoderm).
General vs. Special nerve fibers:
The nerve fibers that carry sensation or motor message, that are found all over the body and we can literally say there is nothing special about such sensations or motor messages, are termed as general nerve fibers. For example, touch or pain sensation that are present almost everywhere in our body are carried by general afferent fibers. Similarly a message to move any of your skeletal muscle like biceps of your arm are carried by general efferent nerve fibers.
On the other hand, fibers that carry sensations or motor message to some organs, that are exclusive to certain specific places in our body and are unique in many ways, are termed as “special” nerve fibers. For example, Visual sensation from your eyes being carried by the optic nerve fibers, is a classical example of “special” afferent fibers. Similarly, skeletal muscles are typically derived from the somites, but some skeletal muscles are unique in a sense that they are derived from branchial arches, hence their motor fibers are special efferent fibers and are not “general ”.