Concepts on Peripheral Innervation
Neurons are the structural and functional units of the entire nervous system. Their fiber processes (axons) form cranial and spinal nerves.
Each paired somite is innervated bilaterally by a spinal nerve. When somites appear to "migrate" through the embryonic process of differential growth, they take their nerve supply with them. This concept also applies to the component parts of the somite, i.e., sclerotome, myotome, and dermatome.
Spinal nerves are redundant. There is motor and sensory overlap. One must destroy or anesthetize two or more spinal nerves to cause a complete motor/sensory deficit, i.e., paralysis/anesthesia.
For motor (efferent) nerves the overlap is within the named skeletal muscles, i.e., two or more myotomes combine to form each skeletal muscle. Therefore, two or more spinal nerves innervate each named muscle.
For sensory (afferent) nerves the overlap is in the spinal cord. Afferent input is not only to the contiguous spinal cord segment, but it radiates in the cord to the two spinal cord segments above and to the two segments below, e.g., the C6 somatic afferent input (GSA) is distributed not only to the C6 spinal cord segment, but to a total of five segments of the spinal cord, i.e., from C4 through C8.
The cranial nerves are not redundant. There is no motor and sensory overlap. Destroying or anesthetizing a cranial nerve or its branches will provoke a complete motor/sensory deficit, i.e., paralysis/anesthesia.
Sensory modalities are perceived or "felt" within the brain and not within the receptor fields located within the body wall or cavities.
The more proximal an impairment is to the CNS, i.e., the brain and spinal cord, the more global the clinical symptoms and signs are within the body wall and cavities.
Sensory and motor deficits are always realized at and distal to the site of impairment, i.e., at the site of impairment and away from it.
The neurovascular bundles containing spinal nerves are located on the flexor side of joints with one exception, e.g., the ulnar nerve.
Muscular and cutaneous nerve branches arise proximal to the site they innervate, i.e., proximal to the site of joint movement or cutaneous innervation.
For somatic plexuses (e.g., brachial at C5 - T1), the more proximal ventral rami, (e.g., C5) supply the more proximal muscles (e.g., shoulder joint and elbow), while more distal ventral rami (e.g., C8 and T1) innervate the more distal muscles (e.g., joints of the wrist and hand).
Somatic nerve plexuses (e.g., brachial at C5 - T1) also are redundant, i.e., each named peripheral nerve (e.g., radial nerve) and its muscular and cutaneous branches contains two or more ventral rami. Moreover, each ventral ramus (e.g., C5) is distributed by many of the named peripheral nerves (e.g., radial, axillary, musculocutaneous, suprascapular, long thoracic, median, etc.).
Somatic nerve plexuses (e.g., cervical) also supply body wall-derived structures located within body cavities, i.e., the phrenic nerves innervate the pericardium and diaphragm. Innervation includes their coverings, i.e., parietal pericardium, pleura, and peritoneum. Irritation provokes 'referred' shoulder pain and hiccups.
Visceral nerve plexuses supply organs and their coverings, i.e., visceral pericardium, pleura, and peritoneum, with bilateral segmental innervation. Innervation is redundant as destruction of two or more nerves is required to cause neurologic deficit. Irritation provokes 'referred' pain (organ stretching or ischemia) and 'reflex' autonomic symptoms and signs, e.g., tachycardia or bradycardia, hypertension or hypotension, syncope, coughing, nausea, vomiting, etc. Two exceptions are lungs and suprarenal (adrenal) glands that are silent, i.e., they have no pain innervation.
The nerves of the visceral plexuses are named for the organs they innervate, e.g., cardiac nerves supply the cardiac nerve plexus.
Visceral nerve plexuses are 'mixed' nerves formed by autonomic efferents and visceral afferents. 'Wiring' is the 'same' for head and neck, thoracic, or abdominopelvic organs making localization and diagnosis of symptoms and signs difficult.
Cranial nerve III, VII, and IX parasympathetics are distributed by 'hitch-hiking' in trigeminal (V) cranial nerves, while sympathetics are distributed by spinal nerve rami and by 'hitch-hiking' in trigeminal (V) cranial nerves and on systemic blood vessels.
With two exceptions, visceral pain afferents co-localize within sympathetic nerves, while visceral enteroceptive (pH, pO2, stretch, irritant) afferents co-localized within parasympathetic nerves, e.g., facial (VII), glossopharyngeal (IX), vagus (X), and pelvic splanchnic nerves (the latter two nerves are exceptions as both interoception and pain are transmitted in the pharynx and pelvis, respectively).
The 'default' for referred visceral or somatic pain is 'cutaneous', i.e., to dermatomes along the trunk, limbs, head, and neck.
Referred pain is either visceral-somatic, i.e., projected from organs to dermatomes, or somatic-somatic, i.e., projected from somatic wall-derived structures, e.g., the diaphragm and pericardium, to dermatomes.
In the brain, referred pain is not 'felt' identically in everyone. Sensations can range from nothing at all (asymptomatic) to pressure or to an ache or intense pain that is defuse in its somatic (dermatome) distribution along the body wall.
Referred visceral-somatic pain can become purely somatic pain when diseased organs or tissues infect or compress the internal surface of the body wall, e.g., late-stage appendicitis.
Concepts apply to all parts of the human body wall, i.e., the upper and lower limbs, trunk, head, neck, and body cavities.