Variants and anomalies of cranial and spinal nerves

The structure, branching paths, character and branching zones of cranial and spinal nerves are highly variable. The interweaving of nerve trunks, bundles during the formation of plexuses, the places of branching from cranial and spinal nerves, from plexuses are variable. Muscle innervation and the branching zones of cutaneous nerves are also individually variable. Both cranial and spinal nerves are traced by the location and extent of the connection of adjacent nerves with each other, and the exchange of bundles of nerve fibers is diverse. Examples of nerve variants and anomalies are given below.

The zones of sensory innervation of the cranial nerves vary significantly.

The greater occipital nerve sometimes gives off an auricular branch to the skin of the auricle, as well as a communicating branch with the lesser occipital nerve. This nerve may innervate the occipital belly of the occipitofrontalis muscle.

The lesser occipital nerve may be absent or duplicated, replacing the absent greater occipital nerve.

There may be additional phrenic nerves, originating from the anterior branch of the third cervical spinal nerve, from the brachial plexus, or from the subclavian nerve (most often). The phrenic nerve in 38% of cases originates from the fourth cervical spinal nerve, in 16% from the fourth and fifth, in 22% from the third to fifth, and in 19% from the third and fourth cervical spinal nerves.

There are two extreme forms of the structure of the brachial plexus. The first is characterized by a wider arrangement of branches and a large angle of their convergence. A relatively narrow and short brachial plexus is typical for people with a narrow and long neck. The second form is characteristic of people with a short and wide neck: a close arrangement of the nerve branches of the plexus, which connect at an acute angle to each other. The plexus itself is relatively wide and long.

The suprascapular nerve may innervate the middle or posterior scalene muscle. The medial cutaneous nerve of the forearm sometimes gives off sensory branches to the elbow joint. The musculocutaneous nerve is rarely absent, being replaced by branches of the median nerve. Often, the musculocutaneous nerve gives off branches to the elbow joint. The axillary nerve may be located in the thickness of the subscapularis muscle, innervating it and the long head of the triceps brachii muscle.

The median nerve often originates from the cervical spinal nerves.

The ulnar nerve is often formed from the anterior branches of the V-VIII spinal nerves.

The radial nerve is often formed by fibers of the anterior branches of the lower cervical spinal nerves. In almost 50% of cases, the anatomical border of the innervation area of the back of the hand does not correspond to the middle of the third finger, but shifts to one side.

The location of the lumbosacral plexus, its shape and size are variable. The ilioinguinal nerve may be absent. The femoral and genital branches of the genitofemoral nerve may originate directly from the lumbar plexus. The anterior, middle and medial cutaneous nerves of the thigh sometimes originate from the middle part of the lumbar plexus. The lateral cutaneous nerve of the thigh passes together with the femoral nerve under the inguinal ligament in 6% of cases. In 10% of cases, there is an additional obturator nerve passing near the medial edge of the psoas major muscle.

There are two extreme forms of division of the femoral nerve:

1. the nerve divides into a few but large branches;
2. The nerve gives off a significant number of long and thin branches.

The femoral nerve may give off terminal branches above the level of the inguinal ligament.

The sciatic nerve sometimes pierces the piriformis muscle, often dividing into the tibial and common peroneal nerves already in the pelvic cavity or in the region of the greater sciatic foramen. The number and directions of the branches of the common peroneal nerve are variable. Sometimes the intermediate dorsal nerve of the foot ends on the dorsum of the foot, not reaching the toes. The medial plantar nerve, instead of the lateral plantar nerve, can give off branches to the short muscle that flexes the toes.

Age-related features of the peripheral nervous system

After birth, the number of nerve bundles in the peripheral nerves increases: their branching becomes more complex, inter-nerve connections expand, and receptor apparatuses become more complex. With age, the thickness of nerve fibers increases. In old and senile age, the number of neurons in the spinal ganglia decreases by 30%, and some neurons atrophy.