The PNS is divided into two primary functional branches:

Somatic Nervous System
Controls voluntary movements by carrying motor commands to skeletal muscles and transmitting sensory information from the skin and joints to the CNS.





The PNS is further organized into nerve plexuses. A nerve plexus is a complex, branching network of intersecting nerves in the peripheral nervous system, acting as a "junction box" that sorts and recombines nerve fibers from the spinal cord to control specific body areas. Key examples include the brachial, cervical, lumbar, and sacral plexuses, which are essential for movement and sensation.

Key Nerve Plexuses

Brachial Plexus
Located in the neck/shoulder, this network (C5-T1) controls movement and sensation in the shoulders, arms, and hands.

Cervical Plexus
Supplies nerves to the head, neck, and shoulders, including the phrenic nerve for the diaphragm.

Lumbar & Sacral Plexuses
These interconnected networks serve the back, abdomen, and lower extremities.









The PNS includes the cranial nerves that originate from the brain and brainstem, controlling sensory and motor functions for the head, neck, and organs, as well balance and equilibrium.

The cranial nerves include:





The olfactory nerve (I): This is instrumental for the sense of smell, it is one of the few nerves that are capable of regeneration.

The optic nerve (II): This nerve carries visual information from the retina of the eye to the brain.


The oculomotor nerve (III): This controls most of the eye’s movements, the constriction of the pupil, and maintains an open eyelid.


The trochlear nerve (IV): A motor nerve that innervates the superior oblique muscle of the eye, which controls rotational movement.


The trigeminal nerve (V): This is responsible for sensation and motor function in the face and mouth.


The abducens nerve (VI): A motor nerve that innervates the lateral rectus muscle of the eye, which controls lateral movement.


The facial nerve (VII): This controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity.


The vestibulocochlear nerve (VIII): This is responsible for transmitting sound and equilibrium (balance) information from the inner ear to the brain.


The glossopharyngeal nerve (IX): This nerve receives sensory information from the tonsils, the pharynx, the middle ear, and the rest of the tongue.

The vagus nerve (X): This is responsible for many tasks, including heart rate, gastrointestinal peristalsis, sweating, and muscle movements in the mouth, including speech and keeping the larynx open for breathing.


The spinal accessory (XI): This nerve controls specific muscles of the shoulder and neck.


The hypoglossal nerve (XII): This nerve controls the tongue movements of speech, food manipulation, and swallowing.











The primary cause, occurring when the baby's shoulder becomes trapped behind the mother’s pubic bone during birth.

Stretching the neck during delivery (e.g., in breech births or when using forceps/vacuum).


Risk Factors

High birth weight (fetal macrosomia)

Maternal diabetes

Prolonged labor.


Symptoms


Selective Paralysis


"Waiter's Tip" Position: shoulder adduction, and internal rotation, elbow extension, wrist flexion. straight against the body, elbow extended, and forearm pronation.

Weak grip strength


Decreased Moro/Startle reflex on the affected side.

Diminished proximal extremity


Management


With early intervention, often within four weeks of birth, the majority of infants experience significant recovery or complete resolution of symptoms within the first year.


If not treated, it can lead to permanent muscle weakness, shortened limbs, and joint contractures.

Physical Therapy for maintaining joint range of motion and strengthening muscles.

In cases where nerves are torn rather than just stretched, nerve graft surgeries may be required, ideally performed before 6–9 months of age.