Peripheral Nervous System: Structure, Function, and Lab Objectives

Peripheral Nervous System (PNS): Motor Division

Cranial Nerves: Identification, Location, and Function

The cranial nerves are twelve pairs of nerves that emerge directly from the brain, primarily the brainstem. Each cranial nerve has a specific location and function, ranging from sensory to motor or mixed roles.

• Olfactory (I): Located in the nasal cavity; responsible for sense of smell.

• Optic (II): Located in the retina; responsible for vision.

• Oculomotor (III): Originates from the midbrain; controls most eye movements, pupil constriction.

• Facial (VII): Originates from the pons; motor for facial expression, sensory for taste (anterior 2/3 of tongue).

• Vestibulocochlear (VIII): Originates from the inner ear; responsible for hearing and balance.

• Vagus (X): Originates from the medulla; motor and sensory for thoracic and abdominal organs.

Example: The facial nerve (VII) is responsible for both facial expression and taste sensation from the anterior tongue.

Spinal Nerves: Origin and Branches

Spinal nerves arise from the spinal cord and are formed by the union of two roots:

• Dorsal root: Contains sensory (afferent) fibers; cell bodies are located in the dorsal root ganglion.

• Ventral root: Contains motor (efferent) fibers; cell bodies are located in the ventral horn of the spinal cord.

After exiting the vertebral column, each spinal nerve splits into branches called rami:

• Dorsal ramus: Serves the muscles and skin of the back.

• Ventral ramus: Serves the limbs and anterior trunk; often forms nerve plexuses.

Example: The ventral ramus of the cervical spinal nerves forms the cervical plexus.

Spinal Nerves: Regional Identification and Numbering

Spinal nerves are classified by the region of the vertebral column from which they emerge:

• Cervical: 8 pairs (C1–C8)

• Thoracic: 12 pairs (T1–T12)

• Lumbar: 5 pairs (L1–L5)

• Sacral: 5 pairs (S1–S5)

• Coccygeal: 1 pair (Co1)

Example: The lumbar region contains five pairs of spinal nerves, labeled L1–L5.

Spinal Nerve Plexuses: Formation and Major Nerves

Nerve plexuses are networks formed by the ventral rami of spinal nerves. Each plexus gives rise to major nerves that serve specific regions of the body.

• Cervical Plexus: Formed by C1–C4; major nerve: Phrenic nerve (controls diaphragm).

• Brachial Plexus: Formed by C5–T1; major nerves: Radial, Median, Ulnar nerves (serve upper limb).

• Lumbar Plexus: Formed by L1–L4; major nerve: Femoral nerve (serves anterior thigh).

• Sacral Plexus: Formed by L4–S4; major nerve: Sciatic nerve (serves posterior thigh and leg).