Peripheral Nervous System: Nerve Plexuses, Branches, and Skeletal Muscle Innervations

The Peripheral Nervous System

Overview of the Nervous System

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all neural structures outside the CNS, such as cranial and spinal nerves.

• Central Nervous System: Brain and spinal cord

• Peripheral Nervous System: 12 pairs of cranial nerves, 31 pairs of spinal nerves

Basic Structure of Peripheral Nerves

Peripheral nerves are composed of bundles of axons (nerve fibers) organized into fascicles and surrounded by connective tissue layers.

• Axon: The long, slender projection of a neuron that conducts electrical impulses

• Myelin sheath: Insulating layer formed by Schwann cells in the PNS, increasing conduction speed

• Endoneurium: Surrounds individual axons

• Perineurium: Encloses each fascicle

• Epineurium: Outermost layer, encasing the entire nerve

• Blood vessels: Supply nutrients to nerve tissue

Schwann Cells and Myelination

Schwann cells are responsible for myelinating axons in the PNS. Unmyelinated axons are still encased by Schwann cells, but only with a single cell membrane layer.

• Myelinated axons: Wrapped in multiple layers of Schwann cell membrane

• Unmyelinated axons: Surrounded by Schwann cell cytoplasm, but not wrapped in multiple layers

• Oligodendrocytes: Myelinate axons in the CNS (not PNS)

Spinal Nerves and Rami

Spinal Nerves

There are 31 pairs of spinal nerves, each arising from a specific segment of the spinal cord. Each spinal nerve is formed by the union of dorsal and ventral roots.

• Dorsal root: Contains sensory (afferent) fibers

• Ventral root: Contains motor (efferent) fibers

• Spinal nerve: Mixed nerve formed by the joining of dorsal and ventral roots

Rami

After exiting the vertebral column, each spinal nerve splits into dorsal and ventral rami.

• Dorsal rami: Supply the posterior head and back

• Ventral rami: Supply the trunk and limbs; give rise to major nerve plexuses

• Branching: Little branching between spinal segments from T1-T12

Dermatomes

Definition and Clinical Importance

A dermatome is the area of skin innervated by the sensory fibers of a single spinal nerve. Dermatomes are important for diagnosing nerve injuries and neurological disorders.

• Dermatome maps: Show the distribution of spinal nerves across the skin

• Clinical application: Loss of sensation in a dermatome can indicate damage to a specific spinal nerve

Nerve Plexuses and Major Peripheral Nerves

Nerve Plexuses

Nerve plexuses are networks of intersecting nerves formed by the ventral rami of spinal nerves. They provide motor and sensory innervation to the limbs and trunk.

• Cervical Plexus (C1–C5): Innervates neck muscles and diaphragm via the phrenic nerve (C3–C5)

• Brachial Plexus (C5–T1): Innervates pectoral girdle and upper limb

• Lumbar Plexus (T12–L4): Innervates pelvic girdle and lower limb

• Sacral Plexus (L4–S4): Innervates pelvic girdle and lower limb

Major Peripheral Nerves and Their Functions

• Phrenic nerve: Motor innervation to the diaphragm (breathing)

• Axillary nerve: Innervates deltoid and teres minor muscles

• Musculocutaneous nerve: Innervates anterior arm muscles (e.g., biceps brachii)

• Radial nerve: Innervates posterior arm and forearm muscles (e.g., triceps brachii, extensors)

• Median nerve: Innervates most anterior forearm muscles and some hand muscles

• Ulnar nerve: Innervates intrinsic hand muscles and some forearm muscles

• Obturator nerve: Innervates medial thigh muscles (adductors)

• Femoral nerve: Innervates anterior thigh muscles (e.g., quadriceps)

• Sciatic nerve: Largest nerve; innervates posterior thigh and all muscles below the knee via its branches

• Tibial nerve: Innervates posterior leg and plantar foot muscles

• Common fibular (peroneal) nerve: Innervates anterior and lateral leg muscles

Cranial Nerves

Overview

There are 12 pairs of cranial nerves, each with specific sensory, motor, or mixed functions. They emerge directly from the brain.

• Olfactory (CN I): Sensory for smell; composed of bipolar neurons

• Other cranial nerves: Include optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerves

Olfactory Nerve (CN I)

The olfactory nerve is a good example of a nerve composed of bipolar neurons. It transmits sensory information from the olfactory epithelium to the olfactory cortex.

• Olfactory bulb: Not to be confused with the olfactory nerve itself

• Pathway: Olfactory receptor neurons → olfactory nerve fibers → olfactory bulb → olfactory tract → olfactory cortex

Peripheral Nerve Innervation of Skeletal Muscles

Key Muscle-Nerve Relationships

Specific peripheral nerves innervate distinct skeletal muscles, enabling voluntary movement and reflexes.

Muscle	Peripheral Nerve Innervation
Biceps brachii	Musculocutaneous nerve
Flexor carpi ulnaris	Ulnar nerve
Flexor carpi radialis	Median nerve
Triceps brachii	Radial nerve
Deltoid	Axillary nerve
Adductor brevis	Obturator nerve
Psoas major	Femoral nerve
Vastus lateralis	Femoral nerve
Fibularis brevis	Common fibular nerve
Semitendinosus	Sciatic nerve (tibial portion)

Summary Table: Major Nerve Plexuses and Their Regions

Plexus	Spinal Segments	Regions Innervated	Major Nerves
Cervical	C1–C5	Neck, diaphragm	Phrenic
Brachial	C5–T1	Pectoral girdle, upper limb	Axillary, musculocutaneous, radial, median, ulnar
Lumbar	T12–L4	Pelvic girdle, lower limb	Femoral, obturator
Sacral	L4–S4	Pelvic girdle, lower limb	Sciatic, tibial, common fibular

Additional info:

• Understanding the anatomical course and function of peripheral nerves is essential for clinical diagnosis and treatment of nerve injuries.

• Dermatome maps are used in neurological examinations to localize spinal cord or nerve root lesions.

• Reflex arcs involve sensory receptors, afferent neurons, integration in the spinal cord, efferent neurons, and effector organs (muscles).