FHA - Nerve Plexuses

Nerve Plexus

A braided, interwoven network of ventral rami from spinal nerves

Rami which forms Plexuses

Ventral rami

Major Nerve Plexuses

Four: cervical, brachial, lumbar, sacral

Plexus

To braid

Dorsal Root Carries

Incoming sensory (afferent) information

Ventral Root Carries

Outgoing motor (efferent) information

Spinal Nerve

The merging of dorsal and ventral roots, splits into dorsal ramus and ventral ramus

Ramus Participating in plexuses

Ventral ramus

Ventral Ramus Carries

Both sensory and motor fibers

Roots Forming Cervical Plexus

C1–C5, It contributes fibers to cranial nerves XI (accessory) and XII (hypoglossal)

Roots Forming Brachial Plexus

C5–T1

Roots Forming Lumbar Plexus

T12–L4

Roots Forming Sacral Plexus

L4–S4

Lesser Occipital Nerve Arises From

C2, sensory only

Area Lesser Occipital Nerve Supplies

Skin of the neck and scalp posterior/superior to the ear

Roots forming the Ansa Cervicalis

C1–C3 (with some C4), motor only

Muscle Ansa Cervicalis Innervates

Geniohyoid, thyrohyoid, omohyoid, sternohyoid, sternothyroid

Robbie T Drinks Cold Nectar

Pneumonic for Brachial Plexus Organization

Roots → Trunks → Divisions → Cords → Nerves

Roots Forming Superior Trunk

C5–C6

Roots Forming Middle Trunk

C7

Roots Forming Inferior trunk

C8–T1

Each Trunk Divides Into

An anterior and a posterior division

Forms Posterior Cord

All three posterior divisions

Forms Medial Cord

The anterior division of the inferior trunk

Forms Lateral Cord

The anterior divisions of the superior and middle trunks

Roots Contributing to the Axillary Nerve

C5–C6

Axillary Nerve Innervates

Deltoid and teres minor (motor); skin of shoulder (sensory)

Roots Contributing the the Radial nerve

C5–T1, receives all roots because it is the continuation of the posterior cord, mixed sensory and motor

Roots Contributing to the Musculocutaneous Nerve

C5–C7

Roots Contributing to the Median Nerve

C6–T1

Roots Contributing the the Ulnar Nerve

C8–T1

Thoracic Outlet Syndrome

Compression of the brachial plexus beneath the clavicle, can cause neuropathy, weakness, paresthesias, pain, tingling

Neuropathy

A pathology of a nerve causing weakness or sensory changes

Paresthesias

Abnormal sensations like numbness, tingling, or burning

Erb’s Palsy

Injury to C5–C6 roots, often during childbirth

Radial Nerve Palsy

Compression of the radial nerve, often from improper crutch use, associated with wrist drop

Roots Forming Lumbar Plexus

T12–L4

Major Nerves of Lumbar Plexus

Iliohypogastric, ilioinguinal, lateral femoral cutaneous, genitofemoral, femoral, obturator, motor and sensory mixed

Roots Forming Sacral Plexus

L4–S4

Major Nerves of Sacral Plexus

Superior gluteal, inferior gluteal, sciatic, posterior femoral cutaneous, pudendal

Largest Nerve in Body

Sciatic nerve

Sciatic Nerve Splits

In the popliteal region into tibial nerve and common fibular (peroneal) nerve

Sciatic Nerve Innervates

Entire leg, ankle, and foot (via its branches)

Muscle Commonly Compressing Sciatic Nerve

Piriformis

Sciatica

Pain, weakness, or paresthesias caused by sciatic nerve compression, the sciatic nerve passes directly beneath or through the piriformis

Dermatome

An area of skin supplied by a single spinal nerve root, help identify which spinal root is affected in sensory disturbances

Reflex

An immediate, involuntary motor response to a stimulus

Reflex Arc

The neural wiring of a reflex from receptor → CNS → effector

First Step of Reflex Arc

Activation of a sensory receptor

Root Sensory Information Enters the Spinal Cord, Root Stretch Reflex Sensory Signal Enters

Dorsal root

Neurons Relays the Signal Within the Spinal Cord

An interneuron

Root Motor Output Leaves the Signal Cord

Ventral root

Effector in Somatic Reflex

Skeletal muscle

Withdrawal Reflexes

Involuntary reflex, brain only becomes aware after reflex occurs

Activation of Stretch Reflex

Muscle spindle fibers detecting stretch, does not require interneuron

Common Clinical Stretch Reflex

The patellar (knee‑jerk) reflex

Patellar Ligament Tapped

Quadriceps muscle stretches, activating spindle fibers

Effector in Patellar Reflex

Quadriceps muscle

Response of Patellar Reflex

Knee extension