Anatomy 2- Dermatomes, Cutaneous Nn

Overview: Somatic Motor System

Axons migrate with the myotome they innervate

Intervertebral Disc Herniation

If the disc herniates out on the left, the patient will present with sidebending to the right in an attempt to open up the IV space on that side and take pressure off of the compressed nerve root. Radiculopathy= pain that runs down a dermatome...IV disc herniation of the nucleus pulposus can cause this A laterally herniated disk impinges on the nerve root below the herniation. Ex: L5-S1 disc herniation will effect the 1st sacral nerve root.

Dermatomes and Cutaneous nn

*Dermatomes* Clavicle: C4 Lateral epicondyle: C6 Thumb: C6 Middle finger: C7 Medial epicondyle: C8 Ring & Little finger: C8 *Cutaneous nn.* Clavicle: Supraclavicular nn. Digit 5: Palmar digital branches of ulnar n. *Autonomous regions: Distal phalanx of first 3 digits: Median n. Little finger: Ulnar n.

Specific Myotomes Are Associated with Moving the Lower Extremity

*Primary Spinal Segment(s)* Extend hip: L5 Flex hip: L2 ADduct hip: L3,4 ABduct hip: L5 Extend knee: L4 Flex knee: S1 Dorsiflex foot: L5 Plantarflex foot: S1 Invert foot: L4 Evert foot: L5 Toe movements: S2,3

Specific Myotomes Associated with moving the upper extremity

*Primary Spinal Segment(s)* Extend shoulder: C6-8 Flex Shoulder: C5 Lateral rotation: C5 Medial rotation: C6-8 Extend elbow: C7 Flex elbow: C6 Extend wrist: C6 Flex wrist: C7 Extend fingers: C7 Flex fingers: C8 AB- & ADduct fingers: T1 Note: As you move down the extremity, you get to progressively lower spinal cord segments.

Anterior Body Wall Nerve Innervation Pattern: Bony & Soft Tissue Landmarks Vs. Dermatomes

Anterior neck: C3 Clavicle & Jugular notch: C4 Nipple: T4 Umbilicus: T10 Iliac crest: T12 Inguinal ligament: L1

How do motor neuron axons reach their target molecule?

1. Each spinal cord segment sends an individual nerve (containing hundreds of axons) to the muscle, so that each muscle receives numerous nerves, or 2. The axons leave the spinal cord via their respective ventral ramus, but the ventral rami merge together to form a plexus. The plexus bundles axons from different spinal cord segments into large nerves. that course through the extremity. The result is fewer, larger nerves that will innervate multiple muscles.

Clinical Significance of Dermatomes

Knowing where the sensory loss occurs can aid in localizing the site of a spinal cord injury.

Where do the dorsal and ventral rami go?

Note that the Dorsal ramus will innervate the deep back muscles and posterior skin. It splits into the posterior cutaneous branches and innervates cutaneously about a hands width from the spinal cord. The ventral ramus is the intercostal nerve and will split to for the lateral cutaneous branches at midaxillary and then to form the anterior cutaneous branch near the sternum.

Sclerotome

Splits and migrates rostrally and caudally , forming an intervertebral foramen through which the spinal n exits. Spinal n then follows the respective dermamyotome

The Dermamyotome Splits Forming Epaxial and Hypaxial Muscles

The *dorsal rami will innervate the deep back mm.*, while the much larger* ventral rami will innervate the remaining, hypaxial skeletal mm.* (Except those innervated by cranial nn.)

Overview: Somatic Sensory System

The peripheral process of the pseudounipolar neuron will migrate with the dermamyotome.

Skeletal Muscles composed of more than one myotome

The ventral gray horn contains columns of motor neurons - they extend through more than one spinal cord segment These go to innervate a muscle made of more than one myotome For example, the deltoid m. is innervated by motor neurons in the C5 & C6 spinal cord segments, and develops from somites initially associated with the C5 & C6 regions of the developing spinal cord.

Clinical Testing of Dermatomes vs. Cutaneous nn in the hand

Therefore to assure a digit is innervated by only one dermatome you test: Thumb = C6 Middle finger = C7 Little finger = C8 Next slide is the cutaneous nn

Sensory & motor axons in *dorsal* ramus

To EPAXIAL skin and mm

Sensory & motor axons in *ventral* ramus

To HYPAXIAL skin and mm

Herpes Zoster

Viral infection of a DRG. Can follow a specific dermatome.

CAUTION: The sensory distribution/field of a dermatome ...

is not always the same as the sensory distribution/field of a cutaneous nerve. This is especially true for the extremities.