Spinal Nerves, Dermatomes, and Myotomes

Background (development)

Spinal Nerves (beginnings) present

Eventually expand to innervate distinct segments of the body

Orderly innervation of the body (soma) gives rise to dermatomes and myotomes

Background

• Dorsal root- Sensory, afferent

• Ventral Root- Motor, efferent

• Spinal Nerve- Motor and sensory

• Dorsal Primary Ramus (DPR)- Muscles of deep back, posterior head and neck

• Ventral Primary Ramus (VPR)- Anterior trunk and limbs

Typical Spinal Nerve

Dorsal Root Ganglion

Sympathetic Chain Ganglion

Somatic

• General Somatic Efferent (GSE)

  • Innervates skeletal (voluntary) muscle (movement)

• General Somatic Afferent (GSA)

  • Sensory information from the body (hot/cold)

Autonomic

• General Visceral Efferent (GVE) HUNGER

  • Innervates smooth (non-voluntary) muscle, glands, organs

• General Visceral Afferent (GVA) SUPER FULL

  • Sensory information that is conscious and unconscious

Typical Spinal Nerves

• All carry some form of motor and sensory information

• Innervate nearly all the body wall, extremities, and most of the muscles (Except cranial nerves)

• Nearly all motor commands to muscles

• Nearly all sensory information to CNS

Dermatomes

• A band or region of skin supplied (innervated) by a single sensory nerve

• Can be described in two ways

  • With reference to a dorsal nerve root (ex. C4 dorsal root)

  • With reference to a given cutaneous nerve (ex. Lateral Interracial Cutaneous nerve)

• Vital to PT sensory testing

Dermatome: Dorsal Roots

• Body sensation is attributed to/ described by the dorsal nerve root responsible for a given area

• Useful when injury to dorsal root or spinal cord is suspected

Dermatome: Cutaneous Nerve

Myotomes

A group of muscles receiving innervations from a single spinal cord segment or spinal nerve

Often done as part of PT screening testing

Motor Neuron Pool

• Cluster of motor neurons in longitudinal columns

• Correlate to muscle spinal level innervation (C5-C7)

• Some muscles most associated with 1 spinal level

Upper Extremity Myotomes

C1/2: Cervical Flexion

Upper Extremity Myotomes

C3: cervical Sidebending

Upper Extremity Myotomes

C4: Scapular Elevation (Shrug)

Upper Extremity Myotomes

C5: Shoulder Abduction

Upper Extremity Myotomes

C6: Elbow Flexion (or Wrist Extension)

Upper Extremity Myotomes

C7: Elbow Extension (or Wrist Flexion)

Upper Extremity Myotomes

C8: Thumb Extension (or Finger Flexion)

Upper Extremity Myotomes

T1: 5th Finger Abduction

Lower Extremity Myotomes

L2: Hip Flexion (or Adduction)

Lower Extremity Myotomes

L3: Knee Extension

Lower Extremity Myotomes

L4: Ankle Dorsiflexion

Lower Extremity Myotomes

L5: Great Toe MTP Extension

Lower Extremity Myotomes

S1: Eversion ( or Plantarflexion)

Lower Extremity Myotomes

S2: Plantarflexion (or Knee Flexion)