Somatic Innervation #3: Peripheral Nerve Innervation

Vocabulary flashcards covering major terms and definitions related to spinal nerves, rami, plexuses, and patterns of innervation.

Spinal Nerve

One of 31 paired mixed nerves emerging from the spinal cord that immediately splits into dorsal and ventral rami.

Dorsal (Posterior) Ramus

Segmental branch of a spinal nerve that innervates intrinsic back muscles and the overlying skin.

Ventral (Anterior) Ramus

Branch of a spinal nerve that supplies the anterolateral trunk and limbs; forms intercostal nerves in the thorax and plexuses elsewhere.

Intercostal Nerve

Segmental ventral ramus of T1–T12 running in the costal groove, giving anterior and lateral cutaneous branches and motor branches to intercostal muscles.

Peripheral Nerve

Any branch of a dorsal ramus, ventral ramus, or nerve plexus that carries somatic sensory and motor axons to distal targets.

Nerve Plexus

Interlacing network of ventral rami (cervical, brachial, lumbar, sacral) that redistributes axons into peripheral nerves.

Cervical Plexus (C1–C4)

Network of ventral rami C1-C4 deep to sternocleidomastoid; provides phrenic nerve, sensory supply to head/neck skin, and motor supply to deep anterior neck muscles.

Phrenic Nerve

Major branch of the cervical plexus (primarily C3-C5) that supplies sensory and motor innervation to the diaphragm.

Brachial Plexus (C5–T1)

Ventral rami C5-T1 passing between anterior and middle scalene muscles and under the clavicle to the axilla; innervates the upper limb.

Lumbar Plexus (L1–L4)

Ventral rami L1-L4; gives motor supply to portions of the abdominal wall and anterior/medial thigh, and sensory supply to the anterior and medial thigh and leg.

Sacral Plexus (L4–S4)

Ventral rami L4-S4; supplies sensory innervation to posterior thigh, leg, and foot, and motor innervation to posterior thigh and all muscles of the leg and foot; S4–S5 also supply the perineum.

Dermatome

Strip of skin supplied by the sensory fibers of a single spinal nerve level.

Myotome

Group of skeletal muscles innervated by motor fibers from a single spinal nerve level.

Segmental Innervation

Pattern in which a spinal nerve or its ramus supplies structures directly aligned with its segment, without mixing (e.g., dorsal rami, thoracic ventral rami).

Costoclavicular Space

Region beneath the medial two-thirds of the clavicle through which the brachial plexus travels toward the axilla.

Intrinsic Back Muscles

Deep muscles such as erector spinae and transversospinales that are innervated by dorsal rami.

Axilla

Armpit region where the cords and branches of the brachial plexus distribute to the upper limb.

Peripheral vs. Spinal Nerve Lesion

Spinal nerve damage affects only one segment, producing dermatomal/myotomal deficits; peripheral nerve damage affects all axons within that nerve, causing deficits in its specific distal territory.