Spinal Cord and Nerve Anatomy

Overview

This lecture covers the anatomy of the spinal cord and spinal nerves, including their structure, organization, naming, and important nerve plexuses.

Spinal Cord Anatomy

• The spinal cord begins at the foramen magnum where the brain ends.
• It ends at the level of the L1–L2 vertebrae, not extending to the coccyx.
• The cauda equina is a collection of spinal nerves below the spinal cord within the vertebral canal.
• The conus medullaris is the cone-shaped inferior end of the spinal cord.
• Cervical and lumbar enlargements correspond to regions supplying the upper and lower limbs, respectively.
• The filum terminale is a thin extension of pia mater anchoring the spinal cord to the coccyx.

Meninges and Spaces

• The spinal cord is surrounded by three meninges: dura mater (outer), arachnoid mater (middle), and pia mater (inner).
• The epidural space is found between the dura mater and vertebrae, filled with fat; it is absent in the cranium.
• Subarachnoid and subdural spaces also exist around the spinal cord.

Spinal Cord Structure and Function

• In spinal cord cross-section, white matter is peripheral and gray matter is central (butterfly shape).
• Spinal nerves have two roots: dorsal (posterior) root for sensory input (contains ganglion) and ventral (anterior) root for motor output.
• The spinal nerve as a whole is mixed, carrying both sensory and motor fibers.
• Neuron = single nerve cell; nerve = bundle of neurons.

Naming and Organization of Spinal Nerves

• Spinal nerves are named and numbered by region: 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), 1 coccygeal.
• Spinal nerves exit the vertebral canal through intervertebral or sacral foramina.

Nerve Plexuses

• Plexus = network of interweaving spinal nerves.
• Cervical plexus (C1–C4) contains the phrenic nerve (C3–C5), which innervates the diaphragm.
• Brachial plexus supplies the upper limb and gives rise to major nerves (axillary, radial, musculocutaneous, median, ulnar).
• No thoracic plexus; thoracic nerves become intercostal nerves.
• Lumbar plexus gives rise to the femoral nerve (front of thigh).
• Sacral plexus gives rise to the sciatic nerve (back of thigh and leg).

Key Terms & Definitions

• Foramen Magnum — opening at the base of the skull where spinal cord begins.
• Cauda Equina — bundle of spinal nerves below the spinal cord.
• Conus Medullaris — cone-shaped lower end of the spinal cord.
• Filum Terminale — pia mater extension anchoring spinal cord.
• Meninges — protective coverings of brain and spinal cord: dura, arachnoid, pia mater.
• Epidural Space — space above the dura mater in the spinal canal.
• Plexus — network of interwoven spinal nerves.
• Phrenic Nerve — nerve controlling the diaphragm.
• Ganglion — lump containing nerve cell bodies on the dorsal root.
• Intercostal Nerves — thoracic spinal nerves running between the ribs.

Action Items / Next Steps

• Review the structure and names of spinal nerve plexuses.
• Memorize key branches such as the phrenic, axillary, femoral, and sciatic nerves.