Chapter 13 Objectives

Peripheral Nervous System: Spinal Cord and Spinal Nerves: Chapter 13

Spinal Cord Anatomy

The spinal cord is a vital component of the central nervous system, serving as the main pathway for transmitting information between the brain and the rest of the body. It is protected by several anatomical structures and is organized into distinct regions both microscopically and macroscopically.

Microscopic Structures of the Spinal Cord

• Gray Matter: The central, butterfly-shaped region of the spinal cord, primarily composed of neuron cell bodies, dendrites, and unmyelinated axons.

  • Posterior (Dorsal) Horn: Contains interneurons and sensory neuron axons.

  • Anterior (Ventral) Horn: Contains cell bodies of somatic motor neurons.

  • Gray Commissure: A narrow band connecting the two sides of gray matter.

  • Central Canal: A small channel in the center of the gray commissure, filled with cerebrospinal fluid (CSF).

• White Matter: Surrounds the gray matter and consists mainly of myelinated axons, organized into columns (funiculi):

  • Posterior (Dorsal) White Column (Funiculus): Carries sensory information to the brain.

  • Anterior (Ventral) White Column (Funiculus): Contains both ascending sensory and descending motor tracts.

  • Lateral White Column (Funiculus): Contains both sensory and motor tracts.

• Posterior Median Sulcus: A shallow groove on the dorsal side of the spinal cord.

• Anterior Median Fissure: A deeper groove on the ventral side of the spinal cord.

Spinal Nerves and Roots

• Spinal Nerves: Mixed nerves that carry both sensory and motor information. Each is formed by the union of:

  • Posterior (Dorsal) Root: Contains sensory fibers entering the spinal cord.

  • Posterior (Dorsal) Root Ganglion: Contains cell bodies of sensory neurons.

  • Anterior (Ventral) Root: Contains motor fibers exiting the spinal cord.

• Rami: After exiting the vertebral column, each spinal nerve splits into:

  • Anterior Ramus: Supplies the limbs and anterior/lateral trunk.

  • Posterior Ramus: Supplies the muscles and skin of the back.

Protective Structures of the Spinal Cord

The spinal cord is protected by several layers and spaces, which provide physical and chemical protection.

• Vertebrae: The bony vertebral column encases the spinal cord.

• Epidural Space: Space between the vertebrae and dura mater, containing fat and blood vessels.

• Dura Mater: The tough, outermost meningeal layer.

• Subdural Space: A thin space between the dura mater and arachnoid mater.

• Arachnoid Mater: The middle, web-like meningeal layer.

• Subarachnoid Space: Located between the arachnoid and pia mater; filled with cerebrospinal fluid (CSF).

• Pia Mater: The delicate, innermost meningeal layer, adhering closely to the spinal cord.

External Features of the Spinal Cord

The spinal cord exhibits several external features that reflect its functional organization and regional specialization.

• Cervical Enlargement: Expanded region (C3–T1) supplying nerves to the upper limbs.

• Lumbar Enlargement: Expanded region (T9–T12) supplying nerves to the lower limbs.

• Conus Medullaris: The tapered, inferior end of the spinal cord (around L1/L2).

• Cauda Equina: A bundle of spinal nerve roots resembling a horse's tail, extending below the conus medullaris.

• Filum Terminale: A fibrous extension anchoring the spinal cord to the coccyx.

Spinal Nerve Anatomy and Plexuses

Spinal nerves are organized into segments and plexuses, which allow for the distribution of motor and sensory fibers to the body. Most anterior rami form plexuses, except for T2–T12, which become intercostal nerves.

Plexus	Spinal Cord Segments	Major Nerves	Innervation
Cervical Plexus	C1–C5	Phrenic nerve	Diaphragm, neck muscles, skin of neck and upper shoulders
Brachial Plexus	C5–C8, T1	Musculocutaneous, Median, Radial, Ulnar nerves	Muscles and skin of shoulder, arm, forearm, and hand
Lumbar Plexus	L1–L4	Femoral nerve	Anterior and medial thigh muscles, skin of thigh and medial leg
Sacral Plexus	L4–L5, S1–S4	Sciatic nerve	Posterior thigh, most of lower leg and foot

• Intercostal Nerves (T2–T12): These do not form plexuses but supply the muscles and skin between the ribs and some abdominal structures.

Examples of Major Peripheral Nerves

• Phrenic Nerve: Arises from the cervical plexus (C3–C5); innervates the diaphragm, essential for breathing.

• Musculocutaneous Nerve: From the brachial plexus; supplies anterior arm muscles.

• Median Nerve: From the brachial plexus; supplies most anterior forearm muscles and some hand muscles.

• Radial Nerve: From the brachial plexus; supplies posterior arm and forearm muscles.

• Ulnar Nerve: From the brachial plexus; supplies some forearm and most hand muscles.

• Femoral Nerve: From the lumbar plexus; supplies anterior thigh muscles.

• Sciatic Nerve: From the sacral plexus; the largest nerve in the body, supplies the posterior thigh, leg, and foot.

Key Definitions

• Plexus: A network of intersecting nerves.

• Mixed Nerve: A nerve containing both sensory and motor fibers.

• Ramus (pl. Rami): Branch of a spinal nerve; anterior and posterior rami serve different body regions.

Additional info:

• The spinal cord is part of the central nervous system, while the spinal nerves are part of the peripheral nervous system.

• Damage to specific spinal cord segments or nerves can result in characteristic patterns of sensory or motor loss, known as dermatomes and myotomes.