Spinal Cord

Overview

The spinal cord is a vital component of the central nervous system (CNS), working in conjunction with the brain to coordinate sensory and motor functions throughout the body. It serves as a major pathway for nerve impulses and is essential for reflex actions and integration of information.

• Central Nervous System (CNS): Composed of the brain and spinal cord.

• Functions of the Spinal Cord:

  • Spinal cord reflexes

  • Integration of nerve impulses

  • Upward (sensory) and downward (motor) transmission of information

Surface Markings and Coverings of the Spinal Cord

Protection and Meninges

The spinal cord is protected by several layers and structures that shield it from injury and infection. These include the vertebral column, meninges, and cerebrospinal fluid (CSF).

• Vertebral Column: Bony structure encasing the spinal cord.

• Meninges: Three connective tissue membranes:

  • Dura mater: Tough, outermost layer.

  • Arachnoid mater: Middle, web-like layer.

  • Pia mater: Thin, innermost layer adhering to the spinal cord.

• Epidural Space: Contains adipose tissue between vertebrae and dura mater.

• Subdural Space: Contains interstitial fluid between dura mater and arachnoid mater.

• Subarachnoid Space: Contains cerebrospinal fluid (CSF) between arachnoid and pia mater.

Example: A lumbar puncture (spinal tap) is performed in the lower back to access CSF in the subarachnoid space for diagnostic purposes.

External Anatomy of the Spinal Cord

Structure and Landmarks

The spinal cord is a flattened cylindrical structure, typically 16-18 inches long and about 3/4 inch in diameter in adults. It extends from the foramen magnum to the level of the first or second lumbar vertebra (L1-L2) in adults.

• Cervical Enlargement: Supplies nerves to the upper limbs.

• Lumbar Enlargement: Supplies nerves to the lower limbs.

• Growth: The spinal cord stops growing in early childhood; the vertebral column continues to grow.

Internal Anatomy of the Spinal Cord

Gray Matter

The gray matter of the spinal cord is shaped like an 'H' or butterfly and contains neuron cell bodies, unmyelinated axons, and dendrites. It is organized into horns:

• Dorsal (Posterior) Horns: Contain sensory neurons.

• Ventral (Anterior) Horns: Contain motor neurons.

• Lateral Horns: Present in thoracic and upper lumbar regions; contain autonomic motor neurons.

• Central Canal: Runs through the center, filled with CSF and continuous with the brain's ventricular system.

White Matter

White matter surrounds the gray matter and consists of myelinated axons organized into columns:

• Anterior (Ventral) White Columns

• Lateral White Columns

• Posterior (Dorsal) White Columns

These columns contain tracts that carry ascending (sensory) and descending (motor) information.

Tracts of the Spinal Cord

Ascending and Descending Pathways

Tracts are bundles of axons that serve as highways for nerve impulses:

• Ascending Tracts: Carry sensory information from the body to the brain.

• Descending Tracts: Carry motor commands from the brain to the body.

• Example: The anterior spinothalamic tract transmits sensory impulses related to crude touch and pressure from the spinal cord to the thalamus in the brain.

Spinal Reflexes

Types and Components

A spinal reflex is an automatic, rapid response to a stimulus, processed in the spinal cord without direct involvement of the brain.

• Somatic Reflexes: Involve skeletal muscle contraction (e.g., stretch reflex).

• Autonomic (Visceral) Reflexes: Involve smooth muscle, cardiac muscle, or glands (e.g., heart rate, digestion).

Reflex Arc

The reflex arc is the specific nerve pathway involved in a reflex action. Its components include:

1. Receptor: Detects stimulus.

2. Sensory Neuron: Transmits impulse to the spinal cord.

3. Integrating Center: Usually one or more interneurons in the gray matter.

4. Motor Neuron: Transmits impulse from spinal cord to effector.

5. Effector: Muscle or gland that responds to the stimulus.

Example: The stretch reflex (e.g., knee-jerk) helps maintain muscle tone and posture.

Spinal Nerves

Classification and Branching

There are 31 pairs of spinal nerves, each named and numbered according to their origin from the spinal cord:

• 8 pairs cervical (C1–C8)

• 12 pairs thoracic (T1–T12)

• 5 pairs lumbar (L1–L5)

• 5 pairs sacral (S1–S5)

• 1 pair coccygeal (Co)

Each spinal nerve is formed by the union of a dorsal (posterior) root (sensory fibers) and a ventral (anterior) root (motor fibers). The dorsal root ganglion contains cell bodies of sensory neurons.

Branching of Spinal Nerves

• Dorsal Rami: Supply skin and muscles of the back.

• Ventral Rami: Form plexuses that supply the anterior trunk and limbs.

• Meningeal Branches: Supply meninges and vertebrae.

Nerve Plexuses

Overview

Nerve plexuses are networks formed by the ventral rami of spinal nerves. They provide innervation to specific regions of the body.

• Cervical Plexus (C1–C5): Supplies head, neck, and shoulders. The phrenic nerve (C3–C5) controls the diaphragm; injury above C3 causes respiratory arrest.

• Brachial Plexus (C5–T1): Supplies shoulder and upper limb. Major branches:

  • Axillary nerve: Deltoid and teres minor muscles.

  • Musculocutaneous nerve: Elbow flexors.

  • Radial nerve: Shoulder and elbow extensors.

  • Median and Ulnar nerves: Flexors of wrist and hand.

• Lumbar Plexus (L1–L4): Supplies abdominal wall, external genitals, and anterior/medial thigh. Major branches:

  • Femoral nerve: Injury causes inability to extend leg and loss of sensation in thigh.

  • Obturator nerve: Injury causes paralysis of thigh adductors.

• Sacral Plexus (L4–S4): Supplies buttocks, perineum, and lower limb. Major branches:

  • Sciatic nerve (L4–S3): Largest nerve; injury can cause foot drop or numbness.

  • Peroneal nerve: Injury produces foot drop.

  • Tibial nerve: Injury produces calcaneovalgus (loss of flexion on anterior leg and dorsum).

Additional info: No plexus is formed in the thoracic region; intercostal nerves supply the intercostal muscles and abdominal wall.

Table: Major Nerves of Plexuses and Their Functions

Clinical Application

Spinal Cord Injury

Damage to specific regions of the spinal cord or its nerves can result in loss of function in corresponding body areas. For example, injury at the lumbar level may impair leg movement but spare arm function.

Review Questions

• What are two functions of the spinal cord?

• Where is a lumbar tap performed, and why?

• What are the components of a spinal reflex?

• Which parts of the spinal cord are exclusively sensory or motor?

• Complete the table of major nerves, their roots, functions, and damage results.