The Spinal Cord: Structure, Function, and Reflexes

The Spinal Cord

Overview

The spinal cord is a vital organ of the central nervous system (CNS) that serves as a major pathway for transmitting information between the brain and the rest of the body. It is involved in both sensory and motor functions, as well as in mediating reflexes.

• Sensory (afferent) tracts carry messages to the brain through ascending pathways.

• Motor (efferent) tracts carry messages from the brain through descending pathways.

• The spinal cord is essential for spinal reflexes.

Anatomy of the Spinal Cord

Gross Structure

• Length: Approximately 18 inches, extending from the foramen magnum to the level of L1 vertebra.

• The spinal cord is a direct continuation of the brainstem.

• It travels vertically through the vertebral column.

Protective Coverings: Meninges

The spinal cord is surrounded by three layers of connective tissue called meninges, listed from superficial to deep:

1. Dura mater

2. Arachnoid mater

3. Pia mater

• The epidural space is a large space outside the dura mater, important for the administration of anesthesia (epidural anesthesia).

• Cerebrospinal fluid (CSF) is found in the subarachnoid space, between the arachnoid and pia mater.

• The meninges protect the spinal cord and provide a supportive framework for blood vessels.

• Although the spinal cord ends at L1, the dura mater and arachnoid continue to S2, which is significant for procedures such as lumbar puncture.

Cross-Sectional Anatomy

The spinal cord consists of gray matter (central, butterfly-shaped) and white matter (peripheral).

• Gray matter: Contains neuron cell bodies, dendrites, and unmyelinated axons. Organized into horns (anterior, posterior, lateral).

• White matter: Contains myelinated axons organized into columns (anterior, posterior, lateral funiculi).

• The central canal runs through the center and contains CSF.

Key Anatomical Features

• Conus medullaris: The tapered, lower end of the spinal cord near L1.

• Filum terminale: A connective tissue strand anchoring the spinal cord to the coccyx.

• Spinal nerves: 31 pairs, each associated with a segment of the spinal cord. Each is a mixed nerve (contains both sensory and motor fibers).

• Enlargements: Cervical and lumbar regions are enlarged due to the increased number of neurons serving the limbs.

• Cauda equina: A bundle of spinal nerve roots below the conus medullaris.

Spinal Nerves and Plexuses

Spinal Nerves

• Each spinal nerve is formed by the union of a dorsal (sensory) and ventral (motor) root.

• Mixed nerves contain both sensory and motor fibers.

Plexuses

Plexuses are networks of intersecting nerves. There are four main plexuses:

Plexus	Spinal Nerves	Key Nerves	Main Functions
Cervical	C1–C5	Phrenic nerve	Supplies diaphragm
Brachial	C5–T1	Radial, median, ulnar, musculocutaneous, axillary	Supplies upper limb
Lumbar	L1–L4	Femoral nerve	Supplies anterior thigh
Sacral	L4–S4	Sciatic nerve, pudendal nerve	Supplies lower limb, pelvic floor

Additional info: The pudendal nerve is important for continence.

Dermatomes

• A dermatome is an area of skin supplied by a single spinal nerve.

• Dermatomes are clinically significant for diagnosing nerve injuries and conditions like shingles.

Cross-Sectional Anatomy of the Spinal Cord

Gray and White Matter

• Gray matter horns: Contain cell bodies of neurons.

• White matter columns: Contain ascending (sensory) and descending (motor) tracts.

• Central canal: Contains CSF.

Spinal Cord Tracts

Tract Organization

• Tracts are either sensory (ascending) or motor (descending); they are not mixed.

Sensory Tracts

• Carry sensory information to the brain through white columns.

• Example: Spinothalamic tract (pain and temperature sensation).

Motor Tracts

• Carry motor commands from the brain to the body.

• Example: Corticospinal (pyramidal) tract (voluntary movement).

Reflexes

Definition and Classification

• A reflex is an automatic, rapid response to a stimulus.

• Reflexes can be monosynaptic (one synapse) or polysynaptic (multiple synapses).

• They can be ipsilateral (same side) or contralateral (opposite side), and unilateral or bilateral.

• Reflexes are also classified as somatic (skeletal muscle) or autonomic (viscera).

Reflex Arc Components

1. Receptor

2. Sensory (afferent) neuron

3. Integration center (spinal cord or brain)

4. Motor (efferent) neuron

5. Effector (muscle or gland)

Types of Reflexes

• Stretch reflex: Monosynaptic; maintains muscle tone (e.g., patellar reflex).

• Golgi tendon reflex: Polysynaptic; prevents muscle overcontraction.

• Withdrawal (flexor) reflex: Polysynaptic; withdraws limb from painful stimulus.

• Crossed extensor reflex: Contralateral; supports body during withdrawal reflex.

• Superficial reflexes: Elicited by skin stimulation (e.g., plantar reflex, Babinski sign).

Key Reflex Terms

• Monosynaptic: One synapse between sensory and motor neuron.

• Polysynaptic: Multiple synapses; involves interneurons.

• Ipsilateral: Response on the same side as stimulus.

• Contralateral: Response on the opposite side.

• Bilateral: Both sides respond.

Examples and Clinical Relevance

• Patellar reflex: Tests L2–L4 spinal segments.

• Achilles reflex: Tests S1–S2 segments.

• Babinski sign: Abnormal in adults; indicates corticospinal tract damage.

Autonomic Reflexes

Definition and Examples

• Autonomic reflexes involve smooth muscle, cardiac muscle, or glands.

• Examples: Pupillary reflex, salivary reflex, baroreceptor reflex, gastrointestinal reflexes.

Pupillary Reflex

• Receptor: Photoreceptor in the retina.

• Afferent neuron: Optic nerve (CN II).

• Integration center: Midbrain (pretectal area).

• Efferent neuron: Oculomotor nerve (CN III).

• Effector: Smooth muscle of the iris (pupil constriction).

• The response is bilateral (both pupils constrict).

Additional info: The pupillary reflex is used clinically to assess brainstem function.

Summary Table: Spinal Cord Reflexes

Reflex	Type	Synapses	Response	Clinical Use
Patellar	Somatic	Monosynaptic	Knee extension	Tests L2–L4
Golgi tendon	Somatic	Polysynaptic	Muscle relaxation	Prevents overcontraction
Withdrawal	Somatic	Polysynaptic	Flexion withdrawal	Protective
Pupillary	Autonomic	Polysynaptic	Pupil constriction	Brainstem function

Key Terms and Definitions

• Ganglion: A cluster of neuron cell bodies in the PNS.

• Mixed nerve: Contains both sensory and motor fibers.

• Dermatome: Area of skin supplied by a single spinal nerve.

• Reflex arc: The neural pathway mediating a reflex.