Spinal Cord and Spinal Nerves: Structure, Function, and Reflexes

Spinal Cord Structure and Organization

Overview and Segmentation

The spinal cord is a vital component of the central nervous system, extending from the foramen magnum at the base of the skull to the level of the second lumbar vertebra. It is segmented into regions, each associated with a pair of spinal nerves:

• Cervical

• Thoracic

• Lumbar

• Sacral

There are 31 pairs of spinal nerves, which emerge from these segments. The diameter of the spinal cord is not uniform:

• Cervical enlargement: supplies upper limbs

• Lumbar enlargement: supplies lower limbs

• Conus medullaris: tapered inferior end

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

Meninges of the Spinal Cord

The spinal cord is protected by three connective tissue membranes called meninges:

• Dura mater (thecal sac): outermost, tough layer, continuous with the epineurium of spinal nerves

• Arachnoid mater: thin, wispy middle layer

• Pia mater: innermost layer, tightly bound to the surface of the spinal cord and brain; forms the filum terminale and denticulate ligaments

Spaces associated with the meninges:

• Epidural space: contains blood vessels, areolar connective tissue, and fat; site for epidural anesthesia

• Subdural space: contains serous fluid

• Subarachnoid space: contains cerebrospinal fluid (CSF) and blood vessels

Cross Section of the Spinal Cord

The spinal cord is divided by the anterior median fissure and posterior median sulcus. It contains:

• White matter: myelinated axons forming tracts (ventral, dorsal, lateral columns)

• Gray matter: neuron cell bodies, dendrites, axons; organized into horns (posterior/dorsal, anterior/ventral, lateral/ANS)

• Commissures: connections between left and right halves, with a central canal in the center

• Roots: spinal nerves arise as rootlets, combine to form roots; dorsal root (sensory, with ganglion), ventral root (motor)

Relationship of Sensory and Motor Neurons

Dorsal Root Ganglion and Motor Neuron Organization

• Dorsal root ganglion: contains cell bodies of pseudo-unipolar sensory neurons

• Motor neuron cell bodies are in anterior and lateral horns

• Multipolar somatic motor neurons in anterior horn

• Autonomic neurons in lateral horn

• Axons of motor neurons pass through ventral roots into spinal nerves

Spinal Cord Reflexes

Reflexes and Reflex Arcs

A reflex is an automatic, homeostatic response to a stimulus. The reflex arc is the neural pathway controlling a reflex:

• Sensory receptor → sensory neuron → interneuron → motor neuron → effector organ

Classification of Reflexes

• Monosynaptic reflexes: no interneurons

• Polysynaptic reflexes: involve interneurons

• Innate reflexes: present from birth

• Learned reflexes: acquired through repetition

• Somatic reflexes: involve skeletal muscle

• Autonomic (visceral) reflexes: involve smooth muscle, cardiac muscle, glands

• Cranial reflexes: integrated in the brain

• Spinal reflexes: integrated in the spinal cord

Reflex Arc Steps

1. Sensory receptor detects stimulus, generates action potentials

2. Sensory neuron conducts action potentials to spinal cord

3. Action potential carried to integration center (may involve interneuron)

4. Interneuron synapses with motor neuron

5. Motor neuron axon conducts action potentials to effector organ

Major Spinal Reflexes

Stretch Reflex

Muscles contract in response to stretching force. Unique as it does not involve an interneuron.

• Muscle spindle: specialized muscle cells that detect stretch

• Gamma motor neurons: control sensitivity of muscle spindle

• Alpha motor neurons: cause muscle contraction

Example: Patellar reflex (knee-jerk)

Golgi Tendon Reflex

Prevents excessive tension in tendons by causing muscle relaxation.

• Golgi tendon organ: encapsulated nerve endings in tendons

• Increased tension stimulates sensory neurons → inhibitory interneurons → inhibit alpha motor neurons → muscle relaxes

Example: Sudden relaxation when lifting excessive weight

Withdrawal Reflex

Removes body part from painful stimulus.

• Reciprocal innervation: relaxation of extensor muscle when flexor contracts

• Crossed extensor reflex: extension of opposite limb when withdrawal reflex is initiated

Spinal Nerves

Structure and Organization

• Composed of axon bundles, Schwann cells, and connective tissue

• Endoneurium: surrounds individual neurons

• Perineurium: surrounds axon groups (fascicles)

• Epineurium: surrounds entire nerve

Spinal Nerve Pairs and Exit Points

• 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

• First pair exits between skull and atlas; last four pairs via sacral foramina; others through intervertebral foramina

Dermatomal Map

A dermatomal map shows skin areas supplied by sensory innervation from spinal nerves, indicated by capital letter and number.

Branches (Rami) of Spinal Nerves

• Dorsal ramus: innervates deep trunk muscles and skin near midline of back

• Ventral ramus: forms intercostal nerves (thoracic region) and plexuses (other regions)

• Communicating rami: connect with sympathetic chain of ganglia

Spinal Nerve Plexuses

Cervical Plexus

• C1 to C4

• Innervates superficial neck structures, skin of neck, posterior head

• Ansa cervicalis: loop between C1 and C3, serves infrahyoid muscles

• Phrenic nerve: C3 to C5, innervates diaphragm

Brachial Plexus

• C5 to T1

• Five ventral rami form three trunks, six divisions, cords, and five branches

• Branches: Axillary, Radial, Musculocutaneous, Ulnar, Median

• Smaller nerves: pectoral, long thoracic, thoracodorsal, subscapular, suprascapular

Lumbar and Sacral Plexuses

• Lumbar: L1 to L4

• Sacral: L4 to S4

• Major nerves: Obturator, Femoral, Tibial, Common fibular (peroneal)

• Tibial and common fibular nerves together form the sciatic nerve

Coccygeal Plexus

• S5 and coccygeal nerve

• Innervates muscles of pelvic floor and skin over coccyx

Representative Diseases and Disorders

Spinal Cord Disorders

• Injury or disease can affect motor and sensory function

• Examples: trauma, infection, degenerative diseases

Spinal Nerve Disorders

• Can result from compression, injury, or disease

• Symptoms depend on affected nerve and region

Table: Major Spinal Nerve Plexuses and Their Key Nerves

Plexus	Spinal Nerve Roots	Major Nerves	Innervation
Cervical	C1–C4	Ansa cervicalis, Phrenic	Neck muscles, diaphragm
Brachial	C5–T1	Axillary, Radial, Musculocutaneous, Ulnar, Median	Upper limb muscles and skin
Lumbar	L1–L4	Obturator, Femoral	Anterior thigh, medial thigh
Sacral	L4–S4	Tibial, Common fibular (sciatic)	Posterior thigh, leg, foot
Coccygeal	S5, coccygeal	Coccygeal nerve	Pelvic floor, skin over coccyx

Additional info: The above table summarizes the main spinal nerve plexuses, their roots, major nerves, and areas of innervation for quick reference.