Spinal Nerves | VeridianCodex

Spinal Column (Bone)

The spine consists of a series of small bones (vertebrae) that protect the spinal cord (Yu et al., 2020). It includes 24 articulated (mobile) vertebrae separated by intervertebral discs, and 9 non-articulated (fused) vertebrae found in the sacral and coccygeal regions. Details about each vertebra are provided in Table 1 within the appendices.

Spinal Nerves (Neurons)

The spinal cord is a cylindrical structure that emerges from the medulla oblongata (where the spinal cord meets the skull) and travels down the spine. Pairs of spinal nerves exit and enter through each vertebra (left and right), forming part of the peripheral nervous system (PNS) (Yu et al., 2020; Kaiser & Lugo-Pico, 2019). These spinal nerves transmit sensory information to the central nervous system (CNS) and initiate motor responses.

There are 31 pairs of spinal nerves connecting the CNS and PNS:

8 cervical pairs (C1–C8, neck region)
12 thoracic pairs (T1–T12, chest region)
5 lumbar pairs (L1–L5, lower back)
5 sacral pairs (S1–S5, pelvis)
1 coccygeal pair (Co1, tailbone)

Each spinal nerve is a mixed nerve, containing both sensory (afferent) and motor (efferent) fibers, which extend outward to form muscle fibers, receptors, and components of the autonomic nervous system (ANS).

Source: https://commons.wikimedia.org/wiki/File:Spinal_cord_details.jpg

Key Structures of a Spinal Nerve

Spinal Nerve: Formed by the union of dorsal (sensory) and ventral (motor) roots; exits through intervertebral foramina.
Dorsal Root & Spinal Ganglion: Carries sensory input; ganglion contains sensory neuron cell bodies.
Ventral Root: Carries motor output to muscles and glands.
Ventral Median Fissure: Midline groove on the anterior spinal cord.
Epineurium & Perineurium: Protective connective tissue layers surrounding nerves and fascicles.

Meningeal Layers:

The spinal cord is enclosed by three protective membranes (meninges), with associated spaces:

Pia Mater: Innermost layer, closely adheres to the spinal cord, carrying blood vessels.
Arachnoid Mater: Middle layer, enclosing the subarachnoid space, which contains cerebrospinal fluid (CSF).
Dura Mater: Tough outer layer, enclosing the epidural space filled with fat and blood vessels.
Subarachnoid Space: Cushions spinal cord with CSF.
Subdural Space: A potential space with serous fluid, aiding wound healing.
Epidural Space: Provides structural buffer with fat and venous networks.
Denticulate Ligaments: Extensions of the pia mater that anchor the cord to the dura mater for stability.

Source: https://commons.wikimedia.org/wiki/File:Spinal_Cord_Segments_and_body_representation.png

Cervical Plexus

Sensory Functions: Provides cutaneous innervation to the scalp, neck, chest, and axilla, as well as proprioceptive input.

Key sensory nerves:

Lesser occipital nerve (C2-C3): Scalp behind the ear.
Great auricular nerve (C2-C3): Skin around ear and jaw.
Transverse cervical nerve (C2-C3): Anterior and lateral neck.
Supraclavicular nerve (C3-C4): Shoulder and upper chest.

Motor Functions:

Ansa cervicalis (C1–C3): Innervates infrahyoid muscles and sternocleidomastoid for neck movement
Phrenic nerve (C3-C5): Regulates diaphragm for breathing.

Brachial Plexus

Organized into trunks, divisions, cords, and branches; innervates ~50 muscles plus skin of the upper limbs and pectoral region.

Major Mixed Nerves:

Axillary nerve (C5-C6): Deltoid, teres minor
Musculocutaneous nerve (C5-C6): Anterior arm, lateral forearm skin
Radial nerve (C6-C8): Posterior arm/forearm muscles and skin
Median nerve (C5-T1): Forearm flexors, hand muscles
Ulnar nerve (C8-T1): Medial forearm, hand muscles

Thoracic Nerves

Structure: 12 pairs of spinal nerves, one per thoracic segment.

Sensory: Provide cutaneous innervation to skin, musculoskeletal system, and viscera.
Motor: Thorax, abdominal wall, deep back, gut muscles
Autonomic: Connect with sympathetic trunk for vasomotor and visceral regulation

Lumbosacral Plexus

Combines lumbar and sacral plexuses, providing all sensory and motor innervation to the lower limbs and parts of the abdominal wall (~200,000 axons).

Lumbar Plexus

Structure: 12 pairs of spinal nerves, one per thoracic segment.

Iliohypogastric & ilioinguinal nerves (L1–L2): Abdominal muscles; sensory innervation to lower trunk and genitalia
Genitofemoral nerve (L1–L2): Genitalia sensation
Lateral femoral cutaneous nerve (L2–L3): Sensory innervation to lateral thigh
Femoral & obturator nerves (L3–L4): Thigh flexion, adduction, extension; sensory to thigh and medial leg
Autonomic fibers: Preganglionic sympathetic fibers from L1–L2

Sacral Plexus

Superior & inferior gluteal nerves (L4–S1): Gluteal muscles, hip movement
Posterior femoral cutaneous nerve (S1–S3): Sensory innervation to gluteal and perineal regions
Sciatic nerve (L4–S3) → branches:
- Tibial nerve (L4–S2): Posterior thigh, leg, foot (motor + sensory)
- Common peroneal nerve (L4–S1): Anterior/lateral leg, foot (motor + sensory)
Pudendal nerve (S2–S4): Perineum innervation
Autonomic fibers: Preganglionic parasympathetic fibers from S2–S4

References

Kaiser, J. T., & Lugo-Pico, J. G. (2019). Neuroanatomy, spinal nerves. Study Guide from StatPearls Publishing, Treasure Island (FL), 14 Aug 2023 PMID: 31194375
Yu, J., Manouchehri, N., Yamamoto, S., Kwon, B. K., & Oxland, T. R. (2020). Mechanical properties of spinal cord grey matter and white matter in confined compression. Journal of the Mechanical Behavior of Biomedical Materials, 112, 104044.

Appendices

Table of Human Vertebrae (24 Articulating and 9 Non-Articulating)

The human vertebral column consists of 33 vertebrae: 24 articulating (mobile) vertebrae (7 cervical: C1–C7, 12 thoracic: T1–T12, 5 lumbar: L1–L5) and 9 non-articulating (fused) vertebrae (5 sacral: S1–S5, fused into the sacrum; 4 coccygeal: Co1–Co4, typically fused into the coccyx).

Vertebra	Region	Key Features
C1 (Atlas)	Cervical	Lacks a body; lateral masses with superior facets articulate with occipital condyles (nodding); inferior facets articulate with C2; oblique facet orientation maximizes motion.
C2 (Axis)	Cervical	Odontoid process (dens) enables rotation with C1; oblique facet joints support neck flexibility.
C3 – C5	Cervical	Small body, triangular vertebral foramen, bifid spinous process; oblique facet joints allow flexion, extension, and rotation.
C6	Cervical	Features carotid tubercle (anterior tubercle); oblique facet joints allow neck flexibility.
C7	Cervical	Prominent spinous process (vertebra prominens); smaller transverse foramina (may not transmit vertebral arteries); oblique facet joints support neck and head movement.
T1	Thoracic	Full facet for rib 1, demi facet for rib 2; coronal facet joints allow rotation, limit flexion/extension.
T2 – T9	Thoracic	Superior and inferior demi facets for ribs; coronal facet joints and rib articulations restrict movement.
T10	Thoracic	Single full facet for rib articulation; coronal facet joints allow slightly more movement.
T11	Thoracic	Single full facet, no transverse process facets (no rib tubercle articulation); coronal facet joints increase mobility.
T12	Thoracic	Single full facet, no transverse process facets; transitional, resembles lumbar; coronal facet joints allow more flexion.
L1 – L2	Lumbar	Large body; sagittal facet joints support flexion/extension, resist rotation; robust for weight-bearing.
L3 – L4	Lumbar	Large body; sagittal facet joints enable flexion/extension, slight rotation.
L5	Lumbar	Largest body; sagittal facet joints articulate with sacrum, enabling flexion/extension.
S1 – S5	Sacral	Fused into sacrum; articulates with L5 via superior facets and intervertebral disc; no independent movement due to fusion.
Co1 – Co4	Coccygeal	Fused into coccyx; rudimentary, no significant articulation; may articulate slightly with sacrum in some individuals.

Notes:

Cervical (C1–C7): Oblique facet joint orientation allows greatest range of motion (flexion, extension, rotation).
Thoracic (T1–T12): Coronal facet joints and rib articulations prioritize stability, allow limited rotation.
Lumbar (L1–L5): Sagittal facet joints support flexion/extension and weight-bearing, resist rotation.
Sacral (S1–S5): Fused into the sacrum; articulates only with L5 superiorly and coccyx inferiorly; fusion prevents inter-segmental movement.
Coccygeal (Co1–Co4): Fused into the coccyx; vestigial, with minimal or no articulation; slight mobility at sacrococcygeal junction in some cases.
Articulating vertebrae move via intervertebral discs and facet joints; non-articulating vertebrae are fused, providing structural stability.

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