Spinal Nerves and Spinal Cord

Origin and Extent of the Spinal Cord

A continuous extension of the medulla oblongata that exits the cranium through the foramen magnum and runs inside the vertebral canal

How many pairs of Spinal nerves are there?

31 Pairs

8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal

Cervical nerve exit rules

Cervical nerves (C1-C7) exit ABOVE their corresponding vertebrae
C8 exits below C7 and above T1 vertebrae

Thoracic to Sacral nerve exit rules

(T1-S5) exit below their corresponding vertebrae

C4-C5 herniation rules

C4-C5 disc herniation affects the C5 nerve root (exiting above C5)

L4-L5 herniation rules

L4-L5 disc herniation affects the L5 nerve root traveling in the cauda equina

Conus Medullaris

The terminal tapered tip of the spinal cord located at the L1-L2 vertebral level

Cauda Equina

The collection of nerve roots continuing inferiorly through the spinal canal below the conus medullaris before exiting through their respective foramina

Lumbar Puncture (Spinal Tap)

Safely performed at the L3-L4 level into the lumbar cistern, sampling CSF without risking direct injury to the spinal cord

Clinical Red Flag for Lumbar Puncture

Severe herniation compressing autonomic fibers within the cauda equina is a medical emergency requiring urgent decompression to prevent permanent bowel, bladder, and organ dysregulation

Cervical Enlargement (C3-T2)

Serves the cervical and brachial plexuses (C5-T1) for upper limb innervation

Lumbar Enlargement (T9-T12)

Serves the lumbar and sacral plexuses (L1-S4) for lower limb innervation

Enlargements feature expanded ________ due to the high density of ____ and ________ required for the limbs

gray matter, sensory, motor synapses

CNS vs PNS Boundary

Defined at the cellular level by the myelinating cells: oligodendrocytes in the CNS (spinal cord), Schwann cells in the PNS (spinal nerves)

Dorsal Posterior Root

Carries afferent sensory fibers into the cord

Dorsal Root Ganglion (DRG)

Houses the cell bodies of these primary cell fibers

Ventral (Anterior) Roof

Carries efferent motor fibers out of the cord

Rami Branches

Posterior Ramus

Anterior Ramus

Communication Rami (T1-L2/L3)

Posterior Ramus

Innervates intrinsic back muscles and skin of back

Anterior Ramus

Innervates muscles and skin of anterior trunk, limbs, and forms major plexuses

Communicating Rami (T1-L2/L3)

Route sympathetic preganglionic fibers to the sympathetic chain ganglia

Dorsal Horn

Sensory processing (somatic and visceral sensory nuclei)

Ventral Horn

Motor cell bodies (somatic motor)

What do medial nuclei innervate?

Axial and proximal muscles

What do lateral nuclei innervate?

Distal extremity muscles

Where do flexor motor neurons sit?

Centrally

Where do extensor motor neurons sit?

Peripherally

Lateral Horn

Autonomic visceral motor cell bodies present at T1-L2/L3 (sympathetic) and S2-S4 (parasympathetic)

How are white matter funiculi divided?

Divided into posterior, lateral, and anterior funiculi:

Dorsal Columns, anterolateral system, and corticospinal tracts

Dorsal Columns (Gracile/Cuneate)

Discriminative touch, vibration, and proprioception

Anterolateral system (Spinothalamic)

Pain, temperature, and touch

Corticospinal tracts

Descending motor pathways for voluntary movement

Key motor myotomes

C5, C6, C7, C8, T1, L2, L3, L4, L5, S1

C5 myotome

Shoulder abduction

C6 myotome

elbow flexion and wrist extension

C7 myotome

elbow extension and wrist flexion

C8 myotome

Digit flexion

T1 myotome

Digit abduction

L2 myotome

Hip flexion

L3 myotome

Knee extension

L4 myotome

Ankle dorsiflexion

L5 myotome

Great toe extension

S1 myotome

Ankle plantarflexion

Deep Tendon Reflexes

C5, C6, C7, L4, S1

C5 deep tendon reflex

Biceps reflex

C6 deep tendon reflex

Brachioradialis reflex

C7 deep tendon reflex

Triceps reflex

L4 deep tendon reflexion

Quadriceps (patellar reflex)

S1 deep tendon reflex

Gastrocnemius (Achilles) reflex