Spinal cord tracts and reflexes

Gross anatomy

Structures of the spinal cord

Structure Anatomy Function Characteristic features
White matter
  • Peripheral
    • Anterior funiculus
    • Lateral funiculus
    • Posterior funiculus
  • Contains bundles of myelinated axons
    • Tracts or fasciculi
  • Axons ascend or descend in the spinal cord within the white matter
Gray matter
  • Central
    • Butterfly-like shape
Anterior horn
  • Cell body of motor neurons
    • Alpha fibers innervate skeletal muscle (extrafusal fibers)
    • Gamma fibers innervate muscle spindles (intrafusal fibers)
    • Neurons that innervate flexor muscles are posterior
    • Neurons that innervate extensor muscles are anterior
    • Neurons that innervate proximal muscles are medial
    • Neurons that innervate distal muscles are lateral
  • Renshaw cells: Inhibitory interneurons in the anterior horn; inhibit alpha motor neurons
  • Each horn is organized somatotopically
    • Medial portion innervates the proximal muscles of the limbs (shoulder + proximal arm, hip + thigh)
    • Lateral portion innervates distal muscles of the limbs (forearm + hand, lower leg + feet)
  • Contains Rexed laminae VIII–IX
Dorsal horn
  • Contains Rexed laminae I–VI
    • Marginal nucleus (lamina I)
    • Substantia gelatinosa of Rolando (lamina II)
    • Nucleus proprius (laminae III, IV)
    • Spinal lamina V
    • Spinal lamina VI
Lateral horn
  • Contains Rexed lamina VII
Ventral rami
  • Innervate the skin of the anterolateral trunk and limbs
  • Innervate muscles of the anterolateral trunk and limbs
  • Mixed nerve (contains both sensory and motor information)
Dorsal rami
  • Innervate the skin of the back and dorsal neck
  • Innervate deep muscles of the back (e.g., erector spinae)
  • Mixed nerve (contains both sensory and motor information)

Dorsal root ganglia

Cervical region
  • Enlargement at C5–T1 level
Thoracic region
  • T1–T12
  • Origin of the intercostal nerves
Lumbar region
  • Enlargement at L2–S3 level
  • Forms lumbar and sacral plexuses
  • Innervates the lower limbs
  • Innervation to back muscles and the lower limb
Conus medullaris
  • Caudal end of the spinal cord
  • Formed by S2–S5
  • Ends at the L2 vertebral level (in adults)
  • Supplied by 3 arteries
    • Anterior spinal artery
    • Right and left posterior spinal arteries
Cauda equina
Filum terminale
  • Contains the terminal portion of the central canal
Spinal nerves
  • Carry autonomic, motor, and sensory signals between the spinal cord and the body

Renshaw cells are inhibitory interneurons that secrete glycine. These are the neurons targeted by Clostridium tetani toxin.


Vessels Characteristics
  • Vertebral arteries
    • Arise from the subclavian artery
    • Main source of blood supply to the spinal cord
    • Branches
      • Anterior spinal artery
        • Courses on the anterior median fissure of the spinal cord
        • Supplies the anterior two thirds of the spinal cord and the lower medulla at the midline
        • Has penetrating and circumferential branches
      • Posterior spinal arteries
        • Supply the posterior part of the spinal cord bilaterally
      • Anterior and posterior radicular arteries
      • Arterial vasocorona
    • The anterior and posterior spinal arteries fuse together to form the circle of Willis in the base of the brain

The great anterior radiculomedullary artery (artery of Adamkiewicz) is the dominant artery supplying the thoracolumbar region of the spinal cord.References:[1]

Spinal cord tracts

The spinal cord contains ascending and descending tracts. They receive sensory information, or respectively, send motor impulses throughout the body. For more information on lesions of the spinal cord tracts see “Incomplete spinal cord syndromes.

Ascending tracts

Tract 1st neuron Synapses 2nd neuron Trajectory Function
Conscious sensation Spinothalamic tract
  • Ipsilateral
  • A couple of segments below or above 1st neuron
  • Anterior
    • Pressure
    • Crude touch
  • Lateral
Dorsal column
  • Cell body in the lower region of the medulla
    • Gracile nucleus or cuneate nucleus
  • Axon crosses to the contralateral side within the medulla
Unconscious sensation Spinocerebellar tract
Spinoolivary tract
  • Axons arising from the posterior horn of the spinal cord to the olivary nuclei of the medulla

Descending tracts

Tract 1st neuron Synapses 2nd neuron Trajectory Function
Voluntary movement

Corticospinal tract

(pyramidal tracts)

  • Anterior horn cells of the spinal cord
  • (Voluntary) movement of the contralateral side
Involuntary movement Extrapyramidal tract
  • Multiple projections and pathways
  • Rubrospinal pathway
  • Reticulospinal tract
  • Vestibulospinal tract
  • Tectospinal tract
  • Anterior horn cells of the spinal cord
  • Regulate the action of motor neurons
    • Involuntary movement (e.g., equilibratory reflexes, visual and auditory reflexes)
    • Muscle tone
    • Facial expressions


Embryology of the spinal cord

During the 3rd week of gestation begins the process of neurulation, through which the neural plate folds to form the neural tube. Both the spinal cord and the brain develop from the neural tube.

Neurulation derivatives

Structure Characteristics Derivatives
Neural plate
  • Located on the dorsal portion of the trilaminar germ disk
  • Precursor structure for the process of neurulation
Neural folds
  • Located on the free edges of the neural plate bilaterally during the elongation and folding process
  • At the end of the 3rd week of gestation, the neural folds grow dorsally and toward the midline, fuse together, and close the neural tube.
Neural groove
  • Central groove of the neural plate on the floor of the developing neural tube (ventral)
  • It deepens as the neural crests approach each other and begin to fuse.
Neural crest cells
Neural tube

The rostral neuropore closes by day 26, the caudal neuropore closes by day 28. Failure of neuropores to completely close causes neural tube defects (e.g., spina bifida).

Spinal reflexes

Reflex physiology

Clinically important reflexes

Reflex Muscle tested Spinal level
Biceps reflex and brachioradialis reflex Biceps C5–C6
Triceps reflex Triceps C7–C8
Knee reflex (Patellar) Quadriceps L3–L4
Ankle reflex (Achilles) Gastrocnemius S1–S2
Cremasteric reflex Cremaster L1–L2
Anal wink reflex External anal sphincter S3–S4

For more details on mechanoreceptors, see “sensory receptors of the skin” in the “skin and skin appendage” learning card.


Head dermatomes

Dermatome Distribution
  • Maxillary branch of the trigeminal nerve
  • Mid face
    • Lateral nose
    • Anteroinferior part of the nasal septum
    • Upper lip and cheeks
    • Lower eyelid
    • Anterior region of the temple (anterior to the ear)

Body dermatomes

Dermatome Distribution
  • Lower clavicular region and middle clavicular line
    • Supraclavicular fossa
  • Dorsal surface of the thumbs
  • Dorsal surface of the middle finger
  • Dorsal surface of the little finger
  • Level of the nipples
  • Anterior region of the medial thigh
  • Dorsum of the foot

Referred pain: pain can be perceived in a location that differs from the actual site of the stimulus because it is projected to a dermatome via the corresponding spinal cord segment!References:[6]

Diseases of the spinal cord

Complete spinal cord injuries

Incomplete spinal cord injuries

Other spinal cord pathologies