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

  • In the intervertebral foramina
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
  • Innervates the lower limb, perineum, and pelvic organs (including internal and external anal sphincter)
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.

Vasculature

Vessels Characteristics
Arteries
  • 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
        • Arise from spinal branches of the following arteries
          • Ascending cervical arteries
          • Vertebral arteries
          • Deep cervical arteries
          • Intercostal arteries
          • Lumbar arteries
          • Sacral arteries
        • Supply nerve roots and the spinal cord
      • Arterial vasocorona
        • Anastomosis between spinal arteries
    • The anterior and posterior spinal arteries fuse together to form the circle of Willis in the base of the brain
Veins
  • Vertebral veins
    • For more information on the venous system of the spinal cord see vertebral column

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
  • To 3rd neuron in the thalamus
  • Somatosensory cortex
  • 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
Notochord
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
  • Gives rise to the brain and the spinal cord

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

  • Initiation: stimuli (e.g., reflex hammer tap on patellar tendon) → perception by sensory receptors in skeletal muscle → signal travels in sensory neurons to synapsis → stimulation of lower motor neurons (LMNs) → motor response (e.g., muscle contraction or relaxation)
  • Upper motor neurons (UMNs) controls the strength of the reflex
  • Lower motor neurons: innervate extrafusal muscle fibers
    • Motor unit: alpha motor neuron + group of muscle fibers innervated by the alpha motor neuron
      • Controls voluntary movement, postural and reflex activity
  • Gamma motor neurons: innervate intrafusal fibers (modified skeletal muscle fibers)
    • Intrafusal fibers for muscle spindles
      • Detect length and change in length of extrafusal fibers
      • Function as sensory receptors in skeletal muscle reflexes.
  • Myotatic stretch reflex : muscle contraction in response to stretch (e.g., tapping on the patellar tendon stretches the quadriceps muscle by creating tension)
  • Inverse myotatic stretch reflex (inverse of the myotatic stretch reflex)
    • Through Golgi tendon organs (GTOs)
      • Nerve endings located at the junction between tendon and muscle
      • Responds to tension and force within the muscle
        • ↑ force or tension → lengthening the muscle → relaxation of antagonist muscles and inhibition of agonist muscles
          • This prevents excessive muscle tension that can damage the muscle

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 S1S2
Cremasteric reflex Cremaster L1–L2
Anal wink reflex External anal sphincter S3S4

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

Dermatomes

Head dermatomes

Dermatome Distribution
V1
V2
  • 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)
V3
  • Mandibular branch of the trigeminal nerve
  • Lower face
C1–C4

Body dermatomes

Dermatome Distribution
C3
C4
  • Lower clavicular region and middle clavicular line
    • Supraclavicular fossa
C5
C6
  • Dorsal surface of the thumbs
C7
  • Dorsal surface of the middle finger
C8
  • Dorsal surface of the little finger
T1
T2
  • The axilla
T4
  • Level of the nipples
T7
T10
T12–L1
  • Inguinal ligament
L2
  • Anterior region of the medial thigh
L4
L5
  • Dorsum of the foot
S2S4
  • S2: popliteal fossa
  • S3: infragluteal folds
  • S4: perineal region

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!

Diseases of the spinal cord

Complete spinal cord injuries

Incomplete spinal cord injuries

Other spinal cord pathologies