Spinal cord tracts and reflexes

Summary

The spinal cord is the part of the central nervous system that extends from the medulla oblongata to the conus medullaris, entering the spinal canal at the first cervical vertebrae (C1) and terminating between the first and second lumbar vertebra (L1–L2). It is derived from the neural tube. The spinal cord receives blood supply from the branches of the vertebral artery and drains into the vertebral veins. It can be divided into gray matter (mainly neuronal cell bodies) and white matter (mainly myelinated axons). The gray matter of the spinal cord is located centrally, while the white matter is located peripherally, unlike in the brain, where the gray matter is located peripherally and the white matter centrally. The gray matter consists of the anterior horn, which contains the cell bodies of motor neurons; the dorsal horn, which contains the cell bodies of sensory neurons; and the lateral horn, which contains the cell bodies of preganglionic sympathetic neurons. The white matter contains descending tracts, which transmit motor signals to the periphery and ascending tracts, which transmit sensory signals to the brain. The spinal cord gives rise to 31 pairs of spinal nerves that transfer autonomic, motor, and sensory signals between the CNS and the body. Sensory innervation has a dermatomal distribution, with each area of skin of the body receiving sensory innervation from a single spinal root. A spinal reflex is an involuntary motor response following the stimulation of proprioceptors in a muscle. Clinically relevant spinal reflexes include knee (patellar) reflex, ankle reflex, biceps, and triceps reflex. Conditions that affect the spinal cord include spinal shock (related to trauma), central cord syndrome, multiple sclerosis, and tabes dorsalis.

Gross anatomy

Structures of the spinal cord

Structure Anatomy Function Characteristic features
White matter
  • 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
  • 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)
Nerve roots
  • Anterior roots contain motor (efferent) fibers
  • Posterior roots contain sensory (afferent) fibers
Dorsal rami
  • 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
Conus medullaris
  • Caudal end of the spinal cord
  • Formed by S2–S5
  • Ends at the L2 vertebral level (in adults)
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.

Vasculature

Vessels Characteristics
Arteries
Veins

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
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)

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

References:[2][3][4][5]

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

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
  • 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 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.

Dermatomes

Head dermatomes

Dermatome Distribution
Cranial nerve V1
Cranial nerve V2
Cranial nerve V3
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
T4
  • Level of the nipples
T7
T10
T12–L1
L2
L4
L5
  • Dorsum of the foot
S2–S4

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