Spinal cord tracts and reflexes

Summary

The spinal cord is part of the central nervous system and coordinates motor, sensory, and reflex signals. Anatomically, the spinal cord is located within the spinal canal and extends from the bottom of the medulla (at the first cervical vertebra C1) to the conus medullaris (between L1 and L2). Blood is supplied to the spinal cord from the branches of the vertebral artery and drains into the vertebral veins. Internally, the cord can be divided into gray matter centrally and white matter peripherally (unlike in the brain, where this division is inverted). The gray matter is composed 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 and ascending tracts. The descending tracts transmit motor signals to the periphery and the ascending tracts transmit sensory signals to the brain. Thirty-one pairs of peripheral spinal nerves arise segmentally from the spinal cord and conduct autonomic, motor, sensory, and reflex signals between the CNS and the body. Clinically relevant spinal reflexes include the patellar reflex, ankle reflex, biceps reflex, and triceps reflex. Each spinal nerve carries dermatomes, which are somatic sensory fibers that correspond with a specific and identifiable region of skin. Each dermatome is supplied by a single spinal cord level and is, therefore, useful in identifying specific levels of damage in spinal cord injuries. The development of the spinal cord begins around the third week of gestation during the process of neurulation. The spinal cord is derived from the neural tube. Conditions that affect the spinal cord include spinal shock (related to trauma), central cord syndrome, and multiple sclerosis.

Gross anatomy

Structures of the spinal cord

Structure Anatomy Function Characteristic features

White matter

  • Contains bundles of myelinated axons called tracts or fasciculi
  • Axons ascend or descend in the spinal cord within the white matter to transmit signals between the CNS and periphery.
Gray matter
  • Centrally located, butterfly-like shape
  • Surrounds the central canal
  • Gray commissure crosses the midline.
Anterior horn
  • Contains Rexed laminae VIII–IX
  • Organized somatotopically
    • Medial motor neurons innervate proximal musculature (e.g., the trunk and shoulders).
    • Lateral motor neurons innervate distal musculature (e.g., the hands and feet).
    • Motor neurons that innervate flexors are located dorsally and those that innervate extensors are located ventrally.
Dorsal horn
  • Cell bodies of second-order sensory neurons
  • 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

Spinal nerves

  • Transport autonomic, motor, and sensory signals between the spinal cord and the body
Nerve roots
  • Anterior roots carry motor (efferent) fibers that control skeletal and smooth muscle.
  • Dorsal roots carry sensor (afferent) fibers that transmit somatosensory information.
  • Disk herniation is the most common cause of damage to the spinal nerve roots.
Dorsal root ganglia
  • Carry sensory (afferent) fibers
Ventral rami
  • Innervate the skin and skeletal muscle of the anterolateral trunk and limbs
  • Mixed nerves (relay both sensory and motor information)

Dorsal rami

  • Mixed nerves (relay both sensory and motor information)
Cervical region
  • Cervical enlargement at C5–T1 level
Thoracic region
  • Consists of 12 spinal segments (T1–T12); no enlargement at this level
Lumbar region
  • Lumbar enlargement at L2–S3 level
  • Forms lumbar and sacral plexuses, which innervate the lower limbs
Conus medullaris
  • Innervates lower extremities, bowel, and bladder
  • Tapered terminal end of spinal cord; directly attached to filum terminale
Cauda equina
  • Bundle of tapered nerve roots resembles a horse's tail
Filum terminale
  • Slender, string-like appearance

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

Vasculature

The great anterior radiculomedullary artery (artery of Adamkiewicz) is the dominant artery supplying the thoracolumbar region of the spinal cord.

Spinal cord tracts

The spinal cord contains ascending and descending tracts. The primary ascending tracts use three neurons to relay peripheral sensory information to the brain. In contrast, the descending tracts transmit motor impulses from the cerebral cortex throughout the body. For more information on lesions of the spinal cord tracts see the incomplete spinal cord syndromes article.

Ascending tracts [1][2]

Tract Function First-order neuron Synapses Second-order neuron Trajectory
Conscious sensation
Spinothalamic tract
  • Ipsilateral spinal cord in the substantia gelatinosa or nucleus proprius one to two segments below or above the 1st neuron
Dorsal column
  • Axon decussates to the contralateral side within the medulla.
  • Ascends contralaterally in the medial lemniscus
Unconscious sensation [3]
Spinocerebellar tract
Spinoolivary tract
  • Axons arising from the posterior horn of the spinal cord to the olivary nuclei of the medulla

Descending tracts [1][2]

Tract Function First-order neuron Synapses Second-order neuron Trajectory

Corticospinal tract

(part of the pyramidal tract) [4]

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

Legs are represented Laterally in the Lateral spinothalamic and Lateral corticospinal tract.

Remember that fasciculus graciLis carries sensory information from the Lower limbs, and fasciculus cUneatus transmits information from the Upper limbs.

Spinal reflexes

Reflex innervation of skeletal muscles [1][4][2]

  • Definition: Reflexes are involuntary, consistent reactions to specific stimuli.
  • Reflex arc

Muscle proprioceptors

Muscle spindle Golgi tendon organ
Structure
  • Intrafusal muscle fibers (modified skeletal muscle fibers) and nerve fibers surrounded by a capsule
  • Collagen strands and nerve fibers surrounded by a capsule
Location
  • Within skeletal muscle; parallel to extrafusal muscle fibers
  • Within tendons of skeletal muscle

Perception

  • Muscle length and change of muscle length (e.g., muscle stretching)
  • Muscle tension and muscle force
Reflex arc
Purpose
  • Causes stretched muscle to contract
  • Controls tendon tension of activated muscle
  • Causes muscle relaxation of agonist muscle before tendon damage occurs
Example
  • Tapping of reflex hammer on tendon
  • Sudden relaxation when a weightlifter uses extremely heavy weights

Clinically important reflexes

Deep tendon (stretch) reflexes are ipsilateral and only require one synapse (monosynaptic).

Reflex Muscle tested Spinal level
Biceps reflex Biceps C5–C6
Brachioradialis reflex Brachioradialis 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–S5

Think of this poem to remember which nerve roots correspond to which reflexes:
S1–S2
Buckle my shoe (ankle reflex),
L2–L4
Kick the door (knee reflex),
C5–C6
Pick up sticks (biceps and brachioradialis reflexes),
C7–C8
Lay them straight (triceps reflex),
L1–L2
Testicle move (cremasteric reflex),
S3–S5
Winking drive (anal wink reflex).

Dermatomes

Dermatomes are areas of cutaneous innervation that are supplied by a single spinal nerve or cord level (with the exception of cranial nerves V1–3). While there is some overlap in the general distribution of dermatomes, each individual spinal cord level supplies a distinct zone of skin. Testing touch or sensory perception in these areas can be used to localize lesions of the spinal cord to a specific cord level or spinal nerve. [5][2]

Head dermatomes

Dermatome Distribution

Cranial nerve V1

Cranial nerve V2
Cranial nerve V3
C1–C4

Body dermatomes

Dermatome Distribution
C2

Posterior half of skull

C3

Upper neck, directly inferior to the mandible

C4

Above the clavicle, including upper shoulders and trapezius

C5

Below the clavicle, including lower shoulders bilaterally and medial biceps

C6

Lateral aspect of forearms and thumbs

C7

Middle triceps and mid-palm, including index and middle fingers

C8

Medial triceps and medial palm, including ring and small fingers

T4

Level of the nipples

T7

Xiphoid process

T10

Umbilicus

L1

Inguinal ligament

L4

Patella and large toe

L5

Dorsal web space between first and second toes

S2–S4

Penile and anal regions

Referred pain in dermatomes

Referred pain is the perception of sensory information in a location that differs from the actual site of the stimulus. The pain is sensed from a particular spinal cord level but the CNS interprets this pain as coming from another location that is innervated by the same spinal cord level.

Spinal cord level Organ Area of referred pain
C3–C5

Diaphragm via phrenic nerve

Shoulder

T10

Appendix via greater splanchnic nerve

Umbilical region

T1–T4

Heart

Upper thorax and left arm

T5–T9

Foregut (organs supplied by the celiac trunk)

Lower thorax and epigastrium

T10–T12

Midgut (organs supplied by the superior mesenteric artery)

Umbilical region

L1–L2

Hindgut (organs supplied by the inferior mesenteric artery)

Hypogastrium and groin

T12

Kidneys and ureters

Lateral flanks and pubic region


To remember the segments that innervate the diaphragm, think of “C3–C5 keep the diaphragm alive.”

Embryology of the spinal cord

Neurulation begins during the third week of gestation, during which the neural plate folds over at the midline and fuses to form the neural tube. The neural tube then forms both the spinal cord and brain.

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
  • Grow dorsally toward the midline; fuse together to form the neural tube
  • Occurs at the end of the 3rd week (21st day) of gestation
  • Located on the free edges of the neural plate bilaterally during the elongation and folding process
Neural crest cells
Neural tube
  • Gives rise to the brain and the spinal cord
    • Alar plate gives rise to sensory neurons.
    • Basal plate gives rise to motor neurons.

See also “Neurulation” in embryogenesis for more information.

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

Clinical significance

Complete spinal cord injuries

Incomplete spinal cord injuries

Other spinal cord pathologies