Harvey's Garden

Neuroloy physioloy

4 min read

Spinal cord

Cross-sectional anatomy

Drawing 2025-07-26 09.26.44.excalidraw

⚠ Switch to EXCALIDRAW VIEW in the MORE OPTIONS menu of this document. ⚠ You can decompress Drawing data with the command palette: ‘Decompress current Excalidraw file’. For more info check in plugin settings under ‘Saving’

Excalidraw Data

Text Elements

Spinothalamic tract

Dorsal column

Corticospinal tract

Embedded Files

0f69b6f90704f2e1846fca80217e793a8633730b: Pasted Image 20250726092659_518.png

Link to original

  • Gray Matter (Neuron Bodies):
    • Dorsal (Posterior) Horn: Sensory input (pain, temp, touch).
    • Ventral (Anterior) Horn: Motor output (Lower Motor Neurons - LMNs).
    • Lateral (Intermediate) Horn (T1-L2): Sympathetic nervous system.
  • White Matter (Axon Tracts):
    • Dorsal Columns: Ascending sensory (vibration, proprioception).
    • Lateral & Ventral Columns: Corticospinal tract (motor), Spinothalamic tract (pain/temp).
    • Anterior White Commissure: Decussation site for spinothalamic tract.

Mnemonic

  • If it’s in the back (dorsal), think SENSORY.
  • If it’s in the front (ventral), think MOTOR.

Ascending (Sensory) Tracts

  • Dorsal Column-Medial Lemniscus (DCML)
    • Function: Fine touch, vibration, proprioception, pressure.
    • Location: Posterior (dorsal) funiculus.
      • Fasciculus Gracilis: Medial; info from lower body (legs).
      • Fasciculus Cuneatus: Lateral; info from upper body (arms).
    • Decussation: Medulla (as internal arcuate fibers), then ascends as medial lemniscus.
    • Path: 1° neuron (DRG) → ascends ipsilaterally in dorsal column → synapses in nucleus gracilis/cuneatus (medulla) → 2° neuron decussates → ascends to VPL of thalamus → 3° neuron to somatosensory cortex.
    • Somatotopy: Fibers are added laterally. Sacral/leg fibers are most medial (Fasciculus Gracilis), and cervical/arm fibers are most lateral (Fasciculus Cuneatus). “Legs are in the middle.” t
  • Spinothalamic Tract (Anterolateral System)
    • Function: Pain, temperature, crude touch.
    • Location: Anterolateral funiculus.
    • Decussation: Anterior white commissure at/near the level of entry into the spinal cord.
      1. Entry & Lissauer’s Tract: The primary pain/temperature neurons (1st order) enter the spinal cord and travel within the Tract of Lissauer, where they can ascend or descend 1-2 spinal segments before synapsing.
      2. Synapse: They then synapse on the second-order neuron in the dorsal horn (substantia gelatinosa).
      3. Decussation: The axon of this second-order neuron is what immediately crosses to the contralateral side via the anterior white commissure to form the spinothalamic tract.
    • Path: 1° neuron (DRG) → synapses in dorsal horn → 2° neuron decussates → ascends contralaterally → synapses in VPL of thalamus → 3° neuron to somatosensory cortex.
    • Somatotopy: Cervical/arm fibers are most medial, and sacral/leg fibers are most lateral. “Pain in the ass is on the outside.”

Descending (Motor) Tracts

  • Lateral Corticospinal Tract
    • Function: Voluntary motor control of limbs (distal muscles).
    • Location: Lateral funiculus.
    • Decussation: Pyramidal decussation in the caudal medulla.
    • Path: Upper Motor Neuron (UMN) in primary motor cortex → descends ipsilaterally through internal capsule → decussates in medulla → descends contralaterally in lateral corticospinal tract → synapses on LMN in ventral horn.
      • Somatotopy: Cervical/arm fibers are most medial, and sacral/leg fibers are most lateral.
  • Hypothalamospinal Tract (Sympathetic)
    • Function: Provides sympathetic innervation to the body.
    • Location: Lateral funiculus.
    • Decussation: None (descends ipsilaterally).
    • Path: Hypothalamus → descends through brainstem and lateral funiculus of spinal cord → synapses on preganglionic neurons in the lateral horn (T1-L2).
    • Clinical Pearl: A lesion anywhere along this path (e.g., lateral medulla, cervical spinal cord) causes ipsilateral Horner’s Syndrome (Ptosis, Miosis, Anhidrosis).
TractOrganizationReason
Dorsal ColumnLegs Medial, Arms LateralAscends before crossing; new fibers are added laterally.
SpinothalamicArms Medial, Legs LateralCrosses then ascends; existing fibers are pushed laterally.
CorticospinalArms Medial, Legs LateralFibers exiting soonest (arms) are medial; long-traveling fibers (legs) are lateral. (“exit” is on the inside, not the outside.)

Nerve roots

  • Anterior roots carry motor (efferent) fibers that control skeletal and smooth muscle.
  • Dorsal roots carry sensor (afferent) fibers that transmit somatosensory information.

Muscle proprioceptors

  • Muscle Stretch Receptors (Muscle Spindles)
    • Primary function: to regulate muscle tension and prevent damage from excessive force.
    • Pathway (Myotatic Reflex)
      1. Senses an ↑ in muscle length and the speed of the stretch.
      2. The signal travels via Type Ia/II sensory axons to the dorsal root ganglion (DRG).
      3. In the spinal cord, there is direct activation of the α-motor neuron (for the agonist) and an inhibitory interneuron (for the antagonist).
      4. This leads to the simultaneous contraction of the agonist muscle and the inhibition of the antagonist muscle, which prevents overstretching.
    • Location/Innervation
      • Found in the body of the muscle.
      • Innervated by Type Ia and II sensory axons.
    • Activated By
      • ↑ muscle stretch.
      • This mechanism is responsible for Deep Tendon Reflexes (DTRs).
    • Example: The Knee-Jerk Reflex (deep tendon reflex). Tapping the patellar tendon stretches the quadriceps, causing it to contract and the leg to extend.
  • Golgi Tendon Organ (GTO)
    • Primary Function: to resist changes in muscle length; maintains muscle tone and posture.
    • Pathway (Inverse Myotatic Reflex)
      1. Senses an ↑ in muscle tension.
      2. The signal travels via Type Ib sensory axons to the dorsal root ganglion (DRG).
      3. In the spinal cord, it activates an inhibitory interneuron.
      4. This interneuron leads to the inhibition of the agonist muscle, causing it to relax and producing a reduction in tension.
    • Location/Innervation
      • Found in tendons.
      • Innervated by Type Ib sensory axons.
    • Activated By
      • ↑ muscle tension.
      • This is a protective reflex to prevent damage from excessive force.
    • Example: A weightlifter suddenly dropping a very heavy weight when the tension becomes excessive, overriding the voluntary command to lift.

Homunculus

Graph View

Backlinks