Somatotopy, Major Spinal Tracts & Lesion Patterns – PT 7311 Review

Vocabulary flashcards covering key neuroanatomical structures, pathways, and clinical syndromes from the PT 7311 lecture notes.

Somatotopic Organization

Point-to-point mapping within the CNS in which specific body regions project to specific, orderly locations on neural structures (e.g., cortex, tracts).

Motor Homunculus

Somatotopic map of voluntary movement represented along the pre-central gyrus (frontal lobe); larger cortical areas correspond to body parts requiring fine motor control.

Sensory Homunculus

Somatotopic map of bodily sensation laid out along the post-central gyrus (parietal lobe); larger cortical areas represent regions with dense sensory receptors.

Upper Motor Neuron (UMN)

Motor neuron whose cell body lies in the cerebral cortex or brainstem and whose axon descends within CNS tracts (e.g., corticospinal) to synapse on lower motor neurons; lesions produce spasticity, weakness, hyperreflexia.

Lower Motor Neuron (LMN)

Motor neuron in the ventral horn of the spinal cord or cranial nerve nuclei whose axon exits the CNS to innervate skeletal muscle; lesions produce flaccid weakness, atrophy, areflexia.

Monosynaptic Stretch Reflex (MSR) Arc

Simple reflex circuit in which a muscle spindle afferent synapses directly on its corresponding α-motor neuron, producing a rapid contraction (e.g., patellar reflex).

Spinal Cord Gray Matter

Central ‘H-shaped’ region containing neuronal cell bodies, dendrites, synapses; organized into dorsal (sensory), ventral (motor), and lateral horns.

Spinal Cord White Matter

Peripheral region of the cord composed of myelinated ascending and descending axons grouped into funiculi (posterior, lateral, anterior).

Fasciculus Gracilis (FG)

Medial division of dorsal columns carrying proprioception, vibration, discriminative touch from lower trunk and lower extremities (T7 and below) to medulla.

Fasciculus Cuneatus (FC)

Lateral division of dorsal columns conveying proprioception, vibration, discriminative touch from upper trunk and upper extremities (T6 and above) to medulla.

Dorsal Columns

Collective term for fasciculus gracilis & cuneatus; ascending ipsilateral sensory tract that crosses in the medulla and terminates in thalamus → post-central gyrus.

Tract (CNS)

Bundle of axons carrying a single type of information between two specific CNS regions (e.g., corticospinal tract, spinothalamic tract).

Funiculus

Large region of spinal white matter composed of multiple tracts/fasciculi; spinal cord contains posterior, lateral, and anterior funiculi.

Posterior Funiculus

Dorsal white-matter column containing the dorsal columns (FG & FC).

Lateral Funiculus

White-matter column housing the lateral corticospinal and spinothalamic tracts among others.

Anterior Funiculus

Ventral white-matter column containing anterior corticospinal tract and parts of STT.

Corticospinal Tract (CST)

Descending motor pathway from pre-central gyrus to spinal cord; mediates voluntary movement.

Lateral Corticospinal Tract (LCST)

85 % of CST fibers that decussate in medullary pyramids; controls distal limb movement; lesions cause ipsilateral UMN signs below the level of injury.

Anterior Corticospinal Tract (ACST)

15 % of CST fibers that remain uncrossed in medulla; cross within spinal cord to control axial/trunk muscles.

Pyramidal Decussation

Crossing of ~85 % corticospinal fibers at caudal medulla forming the LCST.

Spinothalamic Tract (STT)

Ascending contralateral pathway conveying pain, temperature, and crude/light touch; fibers cross within one–two spinal segments via anterior white commissure.

Decussation (General)

Midline crossing of neural fibers from one side of the CNS to the other.

Dorsal Root Ganglion (DRG)

Peripheral sensory neuron cell bodies whose central processes enter spinal cord to form dorsal roots.

Internal Capsule

White-matter structure through which corticospinal (and other) fibers descend between thalamus & basal ganglia.

Pyramids (Medulla)

Longitudinal swellings on ventral medulla containing corticospinal fibers just before decussation.

Complete Spinal Cord Transection

Full cross-sectional lesion causing bilateral loss of all motor & sensory below the level; UMN signs below, LMN signs at level, absent MSR at level but hyperreflexic below after shock resolves.

Brown-Séquard Syndrome

Hemisection of spinal cord: ipsilateral UMN weakness & dorsal column loss below lesion, ipsilateral LMN signs at level, contralateral pain/temperature loss below lesion.

Amyotrophic Lateral Sclerosis (ALS)

Neurodegenerative disease causing progressive death of UMNs in cortex and LMNs in ventral horn, producing mixed UMN hyperreflexia and LMN atrophy with typically preserved sensation.

Somatosensory Modalities – Dorsal Columns

Proprioception, vibration sense, and discriminative (fine) touch.

Somatosensory Modalities – Spinothalamic

Pain, temperature, and crude/light touch.

Posterior Column–Medial Lemniscal Pathway

Alternate name for dorsal column system emphasizing medial lemniscus segment in brainstem after decussation.

Mononeuronal Chain (4-Neuron Chain)

Typical arrangement of dorsal column and STT pathways: peripheral receptor → DRG neuron → spinal/medullary relay → thalamic relay → cerebral cortex neuron.

Ipsilateral vs. Contralateral

Ipsilateral: same side of body. Contralateral: opposite side; relevant to understanding deficits after tract lesions.

Hyperreflexia

Exaggerated stretch reflexes, characteristic of UMN lesions.

Areflexia/Hyporeflexia

Absent or diminished reflexes, typical of LMN lesions or peripheral nerve damage.