PT 7311 – Neuroanatomy Fundamentals

Functional Organization of the Nervous System

• 2 broad divisions

  • Central Nervous System (CNS)

  • Brain + Spinal Cord

  • Contains sensory, integrative & motor neurons, plus glial cells

  • Peripheral Nervous System (PNS)

  • Ganglia = neuronal cell bodies outside the CNS

  • Nerves = axons outside the CNS

  • Sensory receptors & glial cells

• Motor systems

  • Somatic (voluntary) → skeletal muscle

  • Visceral / Autonomic (involuntary) → cardiac mm, smooth mm, glands

• Sensory systems

  • Somatosensory → pain, temp, touch, proprioception

  • Visceral sensory → information from internal organs

  • Special senses → audition, taste, vestibular, vision, olfaction

Gross Brain Anatomy

• Cerebral hemispheres (R & L) separated by the longitudinal cerebral fissure

• Cortex (≈ 1/8 inch) = superficial gray matter (neuronal cell bodies)

  • Highly convoluted → gyri (ridges) & sulci (grooves); deep sulci called fissures

Five Lobes / Major Gyri

1. Frontal lobe

   • Executive functions, voluntary motor (body, head, eyes, speech)

   • Pre-central gyrus = primary motor cortex → contains Upper Motor Neurons (UMNs)

   • Initiates contralateral voluntary movement

2. Parietal lobe

   • Sensory integration, visual-spatial processing

   • Post-central gyrus = primary somatosensory cortex; receives contralateral input from thalamus

3. Occipital lobe

   • Visual cortex & processing

4. Temporal lobe

   • Language (Wernicke’s area), memory, auditory & visual association

5. Limbic lobe (cingulate gyrus, parahippocampal gyrus, etc.)

   • Drives for survival, emotion, memory

Sub-cortical Gray Structures

• Basal nuclei (basal ganglia)

  • Movement selection, learned motor patterns

• Hippocampus & Amygdala (limbic system)

  • Memory consolidation (hippocampus) & emotion (amygdala)

• Thalamus

  • Bilateral relay station atop midbrain (NOT cerebrum)

  • All sensory modalities except smell synapse here before cortex

  • Functions as sensory “filter” & switchboard

Brainstem & Cerebellum

• Brainstem (anterior to cerebellum)

  • Midbrain → Pons → Medulla

  • Contains cranial nerves III–XII & their nuclei

• Cerebellum (“little brain”)

  • 2 hemispheres + vermis (midline)

  • Modulates limb movements, trunk posture & motor learning

Neuron Anatomy & Cell Types

• Cell body (soma) – protein synthesis, integration

• Dendrites – receive input; 0 to 10^4 per neuron

• Axon – conducts APs ("axons away")

• Synaptic endings – neurotransmitter release

Morphologic Types

• Multipolar motor neuron (many dendrites, one axon)

  • UMN soma in pre-central gyrus → axon descends to spinal cord

  • LMN soma in ventral horn → axon exits to muscle (releases ACh)

• Pseudo-unipolar sensory neuron

  • Cell body in Dorsal Root Ganglion (DRG)

  • Single process splits into:

  • Peripheral process → from receptor to DRG (acts like dendrite)

  • Central process → DRG to spinal cord (acts like axon)

Gray vs White Matter Terminology

• Gray matter = neuronal cell bodies (processing)

  • Nucleus (CNS), Cortex (surface layer), Ganglion (PNS)

• White matter = myelinated axons (communication)

  • Tract = bundle connecting two CNS regions (e.g.
    corticospinal tract)

  • Fasciculus = bundle travelling together (e.g. fasciculus cuneatus)

  • Funiculus = large bundle within spinal cord containing several fasciculi

  • Nerve = bundle of PNS axons

Spinal Cord Overview

• Located in vertebral canal; begins at foramen magnum (spinomedullary junction)

• 31 spinal segments / pairs of mixed spinal nerves:

  • 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

• Ends as conus medullaris ≈ vertebral level L2 → below this the cauda equina (lumbosacral nerve roots)

Internal SC Anatomy

• Central gray matter shaped like a butterfly

  • Dorsal horn (sensory), Ventral horn (LMNs)

  • Each horn services the ipsilateral body

• White matter surrounds gray; organized into ascending (sensory) & descending (motor) tracts

• Roots & nerves

  • Dorsal root = central processes of DRG neurons (sensory)

  • Ventral root = LMN axons (motor)

  • Roots merge to form a mixed spinal nerve

Lesion Principles

• Complete gray-matter lesion at one segment → sensory + LMN loss at that level only

• Complete white-matter lesion at a segment → loss of all ascending & descending functions below that level

Ascending Somatosensory Pathway ("4-Neuron Chain")

1. DRG neuron: receptor → peripheral process → DRG → central process into SC

2. Second-order neuron:

   • Dorsal horn (pain, temperature, light touch) OR

   • Medulla (gracile/cuneate nuclei: discriminative touch, vibration, conscious proprioception)

3. Third-order neuron: Thalamus (axons have crossed midline)

4. Fourth-order neuron: Post-central gyrus (primary somatosensory cortex)

Descending Motor Pathway (Corticospinal Tract – "Big Three")

• Only two neurons needed to activate skeletal muscle

  1. Upper Motor Neuron (UMN)

  • Soma: pre-central gyrus (frontal lobe)

  • Axon: passes through internal capsule → brainstem → decussates → descends in lateral corticospinal tract → synapses in ventral horn

  1. Lower Motor Neuron (LMN)

  • Soma: ventral horn

  • Axon: exits via ventral root → peripheral nerve → neuromuscular junction

UMNs modulate LMNs through constant excitation and inhibition.

Monosynaptic Stretch Reflex (MSR)

• Simplest spinal reflex; involuntary & stereotyped

• Components that must be intact:

  1. Muscle spindle (length detector)

  2. Sensory axon (peripheral + central processes)

  3. Intact LMN & its axon

Reflex Arc Sequence

1. Muscle stretch → spindle fires AP along peripheral process

2. DRG transmits AP to central process → enters spinal cord

3. Monosynaptic synapse onto ipsilateral LMN in ventral horn (same segment)

4. LMN fires → muscle contraction counteracts stretch (postural stability, smooth gait)

Example: Patellar (Knee-Jerk) Reflex

• Tests L2–L4 spinal levels (predominantly L4)

• Hammer taps patellar tendon ⇨ stretches quadriceps & spindles

• Sequence above elicits quad contraction → knee extension

• Clinical interpretations:

  • Normal or hyperactive reflex → intact reflex arc

  • Areflexia → lesion in any arc component (DRG, root, LMN, muscle, etc.)

  • Hyperreflexia + spasticity → UMN lesion (loss of inhibitory modulation)

Upper vs Lower Motor Neuron Syndromes

• UMN damage (cortex, tract, spinal white matter)…

  • Voluntary paralysis (loss of excitatory drive)

  • Spasticity & hyperreflexia (loss of inhibitory drive)

• LMN damage (ventral horn, root, peripheral nerve)…

  • Flaccid weakness, fasciculations, atrophy

  • Areflexia at the involved segment only

Key Numerical / Statistical Facts

• Cortex thickness ≈ 1/8\;\text{inch}

• DRG size: 10^4 \text{ – } 10^5 neurons, varies by level

• Spinal cord shorter than vertebral column; conus ends at \text{L}2

Practical & Clinical Connections

• MSR testing quickly distinguishes UMN vs LMN pathology

• Postural sway constantly triggers stretch reflexes to maintain stance

• Disruption of sensory tracts impairs reflex modulation & motor learning (cerebellum needs sensory feedback)

• Basal nuclei disorders (e.g. Parkinson’s) → impaired movement selection despite intact reflexes

• Thalamic or cortical lesions disrupt conscious perception but may leave spinal reflexes intact

Preparing for the “Big Three” Lecture

Slides 7, 12, 16, 19, 23, 27–30, 33–36, 38–40 emphasize:

• Complete tracing of corticospinal tract, dorsal column–medial lemniscal pathway & spinothalamic tract

• Cross-sections of spinal cord highlighting white vs gray columns

• Lesion localization based on sensory & motor findings

Exam will cover all material above; mastery of terminology, pathways, lesion consequences & clinical reflex testing is essential.