Motor Tracts
Pyramidal Tracts (Skilled Voluntary Movement)
Corticospinal Tract
- Forms medullary pyramid.
- Influences alpha motor neurons.
- Upper Motor Neuron (UMN):
- Pyramidal + Extrapyramidal tracts.
- Lower Motor Neuron (LMN):
- Alpha motor neuron in spinal cord
- Final Common Pathway
- Origin:
- Motor cortex (Brodmann area 4) from Betz cells of cerebral cotex
- Primary motor area (area 4): ~30%
- Premotor area (area 6): ~30%
- Primary sensory area (area 3,1,2): ~40%
- Pathway:
- Cerebrum → Diencephalon → Internal capsule (anterior 2/3 of posterior limb) → Midbrain → Pons → Medulla oblongata
- Pyramidal Decussation at lower medulla
- Lesion:
- C/L Hemiplegia/paralysis
Pyramidal Decussation
- Occurs in the lower medulla.
- 90–95% fibers cross
- enter lateral corticospinal tract.
- 5–10% fibers remain uncrossed
- enter anterior corticospinal tract.
- Then decussates at the spinal cord level.
- 100% of pyramidal tract fibers eventually decussate.
Functions
- Responsible for voluntary movements.
Lateral corticospinal tract
- Crossed fibers (85–90%).
- Controls limb muscles.
Anterior corticospinal tract
- Uncrossed initially.
- Controls axial/midline muscles.
Extrapyramidal Tracts
- Guy performing Ridiculous (Reticular) Balancing (Vestibular) Techniques (Tectorial) → got EPS
- hurt himself → Blood came (Red → Rubrospinal)
- VEstibulospinal -
- Estensors
- Rubrospinal
- Rub - Flexors
Tract | Origin | Function | Decussation |
Rubrospinal | Red Nucleus | Facilitates flexors, inhibits extensors Only EPS which is lateral | Ventral Tegmental Decussation Rubro → Red → Blood → Flex the injury |
Pontine & Medullary Reticulospinal | From pons/ medulla nuclei. | Maintain Posture - Antigravity muscles - Paravertebral muscles - Proximal extensors Pontine: Excitatory Medullary: Inhibitory | Ridiculous Posture |
Vestibulospinal | Vestibular nuclei | Lateral: Maintain Posture Medial: Control Neck Facilitates extensors, inhibits flexors | No decussation (ipsilateral) Vestibulo → Balance → Extension |
Tectospinal | Superior colliculus | Coordinates head/neck/eye movements to visual stimuli | Dorsal Tegmental Decussation Techniques involves head neck eye coordination, also back (Dorsal Tegmental) |
- Dorsal Tegmental decussation
- Tectospinal Tract
- Ventral Tegmental decussation
- Rubrospinal Tract
Spinal Cord – Ventral Horn Nuclei
- Nucleus dorsalisis
- aka Clarke's column
- Lateral horn
- Not present in all the segments
Nucleus | Location/Function |
Anterior/Medial nucleus | Controls axial/midline muscles |
Intermediate nucleus | Cervical only: 1. Phrenic nucleus (C3–C5): Diaphragm 2. Spinal accessory nucleus (C1–C5): SCM + Trapezius |
Lateral nucleus | Controls limb muscles |
Intermedio-lateral nucleus | Autonomic: 1. Sympathetic: T1–L2 2. Parasympathetic: S2–S4 |
UMN Lesion vs. LMN Lesion
Feature | UMN Lesion | LMN Lesion |
Atrophy | Disuse atrophy (late), generally not seen | Severe |
Fasciculations | Not present | Present |
Tone | Increased (Hypertonia): spasticity, rigidity | Severely decreased (Hypotonia) |
Stretch Reflex | Hyperactive | Hypoactive or absent |
Muscles Affected | Broad group | Segmental, single muscle; distal dominant |
Babinski sign | Present (dorsiflexion big toe, fanning other toes) | Absent |
UMN lesion assesment
Chaddock sign
Gordon sign
Spasticity vs. Rigidity
Feature | Spasticity (Pyramidal Lesion) | Rigidity (Extrapyramidal Lesion) |
Weakness | Present | Not primary feature |
Muscles | Only anti-gravity (arm flexors, leg extensors) | Both flexor & extensor groups |
Tone Pattern | High in initial movement | High throughout movement range |
Special Sign | Clasp-knife plasticity (initial resistance → release) | Cogwheel (towards the end → tremor + hypertonia - s/o Parkinsonism), Lead pipe - throughout the movement - Neuroleptic malignant syndrome |
Velocity | Velocity Dependent (worse with fast movement) | Independent |
- Clasp Knife Spasticity
- Initial ↑ resistance → Stretch reflex
- Later ↓ resistance → Inverse stretch reflex
- Ankle Clonus
- Trigger: Sudden, sustained stretch
- Mechanism: Continuous cycle of stretch reflex and inverse stretch reflex.
Decerebrate vs. Decorticate Rigidity
Decerebrate vs Decorticate
Decorticate Rigidity
- Mnemonic: Decor → Red ribbon from up, Brown ribbon from down
- Mnemonic: Corticate → Kuuttipidich irikkunnu → Red blood kandapo → hemorrhage → Rubral nucleus/Red nucleus → In midbrain → (Red → Mid) → Midbrain affected
- Cut: Upper midbrain (above red nucleus).
- Lower limb extension
- Pathophysiology:
- Decortication (Cortex removed) → Cortical inhibition lost
- Pontine reticulospinal tract.
- Head Extension
- LL Extension
- Rubrospinal tract is intact (and unopposed)(lesion above red nucleus)
- Upper limb flexion → Decorticate rigidity
- Cause:
- Thrombosis or hemorrhage of internal capsule → Decorticate rigidity
Decerebrate Rigidity / Extensor rigidity / γ rigidity
- Cerebrum prevents all extensor muscle always.
- When it is lost → causes
- Extensor plantar → Babinsky
- Extension of UL, Head, LL → Extensor rigidity / γ rigidity
- It is characterized by rigid extension of the limbs and trunk, not all muscles of body
- Pathophysiology:
- Cut: Upper pons (between superior/inferior colliculi).
- Mechanism:
Cortical inhibition lost
↓
Dorsal sensory roots activated
↓
Pontine Reticulospinal tract overactive (++)
↓
Gamma (γ)-motor neurons overactive (++)
↓
Increased extensor muscle tone
↓
Decerebrate rigidity / γ rigidity
- Cause:
- Uncal herniation (due to space occupying lesion)
Decerebellate rigidity / α rigidity
- Mechanism:
- Anterior cerebellum removed (++)
- → α Motor neurons overactive
- → Exaggeration of extensor activity
Uncal Herniation & Kernohan’s Notch & Hutchinson pupil
- Uncal herniation:
- Herniation of uncus (medial temporal lobe)
- Progression:
- Affect Parasympathetic fibers of ipsilateral cranial nerve III
- Called "ipsilateral blown pupil" or "false localizing sign"
- C/L corticospinal tract/crus cerebri compressed
- I/L UMN palsy
- Kernohan’s notch phenomenon:
- False localizing sign
- Hemiparesis appears ipsilateral to lesion
- instead of expected contralateral
Key signs:
- Ipsilateral pupil dilatation → Hutchinson pupil
- Ipsilateral UMN palsy
Spinalcord Lesions
Romberg sign +
Werdnig hoffman disease
Brown-Sequard Syndrome
(Hemisection of Spinal Cord)
At the Level of Lesion
- Ipsilateral
- Loss of all sensory and motor
- Flaccid paralysis (LMN).
- Loss of all sensory modalities
Below the Level of Lesion
- Ipsilateral
- Spastic paraparesis (due to corticospinal tract damage).
- Loss of vibration and position sense (due to posterior column damage).
- Loss of discriminative touch
- Contralateral
- Loss of pain and temperature
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