What is the Extrapyramidal System?
The extrapyramidal system comprises six specific descending motor pathways originating from brainstem nuclei—reticulospinal (medial and lateral), vestibulospinal (medial and lateral), rubrospinal (lateral only), and tectospinal (medial only)—that control involuntary motor functions including posture, muscle tone, and automatic movements, distinct from the pyramidal tracts that mediate voluntary movement. 1
Anatomical Definition
The extrapyramidal system consists of specific brainstem-to-spinal cord pathways that organize four categories of motor synergies:
- Oculofacial and oculocephalic movements (eye and head coordination) 1
- Faciorespiratory patterns (facial and breathing coordination) 1
- Axial-appendicular movements (trunk and limb coordination) 1
- Plurisegmental motor patterns (multi-level spinal coordination) 1
These pathways provide involuntary motor control to muscles of the head, neck, and limbs, functioning separately from the pyramidal (corticospinal) tracts. 2
Key Neural Structures Involved
Basal Ganglia Components
- The basal ganglia (caudate nucleus, putamen, nucleus accumbens, and globus pallidus) form the major center of the extrapyramidal motor system and receive inputs from all cortical areas. 3
- These structures project through the thalamus to frontal lobe areas (prefrontal, premotor, and supplementary motor areas) concerned with motor planning. 3
- The basal ganglia provide regulatory influence on the cortex for both automatic and voluntary motor responses, predict future events, reinforce wanted behavior, and suppress unwanted behavior. 3
Thalamic Relay Function
- The thalamus serves as a relay center for both sensory and motor mechanisms, with 50-60 nuclei that project to well-defined cortical areas in a contralateral, topographically organized manner. 3
Clinical Distinction from Pyramidal System
Pyramidal Tract Lesions Produce:
- Increased deep tendon reflexes, spasticity, positive Babinski sign, and weakness predominantly affecting extensors in upper limbs and flexors in lower limbs 4
- Hyperreflexia and abnormal plantar reflexes 4
- True paralysis with upper motor neuron signs 4
Extrapyramidal Lesions Produce:
- Movement disorders without true paralysis, often with normal or slightly reduced reflexes 4
- Basal ganglia dysfunction manifesting as tremor, rigidity, and bradykinesia (as seen in Parkinson's disease) 4
- Involuntary movements, dystonia, dyskinesia, and abnormal postures 2
Important Clinical Caveat
The term "extrapyramidal symptoms" (EPS) in psychiatry specifically refers to drug-induced movement disorders from dopamine receptor blockade, not the anatomical extrapyramidal system itself. 5, 6 These medication-induced symptoms include acute dystonia, drug-induced parkinsonism, akathisia, and tardive dyskinesia, which result from antipsychotic medications blocking dopamine receptors in the nigrostriatal pathways. 5, 6
Common Pitfall to Avoid
Do not conflate the anatomical extrapyramidal system (specific brainstem motor pathways) with "extrapyramidal symptoms" (medication side effects). 7 The latter term has been criticized for lacking clinical, anatomical, and physiologic specificity, and movement disorder specialists increasingly recommend using specific phenomenologic descriptors (e.g., "parkinsonism," "dystonia," "akathisia") rather than the vague acronym "EPS." 7