Chapter 15: Disorders of Motor Function

The study of motor function disorders begins with an overview of the complex hierarchical control system, involving the spinal cord's fundamental reflex circuits, brain stem pathways, and the supreme regulatory influence of the cerebral motor cortex, basal ganglia, and cerebellum. Key to this control is the motor unit, comprised of the lower motor neuron (LMN) and the muscle fibers it innervates, which are themselves modulated by upper motor neurons (UMNs) descending from the central nervous system (CNS). Comprehensive assessment requires evaluating posture, muscle characteristics (strength, size, tone, reflexes), and coordination, with specific patterns of paresis, paralysis, or tone abnormalities (hypotonia, flaccidity, rigidity, spasticity) pointing toward UMN or LMN pathology. This chapter details various specific pathologies, categorized by location, including genetic disorders of the muscle tissue like Muscular Dystrophy (e.g., Duchenne Muscular Dystrophy), and disorders of the neuromuscular junction (NMJ), notably Myasthenia Gravis, an autoimmune disease characterized by an antibody-mediated attack on acetylcholine receptors. Peripheral neuropathies, resulting from segmental demyelination or axonal degeneration, range from localized mononeuropathies like Carpal Tunnel Syndrome to generalized polyneuropathies such as Guillain–Barré Syndrome, often manifesting with symmetric distal sensory and motor deficits. Furthermore, dysfunctions originating in the basal ganglia, exemplified by the hypokinetic movement disorder Parkinson Disease (PD), result from the progressive destruction of nigrostriatal dopamine neurons, causing resting tremor, rigidity, and bradykinesia. Cerebellar disorders, in contrast, lead to issues of coordination and timing, such as ataxia and intention tremor. Finally, CNS motor diseases include the devastating mixed UMN and LMN destruction seen in Amyotrophic Lateral Sclerosis (ALS), the progressive demyelination of the CNS characteristic of Multiple Sclerosis (MS), and the varied outcomes of Spinal Cord Injury (SCI), which can lead to spinal shock, distinct cord syndromes (like Brown-Séquard), and severe autonomic instability such as Autonomic Dysreflexia in high-level lesions.