Neurobiology of Motor Systems, Locomotion, and Neuromotor Pathology

Comprehensive vocabulary flashcards covering motor control hierarchy, muscle histology, reflexes, central pattern generators, basal ganglia pathways, cortical planning areas, and associated motor pathologies.

Electromyography (EMG)

A technique that monitors and records the timing and strength of electrical activity produced by muscle contractions.

Sarcomere

The basic functional contractile unit of a muscle cell (myocyte), composed of regularly arranged, overlapping thick (myosin) and thin (actin) filaments.

Antagonistic Pairs

Muscles arranged such that the contraction of one muscle (e.g., a flexor) is coordinated with the relaxation of the opposing muscle (e.g., an extensor) to produce limb movement.

Cross-Bridge Cycle

The cyclic mechanism in which myosin heads bind actin filaments, bend to slide the filaments toward the center of the sarcomere using ATP, and ratchet muscle shortening in the presence of $Ca^{2+}$.

Extrafusal Muscle Fibers

Standard skeletal muscle fibers located outside the muscle spindle that generate mechanical force for movement, innervated strictly by alpha motor neurons.

Intrafusal Muscle Fibers

Specialized muscle fibers enclosed within the muscle spindle that are innervated by gamma motor neurons and sensory receptors to regulate spindle stretch sensitivity.

Primary Sensory Endings

Fast-adapting sensory endings that wrap around the central region of the muscle spindle, exhibiting maximum discharge early during muscle stretch to signal stretch velocity.

Secondary Sensory Endings

Slow-adapting (tonic) sensory receptors terminating near the spindle poles that respond to sustained static stretch to signal overall muscle length.

Golgi Tendon Organs

Proprioceptive sensory receptors interwoven into tendon collagen fibers that monitor muscle tension and force, acting to inhibit motor neurons when overloaded to prevent muscle injury.

Proprioception

The perception of body posture, limb position, and movement, integrated by the brain from muscle spindles, Golgi tendon organs, skin receptors, and the vestibular system.

Primary Motor Cortex (M1)

The precentral gyrus cortical region that initiates voluntary movement commands and encodes both individual muscle activation and coordinated spatial directions of movement.

Motor Homunculus

The somatotopic representation of the human body across the primary motor cortex, featuring disproportionately large areas dedicated to body parts capable of complex fine movements, such as the hands and lips.

Supplementary Motor Area (SMA)

A region of the nonprimary motor cortex vital for planning internally generated movements, coordinating actions between both cerebral hemispheres, and organizing learned sequential actions from memory.

Premotor Cortex

A nonprimary motor cortex region that selects and prepares movement patterns guided primarily by external environmental stimuli and cues.

Mirror Neurons

Specialized neurons located in area F5 of the premotor cortex that discharge both when an individual executes a specific goal-directed movement and when observing another individual perform that same action.

Open-Loop Mechanism

A pre-programmed, ballistic motor control system operating without external sensory feedback to maximize execution speed, exemplified primarily by the basal ganglia.

Closed-Loop Mechanism

A motor control mechanism that continuously integrates sensory feedback into the controlling center to detect errors and maximize movement accuracy, exemplified by the cerebellum.

Basal Ganglia

A subcortical forebrain group consisting of the striatum (caudate nucleus and putamen), globus pallidus, substantia nigra, and subthalamic nucleus that modulates movement initiation, suppresses unwanted movements, and automates procedural habits.

Direct Pathway

The basal ganglia 'go' circuit that facilitates and initiates voluntary movements by relieving the thalamus from tonic inhibitory control, thereby exciting the primary motor cortex.

Indirect Pathway

The basal ganglia 'no-go' circuit that suppresses competing, inappropriate, or unwanted motor movements by increasing inhibitory output to the thalamus.

Cerebellum

The hindbrain structure containing approximately 80% of the brain's neurons that coordinates ongoing motor activity, error correction, and motor learning, without directly initiating muscle movements.

Upper Motor Neurons (UMNs)

Motor neurons whose cell bodies reside in the cerebral cortex (such as M1) or brainstem, sending descending efferent axons to innervate lower motor neurons in the cranial nerve nuclei or spinal cord.

Lower Motor Neurons (LMNs)

Myelinated efferent motor neurons originating in the brainstem or the anterior horn of the spinal cord that directly project axon terminals to skeletal muscle fibers.

Motor Plan (Motor Program)

An established set of neural muscle commands formulated prior to action onset (e.g., postural bracing of the calves prior to pulling an arm lever).

Central Pattern Generator (CPG)

Autonomous neural circuits located in the spinal cord and brainstem capable of producing rhythmic, stereotypic motor outputs (such as walking, breathing, or swallowing) independently of sensory input or descending brain signals.

Motor Unit

The functional motor entity composed of a single lower motor neuron and all individual muscle fibers innervated by its terminal axon branches.

Innervation Ratio

The ratio between a single motor neuron axon and the number of muscle fibers it innervates; low ratios enable delicate, precise control, while high ratios produce large contractile force.

Pyramidal Tracts

Descending motor pathways originating in the cerebral cortex and passing through the medullary pyramids to mediate voluntary contralateral control of the musculature of the body and face.

Lateral Corticospinal Tract

The division of the pyramidal tract comprising roughly 90% of its fibers, decussating at the medulla to innervate motor neurons controlling distal limbs and digits.

Anterior Corticospinal Tract

The division of the pyramidal tract that descends uncrossed until decussating at the spinal cord level, primarily controlling axial muscles of the neck, shoulder, and trunk.

Extrapyramidal System

Descending motor pathways that bypass the medullary pyramids, responsible for indirect modulation, postural stabilization, and regulation of mostly involuntary movement.

Neuromuscular Junction (NMJ)

The specialized, highly reliable chemical synapse formed between the terminal button of an alpha motor neuron and the motor endplate of a muscle fiber.

Sarcoplasmic Reticulum

An internal membrane system within muscle fibers that stores and releases $Ca^{2+}$ into the sarcoplasm when triggered by endplate action potentials traveling down T-tubules.

Cerebral Palsy

A non-progressive physical disorder characterized by impaired movement and posture resulting from injury or abnormal development in brain motor regions before, during, or shortly after birth.

Multiple Sclerosis (MS)

An autoimmune disorder in which the immune system attacks and damages the myelin sheath of CNS neurons, producing variable motor instability and weakness.

Myasthenia Gravis

An autoimmune pathology caused by auto-antibodies that block or destroy nicotinic acetylcholine receptors at the neuromuscular junction, preventing normal endplate depolarization and causing profound muscle weakness.

Acute Anterior Poliomyelitis (Polio)

A viral disease wherein polioviruses infect and destroy spinal lower motor neurons, causing flaccid paralysis and disuse muscle atrophy.

Amyotrophic Lateral Sclerosis (ALS)

A progressive neurodegenerative disorder marked by the destruction of both upper and lower motor neurons and intracellular misfolded protein accumulation, usually leading to fatal respiratory muscle failure within 5 to 10 years.

Parkinson's Disease

A neurodegenerative movement disorder characterized by resting tremor, muscular rigidity, akinesia, bradykinesia, and postural instability, triggered by the loss of dopaminergic neurons projecting from the substantia nigra pars compacta.

Lewy Body

An abnormal spherical cytoplasmic inclusion body featuring a dense protein core containing misfolded alpha-synuclein that displaces cellular organelles within surviving dopaminergic neurons in Parkinson’s disease.

L-DOPA

The standard pharmacological metabolic precursor of dopamine that crosses the blood-brain barrier to enhance dopamine synthesis in surviving striatal projections of Parkinson's patients.

Deprenyl

A monoamine oxidase-B (MAO-B) inhibitor that prevents dopamine degradation in the synapse to alleviate Parkinsonian symptoms and slow symptomatic progression without halting neuronal death.

Huntington's Disease

An inherited neurodegenerative condition linked to an autosomal dominant mutation on chromosome 4 that causes extensive atrophy of the caudate and putamen, producing hyperkinetic chorea, cognitive decline, and emotional changes.

Huntingtin (Htt)

A protein whose mutated form possesses an expanded polyglutamine tract causing protein misfolding, toxic gain-of-function, and cell apoptosis in the basal ganglia.

Apraxia

A higher-order motor planning disorder characterized by the inability to execute purposeful, skilled motor gestures despite normal muscular strength, sensory function, and basic comprehension.

Ideomotor Apraxia

A variant of apraxia marked by the inability to execute simple motor tasks or imitate gestures on verbal command, despite preserving conceptual understanding of the task.

Ideational Apraxia

An impairment in organizing, sequencing, and executing multistep motor actions in their correct logical order, even though each isolated movement component can be performed.

Ataxia

A gross motor coordination impairment characterized by jerky, poorly sequenced, and dysmetric movements, typically resulting from lesions or pathologies within the cerebellum.

Alien Hand Syndrome

A condition in which a patient's limb performs involuntary, purposeful-appearing actions without conscious volition, typically resulting from lesions affecting the corpus callosum, supplementary motor area, or parietal lobe.