🧢Neuroscience Unit 5 – Motor Systems

Key Concepts and Terminology

• Motor systems encompass the neural structures and pathways responsible for controlling and coordinating movement
• Includes the motor cortex, basal ganglia, cerebellum, brainstem, spinal cord, and peripheral nerves
• Key terms: motor neurons, upper motor neurons, lower motor neurons, pyramidal tract, extrapyramidal tract
• Efferent pathways carry motor commands from the central nervous system to the muscles
• Afferent pathways convey sensory information from the muscles and joints back to the central nervous system
• Somatotopic organization maps body parts onto specific areas of the motor cortex
• Plasticity allows the motor system to adapt and learn new skills through experience and practice

Anatomy of Motor Systems

• Primary motor cortex (M1) located in the frontal lobe controls voluntary movements
• Premotor cortex and supplementary motor area involved in planning and coordinating complex movements
• Basal ganglia a group of subcortical nuclei that modulate movement initiation, execution, and termination
  • Includes the striatum (caudate nucleus and putamen), globus pallidus, substantia nigra, and subthalamic nucleus
• Cerebellum coordinates smooth, precise movements and maintains balance and posture
• Brainstem contains nuclei that control reflexes, posture, and automatic functions (breathing, swallowing)
• Spinal cord contains lower motor neurons that directly innervate skeletal muscles
• Peripheral nerves carry motor commands from the spinal cord to the muscles and sensory information back to the spinal cord

Neural Pathways in Motor Control

• Pyramidal tract (corticospinal tract) carries motor commands from the motor cortex directly to the spinal cord
  • Crosses over (decussates) in the medulla, allowing each hemisphere to control the opposite side of the body
• Extrapyramidal tract involves indirect pathways from the motor cortex through the basal ganglia and cerebellum
• Reticulospinal tract originates in the reticular formation of the brainstem and controls posture and balance
• Rubrospinal tract originates in the red nucleus of the midbrain and controls fine motor movements
• Vestibulospinal tract originates in the vestibular nuclei and maintains balance and head position
• Tectospinal tract originates in the superior colliculus and controls head and eye movements
• Propriospinal tract connects different levels of the spinal cord for coordinating limb movements

Neurotransmitters and Motor Function

• Acetylcholine (ACh) is the primary neurotransmitter at the neuromuscular junction
  • Released by motor neurons to stimulate muscle contraction
• Dopamine plays a crucial role in the basal ganglia for initiating and controlling movement
  • Depletion of dopamine in the substantia nigra leads to Parkinson's disease
• Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the basal ganglia and cerebellum
• Glutamate is the primary excitatory neurotransmitter in the motor cortex and corticospinal tract
• Serotonin modulates motor neuron excitability in the spinal cord
• Norepinephrine released by the locus coeruleus in the brainstem regulates arousal and motor activity
• Endorphins released during exercise can modulate pain perception and motor function

Motor Cortex and Planning

• Primary motor cortex (M1) contains a somatotopic representation of the body (motor homunculus)
  • Areas controlling fine motor skills (hands, face) have larger representations
• Premotor cortex is involved in planning and preparing movements based on sensory cues
• Supplementary motor area (SMA) participates in planning and coordinating complex, sequential movements
• Prefrontal cortex is involved in decision-making and goal-directed behavior related to motor actions
• Parietal cortex integrates sensory information to guide motor planning and execution
• Basal ganglia and cerebellum provide feedback to the motor cortex for refining movements
• Mirror neurons in the premotor cortex fire both when performing an action and observing others perform the same action

Spinal Cord and Reflexes

• Spinal cord contains lower motor neurons (alpha and gamma) that directly innervate skeletal muscles
• Alpha motor neurons innervate extrafusal muscle fibers and generate muscle contraction
• Gamma motor neurons innervate intrafusal muscle fibers (muscle spindles) and regulate their sensitivity
• Muscle spindles detect changes in muscle length and provide feedback for maintaining posture and tone
• Golgi tendon organs detect changes in muscle tension and provide feedback for regulating force
• Stretch reflex (myotatic reflex) is a monosynaptic reflex that maintains muscle length and posture
  • Involves Ia afferent fibers from muscle spindles synapsing directly onto alpha motor neurons
• Inverse stretch reflex is a polysynaptic reflex that prevents muscle overextension
  • Involves Ib afferent fibers from Golgi tendon organs inhibiting alpha motor neurons
• Flexor withdrawal reflex is a polysynaptic reflex that protects the body from harmful stimuli
  • Involves nociceptive afferent fibers activating flexor muscles and inhibiting extensor muscles

Disorders and Diseases of Motor Systems

• Parkinson's disease caused by degeneration of dopaminergic neurons in the substantia nigra
  • Characterized by tremor, rigidity, bradykinesia (slowness of movement), and postural instability
• Huntington's disease an inherited disorder caused by degeneration of neurons in the striatum
  • Characterized by chorea (involuntary, jerky movements), cognitive decline, and psychiatric symptoms
• Amyotrophic lateral sclerosis (ALS) a progressive disorder affecting upper and lower motor neurons
  • Leads to muscle weakness, atrophy, and paralysis
• Multiple sclerosis an autoimmune disorder that damages the myelin sheath of motor and sensory axons
  • Causes a wide range of motor, sensory, and cognitive symptoms depending on the affected areas
• Cerebral palsy a group of disorders affecting movement and posture due to damage to the developing brain
  • Can cause spasticity, dystonia, ataxia, and other motor impairments
• Tourette syndrome a neurodevelopmental disorder characterized by motor and vocal tics
  • Involves abnormalities in the basal ganglia and neurotransmitter systems (dopamine, serotonin, GABA)
• Spinal cord injuries can disrupt motor pathways and cause paralysis below the level of the injury
  • Severity depends on the location and extent of the damage (complete vs. incomplete)

Clinical Applications and Research

• Deep brain stimulation (DBS) involves implanting electrodes in specific brain regions (basal ganglia, thalamus) to modulate abnormal neural activity
  • Used to treat Parkinson's disease, essential tremor, dystonia, and other movement disorders
• Transcranial magnetic stimulation (TMS) uses magnetic fields to stimulate or inhibit specific areas of the motor cortex
  • Used to study motor function, plasticity, and as a potential treatment for neurological and psychiatric disorders
• Functional electrical stimulation (FES) applies electrical currents to paralyzed muscles to restore movement
  • Used in rehabilitation for spinal cord injuries, stroke, and other motor impairments
• Robotic exoskeletons and brain-computer interfaces (BCIs) are being developed to assist or restore movement in individuals with motor disabilities
• Stem cell therapy and gene therapy are being investigated as potential treatments for neurodegenerative disorders affecting the motor system
• Animal models (mice, rats, primates) are used to study the mechanisms of motor control and to develop new therapies for motor disorders
• Neuroimaging techniques (fMRI, PET, EEG) are used to investigate the neural basis of motor function and to monitor the effects of interventions
• Computational models and simulations are used to understand the complex interactions between different components of the motor system