Lecture: Reflexive and Voluntary Movement

Overview of Motor Control and Reflex Pathways

• Motor Movement Classification:

  • Reflexive Movements: Involuntary responses executed via spinal cord circuits or subcortical/brainstem loops, requiring minimal to no initial cerebral cortex input.

  • Voluntary Movements: Goal-directed actions initiated and executed by higher cortical centers, modulating lower spinal circuits.

• Objectives of Motor Integration:

  • Understand muscle control governed directly by the spinal cord versus control governed by the brain.

  • Analyze how cortical regions, specifically the supplementary motor area (SMA) and premotor cortex, organize, plan, and initiate motor actions.

• The Unintentional Withdrawal Reflex:

  • Trigger: A high-heat or noxious stimulus applied to a sensory receptor (e.g., a finger touching a hot object).

  • Signal Transmission: Generates a high firing rate of action potentials along a sensory neuron.

  • Spinal Circuitry: The afferent signal enters the spinal cord and excites an interneuron within a local circuit, which in turn excites an alpha motor neuron.

  • Effector Action: The motor neuron delivers an excitatory signal back to the arm musculature, causing rapid muscle contraction and withdrawal of the limb from the noxious stimulus.

  • Brain Independence: This primary withdrawal reflex pathway functions entirely without requiring input or processing from the brain.

• Supraspinal Modulation and Voluntary Override:

  • Context: Humans frequently override pure reflexive withdrawal when executing conscious tasks, such as carrying a hot casserole dish known to be safe despite high temperature.

  • Descending Inhibition: Simultaneously with voluntary motor commands maintaining grip, the brain sends descending inhibitory signals to the relay points (interneurons/synaptic junctions between sensory and motor neurons) in the spinal cord.

  • Functional Result: Inhibitory signals suppress the local withdrawal reflex arc, preventing the individual from dropping the object.

Skeletal Muscle Anatomy and Contraction Dynamics

• Structural Composition of Skeletal Muscle:

  • Skeletal muscles are the primary effectors responsible for physical actions and locomotion.

  • A single muscle fiber comprises a tight bundle of microscopic structural units called myofibrils.

  • Myofibrils: Composed of overlapping protein filaments primarily consisting of actin and myosin.

• Mechanism of Muscular Contraction:

  • Depolarization: An action potential traveling down a motor neuron depolarizes the muscle fiber membrane.

  • Cross-Bridge Cycling: Depolarization triggers myosin filaments to bind, slide, or "row" along adjacent actin filaments.

  • Mechanical Movement: This sliding action shortens the myofibrils and contracts the muscle fiber.

• Motor Units and Force Generation:

  • Motor Unit Definition: A single motor neuron together with all the individual muscle fibers it innervates.

  • Gradation of Force: The absolute strength of a muscular contraction is dictated by the average firing rate of action potentials across the population of active motor units supplying that muscle.

• Sensory Feedback Loop:

  • Muscles continually transmit proprioceptive and mechanical sensory feedback back to the central nervous system (CNS) to adjust dynamic motor output.

Monosynaptic Stretch Reflexes

• Definition and Architecture:

  • The monosynaptic stretch reflex represents the simplest level of central motor integration.

  • Single Synapse Structure: Contains only one direct synaptic connection within the spinal cord gray matter between the sensory afferent neuron and the motor efferent neuron.

• Primary Function:

  • Maintains body posture, stabilizes joints, and dynamically adjusts limb position in response to sudden, unintended external loads or muscle stretches.

• Neuronal Pathway:

  • Afferent Impulses: originate at stretch receptors inside the muscle, pass through the dorsal root ganglion into the dorsal root of the spinal column, and extend into the gray matter of the spinal cord.

  • Terminal Buttons: Synapse directly onto the cell bodies or dendrites of motor neurons within the spinal cord gray matter.

  • Efferent Impulses: The activated motor neuron projects its axon out through the ventral root of the spinal column back to the exact same muscle fibers that were stretched, inducing contraction.

• Classic Examples of Monosynaptic Stretch Reflexes:

  • Patellar Reflex (Knee-Jerk Reflex):

    • Procedure: Leg is allowed to dangle freely, placing the quadriceps (thigh muscle) under mild stretch.

    • Stimulus: Tapping the patellar tendon below the kneecap with a reflex hammer rapidly stretches the attached quadriceps muscle.

    • Response: Monosynaptic circuit fires, sending excitatory signals back to the quadriceps, causing it to contract rapidly and producing an outward kicking motion of the lower leg.

  • Sudden Load Accommodation (Bicep Reflex):

    • Stimulus: Holding an arm out and unexpectedly receiving a heavy object (e.g., a weight or heavy book).

    • Response: The weight stretches the bicep muscle, instantly triggering the monosynaptic stretch reflex to contract the bicep and keep the hand level.

  • Postural Control (Calf Muscle Modulation):

    • Stimulus: When standing upright, leaning forward sways the center of gravity and stretches the calf muscles.

    • Response: The stretch activates monosynaptic circuits in the calf, causing the muscle to contract, pulling the body backward into upright alignment and preventing falling forward.

Polysynaptic Reflexes and Central Modulation

• Characteristics of Polysynaptic Reflexes:

  • Involve multiple synaptic links within the spinal cord or brainstem, incorporating one or more interneurons between the primary afferent input and efferent motor output.

  • Govern more complex, coordinated reflexive behaviors.

  • Subject to deep integration and modulation by brain pathways.

• Distinction Between Withdrawal Reflexes and Pain Reflexes:

  • Withdrawal Reflex: A spinal polysynaptic reflex triggered directly by high-intensity noxious stimulation. The withdrawal response occurs automatically before sensory information reaches cortical awareness.

  • Pain Perception and Reflexes: True pain reflexes require conscious perception of pain, which is processed centrally within higher brain structures.

  • Timing Difference: Movements induced by conscious pain perception (e.g., pulling away after perceiving heat) are significantly slower than automatic, subcortically mediated spinal withdrawal reflexes.

• Clinical Polysynaptic Reflex - Moro Reflex:

  • Application: Used clinically in infants to test brainstem integrity and primitive neurological functioning.

  • Procedure: An infant is supported by its back and briefly given the physical sensation of falling.

  • Response: In infants around $32$ weeks of gestational age or older, this sudden displacement causes them to abduct and extend their arms outward in a characteristic startle motion.

• Inhibitory Mechanics in Spinal Interneurons:

  • Afferent sensory inputs enter the spinal cord via the dorsal root.

  • Terminal buttons of descending or interneuronal paths release inhibitory neurotransmitters onto motor neurons, generating inhibitory postsynaptic potentials (IPSPs).

  • These IPSPs hyperpolarize the motor neuron, preventing it from reaching threshold and thereby suppressing unintended or undesirable motor responses.

Cortical Structures in Voluntary Movement Planning and Execution

• Primary Motor Cortex (M1):

  • Located along the precentral gyrus.

  • Responsible for executing movement of specific body parts via coordinated contraction of multiple muscle groups.

  • Communicates with spinal cord motor neurons via intricate intermediate neural circuits.

  • Somatotopic Organization:

    • Organized as a motor homunculus mapping the spatial structure of the body.

    • Cortical representation proportional to fine motor control: larger areas (more neurons) are dedicated to fine-control effectors (e.g., fingers, hands), while smaller areas are allocated to regions with gross motor control (e.g., the trunk or belly).

  • Transcranial Magnetic Stimulation (TMS):

    • A non-invasive technique utilizing small, focused electromagnetic pulses applied over the scalp.

    • Applying TMS precisely over the primary motor cortex area representing the fingers induces involuntary twitching in the hand muscles.

• Hierarchical Stages of Movement Planning:

  • Prefrontal Cortex: The top of the planning hierarchy; formulates high-level decisions, goals, and intentions based on perceptual inputs received from various sensory modalities.

  • Motor Association Cortex: Receives goal-oriented signals from the prefrontal cortex and translates them into comprehensive motor plans and spatial coordinates. Composed of the supplementary motor area and premotor cortex.

• Supplementary Motor Area (SMA):

  • Anatomical Location: Situated primarily on the medial surface of the cerebral hemisphere.

  • Functional Role: Crucial for learning, sequencing, and executing complex behaviors that consist of sequential motor actions guided by internalized or sensory-driven cues.

• Premotor Cortex:

  • Anatomical Location: Located predominantly on the lateral surface of the cerebral hemisphere.

  • Functional Role: Critical for learning and executing movements triggered by arbitrary visual or sensory stimuli, as well as directing observational learning and imitation.

• Motor Signal Pathway Execution:

  • Prefrontal Cortex (Goal/Decision) $\rightarrow$ Motor Association Cortex (SMA / Premotor Cortex planning) $\rightarrow$ Primary Motor Cortex (Conversion to explicit neural motor commands) $\rightarrow$ Descending spinal cord pathways $\rightarrow$ Alpha motor neurons $\rightarrow$ Muscle activation.

Subcortical Structures for Movement Refinement

• Reticular Formation:

  • A network of brainstem nuclei.

  • Regulates baseline muscle tone, controls postural stability, directs locomotion, and modulates specific automated species-typical behaviors.

• Cerebellum:

  • Located at the posterior base of the brain stem ("little brain").

  • Coordination Functions: Regulates repetitive, rhythmic motor sequences requiring high temporal accuracy and spatial precision.

  • Guidance & Control: Smoothly guides voluntary movements, calculates trajectory, and enforces precise stopping points at intended spatial locations.

  • Integration: Integrates distinct sequential movements into fluid, continuous motor acts.

• Basal Ganglia:

  • Subcortical deep gray matter structures receiving extensive inputs from all regions of the cerebral cortex as well as the dopaminergic projections from the substantia nigra.

  • Modulates motor output generated by the primary motor cortex.

  • Serves an essential gating role by actively inhibiting unwanted, competing motor movements while permitting intended actions.

Descending Motor Pathways

• Pathways Overview:

  • Voluntary motor commands from the cerebral cortex travel down the spinal cord along two major anatomical systems categorized by their anatomical placement within the white matter of the spinal cord.

• Lateral Group Pathway:

  • Anatomical Course: Axons descend through the lateral (outer) column of the spinal cord.

  • Functional Role: Governs independent, non-stereotyped limb movements, specifically fine control of distal extremity muscles in the hands, fingers, and feet.

• Ventromedial Group Pathway:

  • Anatomical Course: Axons descend through the ventromedial (middle and ventral/front side) section of the spinal cord near the midline.

  • Functional Role: Controls automatic, gross motor actions of axial and proximal muscles, including postural adjustments, balance, trunk movements, and locomotion.