Motor control Study Guide

📖 Core Concepts Motor control – the nervous system’s regulation of movement, integrating external sensory cues and internal proprioception to generate neural commands. Motor unit – one motor neuron + all muscle fibers it innervates (all same fiber type). A muscle’s motor pool is the collection of its motor units. Size principle (Henneman) – motor units are recruited from smallest/most excitable (low‑force, slow‑twitch) to largest/least excitable (high‑force, fast‑twitch). Redundancy / Degrees‑of‑Freedom problem – many possible joint/muscle configurations can achieve the same task goal. Feedback (closed‑loop) vs. Feed‑forward (open‑loop) control – feedback uses sensory error signals during movement; feed‑forward plans a trajectory in advance and runs without correction. Motor synergy – a coordinated pattern that couples multiple muscles/elements so errors in one are compensated by others, reducing computational load. Forward model – predicts sensory consequences of a motor command; inverse model – computes the command needed for a desired sensory outcome. Optimal feedback control – theoretical framework that selects motor commands to minimize a cost function (e.g., error + energy). Speed‑accuracy trade‑off – higher speed → larger signal‑dependent noise → lower accuracy (Fitts’ Law, signal‑dependent noise model). --- 📌 Must Remember 150 k motor neurons control 600 muscles in humans. Recruitment order: small → large motor units (size principle). Force production = (number of active motor neurons × their firing rates) + contractile properties of fibers. Reaction time (RT) = stimulus → response onset; movement time (MT) = duration of the movement itself. Hick’s Law: $RT = a + b \log2(N+1)$ (N = number of choices). Fitts’ Law: $MT = a + b \log2\!\left(\frac{2D}{W}\right)$ (D = distance, W = target width). Signal‑dependent noise: variability scales with muscle activation level. Monosynaptic reflex – single synapse (e.g., stretch reflex); polysynaptic reflex – multiple synapses, slower, can involve cortex. Central pattern generator (CPG): neural network that produces rhythmic output without descending input or sensory feedback. --- 🔄 Key Processes Motor‑unit recruitment Detect task demand → increase firing rate of already‑active units → recruit next‑largest unit (size principle). Closed‑loop feedback correction Desired state → compare with actual sensory feedback → compute error → adjust motor commands → repeat until error < threshold. Feed‑forward movement planning Define goal → compute inverse model → generate motor command → execute ballistic movement → (no online correction). Forward‑model learning Execute command → compare predicted sensory outcome (forward model) with actual outcome → generate prediction error → update internal model. Synergy formation Identify task‑relevant performance variable → couple muscle activations so that variability orthogonal to the variable is allowed, while variability affecting the variable is minimized. --- 🔍 Key Comparisons Open‑loop vs. Closed‑loop Open‑loop: pre‑planned, fast, no online sensory correction (e.g., throwing). Closed‑loop: continuous error correction, slower, more accurate for perturbed tasks. Forward model vs. Inverse model Forward: predicts sensory consequences of a given command. Inverse: computes the command needed for a desired sensory outcome. Monosynaptic vs. Polysynaptic reflex Monosynaptic: single synapse, very fast (∼30 ms). Polysynaptic: multiple synapses, slower, can be modulated by higher centers. Motor unit vs. Motor pool Motor unit: one neuron + its fibers. Motor pool: all units belonging to one muscle. --- ⚠️ Common Misunderstandings “More motor units = more force” – true only when the additional units are recruited and fire at sufficient rates; firing frequency also matters. “Reflexes are immutable” – reflex gain can be modulated by context, attention, and prior experience. “Open‑loop movements are inaccurate” – they can be highly accurate when the internal model is well‑trained; inaccuracies arise mainly from unpredicted disturbances. “Redundancy is a problem” – it is a resource; the CNS exploits it via synergies to increase robustness. --- 🧠 Mental Models / Intuition “Size principle = climbing a ladder” – start at the bottom rung (small unit) and step up only as needed; you never skip a rung. “Forward model = crystal ball” – the brain predicts what will happen before it happens; mismatches are the “mistakes” that drive learning. “Synergy = orchestra conductor” – one command (conductor) leads many instruments (muscles) to produce a harmonious performance, even if some play slightly off‑key. --- 🚩 Exceptions & Edge Cases High‑frequency fatigue: Fast‑twitch units fatigue quickly; prolonged high‑force tasks may require intermittent recruitment of slower units. Delayed feedback: Very rapid movements (≤ 100 ms) rely on feed‑forward control because sensory feedback cannot arrive in time. Nonlinear muscle dynamics: At extreme joint angles or high loads, the linear approximation of force‑vs‑activation breaks down; optimal control models must incorporate nonlinearities. --- 📍 When to Use Which Choose feedback control when: Task involves external perturbations, need for high precision, or long movement duration. Choose feed‑forward (open‑loop) control when: Movement is ballistic, time‑critical, and the internal model is reliable (e.g., saccades, throwing). Apply forward model for: Predicting sensory consequences, error detection, and learning. Apply inverse model for: Computing motor commands for a desired endpoint (e.g., gaze stabilization). Use motor synergies when: The task has many redundant degrees of freedom and requires robustness to variability. --- 👀 Patterns to Recognize Increasing task difficulty → split movement (fast initial + slow corrective sub‑movement). Larger targets → single, smooth trajectory; smaller or farther targets → multi‑phase movement. Noise‑dependent variability grows with higher muscle activation; expect larger endpoint spread in high‑force, fast movements. Reaction time grows logarithmically with number of choices (Hick’s law). --- 🗂️ Exam Traps “All reflexes are monosynaptic.” – only the stretch reflex is monosynaptic; many reflexes are polysynaptic. “Open‑loop = no brain involvement.” – open‑loop movements still require central planning and internal models; they just lack online sensory correction. “Motor unit recruitment is random.” – it follows the orderly size principle, not random. “Fitts’ Law only applies to pointing tasks.” – the speed‑accuracy trade‑off described by Fitts’ law extends to any goal‑directed movement where distance and target size define difficulty. “Redundancy always makes control harder.” – redundancy provides flexibility; the CNS resolves it via synergies and optimal control. ---