Bi360 Neurobiology: The Motor System 1. Define a motor unit A motor neuron and the muscle fiber it innervates. This is the basic functional unit by which the nervous system controls movement. 2. What are the determinants of the force exerted by a muscle contraction? a) The number of motor units that are activated b) The rate at which each of the active motor neurons fire action potentials 3. Describe all steps in the excitation-contraction coupling in skeletal muscles a) ACh binds to post-synaptic nicotinic nAChRs b) NAChRs undergo conformational change and nAChR Na+/K+channels open c) A net inward flow of cations (Na+) leads to an End Plate Potential (EPP) d) Muscle depolarizes e) If the EPP is above threshold, the muscle produces an action potential f) Action potential generated at neuromuscular junction travels around sarcolemma and along the membrane of the muscle T-tubules g) The depolarization of the T-tubule membrane signals the sarcoplasmic reticulum (SR) to release Ca2+ into sarcoplasm (muscle cytosol) h) Ca2+ binds to troponin i) Troponin undergoes a conformational change that rolls tropomyosin away from and exposing the myosin-binding site on the actin filament j) The myosin head attaches to the myosin binding site on the actin filament forming a cross- bridge k) After forming a cross-bridge, myosin head moves the actin-myosin complex forward and ADP and Pi are released (known as the power stroke) l) ATP binds with myosin head, which releases actin, and returns to original position (also known as the recovery stroke) m) Returning to the resting state, the myosin head contains ADP and Pi and is ready for another cross-bridge cycle starting with step j. 3. What are the determinants of the order in which motor units are recruited during a muscle contraction? a) Size of the motor neuron cell body b) Axon diameter which determines action potential conduction velocity 4. How does the “Size Principle” affect the control of movement by the nervous system and muscle fatigue? a) The sequence of motor neuron recruitment is determined by spinal mechanisms and not by higher centers. This means the brain cannot selectively activate specific motor units. b) Motor units are activated in order of fatigability, so the least fatigable motor units are activated first, producing the initial force required for a specific task. 5. Discuss the key rules of simple motor reflexes. a) Response is proportional to the stimulus (Not all or none) b) Latency depends on the strength of the stimulus c) There is a threshold for the stimulus 6. Explain the concept of a central pattern generator (CPG). Sketch the lumbar CPG for locomotion and draw its cellular circuit with its specific neuronal connections (excitatory, inhibitory). What is the input? Output? How does sensory information modulate a CPG-mediated behavior? Coordinated motor behaviors are the result of the neuronal activity in a network of cells called a CPG. A central pattern generator is a circuit of neurons capable of producing a sustained rhythmic output in the absence of continuous sensory input from the PNS or descending input from the brain. Some examples of the behavioral output produced by CPGs are walking, breathing, chewing, and swimming. Sensory information from receptors in the PNS is used to modify the pattern of activity produced by the CPG. In this way the animal can concentrate on adjusting motor activity to suit the changing external environment while the CPG does the computations that keep the basic stereotyped pattern going. The properties of a CPG depend upon the properties of the cells in the network, the properties of the synapses that connect them, and the pattern of connectivity in the circuit. The existence of CPG circuits for stereotyped behavior is very beneficial to an organism. First, it decreases the computational load on the brain by making patterned activity more or less automatic, once it is turned on. Secondly, they help to ensure coordinated movements for a group of muscles that may otherwise work against each other. This saves energy and is crucial to producing correct motor output. Lumbar CPG for Walking Located in the lumbar spinal cord (L2), the locomotor CPG is a specialized network of interneurons termed produces alternating activity between flexors and extensors on one side and is coupled with the CPG on the other side contains programs that control movement of hip, knee, and foot muscles. The CPG activates walking by first causing hip extension, then knee extension, then foot extension, followed by foot flexion, knee flexion, and hip flexion. The CPG produces alternating activity between flexors and extensors on one side and is coupled with the CPG on the other side. The specialized interneurons activate flexor (F) and extensor (E) motoneurons. Some interneurons are under the influence of the CPG and respond to various inputs in a phase-dependent manner. This sequence of activation can be initiated by activating the CPG and subsequent reflexive and sensory inputs resulting from the walking motions. Likewise, running, hopping, skipping, jumping, and dancing motions can be initiated, sustained, and modulated by the brain without direct activation of motoneurons. Other interneurons are outside this zone of influence but receive

various inputs as well. Descending inputs and sensory afferents can reach different types of interneurons. Source: W Young, Electrical Stimulation and Motor Recovery (Review), Cell Transplantation, Vol. 24, pp. 429–446, 2015. Rossignol & Frigon, Recovery of Locomotion After Spinal Cord Injury: Some Facts and Mechanisms Annual Reviews in Neuroscience. 2011.34:413-44