Review① Neurons in the spinal cord Ventral = motor neurons; dorsal = sensory neurons Local circuit neurons: long- and short-distance② Types of motor units and force production/fatigue 3 types: I, IIa, and IIb Force Regulation: Motor unit recruitment (size principle); Frequency of APs ; synchronization③ Training and motor unit types animal researchSpinal cord reflex circuits① What is the neural circuitry for basic reflexes?② What is the function of the muscle spindle?③ What is the function of the Golgi tendon organ (GTO)?④ What is a Central Pattern Generator (CPG)?⑤ Lower motor neuron damageOverall organization of neural structures that control movementSpinal cord reflex circuits① What is the neural circuitry for basic reflexes?② What is the function of the muscle spindle?③ What is the function of the Golgi tendon organ (GTO)?④ What is a Central Pattern Generator (CPG)?⑤ Lower motor neuron damageMonosynaptic reflex arcA simple reflex circuit, the knee-jerk responseA simple reflex circuit, the knee-jerk responseIntracellularly recorded responses underlying the myotatic reflexNEUROSCIENCE, Fifth Edition, Figure 1.9Soleus muscle in the catOverview motor and sensory neurons• α (alpha) motor neuron: innnervates muscle fibers•  (gamma) Motor neuron:innervates muscle spindle fibers• Ia sensory afferent: from muscle spindle• Ib sensory afferent: from golgi tendon organ (GTO)Spinal cord reflex circuits① What is the neural circuitry for basic reflexes?② What is the function of the muscle spindle?③ What is the function of the Golgi tendon organ (GTO)?④ What is a Central Pattern Generator (CPG)?⑤ Lower motor neuron damageMuscle spindleGamma motor neurons: regulate spindle length; prevent “slack”A simple reflex circuit, the knee-jerk response Gamma (ϒ) motorneuronFunction of gamma motor neuronsGamma () motor neuron activityGamma () motor neuron activity:  gain or  bias.• Gain refers to the amount of muscle force generated in response to a given stretch of the intrafusal fibers during• Consistent level of active (smaller neuron  smaller excitatory threshold)• High gain: small stretch  greater ↑ in muscle force (↑ α motor neuron recruitment and firing rate)• Low gain: greater stretch needed for similar amount of muscle force.• Animation of reflex arcServo hypothesisSpinal cord reflex circuits① What is the neural circuitry for basic reflexes?② What is the function of the muscle spindle?③ Next class - What is the function of the Golgi tendon organ (GTO)?④ What is a Central Pattern Generator (CPG)?⑤ Lower motor neuron damageGolgi tendon organ (GTO)Negative feedback regulation of muscle tension by Golgi tendon organsGTO:•Sensitive to muscle tension from muscle contraction but not from muscle stretch•Maintains steady force level (fatigue)•Inhibits large forcesSpinal cord circuitry responsible for the flexion-crossed extension reflexFlexion reflexCrossed extensor reflexOverview① What is the neural circuitry for basic reflexes?② What is the function of the muscle spindle③ What is the function of the Golgi tendon organ (GTO)?④ What is a Central Pattern Generator (CPG)?⑤ Lower motor neuron damageFigure 16.15 Central pattern generators in terrestrial mammals (Part 3)https://youtu.be/wPiLLplofYwSpinal Pattern Generators: CPGNeuronal spinal networks capable of generating rhythmic motor activity in the absence of rhythmic input from peripheral receptors• Main feature Many forms of locomotion Rhythmic and alternating movements – appears to be stereotyped repeated action Allows automatic control at lower levels of the nervous system Still requires continuous (and subtable) modifications to adapt to different environments From sensory information From other regions of the CNSDo humans have CPGs?Indirect support:• Evidence of CPGs across number of species (vertebrate and invertebrate)• Some evidence of rhythmic lower limb movements after complete SCI in humans• Evidence of rhythmic stepping movements very early in human development (also in anencephaly)Spinal Cord rehab• Spinal cord retraining based on current knowledge of central pattern generators and reflexes• Injecting L-Dopa in spinal cord: alternating flexion/extension movements (cat/mouse)Overview① What is the neural circuitry for basic reflexes?② What is the function of the muscle spindle③ What is the function of the Golgi tendon organ (GTO)?④ What is a Central Pattern Generator (CPG)?⑤ Lower motor neuron damageLower motor neuron syndromemusclefibersmotor nervefiberCardinal signs of lower motor neuron syndrome1.Paralysis (loss of movement)/Paresis (weakness)2. Loss of reflexes3. Loss of muscle tone4. Atrophy5. Degeneration (MS/ALS)ALS or Lou Gehrig’s disease: Degeneration of a-motor neurons spinal cord – genetic