Motor Systems: Spinal Cord PDF

Motor systems: spinal cord Larger organization of motor systems Alpha motor Neuron control muscle contraction Alpha Motor Neuron anatomy  Rules of Muscle Innervation To allow for precise motor control muscles innervation patterns must be very organized. Each muscle fiber is...

Motor systems: spinal cord Larger organization of motor systems Alpha motor Neuron control muscle contraction Alpha Motor Neuron anatomy  Rules of Muscle Innervation To allow for precise motor control muscles innervation patterns must be very organized. Each muscle fiber is ONLY innervated by one motor neuron. Each motorneuron can innervate multiple muscle fibers but they all must be in the same muscle. A motor neuron + all muscle fibers it innervated=motor unit. Motor units are recruited in size order ( small motor units before large motor units)  Motorneuron / motor unit types Neuron Size: Large Medium Small Threshold: High Medium Low Conduction: Fastest Medium Slow Motor Unit Force: Large Medium Small Fatigue: Fast Intermediate Very Slow  Types of muscles (Force vs. fatigue of Motor Units) Single AP Repetitive AP (FF) (FFR) Sprint (FF) Vs marathon ( slow twitch ) Effect of stimulation rate on muscle tension (Twitch or Tetany) Recruitment of Motor units under different behavioral conditions  Sensory receptors for Motor function Sensory receptor is in the muscle Cell body is in the dorsal root ganglia Enter the spinal cord through dorsal root Synapses on the interneurons in the dorsal horn /alpha motor neuron in ventral horn of spinal cord Stretch reflex circuitry The role of g motor neurons in regulating muscle spindle responses Muscle spindles and Golgi tendon organs (Sense muscle tension or muscle force) Muscle spindles and Golgi tendon organs  Lower Motor Control: Spinal Reflex Reflex needs: Sensor (ex Sensory neurons) Integrator (Spinal cord interneurons) Effector (Motor Neuron/ muscle) ◼ Myotatic/Stretch Reflex ◼ Withdrawal Reflex ◼ Cross extensor Reflex ◼ Spinal regulation of muscles for respiration Medial-lateral organization in cord Flexor Extensor Cross-section from the cervical level of the spinal cord Rostro-caudal organization in cord 17 of 21  Spinal Reflexes 1. Stretch Reflex Muscle contraction in response to stretching of proprioceptors (spindles). Monosynaptic or two neurons. Example Knee Jerk. Complementary excitation & inhibition 18 of 21 Problem to be solved ? Solution  Spinal Reflexes 2. Withdrawal or Flexor Reflex: Polysynaptic reflex Ex: touching hot pan or steps on a nail Also known as reciprocal inhibition (clinical) Ex.Arm flexion  Spinal Reflex 3. Cross Extensor Reflex Connections to the motor neurons for the antagonistic muscles on the contralateral half of the body. Multisynaptic reflex Bilateral coordination: Don’t fall down!  Spinal Interneurons ◼ Most input to alpha motor neurons mediated by spinal interneurons ◼ Synaptic inputs to spinal interneurons  Primary sensory axons  Descending axons from brain  Collaterals of lower motor neuron axons  Other interneurons  Integrative role of interneurons Midline Vestibulospinal tract Cutaneous afferents Flexor reflex afferent Ia In Ia In Flexor reflex afferent EMn FMn muscle spindle muscle spindle afferents afferents Renshaw cells Extensor Flexor  “Simple” reflexes Ex: Hamstring Monosynaptic reflex Ex: Quadracep Ex: Knee reflex /Autogenic inhibition/Golgi tendon organ Ex: Carry heavy weights “Simple” reflexes Simple patterned movements  Central pattern generator neural circuit that generates rhythmic behaviors Rhythmic activities, such as walking, are generated in spinal cord Animals with spinal cord injury can still walk ◼ Spinally transected animal ◼ Partially support the animal and put it on a treadmill ◼ Start the treadmill ◼ Animal starts to walking How can the animal walk? ◼ No descend drive to initiate behavior ◼ Only spinal circuits intact that has locomotor central pattern generator (CPG) Rostro-caudal organization of control  The mammalian cycle of locomotion is organized by central pattern generators in the spinal cord Electromyography The mammalian cycle of locomotion is organized by central pattern generators in the spinal cord Stepping movements for different gaits Pink: Swing-Foot lifted Blue: Stance- Foot planted  The mammalian cycle of locomotion is organized by central pattern generators in the spinal cord After surgery animal is still able to walk. Reciprocal burst of electrical can be recorded from flexors during the swing phase and from extensors during the stance phase of walking.  Case Study 1 It had been a busy week for Jessica. She had taken three exams, and as if that wasn't stressful enough, her sister was ill and Jessica had to spend much of her time helping out at home. By the end of the week, Jessica decided to go to the beach for a walk and some genuine relaxation. The day was warm and Jessica took off her shoes so she could walk barefoot and feel the sand between her toes. Halfway down the beach, she stepped on something extremely sharp and withdrew her foot in pain. As Jessica hopped around the sand on one foot, rubbing the other to ease the pain, she decided to put her shoes back on. This incident is a simple event and may have happened to you. In terms of the neural pathways, this reaction requires a complex neural net. Can you explain what happened?  CASE STUDY 2 Jessica had been on the go the entire day. Her busy schedule had included a job interview, a class presentation, and an unmerciful physics exam she would rather not discuss. Once home, she was determined to head straight for bed, but there was one minor complication. She hadn't managed to eat anything in the midst of all the day's activities. While scrambling to put together a fast and easy meal at home, she inadvertently touched one of the hot burners on the stove. Fortunately, she reacted by jerking back her hand before any damage was done. A short time later Jessica felt pain on her hand where it had been exposed to the hot burner. After telling her mother about her eventful day and her narrow escape, Jessica's mother asked her about her hand. Jessica replied, "I'm fine, but it could have been much worse." Questions: 1. In this relatively simple event involving the hot burner, several neural pathways have contributed to the response of Jessica removing her hand from the potentially dangerous hot burner. Can you explain what happened? 2. Describe the functional anatomy of the spinal cord: white matter, gray matter, tracts, roots, and spinal nerves? 3. What are different components of Reflex arc? 36 of 21 Neuromuscular junction (NMJ)  ◼ Enough Ach is relaeased to depolarize the mucle over threshold by a significant amount (safety factor). ◼ Under normal conditions action potential in motor neuron=muscle contraction. Overview of muscle contraction  Muscle contraction leads to shortening of sarcomere Figure 10.15