Chapter 13 Notes/Summary - Spinal Control of Movement PDF

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Notes and lecture summary for chapter 13 from Prof. Smith. ...

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Chapter 13: Spinal Control of Movement 1. The Somatic Sensory System a. Muscles can be described in two broad categories (Striated and Smooth) i. Smooth 1. Lines the digestive tract, arteries and related structures 2. Is innervated by the autonomic nervous system 3. Plays a role in peristalsis and control of blood pressure and blood flow ii. Striated 1. Two Types (Cardiac and Skeletal) a. Cardiac i. Heart Muscle ii. Contracts rhythmically even with a lack of innervation 1. Innervation from the autonomic nervous system serves to speed up or slow down heart rate b. Skeletal i. Moves around bone and joints ii. Moves eyes within head, controls respiration iii. Each is enclosed in a connective tissue sheath that forms tendons at the end of the muscles iv. Consists of hundreds of muscle fibers (cells of skeletal muscle) 1. Each fiber is innervated by a single axon branch from the CNS v. Derived from Somites vi. Called the Somatic motor System c. Flexion – Flexing of muscle d. Extension – opposite of flexion e. Synergists – muscles that work together f. Antagonists – flexors and extensors that pull in opposite directions g. Axial Muscles – responsible for movements of the trunk h. Proximal (Girdle) muscles – those that move the shoulder, elbow, pelvis and knee i. Distal muscles – move the hands, feet and digits 2. The Lower Motor Neuron a. Somatic musculature is innervated by the somatic motor neurons in the Ventral Horn of the spinal cord i. Called Lower motor neurons ii. Only these lower motor neurons directly command muscle contraction b. Segmental Organization of Lower Motor Neurons i. Axons of lower motor neurons bundle together to form ventral roots ii. Ventral and dorsal roots join to form a spinal nerve that exits through notches in the vertebrae 1. Humans have 30 spinal nerves on a side (30 notches) iii. Called mixed spinal nerves because they contain sensory and motor fibers iv. Spinal Segments (Cervical, Thoracic, Lumbar, Sacral) v. Skeletal muscles aren’t evenly distributed among spinal segments c. Alpha Motor Neurons i. Directly Trigger the generation of force by muscles ii. One alpha motor neuron and all the fibers it innervates make up the motor unit iii. Collection of alpha motor neurons that innervate a single muscle is a motor neuron pool iv. Graded Control of muscle contraction 1. Varying the firing rate of motor neurons a. Alpha motor neuron communicates with a muscle fiber by releasing Ach at the neuromuscular junction b. Ach released in response to one presynaptic action potential is enough to trigger one postsynaptic AP i. Postsynaptic AP causes a twitch – contraction then relaxation ii. Susutained contraction requires a barrage of AP’s 2. Recruiting additional synergistic motor units

a. Muscles with a large number of small motor units can be more finely controlled by the CNS b. Motor units are recruited in the order of smallest first, largest last c. Size Principle – orderly recruitment of motor neurons due to size 3. Inputs to Alpha Motor Neurons a. Lower motor neurons are controlled by synaptic inputs in the ventral horn. b. 3 major sources of input to an alpha motor neuron i. Dorsal Root ganglion cells that innervate a specialized sensory apparatus embedded in the muscle called a Muscle Spindle ii. Upper motor neurons in motor cortex and brain stem iii. Interneurons in the spinal cord (largest input source) 1. Can be excitatory or inhibitory d. Types of Motor Units i. Red Muscle Fibers 1. Large number of mitochondria 2. Enzymes specialized for oxidative energy metabolism 3. Slow to contract but can sustain long contractions 4. Found in the leg ii. White muscle fibers 1. Fewer mitochondria 2. Mainly rely on anaerobic metabolism 3. Contract rapidly and powerfully, but they fatigue rapidly 4. Escape reflexes (arm muscles) iii. Fast Motor Units 1. Rapidly fatiguing white fibers 2. Motor neurons generally bigger and have larger diameter with faster conducting axons 3. Generate occasional high frequency bursts of action potentials (30-60 impulses/second) iv. Slow motor units 1. Slow fatiguing red fibers 2. Smaller diameter motor neurons 3. Slower conducting axons 4. Steady, low-frequency activity (10-20 impulses/second) v. Neuromuscular Matchmaking 1. If nerves of fast and slow muscles are switched, the muscles will assume the properties of the new nerve. a. Including contraction rate and much of the underlying biochemistry b. Switch of muscle phenotype 2. Muscle fibers are also changed by varying the amount of activity a. Increased activity can lead to hypertrophy – exaggerated growth of muscle fibers b. Prolonged inactivity leads to atrophy vi. Amyotrophic Lateral Sclerosis (ALS) 1. Caused by degeneration of the Large Alpha Motor Neurons 2. Riluzole – a blocker of glutamate release that has been used to help treat ALS. 3. Excitation - Contraction Coupling a. Ach produces a large EPSP in the postsynaptic membrane due to the activation of nicotinic Ach receptors b. Membrane of muscle cells contain voltage gated sodium channels, and the EPSP can cause an AP c. Because of the excitation – contraction coupling, this AP (excitation) triggers Ca2+ release from an organelle in the muscle fiber, which leads to contraction of the fiber d. Relaxation occurs when Ca2+ levels are lowered by reuptake in the organelle e. Muscle Fiber Structure i. Formed from fusions of myoblasts early in fetal development (derived from mesoderm) ii. Individual muscle cells are multinucleated and elongated iii. Muscle fibers are enclosed by an excitable membrane called the sarcolemma iv. Myofibrils – cylindrical structures within muscle fiber 1. Contract in response to AP

v. Sacroplasmic reticulum (SR) 1. Surround myofibrils 2. Intracellular sac that stores Ca2+ vi. T tubules 1. Allow AP’s in the sarcolemma to gain access to the SR 2. Network of tunnels 3. Like inside out axons – lumen of each is continuous with extracellular fluid vii. Tetrad – voltage-sensitive cluster of four calcium channels 1. In the T tubule membrane 2. Linked to a calcium release channel in the SR 3. Conformational change in tetrad opens calcium release channels in the SR membrane. a. Increase in free Ca2+ w/in cytosol causes myofibrils to contract 4. Molecular Basis of Muscle Contraction a. Myofibril i. Divided into segments called Z Lines 1. 2 Z Lines and the myofibril in between is a Sarcomere ii. Anchored to each side of the Z line are thin filaments 1. Each Z line’s thin filaments don’t come in contact with one another 2. Between and among the thin filaments are thick filaments iii. Muscle contraction 1. Thin filaments slide along thick filaments, bringing Z lines close to each other (Sarcomere Shortens) iv. Called the Sliding Filament Model 1. Occurs because of the interaction between myosin (thick filament protein) and actin (thin filament protein) 2. Myosin heads bind actin molecules and then undergo a conformational change, which causes them to pivot. 3. Thick filaments then move with respect to the thin filaments 4. Myosin heads disengage at the expense of ATP. a. Repeating this cycle allows the myosin heads to walk along the actin filament 5. When the muscle is a rest, myosin cannot bind the actin because it is covered by the protein troponin a. Ca2+ initiates muscle contraction by binding troponin, which exposes the binding sites on actin b. Contraction continues as long as Ca2+ and ATP are available c. Relaxation occurs when the ca2+ is sequestered by the SR i. This re-uptake depends on action of a calcium pump (and also requires ATP) 6. Excitation a. AP occurs in alpha motor neuron axon b. Ach released by axon terminal of alpha motor neuron at neuromuscular junction c. Nicotinic receptor channels in sarcolemma open, and postsynaptic sarcolemma depolarizes (EPSP) d. Voltage-gated Na+ channels open, and an AP is generated in the muscle fiber, which sweeps down the sarcolemma into the T Tubules e. Depolarization of the T Tubules causes Ca2+ release from the sarcoplasmic reticulum 7. Contraction a. Ca2+ binds to troponin b. Myosin binding sites on actin are exposed c. Myosin heads bind to actin d. Myosin heads pivot e. Myosin heads disengage at the expense of ATP f. Cycle continues as long as Ca2+ and ATP are present 8. Relaxation

a. As EPSP’s end, the sarcolemma and T tubules return to the resting potentials b. Ca2+ is sequestered by the SR by an ATP-driven calcium pump c. Myosin binding sites on actin are covered by troponin v. Rigor Mortis 1. Dead muscles have no ATP to release myosin heads 5. Spinal Control of Motor Units a. The first source of synaptic input to the alpha motor neuron is sensory feedback from the muscles themselves b. Proprioception from Muscle Spindles i. Muscle spindles lie deep within most skeletal muscles and are also called stretch receptors 1. The middle third of the capsule is swollen. 2. In this middle (equatorial) region, 1a sensory axons wrap around the muscle fibers of the spindle a. The spindles and their 1a axons are specialized for changes in muscle length (stretch) and are Proprioceptors. i. Specialized for proprioception (body sense) 3. 1a axons enter spinal cord via dorsal roots, branch repeatedly, and form excitatory synapses with interneurons and alpha motor neurons of the ventral horns c. Myotatic Reflex (monosynaptic myotatic reflex arc) i. 1a axons and the alpha motor neurons on which it synapses ii. only one synapse separates the primary sensory input from the motor neuron output. iii. Muscle spindles stretch when muscle is stretched (weight added) 1. Leads to depolarization of 1a axons due to opening of mechanosensitive ion channels a. Depolarizes alpha motor neurons which increase AP frequency b. Causes muscle to contract iv. Knee-jerk reflex is an example 1. Quad contracts extending leg d. Gamma Motor Neurons i. Intrafusal fibers – modified skeletal muscle fibers in muscle spindles 1. Innervated by gamma motor neurons ii. Extrafusal fibers – lie outside the spindle and form the bulk of muscle 1. Only these are innervated by alpha motor neurons iii. Activation of alpha motor neurons causes extrafusal muscle fibers to shorten 1. If the muscle spindle goes slack, it goes Off the Air and cannot report muscle length 2. Activation of Gamma Motor Neurons causes the spindle poles to contract, tightening the slack and keeping the spindle On the Air (1a axons active) iv. Alpha activation alone decreases 1a activity v. Gamma activation alone increases 1a activity e. Proprioception from Golgi tendon organs i. Acts like a strain gauge 1. Monitors muscle contraction (the force of tension) 2. Located at the junction of the muscle and the tendon a. Innervated by 1b sensory axons ii. Spindles are Situated in parallel (rather than in series) 1. Encodes muscle length info. iii. Golgi tendon organs are situated in series 1. Encodes muscle tension info. iv. 1b axons enter spinal cord, branch repeatedly, and synapse on interneurons in the ventral horn. 1. Some of the interneurons inhibit connections with alpha motor neurons innervating the same muscle v. Reverse myotatic reflex – protects the muscle from being overloaded 1. Also for proper execution of fine motor acts a. Steady, but not too powerful grip f. Proprioception from the joints i. Especially present in joint capsules and ligaments

ii. Respond to changes in movement of a joint iii. Rapidly adapting – sensory info. About a moving joint is plentiful while nerves encoding a resting joint are few 6. Spinal Interneurons a. Spinal interneurons receive synaptic input from i. Primary sensory axons ii. Descending axons from the brain iii. Collaterals of lower motor neuron axons b. Ineterneurons are themselves networked together in away that allows coordinated motor programs to be generated in response to their many outputs c. Actions of 1b inputs are entirely polysynaptic d. Inhibitory Input i. Recipricol inhibition – the contraction of one set of muscles is accompanied by the relaxation of the antagonist muscles e. Excitatory Input i. Flexor Reflex – withdrawals a limb from an aversive stimulus 1. Excite the alpha motor neurons that control the flexor muscles of the affected limb 2. Interneurons inhibit the alphas that control the extensors ii. Coupled with the crossed-extensor reflex 1. Activates extensor muscles and inhibits flexors on the opposite side 2. Another example of reciprocal inhibition 7. Generation of Spinal Motor Programs for Walking a. Crossed-extensor reflex coupled with a timing mechanism b. Circuitry for coordinated control of walking lies in the spinal cord c. Central Pattern Generators i. Simplest are neurons with pacemaker properties ii. Activation of NMDA receptors on spinal interneurons could generate this locomotor activity in the lamprey 1. NMDA receptors are glutamate-gated on channels that a. Allow more current to flow into cell when the postsynaptic membrane is depolarized b. Admit Ca2+ and Na+ into the cell iii. Spinal interneurons also possess calcium-activated k+ channels iv. Cycle when NMDA receptors are activated by glutamate 1. Membrane depolarizes 2. NA+ and ca2+ flow into cell through NMDA receptors 3. Ca2+ activates K+ channels 4. K+ flows out of the cell 5. Membrane hyperpolarizes 6. Ca2+ stops flowing into cell 7. K+ channels close 8. Membrane depolarizes, cycle repeats d. Synaptic interconnections also produce rhythm...