Psyc 6 week 5
Popular in Introduction to Neuroscience
Popular in Psychology (PSYC)
This 6 page Class Notes was uploaded by Sabrina Straus on Friday October 14, 2016. The Class Notes belongs to PSYC 6 at Dartmouth College taught by Catherine Cramer in Fall 2016. Since its upload, it has received 4 views. For similar materials see Introduction to Neuroscience in Psychology (PSYC) at Dartmouth College.
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Date Created: 10/14/16
1. 10-12-16 Intro to Neuro class notes + Chapter notes SPINAL MOTOR CONTROL Terms ● Motor system: consists of all muscles and neurons that control them ○ Controlled by spinal cord and brain ● Smooth muscle: lines digestive tract, arteries, etc ○ Plays a role in peristalsis or movement of material through the intestines and control of blood pressure/flow ● Striated muscle ○ Cardiac: heart muscle which contracts rhythmically (pace is controlled by innervation from autonomic nervous system) ○ Skeletal: moves bones around joints, move eyes, inhale and exhale, control facial expression, and produce speech ■ Enclosed in connective tissue sheath that forms tendons at the ends ■ Part of somatic motor system bc embryologically derived ■ Muscle fibers: cells of skeletal muscle ● Innervated by a single axon branch from CNS ● Synergists: muscles that work together ● Antagonist: flexor and extensor muscles ● Axial muscles: movements of trunk } maintain posture ● proximal/girdle muscles: move shoulder, elbow, pelvis, and knee } locomotion ● Distal muscles: move hands, feet, and digits } manipulation of objects ● Alpha motor neurons: directly trigger generation of force by muscles ● Contraction:initiated by release of ACh from axon terminals of alpha motor neurons ● Excitation-contraction coupling: action potential (depolarization of T tubule membrane) triggers release of Ca2+ from an organelle inside a muscle fiber}sarcoplasmic reticulum->contraction ● Relaxation: Ca2+ levels are lowered by reuptake into the organelle ● Rigor mortis: death stiffens muscles bc no ATP so myosin stays attached on actin ● Proprioceptors: components of somatic sensory system specialized for body sense} informs us of body position ● Central pattern generators: circuits that give rise to rhythmic motor ~muscles attached to skeleton >spinal cord contains motor programs for generation of coordinated movements ~different joints for different connections ● Hinge joint: knee ● Ball and socket: hip ● Ellipsoidal joint: wrist I. Elements of muscles ● Flexor: flex bicep and increase angle ○ Esp brachialis ● Extensor: flex tendons } straight ● Tendon: attach skeletal to muscle ● contractile fibers: make up skeletal muscles ● Myosin heads attach to actin and pull muscle together to contract it ~muscles only actively contract not extend II. Neural control of muscles ● motor neurons ○ Lower motor neurons: alpha and gamma } controlled by ventral horn ■ Only lower neurons directly command muscle contraction ■ Structure ● Neurons bundle together to form ventral roots which join w/ dorsal roots to form spinal nerves (exit through notches)(mixed bc contain sensory and motor fibers) ● Spinal segments: cervical (enlarged ventral horn for arms), thoracic, lumbar(enlarged for legs), sacral ● Lower motor neurons innervating axial muscles=medial to those innervating distal / flexors=dorsal to those of extensors ■ alpha motor neurons -> extrafusal fibers} provide strength of contraction } heavy part of muscle } communicates w/ muscle fiber by releasing ACh-> causes EPSP in muscle fiber (end-plate potential) -> contract and release or continual contraction=twitch } excite skeletal muscles ● motor unit: one motor neuron and 1 group of muscle fibers } must all be contracted together ○ Make up motor control ○ Types: ■ Slow motor Unit: Red muscle fibers (dark): have a large # of mitochondria enzymes specialized for oxidative energy metabolism } slow (S) fibers ● Slow to contract but good at sustaining ● In torso and legs ■ Fast Motor Units: Pale muscle fibers (white): rely on anaerobic metabolism } fast (F) fibers ● Contract rapidly but fatigue quickly ● In escape muscles ● Subtypes: ○ Fatigue-resistant (FR): strong and fast contractions that resist fatigue ○ Fast fatigable (FF): strongest contractions but exhaust quickly ■ Biggest motor neurons+high frequency potentials ○ Neuromuscular matching: neurons to fibers ■ Neurons switch phenotype based on synaptic activity} altering neuron activity ■ Varying absolute amt of activity ● Ex: hypertrophy=exaggerated growth ● Motor neuron pool: collection of alpha motor neurons that innervates a single muscle ● Graded control of muscle contraction: ○ Vary firing rate ○ Recruiting additional synergistic motor units (smallest to largest) ■ Size principle: Smallest have small alpha motor neurons and can be excited easily by signals ● Inputs to alpha motor neurons: ○ Spinal interneurons: largest input ■ Can be excitatory or inhibitory and is part of the circuitry that generates spinal motor programs ■ Inhibitory input ● Reciprocal inhibition: contraction and relaxation ■ Excitatory input ● Flexor reflex: withdrawal reflex } slower than stretch ○ Activated by small myelinated A8 axons->eventually excite alpha motor neuron ○ Muscle spindles: innervated by dorsal root ganglion cells with axons} stretch receptor ■ Contains specialized skeletal muscle fibers ■ Input provides feedback about muscle length ■ varies in how wide it is based on stretching ■ provide input to CNS on how much stretch there is ■ Aa tells spinal cord how much stretch there is-> processed in spinal cord-> motor neurons ○ Upper motor neurons in brain: important for initiation and control of voluntary movement ■ gamma motor neurons -> intrafusal fibers} smaller + contain muscle spindles (activated by stretching and excites receptor->contraction of muscle allows intrafusal fiber to become slack/no signal but contract on either end: maintain signal and ability to hold contraction through gamma motor neurons) } maintaining stretch ● Give motor innervation to intrafusal fibers->contraction of poles to keep Ia active ● Gamma loop: gamma motor neuron → intrafusal muscle fiber → Ia afferent axon → alpha motor neuron → extrafusal muscle fibers ● Muscle fiber structure ○ Formed in fetal development ■ Fusion of muscle precursor cells (derived from mesoderm) ● Leaves each cell with more than one nucleus ● Elongates cells ○ Sarcolemma: excitable cell membrane ○ Myofibrils: contract in response to an action potential sweeping down the sarcolemma ○ Sarcoplasmic reticulum: intracellular sac that stores Ca2+ ■ surrounds myofibrils ○ T tubules: bring action potential to SR / inside-out axons ■ Continuous w/ extracellular fluid ■ Tetrade: voltage sensitive cluster of 4 Ca2+ channels } linked to release in SR and receives AP from tubule -> Ca2+ in cytosol causes myofibril to contract ● Molecular basis muscle contraction ○ Myofibril: divided into segments by disks} z lines ○ Sarcomere: segment of 2 z lines and myofibril in between ○ Thin filaments: bristles anchored to sides of Z lines ○ Thick filaments: between thin filaments ○ Contraction: when thin filaments slide along thick so Z lines come closer ■ Shortens sarcomere ■ Heads of myosin binds to actin (major thin filament protein)-> causes pivot ■ ATP binds to myosin so process of contracting can repeat ■ At rest proteins cover actin (leaves when Ca2+ shifts protein position) ● Relaxation: Ca2+ is sequestered by SR w/ calcium pump and ATP ○ Excitation 1. An action potential occurs in an alpha motor neuron axon. 2. ACh is released by the axon terminal of the alpha motor neuron at the neuromuscular junction. 3. Nicotinic receptor channels in the sarcolemma open, and the postsynaptic sarcolemma depolarizes (EPSP). 4. Voltage-gated sodium channels in the sarcolemma open and an action potential is generated in the muscle fiber, which sweeps down the sarcolemma and into the T tubules. 5. Depolarization of the T tubules causes Ca 2 release from the SR. ○ Contraction 1. Ca 2 binds to troponin. 2. Tropomyosin shifts position and myosin binding sites on actin are exposed. 3. Myosin heads bind actin. 4. Myosin heads pivot. 5. An ATP binds to each myosin head and it disengages from actin. 6. The cycle continues as long as Ca 2 and ATP are present. ○ Relaxation 1. As EPSPs end, the sarcolemma and T tubules return to their resting potentials. 2. Ca 2 is sequestered by the SR by an ATP-driven pump. 3. Myosin binding sites on actin are covered by tropomyosin. ● neuromuscular junction: synapses ● motor end plate: like the terminal >Synapsing onto.. As opposed to another cell ● Proprioceptors >Primary: stretch receptor ● In spinal motor neurons: spindles, golgi, and joints ● Ia axons: fast + myelinated ○ Enter spinal cord via dorsal roots->excitatory synapses on interneurons and alpha motor neurons of ventral horns ● Stretch reflex: increase stretch=increase discharge rate ○ Monosynaptic stretch reflex arc: Ia axon and alpha motor neurons ● Golgi tendon organ: over contract or too much stretch-> muscles get inhibited to protect from ripping it off the bone } sensory organ ○ Monitors contraction ○ Innervated by Ib sensory axons ○ Situated in series unlike parallel spindles ○ Good for fragile materials ■ Ia spindle: muscle length ■ Ib golgi: muscle tension ● inhibitory interneurons: inhibit opposing muscle or that same muscle III. Wiring up a muscle (stretch) reflex arc IV. More complex spinal systems ● crossed-extensor reflex} contralateral - do the opposite on the other foot ex : contract extensor and inhibit flexor -> > reverse on other side ~reciprocal inhibition ● central pattern generator: input from brain-> patterns of circling info at spinal level to generate outputs} for walking ○ Depolaraize Glu-> release of Ca2+ and Na+ -> K+ exits/ hyperpolarize ○ Walking: steady input excites 2 interneurons that connect to motor neurons
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