New Content F. of Neuro Final Exam

starting at lecture 20

when and how are muscle fibers formed?

muscle fibers formed early in fetal development by the fusion of muscle precursor cells

muscle fiber structure—does each cell have more than one cell nucleus? T/F

True

what is the excitable cell membrane muscle fibers are enclosed by?

sarcolemma

what are the number of cylindrical structures within the muscle fiber, that contract, called?

myofibrils

excitation-contraction coupling: muscle contraction

• alpha motor neurons release ACh

• ACh produces large EPSP in muscle fiber

• EPSP evokes muscle action potential

• Action potential triggers CA2+ release

• fiber contracts

• ca2+ reuptake

• fiber relaxes

what are z lines?

division of myofibril into segments by disks

what is a sarcomere?

two z lines and myofibril

what are thin filaments?

series of bristles anchored to z lines

where are thick filaments?

between and among thin filaments

what happens when the thin filaments slide along the thick filaments, bringing adjacent Z lines toward one another?

muscle contraction occurs

steps in excitation-contraction coupling: sliding filament model & death

Sliding filament model

• binding of Ca2+ causes myosin (thick filament protein) to bind to actin (thin filament protein)

• myosin “heads” pivot, cause filaments to slide

• ATP unlocks this interaction

• repetition of process “walks” myosin heads along filament

Death causes stiffening of the muscles

what are specialized structures deep within skeleton muscles?

spindles

what does sensory feedback from muscle spindles do?

stretch receptors

what are spindles and their associated Ia axons, specialized for the detection of changes in muscle length (stretch) examples of?

proprioceptors (“body sense”)

the stretch reflex: an antigravity feedback loop

• a weight is placed on a muscle, the muscle spindles are stretched

• the stretching leads to depolarization of the Ia axon endings due to the opening of mechanosensitive ion channels

• increased action potential discharge of the Ia axons synaptically depolarizes the motor neurons

• this causes the muscle to contract, thereby shortening it

the stretch reflex

stretch reflex: muscle pulled —> tendency to pull back

• discharge of Ia sensory axons is closely related to the length of the muscles. as the muscle is stretched, the discharge rate goes up; as the muscle is shortened and goes back, the discharge rate goes down

• feedback loop

• monosynaptic

• ex) knee-jerk reflex

many inputs to motor neurons are mediated by ___ ____

spinal interneurons

what is reciprocal inhibition?

contraction of one muscle set is accompanied by relaxation of antagonist muscle

• collaterals of the Ia axons synapse on inhibitory spinal interneurons that contact the motor neurons supplying the antagonist muscles

circuitry of the flexor withdrawal reflex

flexor reflect: withdraw a limb from an aversive stimulus

• excitatory input

• activate interneurons in several different spinal segments. these cells eventually excite the motor neurons that control all the flexor muscles of the affected limb

what is the activation of extensor muscles and inhibition of flexors on opposite sides during flexor reflex?

crossed-extensor reflex

• compensates for the extra load imposed by limb withdrawal

central pattern generators and examples?

circuits that give rise to rhythmic motor activity

ex) spinal motor programs for walking

• circuitry for walking resides in spinal cord

• rhythmic activity in spinal interneurons

• coordination depends on multiple mechanisms

possible circuit for rhythmic alternating activity

• a steady input excites 2 interneurons that connect to the motor neurons controlling the flexors and extensors, respectively

• the interneurons respond by bursts of outputs. the activity of these interneurons alternates because they inhibit each other via interneurons

• activity in one interneuron strongly inhibits the other, and vice versa

• the movements of the opposite limb could be coordinated so that flexion on one side is accompanied by extension on the other