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what are synergists?
muscles that work together
what are the somatic motor neurons in the ventral horn of the spinal cord called?
lower motor neurons
what are upper motor neurons?
higher-order neurons of the brain that supply input to the spinal cord.
Explain why there is a cervical and lumbar enlargement of the spinal cord
since the portions of the body corresponding to these spinal cord segments innervate more muscle fibers, they are larger than other segments of the spinal cord
what do alpha motor neurons do?
they directly trigger the generation of force by muscles
what is a motor unit?
one alpha motor neuron and all the muscle fibers it innervates make up a motor unit
what is the collection of alpha motor neurons that innervate a single muscle called?
a motor neuron pool
what are the two ways in which the CNS controls muscle contraction?
1. by varying the firing rate of motor neurons
2. by recruiting additional synergistic motor units
what are the inputs to alpha motor neurons?
there are only three major sources of input to an alpha motor neuron:
1. the first source is the dorsal root ganglion cells with axons that innervate a specialized sensory apparatus embedded within the muscle known as a muscle spindle. This input provides feedback about muscle lenght.
2. The second source of input derives from upper motor neurons in the motor cortex and brain stem. This input is important for the initation and control of voluntary movement.
3. The third and largest input to an alpha motor neuron derives from interneurons in the spinal cord. This input may be excitatory or inhibitory and is part of the circuitry that generates the spinal motor programs.
describe the two types of muscle fibers (red and pale)
the red muscle fibers are characterized by a large number of mitochondria and enzymes specialized for oxidative energy metabolism. These fibers are relatively slow to contract but can maintain contraction for a long time.
The pale muscle fibers contain fewer mitochondria and rely mainly on anaerobic metabolism. These fibers contract rapidly and powerfully, but they also fatigue rapidly
what occurs if the normal innervation of a fast muscle is removed and replaced with a nerve that normally innervates slow muscle?
the muscle acquires slow properties, including not only the type of contraction (slow, fatigue-resistant), but also a switch in the underlying biochemistry of the muscle.
what is hypertrophy?
an exaggerated growth of a muscle (think bodybuilders)
muscle fibers are enclosed by an excitable cell membrane called the ________
within the muscle fiber are a number of cylindrical structures called _______, which contract in response to an action potential sweeping down the sarcolemma
myofibrils are surrounded by the ______ _______, an extensive intracellular sac that stores Ca ions.
what are T tubules? describe their function and location
the T tubules surround myofibrils. The T tubules come in close apposition to the SR. Where the T tubules come into close apposition to the SR, there is a specialized coupling of the proteins in the two membranes. A voltage-sensitive cluster of four calcium channels, called a tetrad, in the T tubule membrane is linked to a calcium release channel in the SR.
The arrival of an action potetnial in the T tubule membrane causes a conformational change in the voltage-sensitive tetrad of channels, which opens the calcium release channel in the SR membrane. Some Ca ions flow through the tetrad, and even more Ca ions flow through the caclium release channel, and the resulting increase in free Ca ions in the cytosol causes the myofibril to contract
what are Z lines?
disks onto which the thin filaments are anchored.
a segment comprised of two Z lines and the myofibril in between is called a _______
anchored to each side of the z lines is a series of bristles called ____ ________
describe where thick filaments are located
they are located between and among the two sets of thin filaments
the thick filament is made up of what major protein?
the thin filament is made up of what major protein?