Muscle Control

Spindle organs in mucle contain two types of fibers, nuclear bag fibers which are not contractile and have many clumped nuclei, and nuclear chain fibers which are contractile and have rows of nuclei. Both of these fibers are intrafusal or inside the spindle.  Sensory fibers run from the spindle organ back to the CNS. Nuclear bag fibers have spiral endings and nuclear chain fibers have "flower spray" endings.  There are also motor fibers that run to the spindle -- gamma fibers, and fibers that run to the surrounding muscle cells -- alpha fibers.

At Rest sensory endings in the spindle organ are tonically active indicating a normal degree of stretch. These fibers synapse with alpha motor fibers in the spinal column so these fibers are tonically active too to keep some tone to the muscle.

When the muscle is stretched ( like trying to lift a very heavy weight), spindle sensory nerves fire. These synapse with alpha motor neurons and contraction of the muscle is increased. In turn the spindle fibers are less stretched.

With relatively weak or slow  stimulation to the muscle alpa motor neurons cause contraction, gamma neurons don't fire and sensory or afferent fibers from the spinde decrease their firing rate. This decrease helps to decrease stimulation to the alpha motor neurons i.e. muscle doesn't contract too much.

With a strong or sharp stimulus, the alpha and gamma fibers fire together. The gamma fibers help to "take the slack" out of the spindle so that sensory fibers from the spindle to not inhibit alpha stimulated firing and the muscle contraction response is stronger .
 
 

In isotonic contraction, the muscle shortens when lifting or moving an object. The lighter the load, the higher the velocity of the muscle. When there is no load, the muscle velocity (lengths/second) is maximal (Vmax), but the muscle is not doing any work so the power is zero. In isometric contraction, the muscle does not shorten but the number of cross bridges between the actin and the myosin does increase. This muscle is trying to move or lift a load that is too heavy for it. The velocity at this high load is zero because the muscle is not shortening, power is zero because a weight is not moved. Note that the max power is generated about half-way between isometric and isotonic contraction.

Different muscle fibers have different characteristics due to differences in myosin ATPase, availabilty of calcium, number of mitochondria etc. Here two muscles are compared. Once has a high Vmax and is like a fast twitch fiber, the other with a low Vmax is similar to a slow oxidative fiber. The high Vmax muscle has more power, but it also uses more energy and at low velocities it is less efficient.  An example of use of these muscles is swimming in a fish. During normal cruising, low Vmax muscle does most of the work, but when an escape response is needed, high Vmax muscle allows a quick burst of speed.

Arrangement of muscle

Class I and Class II levers provide mechanical advantage; the power arm is greater than the weight arm (distance from the pivot point). Most of our muscle are Class III levers. They don't provide mechanical advantage but produce the greatest arc of movement for the smallest shortening distance-- so give us speed and agility.

Smooth Muscle