Muscle Spindles PDF

Muscle Mrs Sinkala skeletal muscles Alpha motor neurons in the spinal cord, innervate the extrafusal fibers of skeletal muscles. These are responsible for the contraction of muscles in upper limbs, trunk and lower part of the body. Muscle spindles Muscle spindles are collections of 6-8...

Muscle Mrs Sinkala skeletal muscles Alpha motor neurons in the spinal cord, innervate the extrafusal fibers of skeletal muscles. These are responsible for the contraction of muscles in upper limbs, trunk and lower part of the body. Muscle spindles Muscle spindles are collections of 6-8 specialized muscle fibers that are located within the muscle mass itself. These fibers do not contribute significantly to the force generated by the muscle. Rather, they are specialized receptors that signal (a) the length and (b) the rate of change of length (velocity) of the muscle. Muscle Spindles Muscle spindles are proprioceptors that consist of intrafusal muscle fibers enclosed in a sheath (spindle). They run parallel to the extrafusal muscle fibers and act as receptors that provide information on muscle length and the rate of change in muscle length. Are composed of a few intrafusal muscle fibers that lack actin and myosin in their central regions, are noncontractile, and serve as receptive surfaces.  Muscle spindles are wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers. The regions of muscle spindles are innervated by gamma (γ) efferent fibers. Contractile muscle fibers are extrafusal fibers and are innervated by alpha (α) efferent fibers. Each muscle contains many muscle spindles; muscles that are necessary for fine movements contain more spindles than muscles that are used for posture or coarse movements.(opp to extrafusal muscle fibers) Muscle Spindles Types of Muscle Spindle Fibers Nuclear Chain fibers. These fibers have nuclei which is aligned in a single row (chain) in the center of the fiber. They signal information about the static length of the muscle. Static Nuclear Bag fibers. These fibers have nuclei which is collected in a bundle in the middle of the fiber. Like the nuclear chain fiber, these fibers signal information about the static length of a muscle. Types of Muscle Spindle Fibers Dynamic Nuclear Bag fibers. These fibers are similar to the static nuclear bag fibers, but they signal primarily information about the rate of change (velocity) of muscle length. A typical muscle spindle is composed of 1 dynamic nuclear bag fiber, 1 static nuclear bag fiber, and ~5 nuclear chain fibers. Sensory Innervation of Muscle Spindles Muscle spindle is located in parallel with the extrafusal fibers, it will stretch along with the muscle. The muscle spindle signals muscle length and velocity to the CNS through two types of specialized sensory fibers that innervate the intrafusal fibers. These sensory fibers have stretch receptors that open and close as a function of the length of the intrafusal fiber. Group Ia afferents (also called primary afferents) wrap around the central portion of all 3 types of intrafusal fibers; these specialized endings are called annulospiral endings. Because they innervate all 3 types of intrafusal fibers, Group Ia afferents provide information about both length and velocity. Group II afferents (also called secondary afferents) Innervate the ends of the nuclear chain fibers and the static nuclear bag fibers at specialized junctions termed flower spray endings. Because they do not innervate the dynamic nuclear bag fibers, Group II afferents signal information about muscle length only. responses of each type of afferent to a linear stretch of the muscle. The Group Ia afferent fires at a very high rate during the stretch, encoding the velocity of the muscle length; at the end of the stretch, its firing decreases, as the muscle is no longer changing length. Note, however, that its firing rate is still higher than it was before the stretch, as it is now encoding the new length of the muscle. The Group II afferent increases its firing rate steadily as the muscle is stretched. Its firing rate does not depend on the rate of change of the muscle; rather, its firing rate depends only on the immediate length of the muscle.  The Group Ia afferent responds at a highest rate when the muscle is actively stretching, but also signals the static length of the muscle because of its innervation of the static nuclear bag fiber and the nuclear chain fiber. The Group II afferent signals only the static length of the muscle, increasing its firing rate linearly as a function of muscle length. Operation of the Muscle Spindles Motor Neuron innervation Although intrafusal fibers do not contribute significantly to muscle contraction, they do have contractile elements at their ends that are innervated by motor neurons. Motor neurons are divided into two groups. Alpha motor neurons innervate extrafusal fibers, the highly contracting fibers that supply the muscle with its power. Gamma motor neurons innervate intrafusal fibers, which contract only slightly. Muscle Spindles  Gamma activation of the intrafusal fiber helps to keep the muscle spindle stretched and therefore sensitive to stretch, over a wide range of muscle lengths. If a resting muscle is stretched, the muscle spindle becomes stretched in parallel, sending signals through the primary and secondary afferents. A subsequent contraction of the muscle, however, removes the pull on the spindle, and it becomes slack and the spindle stops firing.  Activation of gamma motor neurons prevents this temporary insensitivity by causing a weak contraction of the intrafusal fibers, in parallel with the contraction of the muscle. Instructions on alpha innerrvations also instructs gamma motor neuron innervation and this is called alpha-gamma coactivation. Operation of the Muscle Spindles Stretching the muscles activates the muscle spindle There is an increased rate of action potential in Ia fibers Contracting the muscle reduces tension on the muscle spindle There is a decreased rate of action potential on Ia fibers Golgi Tendon Organ It is a specialized receptor that is located between the muscle and the tendon. Golgi tendon organ is located in series with the muscle and signals information about the load or force being applied to the muscle. A Golgi tendon organ is made up of a capsule containing numerous collagen fibers.  The organ is innervated by primary afferents called Group Ib fibers. When force is applied to a muscle, the Golgi tendon organ is stretched, causing the collagen fibers to squeeze and distort the membranes of the primary afferent sensory endings. As a result, the afferent is depolarized, and it fires action potentials to signal the amount of force. For skeletal muscles to perform normally: The Golgi tendon organs (proprioceptors) must constantly inform the brain as to the state of the muscle. Stretch reflexes initiated by muscle spindles must maintain healthy muscle tone. THE END