The lower trace shows the force generated by the isometrically contracting muscle. PLAY 1: Single spikes by the motor neuron produce small twitches of the muscle. PLAY 2: Multiple spikes in succession summate to produce larger contractions. PLAY 3: Very high rates of spikes produce maximal contraction called tetanus. Because motor units are recruited in an orderly fashion, weak inputs onto motor neurons will cause only a few motor units to be active, resulting in a small force exerted by the muscle Play 1.
With stronger inputs, more motor neurons will be recruited, resulting in more force applied to the muscle Play 2 and Play 3. Moreover, different types of muscle fibers are innervated by small and larger motor neurons. Small motor neurons innervate slow-twitch fibers ; intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibers ; and large motor neurons innervate fast-twitch, fatigable muscle fibers.
The slow-twitch fibers generate less force than the fast-twitch fibers, but they are able to maintain these levels of force for long periods. These fibers are used for maintaining posture and making other low-force movements. Fast-twitch, fatigue-resistant fibers are recruited when the input onto motor neurons is large enough to recruit intermediate-sized motor neurons.
These fibers generate more force than slow-twitch fibers, but they are not able to maintain the force as long as the slow-twitch fibers.
Finally, fast-twitch, fatigable fibers are recruited when the largest motor neurons are activated. These fibers produce large amounts of force, but they fatigue very quickly. They are used when the organism must generate a burst of large amounts of force, such as in an escape mechanism.
Most muscles contain both fast- and slow-twitch fibers, but in different proportions. Thus, the white meat of a chicken, used to control the wings, is composed primarily of fast-twitch fibers, whereas the dark meat, used to maintain balance and posture, is composed primarily of slow-twitch fibers.
Upper trace of oscilloscope represents the action potentials of a descending pathway axon. With low rates of activity of the descending pathway, only small alpha motor neurons are activated, producing small amounts of muscle force lower trace of oscilloscope.
With increasing rates of descending pathway activity, intermediate-size alpha motor neurons are activated in addition to the small neurons. Because more motor units are activated, the muscle produces more force. Finally, with the highest rates of descending activity, the largest alpha motor neurons are recruited, producing maximal muscle force. The motor system requires sensory input in order to function properly.
In addition to sensory information about the external environment, the motor system also requires sensory information about the current state of the muscles and limbs themselves. The muscle spindle signals the length of a muscle and changes in the length of a muscle. The Golgi tendon organ signals the amount of force being applied to a muscle. Muscle spindles are collections of specialized muscle fibers that are located within the muscle mass itself Figure 1. 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. Because of the fusiform shape of the muscle spindle, these fibers are referred to as intrafusal fibers. The large majority of muscle fibers that allow the muscle to do work are termed extrafusal 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. There are 3 types of muscle spindle fibers, characterized by their shape and the type of information they convey Figure 1.
Because the 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. The types of muscle tissue have different functions within your body:. Muscle fibers and muscles work to cause movement in the body. But how does this occur? While the exact mechanism is different between striated and smooth muscles, the basic process is similar.
The first thing that occurs is something called depolarization. Depolarization is a change in electric charge. It can be initiated by a stimulatory input like a nerve impulse or, in the case of the heart, by pacemaker cells. Depolarization leads to a complex chain reaction within muscle fibers. This eventually leads to a release of energy, resulting in muscle contraction.
Muscles relax when they stop receiving a stimulatory input. You may have also heard about something called fast-twitch FT and slow-twitch ST muscle. FT and ST refer to skeletal muscle fibers. FT and ST refer to how fast muscles contract. The speed at which a muscle contracts is determined by how quickly it acts on ATP.
So as far as endurance is concerned, the skeletal muscles listed from highest to lowest are:. ST fibers are good for long lasting activities. These can include things like holding a posture and stabilizing bones and joints. FT fibers produce shorter, more explosive bursts of energy. Examples include sprinting and weightlifting. Everyone has both FT and ST muscles throughout their body. Although each subdivision of the system is also called a "nervous system," all of these smaller systems belong to the single, highly integrated nervous system.
Each subdivision has structural and functional characteristics that distinguish it from the others. The nervous system as a whole is divided into two subdivisions: the central nervous system CNS and the peripheral nervous system PNS. The brain and spinal cord are the organs of the central nervous system. Because they are so vitally important, the brain and spinal cord, located in the dorsal body cavity , are encased in bone for protection. Somatomotor innervation of the orbicularis oculi, frontalis, procerus, and corrugator supercili is supplied by the facial nerve CNVII.
The motor neurons originate in the pons. Their fibers hook medially around the abducens nucleus in the medial pons before exiting at the cerebellopontine angle near the anterior inferior cerebellar artery. The motor division then enters the internal auditory canal, passes through the geniculate ganglion, travels next to the mastoid air cells, and exits the skull via the stylomastoid foramen.
After exiting the stylomastoid foramen, the facial nerve ascends enters the parotid gland, crosses the external carotid artery, and divides into the upper temporofacial and lower cervicofacial divisions. The upper temporofacial division divides into the temporal, zygomatic, and buccal branches deep in the orbicularis muscle.
The temporal and zygomatic branches supply the frontalis and orbicularis muscles on their deep surfaces. The temporal branch becomes superficial as it travels towards the lateral orbit and innervates additional muscles of facial expression. Additional innervation of the orbicularis comes from terminal branches of the facial nerve running in the fascial plane posterior to the orbicularis. The zygomatic and buccal branches may also co-innervate orbicularis and upper facial muscles.
The buccal branch may supply further innervation to the eyelid muscles; a superficial branch of the buccal passes medially and superiorly to supply the superomedial orbicularis, procerus, and corrugators. Sympathetic supply to the eyelid muscles originates in the hypohthalamus. Hypothalamic neurons travel ipsilaterally in the brainstem and synapse in the spinal cord.
Second-order neurons exit the spinal cord, travel in the sympathetic chain, and synapse in the superior cervical ganglion. Third-order neurons then travel along the carotid artery, follow the internal carotid artery and enter the cavernous sinus. From here, the fibers most likely travel from the internal carotid artery to the ophthalmic division of the trigeminal and enter the orbit via the superior orbital fissure to supply the superior and inferior tarsal muscles.
However, the exact pathway of the sympathetic fibers distal to the cavernous sinus remains controversial. By Jonathan J. Elsevier Inc. By Myron Yanoff and Jay S. Elsevier Inc, Barry Lee.
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