Muscle spindles are stretch receptors within the body of a muscle that detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers; this information can be processed by the brain as proprioception. The responses of muscle spindles to changes in length play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch; the muscle spindle has both sensory and motor components. Sensory information conveyed by primary type Ia sensory fibers and secondary type II sensory fibers, which spiral around muscle fibres within the spindle Motor action by up to a dozen gamma motor neurons and to a lesser extent by one or two beta motor neurons that activate muscle fibres within the spindle. Muscle spindles are found within the belly between extrafusal muscle fibers; the specialised fibers that constitute the muscle spindle are known as intrafusal fibers, to distinguish themselves from the fibres of the muscle itself which are called extrafusal fibers.
Muscle spindles have a capsule of connective tissue, run parallel to the extrafusal muscle fibers. Muscle spindles are composed of three to twelve muscle fibers, of which there are three types: dynamic nuclear bag fibers, static nuclear bag fibers, nuclear chain fibers and afferent nerve fibers. Sensory fibres spiral around the intrafusal muscle fibres; these fibres, primary type Ia sensory fibers and secondary type II sensory fibers, send information by stretch-sensitive ion-channels of the axons. The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons and one or two beta motor neurons, collectively called fusimotor neurons; these activate the muscle fibres within the spindle. Gamma motor neurons supply only muscle fibres within the spindle, whereas beta motor neurons supply muscle fibres both within and outside of the spindle. Activation of the neurons causes a contraction and stiffening of the end parts of the muscle spindle muscle fibers. Fusimotor neurons are classified as static or dynamic according to the type of muscle fibers they innervate and their effects on the responses of the Ia and II sensory neurons innervating the central, non-contractile part of the muscle spindle.
The static axons innervate bag2 fibers. They increase the firing rate of II afferents at a given muscle length; the dynamic axons innervate the bag1 intrafusal muscle fibers. They increase the stretch-sensitivity of the Ia afferents by stiffening the bag1 intrafusal fibers. Efferent nerve fibers of gamma motoneurons terminate in muscle spindles; when a muscle is stretched, primary type Ia sensory fibers of the muscle spindle respond to both changes in muscle length and velocity and transmit this activity to the spinal cord in the form of changes in the rate of action potentials. Secondary type II sensory fibers respond to muscle length changes and transmit this signal to the spinal cord; the Ia afferent signals are transmitted monosynaptically to many alpha motor neurons of the receptor-bearing muscle. The reflexly evoked activity in the alpha motoneurons is transmitted via their efferent axons to the extrafusal fibers of the muscle, which generate force and thereby resist the stretch; the Ia afferent signal is transmitted polysynaptically through interneurons, which inhibit alpha motorneurons of antagonist muscles, causing them to relax.
The function of the gamma motor neurons is not to supplement the force of muscle contraction provided by the extrafusal fibers, but to modify the sensitivity of the muscle spindle sensory afferents to stretch. Upon release of acetylcholine by the active gamma motor neuron, the end portions of the intrafusal muscle fibers contract, thus elongating the non-contractile central portions; this opens stretch-sensitive ion channels of the sensory endings, leading to an influx of sodium ions. This raises the resting potential of the endings, thereby increasing the probability of action potential firing, thus increasing the stretch-sensitivity of the muscle spindle afferents. How does the central nervous system control gamma fusimotor neurons? It has been difficult to record from gamma motoneurons during normal movement because they have small axons. Several theories have been proposed, based on recordings from spindle afferents. 1) Alpha-gamma coactivation. Here it is posited that gamma motoneurons are activated in parallel with alpha motoneurons to maintain the firing of spindle afferents when the extrafusal muscles shorten.
2) Fusimotor set: Gamma motoneurons are activated according to the novelty or difficulty of a task. Whereas static gamma motoneurons are continuously active during routine movements such as locomotion, dynamic gamma motoneoruns tend to be activated more during difficult tasks, increasing Ia stretch-sensitivity. 3) Fusimotor template of intended movement. Static gamma activity is a "temporal template" of the expected shortening and lengthening of the receptor-bearing muscle. Dynamic gamma activity turns on and off abruptly, sensitizing spindle afferents to the onset of muscle lengthening and departures from the intended movement trajectory, it is believed that muscle spindles play a critical role in sensorimotor development. After stroke or spinal cord injury in humans, spastic hypertonia
A reflex, or reflex action, is an involuntary and nearly instantaneous movement in response to a stimulus. A reflex is made possible by neural pathways called reflex arcs which can act on an impulse before that impulse reaches the brain; the reflex is an automatic response to a stimulus that does not receive or need conscious thought. Myotatic reflexes The myotatic reflexes, provide information on the integrity of the central nervous system and peripheral nervous system. Decreased reflexes indicate a peripheral problem, lively or exaggerated reflexes a central one. A stretch reflex is the contraction of a muscle in response to its lengthwise stretch. Biceps reflex Brachioradialis reflex Extensor digitorum reflex Triceps reflex Patellar reflex or knee-jerk reflex Ankle jerk reflex While the reflexes above are stimulated mechanically, the term H-reflex refers to the analogous reflex stimulated electrically, tonic vibration reflex for those stimulated to vibration. A tendon reflex is the contraction of a muscle in response to striking its tendon.
The Golgi tendon reflex is the inverse of a stretch reflex. Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive reflexes; these automatic reactions to stimuli enable infants to respond to the environment before any learning has taken place. They include: Asymmetrical tonic neck reflex Palmomental reflex Moro reflex known as the startle reflex Palmar grasp reflex Rooting reflex Sucking reflex Symmetrical tonic neck reflex Tonic labyrinthine reflex Other reflexes found in the central nervous system include: Abdominal reflexes Gastrocolic reflex Anocutaneous reflex Baroreflex Cough reflex Cremasteric reflex Diving reflex Muscular defense Photic sneeze reflex Scratch reflex Sneeze Startle reflex Withdrawal reflex Crossed extensor reflexMany of these reflexes are quite complex requiring a number of synapses in a number of different nuclei in the CNS. Others of these involve just a couple of synapses to function. Processes such as breathing and the maintenance of the heartbeat can be regarded as reflex actions, according to some definitions of the term.
In medicine, reflexes are used to assess the health of the nervous system. Doctors will grade the activity of a reflex on a scale from 0 to 4. While 2+ is considered normal, some healthy individuals are hypo-reflexive and register all reflexes at 1+, while others are hyper-reflexive and register all reflexes at 3+. List of reflexes All-or-none law Automatic behavior Conditioned reflex Instinct Jumping Frenchmen of Maine Voluntary action Preflexes
A vibrator is a mechanical device to generate vibrations. The vibration is generated by an electric motor with an unbalanced mass on its driveshaft. There are many different types of vibrator, they are components of larger products such as smartphones, vibrating sex toys, or video game controllers with a "rumble" feature. When smartphones and pagers vibrate, the vibrating alert is produced by a small component, built into the phone or pager. Many older, non-electronic buzzers and doorbells contain a component that vibrates for the purpose of producing a sound. Tattoo machines and some types of electric engraving tools contain a mechanism that vibrates a needle or cutting tool. Vibrators are used in many different industrial applications both as components and as individual pieces of equipment. Vibratory feeders and vibrating hoppers are used extensively in the food and chemical industries to move and position bulk material or small component parts; the application of vibration working with the force of gravity can move materials through a process more than other methods.
Vibration is used to position small components so that they can be gripped mechanically by automated equipment as required for assembly etc. Vibrating screens are used to separate bulk materials in a mixture of different sized particles. For example, gravel, river rock and crushed rock, other aggregates are separated by size using vibrating screens. Vibrating compactors are used for soil compaction in foundations for roads and buildings. Concrete vibrators consolidate freshly poured concrete so that trapped air and excess water are released and the concrete settles in place in the formwork. Improper consolidation of concrete can cause product defects, compromise the concrete strength, produce surface blemishes such as bug holes and honeycombing. An internal concrete vibrator is a steel cylinder about the size of the handle of a baseball bat, with a hose or electrical cord attached to one end; the vibrator head is immersed in the wet concrete. External concrete vibrators attach, to the concrete forms.
There are a wide variety of external concrete vibrators available and some vibrator manufacturers have bracket or clamp systems designed to fit the major brands of concrete forms. External concrete vibrators are available in pneumatic or electric power. Vibrating tables or shake tables are sometimes used to test products to determine or demonstrate their ability to withstand vibration. Testing of this type is done in the automotive and defense industries; these machines are capable of producing three different types of vibration profile sine sweep, random vibration, synthesized shock. In all three of these applications, the part under test will be instrumented with one or more accelerometers to measure component response to the vibration input. A sine sweep vibration profile starts vibrating at low frequency and increases in frequency at a set rate; the vibratory amplitude as measured in gs may decrease as well. A sine sweep will find resonant frequencies in the part. A random vibration profile will excite different frequencies along a spectrum at different times.
Significant calculation goes into making sure that all frequencies get excited to within an acceptable tolerance band. A random vibration test suite may range anywhere from 30 seconds up to several hours, it is intended to synthesize the effect of, for example, a car driving over rough terrain or a rocket taking off. A synthesized shock pulse is a short duration high level vibration calculated as a sum of many half-sine waves covering a range of frequencies, it is intended to simulate the effects of an explosion. A shock pulse test lasts less than a second. Vibrating tables can be used in the packaging process in material handling industries to shake or settle a container so it can hold more product