Which of the following factors are responsible for the high speed of the knee jerk reflex

The patellar reflex, also called the knee reflex or knee-jerk, is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord.

Mechanism[edit]

Representation of the patellar reflex pathway.

Striking of the patellar tendon with a reflex hammer just below the patella stretches the muscle spindle in the quadriceps muscle. This produces a signal which travels back to the spinal cord and synapses (without interneurons) at the level of L3 or L4 in the spinal cord, completely independent of higher centres. From there, an alpha motor neuron conducts an efferent impulse back to the quadriceps femoris muscle, triggering contraction. This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle causes the leg to kick. There is a latency of around 18 ms between stretch of the patellar tendon and the beginning of contraction of the quadriceps femoris muscle. This is a reflex of proprioception which helps maintain posture and balance, allowing to keep one's balance with little effort or conscious thought.

The patellar reflex is a clinical and classic example of the monosynaptic reflex arc. There is no interneuron in the pathway leading to contraction of the quadriceps muscle. Instead, the sensory neuron synapses directly on a motor neuron in the spinal cord. However, there is an inhibitory interneuron used to relax the antagonistic hamstring muscle (reciprocal innervation).

This test of a basic automatic reflex may be influenced by the patient consciously inhibiting or exaggerating the response; the doctor may use the Jendrassik maneuver in order to ensure a more valid reflex test.

Clinical significance[edit]

After the tap of a hammer, the leg is normally extended once and comes to rest. The absence or decrease of this reflex is problematic, and known as Westphal's sign. This reflex may be diminished or absent in lower motor neuron lesions and during sleep. On the other hand, multiple oscillation of the leg (pendular reflex) following the tap may be a sign of cerebellar diseases. Exaggerated (brisk) deep tendon reflexes such as this can be found in upper motor neuron lesions, hyperthyroidism, anxiety or nervousness. The test itself assesses the nervous tissue between and including the L2 and L4 segments of the spinal cord.

The patellar reflex is often tested in infants to test the nervous system.

History[edit]

Wilhelm Heinrich Erb (1840–1921) and Carl Friedrich Westphal (1833–1890) simultaneously reported the patellar tendon or knee reflex in 1875. The term knee-jerk was recorded by Sir Michael Foster in his Textbook of physiology in 1877: "Striking the tendon below the patella gives rise to a sudden extension of the leg, known as the knee-jerk."

In popular culture[edit]

The term began to be used figuratively from the early 20th century onwards. O. O. McIntyre, in his New York Day-By-Day column in The Coshocton Tribune, October 1921, wrote: "Itinerant preacher stemming Broadway on a soap box. And gets only an occasional knee-jerk."

The trigeminal reflexes include the corneal (blink) reflex, and the jaw jerk (masseter) reflex. The corneal reflex, or blink reflex, is the involuntary blinking of the eyelids caused by something touching the cornea of the eye. It can also result from any peripheral stimulus. This utilizes the orbicularis oculi muscles, which are the facial nerve efferents. The reflex is rapid, occurring in just 0.1 s. Its purpose is to protect the eyes from foreign bodies and bright lights. When bright lights are involved, it is also described as the optical reflex, which occurs more slowly, mediated by the visual cortex in the occipital lobe of the brain. The optical reflex is absent in infants less than 9 months of age. The corneal reflex also occurs when sounds louder than 40–60 dB are made. The reflex is mediated by the nasociliary branch of the ophthalmic branch of the trigeminal nerve that senses corneal stimulation via afferent fibers. It is also mediated by the temporal and zygomatic branches of the facial nerve that initiate the motor response via efferent fibers. Additional mediation is from the nucleus in the pons. The use of contact lenses can reduce or stop testing of this reflex. Damage to the ophthalmic branch of the trigeminal nerve results in absent corneal reflex when the affected eye is stimulated. Stimulation of one cornea usually has a consensual response, with both eyelids closing as a result.

The masticatory nucleus is the relay for the jaw jerk reflex, which is the only important supraspinal monosynaptic reflex. This reflex is brought about by lightly tapping the relaxed and open jaw, in a downward direction. It is also described as the masseter reflex or mandibular reflex, and is used to test the status of the trigeminal nerve as well as to help distinguish an upper cervical cord compression from lesions located above the foramen magnum. The mandible is tapped just below the lips, at the chin, while the mouth is held slightly open. The response is an upward jerking of the mandible by the masseter muscles—a very slight or sometimes absent movement. In an individual with upper motor neuron lesions, this reflex may be extremely pronounced. This reflex is classified as a dynamic stretch reflex. Sensory neurons of the trigeminal mesencephalic nucleus send axons to the trigeminal motor nucleus, which innervates the masseter. While not part of a standard neurological examination, testing this reflex is done when there are other signs of damage to the trigeminal nerve. A normal jaw jerk reflex points diagnosis toward cervical spondylotic myelopathy, and away from multiple sclerosis or amyotrophic lateral sclerosis. The jaw jerk reflex is enhanced in spastic bulbar palsy.

Focus on the jaw jerk reflex

Fast stretching of muscles that close the jaw activates muscle spindle afferents. These travel via the trigeminal nerve's mandibular division to the brainstem. The muscles that close the jaw include the masseter, medial pterygoid, and temporalis. Cell bodies of the primary afferent neurons are within the mesencephalic trigeminal nucleus. Collaterals project, monosynaptically, to the trigeminal nerve's motor nucleus within the pons. Motor axons from the nucleus travel along the mandibular nerve, innervating muscles acting upon the temporomandibular joint, to close the jaw.

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Jaw Movement and Its Control

Greg M. Murray, in Functional Occlusion in Restorative Dentistry and Prosthodontics, 2016

Classification of Jaw Movements

•

Voluntary movements: for example, playing the piano, speaking, taking an alginate impression, moving the jaw forward

•

Reflex movements: for example, knee-jerk reflex, jaw-jerk reflex, and jaw-opening reflex

•

Rhythmical movements: for example, chewing, walking, running, and breathing

Voluntary Movements

Voluntary movements are driven by the primary motor cortex (termed MI) and higher motor cortical areas (SMA and premotor cortex; for review, see Hannam & Sessle 1994). The face MI is responsible for driving voluntary movements of the face, jaw, and tongue. When patients are asked to move the tongue forward and open the jaw (as when taking an impression), a set of programs (much like computer programs) is selected and activated (via the basal ganglia), and these programs send signals to the face MI. The motor programs contain the details of those motor units that must be activated and the sequence of activation, to produce a particular movement. The programs probably reside in the SMA and premotor cortical regions. The MI is responsible for activating the various motor units to produce the movement required.

The face MI consists of specific output zones within the cerebral cortex that send fibers in the pyramidal tract to synapse directly or indirectly (via interneurons) onto alpha-motoneurons. Each output zone from the face MI activates a specific elemental movement: for example, movement of the tongue forward or movement of the tongue to the side, or elevation of the corner of the mouth, or jaw opening or jaw movement to the side. The same movement can be produced at a number of different sites throughout the face MI.

The face MI can be considered the “keys of a piano” that the higher motor centers “play” to allow the generation of the required voluntary movement. Combinations of output zones allow the generation of more complex movements (equivalent to the generation of more complex sounds, as when playing chords on a piano).

The cerebellum continuously coordinates movements by controlling the sensory inputs to the motor areas. Corrections to each movement can also occur via shorter pathways that involve fewer neurons, and many of these pathways are located entirely at the brainstem level. These pathways can be demonstrated clinically by evoking reflexes.

Reflex Movements

Reflex movements are largely organized at the brainstem or spinal cord level (for review, see Hannam & Sessle 1994). They are stereotyped movements that are involuntary and are little modified by voluntary will.

The classic reflex is the knee-jerk reflex, where a sharp tap to the knee evokes contraction in the thigh muscles and a brief lifting of the lower leg. In the jaw motor system, reflexes include the jaw-closing or jaw-jerk reflex, and the jaw-opening reflex.

The jaw-closing reflex occurs when the jaw-closing muscles are suddenly stretched by a rapid downward tap on the chin. This tap causes stretching of specialized sensory receptors called muscle spindles that are stretch sensitive. They are present within all the jaw-closing muscles. When spindles are stretched, a burst of action potentials travels along the group Ia primary afferent nerve fibers coming from the primary endings within the spindles. The primary afferents synapse directly onto and cause activation of the alpha-motoneurons of the same jaw-closing muscle. Thus a stretch of a jaw-closing muscle leads to a fast contraction of the same jaw-closing muscle. This reflex assists in preventing the jaw from flopping up and down during running.

Reflexes demonstrate a pathway that can be used by the higher motor centers for the generation of more complex movements. They also allow fast feedback that adjusts a movement to overcome small, unpredicted irregularities in the ongoing movement and adds smoothness to a movement. Thus, for example, unexpected changes in food bolus consistency during chewing can modulate muscle spindle afferent discharge, and this altered discharge can change alpha-motoneuron activity to help overcome the change in food bolus consistency.

The jaw-opening reflex can be evoked by a variety of types of orofacial afferents. Activity in orofacial afferents, for example, from mucosal mechanoreceptors, passes along primary afferent nerve fibers to contact inhibitory interneurons that then synapse on jaw-closing alpha-motoneurons. The inhibitory interneurons reduce the activity of the jaw-closing motoneurons. At the same time, primary afferents activate other interneurons that are excitatory to jaw-opening muscles, such as the digastric. The overall effect is an opening of the jaw.

Rhythmical Movements

These movements share features of both voluntary and reflex movements (for review, see Lund 1991, Hannam & Sessle 1994). The reflex features of rhythmical movements arise because we do not have to think about these movements for them to occur. For example, we can chew, breathe, swallow, and walk without thinking specifically about the task; however, at any time, we can voluntarily alter the rate and magnitude of these movements.

Rhythmical movements are generated and controlled by collections of neurons in the brainstem or spinal cord. Each collection is called a central pattern generator. The central pattern generator for mastication is located in the pontine-medullary reticular formation. Figure 5-2 shows some relations of the central pattern generators in the brainstem. Swallowing is also controlled by a central pattern generator located in the medulla oblongata.

A central pattern generator is essentially equivalent to a computer program. When activated, the central pattern generator for mastication, for example, sends out appropriately timed impulses of the appropriate magnitude to the various jaw, face, and tongue muscle motoneurons so that the rhythmical movement of mastication can occur. We do not have to think about which motor units in which muscles to activate and the relative timing of activation of the motor units to carry out mastication. This is done by the central pattern generator. We can, however, voluntarily start, stop, and change the rate, magnitude, and shape of the chewing movements, and these modifications are done through descending commands to the central pattern generators from the motor cortical regions.

Figure 5-3, A, shows electromyographic (EMG) data from a number of jaw muscles during the right-sided chewing of gum. The associated movement of the midincisor point is shown in the lower panel. Note the regular bursting pattern of EMG activity that occurs in association with each cycle of movement. Note also, in the expanded version in Figure 5-3, B, that the EMG activity from the inferior head of the lateral pterygoid muscle and the submandibular group of muscles is out of phase with the jaw-closing muscles. All muscles are controlled by the central pattern generator, and many other jaw, face, and tongue muscles, not recorded here, are being activated similarly.

Sensory feedback is provided by mechanoreceptors located within orofacial tissues: for example, periodontal mechanoreceptors signal the magnitude and direction of tooth contact; mucosal mechanoreceptors signal food contact with mucosa; muscle spindles signal muscle length and rate of change of muscle length as the jaw closes; Golgi tendon organs signal forces generated within muscles; and temporomandibular (TM) joint mechanoreceptors signal jaw position.

The muscle spindle is a very complicated sensory receptor. Muscle spindle sensitivity is optimized for all lengths of a muscle. During a muscle contraction, both alpha- and gamma-motoneurons are activated. The alpha-motoneurons cause contraction of the main (extrafusal) muscle fibers and are responsible for the force produced by muscles (Fig. 5-4). The gamma-motoneurons are activated at the same time, but they cause contraction of the intrafusal muscle fibers within the muscle spindle and thus maintain the sensitivity of the spindles as the muscle and spindles shorten (Fig. 5-4, C). The spindle is therefore always able to detect small changes in muscle length, irrespective of the length of the muscle.

Sensory information plays a crucial role in adjusting the chewing cycle to accommodate for changes in food bolus consistency (for review, see Lund & Olsson 1983). Chewing is associated with a barrage of sensory information entering the CNS (Fig. 5-5, A). Some of this information travels directly to the cerebral cortex for conscious sensation. Local reflex effects that assist the masticatory process also occur. For example, as food is crushed between the teeth, periodontal mechanoreceptors are activated, and this activity can cause a reflex increase in activity in the jaw-closing muscles to assist in crushing of food.

Many of the orofacial afferents that are activated by food contact during jaw closing can evoke a jaw-opening reflex, as discussed earlier. This would be counterproductive during the closing phase of mastication. Lund & Olsson (1983) have shown that the masticatory central pattern generator depresses the responsiveness of the jaw-opening reflex during the closing phase of the chewing cycle. The low T (i.e., threshold) test reflex response shown in Figure 5-5, B, on the far left is the control jaw-opening reflex response, seen in the digastric muscle, to the activation of orofacial afferents when there is no chewing. During the closing phase of the chewing cycle, the central pattern generator depresses the ability to evoke this reflex. Therefore, in chewing, the excitatory pathway from orofacial afferents to jaw-opening motoneurons is depressed, and this allows the jaw to close unhindered.

An analogous effect occurs during the opening phase of the chewing cycle. During this phase, muscle spindles in jaw-closing muscles will be stretched and will have a tonic excitatory effect on jaw-closing motor units. This would resist jaw opening. However, the central pattern generator hyperpolarizes (i.e., inhibits) jaw-closing motoneurons during the opening phase of the chewing cycle (Fig. 5-5, B). This hyperpolarization makes jaw-closing motoneurons harder to activate in response to excitatory input from muscle spindles.

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The Trigeminal Nerve

Sashank Prasad, Steven Galetta, in Textbook of Clinical Neurology (Third Edition), 2007

Pontine Nucleus and Lower Motor Nucleus

The motor nucleus of cranial nerve V is located in the rostral pons medial to the principal sensory nucleus. The motor nucleus also receives fibers from the mesencephalic nucleus, which receives jaw proprioceptive information, to mediate the efferent portion of the jaw jerk reflex. Motor axons extend from the motor trigeminal nucleus in the pons and travel along the petrous portion of the temporal bone through Meckel's cave. Motor efferents pass through the gasserian ganglion, join sensory V3 fibers, and exit the skull base via the foramen ovale. Motor efferents in the face travel in association with sensory V3 fibers and reach the masticatory muscles via the medial and lateral pterygoid, deep temporal, masseteric, and mylohyoid nerves. These muscles serve to open (lateral pterygoids, digastric, and mylohyoid) and close (masseter, temporalis, and medial pterygoids) the jaw and provide medial and lateral jaw movements necessary for effective mastication. Small motor branches also extend to the tensor veli palatini and tensor tympani muscles. The tensor veli palatini in the posterior pharynx is active during swallowing and tenses the soft palate against the tongue. The tensor tympani is a small muscle in the middle ear recruited during continuous loud sound; it serves to draw the malleus and tympanic membrane toward the medial wall of the middle ear to dampen the vibrations of the tympanic membrane.

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Trigeminal Nerve (Cranial Nerve V)

Perla I. Periut, Virgilio D. Salanga, in Encyclopedia of the Neurological Sciences, 2003

Trigeminal Reflexes

Afferent trigeminal impulses evoke a number of motor reflexes that depend on the polysynaptic pathway in the brainstem tegmentum. Some of these reflexes are proprioceptive or myotactic, whereas others are nociceptive. Stretch stimuli in muscles of mastication produce reflex contraction of jaw muscles (jaw jerk).

Other trigeminal reflexes include the corneal blink, orbicularis stretch, glabellar tap, and palatopalpebral reflex. The blink reflex occurs bilaterally and may be obtained by different stimulations, such as corneal touch, cutaneous stimulation, supraorbital nerve stimulation, or light flash. The afferent fibers are within the ophthalmic branch of the trigeminal nerve, and after several synapses the fibers connect to bilateral facial nuclei to activate the orbicularis oculi in both sides. The jaw jerk reflex is a monosynaptic muscle stretch reflex that is elicited by a brisk tap with a reflex hammer on the front of the chin while the mouth is slightly opened and the jaw is relaxed. The examiner's thumb may be placed on the chin and tapped, or a tongue blade may be placed on the lower teeth and then tapped. The expected response is a contraction of the masseter and temporalis muscles, which causes a sudden closing of the mouth. An increased jaw jerk reflex is characteristic of supranuclear involvement of the motor portion of the trigeminal nerve and, when exaggerated, may result in a sustained jaw clonus. Also, the jaw jerk reflex may be increased by anxiety, usually in association with diffuse physiological hyper-reflexia. A unilateral lesion of the corticobulbar fibers destined to trigeminal motor nuclei does not alter the function of the muscles of mastication because the motor nucleus of each side receives both crossed and uncrossed fibers. However, when the corticobulbar fibers are bilaterally interrupted in the motor cortex, subcortical regions, internal capsule, or midbrain, there is masticatory paresis that is part of the pseudobulbar palsy.

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The sense of touch in the control of ingestion

T. MORIMOTO, K. TAKADA, in Neurophysiology of Ingestion, 1993

(b) Jaw reflexes

Periodontal masseteric reflex. When the labial surface of the upper incisor is lightly tapped or the associated gingival tissue is stimulated electrically during light clenching of teeth, a reflexive electric potential is evoked in the masseter muscle. This is termed the periodontal masseteric reflex. The receptors are in the periodontal ligament (Goldberg, 1971). This reflex is found not only in man but also in rats, cats and monkeys.

In rats, continuous activity with long latency together with a transitory excitation is evoked in the masseter muscle when the upper incisor is pressed continuously in a linguo-labial direction (Fig. 6.7). This reflex is called as the tonic periodontal-jaw muscle reflex (Funakoshi and Amano, 1974). This response is influenced by the direction of a force exerted to the teeth. That is, if the upper incisor is pushed labio-lingually, the jaw-closing muscle activity is inhibited.

The periodontal-masseteric reflex arc may be monosynaptic via the trigeminal mesencephalic nucleus. The pathway for the tonic periodontal-jaw muscle reflex involves the periodontal ligament, the trigeminal ganglion, the trigeminal brain stem sensory complex, and jaw-closing motoneurones. These reflexes possibly contribute to the control of chewing force.

Jaw-jerk reflex. A jerk of the jaw is initiated by lightly tapping a small piece of board placed between the upper and lower teeth or against the chin. The transient stretching of jaw-closing musculatures produces a contraction of the jaw-closing muscle. This is a stretch reflex similar to the knee-jerk reflex. The muscle spindle is the sensory receptor and the reflex pathway goes via the trigeminal mesencephalic nucleus to jaw-closing motoneurones and the jaw-closing musculature (Fig. 6.2B). The reflex is usually monosynaptic, but a polysynaptic pathway also exists. The jaw-jerk reflex differs from other stretch reflexes in lacking reciprocal inhibition on the jaw-opening motoneurones. Moreover, a stretch reflex is not initiated by stretching the jaw-opening musculatures, since they contain very few spindles.

Various suggestions have been made regarding the physiological significance of the connections producing the jaw-jerk reflex. They may control the contraction force of jaw-closing musculatures promptly in response to the loading to the lower jaw produced by a bite during chewing of food or the unloading that is generated at the moment of crushing food (Lamarre and Lund, 1975). The loading reflex is important to control the chewing force in response to the hardness and consistency of food (Lavigne et al., 1987). With an unloading reflex, on the other hand, an immediate reduction of force exerted on the lower jaw would arise at the moment of crushing hard bolus from a decrease in facilitation of jaw-closing musculatures via the jaw-jerk reflex, because muscle spindles have been relaxed by the sudden shortening of jaw-closing musculatures. A possible second function of the jaw-jerk reflex is to maintain jaw posture, i.e. the mandibular resting position. When the mandible is destabilized during running and jumping activities, the muscle spindle is activated and thus stiffens jaw-closing musculatures and resists instability (Goodwin et al., 1978).

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Head

Paul Rea MBChB, MSc, PhD, MIMI, RMIP, FHEA, FRSA, in Essential Clinically Applied Anatomy of the Peripheral Nervous System in the Head and Neck, 2016

Clinical Examination

As with all clinical examinations, introduce yourself to the patient, stating who you are and your purpose for meeting with the patient, especially if you are a student. Always take a comprehensive history before examining the patient. This will guide you to the most appropriate examinations that need to be conducted. Ensure you explain everything to the patient prior to doing it. This ensures a better level of trust and excellent communication exists during the consultation.

There are two aspects that can be tested for clinically with the trigeminal nerve—the motor function of the nerve and if sensation is intact.

Ensure you take a detailed clinical history first.

Always tell the patient what you will be doing and what you expect them to do in helping elicit any signs and/or symptoms.

Observe the skin over the area of temporalis and masseter first to identify if any atrophy or hypertrophy is obvious.

First, palpate the masseter muscles while you instruct the patient to bite down hard. Also note masseter wasting on observation. Do the same with the temporalis muscle.

Then, ask the patient to open their mouth against resistance applied by the instructor at the base of the patient’s chin.

To assess the stretch reflex (jaw jerk reflex), ask the patient to have their mouth half open and half closed. Place an index finger onto the tip of the mandible at the mental protuberance, and tap your finger briskly with a tendon hammer. Normally this reflex is absent or very light. However, for patients with an upper motor neuron lesion, the stretch reflex (jaw jerk reflex) will be exaggerated.

Also ask the patient to move their jaw from side to side.

Next, test gross sensation of the trigeminal nerve. Tell the patient to close their eyes and say “sharp” or “dull” when they feel an object touch their face. Allowing them to see the needle, brush, or cotton wool ball before this examination may alleviate any fear. Using the needle, brush, or cotton wool, randomly touch the patient’s face with the object. Touch the patient above each temple, next to the nose, and on each side of the chin, all bilaterally. You must test each of the territories of distribution of the ophthalmic, maxillary, and mandibular nerves.

Ask the patient to also compare the strength of the sensation of both sides. If the patient has difficulty distinguishing pinprick and light touch, then proceed to check temperature and vibration sensation using the vibration fork. You can heat it up or cool it down in warm or cold water, respectively.

10.

Finally, test the corneal reflex (blink reflex). You can test it with a cotton wool ball rolled to a fine tip. Ask the patient to look at a distant object and then approaching laterally, touching the cornea (and not the sclera) looking for the eyes to blink. Repeat this on opposite eye. If there is possible facial nerve pathology on the side that you are examining, it is imperative to observe the opposite side for the corneal reflex.

Some clinicians omit the corneal reflex unless there is sensory loss on the face elicited from the history or examination, or if cranial nerve palsies are present at the pontine level. It is best to ensure a complete clinical examination is undertaken, however, especially if there is a possible pathology of the trigeminal nerve.

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Alzheimer's Disease, Neuropsychology of

Robin G. Morris, Claire L. Worsley, in Encyclopedia of the Human Brain, 2002

XV. Motor Functioning

The following are features of motor impairment in AD:

Pyramidal signs
Hyperreflexia
Plantar responses
Extrapyramidal signs
Tremor
Rigidity
Bradykinesia
Gait disturbances
Primitive reflexes
Apraxia
Ideational
Ideomotor

Not all people with AD exhibit the full spectrum of deficits. A general feature of AD is that the primary motor cortex is relatively preserved. However, pyramidal signs may be seen in AD; these include hyperreflexia, extensor plantar responses, hyperactive jaw jerk reflex, and ankle clonus. The mechanism for this is unclear but may involve either diffuse white matter or associated vascular dementia. Extrapyramidal signs exist, and the most common are bradykinesia (slowness of movement) and rigidity (increased resistance to passive movement). Tremor is infrequent. Gait abnormalities consist of slowness in walking, with decreased step length, paucity arm swing, and a droop posture, all of which may become more apparent as the dementia progresses. Myoclonus can occur early on, but motor seizures are regarded as happening later. Primitive reflexes of two types, nocioceptive and prehensile, are also seen in AD. The nocioceptive type, which includes the snout reflex, the glabellar blink reflex, and the palmomental reflex, occurs in approximately 20–50% of cases and can be seen early on. Of these, the snout and palmomental reflexes are the most common. The prehensile type, which includes grasping, sucking, and rooting, is less frequent (10–20% of cases) and is associated with late-stage dementia.

The two common types of apraxia are seen in AD, according to the strict definition of this term. Ideational apraxia, which involves impairment of the ability to perform actions appropriate to real objects, is frequently observed in AD, as is ideomotor apraxia. The latter refers to selection of elements that constitute movement, such as in copying gestures and miming usage. It has been estimated that the presence of both ideational and ideomotor apraxia occurs in 35% of patients with mild, 58% with moderate, and 98% with severe AD. Tool action knowledge has also been characterized as conceptual apraxia and found to be dissociable in AD from semantic language impairment.

In the moderate or severe range, another motor feature that can be observed is motor impersistence. This is an impairment in the ability to sustain a voluntary movement (e.g., exerting a steady hand grip or keeping the eyes closed). This is distinguished from apraxia because the movement can be performed and maintained with instruction. This type of motor impairment is strongly related to frontal lobe involvement.

What is responsible for the high speed of the knee

This reflex is a monosynaptic reflex, meaning that one neuron synapses onto a second neuron, leading to a response in the muscle. This monosynaptic connection is part of why the knee-jerk reflex is so fast.

What causes knee to jerk?

The knee-jerk reflex, also known as the patellar reflex, is a simple reflex that causes the contraction of the quadriceps muscle when the patellar tendon is stretched. I describe the course of the reflex arc from muscle spindles in the quadriceps muscle to motor neurons that cause movement of the leg.

Which of the following structures is involved in the human knee

Explanation: During the knee-jerk reflex, the neural signal is initiated by the stretching of the patella tendon, which is transmitted via sensory neuron to the spinal cord.