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The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the nictitating membrane remains in the corner of the eye. Some mammals, such as cats, camels, polar bears, seals and aardvarks, have full nictitating membranes. Often called a third eyelid or haw, it may be referred to in scientific terminology as the plica semilunaris, membrana nictitans, or palpebra tertia.
In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action 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 then an automatic response to a stimulus that does not receive or need conscious thought.
The vestibulo-ocular reflex (VOR) is a reflex acting to stabilize gaze during head movement, with eye movement due to activation of the vestibular system. The reflex acts to stabilize images on the retinas of the eye during head movement. Gaze is held steadily on a location by producing eye movements in the direction opposite that of head movement. For example, when the head moves to the right, the eyes move to the left, meaning the image a person sees stays the same even though the head has turned. Since slight head movement is present all the time, VOR is necessary for stabilizing vision: people with an impaired reflex find it difficult to read using print, because the eyes do not stabilise during small head tremors, and also because damage to reflex can cause nystagmus.
The plantar reflex is a reflex elicited when the sole of the foot is stimulated with a blunt instrument. The reflex can take one of two forms. In healthy adults, the plantar reflex causes a downward response of the hallux (flexion). An upward response (extension) of the hallux is known as the Babinski response or Babinski sign, named after the neurologist Joseph Babinski. The presence of the Babinski sign can identify disease of the spinal cord and brain in adults, and also exists as a primitive reflex in infants.
A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.
In the human brainstem, the solitary nucleus, also called nucleus of the solitary tract, nucleus solitarius, and nucleus tractus solitarii, is a series of purely sensory nuclei forming a vertical column of grey matter embedded in the medulla oblongata. Through the center of the SN runs the solitary tract, a white bundle of nerve fibers, including fibers from the facial, glossopharyngeal and vagus nerves, that innervate the SN. The SN projects to, among other regions, the reticular formation, parasympathetic preganglionic neurons, hypothalamus and thalamus, forming circuits that contribute to autonomic regulation. Cells along the length of the SN are arranged roughly in accordance with function; for instance, cells involved in taste are located in the rostral part, while those receiving information from cardio-respiratory and gastrointestinal processes are found in the caudal part.
Blinking is a bodily function; it is a semi-autonomic rapid closing of the eyelid. A single blink is determined by the forceful closing of the eyelid or inactivation of the levator palpebrae superioris and the activation of the palpebral portion of the orbicularis oculi, not the full open and close. It is an essential function of the eye that helps spread tears across and remove irritants from the surface of the cornea and conjunctiva.
The Exiles are a group of fictional characters appearing in American comic books published by Marvel Comics commonly associated with The X-Men. They feature in three series, Exiles, New Exiles, and Exiles vol. 2. The Exiles consists of characters from different universes, or realities, which have been removed from time and space in order to correct problems in various alternate worlds and divergent timelines in the Marvel Multiverse.
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.
Bell's phenomenon is a medical sign that allows observers to notice an upward and outward movement of the eye, when an attempt is made to close the eyes. The upward movement of the eye is present in the majority of the population, and is a defensive mechanism. The phenomenon is named after the Scottish anatomist, surgeon, and physiologist Charles Bell. Bell's phenomenon is a normal defense reflex present in about 75% of the population, resulting in elevation of the globes when blinking or when threatened. It becomes noticeable only when the orbicularis oculi muscle becomes weak as in, for example, bilateral facial palsy associated with Guillain–Barré syndrome. It is, however, present behind forcibly closed eyelids in most healthy people and should not be regarded as a pathognomonic sign.
In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative affect. Usually the onset of the startle response is a startle reflex reaction. The startle reflex is a brainstem reflectory reaction (reflex) that serves to protect vulnerable parts, such as the back of the neck and the eyes (eyeblink) and facilitates escape from sudden stimuli. It is found across the lifespan of many species. A variety of responses may occur depending on the affected individual's emotional state, body posture, preparation for execution of a motor task, or other activities. The startle response is implicated in the formation of specific phobias.
The corneal reflex, also known as the blink reflex or eyelid reflex, is an involuntary blinking of the eyelids elicited by stimulation of the cornea, though could result from any peripheral stimulus. Stimulation should elicit both a direct and consensual response. The reflex occurs at a rapid rate of 0.1 seconds. The purpose of this reflex is to protect the eyes from foreign bodies and bright lights. The blink reflex also occurs when sounds greater than 40–60 dB are made.
The nasociliary nerve is a branch of the ophthalmic nerve, itself a branch of the trigeminal nerve. It is intermediate in size between the other two branches of the ophthalmic nerve, the frontal nerve and lacrimal nerve.
The vestibulospinal tract is a neural tract in the central nervous system. Specifically, it is a component of the extrapyramidal system and is classified as a component of the medial pathway. Like other descending motor pathways, the vestibulospinal fibers of the tract relay information from nuclei to motor neurons. The vestibular nuclei receive information through the vestibulocochlear nerve about changes in the orientation of the head. The nuclei relay motor commands through the vestibulospinal tract. The function of these motor commands is to alter muscle tone, extend, and change the position of the limbs and head with the goal of supporting posture and maintaining balance of the body and head.
The trigeminal motor nucleus contains motor neurons that innervate muscles of the first branchial arch, namely the muscles of mastication, the tensor tympani, tensor veli palatini, mylohyoid, and anterior belly of the digastric. This nucleus is located in the mid-pons.
The Bainbridge reflex, also called the atrial reflex, is an increase in heart rate due to an increase in central venous pressure. Increased blood volume is detected by stretch receptors located in both sides of atria at the venoatrial junctions.
Central facial palsy is a symptom or finding characterized by paralysis or paresis of the lower half of one side of the face. It usually results from damage to upper motor neurons of the facial nerve.
Hoffmann's reflex is a neurological examination finding elicited by a reflex test which can help verify the presence or absence of issues arising from the corticospinal tract. It is named after neurologist Johann Hoffmann. Usually considered a pathological reflex in a clinical setting, the Hoffmann's reflex has also been used as a measure of spinal reflex processing (adaptation) in response to exercise training.
The Golgi tendon reflex (also called inverse stretch reflex, autogenic inhibition, tendon reflex) is an inhibitory effect on the muscle resulting from the muscle tension stimulating Golgi tendon organs (GTO) of the muscle, and hence it is self-induced. The reflex arc is a negative feedback mechanism preventing too much tension on the muscle and tendon. When the tension is extreme, the inhibition can be so great it overcomes the excitatory effects on the muscle's alpha motoneurons causing the muscle to suddenly relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex.
The menace response is one of three forms of blink reflex. It is the reflex blinking that occurs in response to the rapid approach of an object. The reflex comprises blinking of the eyelids, in order to protect the eyes from potential damage, but may also include turning of the head, neck, or even the trunk away from the optical stimulus that triggers the reflex.
References
- ↑ "Clinical Rotation Template". www.vetmed.wisc.edu. Archived from the original on 2017-09-17. Retrieved 2017-02-04.
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