The+Nervous+System

This link provide you with information and video clips. [|Overview of Nervous system, Brain structure and Neuron Types]



Reflex Arc

The blink reflex is a reflex which is designed to naturally protect the eyes. Most animals with eyes have some form of the blink reflex, and the reflex is present from the time that an animal first opens its eyes. Abnormalities in this reflex can indicate that there is a neurological problem and may increase the risk of incurring eye damage because the eyes are no longer able to protect themselves reflexively. Reflexes are involuntary actions. They allow the body to react very quickly to potentially dangerous situations by taking defensive action of some form. In the case of the blink reflex, the blink is often over before someone is consciously aware that there was a risk. There are many other reflexes which appear at different stages of development. Several different stimuli can trigger this reflex. Anything which touches the [|cornea] will cause someone to blink, and people usually blink when objects appear to be on a collision course with the eye, as for example when something is thrown in the direction of someone's head. Very bright light also stimulates the blink reflex, as do loud noises.
 * The Blinking Reflex**

The **pupillary light reflex** is a [|reflex] that controls the diameter of the [|pupil], in response to the intensity ([|luminance]) of light that falls on the [|retina] of the [|eye], thereby assisting in [|adaptation] to various levels of darkness and light, in addition to [|retinal] sensitivity. Greater intensity light causes the pupil to become smaller (allowing less light in), whereas lower intensity light causes the pupil to become larger (allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye.
 * Pupillary reflex**
 * [|Pupillary light reflex]**

The **pharyngeal reflex** or **gag reflex** is a [|reflex] contraction of the back of the throat,[|[][|1][|]] evoked by touching the [|soft palate] or sometimes the back of the [|tongue]. It prevents something from entering the throat except as part of normal [|swallowing] and helps prevent [|choking]. Different people have different sensitivities to the gag reflex. The [|afferent limb] of the reflex is supplied by the [|glossopharyngeal nerve] (cranial nerve IX), which inputs to the [|nucleus solitarius] and the [|spinal trigeminal nucleus], and the [|efferent limb] is supplied by the [|vagus nerve] (cranial nerve X) from the [|nucleus ambiguus]. Absence of the gag reflex and pharyngeal sensation can be a symptom of a number of severe medical conditions, such as damage to the [|glossopharyngeal nerve], the [|vagus nerve], or [|brain death]. However, in one study, one-third of healthy people did not have a gag reflex, although pharyngeal sensation in these subjects remained intact.[|[][|2][|]] Swallowing unusually large objects or placing objects in the back of the mouth may cause the pharyngeal reflex. Some people, for instance [|sword swallowers], have learned how to suppress it. In contrast, triggering the reflex is sometimes done intentionally to induce vomiting, for example by those who suffer from [|bulimia nervosa].
 * Swallowing reflex** (= gag reflex)

Striking the [|patellar tendon] with a [|tendon hammer] just below the [|patella] stretches the [|muscle spindle] in the [|quadriceps femoris muscle]. This produces a signal which travels back to the spinal cord and synapses (without interneurons) at the level of 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. This reflex 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 bipolar 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.
 * Patellar reflex**

[|Patella reflex and Blink reflex]
 * Knee-jerk reflex and Blink reflex youtube video clip**