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Mesencephalon (Midbrain)

Audio and visual pathways, sensorimotor function, reflexes, nociception (sensing pain) Here we find the tectum, containing the superior colliculus (visual input) and the inferior colliculus (auditory input). The tectum receives audiovisual input, and the tegmentum has structures for controlling eye movements. The mesencephalon also has generalized motor structures: the red nucleus (motor control) and the substantia nigra (discussed in the section “Basal Ganglia” earlier in this chapter). Mesencephalic audiovisual and motor structures support reflexive audiovisual orienting movements for locating the source of a sudden sound. Such action depends on sensory integration, so vision can be sound guided. There are also tactile structures in the mesencephalon, which are similarly integrated into the task of orienting us rapidly. If you have been stung by a buzzing bee and reflexively turned your gaze toward it, you have experienced rapid mesencephalic movements. You might even brush the bee off before thinking about what is going on in such a situation. In this context, the orienting reflex depends not only on sound but also on tactile pain. After the sting, structures in the diencephalon (the periaqueductal gray matter) modulate the pain by releasing opiates.

Mesencephalon Pathologies

Parkinson’s disease As discussed in the section “Basal Ganglia” earlier in this chapter, when a significant proportion of dopamine neurons die in the substantia nigra, motor problems ensue and, with time, cognitive decline occurs.

Coma Insult to the reticular formation often results in a coma. The most common cause is intoxication.

Weber’s syndrome This syndrome manifests as paralysis of one eye, typically due to a lesion in the superior colliculus. The person may also have other symptoms, such as an unresponsive pupil, deviating eye, and drooping eyelid.

Gaze Palsy The patient has an upward gaze paralysis, occasionally with a downward gaze palsy. Other symptoms are an elevated eyelid, dilated pupil, and inability to accommodate.

Mesencephalon Summary

The mesencephalon helps us orient to visual and auditory stimuli. It supports movement and regulates basic life support systems.

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