Cranial Nerves VII to XII MSTN121 - Neurophysiology Session 13 Department of Myotherapy
Cranial Nerve VIII: Vestibulocochlear Sensory nerve with two distinct branches. Vestibular branch transmits information related to head position and head movement. Cochlear branch transmits information related to hearing. Peripheral receptors for these functions are located in the inner ear (the labyrinth). Labyrinth consists of the vestibular apparatus and the cochlea. Image:
Cochlea Snail shell shaped organ formed by a spiraling, fluid-filled tube. Basilar membrane extends almost the full length of the cochlea, dividing the cochlea into upper and lower chambers. Consists of fibers oriented across the width of the cochlea. Scala vestibuli is further divided by a membrane that separates the cochlear duct from the remainder of the upper chamber. Image:
Converting Sound to Neural Signals Sound is converted to neural signals by a sequence of mechanical actions. When sound waves enter the external ear, vibration moves the ossicles. Ossicles vibrate the membrane at the opening of the upper chamber, moving the fluid contained in the upper chamber. Fluid moves inside the cochlea, vibrating the basilar membrane and its attached hair cells. Image:
Downstream Neural Connections of the Cochlea Image:
CNS Role in Auditory Function Auditory information Orients the head and eyes toward sounds. Increases the activity level throughout the CNS. Provides conscious awareness and recognition of sounds. Image:
Processing Sound From the cochlear nuclei, auditory information is transmitted to three structures: Reticular formation: Connections account for the activating effect of sounds on the CNS. Inferior colliculus: Directly and via the superior olive, auditory information is integrated from both ears to detect the location of sounds. Medial geniculate body: Serves as a thalamic relay station for auditory information to the primary auditory cortex. Image:
Auditory Cortex Three cortical areas are dedicated to processing auditory information: Primary auditory cortex is the site of conscious awareness of the intensity of sounds. Auditory association cortex compares sounds with memories of other sounds, then categorizes the sounds as language, music, or noise. Wernicke s area is where comprehension of spoken language occurs. Image:
Cranial Nerve IX: Glossopharyngeal Mixed nerve containing both sensory and motor fibers. Sensory fibers transmit somatosensation from the soft palate and pharynx and information from taste receptors in the posterior tongue. Motor component innervates a pharyngeal muscle and the parotid salivary gland. Provides the afferent limb of the gag and swallowing reflexes, supplies taste information, and innervates a salivary gland Image:
Cranial Nerve X: Vagus Provides afferent and efferent innervation of the larynx, pharynx, and viscera. Far-reaching connections allow the vagus to decrease HR, constrict the bronchi, affect speech production, and increase digestive activity. Motor function can be tested by eliciting the gag reflex or uvula elevation. Image:
Cholinergic Anti-Inflammatory Pathway Recent discoveries have shown that there are three pathways activated by the vagus nerve that inhibit macrophage release of proinflammatory cytokines (Tracey, 2007) Image: (Tracey, 2007)
Cranial Nerve XI: Accessory Provides innervation to the trapezius and sternocleidomastoid muscles. Originates in the spinal accessory nucleus in the upper cervical cord, travels upward through the foramen magnum, and then leaves the skull through the jugular foramen. Cell bodies are in the ventral horn at levels C1 to C4. Image:
Cranial Nerve XII: Hypoglossal Provides innervation to the intrinsic and extrinsic muscles of the ipsilateral tongue. Cell bodies are located in the hypoglossal nucleus of the medulla. Activity of the hypoglossal nerve is controlled by both voluntary and reflexive neural circuits. Image:
Autonomic Nerves of the Brainstem Image:
Speaking and Swallowing Swallowing involves 3 stages: Oral Pharyngeal and laryngeal Esophageal Speaking: - Requires cortical control. - At the CN level, sounds generated by the larynx (CN X) are articulated by the soft palate (CN X), lips (CN VII), jaws (CN V), and tongue (CN XII).
Cranial Nerve Lesions Glossopharyngeal Complete lesion interrupts the afferent limb of both the gag reflex and the swallowing reflex. Salivation is also decreased. Vagus Complete lesion results in difficulty speaking and swallowing, poor digestion, asymmetrical elevation of the palate, and hoarseness.
Dysphagia Dysphagia is difficulty with swallowing. Frequent choking, lack of awareness of food in one side of the mouth, or food coming out of the nose may indicate dysfunctions of CNs V, VII, IX, X, or XII. Upper motor neuron lesions may also cause swallowing dysfunctions.
Dysarthria Dysarthria is poor control of the speech muscles. Only vocal speech is affected; people with dysarthria can understand the spoken language and can write and read. Causes: Lower motor neuron involvement of CNs V, VII, X, or XII Upper motor neuron lesions or muscle dysfunction
References Felten, DL & Shetty, A 2009, Netter s Atlas of Neuroscience, 2 nd edn, Elsevier, Sydney Lundy-Ekman, L 2013, Neuroscience: fundamentals for rehabilitation, 4th edn, Elsevier, Sydney. Tracey, KH 2007, Physiology and immunology of the cholinergic antiinflammatory pathway, Journal of Clinical Investigation, 117(2):289-296, viewed on 26/6/17 https://www.jci.org/
Image References Felten, DL & Shetty, A 2009, Netter s Atlas of Neuroscience, 2 nd edn, Elsevier, Sydney Lundy-Ekman, L 2013, Neuroscience: fundamentals for rehabilitation, 4th edn, Elsevier, Sydney. Tracey, KH 2007, Physiology and immunology of the cholinergic antiinflammatory pathway, Journal of Clinical Investigation, 117(2):289-296, viewed on 26/6/17 https://www.jci.org/
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