Pares 2

Chapter 12 Brainstem 1: Surface Anatomy & Cranial Nerves

Brainstem is a compact stalk through which most information flowing to and from the brain travels. Which 2 sensory systems do not have primary relays in the brainstem? Brainstem is also site of many important nuclei involved with cranial nerve function, level of consciousness, cerebellar circuitry, muscle tone, posture and important homeostatic control systems for respiration and cardiac function. Because of its compact architecture, even small lesions of brainstem can affect multiple systems.

Brainstem: Ventral View

Brainstem: Dorsal View (Cerebellum removed)

Brainstem: Lateral View (Cerebellum removed)

Sensory & Motor Organization of the Cranial Nerves

Cranial Nerve 1: Olfactory Nerve

Anosmia (Olfactory Nerve) Unilateral anosmia is rarely noticed by patients bcz contralateral side functions normally. Each nostril must be tested separately. Patients sometimes complain of loss of taste when it is actually olfaction that is not functioning. Head injury is a common cause of anosmia. Viral infections can damage the olfactory epithelium Intracranial lesions on the ventral surface of the frontal lobes commonly cause olfactory deficits. Meningioma and metastasis are common causes. Tumors growing in this area often show few symptoms until they reach large size. Foster-Kennedy syndrome, typically caused by meningioma, includes anosmia, optic atrophy in one eye and papilledema in the opposite eye.

Cranial Nerve II: Optic Nerve (covered in Chapter 11) Cranial Nerves III, IV, VI: Oculomotor, Trochlear, & Abducens (covered in detail in Chapter 13) Oculomotor Nucleus in midbrain Innervates 4 extraocular muscles and functions in most eye movements Contains parasympathetic axons from Edinger-Westphal nucleus to ciliary ganglion which innervates pupillary constrictor muscles and ciliary muscle of lens Trochlear Nucleus in midbrain Nerve exits dorsal surface and crosses over Moves eye medially and downward Abducens Nucleus in pons Moves eye laterally

Cranial Nerve V: Trigeminal Nerve Trigeminal ganglion Ophthalmic, maxillary and mandibular divisions Touch, pain, temp, proprioception for face, mouth, anterior 2/3 of tongue, nasal sinuses & meninges (supratentorial) Muscles of chewing, tensor tympani muscle

Trigeminal Nuclear Complex Extends from midbrain to upper cervical spinal cord Mesencephalic, main (principal) sensory, and spinal nuclei Mesencephalic nucleus: proprioception Main nucleus: fine touch, dental pressure Spinal nucleus: crude touch, pain, temperature

Trigemino-thalamic Pathway Axons of main sensory nucleus neurons decussate in rostral pons and ascends to terminate in the ventral posterior medial nucleus of the thalamus Axons of spinal nucleus of trigeminal follow similar path VPM neurons project to face area of somatosensory cortex

Positions of Trigeminothalamic, Medial Lemniscus and Spinothalamic Tracts at Level of Midbrain

Somatotopic Organization of the Spinal Trigeminal Nucleus

Mesencephalic Nucleus and Tract Mesencephalic nucleus is only instance where primary sensory neurons are in the CNS instead of peripheral ganglion Proprioception from chewing muscles, tongue & extraocular muscles Mesencephalic neuron axons project to the motor trigeminal nucleus for jaw jerk reflex and function in monitoring motor function

Motor Functions of Trigeminal Motor nucleus located adjacent to main sensory nucleus in pons Innervates chewing muscles (masseter, temporalis, medial and lateral pterygoid) Tensor tympani muscle of middle ear Upper motor neuron innervation is bilateral so unilateral motor cortex or corticobulbar tract injury will not paralyze chewing muscles

Trigeminal Nerve Disorders 1. Trigeminal neuralgia (tic douloureux) most common Recurrent episodes of brief severe pain in the distribution of V2 or V3 Cause unknown, possible pressure from blood vessel Occurs in MS 2. Sensory Loss Caused by damage to CN V by trauma, tumor metastasis, herpes zoster, aneurysms of internal carotid, trigeminal or vestibular schwannoma Lesions of main sensory nucleus cause ipsilateral sensory loss Causes include infarct, demyelination, or tumor Lesions of trigeminal nucleus in pons or medulla often also involve the adjacent spinothalamic tract producing ipsilateral sensory loss in the face and contralateral loss of pain and temperature sense in the body.

Cranial Nerve VII: Facial Nerve Motor function: muscles of facial expression, stapedius muscle and part of digastric muscle. Parasympathetic function: innervation to lacrimal glands and salivary glands except for parotid Visceral sensory function: taste from anterior 2/3 of tongue Somatosensory function: touch sensation from region near external auditory meatus.

Facial Nucleus Location: pons caudal to trigeminal main sensory nucleus Facial nerve axons loop dorsally around the abducens nucleus then course ventrolaterally to exit the ponto-medullary junction Motor cortex or corticobulbar tract lesions cause contralateral face paralysis/weakness sparing the forehead Lesions of facial nucleus/nerve cause ipsilateral paralysis of whole face.

Along the course of the facial nerve the geniculate ganglion has primary sensory neurons for taste in the anterior 2/3 of the tongue and somatic sensation for the external auditory meatus Parasympathetic axons originate in the superior salivatory nucleus and run to the sphenopalatine ganglion and submandibular ganglion which contain neurons innervating glands in the nasal passage, lacrimal glands, submandibular and sublingual glands

Taste Pathway Taste sensory receptors in the taste buds innervated by primary sensory neurons Primary sensory neurons project to the rostral division of the nucleus solitarius (gustatory nucleus); this nucleus also receives taste input from posterior 1/3 of tongue traveling in CN IX & X Nuc solitarius neurons project ipsilaterally into the midbrain where some fibers decussate and terminate in VPM thalamus bilaterally VPM thalamus projects to taste cortex located in the parietal operculum and insula

Somatosensory axons from the external auditory meatus project to the spinal trigeminal nucleus along with small number of somatosensory axons from CN IX and X.

Facial Nerve Lesions 1 Upper motor neuron lesions spare forehead; lower motor neuron lesions cause paralysis in upper and lower face.

Facial Nerve Lesions 2 Bell’s Palsy All divisions of facial nerve are impaired acutely and then gradually recover. Cause unknown, but viral or inflammatory process suspected. Unilateral upper and lower face paralysis Patients often have pain near the ear Hyperacusis common, as is dry eye 80% recover fully within 3 weeks Some have residual weakness Crocodile tears can result from aberrant regeneration of salivary fibers to lacrimal gland Brainstem lesions occasionally involve the facial nucleus or nerve fascicles.

Corneal Reflex Touching the cornea gently with cotton swab typically elicits a blink. Afferent limb travels in the ophthalmic division of the trigeminal nerve to the main sensory nucleus and spinal nucleus. Efferent limb starts at the facial nucleus and travels in the facial nerve to innervate the orbicularis oculi muscles that close the eye. Lesions of trigeminal, facial nucleus or facial nerve may cause loss of the corneal reflex. Sensorimotor cortex lesions can also decrease corneal reflex on the contralateral side. The blink to threat response is carried in the optic nerve.

Cranial Nerve VIII: Vestibulocochlear Nerve Auditory and vestibular sensation Nerve emerges from ponto-medullary junction; cerebellopontine angle Travels with facial nerve Branches go to cochlea, saccule, utricle and semicircular canals

Vestibular and Cochlear Sensory Organs

Cochlea

Central Auditory Pathways Tonotopic organization Hair cells of organ of Corti Primary sensory neurons in the spiral ganglion of cochlea Dorsal and ventral cochlear nuclei Bilateral projections Superior olivary complex Lateral lemniscus Inferior colliculus Medial geniculate body Auditory cortex (41)

Brainstem Sections Through Auditory Relay Areas

Lesions in Auditory Pathway Auditory information ascends bilaterally after leaving the cochlea so damage proximal to the cochlear nuclei do not cause unilateral hearing loss. However, more input ascends contralaterally than does so ipsilaterally. During auditory seizures patients often report tones or roaring sounds from the side opposite the cortical area involved in the seizure. Efferent pathways from brainstem to cochlea can dampen sensitivity of hair cells. Reflex pathways from the cochlear nuclei project to CN V & VII controlling stapedius and tensor tympani muscles that function to dampen middle ear response to loud sounds.

Vestibular System: Semicircular Canals, Saccule and Utricle Function in adjustment of posture, muscle tone, and eye position in response to head movements. Vestibular nuclei have connections with cerebellum, brainstem motor and extrocular muscle control systems Ascending vestibular pathway to cortex provides conscious awareness of head position integrated with visual and tactile spatial information. Semicircular canals detect angular acceleration around 3 orthogonal axes. Saccule and utricle detect linear acceleration and head tilt.

Vestibular Pathways Hair cells of semicircular canals, saccule & utricle Superior and inferior vestibular ganglia Superior, inferior, lateral and medial vestibular nuclei Ventral posterior nuclear complex Vestibular cortex probably in parietal lobe area 5

Important Vestibular System Ascending & Descending Tracts 1. Lateral vestibulospinal tract: extends throughout spinal cord and functions in balance and extensor tone 2. Medial vestibulospinal tract: only projects to cervical spinal levels and controls neck and head position muscles 3. Medial longitudinal fasciculus: connects vestibular nuclei and CN nuclei III, IV, and VI; mediates vestibulo-ocular reflex

Hearing Loss Common causes (unilateral): disorders of external auditory canal, middle ear, cochlea, 8th nerve, or cochlear nuclei. Auditory pathways ascend bilaterally proximal to the cochlear nuclei. Conductive hearing loss caused by problem in external auditory canal or in middle ear. Sensorineural hearing loss caused by damage to cochlea or 8th nerve. Tuning fork in air outside ear tests sensorineural hearing; tuning fork on mastoid bone tests conductive hearing; normals hear better with air conduction than bone conduction. Common causes of conductive hearing loss include accumulation of ear wax, otitis, tympanic membrane perforation and otosclerosis. Causes of sensorineural hearing loss include exposure to loud sounds, meningitis, ototoxic drugs, head trauma, viral infections, aging, Meniere’s disease, tumors and rarely infarct.

Most common tumor is the acoustic neuroma, which is actually a schwannoma and typically originates where the 8th nerve enters the brainstem. Symptoms include unilateral hearing loss, tinnitus (ringing in the ear) and unsteadiness. Trigeminal is often also affected as tumor spreads, leading to facial pain and sensory loss. With large tumors additional deficits may occur due to compression of cerebellar and corticospinal pathways causing ipsilateral ataxia and contralateral hemiparesis. Trigeminal neuroma is also somewhat common. Schwannomas of the other cranial nerves are very rare.

Dizziness & Vertigo Dizziness is a general term used by patients with a number of different conditions. Vertigo is a spinning sensation of movement and is more closely linked with vestibular disease. Vertigo can be caused by damage to vestibular pathways anywhere from the labyrinth to the parietal cortex. Most cases of vertigo are caused by peripheral disorders, fewer by CNS disorders. CNS disorder can often be detected by looking for diplopia or other visual changes, somatosensory changes, weakness, dysarthria, ataxia, or impaired consciousness.

Cranial Nerve IX: Glossopharyngeal Nerve Motor function: stylopharyngeus muscle which elevates pharynx during talking and swallowing and participates in gag reflex. Motor neurons are in the nucleus ambiguus. Parasympathetic function: innervation of parotid salivary gland. Preganglionic neurons in inferior salivatory nucleus; postganglionic neurons in the otic ganglion

Somatic sensory function: sensation from middle ear, external auditory meatus, pharynx and posterior 1/3 of tongue. Primary sensory neurons in the inferior and superior glossopharyngeal ganglion which projects to the spinal nucleus of the trigeminal. Visceral sensory function: taste from posterior 1/3 of tongue. Primary sensory neurons in the inferior glossopharyngeal ganglion project to rostral nucleus solitarius Visceral sensory function: chemoreceptors and baroreceptors in carotid body. Primary sensory neurons in the inferior glossopharyngeal ganglion project to caudal nucleus solitarius.

Cranial Nerve X: Vagus Nerve Motor function: pharyngeal muscles (swallowing) & laryngeal muscles Motor axons from the nucleus ambiguus. Parasympathetic function: innervation of heart, lungs, digestive tract down to the splenic flexure. Motor neurons in the dorsal motor nucleus of vagus in the medulla. Somatic sensation function: sensation from pharynx, meninges, external auditory meatus Primary sensory neurons in inferior and superior vagal ganglia that project to the spinal nucleus of the trigeminal

Visceral sensory function: taste from epiglottis and pharynx. Primary sensory neurons in the vagal ganglia that project to the rostral nucleus solitarius Visceral sensory function: from chemoreceptors and baroreceptors in the aortic arch, cardiorespiratory system and digestive tract. Primary sensory neurons are in vagal ganglia that project to the caudal nucleus solitarius

Cranial Nerve XI: Spinal Accessory Nerve Motor function: sternomastoid and upper trapezius muscles Motor neurons found in lateral intermediate zone of C1-C5/C6 spinal cord segments; axons leave cord lateral surface, ascend and join together forming spinal accessory nerve. Sternomastoid muscle functions in turning head (left muscle turns head right). Trapezius muscle functions in elevating shoulder.

Cranial Nerve XII: Hypoglossal Nerve Motor function: intrinsic and extrinsic tongue muscles. Motor neurons in hypoglossal nucleus near midline in medulla. Upper motor neurons in inferior part of motor cortex. In UMN injury tongue will deviate away from the lesion; in LMN injury tongue will deviate toward the side of the injury.

Disorders of CN IX, X, Xi, and XII Damage to these nerves/nuclei is most commonly a CNS injury. Hoarseness is most often due to vagus injury. Nucleus ambiguus can be damaged in a lateral medullary infarct. Dysarthria is abnormal articulation of speech and can occur from injury to CN V, VII, IX, X, or XII or UMN ; common causes are infarct and MS damaging UMN pathways. Dysphagia is impaired swallowing and can occur following injury to CN IX, X or XII or UMN injury. Brainstem nuclei involved in laughing and crying include CN VII, IX, X, and XII. Lesions of corticobulbar paths in the subcortical white matter can cause pseudobulbar affect in which patients exhibit uncontrollable outbursts of laughing or crying with no accompanying emotions.