Neuroscience/Objectives/Lecture 20

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Cranial nerves: Sensory nuclei for somatic and visceral sensation

Identify the location in the brainstem of the somatosensory nuclei related to cranial nerves V, VII, IX, and X.

Define the functions of the spinal, principal, and mesencephalic trigeminal nuclei.

Spinal trigeminal nucleus

Receives pain and temperature information from the trigeminal nerve, plus similar protopathic information from CNs VII, IX, and X. Input is arranged in a primary somatotopic map so that inputs from CNs VII, IX, and X synapse dorsally and those from V₃, V₂, and V₁ synapse ventrally (with V₃ inputs dorsal to V₁). A second somatotopic map exists in the pars caudalis, such that afferents from the inner lip are most rostral, outer lip afferents are just caudal to those, cheek inputs would be more caudal, and the temples would be still more caudal. The pars caudalis receives pain and temperature afferents from skin and special visceral locations in the throat; the pars interpolaris receives sensory information from the teeth (e.g. dental pain); the pars oralis receives mainly tactile information from the teeth, oral cavity, and mouth.

Principal trigeminal nuclei

Carries afferents for fine tactile and proprioceptive information from the face, oral cavity, and teeth. Also carries some protopathic information (see below and page 190 of the syllabus).

Mesencephalic trigeminal nuclei

Convey proprioceptive information from dental ligaments and muscles of the jaw, forming monosynaptic circuits with neurons in the trigeminal motor nucleus to mediate jaw reflexes. Importantly, there are no synapses in the mesencephalic trigeminal nuclei.

Describe the sensory modalities conveyed by the trigeminal nerve.

The trigeminal nerve carries epicritic and protopathic sensations. Large diameter neurons mediate discriminative touch and properioception from jaw muscles. Small-diameter neurons carry nociceptive and thermoceptive information. Cell bodies for first-order sensory neurons are contained within the trigeminal semilunar ganglion.

Describe the three subdivisions of the spinal nucleus of CN V.

The trigeminal nerve provides sensory (GSA) and motor innervation to the face (SVE). The nerve's three subdivisions include the ophthalmic division (V₁), maxillary division (V₂) and mandibular division (V₃).

Additionally, all branches innervate the dura, conveying painful stimuli that are perceived as headaches.

Identify the central pathways by which axons carrying somatosensory information from the face reach the thalamus.

Ventral trigeminothalamic pathway

Second-order neurons in the spinal trigeminal nucleus (carrying protopathic information) send axons that cross the midline and form the ventral trigeminothalamic tract, which ascends and terminates in the ventral posteromedial thalamic nucleus (VPM). Also a part of this tract are second-order neurons from the ventrolateral region of the principal sensory trigeminal nucleus (carrying epicritic information from the face and oral cavity). These neurons send axons that also cross the midline and join the ventral trigeminothalamic tract.

Dorsal trigeminothalamic tract

Remaining second-order neurons in the dorsomedial region of the principal sensory trigeminal nucleus (carrying epicritic information from the oral cavity and teeth) ascend via an ipsilateral pathway, the dorsal trigeminothalamic tract. These also terminate in the VPM of the thalamus.

Both ventral and dorsal trigeminothalamic pathways project to the VPM where they synapse with third-order thalamic neurons. Because both ventral and dorsal pathways convey sensory information from the oral cavity, the oral cavity has a bilateral representation in the thalamus. The oral cavity is represented medially in the thalamus, and successive areas of the face are located more laterally.

Third-order neurons in the thalamus send axons through the posterior limb of the internal capsule to the lateral aspect of the postcentral gyrus. These axons maintain their somatotopic organization within the facial area of the primary somatosensory neocortex (in the parietal opercular cortex).

Understand sensory deficits that result from injury to the trigeminal nerve, its nuclei, or the central pathways from these nuclei.

Describe the location in the brainstem of the nucleus of the solitary tract and discuss how it is divided into anatomical and functional subregions.

• Location:
  • Extends from caudal to rostral medulla
• Divisions:
  • Rostral: gustatory nucleus
  • Caudal: autonomic
    • Superior: gastrointestinal
    • Inferior:
      • Lateral: respiratory
      • Medial: cardiovascular

Describe the functions of the general and special visceral afferents from cranial nerves VII, IX, and X that terminate in the nucleus solitarius.

Understand the peripheral distribution of general and special visceral afferents to the nucleus solitarius.

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Describe the relationship between the cardiorespiratory centers of the brainstem and cranial nerves IX and X.

The cardiorespiratory centers of the caudal medulla receive general visceral afferent inputs from cranial nerves IX and X, providing baroreceptor and chemoreceptor input.

Relate the neuroanatomy presented in this lecture to sensory findings on a clinical neurological exam.

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