CN I + CN XI + Cervical plexus

Learning objectives

  • Overview of the brain

  • Describe somatomotor and somatosensory pathways in the brain

  • Define the nerve components on CN I, XI, and the cervical plexus

  • Describe the pathways of each of these nerves

  • Describe the functions associated with each of these nerves

  • Describe select clinical correlations associated with each nerve + testing in clinic

Overview of the Brain

  • Gyri = folds

    • Postcentral gyrus: Sensory area

    • Precentral gyrus: Motor area

  • Sulci = grooves

    • Central sulcus: Divides frontal and parietal lobes

    • Lateral sulcus: Divides parietal and temporal lobes

    • Parietod-occipital sulcus: Divides parietal and occipital lobes

  • Fissures = clefts

  • Structures within the diencephalon (center area of the brain)

    • Epithalamus: Emotion, sleep wake cycles, mood

    • Thalamus: Relay point for sensory and motor info (somatic signals)

    • Hypothalamus: Coordination center for homeostasis (autonomic signals)

Cereborospinal Fluid Circulation

  • Function of CSF: Buffer zone to protect brain while the head moves

  • Secreted by epithelial cells in choroid plexus in ventricles of the brain

  • Circulates through ventricles and into subarachnoid space around spinal cord

  • Total volume = 90-150 ml

    • 400-500 ml produced per day → Excess and used CSF absorbed into venous system

  • Hydrocephalus: Increased CSF volume leading to increased intercranial pressure

    • If sutures haven’t fused (infants) → large head (emergency)

Arachnoid Granulations

  • Protrusions of arachnoid mater and subarachnoid space into superior sagittal sinus or plexus around it, including lateral lakes.

    • In humans, not covered by dura

  • Function: Transport CSF into venous sinus system

    • Regulate CSF volume to prevent increased intercranial pressure

  • Newborn distribution: Microscopic (need less CSF because brains are smaller and they have other means to remove CSF)

    • Increase in size and distribution → widespread by 3 yrs old

  • Adult distribution: Concentrated around sagittal sinus

Skull Fracture and CSF Leak

  • Skull fractures, especially in the frontal nasal area, can cause CSF to leak (clear and watery)

    • Emergency situation because don’t want bacterial exposure

    • Clinical presentation: CSG rhinorrhea

  • Requires defect in dura and pathway from nasal sinus into nasal cavity

    • Cribiform plate → Nasal cavity

Somatomotor and Somatosensory Pathways to Brain

  • Skeletal motor signals (from brain) begin in primary motor cortex (precentral gyrus)

  • Somatosensory signals (to brain) are processed in the primary somatosensory cortex (postcentral gyrus)

  • Both somatomotor and somatosensory signals relay through the thalamus

  • Homunculus (motor or sensory): A map of corresponding regions in each gyrus distorted by concentration

Somatomotor

  • Upper motor neurons (UMN): Integrates all excitatory and inhibitory signals from cortex → translates to a movement signal.

    • 3 tracts of somatomotor signal

      • Anterior corticospinal tract: Voluntary mvmt of trunk muscles

      • Lateral cortocospinal tract: Voluntary mvmt of limb muscles

      • Corticonuclear fibers: Motor signals to cranial nerves

  • Lower motor neurons (LMN): Transmits signal to muscle

    • 3 types of LMN (general breakdown = somatomotor and visceromotor)

      • Somatic motor neurons

      • Special visceral efferent brachial motor neurons

      • General visceral motor neurons

  • Axons of UMN relay through thalamus → spinal cord → synapse directly onto LMN → Signal transmitted to muscle

Somatosensory

  • Info gets conveyed up to the brain from primary sensory neurons in peripheral ganglia (outside CNS) → through thalamus → into post-central gyrus

    • Ex/ Pathway when you touch something with your hands

      • Sensory ending in skin → Splanchnic n. → Sympathetic trunk → White communicating ramus → Ventral ramus → Spinal n. → DRG → Dorsal horn → Peripheral ganglia → Thalamus → Post-central gyrus

Hypothalamus

  • Control autonomic signals → Communicate with spinal cord through hypothalamospinal tract

  • Origin symp signals: Lateral horns

Cranial Nerve I: Olfactory Nerve

  • Fiber components: Special sensory (smell)/Special visceral afferent only

  • Transmit smell to brain from receptors in olfactory epithelium

  • No peripheral ganglia

  • Pathway: Cribiform plate (of ethmoid bone) → Brain

    • Unmyelinated processes of the olfactory cells (olfactory axon) pass through the cribiform plate in bundles (to olfactory bulb)

    • Division within the brain:

      • Lateral olfactory stria: Primary olfactory cortex

      • Medial olfactory stria: Autonomic response to olfaction (ex. salivation)

  • Distribution of nerves: Lateral wall and midline (cover superior section)

  • Pathology/Disease

    • Anosmia: Inability to detect smell

    • Hyposmnia: Reduced ability to smell

    • Smell distortion: Change in smell

    • Lesions or tumors, trauma to the cribiform plates (sever olfactory tracts), distortion can be caused by migraine or epilepsy

      • Affect processing

  • Testing: Strong stimulus presented to one nostril at a time and patient asked to sniff

    • Stimulus must be adjacent to nostril

Cranial Nerve XI: Accessory Nerve - Trapezius and SCM

  • Fiber components: Somatomotor ONLY

  • Carries axons of LMN within the ventral horns of C1-C5

  • Pathway: Ventral horn of C1-C5 → Spinal root → Through foramen magnum → Jugular foramen → Muscle

    • Accessory n. formed by connections with dorsal root

  • Testing:

    • SCM - Place hand on side of face and ask patient to rotate against hand (test both sides)

    • Trapezius - Place hand on shoulders and ask patient to shrug (push up against hands)

Sternocleidomastoid (SCM) - Within investing layer

  • Originate: Mastoid process and lateral superior nuchal line

  • Attachment: Manubrium of sternum and medial 1/3 of clavicle

  • Innervation: CN XI

  • Function:

    • Unilateral - mediolateral head rotation

    • Bilateral (posterior fibers) - head extension

    • Bilateral (anterior fibers) - head flexion

Trapezius - Within Investing layer

  • Origin: Median nuchal line, spinous process

  • Attachment: Scapula and clavicle (ends)

  • Innervation: CN XI

  • Function:

    • Upper fibers:

      • Bilateral - head extension

      • Unilateral - lateral head flexion

      • Elevate and rotate scapula

    • Middle fibers - retract and rotate scapula

    • Lower fibers - depress and rotate scapula

Cervical plexus

  • Plexus: Mesh of intersecting nerves

    • 4 main plexuses from spinal nerves (somatoplexus): Cervical, brachial, lumbosacral, coccygeal

    • Autonomic plexuses: celiac plexus-organs, myenteric plexus-GI tract, submucosal plexus-intestinal mucosa, pharyngeal plexus of CN X-palate, pharynx, cardiac plexus-heart

  • Cervical plexus origin: Arises with contributions from C1-C5 ventral rami

    • C5-T1 also contribute to brachial plexus (SM to anterior and middle scalene + levator scapulae)

  • Fiber components: Somatosensory and somatomotor fibers

    • Somatosensory branches: lesser occipital, greater auricular, transverse cervical, supraclavicular, phrenic

    • Somatomotor branches: ansa cervicalis, nerve to thyroid, phrenic

  • Branches of cervical plexus

    1. To meninges - C1 (sensory)

    2. Lesser occipital n. - C2 (sensory)

    3. Greater auricular n. - C2-3 (sensory)

    4. Transverse cervical n. - C2-3 (sensory)

    5. Supraclavicular n. - C3-4 (sensory)

    6. Phrenic n. - C3-5 (mixed)

    7. Inferior root of ansa cervicalis - C2-3 (motor)

    8. Superior root of ansa cervicalis - C1 (motor)

Ansa cervicalis (superior root + inferior root)

  • Innervation: Somatomotor to infrahyoid muscles (except thyroyoid)

    • Motor portion of the cervical plexus

  • Superior root: hitchhiking or relaying of the cranial nerves - axons that don’t originate on a CN but join the CN for a short period of time to get to a target

    • C1 fibers hitchhike on hypoglossal to reach thyrohyoid

Nerves for somatosensation in the neck

  • Lesser occipital n. → SS to back of head

    • Most posterior

  • Transverse cervical n. → SS to anterior neck

    • Anterior neck

  • Supraclavicular n. → SS to superior clavicle (inferior neck)

    • Inferior

  • Greater auricular n. → SS around ear and angle of mandible

    • Ear/gonion

  • Puncta nervosa (Erb’s point): Location for anesthetic-midpoint on the posterior side of SCM muscle

Phrenic nerve

  • C3-5 “keeps the diaphragm alive”

    • SS and SM to diaphragm

  • Location: On top of anterior scalene

Referred Pain - Viscerosensory pain

  • Pain is perceived in a location distant from the actual site of injury or pathology

    • Follows sympathetic pathway except for 1 exception (signal goes to dorsal horn)

  • Examples:

    • Angina (heart pain): sensation to left upper limb

      • Signal mixed around T1-2 area

    • Viscerosensory