FACTS & HINTS

High-Yield Facts

Clinical Points

Meningitis

Inflammation of the arachnoid and pia mater

Can be caused bydrugs or malignancy, but usuallybypathogenic bacteria

Infection can be due to meningococcal or pneumococcal bacteria

Patient maypresent with fever, nonblanching purpuric rash, neck stiffness, and photophobia

Mortalityfrom bacterial causes can be up to 30%

Diagnosis is byexamination and lumbar puncture

Hydrocephalus

Caused byexcess cerebrospinal fluid (CSF) production or, more likely, byabnormal absorption

Classified as obstructive, communicating or normal pressure

Blockage usuallyin cerebral aqueduct bynarrowing

Can be a result of tumor, hemorrhage, and infection

Communicating blocks movement of CSF from ventricles

Can be caused byabsence of arachnoid granulations or subarachnoid hemorrhage

Dilates ventricles, thins cerebral cortex, separates bones of calvaria in infants

Brain tumors

Twenty-five percent of all brain tumors arise from a different site (metastasis) Common sites of original tumor include: breast, bronchus, prostate, thyroid, and kidney

Primarybrain tumors can be benign, such as meningiomas and neurofibromas, or malignant, such as astrocytomas and oligodendrogliomas

Can present as epilepsy, focal neurology, or signs of raised intracranial pressure

Diagnosis is byhistory, examination, and computed tomography(CT) or magnetic resonance imaging (MRI) of the brain

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12 Cranial and Cervical Nerves

STUDYAIMS

At the end of your study, you should be able to:

Know the names and functions of the cranial nerves

State the foramen through which the cranial nerves emerge from the skull

Outline formation of the cervical plexus

Know the sensorynerves arising from the cervical plexus and their distribution

Know the muscles innervated bymotor branches of the cervical plexus

Understand the formation of the ansa cervicalis and know the muscles innervated byits branches

Describe the formation of, and fibers composing, the phrenic nerve

Know the structures innervated bythe various components of the phrenic nerve

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GUIDE

Head and Neck: Cranial and Cervical Nerves

Cranial Nerves

12 pairs of cranial nerves arise from the brain, and theyare identified both bytheir names and byRoman numerals I through XII. The cranial nerves are somewhat unique and can contain multiple functional components:

General: same general functions as spinal nerves

Special: functions found onlyin cranial nerves

Afferent and efferent: sensoryor motor functions, respectively

Somatic and visceral: related to skin and skeletal muscle (somatic), or to smooth muscle and glands (visceral)

Hence, each cranial nerve maypossess multiple functional components, such as GSA(general somatic afferents), meaning it contains nerve fibers that are sensoryfrom the skin, not unlike those of the spinal nerve; GVE (general visceral efferents), meaning it contains motor fibers to visceral structures (smooth muscle and/or glands) like a parasympathetic fiber from the sacral spinal cord (S2-S4 gives rise to parasympathetics); or SSA(special somatic afferents), meaning it contains special sensoryfibers, such as those for vision or hearing.

[Plate 126, Accessory Nerve (XI): Schema]

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[Plate 130, Autonomic Nerves in Head]

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In general, CN I and II arise from the forebrain and are reallytracts of the brain for the special senses of smell and sight. CN III, IV, and VI move the extraocular skeletal muscles of the eyeball. CN Vhas three divisions: V1 and V2 are sensory, and V3 is both motor to skeletal muscle and sensory. CN VII, IXand Xare both motor and sensory. CN VIII is the special sense of hearing and balance. CN XI and XII are motor to skeletal muscle. CN III, VII, IXand Xalso contain parasympathetic fibers of origin (visceral), although manyof theANS fibers will "jump" onto the branches of CN Vto reach their targets. The following table summarizes the types of fibers in each cranial nerve and where each passes through the cranium:

Cranial nerves emerge through foramina or fissures in the cranium

Twelve pairs

Numbered in order of origin from the brain and brain stem, rostral to caudal

Contain one or more of sixdifferent types of fibers

Motor fibers to voluntarymuscles

Somatic motor fibers to striated muscles (1)

a.Orbit

b.Tongue

c.Neck (sternocleidomastoid and trapezius)

Branchial motor (or special visceral efferent fibers) to striated muscles derived from pharyngeal arches (example: muscles of mastication) (2)

Motor fibers to involuntarymuscles = general visceral efferent (parasympathetic fibers) (3) Sensoryfibers

General visceral afferent fibers (4)

a.Carrysensation from viscera

b.Originate in carotid body, sinus, heart, lungs, and gastrointestinal tract

General somatic afferent fibers carrying pain, pressure, temperature, touch information (5)

Special sensoryafferent fibers conveying taste, smell, vision, hearing, and balance (6)

Can be sensory, motor, or mixed

Sensory Innervation of the Dura

Dura of the cranial fossae innervated bymeningeal branches of cranial and cervical nerves Anterior cranial fossa

Anterior meningeal branches of the ethmoidal nerves from ophthalmic nerves (CNV1)

Meningeal branches of the maxillarynerves (cranial nerve [CN] V2)

Meningeal branches of the mandibular nerves (CN V3) Middle cranial fossa

Meningeal branches of the maxillarynerves (CN V2)

Meningeal branches of the mandibular nerves (CN V3) Posterior cranial fossa

Tentorial nerve from ophthalmic nerve (CN V1)

Meningeal branches directlyfrom C2 and C3 spinal nerves or carried byCN X(vagus) or CN XII (hypoglossal)

Cervical Plexus

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Formed from anterior rami of C1-C4 spinal nerves Consists of a series of loops and branches from the loops

Lies deep to sternocleidomastoid (SCM) and anteromedial to levator scapulae and middle scalene muscles Cutaneous branches of the cervical plexus

Emerge from posterior border of SCM

Nerves from loop formed between anterior rami of C2 and C3

Lesser occipital (C2) to skin of neck and scalp posterior to auricle

Great auricular (C2 and C3) to skin over parotid gland, mastoid process, auricle, and between angle of mandible and mastoid process

Transverse cervical nerve (C2 and C3) to skin over anterior cervical region

Supraclavicular nerves

Arise from C3-C4 loop

Emerge from under SCM

Supplyskin over clavicle, superior thoracic wall, and shoulder Motor branches

Are considered deep branches

Innervate prevertebral muscles

Sternocleidomastoid (C2 and C3)

Trapezius (C3 and C4)

Levator scapulae (C3 and C4)

Motor fibers from C1 travel with hypoglossal nerve

Some C1 fibers leave hypoglossal nerve and innervate

a.Thyrohyoid muscle

b.Geniohyoid muscle

Rest leave the hypoglossal as its descending branch

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Motor fibers from C1 and C2 directlyinnervate thyrohyoid (an infrahyoid strap muscle) Motor fibers from C2, 3 form the descending cervical nerve

Ansa cervicalis

Loop formed bydescending branch from hypoglossal nerve (superior root) (C1) and descending cervical nerve (inferior root) (C2,C3)

Branches from ansa innervate remaining infrahyoid strap muscles

a.Omohyoid

b.Sternohyoid

c.Sternothyroid

Motor fibers from C3, C4, and C5 contribute to roots of phrenic nerve Phrenic nerve

Formed bybranches of anterior rami of C3, C4, and C5 spinal nerves

Contains a mixof fibers

Sole motor supplyto the diaphragm

Sensoryfibers from the central part of the diaphragm (sensoryfibers from peripheryprovided byintercostal nerves) Sympathetic nerve fibers from the cervical sympathetic ganglia to smooth muscle of blood vessel walls

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