Anatomy of cranial nervesintroduction

Dr Hytham Nafady

CNS development

The neural tube

• The neural tube gives rise to the CNS (including the brain & spinal cord).

• The brain stem & spinal cord have:

1. An alar plate that gives rise to the sensory neurons.

2. A basal plate that gives rise to sensory neurons.

• The neural tube gives rise to 3 primary vesicles, which develop into 5 secondary vesicles

The neural crest

• Gives rise to the peripheral nervous system (peripheral nerves & sensory & autonomic ganglia).

1. Pseudounipolar ganglion cells of the spinal & cranial nerves ganglia.

2. Schwan cells (which elaborate the myelin sheath).3. Multipolar ganglion cells of the autonomic ganglia.4. Leptomeninges (the pia-arachnoid).5. Chromaffin cells of the suprarenal medulla.6. Melanocytes.7. Odontoblasts (which elaborate predentine).8. Aortopulmonary septum of the heart.9. Parafollicular cells (calcitonin producing C cells).10.Skeletal & connective tissue component of the pharyngeal

arches.

• The anterior neuropore:• The closure of the anterior neuropore gives rise to

the lamina terminalis.

Failure of closure of the anterior neuropore gives rise to anencephaly

Failure of closure of the posterior neuropore gives rise to spina bifida

Branchial arches

N Site Supply

Somatic motorSomatic motor 3, 4, 6 & 12

Lie medially Striate muscles •Eye •Tongue.

Branchiomeric Branchiomeric motormotor

5, 7, 9 & 10

Initially develop in the periventricular grey matter then migrate ventrolaterally

Striate muscles •Jaw.•Face.•Pharynx.•Larynx.

Visceral motorVisceral motor 3, 7, 9 & 10

glands &viscera •Head & neck. •Thorax.•Abdomen.

N Site Supply

Visceral Visceral sensorysensory

9 & 10

Lie lateral to the visceral motor

Chemo & baro receptors

7 & 9 Taste receptors (tongue).

Somatic Somatic sensorysensory

5 Initially develop in the periventricular grey matter then migrate ventrolaterally

Proprioreceptor, nocireceptors and thermo-receptors (in the skin, muscles & joints of the head)

Special Special sensorysensory

8 Lie most lateral in the periventricular zone.

Ear.

Cochlea.

Vesibule.

Ganglia and nuclei

Ganglia:• Aggregation of cell bodies in a peripheral nerve.Nuclei:• Aggregation of cell bodies inside the CNS.

• Ganglia are peripheral and nuclei are central.• Basal ganglia is a misnomer (basal nuclei).

Motor, parasympathetic & sensory nuclei

Motor nuclei contain the cell bodies of lower motor neurons

Parasympathetic nuclei

contain the cell bodies of the pre-ganglionic para-symathetic fibers

Sensory nuclei (somatic or visceral) contain the cell bodies of the secondary sensory neurons

Parasympathetic & sensory ganglia

Sensory ganglia contain cell bodies of the peripheral sensory (somatic or visceral) nerve.

• It also contain somatic motor fibers which pass without relay.

Autonomic ganglia contain the cell bodies of the postganglionic parasympathetic fibers.

• It also contains visceral sensory fibers which pass without relay.

Autonomic ganglia (ganglia with synapses).

Sensory ganglia (ganglia without synapses).

Head & neck ganglia

Parasympathetic fibers

Taste fibers

Cranial nerve attachments

Cranial nerves attached to the forebrain

(telencephalon) 1-Olfactory nerve.

(diencephalon) 2-Optic nerve.

Cranial nerves attached to the midbrain (mesencephalon)

3-Oculomotor.

4-Trochlear.

Cranial nerves attached to the hindbrain (rhombencephalon)

5-Trigeminal

6-Abducent.

7-Facial.

8-Vestibulocochlear.

9-Glosspharyngeal.

10-Vagus.

11-Accessory.

12-Hypoglossal.

Cranial nerve nuclei

Cross section through the medulla

Medial structures

• Pyramid.

• Medial leminscus.

• Hypoglossal nucleus.

Lateral stuctures

• Olive.• Nucleus ambiguus.• Dorsal motor nucleus of vagus.• Solitary nucleus.• Vestibular nuclei.More lateral• Spinal lemeniscus.• Trigeminal tract & spinal nucleus.• Inferior cerebellar peduncle.

Cross section at the pons

Medial structures

• Genu of facial nerve.

• Abducens nucleus.

• Abducens nerve.

• Medial longitudinal fasciculus.

• Medial leminscus.

• Corticospinal tract.

Lateral structures

• Cochlear nuclei.

• Vestibular nuclei.

• Facial nucleus.

• Spinal leminscus.

• Facial nerve.

• Facial nucleus.

Cross section at the midbrain

Functional Classification of CN• Spinal Nerve classification

– General Efferent or Afferent: serve general motor, sensory.

• Cranial Nerves classification– Receptor type:

• General - just like spinal nerves• Special –Use special receptors and neurons to serve additional

specialized functions

– Signal type• Efferent – Sensory• Afferent - Motor

– Voluntary or reflexive?• Somatic. Innervate somatic muscles (muscles that arise from the

soma in the embryological stage – voluntary muscle control)• Visceral. Innervate visceral structures.

Types of nerve fibers within cranial nerves

Direction of impulse: • motor (efferent) or • sensory (afferent). The mode of control: • voluntary or • involuntary.The embryological origin of the structure innervated: • Somatic (somite, body wall) or • Visceral (gut tube, internal organs).The distribution in the body: • General (widespread) or • Special (restricted to the head and neck).

7 Functional Types

General Somatic Efferent GSE Muscles from somites (skeletal, extraocular, glossal)

General Visceral Efferent GVE Smooth muscles of visceral Organs

Special Visceral Efferent SVE Muscles of face, palate, mouth, pharynx and larynx (excludes eye & tongue muscles).

Special Visceral Afferent SVA visceral sensation of taste from tongue Olfaction from Nose

General Visceral Afferent GVA sensory innervation from visceral organs

General Somatic Afferent GSA Sensory innervation from muscles, skin, ligament and joints

Special Somatic Afferent SSA special sensations of vision from retina and audition and equilibrium from inner ear

3-4-6Somatic motor fibers – oculomotor, trochlear and abducens nuclei:Extrinsic ocular muscles are derived from pre-otic somites.Motor fibers innervating them, therefore, are somatic motor fibres and nuclei are somatic motor nuclei.Upper motor neuron input to nuclei of each side is bilateral.Nuclei also receive fibers from medial longitudinal fasciculus for control of eye movements.Oculomotor nucleus: periaqueductal grey matter of midbrain, near midline, immediately ventral to aqueduct of Sylvius. Axons pass ventrally to emerge in the interpeduncular fossa.Trochlear nucleus:in periaqueductal grey matter of midbrain, below oculomotor nuclei. Axons pass dorsally, decussating within midbrain dorsal to aqueduct.Abducens nucleus: in pons, related to VII motor nucleus