Posterior Cranial Fossa with Cranial Nerves Relation

POSTERIOR FOSSA AND RELATED

CRANIAL NERVES Yousuf Tahir Ali4th year MBBS

Dow University of Health Sciences

Posterior Crania Fossa

■ The posterior cranial fossa is part of the cranial cavity, located between the foramen magnum and tentorium cerebelli. It contains the brainstem and cerebellum.

■ This is the most inferior of the fossae. It houses the cerebellum, medulla and pons. Anteriorly it extends to the apex of the petrous temporal. Posteriorly it is enclosed by the occipital bone. Laterally portions of the squamous temporal and mastoid part of the temporal bone form its walls.

The posterior fossa is unique in that it receives 11/12 CNs with the exclusion of CN 1.

Cranial nerve exit via Skull Base

Parts of brain in the post fossa

Cranial nerves of the Brainstem

Cranial Nerve Nuclei

Arrangement of Nuclei

■ The cranial nerve nuclei are located such that the efferent nuclei are located medially and are separated from the sensory nuclei, by the sulcus limitans, that lie laterally as may be seen in cross sectional views.

(SE)

(SVE)

(GVE)

(GVA and SVA)(GSA)(SSA)

GENERAL TERMINOLOGIES

■ “Somatic” refers to all the body areas especially body under voluntary control.

■ “Visceral” refers to all the internal organs in the main cavities of the body e.g the intestines.

■ “Special” refers to anything that dosen’t fit in all to the above two categories. E.g.

1. semi-autonomous head and neck muscles(branchial arch derivatives).(EFFERENTS)

2. Sense of balance and hearing are special senses

As a general rule, we have from medial to lateral, the arrangement of nuclei in the brain at

all levels as;1. General Somatic efferent (GSE)2. Special visceral efferent (SpVE)3. General visceral efferent (GVE)4. Genreal visceral affarent (GVA)5. Special visceral affarent (SpVA)6. General sensory affarent (GSA)7. Special somatic affarent (SpSA)

General Somatic efferent(GSE)

■ For muscles derived from somitomeres(cells derived from paraxial mesoderm)

Special visceral efferent (SpVE)/Nucleus ambiguous■ CNs IX, X and XI.■ Named as “nucleus ambiguous” due to its particular

ambiguous(not clear or decided) position.■ Responsible for branchiomotor outflow(supplies muscles of

the branchial(pharyngeal) arches which are semi-autonomous)i.e. semiautonomous head and neck muscles.

■ The cell bodies for the preganglionic parasympathetic vagal neurons that innervate the heart reside in the nucleus ambiguous.

■ Also branchial efferent motor fibers of the vagus nerve (CN X) terminating in the laryngeal and pharyngeal muscles, and musculus uvulae arise from this nucleus.

General visceral efferent (GVE)

■ It comprises:1. Dorsal nucleus of the vagus(CN-X)2. Inf. Salivatory nucleus(CN-IX)3. Sup. Salivatory nucleus(CN-VII)4. Lacrimatory nucleus(CN-VII)■ Dorsal nucleus of the vagus(CN-X)

serves parasympathetic vagal functions in the GIT, lungs, and other thoracic and abdominal vagal innervations

TRACTUS SOLITARIUS[General Visceral Affarrent(GVA) + Special Visceral Affarent(SVA)

It receives sensory(affarent) fibers from CN-VII,IX and X.

Nucleus of TS(Some general features)

Genereal visceral affarent (GVA)(medial component of Tractus Solitarius)

■ CNs IX and X.■ Sensory information from the ear (Auricular branch

of the vagus nerve(CN-X).■ Chemoreceptors and mechanoreceptors of the

general visceral afferent pathway (GVA) in the carotid body via glossopharyngeal nerve(CN-IX), and aortic bodies, and the atrial pacemaker, via the vagus nerve(CN-X)

■ Chemically and mechanically sensitive neurons of the general visceral afferent pathway (GVA) with endings located in the heart, lungs, airways, gastrointestinal system, pharynx, and liver via the glossopharyngeal(CN-IX) and vagus nerves(CN-X).

■ NOTE: Bodies serve chemoreception and sinus serves baroreception, but the aortic body serves both chemo and baroreception.

Special visceral affarent (SpVA)(lateral component of Tractus Solitarius)

■ Taste is a special sense.■ CNs-VII, IX and X.■ Taste information from the facial nerve(CN-VII)

(anterior 2/3 of the tongue), glossopharyngeal nerve(CN-IX) (posterior 1/3) and vagus nerve(CN-X) (small area on the epiglottis).

General sensory affarent (GSA)

■ Comprises 2, receive■ GSA of Trigeminal nerve comprises:1. Mesencephalic nucleus: proprioception of the face2. Principal/pontine nucleus: fine touch of face+conscious proprioception of the jaw

(Conscious proprioception is described as the ability to locate the different joints of our body consciously).

3. Spinal nucleus: pain and temp.■ The spinal trigeminal nucleus is further subdivided into three parts, from rostral to caudal:■ Pars Oralis (from the Pons to the Hypoglossal nucleus)■ Pars Interpolaris (from the Hypoglossal nucleus to the obex)■ Pars Caudalis (from the obex[the point in the human brain at which the fourth ventricle

narrows to become the central canal of the spinal cord) to C2]. Decussation of sensory fibers occours at obex.

■ The other GSA receives pain, touch and temp. from the entire body.

General sensory affarent (GSA)■ Comprises 2, (i)Trigeminal and (ii)GSA from entire body■ GSA of Trigeminal nerve comprises:1. Mesencephalic nucleus: proprioception of the face2. Principal/pontine nucleus: fine touch of face+conscious proprioception of

the jaw (Conscious proprioception is described as the ability to locate the different joints of our body consciously).

3. Spinal nucleus: pain and temp.■ The spinal trigeminal nucleus is further subdivided into three parts, from

rostral to caudal:■ Pars Oralis (from the Pons to the Hypoglossal nucleus)■ Pars Interpolaris (from the Hypoglossal nucleus to the obex)■ Pars Caudalis (from the obex[the point in the human brain at which the fourth

ventricle narrows to become the central canal of the spinal cord) to C2]. Decussation of sensory fibers occours at obex.

■ The other GSA receives pain, touch and temp. from the entire body.

The sensory trigeminal nerve nuclei are the largest of the cranial nerve nuclei, and extend through the whole of the midbrain, pons and medulla, and into the high cervical spinal cord.

Special somatic affarent (SpSA)

■ This comprises:1. Vestibular nuclei(sup., lat., med. and inf.)2. Cochlear nuclei(dorsal and ventral)■ The vestibular nuclei lie lateral to TS and are

concerned with the sense of balance.■ The dorsal and ventral cochlear nuclei are located

dorsal and ventral to the inf. Cerebellar peduncle and are concerned with the sense of hearing.

As a general rule, we have from medial to lateral, the arrangement of nuclei in the brain at

all levels as;1. General Somatic efferent (SE)2. Somatic visceral efferent (SVE)3. General visceral efferent (GVE)4. Genreal visceral affarent (GVA)5. Special visceral affarent (SVA)6. General sensory affarent (GSA)7. Special somatic affarent (SSA)

Lateral view of Brainstem

LATERAL VIEW OF BRAINSTEM AND NERVES EXITING IT

Lat. diagramatic view showing exit of CNs

10

9

11

11th cranial part11th spinal part

12

3RD VENTRICLE

MIDBRAIN

PONS

MEDIULLA

INFERIOR CEREBELLAR PEDUNCLE

LATERAL VIEW OF BRAINSTEM

Anterior view of Brainstem

ANTERIOR VIEW OF BRAINSTEM

CRANIAL NERVE III