cranial nerves Flashcards
What are cranial nerves?
- nerve bundles from the brain that supply (or innervate) the head and neck region, connect since organs to the brain (Afferent) and connect the brain to the muscles and glands (efferents)
- Most only have effect in the head and neck but some have innervations in the body e.g. vagus nerve
- They have specialisations that make them distinct from other nerves of the body
- Has special since nerves e.g. optic nerve for sight and olfactory nerve for smell
Why are cranial nerves different?
- Specialisations : anatomy and function
- Anatomy:
- cranial nerves have different anatomical trajectories
- connects to the nuclei of the brain and to the effector organs
- function:
- Spinal nerves are more segmented/predictable but each cranial nerve has its own anatomical course, rout and location
- Anatomy:
Give me a general overview of the cranial nerves?
- 12 pairs of cranial nerves
- Mostly bilaterally symmetrical
- All exit from anterior and inferior surface apparat from vestibulocochlear nerve which exits superior and posteriorly
- Short sections attaching nerves to the brain are found within the cranium
- Some only have short exteriors out of the cranium E.g. Olfactory nerve sits on cribitol plate and provides sense of smell as extends into superior aspect of the nose
Give me a general over view of the cranial nerves
- most are typical of “peripheral nerves” and 2 atypical nerves that are true brain tracts/not peripheral
- The 2 atypical =covered in meninges
- Their Head & Neck Targets are: – General Sense organs (afferent) – Skeletal Muscles (somatic efferent) – General Viscera (afferent &
- efferent) – Special Viscera (afferent & efferent) – Special Sense Organs (afferent) Some also supply viscera of the body
- -Special viscera= muscle derived from pharyngeal arches
- outside of the Head & Neck Region
What are the similarities between cranial nerves and spinal nerves?
- Both classes of nerves are constructed from a common
design template- They are largely similar in most respects
- They are both major constituents of the peripheral
nervous system - Has endoneruium, Elineruium and perineruium
There are critical noteworthy differences between them
What are the differences of the cranial nerves
- Differences are dictated by the separate embryological
origins of the head region from that of the spinal cord- Most Supply Head & Neck Structures that are derived from the same embryological branchial arches as themselves
- Head & Neck structures of non-branchial arch origin are also innervated by cranial nerves
- Branchial arches=pharyngeal arches
- Sometimes the nerves and the tissue they inverse are developed from the same pharyngeal arches
Origin of the Cranial Nerves
The olfactory nerve (CN I) and optic nerve (CN II) originate from the cerebrum. Cranial nerves III – XII arise from the brainstem (Figure 1).
They can arise from a specific part of the brainstem (midbrain, pons or medulla), or from a junction between two parts:
* Midbrain – the trochlear nerve (IV) comes from the posterior side of the midbrain. It has the longest intracranial length of all the cranial nerves.
* Midbrain-pontine junction – oculomotor (III).
* Pons – trigeminal (V).
* Pontine-medulla junction – abducens, facial, vestibulocochlear (VI-VIII).
* Medulla oblongata
○ Posterior to the olive: glossopharyngeal, vagus, accessory (IX-XI).
○ Anterior to the olive: hypoglossal (XII).
*
How are the cranial nerves numbered?
○ The cranial nerves are numbered by their location on the brainstem (superior to inferior, then medial to lateral) and the order of their exit from the cranium (anterior to posterior)
○ Last two nerves are swapped around CNXII is in-front if CNX1
○ CN1 most rostral
○ CNX1 most caudal
What are the twelve cranial nerves?
CNI Olfactory
* CNII Optic
* CNIII Oculomotor
* CNIV Trochlear
* CNV Trigeminal
* CNVI Abducent/ Abducens
* CNVII Facial
* CNVIII Vestibulocochlear
* CNIX Glossopharyngeal
* CNX Vagus
* CNXI Accessory
* CNXII Hypoglossal
Where do the cranial nerves exit from
CNI Olfactory- cribriform plate
* CNII Optic-optic canal
* CNIII Oculomotor-superior orbital fissure
* CNIV Trochlear -superior orbital fissure
* CNV trigeminal oplthamic division (v1) -superior orbital fissure
* CNVI Abducent/ Abducens -superior orbital fissure
* CNVII Facial-internal acoustic meatus
* CNVIII Vestibulocochlear -internal acoustic meatus
* CNIX Glossopharyngeal-jugular Foramen
* CNX Vagus-jugular Foramen
CNXI Accessory-jugular Foramen
* CNXII Hypoglossal-Hypoglossal canal
Give me a general overview about most peripheral nerves.
- Most nerve bundles contain upwards of 100s if not 1000s or 1000000 of axons
- Axons of nerve bundles are said to be “mixed”
- Some are myelinated
○ Heavily myelinated-faster conduction
○ Lightly myelinated-slower conduction - Others are unmyelinated
- There are varied conduction velocities amongst nerves
- There are differences in the directions in which impulses are conducted by nerve fibres
○ Diverse Afferent Modalities
○ 2 Major Classes of Efferent Modalities - Somatic Afferent for pain may be unmyelinated
What is the composition and classification of the spinal nerves?
- The general functional classification if nerves e.g. somatic or automictic (visceral)
- The direction of conduction of electrical impulses by the nerves (or modalities) e.g. afferent (or sensory) and efferent (output of the nervous system
What are the modalities of cranial nerves?
- Sensory (afferent) modalities:
○ General somatic sensory (GSS) – general sensation from skin.
○ General visceral sensory (GVS) – general sensation from viscera.
○ Special somatic sensory (SSS) – senses derived from ectoderm (e.g. sight, sound, balance).
○ Special visceral sensory (SVS) – senses derived from endoderm (e.g. taste, smell).- Motor (efferent) modalities:
○ General somatic motor (GSM) – skeletal muscles.
○ General visceral motor (GVM) – smooth muscles of gut and autonomic motor.
○ Special visceral motor (SVM) – muscles derived from pharyngeal arches.
- Motor (efferent) modalities:
What are the general classification of spinal nerves?
- General Somatic Afferents (GSA) - General Sensation
- General Visceral Afferents (GVA) - Visceral Sensation
- General Somatic Efferents (GSE) - Supply Skeletal muscle
- General Autonomic Efferents (GVE) - Autonomic Fibres Innervating Visceral Structures
What are the Functional Classification of Cranial Nerve Fibres?
- Cranial nerves may contain all of the 4 fibre types equivalent to those found in spine
- In addition, they may also contain special components not found in spinal nerves
- The Diverse and Complicated Composition of Cranial Nerves
- General Somatic Afferents - General Sensation
- General Visceral Afferents - Visceral Sensation
- General Somatic Efferents - Supply Skeletal muscle
- General Autonomic Efferents - Autonomic Fibres Innervating Visceral Structures
- Special Visceral Efferents - Muscles derived from branchial arches (CNs V, VII, IX & X)
- Special Somatic Afferents - Equilibration, Hearing, and Sight
- Special Visceral Afferents - Taste
