Neuroanatomy Flashcards
1
Q
What is the role of oligodendrocytes?
A
myelinate axons in the brain (CNS)
2
Q
What is the role of schwann cells?
A
Myelinate axons in the PNS
3
Q
What are commisures?
A
tract connecting one hemisphere to the other, tracts that cross the midline
4
Q
What are lemnisci?
A
narrow strip of fibres
5
Q
What is a fasciculi?
A
bundle e.g. gracile fasiculus
6
Q
What is the difference tetween afferent and efferent fibres?
A
Afferents: axons taking information towards the CNS e.g. sensory fibres
Efferents: axons taking information to another site from the CNS e.g. motor fibres
7
Q
What are gyri?
A
Ridges
8
Q
What are sulci?
A
Grooves
9
Q
What is meant by reticular?
A
‘netlike’, where grey & white matter mix e.g. reticular formation of brainstem
10
Q
What is the coronal plane?
A
vertical/frontal - parallel with coronal suture of skull
11
Q
In the brainstem what are dorsal and ventral?
A
Dorsal Posterior
Ventral Anterior
12
Q
In the cerebrum what are dorsal and ventral?
A
Dorsal superior
Ventral inferior
13
Q
What is the parietal lobe and what are its functions?
A
Extends from the central sulcus anteriorly to the imaginary parietoccipital fissure posteriorly, and contains the primary sensory area (9). There are two parietal lobes, and the dominant lobe (normally the left) is important for perception, interpretation of sensory information (7 and 10) and the formation of the idea of a complex, meaningful motor response. The supramarginal and angular gyrus of the dominant lobe are concerned with language and mathematical operations. The nondominant lobe (normally the right) is important for visuospatial functions. Receives and interprets sensations, including pain, touch, pressure, size and shape and body-part awareness (proprioception)
14
Q
What is the frontal lobe and what are its functions?
A
Involved in motor function (3 and 12), problem solving, spontaneity, memory, language, judgement, personality, impulse control and social and sexual behaviour (13). The anterior portion of the frontal lobe, or the prefrontal cortex, is important for higher cognitive functions and determination of personality. The posterior portion of the frontal lobe contains the motor and premotor areas. Broca’s area (4) is found at the inferior frontal gyrus, and is important for language production and comprehension. Olfaction (8) Voluntary movement on opposite side of body - Frontal lobe of dominant hemisphere controls speech (Broca’s area) & writing (if right handed, then left hemisphere is dominant etc.) - Intellectual functioning, thought processes, reasoning & memory
15
Q
What is the temporal lobe and what are its functions?
A
Contains the primary auditory cortex (5), hippocampus, amygdala and Wernicke’s area (11). Wernicke’s area is located in the superior temporal gyrus of the left hemisphere and is concerned with understanding the spoken word. Area (2) is concerned with short term memory, equilibrium and emotion. Understanding the spoken word (Wernicke’s - understanding), sounds as well as memory and emotion
16
Q
What is the occipital lobe and what are its functions?
A
Is located at the posterior aspect of the brain, and contains the primary visual and visual association cortex (1). Limbic system: The limbic lobe surrounds the medial margin of the hemisphere (6). The limbic system includes the hippocampus, fornix, amygdala etc. and is involved in emotion, memory, behaviour and olfaction. The hippocampus is involved in long term memory formation; the amygdala is important in motivationally significant stimuli, such as those related to reward and fear. The limbic system operates by influencing the endocrine system and the autonomic nervous system, and is highly interconnected with the brains pleasure centre; the nucleus accumbens – which has a role in sexual arousal and the high experienced with recreational drugs. Understanding visual images and meaning of written words
17
Q
Give examples of nerve cells?
A
Many types e.g. pyramidal, stellate, Golgi, Purkinje
18
Q
Give examples of neuroglia?
A
Astrocytes, Oligodendrocytes & Microglia
19
Q
What is the correct name for the hindbrain and what is included in it?
A
Rhombencephalon
a) The MEDULLA OBLONGATA, derived from the myelencephalon;
b) the PONS, derived from the metencephalon;
c) the CEREBELLUM, also derived from the metencephalon
20
Q
What is the correct name for midbrain and what is included in it?
A
Mesencephalon
a) The TECTUM which is that part of the midbrain lying dorsal to the central canal (cerebral aqueduct), and comprising the superior and inferior COLLICULI, also called the corpora quadrigemina;
b) The CEREBRAL PEDUNCLE which is the midbrain ventral to the aqueduct.
21
Q
What is the correct name for the forebrain and what is included in it?
A
(prosencephalon) - forms the majority of the brain
a) The DIENCEPHALON, which derives from the anterior part of the developing neural tube and comprises the THALAMUS and the HYPOTHALAMUS (with the epi- and sub-thalamus) of each side.
b) The TELENCEPHALON, deriving from the cerebral vesicle of each side and consisting of an outer layer of grey matter, the CEREBRAL CORTEX, and deep nuclei, the BASAL GANGLIA which grow into the vesicle. The term CEREBRAL HEMISPHERE is usually used to refer to the telencephalon of each side, though it may be used to mean the telencephalon and the diencephalon of each side together.
22
Q
What is white matter?
A
formed by collections of nerve fibres (axons and dendrites) with few or no neuronal cell bodies
23
Q
What is grey matter?
A
is formed by aggregations of neuronal cell bodies and their local processes
Within the grey matter, the felt work of intermingled and interconnected neuronal processes which occupies the space between neuronal cell bodies is called the NEUROPIL.
o In the fresh brain, large numbers of nerve fibres with fatty myelin sheaths appear white, whereas aggregations of nerve cells with few myelinated fibres amongst them appear grey. This distinction is considerably less clear in formalin fixed brains such as those you will dissect
24
Q
What is the insula?
A
forms the floor of the Lateral Sulcus (insular cortex)
25
What are the opercula?
the parts of the temporal, frontal and parietal lobes that overlie the insula
26
What is the corpus callosum?
a large bundle of white matter connecting the two hemispheres.
27
What are the mamillary bodies?
two rounded eminences behind the optic chiasma
28
What is the hypothalamus and what is its location?
behind the optic chiasma up to and including the mammillary bodies (the only part of the DIENCEPHALON visible on the outside of the brain).
29
What are the THE CRURA CEREBRI of the CEREBRAL PEDUNCLES?
two large masses of white matter emerging, behind the mammillary bodies on each side, from the cerebral hemisphere. They pass backwards, converging in the midline at the upper border of the PONS
30
Where is the interpenduncular fossa?
space between the crura roofed over by arachnoid
31
What is Broca's Area? What is the effect of damage?
Broca’s area is the language area in the DOMINANT (normally left) FRONTAL LOBE responsible for the articulation of speech
Damage to this area can result in expressive aphasias - Individuals with expressive aphasia may have difficulties forming words or sentences
They will understand what you are saying and know what they want to say but just cannot expressive the words in meaningful language
32
What is Wernicke's Area? What is the effect of damage?
Wernicke’s are is the comprehension area in the DOMINANT (normally left) TEMPORAL LOBE and is responsible for understanding speech
Damage to this area results in comprehension aphasias
Individuals with comprehension aphasia may have difficulty understanding spoken or written language even though their hearing & vision are not impaired
They tend to have fluent speech but they may scramble words so that their sentences make no sense, often adding unnecessary words or even creating made-up words
33
What is a berry aneurysm?
The most common type of inter cranial aneurysm
Most common at the anterior cerebral artery (ACA) and anterior communicating artery junction
Produces a subarachnoid haemorrhage
resulting in a thunderclap headache
34
What is MYASTHENIA GRAVIS?
Autoimmune
• Muscle weakness caused by circulating antibodies that block the acetylcholine receptors at the postsynaptic side of the neuromuscular junction
• This blocks the excitatory effect of ACh on the nicotinic receptors resulting in muscle weakness
• More common in women
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Symptoms:
Face getting progressively droopy in the morning
- Tiring or difficult to chew food
- Double vision
- Eyelid drooping
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35
What is Duchenne Muscular Dystrophy? What are the symptoms?
X-linked recessive
• Only affects boys
• Results in muscle degeneration and eventually premature death
• Affects MUSCLES
Symptoms:
Awkward manner of running with frequent falls & more easily fatigued
- Difficulty with motor skills e.g. running & jumping
- Typically in very young boys since most die before they get too old
- Boys will find it difficult to get into standing position
36
What is the dura mater?
has two layers. The outer endosteal layer of the cranial dura mater lines the interior of the skull, adhering to, and sending blood vessels and fibrous processes into the cranial bones.
The inner meningeal layer completely envelopes the central nervous system; it continues as tube of dura seen around the spinal cord and provides tubular sheaths for the cranial nerves. For the most part the two layers are fused. However, in places the inner layer separates from the skull to form dural folds which support the subdivisions of the brain and partially divide the cranial cavity into three areas, the right and left hemispheres and the posterior cranial fossa where the Cerebellum lies. Where these dural folds attach to the skull there is a system of communicating blood filled spaces, the Dural Venous Sinuses.
37
What is falx cerebri?
an arched crescent of dura lying in the longitudinal fissure between the cerebral hemispheres
38
What is the tentorium Cerebelli?
dura forming a thick fibrous roof over the posterior cranial fossa and the cerebellum
39
What is the falx cerebelli?
a small, vertical, sickle-shaped reflection of dura separating the two lobes of the cerebellum
40
What is the superior sagital sinus?
where the falx cerebri attaches to the cranium
41
What is the inferior sagital sinus?
at the free border of the falx cerebri
42
What is the straight sinus?
within the tentorium cerebelli at its attachment to the falx cerebri
43
What is the transverse sinus?
run along the line of attachment of the tentorium cerebelli to the occipital bone
44
What is cavernous sinus?
lies lateral to the body of the sphenoid
45
What is the tentorial inscisure?
a horseshoe-shaped space between the free concave border of the tentorium and the dorsum sellae of the sphenoid
46
What is the trigeminal cave?
lies next to the apex of the petrous part of the temporal bone and envelops the roots of the trigeminal nerve
47
What is THE DIAPHRAGMA SELLAE?
a small, circular, horizontal fold of dura mater which forms the roof of the pituitary fossa
48
What is the arachnoid mater?
This layer encloses the brain loosely following the contour of the meningeal layer of the dura. Where the arachnoid spans the gyri of the brain, spaces exist between the arachnoid and the pia mater called SUBARACHNOID CISTERNS – these cisterns are full of CSF.
49
What is the foramen of magendie?
a midline communication between the IVth ventricle and the subarachnoid space
50
What is the foramen of luschka?
a lateral communication between the IVth ventricle and the subarachnoid space
51
What are subarachnoid cisterns?
The SUBARACHNOID CISTERNS are named after their positions relative to the brain. You need to understand the cisterns and their function both in health and disease (e.g. Meningitis)
52
What is the CEREBELLOMEDULLARY CISTERN?
lies in the angle formed by the dorsal surface of the medulla and the inferior surface of the cerebellum
53
What is the pontine cistern?
on the ventral surface of the pons
54
What is the interpeduncular cistern?
contains the Circle of Willis
55
What is THE CISTERN OF THE LATERAL FISSURE?
contains the Middle Cerebral Artery and bridges the Lateral Sulcus on each side
56
What is the superior cistern?
contains the great cerebral vein (of Galen) and the pineal gland and is found between the posterior end (splenium) of the corpus callosum and the superior surface of the cerebellum
57
What is the cisterna ambiens?
group of subarachnoid cisterns which completely encircle the midbrain
58
What is the pia mater?
This is closely adherent to the underlying nervous tissue, and is indistinguishable with the naked eye, but is functionally very important; it forms part of the blood brain barrier. BLOOD-BRAIN BARRIER On the surface of the brain the arteries lie in the subarachnoid space. As the vessels pass into the substance of the brain they take with them prolongations of the Pia Mater and some of the subarachnoid space, this forms a layer around the vessel. As the vessel penetrates deeper into the Brain tissue, the tunica media thins and the prolongation of the subarachnoid space narrows. At the level of the capillary network, the basement membranes of the endothelial cells and of the pia fuse. The blood brain barrier is a combination of features, unique to the brain and spinal cord that limit the ability of molecules to pass between the blood and the CNS. This has the effect of protecting tissue from toxic substances. These features are:
1. The edges of adjacent endothelial cells that line blood vessels are bonded closely together by ‘tight junctions’ to prevent molecules passing between them.
2. The basement membrane of CNS blood vessels lack ‘fenestrations’ (small holes) that are present elsewhere in the body.
3. Pericytes are cells that are embedded in the basement membrane and wrap around endothelial cells. They regulate capillary blood flow, immunity and vascular permeability.
4. Astrocytes extend processes called end feet that envelop CNS capillaries and restrict the flow of molecules into the CNS parenchyma.
59
Where are the locations of possible haemorrage?
Extradural – between the skull and dura mater – strips the dura from the bone and compresses the brain – typically after head injury – haemorrhage is from the meningeal arteries
Subdural – blood between the dura and arachnoid – appears as a crescent on CT head – typically after high impact injury e.g. RTA – haemorrhage is typically a result of tears in the bridging veins (Chronic subdural haematoma occurs 4-8 weeks following mild/moderate head injury in the elderly)
Subarachnoid – blood between the arachnoid and pia mater – occurs after head injury (traumatic Sub arachnoid Haemorrhage or after rupture of a berry aneurysm – blood surrounds the brain and fills the sulci, predominantly near the site of injury/aneurysm - Pathognomonic symptom: sudden severe headache, high mortality rate!
Intracerebral – rupture of small vessels and microaneurysms in perforating vessels leading to bleeding within the brain tissue - classical location: internal capsule following rupture of the lenticulostriate artery due to high blood pressure
60
What is meningitis?
Inflammation of the meninges, typically caused by infection. Meningism is the triad of headache, neck stiffness and photophobia. Nausea/vomiting and fever are also present with meningitis. Viral meningitis is often mild and self-limiting, bacterial meningitis requires urgent treatment or will lead to brain damage or death. (NB: the rash usually associated with bacterial meningitis is in fact a sign of meningococcal septicaemia not meningitis; the infection is in the blood)
61
What is amaurosis fugax?
Temporary loss of vision to one eye – ‘like a veil across one eye’. Part of a carotid plaque breaks off and occludes the central retinal artery - WARNING of thrombus of the internal carotid artery – potential for an impending stroke.
62
What are features of the the blood brain barrier?
Endothelial tight junctions
Astrocyte end feet
Pericytes
Continuous basement membrane, lacks fenestrations (windows)
Requires specific transported for glucose, essential ions etc.
Certain parts of the brain lack the blood-brain barrier, these are called CIRCUMVENTRULAR ORGANS e.g. posterior pituitary - they need to be in contact with the blood for a sensory role to monitor
63
What is the blood supply to the brain?
Internal Carotid
80%
supply the ANTERIOR and MIDDLE parts of the CEREBRUM and the DIENCEPHALON
Vertebral Arteries
20%
the POSTERIOR CEREBRUM and the contents of the POSTERIOR CRANIAL FOSSA
64
Describe the vertebro-basilar system?
The VERTEBRAL ARTERY on each side arises from the first part of the SUBCLAVIAN ARTERY
Frequently they are of markedly different diameter on the two sides.
They enter the skull through the foramen magnum
At the lower border of the pons, the two vertebral arteries unite in the midline to form the BASILAR ARTERY lying in the ANTERIOR MEDIAN FISSURE on the PONS.
The Vertebral and Basilar Arteries, the branches from them, and the Posterior Cerebral Artery together are often referred to as the POSTERIOR CIRCULATION
The Anterior and Posterior Circulations are linked via Circle of Willis.
65
What is the difference between external cerebral veins and the internal cerebral veins?
INTERNAL CEREBRAL VEINS These run within the substance of the brain tissue and end when they reach the surface of the brain where they become external cerebral veins
EXTERNAL CEREBRAL VEINS These run on the surface of the brain crossing the subarachnoid space to drain into the Dural venous sinuses. Identify the major veins present on the specimen and in your dissected cadaver and trace them to the sinus they adjoin. Identify the Great Cerebral Vein (of Galen) – this drains the deep structures of the brain and drains into the Straight Sinus; the cut end of this vein should be identifiable just above the cerebellum between the two occipital lobes
66
What is the inferior petrosal sinus?
in the groove between the petrous temporal bone and the basal part of the occipital bone
67
What is the sigmoid sinus?
a deep groove in the mastoid part of the temporal bone
68
What is venous sinus thrombosis?
Possible causes include, rare complication of childbirth, clotting disorders and ear infection (septic venous thrombosis).
Obstruction of venous drainage causes cerebral oedema and raised intracranial pressure. Resultant brain damage can present as a combination of headache with epileptic seizures focal motor deficit and deterioration in consciousness.
69
Describe the production of CSF?
The invagination of vessels into the ventricles produces a vascular fold of pia mater covered by an epithelium derived from the ependymal lining of the ventricle, this is the CHOROID PLEXUS.
Tight junctions prevent passage of fluid from the extracellular space of the Choroid Plexus into the ventricle except via the choroidal cells themselves. This enables close control over the volume and composition of the CSF.
Identify the CHOROID PLEXUS and ARACHNOID GRANULATIONS CSF-BRAIN BARRIER The ependyma constitutes the CSF-BRAIN BARRIER.
Resorption of the CSF into the venous drainage of the brain occurs via two routes:
Firstly, tufts of arachnoid mater, called ARACHNOID VILLI or ARACHNOID GRANULATIONS allow CSF to be resorbed into venous structures. With advancing age, these villi tend to calcify and may be visible on Xray. Their presence tends to causes bone to be resorbed along the internal surface of the cranial vault near the midline causing small pit-like structures. The diagram below shows the structure of an arachnoid granulation. The subarachnoid space between the arachnoid and pia mater is highly trabeculated and is continuous with the channel in the centre of the granulation. Narrow channels traverse the cap region of the granulation to come into contact with the endothelium of the venous sinus. The fluid finally drains through the endothelium.
Secondly, CSF drains along nerves to the lymphatics. Most importantly the nasal mucosa lymphatics which are themselves drained by deep cervical lymph nodes.
70
Describe CSF?
Circulates through the subarachnoid space (around the brain and spinal cord) and within ventricles - offers protection by cushioning brain from gentle movements)
There are four ventricles: 1. Lateral (paired) 2. III 3. IV • Ventricles & subarachnoid spaces connect via cisterns
Entries CSF is around 120mls
Its a clear, colourless liquid which contains; protein, urea, glucose & salts
Produced by ependymal cells in the choroid plexuses of the lateral ventricles (mainly)
Choroid plexus: - Formed from modified ependymal cells - They from around a network of capillaries, large surface area
Absorbed via arachnoid granulations (VILLI) e.g. in the superior sagittal sinus
Hydrocephalus: Abnormal accumulation of CSF in ventricular system. Often due to a blocked cerebral aqueduct - Accumulation of fluid leads to a build up of pressure which can damage brain tissue since the skull in hard in adults - In children with soft skull the pressure will cause the soul to bulge and look abnormal as well as damaging the brain
71
Describe the internal carotid arteries?
The internal carotid arteries (ICA) originate at the bifurcation of the left and right common carotid arteries, at the level of the fourth cervical vertebrae (C4).
They move superiorly within the carotid sheath, and enter the brain via the carotid canal of the temporal bone. They do not supply any branches to the face or neck.
Once in the cranial cavity, the internal carotids pass anteriorly through the cavernous sinus. Distal to the cavernous sinus, each ICA gives rise to:
Ophthalmic artery – supplies the structures of the orbit.
Posterior communicating artery – acts as an anastomotic ‘connecting vessel’ in the Circle of Willis (see ‘Circle of Willis’ below).
Anterior choroidal artery – supplies structures in the brain important for motor control and vision.
Anterior cerebral artery – supplies part of the cerebrum.
The internal carotids then continue as the middle cerebral artery, which supplies the lateral portions of the cerebrum.
72
Describe the vertebral arteries?
right and left vertebral arteries arise from the subclavian arteries, medial to the anterior scalene muscle. They then ascend the posterior aspect of the neck, through holes in the transverse processes of the cervical vertebrae, known as foramen transversarium.
The vertebral arteries enter the cranial cavity via the foramen magnum. Within the cranial vault, some branches are given off:
Meningeal branch – supplies the falx cerebelli, a sheet of dura mater.
Anterior and posterior spinal arteries – supplies the spinal cord, spanning its entire length.
Posterior inferior cerebellar artery – supplies the cerebellum.
After this, the two vertebral arteries converge to form the basilar artery. Several branches from the basilar artery originate here, and go onto supply the cerebellum and pons. The basilar artery terminates by bifurcating into the posterior cerebral arteries.
73
Describe the arterial circle of willis?
The terminal branches of the vertebral and internal carotid arteries all anastomose to form a circular blood vessel, called the Circle of Willis.
There are three main (paired) constituents of the Circle of Willis:
Anterior cerebral arteries – terminal branches of the internal carotid arteries.
Internal carotid arteries – located immediately proximal to the origin of the middle cerebral arteries.
Posterior cerebral arteries – terminal branches of the basilar artery
To complete the circle, two ‘connecting vessels’ are also present:
Anterior communicating artery – connects the two anterior cerebral arteries.
Posterior communicating artery – branch of the internal carotid, this artery connects the ICA to the posterior cerebral artery.
74
Describe the regional blood supply of the cerebrum?
There are three cerebral arteries; anterior, middle and posterior. They each supply a different portion of the cerebrum.
The anterior cerebral arteries supply the anteromedial portion of the cerebrum.
The middle cerebral arteries are situated laterally, supplying the majority of the lateral part of the brain.
The posterior cerebral arteries supply both the medial and lateral parts of the posterior cerebrum.
75
Describe the arterial supply to the spinal cord?
The spinal cord is primarily supplied by three longitudinal arteries, as it descends from the brainstem to the conus medullaris
These are:
• Anterior spinal artery – formed from branches of the vertebral arteries, travelling in the anterior median fissure. Gives rise to the sulcal arteries, which enter the spinal cord.
• Two posterior spinal arteries – originate from the vertebral artery or the posteroinferior cerebellar artery, anastomosing with one another in the pia mater.
However, below the cervical level supply from these longitudinal arteries is insufficient. There is support via anastomosis with the segmental medullary and radicular arteries.
The anterior and posterior segmental medullary arteries are derived from spinal branches of a number of arteries, before entering the vertebral canal through the intervertebral foramina.
The great anterior segmental artery of Adamkiewicz reinforces circulation to the inferior 2/3 of the spinal cord, and is found on the left in the majority of individuals.
The radicular arteries supply (and follow the path of) the anterior and posterior nerve roots. Some radicular arteries may also contribute to supplying the spinal cord.
76
Describe the venous drainage of the cerebrum?
Superior Cerebral Veins
Drain the superior surface, carrying blood to the superior sagittal sinus.
Superficial middle cerebral vein
Drains the lateral surface of each hemisphere, carrying blood to the cavernous or sphenopalatine sinuses.
Inferior Cerebral Veins
Drain the inferior aspect of each cerebral hemisphere, depositing blood into cavernous and transverse sinuses.
Superior Anastamotic vein (Trolard)
Connects the superficial middle cerebral vein to the superior sagittal sinus.
Inferior Anastamotic vein (Labbé)
Connects the superficial middle cerebral vein to the transverse sinus.
77
Describe the deep venous drainage?
Subependymal veins
There are numerous subependymal veins, which will not be described here in detail. These receive blood from the medullary veins and carry it to the dural venous sinuses. The great cerebral vein (vein of Galen) is worthy of a mention; it is formed by the union of two of the deep veins, and drains into the straight sinus.
Medullary Veins
Originate 1-2cm below the cortical grey matter, and drain into subependymal veins. These drain the deep areas of the brain.
78
What structures pass through the cavernous sinus?
1. Oculomotor (3)
2. Trochlear (4)
3. Opthalmic trigeminal (5.1)
4. Maxillary trigeminal (5.2)
5. Carotid (INTERNAL)
6. Abducens (6) - only one going medially
7. Trochlear (4)
TO remember O TOM CAT
79
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone for the Cribriform foramina in the cribriform plate?
* Olfactory nerve (CN I)
* Anterior ethmoidal nerves
Anterior cranial fossa
Ethmoid bone
80
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone of the optic canal?
* Optic nerve (CN II)
* Ophthalmic artery
Middle cranial fossa
Sphenoid bone
81
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone of the superior orbital fissure?
* Lacrimal nerve
* Frontal nerve- branch of ophthalmic nerve of trigeminal nerve (CN V)
* Superior ophthalmic vein
* Trochlear nerve (CN IV)
* Superior division of the oculomotor nerve (CN III)
* Nasociliary nerve- branch of ophthalmic nerve (CN V1)
* Inferior division of the oculomotor nerve (CN III)
* Abducens nerve (CN VI)
* A branch of the Inferior ophthalmic vein
Middle cranial fossa
Sphenoid bone
82
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone of the foramen rotundum?
• Maxillary branch of trigeminal nerve (CN V)
Middle cranial fossa
Sphenoid bone
83
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone of the foramen ovale?
• Mandibular branch of trigeminal nerve (CN V)
Middle cranial fossa
Sphenoid bone
84
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone foramen spinosum?
* Middle meningeal artery
* Middle meningeal vein
* Meningeal branch of CN V3
Middle cranial fossa
Sphenoid bone
85
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone of the internal acoustic meatus?
* Facial nerve (CN VII)
* Vestibulocochlear nerve (CN VIII)
* Vestibular ganglion
* Labyrinthine artery
Middle cranial fossa
Petrous part of temporal bone
86
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone of the jugular formamen?
* Glossopharyngeal nerve (CN IX)
* Vagus nerve (CN X)
* Accessory nerve (CN XI)
* Jugular bulb
* Inferior petrosal and sigmoid sinuses
Posterior cranial fossa
Anterior aspect: Petrous portion of the temporal
Posterior aspect: Occipital bone
87
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone of the hypoglossal canal?
• Hypoglossal nerve (CN XII)
Posterior cranial fossa
Occipital bone
88
What Structures are conducted, what is the Cranial Fossa, and what is the Cranial Bone of the foramen magnum?
* Vertebral arteries
* Medulla and meninges
* CN XI (spinal division)
* Dural veins
* Anterior and posterior spinal arteries
Posterior cranial fossa
Occipital bone
89
How big is the midbrain and how does it travel into the skull?
The midbrain is the smallest of the three regions of the brainstem, measuring around 2cm in length. As it ascends, the midbrain travels through the opening in the tentorium cerebelli.
90
How is the midbrain divided?
Divided into two main parts:
Tectum
Located posterior to the cerebral aqueduct
The tectum houses four rounded prominences named colliculi (collectively the corpora quadrigemina) which sit directly inferior to the pineal gland
The colliculi are separated by the cruciform sulcus; there are two superior and two inferior colliculi.
Extending laterally from each colliculi are the quadrigeminal brachium:
Superior quadrigeminal brachium forms a pathway between the superior colliculus and the retina of the eye.
Inferior quadrigeminal brachium conveys fibres from the lateral lemniscus and inferior colliculus to the medial geniculate body.
Inferior to the colliculi, the trochlear nerve (CN IV) emerges before sweeping across to the anterior surface
Paired cerebral peduncles
located anteriorly and laterally.
Internally, the cerebral peduncles are further separated by the substania nigra into the crus cerebri (anterior) and the tegmentum (posterior).
The paired cerebral peduncles extend from the cerebral hemispheres to converge as they meet the pons. They are separated anteriorly in the midline by the interpeduncular fossa, the floor of which is termed the posterior perforated substance (as many perforating blood vessels can be identified).
The oculomotor nerve (CNIII) is seen exiting from between the peduncles while the optic tract runs around the superior border of the midbrain.
The anteriolateral surface of the midbrain houses the paired crus cerebri. Four fibre tracts run within the crus:
Frontopontine fibres – located most medially.
Corticospinal fibres – motor fibres from the primary motor cortex.
Corticobulbar tracts – motor fibres from the primary motor cortex.
Temporopontine fibres – located posterolaterally.
Posteriorly is the substantia nigra – a pigmented nucleus that separates the two regions of the cerebral peduncles. It is further broken down into the pars reticulata (anterior) and pars compacta (posterior).
The tegmentum is located posterior to the substantia nigra. It is continuous with that found in the pons by the same name. It is important to note that unlike the crus cerebri, the tegmentum is continuous at the midline.
The cerebral aqueduct (see ventricles) is a midline structure surrounded by central gray matter – the periaqueductal gray matter. Within this gray matter lies the mesencephalic nucleus of the trigeminal nerve, as well as the trochlear nucleus with its fibres continuing around the gray matter to exit the midbrain. Anterior to this, the medial longitudinal fasciculus can be seen.
The decussation of the superior cerebellar peduncles can be seen centrally at this level with some reticular formation (noted throughout the brainstem) lying lateral.
Between the central gray matter and the substantia nigra are four lemnisci. Moving anterior to posterior they are the medial, spinal, trigeminal, and lateral leminisci.
At the very posterior pole, we find the tectum which, at this level, contains the inferior colliculus.
91
What are the key differences between the cross section of the midbrain at the level of the inferior colliculus and at the level of superior colliculus?
The central portion which previously was occupied by the decussation of the superior cerebellar peduncles now contains the large paired red nuclei with some decussation of the rubrospinal tract occurring anterior to this.
The reticular formation now fans around the posterior borders of the red nuclei.
The trochlear nucleus is replaced with the oculomotor nucleus while the oculomotor nerve projects anteriorly.
The medial, spinal and trigeminal lemnisci are all present in much the same location however the lateral lemnisci does not reach to this level.
The tectum now contains the superior colliculi rather than the inferior colliculi.
92
What is the blood supply of the midbrain?
The midbrain receives vascular supply from the basilar artery and its branches. The major vessels are:
Posterior cerebral artery and its peduncular branch
Superior cerebellar artery
Posterior choroidal artery
Interpeduncular branches of the basilar artery.
93
What are the anatomical borders of the pons?
Posteriorly the cerebellum, separated by the fourth ventricle
Inferiorly the medulla oblongata
Superiorly the midbrain lies immediately above the pons.
94
What is the ventral surface of the pons marked by?
The anterior or ventral surface of the pons is marked by a bulging formed by the transverse pontocerebellar fibres. These fibres wrap around the otherwise vertically oriented brainstem. It measures around 2.5 cm in adults.
The basilar groove demarcates the midline of the ventral surface and is where the basilar artery is located.
95
Which cranial nerves originate from the ventral surface of the pons?
V
Trigeminal
originates from the lateral aspect of mid pons
VI
Abducens
originates from the pontomedullary junction, close to the midline
VII
Facial
originates from the cerebellopontine angle, the more lateral aspect of the pontomedullary junction.
VIII
Vestibulocochlear
originates laterally to the facial nerve.
96
Which important cranial nerve nuclei are housed in the pons?
The main sensory nucleus and the trigeminal motor nucleus are located in the midpons – at the level where the fibres originate from the lateral aspect of the pons. The main sensory nucleus receives somatosensory information from the face. There are two other nuclei that receive sensory information from the trigeminal nerve:
• Spinal trigeminal nucleus – extends caudally towards the medulla.
• Mesencephalic nucleus – extends rostrally all the way to the midbrain.
The abducens nucleus controls the abducens nerve, which innervates the ipsilateral lateral rectus muscle. It is located in the caudal pons, on the medial aspect of its dorsal surface.
At the same level of the abducens nucleus, the facial nucleus is located more anteriorly and laterally. It controls the muscles of facial expression. Its fibres take an unusual course and loop around the abducens nucleus before exiting the brainstem through its ventrolateral surface.
The cochlear and vestibular nuclei sit dorsolaterally from the inferior pons to the superior medulla.
97
What is the blood supply of the pons?
Most of the pons is supplied by the pontine arteries, branches of the basilar artery
A smaller part of its blood supply comes from the anterior inferior cerebellar artery and the superior cerebellar artery (AICA and SCA).
The venous drainage of the pons consists of the anterior pontomesencephalic vein, which drains superiorly into the basal vein, that in turn drains into the cerebral veins. Inferiorly, the pons drains into the inferior petrosal sinus, which drains into the internal jugular veins.
98
Describe the anterior aspect of the medulla oblongata?
The medulla is conical in shape, decreasing in width as it extends inferiorly. It is approximately 3cm long and 2cm wide at its largest point.
The superior margin of the medulla is located at the junction between the medulla and pons, while the inferior margin is marked by the origin of the first pair of cervical spinal nerves. This occurs just as the medulla exits the skull through the foramen magnum.
There are several structures visible on the anterior surface of the medulla – namely the three fissures/sulci, the pyramids, the olives, and five cranial nerves.
In the midline of the medulla is the anterior median fissure, which is continuous along the length of the spinal cord. However, it is interrupted temporarily by the decussation of the pyramids (see below). As we move away from the midline, two sulci are visible – the ventrolateral sulcus and the posterolateral sulcus.
The pyramids are paired swellings found between the anterior median fissure and the ventrolateral sulcus. Information on the pyramids can be found here. The olives are another pair of swellings located laterally to the pyramids – between the ventrolateral and posterolateral sulci.
Arising from the junction between the pons and medulla is the abducens nerve (CN VI). Extending out of the ventrolateral sulcus is the hypoglossal nerve (CN XII). In the posteriolateral sulcus, three more cranial nerves join the medulla (CN IX, CN X, and CN XI).
99
Describe the posterior aspect of the medulla oblongata?
Similar to the anterior surface, the posterior surface has a midline structure – the posterior median sulcus – which is continuous below as the posterior median sulcus of the spinal cord.
Above, the sulcus ends at the point in which the fourth ventricle develops.
As we move lateral from the midline, the fasciculus gracilis and fasciculus cuneatus are seen, separated by the posterior intermediate sulcus.
100
What are the three levels for cross sections of the medulla?
Level of decussation of the pyramids
Level of decussation of the medial lemnisci
Level of the olives
101
Describe the Level of decussation of the pyramids?
This is the major decussation point of the descending motor fibres. Roughly 75% of motor fibres housed within the pyramids cross diagonally and posteriorly, and continue down the spinal column as the lateral corticospinal tracts.
At this level, the central portion of the medulla contains gray matter, while the outer portions consist of white matter.
The posterior white matter contains the fasiculus gracilis and the more lateral fasiculus cuneatus. Corresponding portions of gray matter extend to these regions and are the nucleus gracilis and nucleus cuneatus respectively.
Unchanged from the spinal cord, the spinocerebellar tracts (posterior and anterior) are located laterally, with the lateral spinothalamic tract situated between them. The large trigeminal nucleus and tracts can be found posterior to these tracts. This is a continuation of the substantia gelatinosa of the spinal cord
102
Describe the Level of decussation of medial leminisci?
This level marks the sensory decussation occurs of the medial lemniscus. (Purple lines have been used to represent the internal arcuate fibres as they run from the nucleus gracilis and nucleus cuneatus around and anterior to the central gray matter to form the medial lemniscus.
Lateral to the medial lemniscus, the trigeminal nucleus and spinal tract can once again be seen, as can the spinocerebellar tracts and the lateral spinothalamic tract. Similarly, the posterior structures are much the same at this level.
Centrally, the hypoglossal nucleus and medial longitudinal fasciculus are seen. Moving laterally, the nucleus ambiguous can be seen. Between this structure and the pyramids is the inferior olivary nucleus.
103
Describe the level of the olives?
This level shows significant change in structure both externally and internally when compared with previous levels. The central canal has now expanded into the fourth ventricle and as such makes this region the open medulla.
The large inferior olivary nucleus is responsible for the external expansion of the olives. The related medial and dorsal accessory olivary nuclei can be seen medial and posterior to this structure respectively.
The large inferior cerebellar peduncles come into view and are surrounded by multiple nuclei. The two vestibular nuclei (medial and inferior) are both found towards the midline while the two cochlear nuclei are found somewhat above and below the peduncles. Now a much smaller structure, the trigeminal tract and nucleus is seen adjacent to the peduncle.
The nucleus ambiguous remains as it was previously, while the hypoglossal nucleus has migrated with the central canal posteriorly, joined by the medial longitudinal fasciulus. An additional cranial nucleus comes into view lateral to the hypoglossal – the dorsal vagal nucleus. Moving further lateral, the nucleus of tractus solitarius comes into view.
Centrally, the medial lemniscus hugs the midline posterior to the pyramids, as does the tectospinal tract.
Between the peduncle and the olivary nuclei resides the lateral spinothalamic tract and the more lateral anterior spinocerebellar tract.
104
What is the rhomboid fossa?
diamond shaped floor of the IVth ventricle limited laterally by the Cerebellar Peduncles and posteriorly by the Gracile and Cuneate Tubercles
105
What is the median sulcus?
divides the Rhomboid Fossa into triangular left and right halves
106
What are gracile and cuneate tubercles?
Dorsal column nuclei
gracile-lower limbs
cuneate-upper limbs
107
What is the facial colliculus?
a rounded swelling caused by the fibres of CNVII in the substance of the pons curving around the nucleus of the CN VI at the level of the Superior Fovea
108
What is the medullary striae?
aberrant Ponto-Cerebellar fibres passing from the Pons to the Cerebellum. They divide the floor of the ventricle into a rostral pontine half and a caudal medullary half
109
What is the locus Coeruleus?
bluish-grey pigmented nor-adrenergic cells under the Ependyma at the Rostral half of the Sulcus Limitans (stress response)
110
What is the hypoglossal trigone?
Medial triangular area overlying the XIIth nerve nucleus
111
What is the vagal trigone?
Intermediate triangular area overlying the Xth nerve nucleus
112
What is the vestibular trigone?
Lateral triangular area overlying the VIIth nerve nucleus
113
What is the Obex?
inferior apex of the rhomboid fossa
114
What is the Area postrema?
a small tongue-shaped area immediately rostro-lateral to the Obex. It is a site commonly associated with nausea control – a chemoreceptive trigger zone for the emetic response
115
Name the 12 cranial nerves?
```
I-Olfactory
II-Optic
III-Occulomotor
IV-Trochlear
V-Trigeminal
VI-Abducens
VII-Facial
VIII-Vestibulocochlear
IX- Glossopharyngeal
X-Vagus
XI-Accessory
XII-Hypoglossal
```
116
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Olfactory Nerve?
Sensory
Olfactory Epithelium
The sensory function of the olfactory nerve is achieved via the olfactory mucosa. This mucosal layer not only senses smell, but it also detects the more advanced aspects of taste.
It is located in the roof of the nasal cavity and is composed of pseudostratified columnar epithelium which contains a number of cells:
Basal cells – form the new stem cells from which the new olfactory cells can develop.
Sustentacular cells – tall cells for structural support. These are analogous to the glial cells located in the CNS.
Olfactory receptor cells – bipolar neurons which consist of two processes:
Dendritic process projects to the surface of the epithelium, where they project a number of short cilia, the olfactory hairs, into the mucous membrane. These cilia react to odors in the air and stimulate the olfactory cells.
Central process (also known as the axon) projects in the opposite direction through the basement membrane.
Cribriform plate
117
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Optic Nerve?
Sensory
Retina
The optic nerve (CN II) is the second cranial nerve, responsible for transmitting the special sensory information for vision.
Optic Canal
118
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Occulomotor Nerve?
Motor
Parasympathetic
Motor – Innervates the majority of the extraocular muscles (levator palpebrae superioris, superior rectus, inferior rectus, medial rectus and inferior oblique).
Parasympathetic – Supplies the sphincter pupillae and the ciliary muscles of the eye.
Sympathetic – No direct function, but sympathetic fibres run with the oculomotor nerve to innervate the superior tarsal muscle (helps to raise the eyelid).
The oculomotor nerve innervates many of the extraocular muscles. These muscles move the eyeball and upper eyelid.
Superior Branch
Superior rectus – elevates the eyeball
Levator palpabrae superioris – raises the upper eyelid.
Additionally, there are sympathetic fibres that travel with the superior branch of the oculomotor nerve. They innervate the superior tarsal muscle, which acts to keep the eyelid elevated after the levator palpabrae superioris has raised it.
Inferior Branch:
Inferior rectus – depresses the eyeball
Medial rectus – adducts the eyeball
Inferior oblique – elevates, abducts and laterally rotates the eyeball
There are two structures in the eye that receive parasympathetic innervation from the oculomotor nerve:
Sphincter pupillae – constricts the pupil, reducing the amount of light entering the eye.
Ciliary muscles – contracts, causes the lens to become more spherical, and thus more adapted to short range vision.
The pre-ganglionic parasympathetic fibres travel in the inferior branch of the oculomotor nerve. Within the orbit, they branch off and synapse in the ciliary ganglion. The post-ganglionic fibres are carried to the eye via the short ciliary nerves.
Superior Orbital Fissure
119
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Trochlear Nerve?
Motor
Superior Oblique Muscle
It has a purely somatic motor function.
The trochlear nerve innervates a single muscle – the superior oblique, which is a muscle of oculomotion. As the fibres from the trochlear nucleus cross in the midbrain before they exit, the trochlear neurones innervate the contralateral superior oblique.
The tendon of the superior oblique is tethered by a fibrous structure known as the trochlea, giving the nerve its name. Although the mechanism of action of the superior oblique is complex, in clinical practice it is sufficient to understand that the overall action of the superior oblique is to depress and intort the eyeball.
Superior Orbital Fissure
120
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Trigeminal Nerve?
Both
The trigeminal nerve is associated with derivatives of the 1st pharyngeal arch.
Sensory: The three terminal branches of CN V innervate the skin, mucous membranes and sinuses of the face. Their distribution pattern is similar to the dermatome supply of spinal nerves (except there is little overlap in the supply of the divisions).
Motor: Only the mandibular branch of CN V has motor fibres. It innervates the muscles of mastication: medial pterygoid, lateral pterygoid, masseter and temporalis. The mandibular nerve also supplies other 1st pharyngeal arch derivatives: anterior belly of digastric, mylohyoid, tensor veli palatini and tensor tympani.
Parasympathetic Supply: The post-ganglionic neurones of parasympathetic ganglia travel with branches of the trigeminal nerve. (But note that CN V is NOT part of the cranial outflow of PNS supply)
V1 (Opthalmic branch)- Superior Orbital Fissure
V2 (Maxillary Branch)- Foramen Rotundum
V3 (Mandibular Branch)- Foramen Ovale
121
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Abducens Nerve?
Motor
Lateral Rectus
The abducens nerve provides innervation to the lateral rectus muscle – one of the extraocular muscles.
The lateral rectus originates from the lateral part of the common tendinous ring, and attaches to the anterolateral aspect of the sclera. It acts to abduct the eyeball (i.e. to rotate the gaze away from the midline).
Superior Orbital Fissure
122
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Facial Nerve?
Both
Parasympathetic
The facial nerve is associated with the derivatives of the second pharyngeal arch:
Motor – muscles of facial expression, posterior belly of the digastric, stylohyoid and stapedius muscles.
Sensory – a small area around the concha of the external ear.
Special Sensory – provides special taste sensation to the anterior 2/3 of the tongue via the chorda tympani
Parasympathetic – supplies many of the glands of the head and neck, including:
Submandibular and sublingual salivary glands.
Nasal, palatine and pharyngeal mucous glands.
Lacrimal glands.
Branches of the facial nerve are responsible for innervating many of the muscles of the head and neck. All these muscles are derivatives of the second pharyngeal arch.
The first motor branch arises within the facial canal; the nerve to stapedius. The nerve passes through the pyramidal eminence to supply the stapedius muscle in the middle ear.
Between the stylomastoid foramen, and the parotid gland, three more motor branches are given off:
Posterior auricular nerve – Ascends in front of the mastoid process, and innervates the intrinsic and extrinsic muscles of the outer ear. It also supplies the occipital part of the occipitofrontalis muscle.
Nerve to the posterior belly of the digastric muscle – Innervates the posterior belly of the digastric muscle (a suprahyoid muscle of the neck). It is responsible for raising the hyoid bone.
Nerve to the stylohyoid muscle – Innervates the stylohyoid muscle (a suprahyoid muscle of the neck). It is responsible for raising the hyoid bone.
Within the parotid gland, the facial nerve terminates by bifurcating into five motor branches. These innervate the muscles of facial expression:
Temporal branch – Innervates the frontalis, orbicularis oculi and corrugator supercilii
Zygomatic branch – Innervates the orbicularis oculi.
Buccal branch – Innervates the orbicularis oris, buccinator and zygomaticus muscles.
Marginal Mandibular branch – Innervates the mentalis muscle.
Cervical branch – Innervates the platysma.
The chorda tympani branch of the facial nerve is responsible for innervating the anterior 2/3 of the tongue with the special sense of taste.
The nerve arises in the facial canal, and travels across the bones of the middle ear, exiting via the petrotympanic fissure, and entering the infratemporal fossa. Here, the chorda tympani ‘hitchhikes’ with the lingual nerve. The parasympathetic fibres of the chorda tympani stay with the lingual nerve, but the main body of the nerve leaves to innervate the anterior 2/3 of the tongue.
The parasympathetic fibres of the facial nerve are carried by the greater petrosal and chorda tympani branches.
Greater Petrosal Nerve
The greater petrosal nerve arises immediately distal to the geniculate ganglion within the facial canal. It then moves in anteromedial direction, exiting the temporal bone into the middle cranial fossa. From here, its travels across (but not through) the foramen lacerum, combining with the deep petrosal nerve to form the nerve of the pterygoid canal.
The nerve of pterygoid canal then passes through the pterygoid canal (Vidian canal) to enter the pterygopalatine fossa, and synapses with the pterygopalatine ganglion. Branches from this ganglion then go on to provide parasympathetic innervation to the mucous glands of the oral cavity, nose and pharynx, and the lacrimal gland.
Chorda Tympani
The chorda tympani also carries some parasympathetic fibres. These combine with the lingual nerve (a branch of the trigeminal nerve) in the infratemporal fossa and form the submandibular ganglion. Branches from this ganglion travel to the submandibular and sublingual salivary glands.
Inner Acoustic Meatus (leaves via the stylomastoid foramen)
123
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Vestibulocochlear nerve?
Sensory
The vestibulocochlear nerve is unusual in that it primarily consists of bipolar neurones. It is responsible for the special senses of hearing (via the cochlear nerve), and balance (via the vestibular nerve).
Hearing
The cochlea detects the magnitude and frequency of sound waves. The inner hair cells of the organ of Corti activate ion channels in response to vibrations of the basilar membrane. Action potentials travel from the spiral ganglia, which house the cell bodies of neurones of the cochlear nerve.
The magnitude of the sound determines how much the membrane vibrates and thereby how often action potentials are triggered. Louder sounds cause the basilar membrane to vibrate more, resulting in action potentials being transmitted from the spiral ganglia more often, and vice versa. The frequency of the sound is coded by the position of the activated inner hair cells along the basilar membrane.
Equilibrium (Balance)
The vestibular apparatus senses changes in the position of the head in relation to gravity. The vestibular hair cells are located in the otolith organs (the utricule and saccule), where they detect linear movements of the head, as well as in the three semicircular canals, where they detect rotational movements of the head. The cell bodies of the vestibular nerve are located in the vestibular ganglion which is housed in the outer part of the internal acoustic meatus.
Information about the position of the head is used to coordinate balance and the vestibulo-ocular reflex. The vestibulo-ocular reflex (also called the oculocephalic reflex) allows images on the retina to be stabilised when the head is turning by moving the eyes in the opposite direction. It can be demonstrated by holding one finger still at a comfortable distance in front of you and twisting your head from side to side whilst staying focused on the finger.
Inner Acoustic Meatus( terminates in the petrous temporal bone)
124
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Glossopharyngeal Nerve?
Both
Parasympathetic
Sensory: Innervates the oropharynx, carotid body and sinus, posterior 1/3 of the tongue, middle ear cavity and Eustachian tube.
Special sensory: Provides taste sensation to the posterior 1/3 of the tongue.
Parasympathetic: Provides parasympathetic innervation to the parotid gland.
Motor: Innervates the stylopharyngeus muscle of the pharynx.
The glossopharyngeal nerve provides sensory innervation a variety of structures in the head and neck.
The tympanic nerve arises as the nerve traverses the jugular foramen. It penetrates the temporal bone and enters the cavity of the middle ear. Here, it forms the tympanic plexus – a network of nerves that provide sensory innervation to the middle ear, internal surface of the tympanic membrane and Eustachian tube.
At the level of the stylopharyngeus, the carotid sinus nerve arises. It descends down the neck to innervate both the carotid sinus and carotid body, which provide information about blood pressure and oxygen saturation respectively.
The glossopharyngeal nerve terminates by splitting into several sensory branches:
Pharyngeal branch – combines with fibres of the vagus nerve to form the pharyngeal plexus. It innervates the mucosa of the oropharynx.
Lingual branch – provides the posterior 1/3 of the tongue with general and taste sensation
Tonsillar branch – forms a network of nerves, known as the tonsillar plexus, which innervates the palatine tonsils.
The glossopharyngeal nerve provides taste sensation to the posterior 1/3 of the tongue, via its lingual branch (Note: not to be confused with the lingual nerve).
Motor Functions
The stylopharyngeus muscle of the pharynx is innervated by the glossopharyngeal nerve. This muscle acts to shorten and widen the pharynx and elevate the larynx during swallowing.
Parasympathetic Functions
The glossopharyngeal nerve provides parasympathetic innervation to the parotid gland. These fibres originate in the inferior salivatory nucleus of CN IX. These fibres travel with the tympanic nerve to the middle ear. From the ear, the fibres continue as the lesser petrosal nerve, before synapsing at the otic ganglion.
The fibres then hitchhike on the auriculotemporal nerve to the parotid gland, where they have a secretomotor effect.
Remember – although the facial nerve splits into its five terminal branches in the parotid gland, it is the glossopharyngeal nerve that actually supplies the gland.
Jugular Foramen
125
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Vagus Nerve?
Both
Parasympathetic
Sensory: Innervates the skin of the external acoustic meatus and the internal surfaces of the laryngopharynx and larynx. Provides visceral sensation to the heart and abdominal viscera.
Special Sensory: Provides taste sensation to the epiglottis and root of the tongue.
Motor: Provides motor innervation to the majority of the muscles of the pharynx, soft palate and larynx.
Parasympathetic: Innervates the smooth muscle of the trachea, bronchi and gastro-intestinal tract and regulates heart rhythm.
There are somatic and visceral components to the sensory function of the vagus nerve.
Somatic refers to sensation from the skin and muscles. This is provided by the auricular nerve, which innervates the skin of the posterior part of the external auditory canal and external ear.
Viscera sensation is that from the organs of the body. The vagus nerve innervates:
Laryngopharynx – via the internal laryngeal nerve.
Superior aspect of larynx (above vocal folds) – via the internal laryngeal nerve.
Heart – via cardiac branches of the vagus nerve.
Gastro-intestinal tract (up to the splenic flexure) – via the terminal branches of the vagus nerve.
The vagus nerve has a minor role in taste sensation. It carries afferent fibres from the root of the tongue and epiglottis.
(This is not to be confused with the special sensation of the glossopharyngeal nerve, which provides taste sensation for the posterior 1/3 of the tongue).
The vagus nerve innervates the majority of the muscles associated with the pharynx and larynx. These muscles are responsible for the initiation of swallowing and phonation.
Pharynx
Most of the muscles of the pharynx are innervated by the pharyngeal branches of the vagus nerve:
Superior, middle and inferior pharyngeal constrictor muscles
Palatopharyngeus
Salpingopharyngeus
An additional muscle of the pharynx, the stylopharyngeus, is innervated by the glossopharyngeal nerve.
Innervation to the intrinsic muscles of the larynx is achieved via the recurrent laryngeal nerve and external branch of the superior laryngeal nerve.
Recurrent laryngeal nerve:
Thyro-arytenoid
Posterior crico-arytenoid
Lateral crico-arytenoid
Transverse and oblique arytenoids
Vocalis
External laryngeal nerve:
Cricothyroid
Other Muscles
In addition to the pharynx and larynx, the vagus nerve also innervates the palatoglossus of the tongue, and the majority of the muscles of the soft palate.
Parasympathetic Functions
In the thorax and abdomen, the vagus nerve is the main parasympathetic outflow to the heart and gastro-intestinal organs.
The Heart
Cardiac branches arise in the thorax, conveying parasympathetic innervation to the sino-atrial and atrio-ventricular nodes of the heart (For more heart anatomy, see here).
These branches stimulate a reduction in the resting heart rate. They are constantly active, producing a rhythm of 60 – 80 beats per minute. If the vagus nerve was lesioned, the resting heart rate would be around 100 beats per minute.
Gastro-Intestinal System
The vagus nerve provides parasympathetic innervation to the majority of the abdominal organs. It sends branches to the oesophagus, stomach and most of the intestinal tract – up to the splenic flexure of the large colon.
The function of the vagus nerve is to stimulate smooth muscle contraction and glandular secretions in these organs. For example, in the stomach, the vagus nerve increases the rate of gastric emptying, and stimulates acid production.
Jugular Foramen
126
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Accessory Nerve?
Motor
The spinal accessory nerve innervates two muscles – the sternocleidomastoid and trapezius.
Sternocleidomastoid
Attachments – Runs from the mastoid process of the temporal bone to the manubrium (sternal head) and the medial third of the clavicle (clavicular head).
Actions – Lateral flexion and rotation of the neck when acting unilaterally, and extension of the neck at the atlanto-occipital joints when acting bilaterally.
Trapezius
Attachments – Runs from the base of the skull and the spinous processes of the C7-T12 vertebrae to lateral third of the clavicle and the acromion of the scapula.
Actions – It is made up of upper, middle, and lower fibres. The upper fibres of the trapezius elevate the scapula and rotate it during abduction of the arm. The middle fibres retract the scapula and the lower fibres pull the scapula inferiorly.
Jugular Foramen
127
Describe the component fibres, structures innervated, functions, and exit point from the skull of the Hypoglossal Nerve?
Motor
The hypoglossal nerve is responsible for motor innervation of the vast majority of the muscles of the tongue (except for palatoglossus). These muscles can be subdivided into two groups:
i) Extrinsic muscles
Genioglossus (makes up the bulk of the tongue)
Hyoglossus
Styloglossus
Palatoglossus (innervated by vagus nerve)
ii) Intrinsic muscles
Superior longitudinal
Inferior longitudinal
Transverse
Vertical
Together, these muscles are responsible for all movements of the tongue.
Role of the C1/C2 Roots
The C1/C2 roots that travel with the hypoglossal nerve also have a motor function. They branch off to innervate the geniohyoid (elevates the hyoid bone) and thyrohyoid (depresses the hyoid bone) muscles.
Another branch containing C1/C2 fibres descends to supply the ansa cervicalis – a loop of nerves that is part of the cervical plexus. From the ansa cervicalis, nerves arise to innervate the omohyoid, sternohyoid and sternothyroid muscles. These muscles all act to depress the hyoid bone.
Hypoglossal Canal
