Head and Neck Neuroanatomy (ANA 284) - PDF

Head and Neck and Neuroanatomy ANA 284 Introduction to Head and Neck Auza, M I (BSc, MSc) Department of Human Anatomy Faculty of Basic Medical Sciences Bingham University, Karu  Objectives Scalp Skull Cranial fossa...

Head and Neck and Neuroanatomy ANA 284 Introduction to Head and Neck Auza, M I (BSc, MSc) Department of Human Anatomy Faculty of Basic Medical Sciences Bingham University, Karu  Objectives Scalp Skull Cranial fossa The Brain Meninges Ventricular system Cerebrospinal fluid  Introduction The term scalp is applied to the Soft tissue which covers the calvaria or vault of skull.  Extent The scalp extend from the top of the forehead in front to the superior nuchal line behind. Laterally it projects down to the zygomatic arch and external acoustic meatus.  Layers of Scalp S - skin C - Cutaneous tissue A - Aponeurosis L - Loose areolar tissue P - Pericranium  S = Skin The skin of scalp is thick and hairy except over the forehead. It is firmly adherent to epicranial aponeurosis by dense connective tissue of superficial fascia, as in palms and soles. Being hairy it contains maximum number of hair follicles and associated sebaceous glands. As a result, scalp is the commonest site of sebaceous cysts. It also contains numerous sweat glands.  C = Cutaneous (Superficial/Subcutaneous) The superficial fascia of the scalp is made up of dense fibrous connective tissue that firmly binds the skin to the underlying occipito-frontalis and its aponeurosis. Fibrous septa divide this layer into numerous small pockets containing lobules of fat. The blood vessels and nerves of the scalp lie in this layer. The walls of the vessels are adherent to the fibrous network; hence when blood vessels are torn or cut during an injury, they are unable to retract and cause profuse bleeding. The bleeding, however, can be stopped by pressing against the underlying bone.  A = Aponeurosis This layer is formed by occipito-frontalis muscle and its aponeurosis. The occipito-frontalis muscle consists of four small bellies: Two frontal bellies Two occipital bellies. Since greater part of this layer is formed by aponeurosis, it is called aponeurotic layer. The aponeurosis of occipito-frontalis muscle is also called epicranial aponeurosis or galea aponeurotica (Latin: galea = helmet). The wounds of the scalp do not gape unless epicranial aponeurosis is cut transversely because the aponeurosis is under tension in the antero-posterior direction by the tone of occipito-frontalis muscle Occipito-frontalis muscle: Origin and Insertion Frontal bellies Origin: Arise from skin and subcutaneous tissue over the eyebrows and root of the nose, Insertion: Runs backwards to be inserted into epicranial aponeurosis in front of coronal suture. They have no bony attachment of their own. The front bellies are longer, wider, and partly united with each other along their medial borders. Occipito-frontalis muscle: Origin and Insertion The occipital bellies Origin: Arise from lateral two-third of the superior nuchal lines Insertion: Extend forwards to be inserted into the epicranial aponeurosis. The two occipital bellies are small and separated from each other by a considerable gap. This gap is filled by epicranial aponeurosis  Occipito-frontalis muscle: Nerve supply Innervation The occipital belly on each side is supplied by posterior auricular branch of facial nerve whereas The frontal belly is supplied by temporal branch of the facial nerve. Actions 1. Alternate contractions of frontal and occipital bellies move the scalp forwards and backwards over the vault of the skull. 2. Acting from above, the contraction of frontal bellies raise the eyebrows as in surprise. 3. Acting from below, the contractions of frontal bellies produce transverse wrinkles on the forehead as in fright  L = Loose areolar tissue As the name indicates, this layer is made of loose areolar tissue. It serves as a natural plane of cleavage during craniotomy. This layer is traversed by emissary veins connecting veins in the second layer of scalp with intracranial dural venous sinuses.  Pericranium The fifth layer of scalp is formed by the periosteum of bones of vault of skull called pericranium. It is loosely attached to the bones, but at sutures it is firmly attached to sutural membrane, which in turn attaches it to the endocranium (the periosteum covering inner aspect of the skull bones).  SCALP: NERVE SUPPLY A. Sensory supply of the scalp The scalp on each side of the midline is supplied by eight sensory nerves: (a) four in front of the auricle (b) four behind the auricle. The nerves in front of the auricle are derived from trigeminal nerve, whereas Those behind the auricle are derived from the 2nd and 3rd cervical nerves  Sensory nerve of the scalp In front of the auricle Behind the Auricle Supratrochlear, a branch of Great auricular, derived from frontal nerve from ophthalmic ventral rami of 2nd and 3rd division of trigeminal nerve cervical nerves Supraorbital, a branch of frontal Lesser occipital, derived from nerve from ophthalmic division of ventral ramus of 2nd cervical trigeminal nerve nerve Zygomaticotemporal, a branch of Greater occipital, derived from zygomatic nerve from maxillary dorsal ramus of 2nd cervical nerve division of trigeminal nerve Auriculotemporal, a branch of Third occipital, derived from mandibular division of trigeminal dorsal ramus of 3rd cervical nerve nerve  SCALP: NERVE SUPPLY B. Motor supply of the scalp The scalp on each side of the midline is supplied by two motor nerves: (a) one in front of the ear and (b) one behind the ear; Both of these nerves are derived from the facial nerve. Nerve in front of the ear is temporal branch of the facial nerve. It supplies frontal belly of occipitofrontalis muscle Nerve behind the ear is the. It supplies occipital belly of occipitofrontalis muscle.  ARTERIAL SUPPLY The scalp has very rich blood supply. On each side of midline, It is supplied by five arteries: (a) Three in front of the auricle (b) Two behind the auricle  Arteries supplying the scalp In front of the auricle Behind the Auricle Supratrochlear, a branch Posterior auricular of ophthalmic artery from artery, a branch of internal carotid artery external carotid artery Supraorbital, a branch of Occipital artery, a branch ophthalmic artery from of external carotid artery internal carotid artery Superficial temporal, a branch of external carotid artery  Clinical correlation The scalp wounds bleed profusely but heal quickly due to high vascularity. Since all the superficial arteries supplying the scalp ascend from face and neck. Therefore life-threatening scalp-hemorrhage can be stopped as a first-aid measure, by encircling the head just above the ears and eyebrows with a string or strong-strap of cotton and tied tightly  VENOUS DRAINAGE The scalp on each side of the midline is drained by five veins. The veins of the scalp accompany the arteries and have similar names. These are as follows: Supra-trochlear and supraorbital veins: They join each other at the medial angle of the eye to form the angular vein, which continues downwards as the facial vein behind the facial artery  VENOUS DRAINAGE Superficial temporal vein: It descends in front of tragus to enter the parotid gland where it joins the maxillary vein to form the retromandibular vein, which terminates by dividing into anterior and posterior divisions. The anterior division unites with the facial vein to form common facial vein, which drains into the internal jugular vein.  VENOUS DRAINAGE Posterior auricular vein: It descends behind the auricle and unites with the posterior division of the retro-mandibular vein to form the external jugular vein, which drains into the subclavian vein. Occipital vein: It terminates in the suboccipital venous plexus. NB: The veins of the scalp communicate with intracranial dural venous sinuses through emissary veins.  Clinical Correlation In infants, the veins of the scalp are easily seen deep to the skin, hence they are the favored sites for intravenous infusion.  Emissary Veins The veins connecting the veins outside the cranium with the intracranial dural venous sinuses by passing through foramina in the cranium are called emissary veins. Emissary Veins in the Region of the Scalp On each side of the midline in the region of the scalp two sets of emissary veins are encountered, viz. 1. Parietal emissary vein, which passes through parietal foramen and communicates with the superior sagittal sinus. 2. Mastoid emissary vein, which passes through mastoid foramen and communicates with the sigmoid sinus.  LYMPHATIC DRAINAGE The lymphatics from anterior part of the scalp except the lower half of the forehead drain into preauricular (or superficial parotid) lymph nodes situated on the surface of the parotid gland. The lymphatics from posterior part of the scalp drain into posterior auricular (mastoid) and occipital lymph nodes  The skull The skull is a bony structure that supports the face and forms a protective cavity for the brain. It is comprised of many bones, which are formed by intramembranous ossification, and joined by sutures (fibrous joints). The bones of the skull can be considered as two groups: (A) Those of the cranium (B) Those of the face.  The cranium The cranium (also known as the neurocranium) is formed by the superior aspect of the skull. It encloses and protects the brain, meninges, and cerebral vasculature. Anatomically, the cranium can be subdivided into a roof and a base: Cranial roof – comprised of the frontal, occipital and two parietal bones. It is also known as the calvarium. Cranial base – comprised of the frontal, sphenoid, ethmoid, occipital, parietal, and temporal bones. These cranial base bones articulate with the 1st cervical vertebra (atlas), the facial bones, and the mandible (jaw).  Sutures of the Skull Sutures are a type of fibrous joint that are unique to the skull. They are immovable and fuse completely around the age of 20. These joints are important in the context of trauma, as they represent points of potential weakness in the skull. The main sutures in the adult skull are: Coronal suture – fuses the frontal bone with the two parietal bones. Sagittal suture – fuses both parietal bones to each other. Lambdoid suture – fuses the occipital bone to the two parietal bones.  Sutures of the Skull In neonates, the incompletely fused suture joints give rise to membranous gaps between the bones, known as fontanelles. The two major fontanelles are: Frontal fontanelle – located at the junction of the coronal and sagittal sutures Occipital fontanelle – located at the junction of the sagittal and lambdoid sutures  Cranial Fossa The floor of the cranial cavity is divided into three distinct depressions. They are known as the Anterior cranial fossa Middle cranial fossa and Posterior cranial fossa. Each fossa accommodates a different part of the brain.  Anterior cranial fossa The anterior cranial fossa is the most shallow and superior of the three cranial fossae. It lies superiorly over the nasal and orbital cavities. The fossa accommodates the anteroinferior portions of the frontal lobes of the brain.  Anterior Cranial Fossa: Borders The anterior cranial fossa consists of three bones: the frontal bone, eth moid bone and sphenoid bone.  Anterior Cranial Fossa: Borders It is bounded as follows: Anteriorly and laterally it is bounded by the inner surface of the frontal bone. Posteriorly and medially it is bounded by the limbus of the sphenoid bone. The limbus is a bony ridge that forms the anterior border of the prechiasmatic sulcus (a groove running between the right and left optic canals). Posteriorly and laterally it is bounded by the lesser wings of the sphenoid bone (these are two triangular projections of bone that arise from the central sphenoid body). The floor consists of the frontal bone, ethmoid bone and the anterior aspects of the body and lesser wings of the sphenoid bone  Anterior Cranial Fossa: Contents There are several bony landmarks present in the anterior cranial fossa. The frontal bone is marked in the midline by a body ridge, known as the frontal crest. It projects upwards, and acts as a site of attachment for the falx cerebri (a sheet of dura mater that divides the two cerebral hemispheres). In the midline of the ethmoid bone, the crista galli. This is an upwards projection of bone, which acts as another point of attachment for the falx cerebri. On either side of the crista galli is the cribriform plate which supports the olfactory bulb and has numerous foramina that transmit vessels and nerves.  Anterior Cranial Fossa: Contents The anterior aspect of the sphenoid bone lies within the anterior cranial fossa. From the central body, the lesser wings arise. The rounded ends of the lesser wings are known as the anterior clinoid processes. They serve as a place of attachment for the tentorium cerebelli (a sheet of dura mater that divides the cerebrum from the cerebellum). The lesser wings of the sphenoid bone also separate the anterior and middle cranial fossae.  Anterior Cranial Fossa: Foramina The ethmoid bone in particular contains the main foramina (openings that transmit vessels and nerves) of the anterior cranial fossa. The cribriform plate is a sheet of bone seen either side of the crista galli which contains numerous small foramina – these transmit olfactory nerve fibres (CN I) into the nasal cavity. It also contains two larger foramen: Anterior ethmoidal foramen – transmits the anterior ethmoidal artery, nerve and vein. Posterior ethmoidal foramen – transmits the posterior ethmoidal artery, nerve and vein.  Clinical Relevance: Fracture of the Cribriform Plate The cribriform plate of the ethmoid is the thinnest part of the anterior cranial fossa, and therefore most likely to fracture. There are two major consequences of cribriform plate fracture: Anosmia – the olfactory nerve fibres run through the cribriform plate, and can be ‘sheared’, resulting in loss of sense of smell. CSF rhinorrhoea – the fragments of bone can tear the meningeal coverings of the brain, causing the leakage of cerebrospinal fluid into the nasal cavity. This is visible as a clear fluid.  Middle cranial fossa The middle cranial fossa is located, as its name suggests, centrally in the cranial floor. It is said to be “butterfly shaped”, With a middle part accommodating the pituitary gland and two lateral parts accommodating the temporal lobes of the brain  Middle Cranial Fossa: Borders The anterior cranial fossa consists of three bones: two temporal bone and sphenoid bone.  Middle Cranial Fossa: Borders Anteriorly and laterally it is bounded by the lesser wings of the sphenoid bone. These are two triangular projections of bone that arise from the central sphenoid body. Anteriorly and medially it is bounded by the limbus of the sphenoid bone. The limbus is a bony ridge that forms the anterior border of the chiasmatic sulcus (a groove running between the right and left optic canals). Posteriorly and laterally it is bounded by the superior border of the petrous part of the temporal bone. Posteriorly and medially it is bounded by the dorsum sellae of the sphenoid bone. This is a large superior projection of bone that arises from the sphenoidal body. The floor is formed by the body and greater wing of the sphenoid, and the squamous and petrous parts of the temporal bone.  Contents The middle cranial fossa consists of 1. Central portion, which contains the pituitary gland, and 2. Two lateral portions, which accommodate the temporal lobes of the brain.  Central part of the middle cranial fossa The central part of the middle cranial fossa is formed by the body of the sphenoid bone. It contains the sella turcica, which is a saddle-shaped bony prominence. It acts to hold and support the pituitary gland, and consists of three parts: Tuberculum sellae Hypophysial fossa or pituitary fossa Dorsum sellae  Lateral Parts of the middle cranial fossa The depressed lateral parts of the middle cranial fossa are formed by the greater wings of the sphenoid bone, and the squamous and petrous parts of the temporal bones. They support the temporal lobes of the brain. It is the site of many foramina – small holes by which vessels and nerves enter and leave the cranial cavity.  Middle cranial fossa: Foramina There are many foramina that transmit vessels and nerves into and out of the middle cranial fossa. Foramina of the Sphenoid Bone The optic canals are situated anteriorly in the middle cranial fossa. They transmit the optic nerves (CN II) and ophthalmic arteries into the orbital cavities. Immediately lateral to the central part of the middle cranial fossa are four foramina:  Middle cranial fossa: Foramina Superior orbital fissure opens anteriorly into the orbit. It transmits the oculomotor nerve (CN III), trochlear nerve (CN IV), ophthalmic branch of the trigeminal nerve (CN V1), abducens nerve (CN VI), opthalmic veins and sympathetic fibres. Foramen rotundum opens into the pterygopalatine fossa and transmits the maxillary branch of the trigeminal nerve (CN V2). Foramen ovale opens into the infratemporal fossa, transmitting the mandibular branch of the trigeminal nerve (CN V3) and accessory meningeal artery. Foramen spinosum also opens into the infratemporal fossa. It transmits the middle meningeal artery, middle meningeal vein and a meningeal branch of CN V3.  Middle cranial fossa: Foramina Foramina of the Temporal Bone The temporal bone is marked by three major foramina: Hiatus of the greater petrosal nerve – transmits the greater petrosal nerve (a branch of the facial nerve), and the petrosal branch of the middle meningeal artery. Hiatus of the lesser petrosal nerve – transmits the lesser petrosal nerve (a branch of the glossopharyngeal nerve). Carotid canal – located posteriorly and medially to the foramen ovale. This is traversed by the internal carotid artery, which ascends into the cranium to supply the brain with blood. The deep petrosal nerve also passes through this canal. At the junction of the sphenoid, temporal and occipital bones is the foramen lacerum. In life, this foramen is filled with cartilage, which is pierced only by small blood vessels.  Posterior cranial fossa The posterior cranial fossa is the most posterior and deep of the three cranial fossae. It accommodates the brainstem and cerebellum.  Borders The posterior cranial fossa is comprised of three bones: the occipital bone and the two temporal bones. It is bounded as follows: Anteromedial – dorsum sellae of the sphenoid bone (large projection of bone superiorly that arises from the body of the sphenoid). Anterolateral – superior border of the petrous part of the temporal bone. Posterior – internal surface of the squamous part of the occipital bone. Floor – mastoid part of the temporal bone and the squamous, condylar and basilar parts of the occipital bone.  Posterior Cranial Fossa: Contents The posterior cranial fossa houses the brainstem and cerebellum. The brainstem is comprised of the medulla oblogata, pons and midbrain and continues down through the foramen magnum to become the spinal cord. The cerebellum has an important role in co-ordination and fine motor control  Posterior cranial Fossa: Foramina Foramina present in the posterior cranial fossa allows the passage of blood vessel or nerve. Temporal Bone The internal acoustic meatus is an oval opening in the posterior aspect of the petrous part of the temporal bone. It transmits the facial nerve (CN VII), vestibulocochlear nerve (CN VIII) and labyrinthine artery.  Posterior cranial Fossa: Foramina Occipital Bone A large opening, the foramen magnum, lies centrally in the floor of the posterior cranial fossa. It is the largest foramen in the skull. It transmits the medulla of the brain, meninges, vertebral arteries, spinal accessory nerve (ascending), dural veins and anterior and posterior spinal arteries. Anteriorly an incline, known as the clivus, connects the foramen magnum with the dorsum sellae.  Posterior cranial Fossa: Foramina The jugular foramina are situated either side of the foramen magnum. Each transmits the glossopharyngeal nerve, vagus nerve, spinal accessory nerve (descending), internal jugular vein, inferior petrosal sinus, sigmoid sinus and meningeal branches of the ascending pharyngeal and occipital arteries. Immediately superior to the anterolateral margin of the foramen magnum is the hypoglossal canal. It transmits the hypoglossal nerve through the occipital bone. Posterolaterally to the foramen magnum lies the cerebellar fossae. These are bilateral depressions that house the cerebellum. They are divided medially by a ridge of bone, the internal occipital crest.  The brain The brain consists of the cerebrum, diencephalon, midbrain, pons, cerebellum and medulla oblongata. The midbrain, pons, and medulla oblongata together form the brainstem. The medulla oblongata is continuous below with the spinal cord  MENINGES The brain and the spinal cord are covered by three layers of meninges; the outermost toughest layer is known as dura mater, the next layer is known as arachnoid mater and the innermost thin layer is known as pia mater.  MENINGES: Dura Mater The Dura mater, derived from mesoderm, is made up of dense fibrous tissue and hence it is also known as pachymeninx. The dura mater is the outermost layer of the meninges and is located directly underneath the bones of the skull and vertebral column. It is thick, tough, and inextensible. The dura mater consists of two layered sheets of connective tissue: Periosteal layer – lines the inner surface of the bones of the cranium. Meningeal layer – located deep to the periosteal layer. It is continuous with the dura mater of the spinal cord.  MENINGES: Dura Mater The dural venous sinuses are located between the two periosteal and meningeal layers of dura mater. The dural venous sinueses are responsible for the venous drainage of the cranium and empty into the internal jugular veins. The dura mater receives its own vascular supply – primarily from the middle meningeal artery and vein. It is innervated by the trigeminal nerve (V1, V2 and V3). MENINGES: Arachnoid Mater The Arachnoid mater and the Pia mater are derived from neural crest cells and are quite thin. Hence, these two inner layers are known as leptomeninges. The arachnoid mater is the middle layer of the meninges, lying directly underneath the dura mater. It consists of layers of connective tissue, is avascular, and does not receive any innervation. Underneath the arachnoid is a space known as the sub-arachnoid space. It contains cerebrospinal fluid, which acts to cushion the brain. Small projections of arachnoid mater into the dura (known as arachnoid granulations) allow CSF to re-enter the circulation via the dural venous sinuses. An arachnoid granulation is visible in the centre  MENINGES: Pia mater The pia mater is located underneath the sub-arachnoid space. It is very thin, and tightly adhered to the surface of the brain and spinal cord. It is the only covering to follow the contours of the brain (the gyri and fissures). Like the dura mater, it is highly vascularised, with blood vessels perforating through the membrane to supply the underlying neural tissue.  MENINGES: Spaces The space between the dura mater and the endosteum of the bone covering it is known as epidural or extradural space. The space between the dura mater and the arachnoid mater is known as the subdural space. The space between the arachnoid mater and the pia mater is known as the subarachnoid space and the space between the pia mater and the brain is known as the subpial space  MENINGES: Functions These coverings have two major functions: Provide a supportive framework for the cerebral and cranial vasculature. Acting with cerebrospinal fluid to protect the CNS from mechanical damage. The meninges are often involved cerebral pathology, as a common site of infection (meningitis), and intracranial bleeds.  Clinical Relevance: Meningitis Meningitis refers to inflammation of the meninges. It is usually caused by pathogens, but can be drug induced. Bacteria are the most common infective cause. The most common organisms are Neisseria meningitidis and Streptococcus pneumoniae. The immune response to the infection causes cerebral oedema, consequently raising intra-cranial pressure. This has two main effects: Part of the brain can be forced out of the cranial cavity – this is known as cranial herniation. In combination with systemic hypotension, raised intracranial pressure reduces cerebral perfusion. Both complications can rapidly result in death.  Ventricular system The ventricular system is a set of communicating cavities within the brain These structures are responsible for the production, transport and removal of cerebrospinal fluid, which bathes the central nervous system The ventricles are lined by ependymal cells, which form a structure called the choroid plexus. It is within the choroid plexus that CSF is produced.  Functions of Cerebrospinal Fluid Cerebrospinal fluid is an ultrafiltrate of plasma that surrounds the brain and spinal cord. It serves three main functions: Protection – acts as a cushion for the brain, limiting neural damage in cranial injuries. Buoyancy – by being immersed in CSF, the net weight of the brain is reduced to approximately 25 grams. This prevents excessive pressure on the base of the brain. Chemical stability – the CSF creates an environment to allow for proper functioning of the brain, e.g. maintaining low extracellular K+ for synaptic transmission.  Ventricles of the Brain Embryologically, the ventricular system is derived from the lumen of the neural tube. In total, there are four ventricles; Right and left lateral ventricles, Third ventricle and Fourth ventricle.  Lateral Ventricles The left and right lateral ventricles are located within their respective hemispheres of the cerebrum. They have ‘horns’ which project into the frontal, occipital and temporal lobes. The volume of the lateral ventricles increases with age.  Third Ventricle The lateral ventricles are connected to the third ventricle by the foramen of Monro. The third ventricle is situated in between the right and the left thalamus. The anterior surface of the ventricle contains two protrusions: Supra-optic recess – located above the optic chiasm. Infundibular recess – located above the optic stalk.  Fourth Ventricle The fourth ventricle is the last in the system – it receives CSF from the third ventricle via the cerebral aqueduct. It lies within the brainstem, at the junction between the pons and medulla oblongata. From the 4th ventricle, the fluid drains into two places: Central spinal canal – bathes the spinal cord Subarachnoid cisterns – bathes the brain, between arachnoid mater and pia mater. Here the CSF is reabsorbed back into the circulation.  Production and Reabsorption of Cerebrospinal Fluid Cerebrospinal fluid is produced by the choroid plexus, located in the lining of the ventricles. It consists of capillaries and loose connective tissue, surrounded by cuboidal epithelial cells. Plasma is filtered from the blood by the epithelial cells to produce CSF. In this way, the exact chemical composition of the fluid can be controlled. Drainage of the CSF occurs in the subarachnoid cisterns (or space). Small projections of arachnoid mater (arachnoid granulations) protrude into the dura mater. They allow the fluid to drain into the dural venous sinuses.