Anatomy Skull and Spinal Cord

Transcription

1 1 Anatomy Skull and Spinal Cord 1. Skull The skull is anterior to the spinal column and is the bony structure that encases the brain. Its purpose is to protect the brain and allow attachments for the facial muscles. The two regions of the skull are the cranial and facial region. The cranial portion is the part of the skull that directly houses the brain and the facial portion includes the rest of the bones of the skull. The skull is supported on the summit of the vertebral column, and is of an oval shape, wider behind than in front. It is composed of a series of flattened or irregular bones which, with one exception (the mandible), are immovably jointed together. It is divisible into two parts: (1) the cranium, which lodges and protects the brain, consists of eight bones, and (2) the skeleton of the face, of fourteen, as follows: Skull, 22 bones Cranium, 8 bones - Occipital. - Two Parietals. - Frontal. - Two Temporals - Sphenoidal - Ethmoida Face, 14 bones - Two Nasals - Two Maxillæ - Two Lacrimals - Two Zygomatics - Two Palatines. - Two Inferior Nasal Conchæ - Vomer - Mandible

2 2 SKULL 1. Frontal Bone 2. Supra-Orbital Foramen 3. Orbit (Orbital Cavity) 4. Superior Orbital Fissure 5. Inferior Orbital Fissure 6. Zygomatic Bone 7. Infra-Orbital Foramen 8. Maxilla 9. Mandible 10. Mental Foramen 11. Incisive Fossa 12. Symphysis 13. Vomer 14. Inferior Nasal Concha 15. Middle Nasal Concha 16. Perpendicular Plate of Ethmoid 17. Nasal Bone 18. Lacrimal Bone

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6 6 1. Parietal Bone 2. Temporal Bone 3. Occipital Bone 4. Mandible Maxilla 5. Frontal Bone Sphenoid 6. Bone zygomatic bone A. Frontal bone During development there are two frontal bones which fuse after birth. This bone forms the ridges above the orbits and the roof of the orbit. Branches of the ophthalmic division of the trigeminal nerve emerge from the orbit to innervate the skin of the forehead (supra-orbital and supra-trochlear nerves). The supra-orbital nerve lies in the supra-orbital notch of the frontal bone. B. Parietal bone

7 7 The paired parietal bones form the top and sides of the skull. The suture between them is the sagittal suture. The suture between the pair of parietal bones and the frontal bone is the coronal suture. On the outside the bones carry large muscles which elevate the mandible during chewing (temporalis muscles). On the inside, running along the sagittal suture, lies the superior sagittal sinus, a major route of venous drainage from the brain. At birth there is a soft spot where the sagittal suture meets the coronal suture, the anterior fontanelle. In the adult this fills in and the point is then termed the bregma.

8 8 C. Temporal bone The temporal bone has a vertical squamous part forming part of the side of the skull, and a horizontal base, the petrous temporal bone. On the exterior the temporal bone has an opening which leads into the middle ear, the external auditory meatus. The ear is attached at this point. A process runs forward forming part of the cheek bone, the zygomatic process of the temporal bone. Projecting inferiorly from behind the external auditory meatus is the mastoid process. The mastoid process is a spongy bone containing air cells. The drawing out of the mastoid process is related to the insertion of the sternocleidomastoid muscle, responsible for rotation of the head to the opposite side. Within the substance of the petrous temporal bone lie the components of the inner ear, the cochlear and vestibular systems. On the inside of the squamous part run branches of the middle meningeal artery which may be ruptured in bone fractures. The sigmoid sinus, a continuation of the transverse sinus lies in a groove on the inside of the bone. The head of the mandible articulates with the temporal bone forming the temporomandibular joint (TMJ). Anterior to the joint is the articular tubercle. At the internal auditory meatus the vestibulo-cochlear (VIII) and facial nerves (VII) enter the bone.

9 9 On the inferior surface of the bone between the mastoid and styloid processes is the stylomastoid foramen through which the facial nerve leaves the temporal bone. The temporal bone is commonly fractured, leading to complications. The facial nerve enters the temporal bone at the internal auditory meatus. A common site of injury is close to the geniculate ganglion. The integrity of the greater superficial petrosal nerve and chorda tympani should be tested. The integrity of the motor component of the facial nerve can be tested by testing the muscles of facial expression. Hearing loss is common in fractures of the temporal bone since the auditory mechanism lies entirely within the temporal bone. Vertigo may follow damage to the vestibular apparatus in the temporal. D. Occipital bone This bone forms part of the base and the back of the skull. A fall on the back of the head might cause a fracture of this bone. The basal part of the bone forms a ring (the foramen magnum) through which passes the spinal cord. On the inside surface of the occipital bone, venous drainage from the brain runs in the transverse sinuses. The hypoglossal nerve leaves the skull through the hypoglossal canal.

10 10 E. The sphenoid The sphenoid lies in the base of the skull forming part of the floor of the middle cranial fossa. On its upper surface the bone carries a chamber for the pituitary gland. Anterior to the pituitary fossa the bone is hollowed out as the sphenoidal air sinus. Foramina in the bone transmit nerves and vessels. The superior orbital fissure transmits the ophthalmic division of the trigeminal nerve, the abducent, trochlear and oculomotor nerves. The foramen rotundum transmits the maxillary division of the trigeminal nerve. The pterygoid canal transmits the nerve of the pterygoid canal (sympathetic fibres plus the greater superficial petrosal branch of the facial nerve). The foramen ovale transmits the mandibular division of the trigeminal nerve. The optic foramen transmits the optic nerve and ophthalmic artery. The ophthalmic veins drain back to the cavernous sinus which lies on either side of the sella turcica. The cavernous sinus drains through the superior petrosal sinus to the transverse sinus and through the inferior petrosal sinus to the jugular vein. The internal carotid artery lies for part of its course in the cavernous sinus. Running along the dura of the cavernous sinus are the ophthalmic, oculomotor and trochlear nerves. The abducent nerve runs through the sinus. Although the sphenoid lays centrally it is still prone to fracture. The many neurovascular structures that traverse the sphenoid bone are vulnerable to injury.

11 11 Sphenoidal fractures are frequently associated with vascular injuries such as traumatic pseudo-aneurysm of the internal carotid artery, carotid-cavernous sinus fistula, bleeding from the middle meningeal artery, and nerve palsies due to damage to the optic, trigeminal, oculomotor, abducent and trochlear nerves. F. The maxilla The maxilla forms the upper jaw and part of the cheek bone. It carries the upper set of teeth. The bone forms the medial floor of the orbit and part of the lateral wall and floor of the nasal cavity. The maxillary sinus is a chamber within the bone lined by a mucus producing epithelium. The drain of the maxillary sinus in the adult is above the level of the floor. Cilia in the epithelium move the mucus up and out of the ostium into the hiatus semilunaris below the middle concha. The opening can become blocked resulting in painful sinusitis. The opening can be cleared or another opening created closer to the floor of the maxillary sinus through the lateral wall of the nose below the inferior concha. Several nerves pass through the maxilla, including the infraorbital and superior alveolar nerves. Facial trauma may result in maxillary fractures. The maxillae are hollowed out as the maxillary sinuses. The bone also transmits the maxillary artery and branches of the maxillary nerve.

12 12 G. The mandible The mandible articulates through its head with the articular process of the temporal bone. The bone is flattened as the ramus and changes direction at the angle. On its inside surface the mandible has a foramen for the inferior alveolar nerve and vessels. The mandibular foramen is partly covered over by a spike of bone, the lingula. As the inferior alveolar nerve enters the mandibular foramen it gives off a motor nerve to mylohyoid and the anterior belly of digastric. This nerve lies in a shallow groove in the bone. Anteriorly on the external surface lies the mental foramen. The terminal branches of the inferior alveolar nerve emerge to supply the skin as the mental nerve. The mandible can be dislocated or fractured. The mandible is most commonly dislocated in an anterior direction, with the head of the mandible passing over the articular tubercle. The mandible may be fractured anterior to the masseter muscle. Fracture will result in injury to the inferior alveolar nerve and vessels. Normal action of the mandible in chewing is produced by the muscles of mastication. The temporalis muscle fills the temporal fossa on the side of the skull and inserts onto the coronoid process and anterior edge of the ramus of the mandible. It elevates and retracts the mandible. The masseter muscle arises from the inside of the zygomatic arch and inserts into the lower edge of the body of the mandible at the angle. It elevates the jaw. The medial pterygoid muscle arises from the medial side of the lateral pterygoid plate to insert on the medial side of the angle of the mandible, forming a sling together with masseter.

13 13 The lateral pterygoid muscle arises from the lateral side of the pterygoid plate and neighbouring maxilla to insert into the capsule and disc of the TMJ and the neck of the mandible. It protracts the jaw, and unilaterally moves the jaw from side to side. The mandibular division of the trigeminal nerve supplies all these muscles. H. The zygomatic bone The zygomatic bone forms the lateral part of the orbit and part of the zygomatic arch.

14 14 2. The Vertebrae The vertebral column or spine is made up of a series of bones called vertebrae. The spine is a strong and flexible column of bones made up of the cervical, thoracic, lumbar, sacral, and coccygeal regions. The cervical or neck region is normally made up of 7 bones, the thoracic or chest region has 12 bones, and the lumbar or low back region is made up of 5 bones. The sacral region consists of 5 fused bones, and the coccygeal region 3 to 5 tiny bones. A. Purpose of the Vertebrae Although vertebrae range in size; cervical the smallest, lumbar the largest, vertebral bodies are the weight bearing structures of the spinal column. Upper body weight is distributed through the spine to the sacrum and pelvis. The natural curves in the spine, kyphotic and lordotic, provide resistance and elasticity in distributing body weight and axial loads sustained during movement. The vertebrae are composed of many elements that are critical to the overall function of the spine, which include the intervertebral discs and facet joints.

15 15 B. The Cervical Column The cervical spine is further divided into two parts; the upper cervical region (C1 and C2), and the lower cervical region (C3 through C7). C1 is termed the Atlas and C2 the Axis. The Occiput (CO), also known as the Occipital Bone, is a flat bone that forms the back of the head. 1. Atlas (C1) The Atlas is the first cervical vertebra and therefore abbreviated C1. This vertebra supports the skull. Its appearance is different from the other spinal vertebrae. The atlas is a ring of bone made up of two lateral masses joined at the front and back by the anterior arch and the posterior arch.

16 16 2. Axis (C2) The Axis is the second cervical vertebra or C2. It is a blunt tooth like process that projects upward. It is also referred to as the dens (Latin for tooth ) or odontoid process. The dens provides a type of pivot and collar allowing the head and atlas to rotate around the dens. There are seven vertebrae forming the cervical vertebral column. The first of these, the atlas, forms a ring since the body is absent. It articulates above by a pair of articular facets, with the occipital bone. Nodding movements are possible at these joints. The transverse processes transmit the vertebral arteries which run around the posterior arch to enter the skull through the foramen magnum. The atlas articulates inferiorly with the axis. The dense of the axis projects upwards to articulate with the anterior arch of the atlas. Movements of rotation are possible at the atlanto-axial joint.

17 17 The other cervical vertebrae are more alike in that they all show a small body and perforated transverse processes. They articulate with each other through the intervertebral discs and through facets on the articular processes. The facets lie in the coronal plane, permitting lateral bending and flexion/extension. The spinous processes of the cervical vertebrae are bifid, C7 being easily palpable.

18 18 C. The Spinal column or the Vertebral column The spinal column (or vertebral column) extends from the skull to the pelvis and is made up of 33 individual bones termed vertebrae. The vertebrae are stacked on top of each other group into four regions: 1. Thoracic Vertebrae (T1 T12) The thoracic vertebrae increase in size from T1 through T12. They are characterized by small pedicles, long spinous processes, and relatively large intervertebral foramen (neural passageways), which result in less incidence of nerve compression. The rib cage is joined to the thoracic vertebrae. At T11 and T12, the ribs do not attach and are so are called "floating ribs." The thoracic spine's range of motion is limited due to the many rib/vertebrae connections and the long spinous processes.

19 19 1 Vertebral Body 2 Spinous Process 3 Transverse Process 4 Pedicle 5 Foramin 6 Articular Process 2. Lumbar Vertebrae (L1 L5) The lumbar vertebrae graduate in size from L1 through L5. These vertebrae bear much of the body's weight and related biomechanical stress. The pedicles are longer and wider than those in the thoracic spine. The spinous processes are horizontal and more squared in shape. The intervertebral foramen (neural passageways) are relatively large but nerve root compression is more common than in the thoracic spine.

20 20 Functions of the Vertebral or Spinal Column Include Protection Base for Attachment Structural Support Flexibility and Mobility Other Spinal Cord and Nerve Roots Many internal organs Ligaments Tendons Muscles Head, shoulders, chest Connects upper and lower body Balance and weight distribution Flexion (forward bending) Extension (backward bending) Side bending (left and right) Rotation (left and right) Combination of above Bones produce red blood cells Mineral storage The spine has several special roles in the human body. It protects the spinal cord (which connects nerves to the brain); provides the support needed to walk upright; enables the torso to bend; and supports the head. Viewed from the side, the spine has a natural "S" curve.

21 21 3. There are three main parts to a vertebra: Vertebral Body (weight bearing structure) Vertebral Arch (protective in function) Vertebral Processes A. Vertebral Body The vertebral body is a thick, disc shaped cylindrical block of bone flattened at the back and with roughened top and bottom surfaces. It is made up of spongy bone on the inside, which enables it to resist compression, and a thin outer covering of compact bone. It is the weight bearing part of a vertebrae. VERTEBRA Side View View From Above B. Vertebral arch The vertebral arch, or neural arch as it is sometimes called, extends backwards from the vertebral body. It is made up of two short, thick processes called pedicles that stick out backwards from the vertebral body and join the laminae. The laminae are flat and join together to form the back part of the vertebral arch. Together the vertebral arch and vertebral body surround the spinal column. The space occupied by the spinal column is called the vertebral foramen. The vertebral foramen, when stacked on top of each other, forms the vertebral or spinal canal. On each side of the vertebral column there is an opening between each vertebra called the intervertebral foramen which enables the spinal nerves to pass through.

22 22 VERTEBRA View From Above Side View C. Vertebral processes There are seven different processes that come from the vertebral arch. A transverse process extends sideways on each side from the junction of a lamina and pedicle. A single spinous process extends back and downwards from the junction of the laminae. These three processes have spinal muscles attached to them. The other four articular processes form joints with other vertebrae. The two articular processes on top form a joint with the two articular processes on the bottom of the vertebrae above, and the two bottom processes form a joint with the two processes on top of the vertebrae below. Vertebral discs Adjacent vertebral bodies are attached to each other by an intervertebral disc. The disc is made up of a central mass of pulpy tissue called the nucleus pulposus and a tough outer covering of fibrocartilage called the annulous fibrosis. The layers of the annulus are thinner at the back, making this area potentially more prone to damage.

23 23 Intervertebral disc Front View View from Above Side View The discs are shock absorbers, giving resilience to the spinal column as well as flexibility. When a body is erect, the various parts of the disc are under uniform pressure; but when the spine is flexed, extended or bent to the side, one part of the disc is under increased compression whereas another part is under tension.

24 24 Vertebral Discs under Various Loads Spinal cord & spinal nerves The spinal cord is cylindrical in shape and starts as an extension of the lower part of the brain stem and finishes at around the second lumbar vertebrae. The cord is made up of some 31 segments that each give rise to a pair of spinal nerves. The spinal cord is protected by the spinal column, the spinal meninges (dense fibrous covering), the cerebrospinal fluid (special fluid surrounding the brain and spine), and the vertebral ligaments. The main function of the spinal cord is to convey motor impulses from the brain to the muscles, and sensory information from the body to the brain. There are 31 pairs of spinal nerves named and numbered according to the level and region of the spine from which they emerge. There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar and sacral nerves, and 1 pair of coccygeal nerves. With the exception of the first pair of cervical nerves, all spinal nerves leave the vertebral column from the intervertebral foramen.

25 25 SPINAL CORD AND NERVES - BACK VIEW

26 26 4. Sacrum and Coccyx Sacral/Sacrum There are five vertebrae that join together to form the sacrum, a wedgeshaped part of the spine that rests at the top of the pelvis. In the female the sacrum is shorter and wider than in the male. Coccyx The coccyx is often referred to as the tailbone. It consists of four vertebrae. Lateral surfaces of sacrum and coccyx.

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