Human Anatomy Lesson 16
Brain, Meninges, Vasculature
The brain sits in the cranial cavity, resting on top of the cranial base and surrounded on all sides by the cranial vault. LIke the spinal cord, the brain is made up of gray matter and white matter, but in the brain the pattern is reversed such that an outer shell of gray matter surrounds an inner core of white matter. The brain is surrounded by the same meningeal layers that surround the spinal cord - dura mater, arachnoid mater, and pia mater. The dura mater attaches to the internal surface of the cranial cavity and forms folds and inflections that create channels and folds within the brain, or between the surface of the brain and bone of the endocranium. Venous drainage of the brain and cranial cavity occurs through some of these channels. Arterial supply of the brain is via branches of the subclavian and internal carotid arteries, and blood supply to the dura is via branches of the external carotid arteries.
A classic neuroanatomy text. You will need this book if you go on to pursue any medical field dealing with the brain and how the brain works.
Learning Objectives - By the end of this lesson, you should be able to ...
- List and describe the meninges surrounding the brain, and understand how these tissues relate to the meninges covering the spinal cord
- Understand the relationship between embryological divisions of the brain and lobes/regions in the adult
- List and describe the dural partitions, and understand how these partitions are related to dural venous drainage in the head
- List and describe the sinuses that drain blood from the brain, and understand how these sinuses are related to folds and partitions of dura mater
- List and describe the arteries that supply blood to the brain, dura, and cranial cavity
The meninges covering the brain are the same that cover the spinal cord, so you should already know this information from a previous lesson. Pia mater invests the brain directly, dura mater is attached to the endocranial surface of the cranial cavity, and arachnoid mater is between the two with the subarachnoid space between it and pia mater.
- Dura mater: tough fibrous layer that attaches to the endocranial side of cranial bone in the cranial cavity. Dura in the cranial cavity is made up of an outer periosteal layer and an inner meningeal layer, with a potential space in between. These two layers separate in several places to form dural partitions (see "Dural Partitions" below) and channels called intracranial dural sinuses, where venous blood flows in the space between the periosteal and meningeal layers (see "Venous Drainage" below).
- Arachnoid mater: As in the spinal cord, arachnoid mater butts up against dura mater, but is not adherent to it. Veins and arteries run through the subarachnoid space, which is filled with cerebrospinal fluid (CSF). CSF drains through the venous system through arachnoid villi, which project into the superior sagittal sinus (see "Dural Partitions" below).
- Pia mater: thin, delicate membrane that closely invests the surface of the brain The relationship between the dura mater and cranial cavity on one hand and the pia mater and brain on the other closely mirrors the relationship between parietal pleura and visceral pleura, between fibrous pericardium and serous pericardium, and between parietal peritoneum and visceral peritoneum - that is, dura is adherent to the outer wall and pia mater is adherent to the organ (in this case, the brain).
Lobes and Parts of the Brain
In this lesson, you will learn some basic information about the brain, including the names for its various lobes and parts. This is appropriate for an undergraduate course in human anatomy. You will undoubtedly learn more about the brain if you pursue a medical or professional degree and take courses in histology, neurology, neuroanatomy, or physiology. Names for the embryological divisions of the brain are used often, so much so that embryological terms are often used in lieu of adult terms. Below, I present each of the embryological divisions along with what structures these divisions become in the adult.
- Telencephalon: precursor of the cerebral hemispheres that fills the area superior to the tentorium cerebelli, and can be broken down into two sets of frontal lobes, parietal lobes, temporal lobes, and occipital lobes (one left, one right) connected to one another via the corpus callosum. The central sulcus separates the frontal and parietal lobes and the lateral (Sylvian) sulcus separates the frontal and parietal lobes from the temporal lobes. The frontal lobes contain the primary motor cortex, which is associated with voluntary skeletal muscle movement, including motor coordination of speech (in Broca's area). The parietal lobes contain the primary sensory cortex, which receives sensory information from the body. The temporal lobes are involved in hearing and equilibrium, and contain Wernicke's area, which is implicated in the understanding of spoken language. The occipital lobe is implicated in visual processing and the understanding of written language. One cranial nerve, CN I (olfactory nerve), originates on the telencephalon.
- Diencephalon: part of the forebrain situated between the brainstem and telencephalon that becomes the thalamus, hypothalamus, subthalamus, and related structures. The diencephalon is situated deep within the forebrain, forming the walls and floor of the 3rd ventricle. The thalami (plural of thalamus) are processing centers for sensory impulses, the hypothalamus modulates autonomic responses like blood pressure and heart beat, body temperature, electrolyte balance, and emotional behaviors, and the subthalamus regulates the output of the basal ganglia. The telencephalon and diencephalon are both parts of the embryonic forebrain. One cranial nerve, CN II (optic nerve), originates on the diencephalon.
- Mesencephalon: becomes the adult midbrain, which is situated between the diencephalon anteriorly and the pons of the metencephalon posteriorly at the junction between the middle and posterior cranial fossae. It consists of the cerebral peduncles anteriorly and the tectum posteriorly, the latter of which houses four colliculi, or nerve reflex centers, for eye movements (superior colliculi) and auditory reflexes (inferior colliculi). The midbrain houses the cerebral aqueduct that connects the 3rd ventricle (in the diencephalon) with the 4th ventricle (in the hindbrain - see below). Cranial nerve IV (trochlear nerve) originates on the midbrain, and CN III (oculomotor nerve) comes from between the midbrain and pons.
- Metencephalon: becomes the adult cerebellum and pons. The cerebellum is positioned inferior to the tentorium cerebelli (see below), and the pons sits in the anterior part of the posterior cranial fossa against the dorsum sellae and clivus. The pons forms the bridge between the medulla oblongata and midbrain, and its posterior surface forms the floor of the 4th ventricle. The cerebellum is implicated in balance, coordination of motor control, and body posture. Cranial nerve V (trigeminal nerve) originates from the pons, and cranial nerves VI (abducent), VII (facial), and VIII (vestibulocochlear) arise from between the pons and medulla.
- Myelencephalon: becomes the adult medulla oblongata, and is continuous with the spinal cord as it runs through the foramen magnum. The medulla contains the cardiac, vasomotor, and respiratory nerve centers, and controls breathing, coughing, sneezing, swallowing, and vomiting, among other things. As noted above, the pons connects the medulla to the midbrain. Together, the medulla oblongata, pons, and midbrain make up the brainstem. The metencephalon and myelencephalon together are from the hindbrain during even earlier embryonic development. Cranial nerves IX (glossopharyngeal), X (vagus), XI (spinal accessory), and XII (hypoglossal) all originate on the medulla oblongata.
The dura mater attaches to the endocranial surface of the cranial cavity, and is reflected into the cranial cavity at several points to form a sheath of dura which projects inward between different parts of the brain.
- Falx cerebri: fold of dura inserted between the two cerebral hemispheres that runs in the midsagittal plane from the crista galli and frontal crest of the frontal bone anteriorly to blend with the tentorium cerebelli posteriorly.
- Tentorium cerebelli: fold of dura that runs from attachment to the occipital bone at the internal occipital protuberance and along the grooves for the transverse sinuses on the endocranial side of the occipital bone to an attachment area on the petrous part of the temporal bone and the anterior and posterior clinoid processes. It inserts between the cerebellum and the cerebral hemispheres in the posterior cranial fossa. The anteromedial border of the tentorium is free, forming an opening through which the midbrain passes.
- Falx cerebelli: small fold of dura in the posterior cranial fossa that runs along the internal occipital crest in the midsagittal plane from an attachment to the tentorium cerebelli to the foramen magnum, and inserts between the two cerebellar hemispheres.
- Diaphragma sellae: small covering of dura that covers the sella turcica of the sphenoid bone. The infundibulum, which connects the pituitary gland to the base of the brain, runs through an opening in the center of the diaphragma sellae.
Besides making folds and partitions like those described above, dura mater also splits at some attachment sites, so that the periosteal layer and meningeal layer are separated from one another, creating a potential space. These spaces are called dural sinuses, and they carry venous blood from the brain out of the cranial cavity. Some dural sinuses (like, for example, the superior sagittal sinus) are created by the attachment of dura to the internal surface of the cranial cavity, whereas other dural sinuses (e.g., inferior sagittal sinus) are "free" in the sense that they are formed between layers of dura, and do not butt up against the endocranial wall. There are a large number of named venous sinuses, but the ones listed below are the most important.
- Superior sagittal sinus: runs in the space created by the attachment of the falx cerebri to the internal surface of the cranial cavity, running in a sagittal direction from an attachment to the crista galli of the ethmoid bone. Drainage begins in this area at the foramen caecum, and the sinus runs posteriorly to meet up with right or left transverse sinuses and the straight sinus at the confluence of sinuses.
- Inferior sagittal sinus: runs in the inferior margin of the falx cerebri, and ends posteriorly at the anterior edge of the tentorium cerebelli, where it joins the great cerebral vein which runs anteroposteriorly through the middle of the brain to form the straight sinus.
- Straight sinus: runs from its origin anteriorly at the the junction between the inferior sagittal sinus and great cerebral vein to the confluence of sinuses posterirly, where it generally empties into the left transverse sinus. This sinus runs in the space formed by the attachment of the falx cerebri to the tentorium cerebelli.
- Confluence of sinuses: the superior sagittal sinus, straight sinus, and occipital sinus in the falx cerebri all empty into the confluence of sinuses, blood from which drains into the right and left transverse sinuses. In the image above, the confluence of sinuses is labeled "torcular hirophili."
- Transverse and sigmoid sinuses: transverse sinuses run along the internal part of the cranial cavity in the space created by the attachment of the tentorium cerebelli to the endocranial walls of the occipital bone, and turn into sigmoid sinuses on each side as they run around the petrous temporal bones to reach the internal jugular veins. The right transverse sinus usually receives blood from the superior sagittal sinus and the left transverse sinus usually receives blood from the straight sinus.
- Cavernous sinuses: an enormous number of structures pass through the cavernous sinuses, which are created on either side of the sella turcica by the attachment of dura to the lateral aspects of the sphenoid body. Dural sinuses, ophthalmic veins from the orbit, and emissary veins from the pterygoid plexus all empty into the cavernous sinuses. The internal carotid artery and CN VI (abducent nerve) pass through, and cranial nerves III (oculomotor), IV (trochlear), V1 (ophthalmic division of trigeminal) and V2 (maxillary division of trigeminal) run in the lateral wall of each sinus on their way to the foramen rotundum (V2) or superior orbital fissure (III, IV, V1, VI). The right and left cavernous sinuses are connected to one another by the intercavernous sinus.
- Superior and inferior petrosal sinuses: the cavernous sinuses drain into the transverse sinuses through the superior petrosal sinuses, which run in the space created by the attachment of the tentorium cerebelli to the petrous temporal bone along its crest. The inferior petrosal sinuses pass posteroinferiorly between the petrous temporal and basioccipital, and assist in draining the cavernous sinuses.
Blood supply to the brain is from vertebral arteries, which are branches off the right and left subclavian arteries, and branches of the internal carotid arteries. These two sets of arteries connect to each other at the base of the brain to form the arterial circle of Willis around the infundibulum and optic chiasm.
- Vertebral arteries: these arteries are branches of the right and left subclavian arteries, and pass superiorly through the transverse foramina of the upper six cervical vertebrae to pass through the foramen magnum. After passing through the foramen magnum, vertebral arteries give off (1) a small meningeal branch; (2) one branch on each side which meets to form the anterior spinal artery; (3) posterior inferior cerebellar arteries, which run from the anterolateral side to the posterior side of the medulla oblongata; and (4) posterior spinal arteries which most commonly branch off the posterior inferior cerebellar arteries and work their way down the medulla to exit the cranial cavity through the foramen magnum, and run down the spinal cord in close approximation to the posterior roots of spinal nerves. After giving off these 4 branches, the vertebral arteries meet to create the (5) basilar artery, which runs in an anterior direction along the pons, giving off (6) right and left anterior inferior cerebellar arteries at the junction between the medulla and pons and (7) pontine arteries and right and left (8) superior cerebellar arteries along the pons, before ending in right and left (9) posterior cerebral arteries at the anterior border of the pons.
- Carotid arteries: the right and left internal carotid arteries arise as branches of the right and left common carotid arteries, respectively. They run superiorly in the neck and enter the carotid canal at the base of the skull, where they further branch to produce (1) ophthalmic arteries, posterior communicating arteries, middle cerebral arteries, and anterior cerebral arteries. The anterior, middle, and posterior communicating arteries surround the optic tracts and infundibulum on 3 sides and contribute to the arterial circle of Willis (see below).
- Arterial circle of Willis: made up of the anterior, middle, and posterior communicating arteries, which are branches of the internal carotid arteries; the anterior communicating artery, which connects the right and left anterior cerebral arteries; and the posterior cerebral arteries, which are branches of the basilar artery (formed by the fused right and left vertebral arteries, remember). The arterial circle of Willis surrounds the optic chiasm and infundibulum at the midbrain and anterior part of the pons.
- Arterial supply of the dura mater: the dura mater is supplied separately from the brain, since relevant arteries run in the periosteal layer of dura. Anterior meningeal arteries, branches of the ethmoidal arteries, supply the ACF; middle meningeal arteries and accessory meningeal arteries, branches of the maxillary artery, supply the MCF; and the posterior meningeal artery, the terminal branch of the ascending pharyngeal artery, supplies the PCF along with other branches from the occipital artery and vertebral artery. The anteiror meningeal artery enters the cranial cavity through the cribriform plate, the middle meningeal artery enters the cranial cavity through the foramen spinosum, and meningeal branches that supply the PCF enter the cranial cavity through the jugular foramen, hypoglossal canal, mastoid foramen, and foramen magnum.
After this lesson, we turn to the TMJ, temporal and infratemporal fossae, then the orbit, eye, and ear. After that, we turn to the neck.
© 2014 Robert McCarthy