Bones of the cranium (Used with permissions from Gallici et al. )
The bones of the skull articulate to form three distinct fossae: anterior, middle, and posterior (Fig. 2.3). The anterior fossa is formed by the frontal, ethmoid, and sphenoid bones and contains the anterior and inferior aspects of the frontal lobes. The middle fossa is formed by the sphenoid and temporal bones and contains the temporal lobes. Additionally, the sella turcica of the sphenoid bone provides a protective seat for the pituitary gland within the hypophysial fossa. The posterior fossa is predominantly formed by the occipital bone with small contributions from the sphenoid and temporal bones—it contains the brainstem and the cerebellum.
The brain is covered in three layers of protective meninges, which work with the skull and cerebrospinal fluid (CSF) to blunt the effects of insults to the brain. The dura mater is the thickest fibrous external layer, which adheres to the internal surface of the cranium. The dura can be dissected into two distinct layers: the periosteal layer, which connects the dura to the skull, and the meningeal layer, which lies more medially. The dura mater folds in on itself in the interhemispheric fissure to create the falx cerebri. An additional dural fold creates the tentorium cerebelli, separating the cerebral hemispheres from the cerebellum. While these dural folds provide structure to the brain, they constitute sites of potential herniation in the setting of space occupying lesions or cerebral edema.
The arachnoid mater lies medial to the dura mater. The subarachnoid space separates the arachnoid and pia mater. Small fibrous strands called trabeculae tether the arachnoid and pia to one another. The CSF in this space serves as another protective buffer for the brain. The pia mater is the thinnest meningeal layer and is adherent to the brain. This layer is highly vascular and provides oxygen and nutrients to the brain [6, 7, 15].
With traumatic injury, there is potential for bleeding between the skull and dura (epidural hematoma), between the dura and arachnoid meninges (subdural hematoma), or within the subarachnoid space (subarachnoid hemorrhage). (See Chap. 10 for further clinical information).
An epidural hematoma occurs most commonly when a temporal bone fracture severs the middle meningeal artery, although venous bleeding can also be a cause.
A subdural hematoma is most often caused by tearing of the bridging veins in the subdural space.
Subarachnoid hemorrhage can occur in a number of conditions, including rupture of a cerebral aneurysm and trauma.
The cerebrum constitutes the bulk of the brain and is the area responsible for intellectual thought and function. The cerebral cortex is the circumferential gray matter on the surface of the brain that covers the white matter and the deeper gray matter structures. The cortex folds to create raised gyri and sunken grooves called sulci.
The cerebrum is separated into two hemispheres by the interhemispheric fissure and connected by a bundle of nerves called the corpus callosum. Each hemisphere contains a frontal, parietal, temporal, and occipital lobe (Fig. 2.4). The frontal lobe is anterior to the central sulcus that separates the frontal and parietal lobes. The frontal lobe is the site of abstract reasoning, judgment, behavior, creativity, and initiative. The parietal lobe is involved in language, maintaining attention, memory, spatial awareness, and integrating sensory information including tactile, visual, and auditory senses . The lateral (or Sylvian) fissure separates the parietal and frontal lobes from the temporal lobe. The temporal lobe processes sensory input such as language, visual input, and emotions. Tucked deep within the lateral fissure lays the insula, which is involved with emotion and consciousness. The occipital lobe is the most posterior lobe of the cerebrum and is separated from the parietal and temporal lobes by the parieto-occipital fissure. The occipital lobe contains the primary visual cortex and is involved in sight and interpretation of visual stimuli. On the medial surface of each cerebral hemisphere, the limbic cortex modulates emotion, behavior, and long-term memory .
Cerebrum (Flair sequence MRI brain)
In a majority of people, the left hemisphere is dominant, being responsible for language production and comprehension. This is true for both right-handed (90% left dominance) and left-handed individuals (70% left dominance).
In the dominant hemisphere, Broca’s area in the frontal lobe is responsible for fluent speech. Damage to this region causes expressive aphasia. Wernicke’s area, located in the temporal lobe of the dominant hemisphere, is responsible for comprehension. Damage to Wernicke’s area causes receptive aphasia.
Damage to the nondominant hemisphere can cause unilateral neglect of the contralateral side and apraxia, which can impact activities of daily living and lead to spatial disorientation.
The diencephalon is composed of the thalamus and hypothalamus. The thalami are bilateral relay stations for sensory information located medial to the internal capsule and lateral to the third ventricle. They initiate reflexes in response to visual and auditory stimuli. Sensory fibers ascend from the brainstem to the thalamus and then their signals are relayed to the cortex.
The hypothalamus is connected inferiorly to the pituitary gland; together, these structures regulate many hormonal activities within the body. The anterior lobe of the pituitary gland (adenohypophysis) secretes hormones including adrenocorticotrophic hormone, thyroid-stimulating hormone, luteinizing hormone, follicle-stimulating hormone, prolactin, and melanocyte-stimulating hormone in response to signals from the hypothalamus. The posterior lobe (neurohypophysis) contains axons extending from the hypothalamus that secrete oxytocin and vasopressin .
After pituitary surgery, central diabetes insipidus can develop due to reduced secretion of antidiuretic hormone (vasopressin). Patients develop excessive urine output with resultant hypovolemia and hypernatremia.
2.4 Basal Ganglia
The basal ganglia are the deep gray matter structures consisting of the caudate nucleus, globus pallidus, and putamen (Fig. 2.5). The basal ganglia relay information from the cortex and work with the cerebellum to coordinate movement. They are responsible for the initiation and termination of movements, prevention of unnecessary movement, and modulation of muscle tone.
Basal ganglia (Flair sequence MRI brain)
The brainstem consists of three components: midbrain, pons, and medulla. It contains critical structures, such as the cranial nerve nuclei, regulates several autonomic functions and basic reflexes, and determines the level of consciousness (Figs. 2.6–2.9). The descending motor and ascending sensory pathways pass through the brainstem. The reticular activating system resides in the rostral brainstem and projects to the thalami and then the cortex to maintain wakefulness. Damage to this structure results in decreased level of arousal or coma.
Midbrain and cisterns (Flair sequence MRI brain)
The cerebellum is located posterior to the brainstem (Figs. 2.7, 2.8 and 2.9). The cerebellum works in tandem with the basal ganglia to provide smooth coordinated movement. Damage to the cerebellum causes limb ataxia, vertigo, and gait disturbances.
Pons and posterior fossa (Flair sequence MRI brain)
Medulla and posterior fossa (Flair sequence MRI brain)
Sagittal view (Flair sequence MRI brain)
2.7 Cerebral Vasculature
The arterial supply to the brain is divided into anterior and posterior circulations. The anterior circulation originates from bilateral internal carotid arteries (ICA). Each ICA travels superiorly through the neck and enters the cranium via the carotid canal within the temporal bone. The ICA then bifurcates into the anterior cerebral artery (ACA) and the middle cerebral artery (MCA). The ACA supplies the anterior medial surface of the brain, which includes the frontal and anterior parietal lobes. The MCA supplies the bulk of the cerebral hemisphere. It typically divides into superior and inferior divisions as it passes through the lateral fissure. These divisions supply the cortex superior and inferior to the lateral fissure, respectively. Prior to this bifurcation, several small vessels called the lenticulostriate arteries arise from the MCA. These vessels provide the blood supply for a majority of the basal ganglia and internal capsule.