Neuroanatomy: The Basics



Neuroanatomy: The Basics


Amy D. Costigan

Brian Silver



A solid foundational understanding of neuroanatomy is critical to the recognition and management of neurologic emergencies. This chapter discusses the most important anatomic structures involved in emergent neurologic disease processes and injuries. Subsequent chapters will build on these concepts and how they apply to the physical exam, neuroimaging, and to specific neurologic emergency conditions. Neuroanatomy encompasses all structures of the nervous system and can be divided into two main categories: the central nervous system and the peripheral nervous system. The main components of the central nervous system, including the brain, spinal cord, vasculature, and dural spaces, are presented as well as the somatic and autonomic components of the peripheral nervous system.


CENTRAL NERVOUS SYSTEM

The central nervous system consists of the brain and the spinal cord. The brain is, essentially, the command center for the entire human body. It is a large and complex organ that controls and regulates virtually every action of the human body, including thought, emotion, vision, speech, breathing, and movement. The brain is estimated to contain 100 billion neurons (with approximately 10-20 billion in the cerebral cortex and 55-70 billion in the cerebellum) and an additional 100 glial cells to support neuronal function. The number of synapses in the brain is approximately 100 trillion, representing 1000 synapses per neuron. The brain has three main parts: the cerebrum, the cerebellum, and the brainstem. Although the brain makes up only 2% of total body weight, it receives 20% of the body’s cardiac output. The brain depends on this constant blood flow for nutrition and function because it does not store any fuel.


Cerebrum

The cerebrum is the largest portion of the brain and is composed of the right and left hemispheres that are separated by the longitudinal fissure. The cerebral hemispheres are covered by a thin outer layer of gray matter, called the cerebral cortex, which contains billions of neurons to control essential functions such as consciousness, language, memory, and attention. White matter lies below the cerebral cortex and is made up of myelinated tracts that distribute information between brain regions. In general, the left hemisphere is the “dominant” hemisphere in most people and controls language and speech. The right hemisphere is important in the interpretation of visual and spatial information. Tracts that control motor and sensory function mostly (approximately 95%) cross in the lower medulla to the contralateral arm and leg. Therefore, the right hemisphere controls the left hemibody, and the left hemisphere controls the right hemibody. The two hemispheres are
connected by a large bundle of nerve fibers, called the corpus callosum, as well as smaller commissures, including the anterior commissure, posterior commissure, and fornix. These are important because they transfer and coordinate information between the two hemispheres.

The cortex folds into peaks, called gyri, and grooves, called sulci, allowing for a larger surface area of brain to fit within the skull. Larger sulci and gyri separate the cerebral hemispheres into four distinct areas, called the frontal, temporal, parietal, and occipital lobes (Figure 1.1). Each area has different, although sometimes overlapping, functions.

The frontal lobe is the most anterior portion of the brain and is separated from the parietal lobe by the central sulcus and from the temporal lobe by the lateral sulcus (Sylvian fissure) (Figure 1.1). The frontal lobe controls personality, behavior, speech, body movement, concentration, and intelligence. A clinically important portion of the inferior frontal lobe is called Broca area, which is in the dominant hemisphere and is integral in language processing (Figure 1.2). Strokes or damage to this area will cause expressive aphasia, also known as Broca aphasia. This results in impairment in the production of speech but a preservation of the understanding of spoken and written language. Control of gaze is also mediated through the frontal eye fields. Abnormal excitation of one frontal field, such as in a lateralized seizure, will result in gaze away from the excitatory focus and toward the jerking limbs. Destruction of the frontal eye fields, as in a stroke, will result in gaze toward the destruction and away from the hemiparesis. In clinical scenarios where the eyes appear to be deviated toward hemiparetic limbs, consideration of a seizure should also be undertaken with careful examination of the eyes to look for subtle nystagmoid jerking, which confirms the diagnosis.

The temporal lobe, which is separated from the frontal and parietal lobes by the lateral sulcus, also has a language component, called Wernicke area (Figure 1.2). The ability to understand written and spoken language is controlled by this area. Damage to the superior temporal lobe causes a type of receptive aphasia, called Wernicke aphasia. Because patients have difficulty understanding language, they may produce speech that has no meaning and can colloquially be referred to as “word salad.” Broca and Wernicke area are connected via a bundle called the arcuate fasciculus (Figure 1.2). The temporal lobe also helps controls memory, hearing, and organization.

The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus (Figure 1.1). It has important functions, including the integration of sensory information and visual and spatial processing.












Finally, the occipital lobe is the visual processing center of the brain and is located posteriorly behind the parietal lobe and temporal lobe (Figure 1.1). It contains the primary visual cortex as well as other functional visual areas. Damage to the occipital lobe can cause visual field cuts, visual hallucinations, or even blindness. The calcarine fissure divides the occipital cortex into superior and inferior portions. Damage to the occipital cortex above the calcarine fissure produces lower visual field defects, whereas injury to the occipital cortex below the calcarine fissure produces upper visual field defects. Like motor control, representation of the left visual field (of both eyes) is in the right occipital cortex and that of the right visual field (of both eyes) in the left occipital cortex. Thus, for example, a right occipital injury below the calcarine cortex will produce a left upper visual field defect in both eyes (Figure 1.3).


Subcortical Structures

Deep below the cerebral cortex lie the subcortical structures, which include the diencephalon, pituitary gland, limbic structures, and the basal ganglia. These essential structures are integral in memory, hormone production, and emotion.

The diencephalon, the brain between the cerebrum and the brainstem, is found deep within the cerebrum and consists of the thalamus, epithalamus, subthalamus, and hypothalamus. The thalamus is a sensory integration center that has connections to many areas of the brain as well as the reticular activating system, which regulates arousal and sleep-wake transitions. Damage to the anterior thalamus, particularly if bilateral in the setting of deep venous sinus thrombosis, can produce disruption in awareness or even coma. This diagnosis should be considered as a possibility in someone presenting with sudden unresponsiveness, particularly if there is no obvious cardiac or toxic cause. The epithalamus consists mostly of the pineal gland and helps secrete melatonin for circadian rhythms. The subthalamus is involved in the integration of somatic motor function. The hypothalamus is a small structure that connects to the pituitary gland via the infundibular stalk. It contains several small nuclei and is an important control center for the autonomic nervous system and endocrine system, including body temperature, hunger, thirst, fatigue, and sleep.













Pituitary Gland

The pituitary gland is an endocrine gland that is located off the bottom of the hypothalamus (Figure 1.4). Although it is only about the size of a pea, it secretes hormones that have a vast array of important functions such as growth, metabolism, temperature regulation, and pain relief. It helps regulate thyroid and kidney function as well as sex, pregnancy, child birth, and breastfeeding.


The Limbic System

The limbic system is a set of brain structures located next to the thalamus and beneath the medial temporal lobe (Figure 1.5). It has many important functions, including emotion, motivation, memory, and behavior. Acute alteration in personality may be caused by processes affecting the limbic system, such as limbic encephalitis, an autoimmune disorder that can produce a rapid onset neuropsychiatric disorder. Two important structures in the limbic system are the hippocampus and amygdala. The hippocampus is involved in the processing of spatial memory and learning. The
amygdala is the deepest part of the limbic system and is involved in cognitive processes such as memory, attention, emotion, and social processing.







Basal Ganglia

The basal ganglia (Figure 1.6) are a group of subcortical nuclei at the base of the forebrain and the top of the midbrain. They consist of striatum (caudate nucleus, putamen, nucleus accumbens, and olfactory tubercle), globus pallidus, ventral pallidum, substantia nigra, and subthalamic nucleus.
The basal ganglia are important in the control of voluntary motor movements, eye movements, cognition, emotion, and learning. Some disorders of behavior control and movement are rooted in the basal ganglia, including Parkinson and Huntington. Acute injuries of the basal ganglia, for example, to the subthalamic nucleus, can produce unusual syndromes such as uncontrolled ballistic movements of the contralateral limb. The basal ganglia also play a large role in addiction physiology.







The Cerebellum

The cerebellum is a vital structure that gives human beings motor control and coordination. It is attached to the bottom of the brain and lies underneath the cerebral hemispheres in the posterior cranial fossa. It consists of a much more tightly folded cortex than the cerebrum and gives the appearance of parallel folds. Like the cerebrum, the cerebellum is divided into two hemispheres. Between these is a midline section called the vermis (Figure 1.7). The cerebellum is connected to the rest of the brain and spinal cord via three pairs of cerebellar peduncles, called the superior, middle, and inferior cerebellar peduncles. The superior cerebellar peduncles connect efferent fibers to the cerebral cortex. The middle cerebellar peduncle connects to the pons, and the inferior cerebellar peduncle receives signals from the vestibular nuclei, spinal cord, and tegmentum. When patients have damage to the cerebellum, they typically exhibit ipsilateral decreased motor control. Although they may be able to accomplish gross motor tasks, they have issues with precision and coordination. Patients with cerebellar lesions may exhibit ataxia, ocular skew deviation, poor coordination, dysmetria, dysdiadochokinesia, or a tendency to fall toward the side of the lesion.








The Brainstem

The most posterior portion of the brain that is continuous with the spinal cord is called the brainstem. It consists of three main parts: the midbrain, the pons, and the medulla oblongata (see Figure 1.7). The brainstem transmits motor and sensory information between the brain and the body. Ascending sensory tracts and descending motor tracts are located here, and ten of the body’s cranial nerves also emerge from the brainstem. It also integrates crucial information for cardiovascular control, respiratory control, pain, alertness, and consciousness.


Midbrain

The rostral most portion of the brainstem is called the midbrain and is separated into four structures called the tectum, cerebral aqueduct, tegmentum, and cerebral peduncles. The tectum is located on the dorsal side of the midbrain and is responsible for reflexes to auditory and visual stimuli. The cerebral aqueduct is part of the ventricular system and drains cerebrospinal fluid from the third ventricle to the fourth ventricle. Cranial nerve nuclei for the oculomotor nerve (III) and trochlear nerve (IV) are located on the ventral side of the gray matter surrounding the cerebral aqueduct. The midbrain tegmentum is ventral to the cerebral aqueduct and communicates with the cerebellum via the superior cerebellar peduncles. The tegmentum contains a large network of neural synapses and nuclei with white matter tracts. The cerebral peduncles form lobes ventrally to the tegmentum and contain additional white matter tracts. Syndromes such as the top of the basilar syndrome caused by a distal basilar artery occlusion, around the level of the midbrain, will
produce a variety of syndromes, including alternating hemiparesis, visual hallucinations, and coma. Mortality is approximately 80% if recanalization is not achieved.


Pons

The pons lies between the midbrain and the medulla (see Figure 1.7). The pons helps control sleep as well as respiratory rate. It can be separated into the basilar part of the pons ventrally and the pontine tegmentum. Posteriorly, it contains the cerebellar peduncles, which connect the pons and the cerebellum and midbrain. Because it acts as a connection point between these different areas, damage to the pons can cause issues with autonomic functions, movement, sensory problems, dysfunction in arousal, and coma. The pons contains several cranial nerve nuclei, including trigeminal nerve (V), abducens nerve (VI), facial nerve (VII), and vestibulocochlear nerve (VIII). Because the nerve roots to the face exit above the decussation, strokes affecting the pons can result in ipsilateral facial weakness and contralateral arm and leg weakness. A brainstem stroke should be considered if there is clear facial weakness on the side opposite to limb weakness.


Medulla

The medulla is a long cone-shaped structure that is responsible for several autonomic functions, including heart rate, blood pressure, breathing, vomiting, and sleep. The upper open part of the medulla forms the fourth ventricle, and the lower closed portion surrounds the central canal of the spinal cord. Several white matter tracts synapse in the medulla. The nuclei for cranial nerves IX to XII are also present here.


THE ARTERIAL VASCULAR SYSTEM

A continuous supply of arterial blood flow is vital for the brain because it does not have any energy or fuel stores of its own. The arterial blood supply to the brain is a made up of a complex series of vessels that ultimately anastomose in a ring called the circle of Willis (Figure 1.8). The arterial blood is supplied by two main pairs of arteries: the bilateral internal carotid arteries and the bilateral vertebral arteries. The internal carotid arteries supply the anterior circulation to the cerebrum, whereas the vertebral arteries join to form the basilar artery and supply the posterior circulation to the cerebellum and brainstem. The circle of Willis creates a connection at the base of the skull between the anterior and the posterior circulatory systems of the brain.

The anterior circulation of the brain begins at the bilateral internal carotid arteries, which first branch with the ophthalmic artery and then branch into the anterior cerebral artery (ACA) and the much larger middle cerebral artery (MCA). The anterior cerebral arteries supply blood to the entire midline of the cerebral hemispheres (Figure 1.9). They connect with their contralateral counterpart via the anterior communicating artery (A Comm) to complete the anterior ring of the circle of Willis (Figure 1.8). Strokes in the anterior cerebral artery are less common; acute isolated or predominant contralateral lower extremity motor deficits should raise suspicion of this kind of stroke. This is illustrated in Figure 1.10, which shows the motor homunculus of the cerebral cortex. The homunculus (“little man”) is a famous graphical representation of a man lying within the brain and diagrams the areas of the body controlled by each region. It demonstrates how the midline cerebral hemisphere, which is supplied by the ACA, primarily controls the lower extremity.

The middle cerebral artery (MCA) courses into the lateral sulcus and perfuses most of the lateral surface of the cerebral cortex (Figure 1.9). It is divided into four segments (M1-M4). Owing to the direct flow of blood from the internal carotid artery and the large amount of cerebrum supplied by the MCA, this artery accounts for the most common territory in the cerebrum affected by acute stroke. Neurologic deficits vary depending on the extent and hemisphere, but they include aphasia (typically left hemisphere), neglect (most often in right hemisphere strokes but can occur less commonly with left hemisphere strokes), hemianopia, contralateral hemiparesis, or contralateral hemisensory loss. The MCA is the main artery typically amenable to mechanical thrombectomy for large vessel occlusion during acute stroke.

The posterior circulation originates in the bilateral vertebral arteries, which enter the skull through the foramen magnum. The posterior inferior cerebellar artery (PICA) branches off the vertebral artery to supply the posterior and inferior cerebellum. The two vertebral arteries join to form the basilar artery, which is responsible for supplying the pons, cerebellum, and inner ear

via the anterior inferior cerebellar artery (AICA), pontine arteries, the superior cerebellar artery, and the internal auditory artery. The differential diagnosis for acute unilateral hearing loss should include an AICA territory infarction, particularly if there is associated dizziness and/or dysmetria. The basilar artery terminates when it divides into the two posterior cerebral arteries (PCA). These two vessels supply the inferior and posterior cerebrum and are joined by the posterior communicating artery (P Comm) to complete the posterior ring of the circle of Willis (Figure 1.8).

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Jun 23, 2022 | Posted by in EMERGENCY MEDICINE | Comments Off on Neuroanatomy: The Basics
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