Abstract
In this chapter, we present correlative clinical neuroanatomy for neuroanesthetists. This is written with the aim of helping doctors cope with the ever-evolving field of neuroanesthesia while armed with a rock-solid foundation of anatomy that would help in maintaining safe clinical practices.
Keywords
Anatomy, Brain, Spinal cord
Outline
Introduction 3
Embryological Differentiation of Different Parts of Brain 4
Anatomy of Brain 4
Cerebrum 4
Frontal Lobe 6
Temporal Lobes 6
Parietal Lobe 7
Functional Areas (of Cerebral Cortex) 8
Premotor Area 10
Sensory Areas 10
Visual Area 11
Acoustic (Auditory) Area 11
Association Areas 11
Diencephalon 11
Habenular Nucleus 15
Nucleus Subthalamicus 16
Zona Incerta 16
Basal Ganglia 16
Internal Capsule 17
White Matter 18
Corpus Callosum 19
Ventricular System 19
Limbic System 22
Midbrain (Mesencephalon) 23
Pons 25
Medulla 26
Reticular Formation 29
Cerebellum 29
Vascular Supply of the Brain 30
The Meninges and Cerebrospinal Fluid 33
The Meninges 33
Dura Mater 33
The Spinal Cord 34
Ascending Tracts of Spinal Cord (Sensory Tracts) 36
Acknowledgment [CR]
References 40
Outline
Introduction 3
Embryological Differentiation of Different Parts of Brain 4
Anatomy of Brain 4
Cerebrum 4
Frontal Lobe 6
Temporal Lobes 6
Parietal Lobe 7
Functional Areas (of Cerebral Cortex) 8
Premotor Area 10
Sensory Areas 10
Visual Area 11
Acoustic (Auditory) Area 11
Association Areas 11
Diencephalon 11
Habenular Nucleus 15
Nucleus Subthalamicus 16
Zona Incerta 16
Basal Ganglia 16
Internal Capsule 17
White Matter 18
Corpus Callosum 19
Ventricular System 19
Limbic System 22
Midbrain (Mesencephalon) 23
Pons 25
Medulla 26
Reticular Formation 29
Cerebellum 29
Vascular Supply of the Brain 30
The Meninges and Cerebrospinal Fluid 33
The Meninges 33
Dura Mater 33
The Spinal Cord 34
Ascending Tracts of Spinal Cord (Sensory Tracts) 36
Acknowledgment [CR]
References 40
Acknowledgment
I am thankful to Mr. Anil Kumar, a senior artist from SGPGIMS, Lucknow, for drawing all diagrams given in this chapter.
Introduction
Why should a well-established neuroanesthetist study clinical neuroanatomy? This question, albeit a vexing one, is very pertinent in the present day scenario. The answer is evident. A tower of knowledge built on broad and diverse information helps one to prepare for all the eventualities that one may encounter. Anatomy is the basis of every procedure that we perform. An anesthetist who embarks on a new journey into the anatomical basis of his or her clinical practice has adapted well to a vista that emphasizes fundamental sciences as the basis of all medical education. It helps the person improve his or her procedural skills. Finally, it helps him or her to be better equipped to deal with a changing and unpredictable world where knowledge empowers and also acts as a haven of safety.
About 100 billion neurons and 10–50 trillion neuroglias make up the brain, which has a mass of about 1300–1500 g in adult. On average each neuron forms 1000 synapses with other neurons . The total number of synapses, about 1000 trillion, is larger than the number of stars in the galaxy. The central nervous system (CNS) included the brain and the spinal cord and is composed of (1) cerebral hemisphere, (2) diencephalon, (3) basal ganglion, (4) midbrain, (5) pons, (6) medulla, (7) cerebellum, and (8) spinal cord. This chapter will provide information of these parts individually, that is integrated, informative, and relevant to educational need of the neuroanesthesiologists.
Anatomy of Brain
Cerebrum
The cerebrum consists of two cerebral hemispheres that are partially connected with each other by corpus callosum. Each hemisphere contains a cavity called the lateral ventricle. The cerebrum is arbitrarily divided into five lobes: frontal, parietal, temporal, occipital, and insula. On the lateral surface three sulci (central, lateral or Sylvian, and parietooccipital sulci) and two imaginary lines divide the cerebrum into four lobes ( Fig. 1.1 ). The first imaginary line (lateral parietotemporal line) is drawn from parietooccipital sulcus to preoccipital notch and second (temporo-occipital line) backward continuation of posterior ramus of lateral sulcus before it turns upward to meet first line. The central sulcus and posterior ramus of Sylvian fissure (SF) separate frontal lobe from parietal lobe and temporal lobe. Posteriorly parietooccipital sulcus and lateral parietotemporal line separate occipital lobe from parietal lobe and temporal lobe. Temporal and parietal lobes are separate by posterior ramus of SF and temporo-occipital line ( Fig. 1.1 ).
The cerebral cortex is the outermost sheet of neural tissue of the cerebrum whereas white matter lies in the center. Cerebral cortex is folded into sulci and gyri, which actually increases the surface area of cortex. Sulci include the central lateral and parietooccipital.
The central sulcus begins by cutting the superomedial border of the hemisphere a little behind the midpoint between the frontal and parietal lobe. It runs on the superolateral surface obliquely downward and forward for about 8–10 cm and ends a slight above the posterior ramus of lateral sulcus. It separates precentral gyrus (motor area) from postcentral gyrus (sensory area) ( Figs. 1.2 and 1.3 ). It was originally called the fissure of Rolando or the Rolandic fissure.
The lateral sulcus or Sylvian fissure (SF) is one of the earliest-developing sulci of the human brain. It first appears around the 14th gestational week. It is the deepest and most prominent of the cortical sulci. The lateral sulcus (SF) separates frontal and parietal lobes from temporal lobe. It begins on the superomedial margin. The SF starts on basal and extends to the lateral surface of the brain. It has both a superficial part and a deep part. Superficial part has a stem and three rami ( Figs. 1.2 and 1.3 ). The anterior portion of the deep part (Sylvian cistern) is called the sphenoidal compartment and the posterior part is called the operculoinsular compartment. SF is an important corridor in neurosurgery as it connects the surface of anterior part of brain to its depth with all the neural and vascular components along the way. The structures within the reach through the Transylvian approach include middle cerebral artery; optic nerves; internal carotid artery; and its branched lamina terminalis, insula, basal ganglia, and interpeduncular fossa.
Parietooccipital sulcus begins on the medial surface of hemisphere nearly 5 cm in front of the occipital pole ( Fig. 1.4 ). The upper end of the sulcus reaches the superomedial border to meet the calcarine sulcus, and a small part of it is seen on the superolateral surface.
Frontal Lobe
The frontal lobe is an area in the brain of mammals, located at the front of each cerebral hemisphere and positioned anterior to (in front of) the parietal lobe and superior and anterior to the temporal lobes. A prefrontal sulcus runs downward and forward parallel to the central sulcus. The area between it and central sulcus is the precentral gyrus. Two sulcus run horizontally anterior to precentral gyrus, i.e., superior and inferior frontal sulcus and divide the region into superior, middle, and inferior frontal gyri ( Fig. 1.3 ). The frontal lobe contains most of the dopamine-sensitive neurons in the cerebral cortex associated with reward, attention, short-term memory tasks, planning, and motivation.
Temporal Lobes
Temporal lobes are bounded by SF superiorly and temporo-occipital and lateral parietotemporal line posteriorly ( Fig. 1.3 ). The temporal lobe has two sulci, superior and inferior, that run parallel to the posterior ramus of the lateral sulcus and divide superiolateral surface into superior, middle, and inferior temporal gyri ( Fig. 1.3 ). The temporal lobes are involved in the retention of visual memories, processing sensory input, comprehending language, storing new memories, emotion, and deriving meaning.
Parietal Lobe
The parietal lobe is positioned superior to the occipital lobe and posterior to the frontal lobe. The parietal lobe is bounded anteriorly by central sulcus, inferiorly by SF and temporo-occipital line, medially by interhemispheric fissure, and posteriorly by parietotemporal line. The two main sulci are postcentral sulcus, which run downward and forward parallel to central sulcus, and intraparietal sulci, which are directed posteriorly and inferiorly toward occipital pole. Thus divide the parietal lobe into postcentral gyrus, superior parietal lobule, and inferior parietal lobule ( Fig. 1.3 ). The upturn posterior end of the posterior ramus of lateral sulcus or SF extends into inferior parietal lobule, also superior and inferior temporal sulci turn upward to enter into this lobule and constitute supramarginal, angular gyri, and arcus temporo-occipitalis ( Fig. 1.3 ).
The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the dorsal stream of the visual system. This enables regions of the parietal cortex to map objects perceived visually into body coordinate positions. Several portions of the parietal lobe are important in language processing. Just posterior to the central sulcus lies the postcentral gyrus. This area of the cortex is responsible for somatosensation.
The occipital lobule occupies space behind the lateral parietotemporal line. It has a number of short lobules divided by short sulci. A horizontal sulci, lateral to occipital sulcus divides the lobe into superior and inferior occipital gyri ( Fig. 1.3 ). A vertical strip anterior to curved lunate sulcus is the gyrus descendens. The transverse occipital sulcus is located in the uppermost part of the occipital lobe. A strip superiolateral to this sulcus is arcus parietooccipitalis. The occipital lobe is the visual processing center of the brain containing most of the anatomical region of the visual cortex.
Insula is a portion of the cerebral cortex folded deep within the lateral sulcus. This area grows less than its surrounding areas during development and thus lies deep and not seen from surface view ( Fig. 1.5 ). The surrounding cortical areas are called opercula such as frontal opercula, frontoparietal opercula, and temporal opercula. The insula are believed to be involved in consciousness and play a role in diverse functions usually associated to emotion or the regulation of the body’s homeostasis. These functions include perception, self-awareness, cognitive functioning, and interpersonal experience.
Two hemispheres are attached with each other by corpus callosum. On the medial surface above the corpus callosum there are many sulci and gyri ( Fig. 1.4 ). The most prominent sulcus is the cingulate sulcus, which follows the curve course parallel of corpus callosum. The area between the cingulate sulcus and corpus callosum is the gyrus cinguli. Above the cingulate sulcus, large anterior part is medial frontal gyrus and posteriorly paracentral lobule ( Fig. 1.4 ). Behind the paracentral lobule, two major sulci, parietooccipital sulcus and calcarine sulcus, cut the area into a triangular area called the cuneus. Between parietooccipital sulcus and paracentral lobule, a quadrangular area is called precuneus, which is anteriorly separated from gyrus cinguli by suprasplenial sulcus.
The majority of the space of cerebral hemisphere deep to the cortex is full of the white matter. There are some important structures that are embedded within the white matter. On coronal section the corpus callosum is seen as a strip connecting both the hemispheres ( Fig. 1.5 ). Third ventricle is situated in midline just below corpus callosum. Thalamus and hypothalamus, which are derived from diencephalon, lie adjacent to lateral wall of the third ventricle. Caudate nucleus is situated above and lateral to thalamus. Another gray matter mass lentiform nucleus lies more lateral and just deep to insula. There is a strip of gray matter between insula and lentiform nucleus called claustrum ( Fig. 1.5 ). The caudate nucleus, lentiform nucleus, claustrum along with some other gray matter nucleus are (derived from telencephalon) collectively mentioned as basal nuclei or basal ganglia. There is a white matter, an internal capsule that lies between thalamus and lentiform nucleus ( Fig. 1.5 ). The white matter that radiates from the upper part of internal capsule to the cortex is called corona radiate.
Functional Areas (of Cerebral Cortex)
Korbinian Brodmann was a German neurologist who studied the brain in the early part of the 20th century. Brodmann originally defined and numbered (from 1 to 52) different areas of cerebral cortex based on cytoarchitecture or how the cells were functionally organized ( Box 1.1 ). Brodmann areas have been discussed, debated, refined, and renamed exhaustively for nearly a century and remain the most widely used and frequently cited cytoarchitectural organization of the human cortex.
On the basis of function, regions of the cerebral cortex are divided into three functional categories of areas ( Fig. 1.6 ). (1) Primary sensory areas, which receive signals from the sensory nerves and tracts by way of relay nuclei in the thalamus. Primary sensory areas include the somatosensory cortex in the parietal lobe, visual area of the occipital lobe, and the auditory area in parts of the temporal lobe and insular cortex. (2) Primary motor cortex, which sends axons down to motor neurons in the brain stem and spinal cord and finally innervate voluntary skeletal muscles; (3) remaining parts of the cortex, which are called the association areas. These areas receive input from the sensory areas and lower parts of the brain which integrate sensory information with emotional states, memories, learning, and rational thought processes that we call perception, thought, and decision-making.
Motor areas —The motor area is classically located in precentral gyrus on the superiolateral surface of the hemisphere and in anterior part of paracentral lobule. It is shaped like a pair of headphones stretching from ear to ear ( Fig. 1.6 ). Specific area within the motor cortex controls voluntary muscle activity on the opposite part of body. The body is represented on the motor strip in an upside–down fashion ( Fig. 1.7 ). The lower parts of the body, such as the feet and the legs, receive motor movement commands from the superior part of the precentral gyrus (motor strip). Parts of the face, on the other hand are innervated by the inferior part of the motor strip. The motor strip extends down some distance into the longitudinal cerebral fissure. The portion inside this fissure is its medial aspect . The part on the lateral surface of the hemisphere is called its lateral aspect . The medial cortex controls the movements of the body from the hips on down while the lateral aspect sends commands to the upper body including the larynx, face, hands, shoulders, and trunk ( Fig. 1.7 ). The medial and lateral aspects of the motor strip have different blood supplies. Blood comes to the medial area from the anterior cerebral artery while the lateral cortex is supplied by the middle cerebral artery .
Premotor Area
There is supplementary motor area on and above the superior part of cingulate sulcus on the medial aspect hemisphere that reaches to the premotor cortex (Brodmann areas 6 and 8) on the lateral surface of brain. The cortical area in inferior frontal gyrus corresponds to motor speech area or speech area of Broca (Brodmann areas 44 and 45) and frontal eye area ( Fig. 1.8 ). Lesion into the motor speech area of Broca results in aphasia even the muscles concerned are intact. In 95% of right-handers do have left-hemisphere dominance for language functions, only around 19% of left-handers have right-hemisphere language dominance, with another 20% or so processing language functions in both hemispheres.
Sensory Areas
From the specific nuclei of the thalamus, neurons are projected into two somatosensory areas of the cortex: somatosensory area I in postcentral gyrus and somatosensory area II in the wall of the SF ( Fig. 1.6 ). The arrangement of the thalamic fibers in somatosensory area I is such that the part of the body is represented in order along the postcentral gyrus, with leg on the top and head at the foot of the gyrus. The area of the cortex that receives sensation from a part of the body is not proportional to the size of that part rather the complexity of sensation received from it, as cortical areas for the sensation from the trunk and back are small whereas, hand and part of mouth concerned in speech are very large.
Visual Area
The primary visual receiving area (visual cortex, Brodmann area 17) is located primarily in occipital lobe on the sides of the calcarine fissure ( Fig. 1.6 ). This area also extends into the cuneus and into the lingual gyrus. The visual area is continuous, above and below, with area 18 and beyond this 19. These areas often described as psychovisual areas are responsible mainly for interpretation of visual impulses reaching visual area ( Fig. 1.6 ).
Acoustic (Auditory) Area
The primary auditory cortex (Brodmann area 41) is in the superior portion of temporal lobe. It is located in the part of temporal gyrus which forms the inferior wall of the posterior ramus of the lateral sulcus ( Fig. 1.6 ). The auditory associated areas adjacent to the primary auditory receiving areas are widespread and extend into insula.
Association Areas
Specific areas of the cerebral cortex integrate sensory information with emotional states, memories, learning, and rational thought processes. Primary motor cortex (precentral gyrus of frontal lobe) is located just anterior to the central sulcus in the frontal lobe of the cerebral cortex, of the gray matter motor neurons, which initiates impulses routed through the medulla and spinal cord. It represents the conscious voluntary commands to the prime movers of skeletal muscle groups for specific actions; it is highly organized with specific regions representing each part of the body. Primary sensory cortex ( postcentral gyrus of parietal lobe ) is located just posterior to the central sulcus in the parietal lobe of the cerebral cortex, of the somatic sensory neurons and receives impulses from the thalamus, medulla, and spinal cord. It responds with the first conscious perceptions/awareness of cutaneous sensations arriving from stimulated receptors in the skin and subcutaneous tissues; it is highly organized with specific regions representing each part of the body. Frontal eye field —a specific motor area within the frontal cortex which controls the voluntary scanning movements of the eyes, such as tracking a bird in flight, by sending impulses to the extrinsic muscles of the eyes .
Frontal lobe contains areas that Brodmann identified as involved in cognitive functioning and in speech and language ( Fig. 1.8 ).
- •
Area 4 corresponds to the precentral gyrus or primary motor area.
- •
Area 6 is the premotor or supplemental motor area.
- •
Area 8 is anterior of the premotor cortex. It facilitates eye movements and is involved in visual reflexes as well as pupil dilation and constriction.
- •
Areas 9, 10, and 11 are anterior to area 8. They are involved in cognitive processes such as reasoning and judgment which may be collectively called biological intelligence.
- •
Area 44 is Broca’s area.
Parietal lobe plays a role in somatosensory processes ( Fig. 1.8 ).
- •
Areas 3, 2, and 1 are located on the primary sensory strip, with area 3 being superior to the other two. These are somastosthetic areas, meaning that they are the primary sensory areas for touch and kinesthesia.
- •
Areas 5, 7, and 40 are found posterior to the primary sensory strip and correspond to the presensory to sensory association areas.
- •
Area 39 is the angular gyrus.
Temporal lobe : Areas that are involved in the processing of auditory information and semantics as well as the appreciation of smell ( Fig. 1.8 ).
- •
Area 41 is the primary auditory area.
- •
Area 42 is immediately inferior to area 41 and also involved in the detection and recognition of speech. The processing done in this area of the cortex provides a more detailed analysis than that done in area 41.
- •
Areas 21 and 22 are the auditory association areas. Both areas are divided into two parts; one half of each area lies on either side of area 42.
- •
Area 37 is found on the posterior–inferior part of the temporal lobe.
Occipital lobe contains areas that process visual stimuli ( Fig. 1.8 ).
- •
Area 17 is the primary visual area.
- •
Areas 18 and 19 are the secondary visual areas.
Diencephalon
The diencephalon is midline structure and embedded in the cerebrum. The third ventricle is considered as the cavity of diencephalon. Diencephalon is bounded anteriorly from the plane through the optic chiasm and anterior commissure; caudally from plane through the posterior commissure and the caudal edge of the mammillary bodies; medially from wall of the third ventricle, stria medullaris thalami, and mass intermedia; laterally from the internal capsule, tail of caudate nucleus, and stria terminalis; and dorsally by the fornix and floor of the lateral ventricles ( Figs. 1.4 and 1.5 ).
Diencephalon consists of the following parts:
- 1.
Thalamus
- 2.
Hypothalamus
- 3.
Subthalamus
- 4.
Epithalamus
The Thalamus
It is a large, egg-shaped (ovoid), 4 × 1.5-cm nuclear mass. It makes up about 80% of the mass of the diencephalon. It consists mainly of gray matter, but its superior and lateral surfaces are covered by thin layers of white matter termed the stratum zonale and the external medullary lamina, respectively. The gray matter is incompletely divided into anterior, medial, and ventrolateral nuclei by a Y-shaped lamina of white matter called the internal medullary lamina. It has two ends (anterior and posterior) and four surfaces (superior, inferior, medial, and lateral) ( Fig. 1.9 ).
It extends anteriorly to the interventricular foramen; superiorly to the transverse cerebral fissure (between corpus callosum and fornix); inferiorly to the hypothalamic sulcus; and posteriorly it overlaps the midbrain (pulvinar).
Anterior end of thalamus is smaller than posterior end and lies behind interventricular foramen which connects the lateral ventricle and the third ventricle. Posterior end is large and expanded, called as pulvinar ( Fig. 1.9 ). It is projected backwards and laterally over superior colliculus of midbrain. There are two small swellings on inferior surface of pulvinar called the medial and lateral geniculate bodies. Superior surface is not clearly demarcated from lateral surface. Stria medullaris thalami marks the junction between the superior and medial surfaces. It is separated from the ventricular surface of caudate nucleus by the stria terminalis and thalamostriate vein. It is divided into two areas by an impression produced by the lateral margin of fornix. The lateral area is covered by ependyma and forms part of the floor of the body of lateral ventricle ( Fig. 1.5 ). The median area is covered by the tela choroidea of the third ventricle (double fold of pia matter) ( Fig. 1.4 ). Inferior surface lies upon the subthalamic tegmental region [i.e., hypothalamus, subthalamus, and midbrain (from before backwards)]. Medial surface forms part of lateral wall of the third ventricle separated from corresponding surface of opposite thalamus by a narrow interval.
The two thalami are connected by a short band called the interthalamic adhesion ( Fig. 1.4 ). Lateral surface separated from lentiform nucleus by posterior limb of internal capsule. Many fibers stream out of this surface and enter internal capsule en route for cerebral cortex and form the thalamic radiation.
Functions of Thalamus
- 1.
Relay station
- a.
Most somatic sensory pathways except olfaction
- b.
Few motor pathways (e.g., cerebellar)
- a.
- 2.
Integrating center
For impulses from many sources (e.g., somatic sensory; visual; visceral; some motor, e.g., cerebellar, corpus striatum)
- 3.
Maintenance and regulation of state of:
Consciousness, alertness, attention (through influence upon cerebral cortex)
- 4.
Emotional connotations
(Which accompany most sensory experiences?)
- 5.
Crude sensations
(For example, pain which may reach consciousness at this level even when all connections between thalamus and cortex are destroyed.)
Thalamus is not simply a relay station where information is passed onto the neocortex but thalamus acts as receptionist for information to the cerebral cortex, preventing or enhancing the passage of specific information depending on behavioral state of the individual. Though it has more than 50 nuclei, however, classically they are divided into four groups depending on their position in relation to the internal medullar lamina.
Anterior group is connected from mammillary bodies and subiculum of the hippocampal formation and closely associated with the limbic system ( Fig. 1.9 ). This connection is concerned with emotional tone and mechanism of recent memory. Stimulation or ablation of mammillothalamic tract causes alteration in autonomic control and loss of recent memory.
The medial group receives input from basal ganglion, amygdala, and midbrain and its major output is to the frontal cortex ( Fig. 1.9 ). It provides mechanisms for the integration of certain somatovisceral impulses projecting to prefrontal cortex. It mediates impulses of an affective nature which contributes to the formation of personality. Stimulation, disease, or surgical ablation of medial nuclei results in changes in (1) motivational drive, (2) ability to solve problems, (3) consciousness level, (4) general personality, (5) subjective feeling status (affective tone), (6) pain perception (indifference to pain), (7) emotional content.
The ventral anterior and anterior lateral nuclei transmit information basal ganglia and cerebellum to the motor cortex ( Fig. 1.9 ). These make important contribution to initiation of movements, control of muscle tone, regulation of cortical reflexes.
The posterior group includes medial and lateral geniculate nucleus, lateral posterior nuclei, and pulvinar ( Fig. 1.9 ). Lateral geniculate nucleus receives information from the retina and conveys to the primary visual cortex; medial geniculate nucleus is a component of the auditory system.
The nonspecific projecting nuclei are located either in the midline or within the internal medullary lamina. The largest intralaminar nuclei, centromedial nucleus, is projected to amygdala, hippocampus, and basal ganglia.
Hypothalamus
Hypothalamus is a part of the diencephalon which forms lateral wall and floor of the third ventricle. Laterally it is in contact with internal capsule and ventral thalamus; posteriorly it merges with ventral thalamus and tegmentum of the midbrain; anteriorly it extends up to lamina terminalis; and inferiorly it is related with structures in the floor of the third ventricle (tuber cinereum, infundibulum, and mammillary bodies).
Hypothalamus receives afferent from retina, frontal lobe, hippocampus, corpus striatum, and reticular formation of brain stem. It has efferent to supraopticohypophyseal tract from optic nuclei to the posterior pituitary, pars tuberalis, and pars intermedius. Thus posterior pituitary is brought under the retinal control. It also has efferent to mammillothalamic tract and mammillotegmental tract. The Hypothalamus is also subdivided anterior-posteriorly into three regions ( Box 1.2 ) .
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