Brain Anatomy

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Brain anatomy

Definition

The brain is a large mass of soft nervous tissue made up of both neurons and supporting glial cells lying within the cranium of the skull. The brain contains both gray and white matter. Gray matter is primarily nerve cell bodies, whereas white matter contains myelinated nerve cell processes, giving it a white appearance. White matter is mostly found in the cortex (shell) of the cerebral hemispheres. The brain has a highly complex appearance, with convolutions referred to as gyri and valleys referred to as sulci. These convolutions create a greater surface area within the same size skull.

Description

Central nervous system

The central nervous system is made up of the brain and spinal cord. The major divisions of the human brain are the brainstem, cerebellum , diencephalon , and cerebral hemispheres. The meninges cover and protect the brain and spinal cord.

BRAINSTEM The brainstem, made up of the midbrain, pons, and medulla, sits at the base of the brain. The brainstem is involved in sensory input and motor output. Sensory input enters the brainstem from the head, neck, and face area, while motor output from the brainstem controls muscle movements in these areas as well. The brainstem also receives sensory input from specialized cranial

nerves for olfaction (smell), vision, hearing, gustation (taste), and balance. The brainstem contains ascending and descending nerve pathways that carry sensory input and motor output information to and from higher brain regions, like a relay center. Ascending nerve pathways bring information through the brainstem into the rest of the brain, and descending nerve pathways send information back that coordinates many activities, including motor function. The brainstem also plays a role in vital functions such as cardiovascular and respiratory activity and consciousness.

The medulla is a structure in the brainstem closest to the spinal cord. It is vaguely scoop shaped, with longitudinal grooves indicating the presence of many nerve tracts. It is responsible for maintaining vital body functions such as breathing and heart rate.

The pons is named after the Latin word for bridge. In appearance, the pons seems to be a bridge connecting the two hemispheres, but in reality the connection is indirect through a complicated nerve pathway. The pons is involved in motor control, sensory analysis, and levels of consciousness and sleep. Some structures within the pons are linked to the cerebellum, involving them in movement and posture.

The midbrain, also called the mesencephalon, is the smallest and most anterior part of the brainstem with a tubular appearance. It is involved in functions such as vision, hearing, movement of the eyes, and body motor function. The anterior part of the midbrain contains the cerebral peduncle, a large bundle of axons traveling from the cerebral cortex through the brainstem. These nerve fibers (along with other structures) are important for voluntary motor function.

CEREBELLUM The cerebellum, or "little brain," wraps around the brainstem. It is similar to the cerebrum in that it has two hemispheres with a highly folded surface (cortex). The cerebellum is involved in regulation and coordination of movement, posture, balance, and also some cognitive function.

DIENCEPHALON The diencephalon, or "between brain," lies between the cerebral hemispheres and the midbrain. It is formed by the thalamus and hypothalamus, and has connections to the limbic system and cerebral hemispheres.

The thalamus is a large body of gray matter at the top of the diencephalon, positioned deep within the forebrain. The thalamus has sensory and motor functions. Almost all sensory information enters this structure, where it is relayed to the cortex. Axons, or nerve endings, from every sensory system except olfaction come together (synapse) here as the last relay site before the information reaches the cerebral cortex. The synapse is the junction where nerve endings meet and communicate with each other using chemical messengers that cross the junction.

The hypothalamus is a part of the diencephalon lying next to the thalamus. The hypothalamus is involved in homeostasis, emotional responses, coordinating drive-related behavior such as thirst and hunger, circadian rhythms, control of the autonomic nervous system, and control of the pituitary gland.

MENINGES AND VENTRICULAR SYSTEMS The meninges are membranes that cover and protect the central nervous system (CNS) along with a fluid called cerebrospinal fluid (CSF) that buoys up the brain. The brain is very soft and mushy; without the meninges and CSF, it would be easily distorted and torn under the effects of gravity. The meninges are divided into three membranes: the thick external dura mater provides mechanical strength; the middle web-like, delicate arachnoid mater forms a protective barrier and a space for CSF circulation; and the internal pia mater is continuous with all the contours of the brain and forms CSF. The dura mater contains six major venous sinuses that drain the cerebral veins and several smaller sinuses.

Dural venous sinuses are formed in areas where the two layers of the dura mater separate, forming spaces. The sinuses are triangular in cross-section and lined with endothelium. There are six major dural sinuses that receive cerebral veins. The superior sagittal sinus, straight sinus, and right and left transverse sinuses meet in a structure known as the confluence of the sinuses. Venous blood circulation follows a pathway through the superior sagittal and straight sinuses into the confluence, and then through the transverse sinuses. Each transverse sinus then continues as a sigmoid sinus, carrying the venous blood flow along an S-shaped course until it empties into the internal jugular vein. The major dural sinuses also connect with several smaller sinuses. The inferior sagittal sinus, occipital sinus, and superior and inferior petrosal sinuses all empty into different parts of the major sinus system.

The arachnoid mater follows the general shape of the brain, creating a space between the two membranes. The space between the arachnoid and pia mater is called the subarachnoid space and contains CSF. CSF enters venous circulation through small protrusions into the venous sinus called arachnoid villi. The pia mater forms part of the choroid plexus, a highly convoluted and vascular membranous material that lies within the ventricular system of the brain and is responsible for most CSF production.

The brain contains four ventricles. A pair of long, C-shaped lateral ventricles lies within the cerebral hemispheres. The lateral ventricles communicate with the narrow, slit-shaped third ventricle of the diencephalon. The third ventricle then communicates with the tent-shaped fourth ventricle of the pons and medulla, which protrudes into the cerebellum. The CSF of the brain flows in a specific pattern that allows newly formed CSF to replace the old CSF several times a day. The basic pattern of circulation is formation in lateral ventricles, flow into the third and then fourth ventricles, into basal cisterns, up and over the cerebral hemispheres, into the arachnoid villi, where drainage occurs into a venous sinus to return to the venous system. Some CSF diverts from the basal cisterns into the subarachnoid space of the spinal cord. Blockage of the circulation of CSF can cause a condition called hydrocephalus , where the CSF pressure rises high enough to expand the ventricles at the sacrifice of the surrounding brain. Blockage of CSF circulation can occur at any point in the pathway. Hydrocephalus conditions are divided into two types, communicating and noncommunicating. The classification depends on whether both lateral ventricles are in communication with the subarachnoid space. Noncommunicating hydrocephalus involves blockage in the ventricular system, which prevents the flow of CSF to the subarachnoid space. Tumors sometimes cause hydrocephalus, through instigating either overproduction or physical obstruction of CSF. CSF circulation may also be obstructed in the subarachnoid space by adhesions that form as a result of meningitis.

CEREBRAL HEMISPHERES The cerebral hemispheres are made up of the cerebral cortex, hippocampus, and basal ganglia containing the amygdala of the limbic system. The cerebral hemispheres are divided by the interhemispheric fissure and are involved in higher motor functions, perception, cognition (pertaining to thought and reasoning), emotion, and memory. The cerebral cortex is divided into four major lobes. The frontal lobe contains the primary motor cortex and premotor area involved in voluntary movement, Broca's area involved in writing and speech, and the prefrontal cortex involved in personality, insight, and foresight. The parietal lobe contains the primary somatosensory cortex involved in tactile and positioning information, while remaining sections are involved in spatial orientation and language comprehension. The temporal lobe contains the primary auditory cortex, Wernicke's area involved in language comprehension, and areas involved in the higher processing of visual input, along with aspects of learning and memory associated with the limbic system. The occipital lobe contains the primary visual cortex and the visual association cortex.

The limbic lobe is a subdivision consisting of portions of the frontal, parietal, and temporal lobes that form a continuous band called the limbic system.

The limbic system, buried within the cerebrum, is also referred to as the "emotional brain." It includes the thalamus, hypothalamus, amygdala, and hippocampus. Through these structures, the limbic system is involved in drive-related behavior, memory, and emotional responses such as feeding, defense, and sexual behavior. The thalamus and hypothalamus are parts of the diencephalon, while the amygdala and hippocampus are parts of the cerebral hemispheres.

The left and right cerebral hemispheres are not equal in their functionality. In the human brain, the left hemisphere is more important for the production and comprehension of language than the right hemisphere. Damage to the left hemisphere is more likely to cause language deficits than damage to the right hemisphere. Because of this variation in hemisphere contribution, the left hemisphere is most commonly referred to as the dominant hemisphere and the right hemisphere is referred to as the nondominant hemisphere. Nearly all right-handed people and most left-handed people have a left-dominant brain. However, some people have a right-dominant brain or comparable language representation in both hemispheres.

The hippocampus is a curved sheet of cortex folded in the basal medial part of the temporal lobe. It is divided into three multilayered sections, the dentate gyrus, hippocampus proper, and the subiculum acting as a transitional zone between the two. The dentate gyrus receives input from the cortex, and sends output to the hippocampus proper. The hippocampus proper then sends output to the subiculum, which is the principal source of hippocampal output. The hippocampus, referred to as the gateway to memory, is involved in learning and memory functions. The hippocampus converts short-term memory to more permanent memory, is involved in the storage and retrieval of long-term memory, and recalling learned spatial associations.

The basal ganglia are masses of gray matter located deep in the cerebral hemispheres. The basal ganglia contain the corpus striatum, which is involved mostly in motor activity. The striatum is the major point of entry into basal ganglia circuitry, receiving input from almost all cortical areas. It is subdivided into three further divisions called the caudate nucleus, putamen, and globus pallidus. The caudate nucleus is involved more with cognitive function than with motor function. Of all the striatum subdivisions, the putamen is the most highly associated with motor functions of the basal ganglia. The globus pallidus is a wedge-shaped section of the striatum responsible for most basal ganglia output. The basal ganglia also contain the amygdala, a portion of the limbic system involved in memory, emotion, and fear. The amygdala lies beneath the surface of the temporal lobe where it causes a bulge called the uncus. The basal ganglia collectively modulate the output of the frontal cortex involving motor function, but also cognition and motivation.

SPINAL CORD The spinal cord is a cord-like bundle of nerves comprising a major part of the central nervous system, which conducts sensory and motor nerve impulses to and from the brain and the periphery. It is a long tube-like structure extending from the base of the brain, through a string of skeletal vertebrae, to the small of the back. The spinal cord is continuous with the brainstem, and like the brain, it is encased in a triple sheath of membranes. Thirty-one pairs of spinal nerves belonging to the peripheral nervous system (PNS) arise from the sides of the spinal cord and branch out to both sides of the body. In addition to carrying impulses to and from the brain, the spinal cord regulates reflexes. Reflexes produce a rapid motor response to a stimulus because the sensory neuron synapses directly with the motor neuron in the spinal cord, so the impulse does not need to travel to and from the brain.

NERVOUS TRACTS Tracts are groups or bundles of nerve fibers that constitute an anatomical and functional unit. Commissural tracts such as the corpus callosum connect the two cerebral hemispheres. Association tracts make connections within the same hemisphere. Projection tracts connect the brain with the spinal cord. Sensory tracts project upward from the spinal cord into regions of the brain, bringing sensory input from the periphery via ascending pathways. Motor tracts project down from the brain into the spinal cord, bringing motor output information to the periphery via descending pathways. The internal capsule is the major structure carrying ascending and descending nerve projection fibers to and from the cerebral cortex. It is a curved, funnel-shaped group of cortical projection fibers divided into five regions, based on each region's relationship to the putamen and globus pallidus of the striatum.

Peripheral nervous system

The peripheral nervous system (PNS) is all of the nervous system outside the brain and spinal cord, including the spinal and cranial nerves. The PNS is divided into the somatic and autonomic subdivisions. The somatic nervous system, regulating activities that are under conscious control such as the voluntary movement of skeletal muscles, includes the spinal and cranial nerves and peripheral sensory receptors. Peripheral neurons that transmit information from the periphery toward the CNS are called afferent neurons, whereas those that transmit information away from the CNS toward the periphery are called efferent neurons.

The 31 pairs of spinal nerves are each named according to the location of their respective vertebrae. Each spinal nerve consists of a dorsal root and a ventral root. The dorsal roots contain afferent neurons transmitting information to the CNS from various kinds of sensory neurons. The ventral roots contain the axons of efferent motor neurons transmitting information to the periphery. Information travels great distances via interneurons, which are neurons that connect neurons to each other. Spinal nerves have sensory fibers and motor fibers. The sensory fibers supply nerves to specific areas of skin, while the motor fibers supply nerves to specific muscles. A dermatome, which means "skin-cutting," is an area of skin supplied by nerve fibers originating from a single dorsal nerve root. The dermatomes are named with respect to the spinal nerves that supply them. Dermatomes form bands around the body. In the limbs, dermatome organization is more complex as a result of being "stretched out" during embryological development. There is a high degree of overlap of nerves between adjacent dermatomes. If one spinal nerve loses sensation from the dermatome that it supplies, compensatory overlap from adjacent spinal nerves occurs with reduced sensitivity. In addition to dermatomes supplying the skin, each muscle in the body is supplied by a particular level or segment of the spinal cord and by its corresponding spinal nerve. The muscle, in conjunction with its nerve, makes up a myotome. Although slight variations do exist, dermatome and myotome patterns of distribution are relatively consistent from person to person.

Cranial nerves also carry sensory information from the periphery to the brain, and motor information away from the brain to the periphery. Humans have 12 pairs of cranial nerves numbered by the level at which they enter the brain. Seven of the cranial nerves specialize in information about olfaction, vision, hearing, gustation, and balance. The other cranial nerves control eye and mouth movements, swallowing, and facial expressions. Cranial nerve X is called the vagus nerve; it has effects on visceral gut function and has the ability to slow the heart when stimulated through the parasympathetic nervous system.

The autonomic nervous system includes further sympathetic, parasympathetic, and enteric subdivisions. The autonomic nervous system regulates activities that are not under conscious control but rather are involuntary, such as contractions of the heart and digestion of food. The autonomic nervous system is involved in maintaining homeostasis in the body. The sympathetic and parasympathetic subdivisions of the autonomic nervous system have opposite effects on the organs they control. Most organs controlled by the autonomic nervous system are under the influence of both the sympathetic and parasympathetic nervous systems, which strike a balance with each other to maintain proper body function. The sympathetic nervous system generally stimulates organs, whereas the parasympathetic nervous system generally suppresses organ function or slows it down. An example of this coordination of activity is seen in the fight-or-flight response, which is the body's response to a sudden threatening or stressful situation in which excessive energy is needed to either deal with such an attack or run from it. In the fight-or-flight response, both the sympathetic and parasympathetic nervous systems work in coordination with each other to produce the appropriate results. The sympathetic and parasympathetic nervous systems increase blood pressure and heart rate and slow digestion to enable the physical exertion necessary to respond to the threatening circumstance.

The digestive system contains its own, local nervous system referred to as the enteric, or intrinsic, nervous system. The enteric nervous system is extremely complex and contains as many neurons as does the spinal cord. The enteric nervous system is divided into two networks, or plexuses, of neurons, both of which are embedded in the walls of the digestive tract and extend from the esophagus to the anus. The myenteric plexus is located between the longitudinal and circular layers of muscle in the tunica muscularis and is involved in digestive tract motility. The submucous plexus lies buried in the submucosa. Its principal role is regulating gastrointestinal blood flow and controlling epithelial cell function in response to the environment within the lumen. In regions where these functions are minimal, such as the esophagus, the submucous plexus is sparse. The enteric nervous system functions independently from other nervous systems, but normal digestive function requires communication between the enteric system, other PNS systems, and the CNS. Stimulation of the sympathetic nervous system causes inhibition of gastrointestinal secretions and motor activity, while the parasympathetic nervous system stimulates the same functions. Parasympathetic and sympathetic fibers connect either the central and enteric nervous systems or connect the CNS directly within the digestive tract. In this manner, the digestive system provides sensory information to the CNS, and the CNS is involved in gastrointestinal function. The CNS can also relay input from outside of the digestive system to the digestive system. An example is the sight or smell of food stimulating stomach secretions.

Resources

BOOKS

Nolte, John, PhD. The Human Brain: An Introduction to Its Functional Anatomy. St. Louis, MO: Mosby, Inc., 2002.

Thomas, Clayton L., MD, MPH, ed. Taber's Cyclopedic Medical Dictionary. Philadelphia, PA: F. A. Davis Company, 1993.

Zigmond, Michael J., PhD, Floyd E. Bloom, MD, Story C. Landis, PhD, James L. Roberts, PhD, and Larry R. Squire, PhD, eds. Fundamental Neuroscience. New York: Academic Press, 1999.