Chapter 15 - The Brain

 

Development of the Brain
      The central nervous system (CNS) begins as a hollow neural tube with a fluid-filled internal cavity. In the fourth week of development the cephalic (head) portion of the tube expands to form three primary brain vesicles:
  1. Prosencephalon – forebrain
  2. Mesencephalon – midbrain
  3. Rhombencephalon – hindbrain
     As development proceeds, the prosencephalon divides into two new expanded portions called secondary brain vesicles:  
  a. Telencephalon – This portion develops into the cerebral hemispheres.
  b. Diencephalon – This portion retains the name diencephalon and subdivides into the  epithalamus, thalamus and hypothalamus. The eyes will also develop from optic vesicles that extend laterally from the diencephalons.
     The mesencephalon does not divide and is also called the midbrain.
     The rhombencephalon undergoes subdivision into the:
  a. Metencephalon – The portion is next to the midbrain. The ventral portion of the metencephalon develops into the pons and the dorsal portion expands to form the cerebellum (“little cerebrum”).
  b. Myelencephalon – The portion closest to the spinal cord forms the medulla oblongata.
     The adult brain has 6 major landmarks: 1. cerebrum, 2. diencephalon, 3. mesencephalon, 4. pons, 5. cerebellum, 6. medulla oblongata. The mesencephalon, pons and medulla oblongata are often collectively referred to as the brainstem.
 
Ventricles of the Brain
     The fluid-filled interior of the embryo’s neural tube becomes the fluid-filled ventricles of the adult brain. The ventricles are lined by ependymal cells and are filled with cerebrospinal fluid (CSF). There are four ventricles in the brain:
       The first and second ventricles are contained within the cerebral hemispheres and have a complex shape due to the expansion of the cerebral hemispheres. These ventricles are called the lateral ventricles because they are side-by-side, separated in the middle by a thin partition called the septum pellucidum. Each lateral ventricle communicates (in the sense of being interconnected) with the third ventricle by the interventricular foramina (sing. foramen).
       The third ventricle is within the diencephalons and communicates with the fourth ventricle by a narrow passage within the mesencephalon called the aqueduct of the midbrain (a.k.a. aqueduct of Sylvius, or, cerebral aqueduct).
       The fourth ventricle is between the pons and the cerebellum and within the superior portion of the medulla oblongata. The fourth ventricle communicates with the central canal of the spinal cord.
 
Cranial Meninges
     The brain is protected from damage by the cranium but also needs protection from the hard interior of the skull during sudden changes in movement. The cranial meninges provide this protection by securing the brain in position inside the fluid-filled space created by the cerebrospinal fluid.
     The cranial meninges have three layers:
  Dura Mater
  The dura mater is the outermost layer and consists of two fibrous layers:
  1. Endosteal Layer – fused to the periosteum lining the cranial bones.
  2. Meningeal Layer – the innermost layer.
   In certain areas the endosteal and meningeal layers separate and are lined by endothelium to form the dural sinuses.
  At four locations the dura mater extends into the cranial cavity to provide greater support for the brain:
  1. Falx cerebri – projects in between the cerebral hemispheres along the longitudinal fissure. The superior sagittal sinus and inferior sagittal sinus are found within this fold.
  2. Tentorium cerebelli – separates and protects the cerebellar hemispheres from those of the cerebrum. The transverse sinus is found within the tentorium cerebelli.
  3. Falx cerebelli – extends from the midsagittal line and divides the cerebellar hemispheres.
  4. Diaphragma sellae – is an extension of the dura mater from the rim of the depression (hypophyseal fossa) formed by the sella turcica. It surrounds the stalk that attaches the pituitary gland to the hypothalamus.
  Arachnoid Mater
  The outermost part of the arachnoid mater lines the inner surface of the dura mater. From this lining a web-like mesh of collagen and elastic fibers extend across a fluid-filled space, called the subarachnoid space, and attach to the pia mater. Blood vessels are found in the subarachnoid space.
  Along the superior sagittal sinus, arachnoid extends through the meningeal layer of the dura mater and projects into the sinus. These projections are called arachnoid granulations and are places where CSF flows into the sinus.
  Pia Mater
  The pia mater is a membrane that is tightly attached to the surface of the brain by astrocytes. The pia mater follows the contour of the brain and forms the floor of the subarachnoid space and supports the cerebral vessels found there.
 
The Blood-Brain Barrier (BBB)
     The BBB protects the nervous tissue from substances in the blood that would interfere with its normal activity. For example, glycine, which is a neurotransmitter, is selectively transported out of the nervous tissue into the blood.
     At the same time the BBB protects the proper working of the nerves by selectively transporting materials from the blood into the nervous tissue. An example of this is glucose, the preferred fuel for nerve cells.
     The BBB is created by the tight junctions between the endothelial cells and the association of the endothelial cells with astrocytes.
 
Cerebrospinal Fluid (CSF)
    The CSF completely surrounds and bathes the surfaces of the central nervous system. Its functions include:
  1. Cushioning the delicate neural tissue.
  2. Supporting the physical mass of the brain as the brain essentially floats in a fluid-filled container.
  3. Transporting nutrients, chemical messengers and waste products.
  Formation of CSF
     The CSF originates from the choroid plexus. The choroids plexus consists of permeable capillaries associated with specialized ependymal cells that have tight junctions between them.
     Choroid plexuses are found in:
  a. The roof of the third ventricle and floors of the lateral ventricles. Folds that originate in the third ventricle extend in the lateral ventricles through the interventricular foramina.
  b. The roof of the fourth ventricle and extending between the cerebellum and pons.
     The ependymal cells of the choroids plexus control the composition of the CSF by active transport mechanisms.
  Circulation of CSF
     CSF is continually produced in the choroids plexus at the rate of 500 ml/day. The CSF that reaches the fourth ventricle enters the subarachnoid space by passing through the lateral apertures (openings) and median aperture of the roof of the 4th ventricle. The CSF circulates through the subarachnoid space surrounding the brain and spinal cord. The CSF finally reenters the circulation through the arachnoid granulations.
 
Cerebrum
     The cerebrum is the largest region of the brain.
     The many functions of the cerebrum include:
  1. Controlling thought processes and intellectual functions
  2. Processing somatic sensory and motor information
  3. Exerting voluntary and involuntary control over somatic motor neurons
  Cerebral Hemispheres
     The cerebrum consists of paired cerebral hemispheres. The gray matter on the surface is called cerebral cortex. The cerebral cortex is where the most complicated higher level functions of the brain are performed. The surface area of the cerebral cortex is about 2200 cm2 (2.5 ft2) and fits into the cranium because of its crumpled appearance. This results in the cortex being thrown into folds and creases.
      The folds of the cortex are called gyri (sing. gyrus) and the creases or grooves are called sulci (sing. sulcus). The deeper sulci are called fissures.
  Cerebral Lobes
     A deep longitudinal fissure separates the two cerebral hemispheres and each cerebral hemisphere is divided into lobes whose names relate to the overlying skull bones:
  1. Frontal lobe is delimited by the central sulcus and the lateral sulcus.
  2. Temporal lobe is inferior to the lateral sulcus.
  3. Parietal lobe extends posteriorly from the central sulcus to the parieto-occipital sulcus.
  4. Occipital lobe is the region posterior to the parieto-occipital sulcus.
     Surfaces of the cerebral cortex that are hidden from view include the:
  Insula – This region of cortex is concealed inside the lateral sulcus by overlying folds of the temporal, frontal and parietal lobes (It is almost as if the surface were sucked inward.)
  Cingulate lobe – The cingulated lobe borders the corpus callosum and is only visible in the hemisected, sagittal view of the brain.
     Three important points about the function of the cerebral hemispheres are:
  1. Each cerebral hemisphere is concerned with the sensory and motor functions of the opposite side of the body.
  2. Some functions are present predominantly in one cerebral hemisphere. This phenomenon is described as laterality.
  3. The assignment of a specific function to a specific region of the cortex is imprecise.
  Motor and Sensory Areas
  The central sulcus separates the primary motor and sensory areas of the cortex.
  The primary motor cortex is found on the precentral gyrus of the frontal lobes. Neurons called pyramidal cells that direct muscle contraction originate here.
  The primary somatosensory cortex is found on the postcentral gyrus of the parietal lobes. Neurons here receive somatic sensory information including, touch, pressure, tissue damage (pain) and temperature.
  The visual cortex of the occipital lobes processes visual information.
  The auditory cortex and olfactory cortex is found in the temporal lobes and processes hearing and smell, respectively.
  The gustatory cortex lies in the anterior portion of the insula and adjacent parts of the frontal lobes and processes taste information.
  Association Areas
  The association areas are where the individual bits of information or individual commands are brought together (associated) for interpretation and coordination.
  The somatosomatic sensory association areas in the parietal lobes use the somatic sensory input received by the somatic cortex to interpret size, form and texture.
  The somatic motor association areas (a.k.a. premotor cortex) in the frontal lobes plan and coordinate movements involving different muscle groups.
  The visual association areas in the occipital lobes interpret the individual points of light as lines, contours and colors.
  Integrative Centers
     The integrative centers brings together (integrates) information and commands from different association areas to perform higher order analytical functions. For example, the prefrontal cortex of the frontal lobe enables you to predict the consequences of different behaviors, plan future behaviors and to perform tasks using working memory. 
     The functions of integrative centers include those devoted to language production (Broca’s area), language comprehension (Wernicke’s area), mathematical computation, and spatial awareness. These centers are located predominantly in one cerebral hemisphere. For example, language production and comprehension are located in the left cerebral hemisphere in the vast majority of people.
  Central White Matter
     Different regions of the surface cerebral cortex communicate with each other and with other regions of the CNS by myelinated fibers that form the central white matter of the cerebrum.
     The myelinated fibers form bundles of fibers that can be divided into three types:
  1. Association fibers – interconnect portions of the cerebral cortex with those in the same cerebral hemisphere.
  2. Commissural fibers – cross from one hemisphere to the other and permit communication between them. The corpus callosum and the anterior commissure are examples.
   3. Projection fibers – connect the cerebral cortex with other parts of the CNS including diencephalon, brain stem, cerebellum and spinal cord.
  Basal Ganglia (a.k.a. Basal Nuclei)
     The basal ganglia are actually nuclei that are found in the cerebral hemispheres, diencephalon and midbrain. The basal ganglia include:
  1. Caudate Nucleus (“tailed nucleus”)
  2. Putamen (“husk”)
  3. Globus Pallidus (“pale globe”)
  4. Subthalamus (below the thalamus)
  5. Substantia Nigra (“black substance”)
     The caudate nucleus and putamen are similar histologically and are together called the striatum because of the myelinated fibers that run through them.
     The putamen and globis pallidus are together referred to as the lentiform (lenticular) nucleus because together they reminded the anatomist of a lens.
     The functions of the basal ganglia are not completely understood but with the cerebral cortex and thalamus they form circuits which influence affect (emotions), memory, cognitive (thinking) functions and sensorimotor functions
     The best know diseases associated with the basal ganglia are:
  Parkinson’s disease – A disease manifested by involuntary tremors, bradykinesia and abnormalities in muscle tone and caused by death of cells in the substantia nigra that secrete the neurotransmitter dopamine.
  Huntington’s chorea – A disease whose signs include involuntary movements and dementia.
  Limbic System
     The limbic system consists of cerebral and diencephalic components with their interconnecting tracts. The name limbic (border) derives from the fact that these components are located around the border between the cerebrum and diencephalon.
     The functions of the limbic system include:
  1. Establishment of emotional states and behavioral drives.
  2. Linking the conscious, intellectual functions of the cerebral cortex with unconscious and autonomic functions.
  3. Facilitating memory storage and retrieval.
 
Diencephalon
     The diencephalon connects the cerebral hemispheres to the midbrain and includes the:
  Epithalamus
     The epithalamus forms the roof of the third ventricle. The anterior portion is membranous and is the location of the choroid plexus which protrudes into the third ventricle and extends by way of the interventricular foramina into the lateral ventricles.
     The posterior portion of the epithalamus contains the pineal gland which is involved with the control of sleep-wake cycles.
  Thalamus
     The thalamus consists of a pair of egg-shaped concentrations of nuclei that flank the third ventricle. The paired right and left thalami are often connected by a bridge of neural tissue called the interthalamic adhesion.
     The thalami have both sensory and motor functions. All sensory information, with the notable exception of olfaction, is processed by the thalami before being finally relayed to the cerebral cortex. In this position the thalami may act as information filters. The thalami also participate with the sensorimotor cortex and basal ganglia in the initiation, control and coordination of movement.
  Hypothalamus
     The hypothalamus forms the floor of the third ventricle and extends from the area superior to the optic chiasm to the posterior margins of the mammillary bodies. Posterior to the optic chiasm, the floor of the third ventricle forms a funnel-shaped extension called the infundibulum which connects the pituitary gland to the hypothalamus.
     Functions of the hypothalamus include:
  1. Control of Autonomic Function
  The hypothalamus regulates heart rate, blood pressure, respiration and digestive functions
  2. Control of Pituitary Gland Function
  Chemical messengers released by neurons in the tuberal area (between the optic chiasm and mammillary bodies) control secretion of hormones by the anterior pituitary.
  3. Secretion of Hormones
  Neurons of the hypothalamus secrete two hormones, antidiuretic hormone and oxytocin, which are released at the posterior pituitary.
  4. Production of Emotions and Drives
  Regions of the hypothalamus are associated with emotions such as aggression and contentment and drives such as hunger and thirst.
  5. Regulation of Body Temperature
  The "thermostat" that enables the nervous system to produce physiological responses that adjust body temperature is in the hypothalamus.
  6. Control of Circadian (Daily) Rhythms
  The hypothalamus receives input from the eyes that enable it to adjust daily rhythms.
 
Mesencephalon
     The cerebral aqueduct (a.k.a. mesencephalic aqueduct or aqueduct of Sylvius) runs through the mesencephalon, or midbrain. The region dorsal to the cerebral aqueduct is called the tectum ("roof") and the region below the aqueduct is called the tegmentum ("covering").
  Tectum
     The tectum contains two pairs of sensory nuclei that are collectively called the corpora quadrigemina (four "twin" bodies) that are functionally divided into two pairs:
  Superior colliculi (sing. colliculus)
  The superior colliculi receive visual inputs from the lateral geniculate nuclei of the thalamus and generates reflexive responses to this information that particularly involve eye movements.
  Inferior colliculi
  The inferior colliculi receive auditory inputs from the medial geniculate nuclei of the thalamus and generates reflexive responses to this information.
  Tegmentum
     The important nuclei found in the tegmentum include:
  Red nuclei
  The red nuclei appear red because of the numerous blood vessels the course within it. These nuclei are associated with the control of skeletal movements.
  Substantia nigra
  The substantia nigra ("black substance") consists of gray matter containing darkly pigmented cells that give rise to its name. The substantia nigra, as part of the basal nuclei, plays a role in the initiation and control of skeletal movement.
  Cerebral Peduncles (Crus Cerebri)
     The cerebral peduncles are located on the ventrolateral surfaces of the midbrain and contain primarily descending fibers including  motor commands descending to the spinal cord and sensory information going to the thalamus.
 
Pons
     The pons is a prominent bulge of the anterior surface of the brain stem. The pons is ventral to the anterior part of the 4th ventricle and is attached to the cerebellum by the middle cerebellar peduncles.
     The pons contains:
  1. Nuclei for Cranial nerves V, VI, VII and VIII
  2. Nuclei involved with the control of respiration
  3. Nuclei that process and transfer cerebral inputs to the opposite side of the cerebellum by means of transverse fibers.
  4. Ascending and descending tracts.
 
Cerebellum
     The cerebellum consists of two cerebellar hemispheres that contain neural cortex folded into a smaller space. The smaller folds are called folia ("leaves") deeper folds include the primary fissure that separates the cerebellar hemispheres into anterior and posterior lobes. The cerebellum is attached to the pons and the rest of the brain stem by the superior, middle and inferior cerebellar peduncles.
      The functions of the cerebellum include:
  1. Maintaining equilibrium
  2. Controlling eye movements
  3. Adjusting the postural muscles
  4. Programming and fine tuning movements
 
Medulla Oblongata
     The medulla oblongata is the part of the brainstem that connects to the spinal cord.
     The medulla oblongata includes:
  Nuclei that serve as relay stations along sensory and motor pathways. These nuclei include:
    Nucleus gracilis and nucleus cuneatus which pass somatic sensory information from the spinal cord to the thalamus.
    Olivary nuclei which are located deep to prominent bulges along the ventrolateral surface of the medulla called the olives. These nuclei relay information from the spinal cord, diencephalon, cerebral cortex and brain stem to the cerebellar cortex.
    Nuceli of cranial nerves N VIII to N XII.
    Autonomic nuclei that include:
  Cardiovascular centers that adjust and control heart functions and blood pressure.
  Respiratory rhythmicity center that affects the respiratory rate.
  Fibers that descend from higher centers to the spinal cord are found on the ventral surface of the medulla in structures called pyramids.