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A. Overview of Brain Organization I. Principal Parts of the Brain:
- The adult brain consists of about 100 billion neurons and 10-50 trillion neuroglia; it can be divided into four major regions (see Table 19.1): i. brain stem (continuous with the spinal cord), which consists of: a. medulla oblongata b. pons c. midbrain ii. cerebellum (posterior to the brain stem) iii. diencephalon (superior to the brain stem) , which consists of: a. thalamus b. hypothalamus c. epithalamus d. subthalamus iv. cerebrum (spreads over the diencephalon like a mushroom cap)
II. Protective Coverings of the Brain:
- The brain is protected by: i. cranial bones ii. cranial meninges (that are continuous with the spinal meninges): a. outer dura mater - three extensions of the dura mater separate parts of the brain:
- falx cerebri separates the twocerebral hemispheres
- falx cerebelli separates the two cerebellar hemispheres
- tentorium cerebelli separates the cerebrum from the cerebellum b. middle arachnoid mater c. inner pia mater iii. cerebrospinal fluid
III. Brain Blood Flow and the Blood-Brain Barrier:
- Blood supply to the brain is provided primarily by the internal carotid and vertebral arteries.
- The blood-brain barrier (BBB) regulates the passage of substances from the blood into brain tissue and thereby protects brain cells from harmful substances and pathogens; it is formed by: i. tight junctions that seal together the endothelial cells of brain capillaries ii. basement membrane that surrounds the brain capillaries iii. processes of astrocytes that press up against the capillaries
IV. Cerebrospinal Fluid Production and Circulation in Ventricles:
- The cerebrospinal fluid (CSF) : i. is a clear, colorless liquid that protects the brain and spinal cord against chemical and physical injuries; it carries oxygen, glucose, and other needed substances from the blood to neurons and neuroglia ii. circulates through the subarachnoid space and four ventricles which include: a. right and left lateral ventricles (separated anteriorly by the septum pellucidum ) located in the two cerebral hemispheres b. third ventricle located between the right and left halves of the thalamus c. fourth ventricle located between the brain stem and the cerebellum iii. contributes to homeostasis by performing three major functions: a. mechanical protection by serving as a shock-absorbing liquid b. chemical protection by providing an optimal chemical environment c. circulation , i.e., by exchanging nutrients and wastes between the blood and nervous tissue iv. is formed as blood flows through choroid plexuses located in the walls of ventricles; these networks of capillaries are covered by ependymal cells which produce the CSF from blood plasma by filtration and secretion v. does not contain harmful blood-borne substances due to the presence of the blood-cerebrospinal fluid barrier formed by tight junctions located between the ependymal cells vi. flows along the following route: a. CSF produced in the choroid plexuses of the lateral ventricles flows into the third ventricle via the interventricular foramina b. more CSF is added by the choroid plexus of the third ventricle c. the fluid then flows into the fourth ventricle via the aqueduct of the midbrain ( cerebral aqueduct) d. more CSF is added by the choroid plexus of the fourth ventricle e. CSF then flows into the subarachnoid space via: -one median aperture -two lateral apertures f. after flowing through the subarachnoid space and the spinal cord’s central canal, CSF is gradually reabsorbed into the blood through fingerlike extensions of the arachnoid called arachnoid villi that project into the dural venous sinuses, especially the superior sagittal sinus
V. Brain Stem consists of:
- medulla oblongata (or more simply the medulla ): i. It is a continuation of the upper part of the spinal cord and forms the inferior part of the brain stem; it lies just superior to the foramen magnum and extends upward about 3 cm to the inferior portion of the pons.
b. sensory fibers that pass from the medulla to the thalamus iii. The posterior portion of the midbrain is called the tectum and contains four rounded elevations: a. two superior colliculi that serve as reflex centers for movements of the eyes, head, and neck in response to visual and other stimuli b. two inferior colliculi that help relay auditory information from the ears to the thalamus and also serve as reflex centers for movements of the head and trunk in response to auditory stimuli iv. The midbrain also contains: a. the right and left substantia nigra which control subconscious muscle activities b. the right and left red nuclei which function with the cerebellum to coordinate muscle movements c. the medial lemniscus that extends through the medulla, pons, and midbrain; it conveys impulses for discriminative touch, proprioception, pressure, and vibrations from the medulla to the thalamus v. The midbrain also contains the nuclei of origin for cranial nerves III and IV.
- reticular formation: i. A large portion of the brain stem (medulla, pons, and midbrain) is a region called the reticular formation (which also extends into the spinal cord and the diencephalon); it consists of small areas of gray matter interspersed among small bundles of white matter. ii. It has both motor and sensory functions. iii. Its main motor function is to help regulate muscle tone. iv. Its main sensory function is to alert the cerebral cortex to incoming sensory signals. v. It contains reticular activating system (RAS) the which is responsible for maintaining consciousness and awakening from sleep; it does so by arousing the cerebral cortex in response to incoming sensory signals.
VI. Cerebellum:
- The cerebellum is located in the inferior and posterior aspects of the cranial cavity; it is posterior to the medulla and pons and inferior to the posterior portion of the cerebrum.
- It is separated from the cerebrum by the transverse fissure and by an extension of the cranial dura mater called the tentorium cerebelli.
- The cerebellum resembles a butterfly; it consists of a central constricted area called the vermis located between two cerebellar hemispheres.
- Each hemisphere consists of lobes that are separated by fissures; the lobes are: i. anterior lobe is concerned with subconscious movements of skeletal muscles ii. posterior lobe is also concerned with subconscious movements of skeletal muscles iii. flocculonodular lobe is concerned with the sense of equilibrium
Between the hemispheres is an extension of the cranial dura mater called the falx cerebelli.
- The cerebellar cortex consists of gray matter organized in a series of slender, parallel ridges called folia.
- Beneath the gray matter are tracts called arbor vitae that resemble branches of a tree.
- Within the underlying white matter are masses of gray matter called cerebellar nuclei which give rise to nerve fibers that transmit information from the cerebellum to other brain centers and to the spinal cord.
- The cerebellum is attached to the brain stem by three pairs of cerebellar peduncles: i. inferior cerebellar peduncles connect the cerebellum to the medulla; these peduncles contain axons extending from the inferior olivary nucleus of the medulla and from spinocerebellar tracts of the spinal cord to the cerebellum ii. middle cerebellar peduncles contain axons that extend from the pons to the cerebellum iii. superior cerebellar peduncles contain axons that extend from the cerebellum into the midbrain and to the thalamus
- The main functions of the cerebellum is to smooth and coordinate sequences of skeletal muscle contractions required for skilled movements and to regulate posture and balance.
VII. Diencephalon consists of
- Thalamus: i. The thalamus is superior to the midbrain, is about 3 cm long, and constitutes about 80% of the diencephalon. ii. It consists of paired oval masses of mostly gray matter organized into nuclei with interspersed tracts of white matter. iii. In about 70% of human brains, the right and left portions of the thalamus are connected by a bridge of gray matter called the intermediate mass (interthalamic adhesion) that crosses the third ventricle. iv. The thalamus is the major relay station for sensory impulses transmitted to the cerebral cortex from the spinal cord, brain stem, cerebellum, and other parts of the cerebrum. v. The thalamus: a. provides crude perception of pain, temperature and pressure b. mediates motor functions by transmitting information from the cerebellum and basal ganglia to the primary motor area in the cerebral cortex c. helps regulate autonomic activities and the maintenance of consciousness d. is connected to the cerebral cortex by the internal capsule (which also connects the cerebral cortex to the brain stem and spinal cord) vi. The gray matter of the thalamus is divided by the internal medullary lamina which separates the thalamic nuclei into six major groups: a. anterior nuclei b. medial nuclei
- Circumventricular organs i. The circumventricular organs (CVOs) are located in the walls of the third and fourth ventricles. ii. They lack the blood-brain barrier and therefore can monitor chemical changes in the blood. iii. CVOs include part of the hypothalamus, the pineal gland, pituitary gland, and several other nearby structures.
VIII. Cerebrum:
- The cerebrum is the “seat of intelligence”, performing many mental tasks such as reading, writing, speaking, calculating, composing, imagining, etc.
- The surface of the cerebrum is composed of gray matter (2 to 4 mm thick) and is called the cerebral cortex; beneath the cortex is the cerebral white matter which contains gray matter nuclei.
- The cortex contains folds called gyri or convolutions; deep grooves between folds are called fissures and shallow grooves between folds are called sulci.
- The longitudinal fissure separates the cerebrum into right and left cerebral hemispheres; the hemispheres are connected internally by the corpus callosum.
- Between the hemispheres is an extension of the cranial dura mater called the falx cerebri.
- Lobes of the cerebrum: i. Each cerebral hemisphere is subdivided into four lobes that are named after the overlying bones: a. frontal lobe b. parietal lobe c. temporal lobe d. occipital lobe ii. The central sulcus separates the frontal lobe from the parietal lobe; a. anterior to the central sulcus is the precentral gyrus which contains the primary motor area of the cerebral cortex b. posterior to the central sulcus is the postcentral gyrus which contains the primary somatosensory area of the cerebral cortex iii. The lateral cerebral sulcus separates the frontal lobe from the temporal lobe. iv. The parieto-occipital sulcus separates the parietal lobe from the occipital lobe. v. The transverse fissure separates the cerebrum from the cerebellum. vi. A fifth part of the cerebrum, the insula, is located deep within the lateral cerebral sulcus, under the parietal, frontal, and temporal lobes.
- Cerebral White Matter:
i. The white matter underlying the cortex consists of myelinated and unmyelinated axons in three types of tracts: a. association tracts b. commissural tracts including the corpus callosum , anterior commissure , and posterior commissure c. projection tracts
- Basal Ganglia: i. The basal ganglia are several pairs of nuclei located in the cerebral hemispheres. ii. The largest nucleus is the corpus striatum , which consists of: a. caudate nucleus b. lenticular nucleus , which consists of: - lateral putamen -medial globus pallidus iii. Other structures that are functionally linked to the basal ganglia are the substantia nigra and the subthalamic nuclei. iv. The basal ganglia control subconscious movements of skeletal muscles, such as swinging the arms while walking. v. The globus pallidus helps regulate the muscle tone required for specific body movements. vi. The basal ganglia also help initiate and terminate some cognitive processes and may help regulate emotional behaviors.
- Limbic System: i. The limbic system is a ring of structures on the inner border of the cerebrum and floor of the diencephalon that encircle the upper part of the brain stem and the corpus callosum. ii. Among its components are the following structures: a. limbic lobe, which includes the parahippocampal and cingulate gyri and the hippocampus b. dentate gyrus c. amygdala d. septal nuclei e. mammillary bodies of the hypothalamus f. anterior nucleus and medial nucleus of the thalamus g. olfactory bulbs h. fornix, stria terminalis, stria medullaris, medial forebrain bundle, and mammillothalamic tract are linked by bundles of interconnecting myelinated axons iii. The limbic system functions in emotional aspects of behavior related to survival; it also functions in olfaction and memory.
- Functional Organization of the Cerebral Cortex: i. Specific types of sensory, motor, and integrative signals are processed in certain cerebral regions. ii. Sensory areas receive and interpret sensory information; they include: a. primary somatosensory area (areas 1,2, and 3) b. primary visual area (area 17) c. primary auditory area (areas 41 and 42)
B. Cranial Nerves
- The12 pairs of cranial nerves are part of the peripheral nervous system.
- All cranial nerves travel through foramina of the skull; 10 pairs originate from the brain stem.
- The cranial nerves are designated by: i. roman numerals which indicate the order in which the nerves arise from the brain from anterior to posterior ii. names which indicate the distribution or function
- Two cranial nerves (I and II) contain only sensory fibers and are therefore called sensory nerves.
- The other cranial nerves contain both sensory and motor fibers and are therefore called mixed nerves; some of the mixed nerves are primarily motor in function.
- The cell bodies of sensory fibers are located in ganglia outside the brain.
- The cell bodies of motor fibers are located in nuclei within the brain; some cranial nerves include both somatic motor and parasympathetic fibers of the autonomic nervous system.
- The individual names (and roman numeral designations) of the 12 pairs of cranial nerves and their major characteristics are: i. olfactory (I) nerve: a. entirely sensory b. transmits nerve impulses related to smell c. arises as bipolar neurons from the olfactory epithelium of the nasal cavity d. axons from these neurons pass through the cribriform plate of the ethmoid bone and synapse with other neurons in the olfactory bulb (an extension of the brain) e. the axons of the latter neurons form the olfactory tract and travel to the primary olfactory area in the cerebral cortex
ii. optic (II) nerve: a. entirely sensory b. transmits nerve impulses related to vision c. signals initiated by the rods and cones of the retina are relayed by bipolar neurons to ganglion cells d. axons of the ganglion cells converge to form the optic nerve which exits the orbit via the optic foramen e. the two optic nerves unite to form the optic chiasm where fibers from the medial half of each retina cross to the opposite side and fibers from the lateral side remain on the same side f. posterior to the chiasm, the regrouped fibers form the optic tracts g. from the optic tracts most fibers travel to the lateral geniculate nuclei in the thalamus where they synapse with neurons that pass to the primary visual areas of the cerebral cortex h. some fibers from the optic chiasm travel to the superior colliculi of the midbrain; here they synapse with motor neurons that control the extrinsic and intrinsic eye muscles
iii. oculomotor (III) nerve: a. mixed nerve; but principally motor, serving to stimulate (primarily skeletal) muscle contractions b. originates from nucleus in the ventral portion of the midbrain c. travels forward and divides into superior and inferior branches, both of which enter the orbit via the superior orbital fissure d. the superior branch innervates the superior rectus and levator palpebrae superioris muscles e. the inferior branch innervates the medial rectus, inferior rectus, and inferior oblique muscles f. the inferior branch also provides parasympathetic innervation to the ciliary ganglion, a relay center of the autonomic nervous system that connects a nucleus in the midbrain with the intrinsic eye muscles g. therefore, major functions are regulating movements of upper eyelid and eyeball, adjustment of lens for near vision, and constriction of pupil h. the sensory portion of the oculomotor nerve consists of afferent fibers from proprioceptors in the eyeball muscles supplied by the nerve to the midbrain iv. trochlear (IV) nerve: a. mixed nerve; but primarily motor, serving to stimulate skeletal muscle contractions b. smallest of the cranial nerves and only one to arise from the posterior aspect of the brain stem c. the motor portion originates in nucleus in midbrain, and axons from the nucleus enter the orbit via the superior orbital fissure d. the motor fibers innervate the superior oblique muscle e. therefore, major function is regulating movement of eyeball f. the sensory portion of the trochlear nerve consists of afferent fibers that travel from proprioceptors in the superior oblique muscle to a nucleus of the nerve in the midbrain v. trigeminal (V) nerve: a. mixed nerve b. largest of the cranial nerves c. has three branches:
- ophthalmic nerve
- maxillary nerve
- mandibular nerve d. emerges from two roots on the ventrolateral surface of the pons e. the large sensory root has a swelling called the trigeminal ganglion which contains the cell bodies of most of the primary sensory neurons f. from this ganglion:
- the ophthalmic nerve enters the orbit via the superior orbital fissure
- the maxillary nerve enters the foramen rotundum
- the mandibular nerve exits through the foramen ovale
ix. glossopharyngeal (IX) nerve : a. mixed nerve b. nerve exits the skull via the jugular foramen c. motor fibers originate from nuclei in the medulla and innervate the stylopharyngeus muscle and parotid gland (some of the cell bodies of parasympathetic motor neurons are located in the otic ganglion ) d. therefore, major motor function is regulation of secretion of saliva e. sensory fibers originate from the pharynx, taste buds of the posterior third of the tongue, carotid sinus baroreceptors, proprioceptors in swallowing muscles, and carotid body chemoreceptors; these sensory fibers travel to the medulla f. therefore, major sensory function is taste, monitoring of blood pressure, proprioception, and monitoring blood gas levels x. vagus (X) nerve : a. mixed cranial nerve that is widely distributed from the head and neck into the thorax and abdomen b. motor fibers originate in nuclei of the medulla and travel to muscles in the respiratory passageways, lungs, heart, esophagus, stomach, small intestine, most of the large intestine and the gallbladder; nerve impulses are delivered to smooth, cardiac, and skeletal muscle tissues as well as to glands of the gastrointestinal tract c. sensory fibers travel from visceral sensory receptors of thoracic and abdominal organs, the ear, some taste buds, proprioceptors in neck and throat muscles, carotid sinus baroreceptors, and carotid body chemoreceptors to the medulla and pons xi. accessory (XI) nerve : a. formerly called the spinal accessory nerve b. mixed nerve; but primarily motor, serving to stimulate skeletal muscle contractions c. originates from both the brain stem and the spinal cord d. cranial root originates from nuclei in the medulla and innervates the skeletal muscles of the pharynx, larynx, and soft palate that are used in swallowing e. spinal root originates in the anterior gray horn of the first five segments of the cervical spinal cord; the fibers from the segments converge, enter the foramen magnum, and exit via the jugular foramen along with the cranial root f. the spinal portion transmits motor impulses to the sternocleidomastoid and trapezius muscles to coordinate head movements; its sensory fibers transmit nerve impulses from proprioceptors in these muscles xii. hypoglossal (XII) nerve : a. mixed nerve; but primarily motor, serving to stimulate skeletal muscle contractions b. motor fibers originate in a nucleus in the medulla, travel through the hypoglossal canal, and innervate muscles of the
tongue; these fibers transmit nerve impulses related to speech and swallowing c. sensory fibers travel from proprioceptors in the tongue muscles to the medulla
- An excellent summary of cranial nerves (their types, locations, functions, and clinical applications related to dysfunctions)
C. Development of the Nervous System
- The development of the nervous system begins with a thickening of the ectoderm called the neural plate which folds inward and forms the neural groove; the raised edges of the neural plate are called neural folds and the latter increase in height and meet to form the neural tube.
- The cells of the wall that encloses the neural tube differentiate to form three layers: i. outer or marginal layer, which develops into the white matter ii. middle mantle layer, or which develops into the gray matter iii. inner or ependymal layer, which develops into thelining of the ventricles and lining of the spinal cord’s central canal
- The neural crest is a mass of tissue between the neural tube and the skin ectoderm; it differentiates into the: i. posterior (dorsal) root ganglia of spinal nerves ii. spinal nerves iii. ganglia of cranial nerves iv. cranial nerves v. ganglia of the autonomic nervous system vi. adrenal medulla vii. meninges
- The anterior portion of the neural tube develops into three enlarged fluid-filled areas called primary brain vesicles : i. prosencephalon (forebrain) ii. mesencephalon (midbrain) iii. rhombencephalon (hindbrain)
- As development proceeds, the three primary vesicles develop into four secondary brain vesicles : i. the prosencephalon develops into: a. an anterior telencephalon, which develops into the cerebral hemispheres, basal ganglia, and lateral ventricles b. a posterior diencephalon, which develops into the epithalamus, thalamus, subthalamus, hypothalamus, and pineal gland, and houses the third ventricle ii. the rhombencephalon develops into: a. an anterior metencephalon, which develops into the pons, cerebellum, and houses upper part of the fourth ventricle b. a posterior myelencephalon, which develops into the medulla oblongata, and houses the rest of the fourth ventricle iii. the mesencephalon remains unchanged and develops into the midbrain
- The area of the neural tube inferior to the myelencephalon develops into the spinal cord.
D. Aging and the Nervous System
- The effects of aging on the nervous system include:
