Chapter 19 The Brain and the Cranial Nerves

Transcription

1 Chapter 19 The Brain and the Cranial Nerves Introduction Brain- portion of the central nervous system contained within the cranium. About 100 billion neurons and trillion neuroglia with a mass of about 1300 g (almost 3 lb) in adults. On average, each neuron forms 1000 synapses with other neurons. Thus, the total number of synapses, about a thousand trillion (1015), is larger than the number of stars in the galaxy. The Brain 1

2 Brain Organization, Protection, and Blood Supply The brain and spinal cord develop from ectoderm arranged in a tubular structure called the neural tube. The anterior part of the neural tube creates three regions called primary brain vesicles: prosencephalon, mesencephalon, and rhombencephalon. Development of the brain Development of the Brain Mesencephalon becomes midbrain and aqueduct of the midbrain (cerebral aqueduct). Prosencephalon becomes the telencephalon and diencephalon. Telencephalon becomes the cerebrum and lateral ventricles. Diencephalon forms the thalamus, hypothalamus, epithalamus, third ventricle. Rhombencephalon becomes the metencephalon and myelencephalon. Metencephalon becomes the pons, cerebellum, upper part of the fourth ventricle. Myelencephalon forms the medulla oblongata, lower part of the fourth ventricle. 2

3 The Brain Four major parts: Brain stem- continuous with the spinal cord Consists of the medulla oblongata, pons, and midbrain. Cerebellum- posterior to the brain stem Diencephalon- superior to the brain stem Consists of the thalamus, hypothalamus, and epithalamus. Cerebrum- supported on the diencephalon and brain stem Protective Coverings of the Brain Cranial meninges: dura mater, arachnoid mater, and pia mater. continuous with the spinal meninges Cranial dura mater has two layers (spinal dura mater has only one) fused together except where they enclose the dural venous sinuses (endothelial lined venous channels) that drain venous blood from the brain. no epidural space Protective Coverings of the Brain Three extensions of the dura mater separate parts of the brain. 1. Falx cerebrai separates the two hemispheres of the cerebrum. 2. Falx cerebelli separates the two hemispheres of the cerebellum. 3. Tentorium cerebelli separates the cerebrum from the cerebellum. 3

4 Protective Coverings of the Brain Brain Blood Flow and the Blood Brain Barrier The blood brain barrier (BBB) protects brain cells from harmful substances and pathogens. Tight junctions seal together the endothelial cells of brain capillaries. Astrocytes secrete chemicals that maintain the permeability of the tight junctions. A few water-soluble substances cross very slowly glucose, creatine, urea, and most ions Proteins and most antibiotic drugs do not pass from the blood into brain tissue. Lipid- soluble substances access brain tissue freely oxygen, carbon dioxide, alcohol, and most anesthetic agents Trauma, certain toxins, and inflammation can cause a breakdown of the blood brain barrier. Cerebrospinal fluid (CSF) Cerebrospinal fluid (CSF) a clear, colorless liquid protects the brain and spinal cord against chemical and physical injuries. carries oxygen, glucose, and other needed chemicals from the blood to neurons and neuroglia. continuously circulates through cavities in the brain and spinal cord and around the brain and spinal cord in the subarachnoid space. 4

5 Formation of CSF in the Ventricles Lateral ventricle- in each hemisphere of the cerebrum. Separated by a thin membrane, septum pellucidum. Third ventricle- narrow cavity along the midline superior to the hypothalamus and between the right and left halves of the thalamus. Fourth ventricle- between the brain stem and the cerebellum. The total volume of CSF is 80 to 150 ml (3 to 5 oz) in an adult. CSF contains glucose, proteins, lactic acid, urea, cations (Na +, K +, Ca +, Mg + ), anions (Cl - and HCO3 - ), and some white blood cells. Location of ventricles within a transparent brain. CSF Contributes to Homeostasis Mechanical protection- shock- absorbing medium that protects the brain and spinal cord. Chemical protection- provides an optimal chemical environment for accurate neuronal signaling in neurons that border the fluid. Circulation- a medium for minor exchange of nutrients and waste products between the blood and nervous tissue. 5

6 CSF Production Choroid plexuses- networks of blood capillaries in the walls of the ventricles covered by ependymal cells joined tightly by tight junctions. Ependymal cells form CSF from blood plasma by filtration and secretion. Blood cerebrospinal fluid barrier permits certain substances to enter the CSF but excludes others Circulation of CSF The CSF- formed in the choroid plexuses of each lateral ventricle: flows into the third ventricle via two oval openingsinterventricular foramina. through the aqueduct of the midbrain (cerebral aqueduct), through the midbrain, into the fourth ventricle. enters the subarachnoid space through three openings in the roof of the fourth ventricle median aperture and the paired lateral apertures, one on each side. through the subarachnoid space around the surface of the brain and spinal cord. Circulation of CSF CSF is reabsorbed into the blood via arachnoid villi fingerlike extensions that project into the dural venous sinuses, especially the superior sagittal sinus. Normally, CSF is reabsorbed as rapidly as it is formed by the choroid plexuses rate of about 20 ml/hr (480 ml/day) helps maintains a constant pressure 6

7 Pathway of circulating cerebral spinal fluid Pathway of circulating cerebral spinal fluid, sagittal section of brain and spinal cord Brainstem Between the spinal cord and diencephalon Medulla, Pons, Midbrain, and Reticular Formation 7

8 Medulla Oblongata in relation to the rest of the Brain Stem The medulla oblongata- continuation of the superior part of the spinal cord; forms the inferior part of the brain stem begins at the foramen magnum and extends to the inferior border of the pons. Internal Anatomy of Medulla Oblongata Internal Anatomy of Medulla Oblongata Pyramids- large bulge Consists of descending motor tracts Nuclei- cardiovascular and reparatory control areas Olive- lateral to Pyramids Inferior Olivary Nucleus- contains proprioceptors for joints and muscles 8

9 Pons Consists of both nuclei and tracts- about 2.5 cm (1 in.) Bridge that connects different parts of the brain. Connect the right and left sides of the cerebellum. Two major structural components: Ventral region forms the pontine nuclei. Dorsal region is more like the brainstem, the medulla and midbrain. Contains ascending and descending tracts and nuclei of cranial nerves. Peumotaxic and apneustic areas- Other nuclei in the pons With the medullary rhythmicity area they help control breathing. Midbrain Midbrain (mesencephalon) extends from the pons to the diencephalon and is about 2.5 cm (1 in.) long. Aqueduct of the midbrain (cerebral aqueduct) passes through the midbrain connecting the third ventricle with the fourth ventricle. midbrain contains both tracts and nuclei. Cerebral peduncles- pair of tracts in anterior part of the midbrain Contain axons of corticospinal, corticopontine, and corticobulbar motor neurons conduct nerve impulses from the cerebrum to the spinal cord, medulla, and pons. Contain axons of sensory neurons that extend from the medulla to the thalamus. Midbrain Tectum- posterior part of the midbrain. Superior colliculi- two superior elevations. Serve as reflex centers for visual activities- tracking, image scanning, accommodation, movement of eyes, head, and neck due to vision Inferior colliculi- two inferior elevations Part of the auditory pathway Relaying impulses from the receptors for hearing in the ear to the thalamus. Contains several nuclei: Left and right substantia nigra- large, darkly pigmented nuclei Red nuclei- synapses bet axons from the cerebellum and cerebral cortex Function with the cerebellum to coordinate muscular movements. 9

10 Midbrain Reticular Formation Broad region where white matter and gray matter exhibit a netlike arrangement. Ascending (sensory) and descending (motor) functions. Reticular activating system(ras)- Part of the reticular formation consists of sensory axons that project to the cerebral cortex. Helps maintain consciousness and is active during awakening from sleep. Main descending function- help regulate muscle tone, the slight degree of contraction in normal resting muscles. The Cerebellum Flocculonodular lobe- inferior surface contributes to equilibrium and balance. Cerebellar cortex- superficial consists of gray matter in a series of slender, parallel ridges called folia. Arbor vitae- tracts of white matter, deep to the gray matter Cerebellar nuclei- gray matter form axons carrying impulses from the cerebellum to other brain centers and the spinal cord. Cerebellar peduncles- three paired- inferior, middle, superior attach the cerebellum to the brain stem. bundles of white matter consist of axons that conduct impulses between the cerebellum and other parts of the brain. 10

11 The Cerebellum Transverse fissure (a deep groove), and the Tentorium cerebelli, (supports the posterior part of the cerebrum)- separate the cerebellum from the cerebrum. Vermis (central constricted area) and the lateral wings or lobes are the cerebellar hemispheres. Each hemisphere consists of lobes separated by deep and distinct fissures. Anterior lobe and posterior lobe govern subconscious aspects of skeletal muscle contraction makes possible all skilled muscular activities Ex. catching a baseball, dancing, speaking. Cerebellum The Diencephalon Forms a central core of brain tissue just superior to the midbrain. Surrounded by the cerebral hemispheres Contains numerous nuclei Involved in sensory and motor processing between higher and lower brain centers. Extends from the brain stem to the cerebrum and surrounds the third ventricle Includes the thalamus, hypothalamus, and epithalamus. Hypophysis or Pituitary gland- Projects from the hypothalamus Optic tracts carrying neurons from the retina enter this region of the brain. 11

12 Thalamus Measures about 3 cm (1.2 in.) in length. Intermediate mass (interthalamic adhesion) (gray matter) joins the right and left halves of the thalamus in 70% of human brains. Internal medullary lamina- (white matter) divides the gray matter into right and left halves Major relay station- Sensory impulses from the spinal cord, brain stem, and midbrain to the primary sensory areas of the cerebral cortex Transmitting information from the cerebellum and basal ganglia to the primary motor area of the cerebral cortex. Role in the regulation of autonomic activities and maintenance of consciousness. Axons connecting the thalamus and cerebral cortex pass through the internal capsule. Thalamus Seven major groups of nuclei are on each side of the thalamus: 1. Anterior nucleus connects to the hypothalamus and limbic system. Emotions, regulation of alertness and memory 2. Medial nuclei connect to the cerebral cortex, limbic system, and basal ganglia. Emotion, memory, learning, awareness, cognition 3. Nuclei in the lateral group connect to the superior colliculi, limbic system, and cortex in all lobes of the cerebrum. Expression of emotions 4. Five different nuclei are part of the ventral group. Motor function, somatic sensation - touch, pressure, proprioception, vibration, hot, cold, pain -auditory and visual Thalamus 5. Intralaminar nuclei within the internal medullary lamina and make connections with the reticular formation, cerebellum, basal ganglia, and wide areas of the cerebral cortex. Pain perception, integration of sensory and motor information, arousal (RAS) 6. Midline nucleus forms a thin band adjacent to the third ventricle presumed function in memory and olfaction. 7. Reticular nucleus surrounds the lateral aspect of the thalamus, next to the internal capsule. Monitors, filters, and integrates activities 12

13 Thalamus Hypothalamus: 4 Major Regions It is composed of a dozen or so nuclei in four major regions: 1. The mammillary region, and mammilary bodies adjacent to the midbrain, is the most posterior part of the hypothalamus. - relay for reflexes and smell 2. The tuberal region, the widest part of the hypothalamus, includes the dorsomedial, ventromedial, and arcuate nuclei, plus the stalklike infundibulum, median emminence 3. The supraoptic region, lies superior to the optic chiasm, contains the paraventricular nucleus, supraoptic nucleus, anterior hypothalamic nucleus, and suprachiasmatic nucleus. 4. The preoptic region contains the medial and lateral preoptic nuclei - autonomic activities Hypothalamus Controls many body activities. One of the major regulators of homeostasis. Receives sensory impulses related to both somatic and visceral senses. Receives impulses from receptors for vision, taste, and smell. 13

14 Epithalamus Posterior to the thalamus Consists of the pineal gland and habenular nuclei. Size of a small pea Protrudes from the posterior midline of the third ventricle. Part of the endocrine system because it secretes the hormone melatonin. Circumventricular Organs (CVOs) Parts of the diencephalon because they lie in the wall of the third ventricle monitor chemical changes in the blood because lack a blood brain barrier. Includes part of the hypothalamus, pineal gland, pituitary gland, and a few other nearby structures. Function- coordination of homeostatic activities of the endocrine and nervous systems Regulate blood pressure, hunger, thirst, fluids The Cerebrum The largest portion of the human brain Consists of the cerebral hemispheres and the basal ganglia. Cerebral cortex- an outer rim of gray matter an internal region of cerebral white matter, and gray matter nuclei deep within the white matter. The folds are called gyri or convolutions. The deepest grooves between folds are known as fissures; the shallower grooves between folds are termed sulci Longitudinal fissure- separates the right and left cerebral hemispheres Falx cerebri- dural covering, lines the longitudinal fissure 14

15 The Cerebrum Lobes of the Cerebrum The lobes: frontal, parietal, temporal, and occipital lobes. Central sulcus separates the frontal lobe from the parietal lobe. Precentral gyrus- a major gyrus located immediately anterior to the central sulcus contains the primary motor area of the cerebral cortex. Postcentral gyrus located immediately posterior to the central sulcus contains the primary somatosensory area of the cerebral cortex. Lateral cerebral sulcus (fissure) separates the frontal lobe from the temporal lobe. Insula- below the surface within the lateral cerebral sulcus Parieto-occipital sulcus separates the parietal lobe from the occipital lobe. Cerebral White Matter The cerebral white matter consists primarily of myelinated axons in three types: 1. Association tracts- axons that conduct nerve impulses between gyri in the same hemisphere. 2. Commissural tracts- axons that conduct nerve impulses from gyri in one cerebral hemisphere to corresponding gyri in the other cerebral hemisphere. Three important groups of commissural tracts- corpus callosum, anterior commissure, and posterior commissure. 3. Projection tracts- axons that conduct nerve impulses from the cerebrum to lower parts of the CNS (thalamus, brainstem, or spinal cord) or from lower parts of the CNS to the cerebrum. 15

16 Basal Ganglia Basal ganglia- three nuclei (masses of gray matter) Deep within each cerebral hemisphere. Two are side-by-side, just lateral to the thalamus. Globus pallidus and putamen. Together they are the lentiform nucleus. Caudate nucleus- third Corpus striatum- lentiform + caudate nuclei. (refers to striations of the internal capsule as it passes among the basal ganglia). Receives input from sensory cortex-output to motor cortex, and works with the limbic system Regulates starting and stopping movements Control subcutaneous contractions Start and stop cognition Basal Ganglia The Limbic System 1. Cingulate gyrus, (above the corpus callosum), and parahippocampal gyrus, (in the temporal lobe below). 1. Hippocampus- part of parahippocampal gyrus extends into floor of lateral ventricle. 2. Dentate gyrus- between hippocampus and parahippocampal gyrus. 3. Amygdala- several groups of neurons near the tail of the caudate nucleus. 4. Septal nuclei- in the septal area formed by regions under the corpus callosum and paraterminal gyrus (a cerebral gyrus). 5. Mammillary bodies of the hypothalamus- two round masses close to midline, near the cerebral peduncles. 6. Two nuclei of the thalamus- anterior nucleus and medial nucleus, participate in limbic circuits. 7. Olfactory bulbs- flattened bodies of the olfactory pathway, rest on the cribriform plate. 8. Fornix, stria terminalis, stria medullaris, medial forebrain bundle, and mammillothalamic tract. 16

17 The Limbic System The limbic system is sometimes called the emotional brain plays a primary role in a range of emotions, including pain, pleasure, docility, affection, and anger. Involved in olfaction (smell) and memory. Experiments have shown that when different areas of an animals limbic system are stimulated, the animals reactions indicate that they are experiencing intense pain or extreme pleasure. Functional Areas of the Cerebrum Sensory and Motor Areas Sensory Areas Sensory impulses arrive in the posterior half of both cerebral hemispheres Primary sensory areas have the most direct connections with peripheral sensory receptors Secondary and Association Sensory areas integrate sensations Primary Motor Area Located in the precentral gyrus of the frontal lobe Control voluntary contractions Primary somatosensory area, body parts do not map to the primary motor area in proportion to their size. Broca s speech area is located in the frontal lobe close to the lateral cerebral sulcus. 17

18 Lateralization Brain is almost symmetrical Subtle anatomical differences between the two hemispheres exist. For example, about two- thirds of the population, the planum temporale, a region of the temporal lobe that includes Wernicke s area, is 50% larger on the left side than on the right side. This asymmetry appears in the human fetus at about 30 weeks of gestation. Physiological differences also exist; although the two hemispheres share performance of many functions Each hemisphere also specializes in performing certain unique functions- hemispheric lateralization. Brain Waves At any instant, brain neurons generate millions of nerve impulses (action potentials)- brain waves. Brain waves generated by neurons close to the brain surface, mainly in the cerebral cortex Can be detected by sensors, electrodes placed on the forehead and scalp- electroencephalogram or EEG. Study normal brain functions, such as changes that occur during sleep Diagnose variety of brain disorders, such as epilepsy, tumors, metabolic abnormalities, sites of trauma, and degenerative diseases. Cranial Nerves-PNS The 12 pairs of cranial nerves pass through foramina cranial and arise from the brain inside the cranial cavity Five are mixed nerves because they contain axons of both sensory and motor neurons. The cell bodies of motor neurons lie in nuclei within the brain. Cranial nerves III, VII, IX, and X include both somatic and autonomic motor axons. The cell bodies of sensory neurons are located in ganglia outside the brain (except all proprioceptive sensory neurons in the head region have their cell bodies in the mesencephalic ganglion nucleus. 18

19 Olfactory (I) Nerve Olfactory (I) nerve (sensory); contains axons that conduct nerve impulses for olfaction. Olfactory epithelium occupy the superior part of the nasal cavity, cover inferior surface of the cribriform plate and extend down the superior nasal concha. Olfactory nerves end in the brain, in paired masses of gray matterolfactory bulbs, two extensions of the brain that rest on the cribriform plate. Within the olfactory bulbs, axon terminals of olfactory receptors form synapses with the dendrites. The axons neurons make up the olfactory tracts, which extend posteriorly from the olfactory bulbs Primary Olfactory area in the Temporal Lobe. Olfactory (I) Nerve Optic (II) Nerve Optic (II) nerve (sensory)- contains axons that conduct nerve impulses for vision. In the retina, rods and cones initiate visual signals and relay them to bipolar cells, which pass the signals on to ganglion cells. Axons of all the ganglion cells in the retina of each eye join to form an optic nerve, which passes through the optic foramen. The two optic nerves merge to form the optic chiasm Within the chiasm, axons from the medial half of each eye cross to the opposite side; axons from the lateral half remain on the same side. Posterior to the chiasm, the regrouped axons, some from each eye, form the optic tracts. Most axons in the optic tracts end in the lateral geniculate nucleus of the thalamus. Primary Visual area is in the Occipital lobe 19

20 Optic (II) Nerve Oculomotor (III) Nerve Oculomotor- motor nerve. Extends anteriorly and divides into superior and inferior branches, both pass through the superior orbital fissure into the orbit. Axons in the superior branch innervate superior rectus (an extrinsic eyeball muscle) and levator palpebrae superioris (the muscle of the upper eyelid). Axons in the inferior branch supply Medial rectus, inferior rectus, and inferior oblique muscles all extrinsic eyeball muscles Ciliary muscle attached to lens of eye and Circular muscle of the iris Trochlear (IV) Nerve The trochlear (motor)- smallest and the only one from the posterior of the brain stem. Originates in the trochlear nucleus in the midbrain, and axons from the nucleus exit the brain on its posterior aspect and pass through the superior orbital fissure. These somatic motor axons innervate the superior oblique muscle of the eyeball,(extrinsic eyeball muscle)- controls movement of the eyeball. Proprioceptive sensory axons from the superior oblique muscle begin their course toward the brain in the trochlear nerve but eventually leave the nerve to join the ophthalmic branch of the trigeminal nerve. 20

21 Abducens (VI) Nerve The abducens (VI) nerve (motor)- originates from the abducens nucleus in the pons. Somatic motor axons extend from the nucleus to the lateral rectus muscle of the eyeball (extrinsic eyeball muscle) through the superior orbital fissure of the orbit. The abducens nerve is named because nerve impulses cause abduction (lateral rotation) of the eyeball. Oculomotor (III) Nerve, Troclear (IV) and Abducens (VI) nerves Trigeminal (V) Nerve The trigeminal (V) nerve (mixed nerve)- largest of the cranial nerves. The trigeminal nerve has three branches: ophthalmic, maxillary, and mandibular. Ophthalmic nerve, smallest branch, passes into the orbit via the superior orbital fissure. Maxillary nerve intermediate in size, between the ophthalmic and mandibular nerves and passes through the foramen rotundum. Mandibular nerve, largest branch, passes through the foramen ovale. 21

22 Trigeminal (V) Nerve Facial (VII)Nerve Facial (VII) nerve (mixed cranial nerve). Its sensory axons extend from the taste buds of the anterior two thirds of the tongue through the geniculate ganglion, a cluster of cell bodies of sensory neurons that lie beside the facial nerve, and end in the pons. Axons of branchial motor neurons arise from a nucleus in the pons, pass through the petrous portion of the temporal bone, and innervate facial, scalp, and neck muscles. Nerve impulses along these axons cause contraction of the muscles of facial expression, plus the stylohyoid muscle, the posterior belly of the digastric muscle, and the stapedius muscle. Parasympatheric neurons pass through pterygopalatine and submandibular ganglia to the lacrimal, nasal, palatine, sublingual, and submandibular salivary glands. Facial (VII)Nerve 22

23 Vestibulocochlear (VIII) Nerve Vestibulocochlear (VIII) nerve formerly known as the acoustic or auditory nerve. (sensory) Has two branches, the vestibular branch and the cochlear branch. Vestibular branch carries impulses for equilibrium Cochlear branch carries impulses for hearing. Vestibulocochlear (VIII) Nerve Glossopharyngeal (IX) Nerve The glossopharyneal nerve is a mixed nerve. Sensory axons from taste buds and somatic sensory receptors on the posterior one third of the tongue, from proprioceptors in swallowing muscles suppied by the motor portion, from baroreceptors in the carotid sinus, and chemoreceptors from the carotid body. Sensory neuron cell bodies are in the superior and inferior ganglia, and pass through the jugular foramen to the medulla. Motor neurons begin in the medulla and pass through the jugular foramen, and innervate the Stylopharyngeus muscle. 23

24 Vagus (X) Nerve A mixed cranial nerve from the head and neck into the thorax and abdomen. Sensory axons in the vagus nerve arise from: the skin of the external ear, a few taste buds in the epiglottis and pharynx proprioceptors in muscles of the neck and throat. Somatic motor neurons, arise from nuclei in the medulla oblongata and supply muscles of the pharynx, larynx, and soft palate used in swallowing and vocalization. Axons of autonomic motor neurons (parasympathetic) in the vagus nerve originate in nuclei of the medulla and end in the lungs and heart. Also supply glands of the gastrointestinal tract and smooth muscle of the respiratory passageways, esophagus, stomach, gallbladder, small intestine, and most of the large intestine. Vagus (X) Nerve Accessory (XI) Nerve The accessory (XI) nerve is a mixed cranial nerve. The cranial portion is part of the vagus nerve. The accessory nerve is the old spinal part of the nerve. Motor axons arise in the anterior gray horn of the first five segments of the cervical portion of the spinal cord. The accessory nerve conveys motor impulses to the sternocleidomastoid and trapezius muscles to coordinate head movements. 24

25 Hypoglossal (XII) Nerve The hypoglossal (XII) nerve is motor cranial nerve. The somatic motor axons originate in the hypoglossal nucleus in the medulla blongata, pass through the hypoglossal canal, and supply the muscles of the tongue. Conduct nerve impulses for speech and swallowing. Sensory axons that originate from proprioceptors in the tongue muscles begin their course toward the brain in the hypoglossal nerve and end in the medulla oblongata. Origin of the Nervous System Process Diagram Step-by-Step 1. Future neural crest Neural plate Ectoderm HEAD END Notochord Endoderm Mesoderm Neural plate Neural folds Neural groove Neural tube 2. Neural crest Ectoderm Neural folds Somite Cut edge of amnion Notochord Endoderm Neural groove TAIL END (a) Dorsal view 3. Neural crest Neural tube Somite Notochord Endoderm (b) Transverse sections Ectoderm 25

26 Development of the Brain and Spinal Cord (Fig ) 26