Chapter 14 The Brain and Cranial Nerves PDF

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Chapter 14 The Brain and Cranial Nerves 1. Major Landmarks

a. Rostral – means toward the forehead in human anatomy i. In the spinal cord and brainstem, which are vertically oriented, rostral means higher b. Caudal means toward the spinal column.

i. In the spinal cord and brainstem, which are vertically oriented, caudal means lower.

c. Brain divided into 3 major portions Cerebrum, Cerebellum and Brainstem i. Cerebrum 1. 83% of brains’ volume 2. Consists of a pair of half globes called cerebral hemispheres a. Each is marked by thick folds called gyri separated by shallow grooves called sulci b. Longitudinal fissure – separates the right and left hemispheres from each other. c. Corpus callosum – located bottom of fissure which is a bundle of nerve fibers that connects the hemispheres. ii. Cerebellum 1. Occupies the posterior cranial fossa inferior to the cerebrum. 2. It is separated from the cerebrum by the transverse cerebral fissure. 3. Second largest region of the brain. About 10% of the volume of a brain. 4. Contains about 50% of the brains neurons. iii. Brainstem – all of the brain except the Cerebrum and Cerebellum. 1. Major components from higher to lower (rostral to caudal) are: a. Diencephalon b. Midbrain c. Pons

d. Medulla oblongata 2. In a human, brainstem is oriented like a vertical stalk, with the cerebrum perched on top like a mushroom cap. 3. Caudally, the brainstem ends at the foramen magnum of the skull and the CNS continues below this as the spinal cord

2. Gray and White Matter

a. Brain composed of gray and white matter. i. Gray matter forms a surface layer called the cortex over the cerebrum and cerebellum. ii. Deeper masses surrounded by white matter are called nuclei. iii. White matter lies deep to the cortical gray matter 3. Embryonic Development a. Mature brain described in terms of forebrain, midbrain and hindbrain i. The forebrain (prosencephalon) – divided into two parts 1. Telencephalon 2. Diencephalon ii. The midbrain is undivided and is named mesencephalon iii. The hindbrain divides into two vesicles 1. The metencephalon 2. The myelencephalon

4. Meninges

a. Three membranes the surround the brain, called meninges. (Dura mater, Arachnoid mater, Pia mater) b. Lie between the nervous tissue and bone c. Protect the brain d. Provide structural framework for brain’s arteries and veins. i. Names of membranes 1. Dura mater a. Consist of two layers i. Outer periosteal layer ii. Inner meningeal layer b. The two layers of dura separated by dural sinuses i. Dural sinuses – spaces that collect blood that circulated through the brain ii. Two major sinuses 1. Superior sagittal sinus – found under the cranium along the median line 2. Transvers sinus – runs horizontally from the rear of the head toward each ear.

3. These sinuses meet like upside down T at back of brain and empty into jugular veins of the neck 2. Arachnoid matter 3. Pia mater 5. Ventricles and Cerebrospinal Fluid

a. Brain has 4 internal chambers called ventricles i. Lateral ventricles (two of them) 1. Form an arc in each cerebral hemisphere. 2. A tiny pore called the interventricular foramen, each lateral ventricle is connected to the third ventricle.

b. Cerebral Fluid (CSF)

i. Clear, colorless liquid that fills the ventricles and canals of the CNS (Central Nervous System). ii. 500 mL of CSF produced per day.. iii. CSF continually flows through and around the Central Nervous System (flows because of pressure, and beating of the ependymal cilia, and rhythmic pulsations of the brain produced by each heart beat. iv. Serves three purposes 1. Buoyancy – brain floats in CSF 2. Protection – protects the brain from striking the cranium when the head is jolted. 3. Chemical stability – CSF rinses metabolic wastes from the nervous tissue and regulates its chemical environment. Slight changes in CSF composition can cause malfunctions of the nervous system. Ex – high glycine concentration disrupts the control of body and temperature and blood pressure, and a high pH causes dizziness and fainting.

6. Blood Supply and the Brain Barrier System a. Regulates what can get from the bloodstream into the tissue fluid of the brain. b. Two points of entry that are guarded to protect the brain i. Blood capillaries throughout the brain tissue 1. There are tight junctions that completely seal off the gaps and force anything leaving the blood to pass through the cells and not between them. These endothelial cells are more selective than the gaps between them would be and exclude harmful substances from the brain tissue while allowing necessary ones to pass. ii. Capillaries of the choroid plexuses. 1. Brain is protected by similar blood-CSF barrier formed by tight junctions between the ependymal cells. These junctions are absent from ependymal cells elsewhere because needs to allow exchanges between the brain tissue and CSF. c. The BBS (brain barrier system) is permeable to water, glucose and lipid-soluble substances such as oxygen, carbon dioxide, alcohol, caffeine, nicotine, and anesthetics. Sometimes the BBS is an obstacle because medications such as antibiotics and cancer drugs can’t get through the BBS and complicates some treatments for brain diseases. 7. The Hind Brain and midbrain

a. The Medulla Oblongata i. The myelencephalon becomes the medulla oblongata. It begins at the foramen magnum of the skull and extends for about 3 cm rostrally, ending at the transverse groove between the medulla and pons. ii. The anterior surface has pair of ridges called pyramids. Look like side-by-side baseball bats. iii. All nerve fibers connecting brain to spinal cord pass through medulla. iv. Included in the medulla is the 1. Cardiac center – regulates the rate and force of the heartbeat 2. Vasomotor center – regulates blood pressure and flow by dilating and constricting blood vessels 3. Two respiratory centers – regulate the rhythm and depth of breathing b. The Pons i. The metencephalon develops into 2 structures, pons and cerebellum. ii. The pons appears as a broad anterior bulge rostral to the medulla. iii. Posteriorly, consists of two pairs of thick stalks called cerebellar peduncles. They connect the cerebellum to the pons and the midbrain. iv. Anterior half of pons (lower half) dominated by tracts of white matter. – carries sensory and motor signals up and down the brainstem. v. Cranial nerves V to VIII begin or end in the pons. These nerves included sensory roles in hearing, equilibrium, taste; facial sensations such as touch and pain; motor roles in eye movement, facial expressions, chewing, swallowing, urination, and secretion of saliva and tears.

vi. Reticular formation in the pons contains nuclei concerned with sleep, respiration and posture. 8. The Midbrain a. The mesencephalon becomes the mature brain structure known as midbrain. b. Midbrain is a short segment of brainstem that connects the hindbrain and the forebrain. c. Contains cerebral aqueduct, continuations of medial lemniscus and reticular formation, and cranial nerves III (oculomotor) and IV (trochlear) d. Tectum – roof like part of midbrain posterior to cerebral aqueduct. i. 4 bulges (corpora quadrigeminal) 1. Superior colliculi (upper pair) – function in a. Visual attention b. Visually tracking moving objects c. Blinking, focusing, pupillary dilation and constriction d. Turning the eyes and head in response to visual stimulus 2. Inferior colliculi (lower pair) receive signals from a. Inner ear and relays them to other parts of brain, especially the thalamus. b. Mediate the reflexive turning of the head in response to sound c. One’s tendency to jump when startled by a sudden noise. e. Cerebral peduncles – anterior to cerebral aqueduct. i. Two stalks that anchor the cerebrum to the brainstem. ii. 3 main components: 1. Tegmentum – dominated by the red nucleus. High density of blood vessels. Connects to and from the cerebellum which it works with fine motor control. 2. The substantia nigra – dark gray to black nucleus pigmented with melanin. Is a motor center that relays inhibitory signals to thalamus and basal nuclei, preventing unwanted body movement. Degeneration of the neurons in the substantia nigra leads to muscle tremors of Parkinson disease. 3. Cerebral crus – bundle of nerve fibers that connect the cerebrum to the pons and carry the corticospinal nerve tracts. iii. Central (periaqueductal) gray matter encircles the cerebral aqueduct. Involved with the reticulospinal tracts in controlling awareness of pain.

9. The Reticular Formation

a. Loose web of gray matter that runs vertically through all levels of the brainstem. b. Occupies much of the space between the white fiber tracts and the more anatomically distinct brainstem nuclei, and has connections with many areas of the cerebrum. c. Has more than 100 small neural networks d. Functions of Reticular formation i. Somatic motor control – muscle tension to maintain tone, balance, posture. ii. Relays signals from eyes and ears to cerebellum so visual, auditory and vestibular (balance & motion) stimuli are integrated to cerebellum in its role of motor coordination. iii. Gaze centers – enable eyes to track and fixate objects

iv. Central pattern generator pool that produce rhythmic signals to the muscles of breathing and swallowing v. Cardiovascular control vi. Pain modulation – a route which pain signals from lower body reach the cerebral cortex. Under some circumstances, the nerve fibers in these pathways act in spinal cord to deaden one’s awareness of pain. vii. Sleep and consciousness – projects to thalamus and cerebral cortex that allow it some control over sensory signals that reach the cerebrum and come to our conscious attention. Plays a role in states of consciousness such as alertness and sleep. Injury can result in irreversible coma. viii. Habituation – process in which brain learns to ignore repetitive inconsequential stimuli while remain sensitive to others. Example - Noisy city sounds ignored, but alarm clock wakes one up. 10. The Cerebellum

a. b. c. d.

Largest part of the hindbrain. Second largest part of the brain as a whole 2 hemispheres connected by wormlike bridge called vermis. Each hemisphere has slender, transverse, parallel folds called folia separated by shallow sulci.

e. Surface cortex of gray matter f. Deeper layer of white matter. i. White matter branches in a fernlike pattern called the harbor vitae. g. Each hemisphere has 4 masses of gray matter called deep nuclei embedded in white matter. h. Input to cerebellum goes to the cortex i. Output comes from the deep nuclei j. Contains more than ½ of all brain neurons. k. Granule cells are the most abundant type of neuron. l. The large, globus Purkinje cells are the most distinctive neurons. i. Arranged in a single plane like a flat tree. ii. Are arranged in a single file, with thick dendritic planes parallel to each other. m. Connected to brainstem by 3 pairs of stalks called cerebellar peduncles. i. Inferior peduncles (connected to the medulla oblongata) ii. Middle peduncles (connected to the pons) iii. Superior peduncles (connected to the midbrain) n. Function i. Center for monitoring muscle contractions and aiding in motor coordination. ii. Is a timekeeper. PET scans show cerebellum activity when person is required to judge the elapsed time between two stimuli. iii. Helps predict how much the eyes must move in order to compensate for head movements and remain fixed on an object. iv. Hearing – ability to judge differences in pitch between two tones and to distinguish between similar sounding words such as rabbit and rapid. v. People with lesions on cerebellum have difficulty planning and scheduling tasks. They tend to overreact emotionally and have difficulty with impulse control. Children with adhd have abnormally small cerebellums.

11. The Diencephalon

a. Three parts (Thalamus, Hypothalamus, Epithalamus) i. Thalamus 1. Each side of brain has a thalamus. 2. Ovoid mass sitting on the superior end of the brainstem beneath the cerebral hemisphere. 3. Form about 4/5 of the diencephalon. 4. Consists of 23 nuclei. a. Five groups i. Anterior ii. Posterior

iii. Medial iv. Lateral v. Ventral 5. Gateway to the cerebral cortex. All input to cerebrum passes by the way of the synapses in the thalamus. a. Taste b. Smell c. Hearing d. Equilibrium e. Vision f. Touch, pain, pressure, heat and cold g. Process this information and relay small portion of it to the cerebral cortex h. Also serves in motor control by relaying signals from cerebellum to the cerebrum and providing feed back loops. i. Also involved in memory and emotional functions of the limbic system. ii. Hypothalamus 1. Forms the floor and part of the walls of the third ventricle. Extends anteriorly to the optic chiasm where the optic nerves meet. 2. Primary function is to relay signals from limbic system to thalamus. 3. Pituitary gland is attached to the hypothalamus by a stalk between the optic chiasm and mammillary bodies. 4. Control center of the endocrine and autonomic nervous system. 5. Responsible for a. Hormone secretion concerned with labor contractions, lactation, and water conservation. b. Autonomic effects – influence heart rate, blood pressure, gastrointestinal secretion and motility and pupillary diameter c. Thermoregulation monitory body temperature d. Food and water intake. Contains receptors for hormones that increase hunger and energy expenditure. e. Sleep and circadian rhythms – regulate the rhythm of sleep and waking. f. Memory – pathway of signals traveling from hippocampus, an important memory center of the brain to the thalamus. Lesions to mammillary nuclei cause memory deficits g. Emotional behavior and sexual response iii. The Epithalamus 1. Small mass of tissue composed of the pineal gland, the habenula (a relay from the limbic system to the midbrain), and a thin roof over the 3rd ventricle

12. The Cerebrum

a. b. c. d.

Embryonic telencephalon becomes cerebrum. Largest part of human brain. Seat of voluntary motor control. Gross Anatomy i. Most important are the 2 cerebral hemispheres which are separated by the longitudinal fissure and connected by the corpus callosum ii. Has wrinkles known as gyri, and grooves known as sulci. iii. Folding allows it to fit in cranial cavity. iv. Frontal Lobe 1. Behind the frontal bone, superior to the eyes. From forehead, extends caudally to a wavy vertical groove, the central sulcus. 2. Abstract through, explicit or declarative memory (long term memory); mood, motivation, foresight, planning, decision making, emotional control and judging socially appropriate behavior, speech production and voluntary motor control v. Parietal lobe 1. Uppermost part of brain and underlies the parietal bone. Visible on the medial surface of hemisphere. 2. Concerned with taste, somatic sensation (touch, heat, pain), visual processing, integration between vision and sound (comprehend sensory world), spatial perception and awareness of body orientation, language processing, numerical awareness (sense of the quantity of things around us) vi. Occipital lobe 1. Rear of head, caudal to the parieto-occipital sulcus and underlying the occipital bone. Principal visual center of the brain vii. Temporal lobe

1. Lateral, horizontal lobe dep to the temporal bone, separated from the frontal and parietal lobes above it by a deep lateral sulcus 2. Hearing, smell, emotion, learning, language comprehension and memory of the grammar vocabulary of languages we speak, formation of new long-term memories, storage of verbal, visual and auditory memories viii. Insula 1. Small mass of cortex deep to the lateral sulcus. 2. Plays role in taste; pain; visceral sensation, consciousness, emotional responses and empathy, and cardiovascular homeostasis (heart rate and blood pressure responses to exercise)

e. The Cerebral White Matter

i. ii. iii. iv.

Most volume of cerebrum is white matter. White matter composed of glia and myelinated nerve fibers Transmit signals from one region of cerebrum to another Three kinds of these fibers (tracts) (Projection Tracts, Commissural tracts, Association Tracts) 1. Projection Tracts a. Extend vertically between higher and lower brain and spinal cord centers. b. Carry information between cerebrum and rest of body. c. One of these project tracts is corticospinal tract

i. Carries motor signals from cerebrum to brainstem and spinal cord. 2. Commissural tracts a. Cross from one cerebral hemisphere to the other through bridges b. Most pass through the large corpus callosum. c. Enable the two sides of the cerebrum to communicate with each other 3. Association tracts a. Connect different regions with in same hemisphere of cerebrum. b. Long association fibers connect different lobes of one hemisphere to each other c. Short association fibers connect different gyri with in a single lobe. d. Link perceptual and memory centers of the brain. Ex: see a rose, name it, and imagine its scent.

f.

The Cerebral Cortex

i. ii. iii. iv. v.

Gray matter of the cerebrum A layer covering the surface of the hemispheres. Constitutes 40% of mass of brain. 14 to 16 billion neurons. Two types of neurons 1. Stellate cells- spheroidal somas with short axons and dendrites projecting in all directions.

a. Receiving sensory input and processing information on a local level 2. Pyramidal cells – tall and conical a. Thick dendrite with many branches and small, knobby dendrite spines. b. Pass into white matter. c. Include output neurons of the cerebrum. d. Only neurons whose fibers leave the cortex and connect with other parts of the CNS (central nervous system) vi. Neocortex – six layered tissue that is about 90% of cerebral cortex. 1. Layer IV – thickest in sensory regions 2. Layer V – motor regions. 3. All axons that leave the cortex and enter the white matter arise from layers III, V and VI g. Limbic System

i. Center of emotion and learning. ii. Ring of cortex on the medial side of each hemisphere, encircling the corpus callosum and thalamus of the cerebrum iii. Components are 1. Cingulate gyrus a. Arches over the top of the corpus callosum in the frontal and parietal lobes 2. Hippocampus a. Medial temporal lobe 3. Amygdala a. Immediately rostral to the hippocampus. iv. The components are connected through a complex loop of fiber tracts allowing for circular patterns of feedback among its nuclei and cortical neurons. v. Play role in emotion and memory vi. Centers for gratification and aversion.

h. Basal Nuclei

i. Masses of cerebral gray matter buried deep in the white matter, lateral to the thalamus. ii. Three brain centers of basal nuclei 1. Caudate 2. Nucleus 3. Putamen 4. Globus pallidus 5. All together, are called corups striatum. 6. Involved in motor control 7. Higher brain functions such as sleep, memory, cognition, emotion, sensation, motor control and language 13. The Electroencephalogram (EEG) a. Monitors electrical activity called brain waves b. Useful in studying normal brain functions such as sleep and consciousness and in diagnosing brain disease c. Four types of brain waves i. Alpha – frequency of 8 to 13 hz. 1. Occur in Parieto-occipital area. 2. Dominate the EEG when person is awake and resting with eyes closed 3. Suppressed when eyes opened. 4. Absent during deep sleep ii. Beta – frequency of 14 - 30 hz. 1. Occur in frontal to parietal region 2. Accentuated during mental activity and sensory stimulation iii. Theta – 4 to 7 hz 1. Normal in children and drowsy or sleeping adults

2. If predominance is shown in awake adults – suggests emotional stress or brain disorders. iv. Delta waves frequency less than 3.5 hz. 1. Infants exhibit delta waves when awake 2. Adults in deep sleep. 3. A predominance of delta waves in awake adults indicates serious brain damage. 14. Sleep a. Temporary state of unconsciousness from which one can awaken when stimulated. b. Occur in cycles called circadian rhythms c. Four stages i. Stage 1 – feels drowsy, closes eyes, r...