Chapter 14: The Brain and Cranial Nerves

Transcription

1 Chapter 14: The Brain and Cranial Nerves Chapter Objectives BRAIN ORGANIZATION 1. Introduce the principle parts of the brain in terms of their relative positions. 2. Describe the ventricles and where in the brain they are located THE CEREBRUM 3. Define gyrus, sulcus, and fissure. Give the location of the Central sulcus and its surrounding gyri. 4. Discuss the locations of the cerebral lobes. 5. Describe the cerebral cortex. 6. Define the location and purpose of the motor regions of the cerebral cortex including the following regions: primary motor region, premotor area, Broca s area, and primary eye field area. 7. Define the location and purpose of the sensory regions of the cerebral cortex including the following regions: somatosensory region, somatosensory association area, and the primary sensory areas for the visual, auditory, gustatory, and olfactory systems. 8. Define the location and purpose of the association regions of the cerebral cortex including the following regions: Prefrontal area and Wernicke s area. 9. Describe the pathway destinations for the three types of white matter tracts within the cerebrum (Association, Commissural, and Projection). 10. Identify the nuclei of the Basal nuclei (ganglia), describe the functions of the Basal nuclei and describe one disorder associated with the Basal nuclei. DIENCEPHALON 11. Discuss the structures of the Diencephalon and functions for each structure. THE BRAIN STEM 12. List the structures that make up the brain stem. 13. Discuss the structures of the Midbrain and functions for each structure. 14. Describe the Pons. 15. Discuss the structures of the Medulla and functions for each structure. 16. Identify the vital reflex centers of the medulla oblongata and their functions. CEREBELLUM 17. Describe the location of the Cerebellum and its functions. FUNCTIONAL BRAIN SYSTEMS

2 18. List the component structures of the limbic system, and discuss their contribution to behavioral responses and mental functions. 19. Indicate the distribution of nuclei that constitute the reticular formation and describe the function of the reticular formation. MENINGES 20. Identify the three layers of the meninges and their extensions that surround the brain and spinal cord. 21. List the functions of CSF and where it is formed in the brain. CRANIAL NERVES 22. Define a cranial nerve and identify the 12 pairs of cranial nerves by name, number, type, which hole in the skull they pass through and function. Chapter Lecture Notes Regions of the Brain Cerebrum (Fig 14.1 & Table 14.2) Diencephalon Brain Stem Cerebellum Ventricles Ventricles hollow chambers within the brain that are filled with cerebrospinal fluid and lined with ependymal cells (Fig 14.3 & 14.4) Four ventricles Paired Lateral ventricles - cerebrum Third ventricle - diencephalon Fourth ventricle - brain stem Cerebrum Cerebrum - supported on brain stem and form bulk of brain (Fig 14.11) Lateral ventricles Cranial nerves 1 & 2

3 Surface features Lobes - frontal, parietal, temporal, occipital Gyrus - upfolds - increases surface area (surface area = 25 square feet) Sulcus - shallow grooves Central sulcus - divides parietal from frontal lobe Precentral gyrus - motor in function Postcentral gyrus - sensory in function Lateral cerebral sulcus - divides frontal from temporal lobe Fissures - deep grooves Longitudinal fissure - divides the right from left hemispheres Cortex (bark of tree) = outer gray - 2-4mm thick - 6 layers of cell bodies; divided into 3 functional areas (Fig & 14.15) EEG - electrical activity of cortex (Fig 14.16) Motor - control voluntary muscle movements -in frontal lobe anterior to central sulcus Primary motor area - precentral gyrus - controls muscle movement on opposite side of body (pathway is pyramidal system) (Fig 16.8) Premotor area - anterior to primary motor Controls learned skilled movements - generates action potentials that enable specific groups of muscles to contract in specific sequence like writing, typing, playing musical instruments (deals with learned motor activities of a complex and sequential nature) Broca's area - generally on left side - in frontal lobe just superior to lateral cerebral sulcus Broca's area programs motor cortex to move tongue, lips and speech muscles to articulate words Frontal eye field area - just anterior to premotor area

4 Controls voluntary scanning movements of eye - searching for a word in a dictionary or looking for a book on a shelf or a person in a crowd Sensory - interpretation of sensory impulses; posterior to central sulcus Postcentral gyrus - primary somatosensory (sensory) - localize points on body where sensations such as touch, pressure, pain, taste, & temperature originate (Fig 16.8) Somatosensory association area - Integrate and interpret sensations - permit you to determine shape and texture of object without looking at it - store memories of past experiences - can compare sensations with previous experience Primary visual area = occipital lobe Visual association area = relates present to past visual experience with recognition and evaluation of what is seen Primary auditory area - temporal lobe - interprets basic characteristics of sound such as pitch and rhythm Auditory association area - determines if sound is speech, music, or noise Primary olfactory area - temporal lobe on medial side - interprets sensations related to smell Primary gustatory area - in parietal lobe at base of postcentral gyrus - interprets sensations related to taste Association areas - intellectual processes - occupies greater portions of lateral surfaces of occipital, parietal, temporal, and frontal lobes anterior to motor areas Association areas are concerned with memory, emotions, reasoning, will, judgment, personality and intelligence Prefrontal area executive decision area Allows moral judgments Motivation and foresight to plan; also allows us to predict consequences of our actions

5 Regulation of emotional behavior and mood Functional center for aggression - one method used to eliminate uncontrollable aggression in mental hospital was to surgically remove or destroy the prefrontal regions (prefrontal lobotomy) - it eliminates aggression but also eliminates motivation Wernicke's area - stores information for speech content, arranges words of vocabulary into meaningful speech according to rules of grammar; it plans what to say Middle White Matter = tracts of myelinated nerve fibers (axons) (Fig 14.12) Association fibers - connect and transmit nerve impulses between gyri in same hemisphere Commissural fibers - transmit nerve impulses from gyri in one hemisphere to corresponding gyri in opposite hemisphere Example: corpus callosum Projection fibers - form ascending and descending tracts that transmit impulses to and from cerebrum to other parts of brain and spinal cord Basal Nuclei - paired gray inner matter of cerebrum (Fig 14.13) Technically it is nuclei but common usage is basal ganglia Parts include: Lentiform nucleus - receives input from substantia nigra in brain stem Putamen Globus pallidus Caudate nucleus - receives input from substantia nigra Substantia nigra (in midbrain) - inhibits unwanted motor activity from basal nuclei by releasing dopamine Basal nuclei help control muscle tone by inhibiting contraction of unwanted muscular activity

6 Associated with control of large subconscious movements like swinging arms when walking/facial expression Basal nuclei coordinate slower contractions, whereas cerebellum is more involved with coordination of quick movements Disorder - Parkinson's - decrease in neurotransmitter dopamine from Substantia nigra to Basal nuclei; result is tremors Diencephalon Thalamus - "inner room" - 2 oval masses joined by a bridge called intermediate mass (interthalamic adhesion) (Fig 14.1 & 14.9) 2 masses form lateral walls of 3rd ventricle Thalamus is bounded laterally by internal capsule Gateway to cerebral cortex Incoming sensory neurons are sorted, regrouped and then sent onto proper area of cerebral cortex where interpretation is made All sensory except olfactory synapse here before being relayed to sensory part of cerebrum Thalamus could also be referred to as the "sensory relay station" Hypothalamus (Fig 14.1 & 14.10) Connected to pituitary gland via infundibulum Mammillary body - olfactory reflexes as related to emotions Controls many functions related to homeostasis: (main visceral control center) Controls autonomic nervous system Mind over body phenomenon - extensive connections between hypothalamus and cortex - thoughts influence our visceral functions - "the thought of makes me sick to my stomach" Serves as a link between nervous system and endocrine system

7 Control endocrine system: Stimulates anterior pituitary to release its hormones Control endocrine system: Produces hormones found in posterior pituitary Control body temperature - initiates sweating (cooling) or shivering (warming) Governs eating habits Governs thirst Pacemaker to drive biological rhythms Maintain waking state and sleeping patterns Center for emotional response and behavior - it lies at the "heart" of the limbic system Pleasure center Rage and aggression Pineal gland - reproductive function in most animals; in humans it produces melatonin that helps regulate sleep/wake cycle and some aspects of mood Brain Stem Brain stem - control of involuntary function Midbrain - cranial nerves 3, 4 (Fig 14.7) Corpora quadrigemina - Posterior side (tectum = roof) Superior colliculi - reflex center for movement of eye, head, neck and trunk in response to visual stimuli (contains fibers from optic tract) Inferior colliculi - reflex center for movement of eye, head, neck and trunk in response to auditory stimuli (contains fibers from auditory tract) Cerebral aqueduct - connects 3rd and 4th ventricles Cerebral peduncles - Anterior side Contain motor and sensory tracts and are the main connections for tracts between upper parts of brain and lower parts of brain and spinal cord Peduncles were thought to form vertical pillars that seem to hold up cerebrum; hence "little feet" that hold up the cerebrum

8 Red nucleus - has numerous blood vessels that give it red color Center for integrating information about muscle tone and posture Substantia nigra - pigmented nuclei included as part of basal nuclei Pons - connects medulla with midbrain and connects cerebellum with cerebrum (Fig 14.1) Cranial nerves 5-8 4th ventricle Medulla (Fig 14.6) Cranial nerves 8-12 Pyramids Anterior side Pyramids are composed of largest motor (descending) tracts that pass from cerebral cortex to spinal cord Decussation of pyramids crossing of tracts from one side to another (80% of motor tracts cross in medulla) Contains all sensory (ascending) tracts that connect the spinal cord & brain Vital reflex centers of medulla: Cardiac center - regulate heart rate and force of contraction Respiratory center - controls rate and depth of respiration Vasomotor center - adjusts blood vessel diameter to regulate blood pressure Nonvital centers: Swallowing, vomiting, coughing, sneezing, hiccupping Cerebellum Coordinates movement of skeletal muscle, especially quick movements (Fig 14.8) Maintenance of balance and equilibrium Helps in maintenance of posture Processes input from sensory receptors - through connections with cerebral cortex, cerebellum refines and coordinates muscle movements

9 Does not initiate movement Not involved in sensory perception Injury or removal of cerebellum results in impairment of muscle coordination and not paralysis Hand-eye coordination is one example of cerebellum function Functional Brain Systems Functional Brain Systems - networks of neurons that work together but span large distances within brain, so cannot be localized to a specific region Limbic system - mostly interconnected gray matter that includes components of cerebrum and diencephalons (Fig 14.14) Wishbone shaped and encircles brainstem (limbus = ring) Limbic system has extensive connections to the prefrontal cortex Sometimes called emotional brain Functions in emotional aspects of behavior related to survival Limbic system associated with pleasure, pain, rage, fear, aggressiveness Communication between the cerebral cortex and limbic system explains why emotions sometimes overrides logic and conversely why reason can stop us from expressing emotions in inappropriate situations Plays role in memory processing Because of relationship of memory to emotions, events that cause strong emotional response are remembered more efficiently that those that do not Parts include: Hippocampus - (sea horse) gyrus of the temporal lobe - in floor of the lateral ventricle Hippocampus along with cortex function in memory Amygdala - end of caudate nucleus - role in memory

10 Fornix - tracts that link the limbic system together, specifically connects hypothalamus to hippocampus (fornix = arch) Cingulate gyrus - above the corpus callosum (cingulate = belt) Olfactory bulb Mammillary bodies - part of hypothalamus (odors can trigger memory) Parts of thalamus and hypothalamus Pathway from limbic system project into hypothalamus and exert widespread effect on body via autonomic nervous system and endocrine system (most limbic output is relayed through hypothalamus) Since the hypothalamus is clearinghouse for emotional response and autonomic nervous system, it is not surprising that some people under unrelenting emotional stress fall prey to emotional-induced illness such as increased blood pressure, irritable bowel syndrome, heartburn Reticular formation - gray matter interspersed among white fibers that are found in spinal cord, medulla, pons, midbrain and diencephalon; has both sensory and motor functions (over 90 separate nuclei) (Fig 14.7) Fibers (axons) make extensive connections to diencephalon, cerebellum, cerebrum RAS - reticular activating system - control general wakefulness or alertness of brain Injury to formation results in unconsciousness or deep coma (coma can result from poison, hypoxia, hypoglycemia, ischemia, infection, acid-base disorder, trauma) Alerts cerebral cortex to incoming sensory signals from ears, eyes and skin to activate person Filters sensory impulses entering the brain and accepts and enhances relay of certain sensory impulses to cerebrum while rejecting others Protects brain from constant barrage of sensory "noise" that would be overwhelming (being aware of all stimuli in environment; colors, shapes, odors, sounds)

11 Filters out 99% of sensory and enhance only important Hyperactivity in children is due in part to inability to filter out unnecessary sensory input Meninges Dura mater outermost layer that is double membraned where it surrounds skull (Fig 14.2) Periosteal layer - outer layer - adheres to inner surface of skull (does not exist around spinal cord) Meningeal layer - inner layer - forms true external covering of brain and continues around cord Dura layers are fused together except where they form the dural sinuses that collect venous blood from brain and cerebrospinal fluid from arachnoid villi and then direct the blood and cerebrospinal fluid into the internal jugular of neck Meningeal dural mater extends inward to form flat septa (wall) that anchor brain to skull Falx cerebri (falx = sickle) - dips into longitudinal fissure between cerebrum (attached to crista galli of the ethmoid) Falx cerebelli between cerebrum and cerebellum Arachnoid mater middle layer Weblike extensions of collagen and elastic fibers never dips into sulcus Arachnoid villi protrudes through the dura mater and into dural sinuses Pia mater innermost layer composed of delicate connective tissue and is richly invested with tiny blood vessels clings tightly to brain and into sulcus Subarachnoid space space between arachnoid mater and pia mater contains cerebrospinal fluid (CSF) and blood vessels serving brain Cerebrospinal fluid - cushions, protects and nourishes brain and spinal cord

12 contains O 2, glucose, proteins, Na +, K +, Ca 2+, Cl - Choroid plexus in the brain ventricles form cerebrospinal fluid (Fig 14.4) Circulates from ventricles, through and around the spinal cord and back to the brain where it is reabsorbed at the superior sagittal sinus Volume ml (1/2 cup) Replaced every 3-4 hours Clear fluid Cranial Nerves 12 pairs - know names, numbers, functions and where they pass through the skull Oh, Oh, Oh, To Touch And Feel Very Green Vegetables AH (Fig 14.5 & Table 14.4) On Old Olympic Towering Tops A Finn And German Viewed Some Hops I. Olfactory (Fig 14.17) Passes through cribriform plate Sensory - Smell II. Optic (Fig 14.18) Passes through optic foramen Sensory - Sight III. Oculomotor (Fig 14.19) Passes through superior orbital fissure Motor - Eye movement IV. Trochlear (Fig 14.19) Passes through superior orbital fissure Motor - Eye movement V. Trigeminal (Fig 14.20) Ophthalmic - passes through superior orbital fissure Maxillary - passes through foramen rotundum

13 Mandibular - passes through foramen ovale Mixed Sensory - facial sensations Motor- chewing - (masseter/temporalis) VI. Abducens (Fig 14.19) Passes through superior orbital fissure Motor - Eye movement VII. Facial (Fig 14.21) Passes through stylomastoid foramen Mixed Sensory - anterior 2/3 taste Motor salivation and facial expression VIII. Vestibulocochlear (Auditory) (Fig 14.22) Passes through internal auditory (acoustic) meatus Sensory Vestibular - for equilibrium Cochlear- for hearing IX. Glossopharyngeal (Fig 14.23) Passes through jugular foramen Mixed Sensory - posterior 1/3 taste Motor salivation and swallowing X. Vagus (vagrant) (Fig 14.24) Passes through jugular foramen Mixed Sensory monitors functioning of the organs of thorax and abdomen

14 Motor parasympathetic regulation of the organs of the thorax and abdomen XI. Accessory (Spinal) (Fig 14.25) Passes through jugular foramen Motor - Controls the Trapezius and Sternocleidomastoid muscles XII. Hypoglossal (Fig 14.26) Passes through hypoglossal canal Motor - Tongue movements during speech and swallowing III, IV, VI, XI, XII are primarily motor but have a sensory part that is for proprioception