Note to Self. The Brain and Cranial Nerves. Organization of the Brain

Transcription

1 Note to Self Despite what you think you can get through this chapter in 1 class period, or one with only a few slides left over The Brain and Cranial Nerves Chapter 14 Organization of the Brain The brain is divided into six major regions Cerebrum Diencephalon Mesencephalon Pons Cerebellum Medulla oblongata In addition, the brain contains a layer of gray matter on the surface of the cerebellum and cerebrum called the neural cortex 1

2 Embryology of the brain The brain forms from three swellings (the primary brain vesicles) at the tip of the developing neural tube The prosencephalon forms the telencephalon (cerebrum) and diencephalon Mesencephalon (midbrain) The rhombencephalon forms the metencephalon (cerebellum and pons) and myelencephalon (medulla oblongata) Summarized in table 14-1 p 451 Ventricles of The Brain During development the nueroceol expands to form chambers that, in the mature brain, are called ventricles In the mature brain, the ventricles are filled with cerebrospinal fluid (CSF) which circulates from the ventricles and central canal of the brain into the subarachnoid space Protection and support of The Brain The brain is protected from mechanical forces by 1. The bones of the cranium 2 The cranial meninges 2. The cranial meninges 3. The Cerebrospinal Fluid In addition, the neural tissue of the brain is isolated from general circulation by the blood-brain barrier 2

3 Cranial Meninges The cranial meninges are continuous with those of the spinal cord, they do, however, have distinctive i i anatomical and functional characteristics Dura Mater The cranial dura mater consists of outer and inner fibrous layers The outer layer fuses with the periosteum of the cranial bones, thus there is no epidural space The inner layer of the dura mater extends via dural folds into the cranial cavity which provide support and stabilization to the brain The three largest dural folds are the falx cerebri (between the hemispheres), the tentorium cerebelli (above the cerebellum), and the falx cerebelli (below the cerebellum) The Arachnoid The cranial arachnoid covers the brain, providing a smooth surface that does not follow the folds of the brain This membrane is in contact externally with the inner epithelium of the dura mater. Internally, the subarachnoid space separates the arachnoid from the pia mater Pia Mater The pia mater is closely appressed to the surface of the brain It is anchored to the brain by astrocytes and extends into every fold of the brain 3

4 Cerebrospinal Fluid Cerebrospinal fluid (CSF) completely surrounds and bathes the exposed surfaces of the CNS The CSF has several functions, including Cushioning the neural structures Supporting the brain, the brain is almost neutrally buoyant while floating in the CSF Transporting nutrients, chemical messengers, and waste products Circulation of CSF Inside the brain, the choroid plexus in the third ventricle, contains ependymal cells that produce about 500 ml of CSF per day CSF reaches the subarachnoid space via two lateral apertures and one medial aperture in the roof of the fourth ventricle The CSF then flows within the subarachnoid space around the brain, spinal cord, and cauda equina CSF then is absorbed into the venous circulation at arachnoid granulations The entire volume of CSF is replaced roughly every eight hours Blood Supply To The Brain The brain has a high demand for energy, but no energy or oxygen reserves As a result, the brain has an extensive circulatory supply Arterial blood enters the brain via the internal carotid arteries and vertebral arteries and venous blood leaves via the internal jugular veins which drain the dural sinuses A cerebrovascular accident, or stroke, occurs when the blood supply to a portion of the brain is interrupted. The blood-brain barrier The blood-brain barrier isolates neural tissue from general circulation This barrier exists because the endothelial cells lining the capillaries of the CNS are connected by tight junctions that prevent the diffusion of non-lipid soluble compounds into the interstitial fluid of the CNS Such a restriction is necessary to prevent uncontrolled stimulation of neurons in the brain by circulating hormones 4

5 The blood-brain barrier The blood-brain barrier is an incomplete barrier in some areas Parts of the hypothalamus Pituitary i gland Pineal gland Choroid plexus (read excerpt p456 in Martini) 456 The blood-brain barrier remains intact throughout the CNS, with four exceptions: 1. In portions of the hypothalamus, the capillary endothelium is extremely permeable. This permeability exposes hypothalamic nuclei to circulating hormones and permits the diffusion of hypothalamic hormones into the circulation Capillaries in the posterior lobe of the pituitary gland, which is continuous with the floor of the hypothalamus, are highly permeable. At this site, the hormones ADH and oxytocin, produced by hypothalamic neurons, are released into the circulation 3. Capillaries in the pineal gland are also permeable. The pineal gland is located on the posterior, superior surface of the diencephalon. Capillary permeability here allows pineal hormones into the general circulation Capillaries at the choroid plexus are extremely permeable. Although the capillary characteristics of the blood-brain barrier are lost there, the transport activities of the ependymal cells in the choroid plexus maintain the blood- CSF barrier The Medulla Oblongata The medulla oblongata is continuous with the spinal cord All communication between the brain and spinal cord passes through the medulla oblongata It is a center for the coordination of complex autonomic reflexes and visceral function See 14-6 a & b and table 14-2 Reminder Slide A collection of neuron cell bodies with a common function is a center A center with a discrete boundary is termed a nucleus The neural cortex is a thick layer of gray matter covering the surface of the brain Bundles of axons in the CNS are termed tracts Tracts in the spinal cord form columns 5

6 The Medulla Oblongata The medulla oblongata contains three important groups of nuclei Autonomic nuclei controlling visceral activities (heart rate, peripheral bloodflow, respiratory movements) Sensory and motor nuclei of cranial nerves Relay stations along sensory and motor pathways The Pons The pons contains four groups of components Sensory and motor nuclei of cranial nerves Nuclei involved with the control of respiration Nuclei and tracts that process and relay information to/from the cerebellum Ascending, descending and transverse tracts And see table

7 The Cerebellum The cerebellum is an automatic processing center and has two primary functions Adjusting the postural muscles of the body (balance and equilibrium) Programming and fine tuning movements controlled at the subconscious levels The Mesencephelon The tectum or roof of the mesencephelon, or midbrain, contains two pairs of sensory nuclei called the corpora quadrigemina These nuclei, the superior and inferior colliculi, process visual and auditory sensation respectively, and control movement of the eyes, head, and neck in response to those stimuli And see table 14-4 Table 14-4 Components and Functions of the Midbrain 7

8 The Diencephalon The diencephalon is composed of The epithalamus The hypothalamus The thalamus The epithalamus is the roof of the diencephalon superior to the third ventricle The posterior portion of the epithalamus contains the pineal gland, an endocrine gland that secretes melatonin The Thalamus The thalamus is the final relay point for ascending sensory information that will be projected to the sensory cortex It acts as a filter, passing only a small portion of the arriving information The left and right thalamus are separated by the third ventricle Each thalamus consists of a rounded mass of thalamic nuclei that project information to specific regions of the sensory cortex The Hypothalamus The hypothalamus extends from the area superior to the optic chiasm to the posterior margins of the mamillary bodies Immediately yposterior to the optic chiasm, a narrow stalk, the infundibulum connects the floor of the hypothalamus to the pituitary gland The hypothalamus contains important control and integrative systems with numerous functions The Hypothalamus Controls somatic motor activities at the subconscious level Controls autonomic function (HR, BP, Resp, Dig) Coordinates activities of the endocrine and nervous systems Secretes hormones (ADH and Oxytocin) Produces emotions and behavioral drives (hunger, thirst) Coordinates voluntary and autonomic functions Regulates body temperature Coordinates circadian cycles of activity 8

9 The Limbic System The limbic system includes nuclei and tracts along the border between the cerebrum and diencephalon Its functions include Establishing emotional states Linking the conscious functions of the cerebral cortex with the unconscious functions of the brain stem Facilitating memory retrieval and storage The limbic system is known as the motivational system because it provides desire to perform the complex tasks that the complexity of our brains makes possible The Cerebrum The cerebrum is the largest region of the brain. Conscious thought and all intellectual functions originate in the cerebral hemispheres Much of the cerebrum is involved din processing somatic sensory and motor information See figure p 469 Figure 14-12a The Brain in Lateral View Figure 14-12b The Brain in Lateral View FRONTAL Central sulcus PARIETAL FRONTAL Precentral gyrus Central sulcus Postcentral gyrus PARIETAL OCCIPITAL OCCIPITAL Lateral sulcus TEMPORAL Pons Medulla oblongata Lateral view, cadaver brain Cerebellum Lateral sulcus Pons Medulla oblongata Lateral view TEMPORAL Cerebellum 9

10 The Cerebral Cortex Surface contains gyri (folds) and sulci (fissures) A longitudinal fissure separates two cerebral hemispheres The central sulcus separates frontal and parietal lobes The temporal and occipital lobes are also bounded by sulci The Cerebral Cortex Each lobe contains functional regions whose boundaries are less clearly defined Some things to keep in mind Each cerebral hemisphere receives sensory if information i from and sends motor commands to, the opposite side of the body The 2 hemispheres have different functions even though they appear identical The assignment of specific functions to specific regions of the cerebral cortex is imprecise White matter of the Cerebrum The interior of the cerebrum is principally composed of white matter The axons are classified as: Association i fibers, which h interconnect areas of the cerebral cortex within a single hemisphere Shorter arcuate fibers connect adjacent gyri Longer longitudinal fasciculi connect the frontal lobe and other lobes of the same hemisphere Figure 14-13a Fibers of the White Matter of the Cerebrum Arcuate fibers Longitudinal fasciculi White matter of the Cerebrum Commisural fibers interconnect the cerebral hemispheres Includes the corpus callosum and anterior commisure Projection fibers link the cerebral cortex and the diencephalon, brain stem, cerebellum, and spinal cord Lateral view 10

11 Longitudinal fissure Corpus callosum Projection fibers of internal capsule Anterior commissure Basal Nuclei The basal nuclei are involved with the subconscious control of skeletal muscle and the coordination of learned movements They do not initiate the movement but rather control the general pattern and rhythm, especially for movements of the trunk and proximal limb muscles Anterior view Basal Nuclei The basal nuclei are masses of gray matter that lie within each hemisphere. They are embedded in the white matter of the cerebrum, and radiating gprojection fibers and commissural fibers travel between them The principal nuclei are: The caudate nucleus The lentiform nucleus The globus palidus The putamen Motor and Sensory areas of the Cortex The central sulcus separates the motor and sensory areas of the cortex The surface of the pre-central gyrus of the frontal lobe is the primary motor cortex Nuerons here direct voluntary movement by controlling somatic motor neurons in the brain stem and spinal cord Specific regions of the primary motor cortex correspond to specific skeletal muscles 11

12 Primary motor cortex (precentral gyrus) Somatic motor association area (premotor cortex) Central sulcus Primary sensory cortex (postcentral gyrus) PARIETAL FRONTAL Prefrontal cortex Gustatory cortex Insula Lateral sulcus Olfactory cortex TEMPORAL Somatic sensory association area Visual association area OCCIPITAL Visual cortex Auditory association area Auditory cortex Frontal eye field Speech center Prefrontal cortex Major anatomical landmarks on the surface of the left cerebral hemisphere. The lateral sulcus has been pulled apart to expose the insula. General interpretive area The left hemisphere generally contains the general interpretive area and the speech center. The prefrontal cortex of each hemisphere is involved with conscious intellectual functions. Regions of the cerebral cortex as determined by histological analysis. Several of the 47 Brodmann areas are shown for comparison with the results of functional mapping. Motor and Sensory areas of the Cortex The surface of the post-central gyrus of the parietal lobe contains the primary sensory cortex Neurons here receive sensory info from receptors for touch, pressure, pain, vibration, taste, or temperature We are aware of these sensations only when the thalamus relays it Primary motor cortex (precentral gyrus) Somatic motor association area (premotor cortex) FRONTAL Prefrontal cortex Gustatory cortex Insula Lateral sulcus Olfactory cortex Figure 14-15a Motor and Sensory Regions of the Cerebral Cortex Central sulcus TEMPORAL Major anatomical landmarks on the surface of the left cerebral hemisphere. The lateral sulcus has been pulled apart to expose the insula. Primary sensory cortex (postcentral gyrus) PARIETAL Somatic sensory association area Visual association area OCCIPITAL Visual cortex Auditory association area Auditory cortex Motor and Sensory areas of the Cortex Sensations of light, sound, smell, and taste arrive at other areas of the cerebral cortex The visual cortex is in the occipital lobe The auditory and olfactory cortices are in the temporal lobe The gustatory cortex lies in the anterior portion of the insula, a covered region of the cortex near the frontal lobe Primary motor cortex (precentral gyrus) Somatic motor association area (premotor cortex) FRONTAL Prefrontal cortex Gustatory cortex Insula Lateral sulcus Olfactory cortex Figure 14-15a Motor and Sensory Regions of the Cerebral Cortex Central sulcus TEMPORAL Major anatomical landmarks on the surface of the left cerebral hemisphere. The lateral sulcus has been pulled apart to expose the insula. Primary sensory cortex (postcentral gyrus) PARIETAL Somatic sensory association area Visual association area OCCIPITAL Visual cortex Auditory association area Auditory cortex 12

13 Association Areas The sensory and motor regions of the cortex are connected to nearby association areas Association areas are regions of the cortex that interpret incoming data and coordinate a motor response The somatic sensory area monitors activity in the primary sensory cortex The visual association area monitors the patterns of activity in the visual cortex and interprets the results; the ability to read is governed by this area Association Areas The auditory association area monitors sensory activity in the auditory cortex, word recognition occurs here The somatic motor association area or premotor cortex is responsible for the coordination of learned movements The premotor cortex relays the instructions to the primary motor cortex Primary motor cortex (precentral gyrus) Somatic motor association area (premotor cortex) FRONTAL Prefrontal cortex Gustatory cortex Insula Lateral sulcus Olfactory cortex Figure 14-15a Motor and Sensory Regions of the Cerebral Cortex Central sulcus TEMPORAL Major anatomical landmarks on the surface of the left cerebral hemisphere. The lateral sulcus has been pulled apart to expose the insula. Primary sensory cortex (postcentral gyrus) PARIETAL Somatic sensory association area Visual association area OCCIPITAL Visual cortex Auditory association area Auditory cortex Integrative Centers Integrative centers are areas that receive information from many association areas and direct extremely complex motor activities The prefrontal cortex performs abstract intellectual functions, such as predicting the consequences of possible responses Feelings of frustration, tension and anxiety are generated in the prefrontal cortex as it makes predictions about future situations and consequences Integrative Centers The general interpretive area or Wernicke s area receives information from all of the sensory association areas and plays an important role in personality by integrating sensory data and coordinating access to visual and auditory memories Wernicke s area is typically restricted to the left hemisphere Integrative Centers The speech center or Broca s area lies along the edge of the premotor cortex in the same hemisphere as the general interpretive area (left) The speech center regulates the pattern of breathing and vocalization needed for speech 13

14 Frontal eye field Speech center Prefrontal cortex Figure 14-15b Motor and Sensory Regions of the Cerebral Cortex General interpretive area The left hemisphere generally contains the general interpretive area and the speech center. The prefrontal cortex of each hemisphere is involved with conscious intellectual functions. Hemispheric Lateralization In most people the left hemisphere contains the general interpretive and speech centers responsible for language based skills; reading, writing, speaking The left hemisphere also is important in performing analytical tasks, such as mathematics or logical decision making The right hemisphere analyzes spatial relationships and allows the recognition of familiar objects; face recognition. It is also important in interpreting the emotional context of a conversation Left Cerebral Hemisphere Right Cerebral Hemisphere The Electroencephalogram LEFT HAND Prefrontal cortex Speech center Writing Auditory cortex General interpretive center (language and mathematical calculation) C O RP U S C A L L O SU M RIGHT HAND Prefrontal cortex Anterior commissure Analysis by touch Auditory cortex Spatial visualization and analysis The activity of neurons generates electrical activity that can be monitored by placing electrodes on the skull This electrical activity of the brain is commonly monitored to assess brain activity A graphical report of such activity is termed and electroencephalogram or EEG The electrical patterns observed are termed brain waves Visual cortex (right visual field) Visual cortex (left visual field) The Electroencephalogram Apha waves occur in the brains of healthy adults when they are awake and resting with their eyes closed Beta waves are typical of individuals who are concentrating, or under stress, or in a state of psychological tension Theta waves are observed transiently in sleeping adults, but are common in children and frustrated adults Delta waves are normally seen in deeply sleeping adults, infants, and in waking adults with brain damage Figure Brain Waves Patient being wired for EEG monitoring Alpha waves are characteristic of normal resting adults Beta waves typically accompany intense concentration Theta waves are seen in children and in frustrated adults Delta waves occur in deep sleep and in certain pathological conditions 0 Seconds

15 The Cranial Nerves Cranial nerves are PNS components that connect directly to the brain rather than to the spinal cord The 12 pairs of cranial nerves are named either by appearance or function of the nerve The number assigned to each nerve roughly corresponds to its position along the longitudinal axis of the brain beginning at the cerebrum and moving downwards Olfactory bulb: termination of olfactory nerve (I) Olfactory tract Optic nerve (II) Infundibulum Oculomotor nerve (III) Pons Basilar artery Vertebral artery Cerebellum Medulla oblongata Spinal cord Figure Origins of the Cranial Nerves Optic chiasm Optic tract Mamillary body Trochlear nerve (IV) Trigeminal nerve (V) Abducens nerve (VI) Facial nerve (VII) Vestibulocochlear nerve (VIII) Glossopharyngeal nerve (IX) Vagus nerve (X) Hypoglossal nerve (XII) Accessory nerve (XI) The Mnemonic Oh, Once One Takes The Anatomy Final, Very Good Vacations Are Heavenly Old Opie Occasionally Tries Trigonometry And Feels Very y Gloomy y Vague g And Hypoactive Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal Olfactory nerves (I) Carry sensory information responsible for the sense of smell Synapse within the olfactory bulb 15

16 Cranial nerves II, III, IV Optic nerves (II) Carry visual information from special sensory receptors in the eyes Occulomotor nerves (III) Primary source of innervation for 4 of the extraocular muscles Trochlear nerves (IV) Innervate the superior oblique muscles Cranial nerves V, VI, VII Trigeminal nerves (V) Mixed nerves (sensory and motor) with ophthalmic, maxillary and mandibular branches Abducens nerve (VI) Innervates the lateral rectus muscles Facial nerves (VII) Mixed nerves that control muscles of the face and scalp Provide pressure sensations over the face Receive taste information from the tongue Cranial nerves VIII, IX Vestibulocochlear nerves (VIII) Vestibular branch monitors balance, position and movement Cochlear branch monitors hearing Glossopharyngeal nerves (IX) Mixed nerves that innervate the tongue and pharynx Control the action of swallowing 16

17 Cranial nerves X Vagus nerves (X) Mixed nerves that are vital to the autonomic control of visceral function Cranial nerves XI, XII Accessory nerves (XI) Internal branches innervate voluntary swallowing muscles of the soft palate and pharynx External branches control muscles associates with the pectoral girdle Hypoglossal nerves (XII) Provide voluntary motor control over tongue movement 17