Chapter 14: The Brain and Cranial Nerves
Development of the Brain Three to four-week embryo: prosencephalon, mesencephalon and rhombencephalon. Five-week embryo: telencephalon (cerebrum), diencephalon (thalamus, hypothalamus and epithalamus), mesencephalon (midbrain), metencephalon (pons and cerebellum) and myelencephalon (medulla oblongata).
Development of the Brain
Major Parts of the Brain Brain stem- continuation of the spinal cord; consists of the medulla oblongata, pons and midbrain. Cerebellum- second largest part of the brain. Diencephalon- gives rise to thalamus, hypothalamus and epithalamus. Cerebrum- largest part of the brain.
The Brain
Protective Coverings of the Brain The cranium The cranial meninges: dura mater, arachnoid mater and pia mater.
Three extensions of the dura mater separate parts of the brain: a. Falx cerebri separate the two cerebral hemispheres. b. Falx cerebelli separate the two cerebellar hemispheres. c. Tentorium cerebelli separate the cerebrum from the cerebellum.
Extensions of the Dura Mater
Brain Blood Flow and the Blood-Brain Barrier Brain receives approximately 20% of the total blood supply. Internal carotid and vertebral arteries carry blood to the brain. Internal jugular veins return blood from the brain. Blood-brain barrier (BBB) protects brain from harmful substances.
Cerebrospinal Fluid (CSF) Clear fluid. Circulates through cavities in the brain (ventricles) and the spinal cord (central canal) and also in the subarachnoid space. Absorbs shock and protects the brain and the spinal cord. Helps transport nutrients and wastes from the blood and the nervous tissue.
Ventricles CSF-filled cavities within the brain. Lateral ventricles: cerebral hemispheres. Third ventricle: diencephalon. Cerebral aqueduct: midbrain. Fourth ventricle: brain stem and the cerebellum.
Formation and Circulation of CSF in the Ventricles Choroid plexuses- networks of capillaries in the walls of the ventricles. Ventricles are lined by ependymal cells. Plasma is drawn from the choroid plexuses through ependymal cells into the ventricles to produce CSF.
Formation and Circulation of CSF in the Ventricles
Circulation of CSF CSF from the lateral ventricles interventricular foramina third ventricle cerebral aqueduct fourth ventricle subarachnoid space or central canal. CSF is reabsorbed into the blood by arachnoid villi.
Medulla Oblongata Pyramids- Bulges on the anterior aspect of the medulla. Formed by the large corticospinal tracts that pass from the cerebrum to the spinal cord. A common site for decussation of ascending and descending tracts.
Centers of the Medulla Oblongata Vital centers: Cardiovascular center- Respiratory center- Also includes centers for vomiting, swallowing, sneezing, coughing and hiccupping. Houses five pairs of cranial nerves, VIII-XII. Portion of the ventricle found here is the fourth ventricle.
Midbrain or Mesencephalon Extends from the pons to the diencephalon. Part of the ventricle found here- cerebral aqueduct. Cerebral peduncles: axons of the corticospinal, corticopontine and corticobulbar tracts. Tectum- situated posteriorly and contains four rounded elevations: two superior ones called superior colliculi and two inferior ones called inferior colliculi.
Midbrain or Mesencephalon
Midbrain Substantia nigra: large area with dark pigments. Help control subconscious muscle activities. Loss of neurons here is associated with Parkinson disease. Red nucleus: Help control voluntary movements of the limbs. Contains cranial nerves III-IV.
Midbrain
Reticular formation Extends from the upper part of the spinal cord, throughout the brain stem, and into the lower part of the diencephalon. Part of the reticular formation called the reticular activating system (RAS) consists of sensory axons that project to the cerebral cortex. The RAS helps maintain consciousness.
The Cerebellum Second largest part of the brain. The central constricted area is the vermis. The anterior and posterior lobes control subconscious aspects of skeletal movement. The flocculonodular lobe on the inferior side contributes to the equilibrium and balance.
The Cerebellum
The Cerebellum Cerebellar cortex- gray matter in the form of parallel folds called folia. Arbor vitae- tracts of white matter. Cerebellar peduncles- three pairs: superior, middle and inferior. Attach cerebellum to the brain stem. Functions- coordinate movements, regulate posture and balance.
Thalamus Intermediate mass Several nuclei: Major relay station for most sensory impulses.
Hypothalamus Inferior to the thalamus. Consists of mammillary body, median eminence, infundibulum, and a number of nuclei.
Functions of the Hypothalamus Control of the ANS. Production of hormones Regulation of emotional and behavioral patterns, eating and drinking, body temperature, and circadian rhythms.
Epithalamus Small region superior to the thalamus. Consists of pineal gland which secretes a hormone called melatonin. Melatonin induces sleep.
The Cerebrum seat of intelligence Cerebral cortex- gray matter. Gyri- Sulci- Longitudinal fissure- Cerebral hemispheres-
The Cerebrum
Lobes of the Cerebrum Four lobes: frontal lobe, parietal lobe, temporal lobe and occipital lobe. Central sulcus- separates the frontal and parietal lobes. Precentral gyrus- primary motor area. Postcentral gyrus- primary somatosensory area.
Cerebral white matter Commissural tracts- Corpus callosum: Association tracts- Projection tracts-
Basal Ganglia Three nuclei deep within each cerebral hemisphere make up basal ganglia. They are globus pallidus, putamen, and caudate nucleus. Help initiate and terminate movements, suppress unwanted movements and regulate muscle tone.
Basal Ganglia
The Limbic System A ring of structures on the inner border of the cerebrum and floor of the diencephalon. Includes cingulate gyrus, hippocampus, dentate gyrus, amygdala, mammillary bodies, thalamus, and the olfactory bulb. emotional brain as it governs emotional aspects of behavior. Also involved in olfaction and memory.
The Limbic System
Functional Organization of the Cerebral Cortex: Sensory Areas Primary somatosensory area- postcentral gyrus. Primary visual area- occipital lobe. Primary auditory area- temporal lobe. Primary gustatory area- base of the postcentral gyrus. Primary olfactory area- temporal lobe.
Functional Organization of the Cerebral Cortex: Sensory Areas
Functional Organization of the Cerebral Cortex: Motor Areas Primary motor areaprecentral gyrus. Broca s speech arealeft cerebral hemisphere.
Functional Organization of the Cerebral Cortex: Association Areas Somatosensory association area- posterior to primary somatosensory area. Visual association area- occipital lobe. Auditory association area- temporal lobe. Wernicke s area- left temporal and parietal lobes. Prefrontal cortex- anterior portion of the frontal lobe.
Hemispheric Lateralization
Brain Waves Electroencephalogram: Recording of the electrical activity within the brain. Alpha waves Beta waves Theta waves Delta waves
Cranial Nerves 12 pairs. Sensory, motor and mixed nerves. Name as well as roman numeric numbers to identify the nerves.
Olfactory (I) Nerve Sensory nerve. Sense of smell. Olfactory cells converge to become olfactory nerve.
Optic (II) Nerve Sensory nerve. Ganglion cells in the retina of each eye join to form an optic nerve. Nerve of vision.
Oculomotor (III) Nerve Motor cranial nerve. Originates in the midbrain. Supply extrinsic eye muscles to control movements of the eyeball and upper eyelid.
Trochlear (IV) Nerve Motor cranial nerve. Smallest of the 12 cranial nerves. Origin: midbrain. Controls movement of the eyeball.
Trigeminal (V) nerve Largest cranial nerve. Mixed nerve. Three branches: opthalmic, maxillary and mandibular. Deal with sensation of touch, pain and temperature. Motor axons supply muscles of mastication.
Abducens (VI) Nerve Motor cranial nerve. Originates from the pons. Cause abduction of the eyeball (lateral rotation).
Facial (VII) Nerve Mixed cranial nerve. Sensory portion extends from the taste buds of the anterior two-thirds of the tongue. Motor portion arises from the pons and deal with facial expression.
Vestibulocochlear (VIII) Nerve Sensory cranial nerve. Originates in the inner ear. Vestibular branch carries impulses for equilibrium. Cochlear branch carries impulses for hearing.
Glossopharyngeal (IX) Nerve Mixed cranial nerve. Sensory axons carry signals from the taste buds of the posterior one-third of the tongue. Motor neurons arise from the medulla and deal with the release of saliva.
Vagus (X) Nerve
Vagus (X) Nerve Mixed cranial nerve. Distributed from the head and neck into the thorax and abdomen. Sensory neurons deal with a variety of sensations such as proprioception, and stretching. Motor neurons arise from the medulla and supply muscles of the pharynx, larynx, and soft palate that are involved in swallowing and vocalization.
Accessory (XI) Nerve Motor cranial nerve. Divided into cranial accessory and spinal accessory nerves. Supplies sternocleidomastoid and trapezius muscles to coordinate head movements.
Hypoglossal (XII) Nerve Motor cranial nerve. Conduct nerve impulses for speech and swallowing.
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