The Brain and the Cranial Nerves: An Overview of Structure, Function, and Higher-Order Processes

Cranial Nerves • 12 pair of nerves that arise from brain and exit through cranial foramina leading to muscles, glands and sense organs in head and neck • Learn Number, Name and Function for a matching section on the lab practical. Cranial Nerves Language • Includes reading, writing, speaking and understanding words. • Wernicke’s Area is used to recognize spoken and written language and creates the plan of speech. • Angular Gyrus allows us to read out loud (processes text into a form we can speak). (left hemisphere) • Broca’s Area generates a motor program for the muscles of the larynx, tongue, cheeks and lips and transmits it to the speech Reticular Activating System • Nuclei scattered throughout pons, medulla and midbrain relay information from eyes and ears to cerebral cortex. • Some tracts regulate balance and posture. • Gaze centers allow you to track moving objects. • Initiates habituation to repetitive, unimportant stimuli (background noise). • Regulates sleep and conscious attention. • Injury leads to irreversible coma. • Basal Nuclei • Masses of gray matter deep to cerebral cortex that receive input from substantia nigra and motor cortex and send signals back to these regions. • Important in motor control. • Longitudinal fissure separates 2 cerebral hemispheres. • Surface features: • gyri (singular = gyrus) are the raised folds • sulci (singular = sulcus) are the grooves in between gyri Inferior View of the Brain Median Section of the Brain Photo of Sagittal Section of Brain Brain – Directional Terms and Landmarks • Rostral (toward the forehead) - Caudal (toward the cord) • Major parts of the brain - cerebrum, cerebellum, brainstem – cerebrum is 83% of brain volume; cerebellum contains 50% of the neurons – brain weighs 3 to 3.5 pounds Blood-Brain and Blood-CSF Barriers • Blood-brain barrier is tightly joined endothelium – permeable to lipid-soluble materials (alcohol, O2, CO2, nicotine and anesthetics) – circumventricular organs in 3rd & 4th ventricles at breaks in the barrier where blood has direct access • monitoring of glucose, pH, osmolarity & other variations • allows route for HIV virus to invade the brain • Blood-CSF barrier at choroid plexus is ependymal cells joined by tight junctions Olfactory Nerve • Provides sense of smell • Damage causes impaired sense of smell Optic Nerve • Provides vision • Damage causes blindness in visual field Oculomotor Nerve • Provides some eye movement, opening of eyelid, constriction of pupil, focusing • Damage causes drooping eyelid, dilated pupil, double vision, difficulty focusing & inability to move eye in certain directions Trochlear Nerve • Provides eye movement • Damage causes double vision & inability to rotate eye inferolaterally Trigeminal Nerve • Main sensory nerve to face (touch, pain and temperature) and muscles of mastication • Damage produces loss of sensation & impaired chewing Abducens Nerve • Provides eye movement • Damage results in inability to rotate eye laterally & at rest eye rotates medially Facial Nerve • Provides facial expressions, sense of taste on anterior 2/3’s of tongue, salivary glands and tear, nasal & palatine glands • Damage produces sagging facial muscles & disturbed sense of taste (missing sweet & salty) Branches of Facial Nerve Clinical test: Test anterior 2/3’s of tongue with substances such as sugar, salt, vinegar, and quinine; test response of tear glands to ammonia fumes; test motor functions by asking subject to close eyes, smile, whistle, frown, raise eyebrows, etc. Vestibulocochlear Nerve • Provides hearing & sense of balance • Damage produces deafness, dizziness, nausea, loss of balance & nystagmus Glossopharyngeal Nerve • Provides control over swallowing, salivation, gagging, sensations from posterior 1/3 of tongue, control of BP and respiration • Damage results in loss of bitter & sour taste & impaired swallowing Vagus Nerve • Provides swallowing, speech, regulation of viscera • Damage causes hoarseness or loss of voice, impaired swallowing & fatal if both are cut Accessory Nerve • Provides swallowing, head, neck & shoulder movement • Damage causes impaired head, neck & shoulder movement, head turns towards injured side Hypoglossal Nerve • Provides tongue movements of speech, food manipulation & swallowing • Damage results in inability to protrude tongue if both are damaged or deviation towards injured side & ipsilateral atrophy if one side is damaged Cranial Nerve Disorders • Trigeminal neuralgia (tic douloureux) – recurring episodes of intense stabbing pain in trigeminal nerve area (near mouth or nose) – pain triggered by touch, drinking, washing face – treatment is cutting of nerve • Bell palsy – degenerative disorder of facial nerve – paralysis of facial muscles on one side – may appear abruptly & disappear within 3-5 weeks Meninges • Dura mater -- outermost, tough membrane – outer periosteal layer against bone – where separated from inner meningeal layer forms dural venous sinuses draining blood from brain – supportive structures formed by dura mater • falx cerebri, falx cerebelli and tentorium cerebelli – epidural space filled with fat in lower back region • epidural anaesthesia during childbirth • Arachnoid mater is spider web filamentous layer • Pia mater is a thin vascular layer adherent to contours of brain Ventricles of the Brain NOTE bad lines Medulla and Pons Olive Cerebellum EEG and Brain Waves • Electroencephalogram records voltage changes from postsynaptic potentials in cerebral cortex • Differences in amplitude & frequency distinguish 4 types of brain waves Brain Waves & Sleep • States of consciousness can be correlated with EEG • 4 types of brain waves – – – – alpha occur when awake & resting with eyes closed beta occur with eyes open performing mental tasks theta occur during sleep or emotional stress delta occur during deep sleep • Sleep is temporary state of unconsciousness – coma is state of unconsciousness with no possible arousal – reticular formation seems to regulate state of alertness – suprachiasmatic nucleus acts as biological clock to set our circadian rhythm of sleep and waking Stages of Sleep • Non-REM sleep occurs in stages – 4 stages occurring in first 30 to 45 minutes of sleep • stage 1 is drifting sensation (would claim was not sleeping) • stage 2 still easily aroused • stage 3 vital signs change -- BP, pulse & breathing rates drop – reached in 20 minutes • stage 4 is deep sleep -- difficult to arouse – seems to have a restorative effect • REM sleep occurs about 5 times a night – rapid eye movements under the eyelids, vital signs increase, EEG resembles awake person, dreams and penile erections occur – may help sort & strengthen information from memory Sleep Stages and Brain Waves • Brain waves change as we pass through 4 stages of sleep – – – – alpha waves sleep spindles theta delta waves Sleep Stages • Notice how REM sleep periods become longer and more frequent in the second half of the night Cognition • Cognition is mental processes such as awareness, perception, thinking, knowledge & memory – 75% of brain is association areas where integration of sensory & motor information occurs • Examples of effects of brain lesions – parietal lobe -- contralateral neglect syndrome – temporal lobe -- agnosia (inability to recognize objects) or prosopagnosia (inability to recognize faces) – frontal lobe -- problems with personality (inability to plan & execute appropriate behavior) Memory • Information management requires learning, memory & forgetting (eliminating the trivia) – pathological inability to forget have trouble with reading comprehension – anterograde amnesia -- can not store new data – retrograde amnesia -- can not remember old data • Hippocampus is important in organizing sensory & cognitive information into a memory – lesion to it causes inability to form new memories • Cerebellum helps learn motor skills • Amygdala important in emotional memory Emotion • Prefrontal cortex controls how emotions are expressed (seat of judgement) • Emotions form in hypothalamus & amygdala – artificial stimulation produces fear, anger, pleasure, love, parental affection, etc. – electrode in median forebrain bundle in rat or human and a foot pedal • press all day to the exclusion of food (report a quiet, relaxed feeling) • Much of our behavior is learned by rewards and punishments or responses of others to them PET Scans during a Language Task Functional Regions of Cerebral Cortex Input and Output to Cerebellum • Smoothes muscle contractions, maintains muscle tone & posture, coordinates motions of different joints, aids in learning motor skills & coordinates eye movements