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The Brain • 100 billion neurons • ~1000 trillion synapses • Brain functions: • sensations • memory • emotions • decision making • behavior Chapter 14 The Brain and Cranial Nerves Overview: Organization, protection, blood supply, parts of the brain and their functions 2 1 Synaptic terminals Dendrites Inhibitory Principal Parts of the Brain Excitatory • Cerebrum • Diencephalon – thalamus & hypothalamus Cerebellum • Brainstem • – medulla, pons & midbrain Myelin sheath Receiving cell body Axon Synaptic termini 3 4 Protective Coverings of the Brain Blood Supply to Brain • Uses 20% of our bodies oxygen & glucose needs • Bone, meninges & fluid • Meninges same as around the spinal cord – blood flow to an area increases with activity in that area – because carbohydrate storage in the brain is limited, the supply of glucose to the brain must be continuous – deprivation of O2 for 4 min causes permanent damage – dura mater – arachnoid mater – pia mater • Blood-brain barrier (BBB) – formed by tight junctions between epithelial cells, thick basement membrane & astrocytes covering capillaries – allows passage of some molecules (e.g., O2, glucose, alcohol, anesthetics) – but not others (e.g., proteins, antibiotics, many drugs) – protects cells from some toxins and pathogens – injury to the brain due can cause a breakdown of the BBB • Dura mater extensions – falx cerebri – tentorium cerebelli – falx cerebelli 5 6 Ventricles Cerebrospinal Fluid (CSF) • 80-150 ml (3-5 oz., ½ cup) • Clear liquid containing glucose, proteins, & ions • Functions – mechanical protection • floats brain & softens impact with bony walls – chemical protection • optimal ionic concentrations for action potentials – circulation • nutrients and waste products to and from bloodstream • There are four CSF filled cavities within the brain called ventricles – 2 lateral ventricles, one within each cerebral hemisphere – third ventricle – fourth ventricle • Choroid plexus = capillaries covered by ependymal cells – produce CSF 7 8 THE BRAIN STEM Medulla Oblongata • Medulla oblongata • Pons • Midbrain (mesencephalon) • • • • • Continuation of spinal cord Ascending sensory tracts Descending motor tracts Nuclei of 5 cranial nerves Cardiovascular center – force & rate of heart beat – diameter of blood vessels • Respiratory center – sets basic rhythm of breathing • Reflex centers for coughing, sneezing, swallowing, etc. 9 Medulla Oblongata 10 Pons • Links parts of the brain with one another • Helps control breathing • Cranial nerves 5 through 7 • Decussation (crossing over) of sensory and motor fiber tracts – left cortex controls right muscles • Hard blow to the back of the head may be fatal • Loss of sensation or paralysis of throat or tongue; irregularities in breathing and heart rhythm 11 12 Dorsal Surface of Midbrain Midbrain • Extends from pons to diencephalon • Cerebral aqueduct connects 3rd ventricle above to 4th ventricle below • Corpora quadrigemina = superior & inferior colliculi – coordinate eye movements with visual stimuli – coordinate head movements with auditory stimuli 13 Reticular Formation 14 Cerebellum • Second largest part of brain • 2 cerebellar hemispheres and vermis (central area) • Function • Scattered nuclei in medulla, pons & midbrain • Reticular activating system – alerts cerebral cortex to sensory signals (sound of alarm, flash light, smoke or intruder) to awaken from sleep – maintains consciousness & helps keep you awake with stimuli from ears, eyes, skin and muscles – correct voluntary muscle contraction and posture based on sensory data from body about actual movements – sense of equilibrium & balance – coordinate & regulate motor impulses initiated in cerebrum – govern subconscious aspects of skeletal muscle movements • Helps maintain muscle tone 15 16 Diencephalon Thalamus • Serves as a relay station for all sensory impulses (except smell) to the cerebral cortex • Registers crude perception of some sensations • Plays an essential role in awareness and the acquisition of knowledge (cognition) • Thalamus • Hypothalamus 17 Hypothalamus 18 THE CEREBRUM • A major regulator of homeostasis • Has numerous functions: • Largest part of the brain • Cerebral cortex (surface layer) is gray matter – controls the ANS – produces hormones – functions in regulation of emotional and behavioral patterns – regulates hunger and thirst sensations – aids in controlling body temperature – regulates circadian rhythms and states of consciousness – pineal gland secretes melatonin to influence diurnal cycles – Folds (gyri) – grooves (sulci or fissures) • Longitudinal fissure separates left & right cerebral hemispheres – Corpus callosum is a commissure (band of white matter) connecting left and right cerebral hemispheres 19 20 Lobes and Fissures Lobes • Longitudinal fissure • Central sulcus • Each cerebral hemisphere is further subdivided into four lobes by sulci or fissures – precentral & postcentral gyrus – frontal, parietal, temporal, and occipital • Lateral sulcus • A fifth part of the cerebrum, the insula, lies deep to the parietal, frontal, and temporal lobes and cannot be seen in an external view of the brain 21 22 Basal Ganglia Cerebral White Matter • The white matter is under the cortex and consists of myelinated axons running in three principal directions – Association fibers between gyri in same hemisphere – Commissural fibers from one hemisphere to other – Projection fibers form descending & ascending tracts 23 • The basal ganglia are paired masses of gray matter in each cerebral hemisphere 24 Limbic System Basal Ganglia • Control large automatic movements of skeletal muscles – Arm swinging while walking – Laughter • Damage to the basal ganglia results in tremor, rigidity, and involuntary muscle movements – Parkinson s disease • Basal ganglia also help initiate and terminate some cognitive processes – Obsessive compulsive disorder, schizophrenia & chronic anxiety are thought to involve dysfunction of the circuits between the basal ganglia and limbic system • Emotional brain -- intense pleasure & intense pain • Strong emotions increase efficiency of memory • Damage to hippocampus prevents formation of new memories 25 26 Motor Areas of Cerebral Cortex Sensory Areas of Cerebral Cortex • Motor areas are the regions that govern muscular movement • Sensory areas of the cerebral cortex are concerned with the reception and interpretation of sensory impulses – Primary motor area = 4 = precentral gyrus • controls voluntary contractions of skeletal muscles on other side – Motor speech area = 44 = Broca s area • production of speech -- control of tongue & airway Primary somatosensory area = postcentral gyrus = 1, 2, 3 Primary visual area = 17 Primary auditory area = 41 & 42 27 28 Association Areas of Cerebral Cortex Hemispheric Lateralization • The two cerebral hemispheres share many functions • Each hemisphere also performs unique functions • Functional specialization of each hemisphere is more pronounced in men • Females generally have larger connections between 2 sides • Association areas are concerned with complex integrative functions such as memory, emotions, reasoning, will, judgment, personality traits, and intelligence • Visual association area = 18 & 19 (recognize & evaluate) • Auditory association area (Wernicke s) = 22 (words become speech) 29 Brain Injuries Electroencephalogram (EEG) • Brain waves are millions of nerve action potentials in cerebral cortex – diagnosis of brain disorders (epilepsy) – brain death (absence of activity in 2 EEGs 24 hours apart) • • • • 30 • Causes of damage – displacement or distortion of tissue at impact – increased intracranial pressure – infections – free radical damage after ischemia • Concussion -- temporary loss of consciousness – headache, drowsiness, confusion, lack of concentration • Contusion -- bruising of brain Alpha -- awake & resting Beta -- mental activity Theta -- emotional stress Delta -- deep sleep – brief unconsciousness – blood in CSF • Laceration -- tearing of brain (fracture or bullet) – increased intracranial pressure from hematoma 31 32 Cerebrovascular Accident (CVA or Stroke) Transient Ischemic Attack (TIA) • The most common brain disorder • Third leading cause of death after heart attacks and cancer • CVAs are classified into two principal types: • Episode of temporary cerebral dysfunction • Cause – impaired blood flow to the brain – ischemic (the most common type), due to a decreased blood supply – hemorrhagic, due to a blood vessel in the brain that bursts • Common causes of CVAs are intracerebral hemorrhage, emboli (blood clots), and atherosclerosis • Tissue plasminogen activator (t-PA) used within 3 hours of ischemic CVA onset will decrease permanent disability • Symptoms – dizziness, slurred speech, numbness, paralysis on one side, double vision – reach maximum intensity almost immediately – persists for 5-10 minutes & leaves no deficits • Treatment is aspirin or anticoagulants; artery bypass grafting or carotid endarterectomy (removal of plaque) 33 34 Cranial Nerves Alzheimer Disease (AD) • Dementia = loss of memory, reasoning, ability to read, write, talk, eat & walk • Afflicts 11% of population over 65 • Loss of neurons in specific regions (e.g., hippocampus and cerebral cortex) • Loss of neurons that release acetylcholine • Twelve pairs of cranial nerves originate from the brain – numbered by order of attachment to the brain • Cranial nerves I, II, VIII contain only sensory fibers • Cranial nerves III, IV, VI, XI, XII are motor nerves • The rest (V, VII, IX, X) are mixed nerves – they contain both sensory and motor fibers 35 36