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2/24/13 Overall Organization The Central Nervous System Central nervous system (CNS) – integration and command center Keri Muma Bio 6 Neural Tissue Organization Types Of Tracts Tracts –axons running to or from Projection – vertical tracts, responsible for the communication between the cerebral cortex and lower CNS Association (arcuate) – connect gyri within the same cerebral hemisphere Commissural – connects gyri between left and right hemispheres Protection of the CNS Protection of CNS Bones of the skull and vertebral column Meninges – three CT membranes surrounding the brain Dura Mater – superficial layer, consists of two layers of fibrous CT Arachnoid Mater – loose middle layer Pia Mater – deepest, clings tightly to the brain following every convolution. Brain Spinal cord Cerebrospinal fluid -found in the ventricles and the subarachnoid space around the brain and spinal cord Functions Gives buoyancy to the brain and spinal cord – keeps it from being damaged by its own weight Cushions and protects Transports materials 1 2/24/13 Protection of the CNS Blood Brain Barrier - capillaries are less permeable due to tight junctions between endothelial cells and between astrocytes Helps maintain a constant environment for the brain Very selective- fat soluble materials, glucose, and select ions and amino acids are allowed to pass, others are not Metabolic Requirements of the CNS Neurons rely on a constant supply of oxygen and glucose to produce ATP for active transport of ions and neurotransmitters. Oxygen diffuses across the BBB Under normal circumstances glucose is the only energy source for neurons Regions Of CNS Cerebrum – integration of sensory/motor, higher functions Diencephalon – innate drives and emotions Epithalamus Thalamus Hypothalamus Pineal gland Pituitary gland Brainstem –basic functions to maintain life Midbrain Pons Medulla oblongata Cerebellum – coordination Spinal cord - reflexes Plasticity of the Brain The architecture of the cortex is determined by genetic and developmental processes but it can be modified due to “use-dependent competition” for cortical space Formation of new neural pathways and connections between existing neurons Some cortical regions can be remodeled throughout life while other can be for only a limited time Functional Areas of the Cerebral Cortex Sensory areas – process afferent impulses, interpret sensations Motor areas – involved in planning and initiating muscular movement Association areas – involve combining information from multiple areas and processing them together, involved in higher functions Glucose is transported from the plasma into the interstitial fluid by insulin independent membrane transporters Hypoglycemia leads to confusion, unconsciousness and death Cerebral Cortex: Sensory Areas Primary somatosensory area – postcentral gyrus in the parietal lobe Receives impulses involved in touch, pain, pressure, stretch Somatosensory association – integrates sensory input into understanding by analyzing sensory based on past experiences Lies posteriorly to primary somatosensory 2 2/24/13 Cerebral Cortex: Sensory Areas Cerebral Cortex: Sensory Areas Primary visual cortex – receives sensory input Primary auditory – receives sensory input from the vestibulocochlear nerve. Temporal lobe. Auditory association area – lies posterior to primary, interprets sound into context Primary motor cortex – controls somatic motor neuron output from the retina to the occipital lobe. Data only. Visual association area – interprets the raw data and puts it into context. Surrounds the primary. Cerebral Cortex: Sensory Areas Olfactory cortex –found on medial aspect of temporal lobe. Sensory input from olfactory nerves. Gustatory cortex – perception of taste, found in the insula Cerebral Cortex: Motor Areas Linked limbic system – tied to emotions and memories Cerebral Cortex: Motor Areas Premotor cortex – anterior to the primary motor cortex in the frontal lobe Responsible for coordinating learned motor skills Examples: typing, driving, playing the piano Lies in the precentral gyrus of the frontal lobe Output is usually controlled by higher motor areas and input from the cerebellum Amount of cortex devoted to each area relates to which regions have the most precise control. Control is contra lateral Basal Nuclei Basal Nuclei (basal ganglia) – gray matter Adjust the stopping, starting and intensity of movements after receiving input from cerebral cortex and substantia nigra (midbrain) Lesions of nuclei lead to increase motor output leading to increased muscle tone, difficulty initiating movement, and involuntary muscle movement 3 2/24/13 Cerebral Cortex: Association Areas Cerebral Cortex: Language Areas Prefrontal cortex – involved with intellect, recall, reasoning, judgment, concern for others, personality traits, and management of emotions Language areas – surrounds lateral sulcus, usually in the left hemisphere only Develops later in life and is impacted by social environment Linked to emotions (limbic system) Receives input from auditory and visual senses Coordinates with the motor cortex to carry out motor skills involved in speech and writing Insert fig 9-23 Language Areas Language involves both expression and comprehension Two cortical areas specializing in language are: Wernicke’s area – language comprehension and formulation of coherent patterns of speech Broca’s area – speech production and word formation, associated with motor cortex Cerebral Lateralization Limbic System Limbic system – emotional brain, consists of tracts and nuclei of the medial cerebrum, anterior thalamus, and hypothalamus Functions: Establishes emotional state and behavioral drive Linked to prefrontal cortex, sometimes logic overrides emotion or vice versa Long term memory storage and retrieval 4 2/24/13 Functional Brain Systems Limbic system structures How Emotions Influence Physiological Functions Amygdala – recognizes angry or fearful expressions and assesses danger Cingulate gyrus – role in expressing emotion and resolving mental conflicts Hippocampus – role in memory along with amygdala Memory Memory is the storage and retrieval of information The three principles of memory are: Storage – occurs in stages and is continually changing (stored in regions that need them) Processing – accomplished by the hippocampus and surrounding structures Memory traces – chemical or structural changes that encode memory Transfer from STM to LTM Stages of Memory The two stages of memory are short-term memory and long-term memory Short-term memory (working memory) STM lasts seconds to hours and is limited to 7 or 8 pieces of information Only 5% of sensory input is transferred to STM Long-term memory (LTM) has limitless capacity Memory Processing Factors that affect transfer of memory from STM to LTM include: Emotional state – we learn best when we are alert, motivated, and aroused Rehearsal – repeating or rehearsing material enhances memory Association – associating new information with old memories in LTM enhances memory Automatic memory – subconscious information stored in LTM Figure 12.21 5 2/24/13 Categories of Memory The two categories of memory are fact memory and skill memory Fact (declarative) memory: Skill Memory Entails learning explicit information (names, dates) Is related to our conscious thoughts and our language ability Is stored with the context in which it was learned Skill (procedural) memory is less conscious than fact memory and involves motor activity (example: riding a bike) How Memories are Formed Long-term potentiation (LTP) - prolonged increase in synaptic strength It is acquired through practice or repetition Skill memories do not retain the context in which they were learned Hard to unlearn Stored in the premotor cortex Aspects of Long-term potentiation Synaptic modifications that can occur as a result of LTPs Repetitive stimulation results in modification of synapses that increase the ability of pre-synaptic neurons to stimulate post-synaptic neurons Necessary for memory trace formation Aspects of Long-term potentiation Number and size of presynaptic terminals may increase More neurotransmitter is released by presynaptic neurons Dendritic spines change shape Extracellular proteins are deposited at synapses Diencephalon Thalamus – forms lateral walls of the 3rd ventricle Acts as a relay station for all incoming sensory impulses except olfactory Screens sensory impulses and decides if it should be passed onto the cortex and where it should be sent Crude awareness of sensation 6 2/24/13 Diencephalon Hypothalamus – slightly anterior and inferior to the thalamus Hypothalamus Important in maintaining body homeostasis and behavioral drives Autonomic control center – Controls release of catecholamine from the adrenal medulla Controls ANS centers in the brain stem and spinal cord, BP, HR, digestive tract, respiration rate, pupil size Emotions – heart of limbic system, basic primitive drives such as fear, anger, pleasure Regulates body temperature – thermostat, initiates cooling or heating mechanisms Sleep-wake cycles – acts with pineal gland to set cycles in response to light and dark Hypothalamus Brain Stem Midbrain – superior portion of the brain stem Food intake – responds to changes in levels of nutrients and hormones. Contains satiety and feeding centers. Water balance and thirst – osmoreceptors that detect concentrations of body fluids, triggers antidiuretic hormone (ADH) and thirst centers Corpora quadrigemina – four protrusions on the dorsal surface, contain sensory nuclei Superior colliculi – visual reflexes Inferior colliculi – auditory reflexes Substantia nigra – axons linked to cerebral basal nuclei, release dopamine, controls motor output Degeneration of these neurons causes Parkinson’s disease Hormones Produces releasing or inhibiting factors which controls the release of hormones from the anterior pituitary Produces posterior pituitary hormones, ADH and oxytocin Brain Stem Brain Stem Pons – bulging region between midbrain and medulla, anterior to cerebellum Pontine nuclei – relay station for tracts between motor cortex and cerebellum Pneumotaxic and apneustic respiratory center – works with medulla to maintain rhythmic breathing Medulla Oblongata – base of brain stem, blends inferiorly with the spinal cord Pyramids – longitudinal ridges on the ventral surface Contains motor tracts that cross over (decussation) before they continue down the spinal cord Insert 9-9 7 2/24/13 Medulla Oblongata Autonomic Nuclei Cardiovascular center – adjusts force and rate of heart contraction and blood pressure Respiratory center – controls rate and depth of breathing, works with pons for rhythm Vomiting, swallowing, coughing, sneezing, hiccups Reticular Formation Reticular Activation System (RAS) - neurons sending constant impulses to the cortex via the thalamus to keep the cortex conscious and alert Inhibited by the hypothalamus sleep center, adenosine, alcohol, and tranquilizers Damage suffered by a jolt to the brain stem may result in permanent unconsciousness (coma) Types and Stages of Sleep: REM Characteristics of REM sleep Vital signs increase Neuronal activity is high Skeletal muscles (except ocular muscles) are inhibited Most dreaming takes place Brain Stem Reticular Formation – loose cluster of neurons extending through the brain stem to the thalamus, hypothalamus, cerebellum and spinal cord Responsible for the arousal (alertness) of the brain Types of Sleep There are two major types of sleep: Non-rapid eye movement (NREM) Rapid eye movement (REM) One passes through four stages of NREM during the first 30-45 minutes of sleep REM sleep occurs after the fourth NREM stage has been achieved A typical sleep pattern alternates between REM and NREM sleep Sleep Patterns Alternating cycles of sleep and wakefulness reflect a natural circadian rhythm The suprachiasmatic and preoptic nuclei of the hypothalamus regulate the sleep cycle Adenosine appears to be a sleep inducing chemical that accumulates in the brain Inhibits RAS neurons Caffeine blocks adenosine receptors 8 2/24/13 Importance of Sleep Sleep Disorders NREM sleep is presumed to be the restorative stage REM sleep may be a reverse learning process where superfluous information is purged from the brain (one hypothesis) Those deprived of REM sleep become moody and depressed Daily sleep requirements decline with age Physiological Rhythms" Narcolepsy – lapsing abruptly into REM sleep from the awake state Insomnia – chronic inability to obtain the amount or quality of sleep needed Sleep apnea – temporary cessation of breathing during sleep Examples of Circadian Rhythms Dynamic Equilibrium – variables are continuously fluctuating within their narrow limits " These fluctuations tend to be in wave patterns called biorhythms" If they fluctuate in a cycle every 24 hours they are referred to as circadian rhythms! Controlled by the suprachiasmatic nucleus (SCN) in the hypothalamus" Suprachiasmatic Nucleus Secretes clock proteins. " Cyclic changes in their concentration throughout the day changes the neural output from the SCN. " This neural output produce cyclic changes in effector organs through the day" Circadian Rhythms The SCN cycle is a little longer than 24 hours" Internal clock must be reset daily by external cues in order to stay in sync." Light and dark cycle" Melatonin secreted from the pineal gland keeps the SCN in tune with the environment and regulates sleep/wake cycles" SCN 9 2/24/13 Circadian Rhythms Cerebellum Desynchronosis – internal clock is out of synchronization with the external environment Jet lag, work rotations, change in sleep schedule Effects: decreased cognitive function, depression, foggy head, can’t sleep or wake up Seasonal affective disorder: depression during short winter days due to decreased sunlight Can be treated with bright light or melatonin Cerebellum Maintains posture, balance, and plays a role in learning and executing skilled motor movements Body map – sensory and motor maps of the body so it is aware of what each skeletal muscle is doing Functions of the Cerebellum Monitors intended movements from the motor cortex and basal nuclei Monitors current movements – receiving input from proprioceptors and the vestibule (equilibrium) Compares intended movements, sensory input, and the actual movement and placement of muscles at that time Sends corrective feedback to the upper motor centers if there is a discrepancy between the intended movement and the actual movement Cerebellum Motor cortex & Basal Nuclei Sends intended muscle Movement to cerebellum Adjustments made by Cerebellum sent back to Motor cortex Cerebellum Coordinate motor intent with sensory input Sensory input from proprioceptors, visual and equilibrium pathways Cerebellum & Coordination of Muscles Cerebellum Ataxia- disruption of muscle coordination resulting in inaccurate movements Caused by damage to the cerebellum through trauma or genetic disease Abnormal walking movements, uncoordinated speech, overshoot objects 10