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Motor Functions of the Spinal Cord BIEN 500 Steven A. Jones Spinal Cord Function • Conduct signals to the periphery of the body. • Process these signals as necessary – Reflex actions – More complicated actions, such as walking Spinal Cord Divisions • Anterior Motor Neurons – Alpha Motor Neurons (large skeletal muscles) • Type A nerve fibers • Enervate large skeletal muscles • Stimulate 3 to several hundred fibers each – Gamma Motor Neurons • Type A nerve fibers • Send signals to muscle spindles • Interneurons – Perform Integrative functions – Most signals in spine are processed through these • Renshaw Cell Inhibitory System – Suppresses spread of signals – Located in the ventral horn Motor Spindles • Stimulated by stretching • Send signals as a result of: – Change in length of muscle. – Change in tension on the muslce. • Primary ending – Group Ia fiber – Important for sensing dynamic changes • Secondary ending – Group II fiber – With Primary ending, sends a static response Muscle Stretch Reflex • Muscle stretch causes contraction – First dynamic (short term), then static (long term) – Shortening of muscle will cause inhibition Anterior Horn Motor Nerve Proprioceptor Nerve (Ia) Muscle Spindle Posterior Horn Force of Contraction Feedback Function of Spindle This effect is not surprising, is it? With Spindle 0 Without Spindle Time (s) 3 31% of the motor nerve fibers are gamma efferent fibers Gamma Motor System • Excited by bulboreticular facilitory region of the brain stem via: – Cerebellum – Basal ganglia (in the brain) • • • • Caudate nucleus Putamen Globus pallidus Substantia nigra subthalamic nucleus – Crebral cortex Muscle Length Knee Jerk and Clonus Clonus (think time delay) Knee Jerk 0 E.g. fatigue when standing on tip toes. Time (ms) 400 Golgi Tendon Organ • • • • • Detects tension rather than length. Found between tendon and muscle. Signals transmitted by type Ib nerve fibers. Negative feedback to prevent overstimulation. May help even out muscle load on fibers. Tendon Muscle Golgi Tendon Organ Flexor/Withdrawal Reflex • Also called nociceptive reflex • Pain initiates flexion, without passing through brain • Passes through interneurons • Get extension in opposite limb Flexor Reflex Extensor Inhibition Extensor Stimulation Flexor Stimulation Flexor Inhibition Interneurons Pain Signal from Hand Reciprocal Inhibition: Stimulus exciting one muscle will inhibit the antagonist muscle. Spinal Cord Reflexes • These responses occur in decerebrate animlas. – Walking – Galloping – Scratching Muscle Spasm • Broken bone • Abdominal muscle spasm • Muscle cramps – Severe cold – Lack of blood flow – Overexercise • Contraction may stimulate more contraction – positive feedback. Autonomic Spinal Cord Reflexes • Vascular tone (e.g. change in skin perfusion) • Sweating • Intestinal reflexes – Control of gut muscles – Inhibition of gastrointenstinal motility (e.g. irritation) • Bladder/Bowel control Spinal Shock • Occurs when spinal cord is deprived of brain input. • Spine recovers after days. • Similar mechanism applies to most nerves • Recovery may be excessive • Affected functions: – Arterial blood pressure – Skeletal muscle reflexes are blocked – Bladder/bowel control are lost (but recover) Cortical and Brain Stem Control of Motor Function BIEN 500 Steven A. Jones The Cerebellum, the Basal Ganglia, and Overall Motor Control BIEN 500 Steven A. Jones Organization of the Brain Thalamus Cerebellum Regions of the Basal Ganglia Motor Functions of the Cerebellum • • • • • • Running Playing a musical instrument Talking Writing Typing I.e. Repeated, learned actions. Mechanism of Action of Cerebellum • Sequences motor activities • Monitors and corrects motor activities to conform to signals from the motor cortex & other parts of the brain. • A negative feedback system. Anatomy Anterior Lobe • Afferent – Incoming – From brain – From periphery Posterior Lobe • Efferent – Outgoing – From brain – From periphery Flocculonnodular Lobe Oldest, Vestibular (equilibrium) control The Purkinje Cell Molecular Layer Billions of Parallel Nerve Fibers Purkinje (Inhibitory) Constantly firing Purkinje Layer Granule Layer Granule Cells Input from Inferior Olive (Climbing Fiber) strong stimuli Output Deep Nuclear Cell Constantly firing Multiply by 30,000,000 Deep Nuclei Input from Elsewhere (Mossy Fiber) weak stimuli Learning (Perkinje Cells) • Inferior olivary complex receives intent and execution information – sends error signal. • Climbing fiber rate – 1 per second • Mismatch between desired movement & actual movement increases/decreases rate. • Firing rate changes sensitivity of perkinje cells. • Error signal is then stopped. Feedback Control of the Distal Limb Motor Cortex Thalamus Intermediate zone of cerebellum Red Nucleus Feedback from Cerebellum Intended Movements Actual Movements Muscles Generally important for movements that are too fast to be corrected on the fly. Effects of Cerebellar Damage • Slow development of movements • Weak development of force • Overshoot of motion Thus, the cerebellum functions to plan, sequence and time complex motions. Non-Motor Functions of the Cerebellum • Estimation of speed. • Prediction of timing. • Interpreting spatiotemporal relations. Abnormalities of the Cerebellum • Dysmetria and Ataxia/Past Pointing – Discoordinated movements/Overshoot • Dysdiadochokinesia – don’t know where your body parts are in fast motion. • Dysarthria – cannot coordinate speech • Intention tremor – movement overshoot • Cerebellar nystagmus – tremor of the eyeballs • Hypotonia – loss of muscle tone. Basal Ganglia • Include the following components of the brain: – – – – – Caudate nucleus Putamen Globus pallidus Substantia nigra Subthalamic nucleus • Located lateral to the thalamus • Nearly all motor & sensory nerves from the crerebral cortex pass between the caudate nucleus and putamen. Basal Ganglia Thalamus Putamen and Globus Palidus Amygdala Cerebellum Caudate Nucleus Basal Ganglia • Control complex motor activity – E.g. writing letters of the alphabet – Cutting paper with scissors – Shooting a basketball – Hammering nails – Throwing a football/baseball Parkinson’s Disease • Destruction of the pars compacta in the substantia nigra • Prevents activity of dopamine-secreting nerve fibers to the caudate nucleus. • Dopamine = Inhibitor • Causes – Rigidity of musculature – Involuntary tremor (3-6 Hz) – Difficulty initiating movements Treatment for Parkinson’s • L-Dopa – Gets converted to dopamine – Dopamine will not pass the blood-brain barrier • L-Deprenyl – Inhibits monoamine oxidase, which destroys dopamine. – Helps slow destruction of dopamine producing neurons – Can be combined with L-Dopa • Transplanted fetal dopamine cells – Cells do not persist for more than a few months. – Raises the abortion issue. • Destruction of feedback circuitry in the basal ganglia The Cerebral Cortex; Intellectual Functions of the Brain; and Learning and Memory BIEN 500 Steven A. Jones Cerebral Cortex • Largest part of the nervous system • Functional part: 2 – 5 mm thick layer of neurons • Total area ~ 1 m2 • ~ 100 billion neurons • Three types of cells – Granular (or stellate) – Fusiform – pyramidal Granular Cells • Pyramidal in shape • Short axons – function as intracortical neurons • Excitatory – Glutamate • Inhibiroty - GABA Pyramidal Cells • • • • Source of output fibers Larger, more numerous than fusiform cells Nerve fibers go down to spinal cord. Fibers also connect major regions of the brain. • Fusiform cells provide similar functions. Layers of the Cerebral Cortex I.Molecular Layer Large Numbers of Neurons II. External Granular Layer Intracortical Association Functions III.Pyramidal Cells IV. Granular Cells, Output fibers to thalamus V. Large fibers to brain stem and cord VI. Fusiform or polymorphic cells. Output signals VII. Output signals Divisions of the Cerebral Cortex Eye Tuning Thought Supplementary Motor Synergies Hand Skills Speech Bilateral Vision Contralateral Vison Functional Areas of the Cerebral Cortex Motor Planning Movement, Thought Spatial Coordinates of Body and Surroundings Word Formation Vision Broca’s Area Behavior, Emotion, Motivation (Limbic) Language Comprehension, Intelligence Interesting Brain Regions • Broca’s Area – word formation • Angular Gyrus – Interpretation of visual information – Associated with dyslexia • Wernicke’s Area – More developed in dominant side of brain – General interpretive area – Stimulation may produce complex visualization, hallucinations, complex statements, hearing a musical piece • Limbic Association Area – behavior, emotion, motivation. • Facial Recognition Area (large) Dominant Hemisphere • Wernicke’s area more developed on one side. – Important for language, mathematics, logic. • Left hemisphere more dominant for 95% – More than 50% larger in over 50% of neonates. – Other 5% is either dual- or right-dominant. – Related to right-handedness. • Hemispheres communicate via corpus callosum Non-Dominant Hemisphere • • • • • • • Understanding/Interpreting music Nonverbal visual experiences Spatial relations with surroundings Interpretation of “body language” Interpretation of vocal intonation Somatic experiences Non-symbolic interpretation Effects of Prefrontal Lobotomy • • • • • Inability to solve complex problems Inability to combine tasks to reach goals Inability to multitask Loss of aggressiveness/ambition Inappropriate social responses (w.r.t. morals/sex/body functions) • Inability to produce trains of thought • Rapid mood changes • Normal motor function, but often without purpose Interpretation of Prefrontal Lobotomy • Passiveness/Inappropriate social responses – Probably relates to limbic system • Inability to follow through sequences – Easily distracted from the central theme – Lack of “working” (Cache) memory disables • • • • • Prognostication Planning for the future Delay of impulses Consideration of consequences of actions Solving complicated problems Aphasia • Wernicke’s Aphasia – Damage to Wernicke’s area in the dominant hemisphere – Can understand the words, but not the thought. • Broca’s Aphasia – Damage to Broca’s area – Unable to form motor control • Other areas – Cerebellum, basal ganglia, sensory cortex – Total or partial inability to speak Contralateral Control • Right side of brain controls left side motor coordination & vice versa Corpus Callosum Corpus Callosum • Transfers information from Wernicke’s area to contralateral motor cortex • Prevents somatosensory information from contralateral hemisphere from reachig Wernicke’s area • Severing is a treatment for epilepsy • If severed – Can still perform subconscious motor functions on ipsilateral side – May do things without knowing why Thought • Holistic theory – Thought results from a pattern of stimulation – Involves cerebral cortex, thalamus, limbic system, upper reticular formation (URF) – Thalamus, limbic system and URF determine: • • • • Pleasure Pain comfort Modalities of sensation Crude localization on the body – Cerebral cortex determines: • Fine localization on the body & in space • Texture • Recognition of geometric patterns Memory • Caused by new neural pathways or facilitated pathways. • Occur at all levels of the nervous system. • Intellectual memory mostly in cerebral cortex • Mind ignores much (unimportant information) • Mind remembers important information – Pain – Pleasure Classification of Memories • Short term – Last seconds or minutes – May be converted to long-term memory – Classic example – phone number • Intermediate long-term – Last days to weeks, but fade • Long-term – Can be recalled years later – Pathways to complex memories may be difficult to find Explanations of Short-Term Memory • Reverberating neurons • Presynaptic facilitation or inhibition • Enhanced synaptic conduction/ accumulation of calcium Mechanism for Intermediate Memory • Temporary chemical or physical changes at presynaptic terminals or postsynaptic membranes Facilitator Neuron Sensory Neuron cAMP Serotonin Ca++ Facilitatory neuron prevents habilitation and enhances synaptic response. Molecular Path in Intermediate Memory • Habituation caused by closure of Ca++ channels. • Serotonin activates adenylcyclase, forms cAMP. • cAMP activates protein kinase, blocks K+ conductance (up to several weeks), prolongs action potential (AP). Facilitator Neuron • Prolonged AP releases Ca++ Channel large amounts of Ca++. Serotonin • Facilitatory neuron can cAMP also act on postsynnaptic membrane. ++ K+ Channel Ca Long-Term Memory • Caused by structural changes. – Increase in no. of neural transfer vesicles – Increase in no. of neural transfer vesicle release sites – Increase in no. of presynaptic terminals/length of dendrites Neural Development • Excess neurons at birth • Axons that do not connect or connect with wrong type of cell dissolve • Nerves will not develop for a blocked eye. • 50% or more of original neurons in parts of cerebral cortex are eliminated. • This is a type of memory. • Plasticity continues to a lesser extent in later life. – E.g. can recover after stroke (sensory and motor). Consolidation Time • Minimal consolidation 5-10 minutes • Strong consolidation 1 hour • Determined by shock experiments – Provide strong sensory input – Convulsive shock after a time period – Determine whether subject remembers or not. Rehearsal • Repeating something over in the mind. • People tend to do this with “interesting” things. • If wide awake, remember better than when in mental fatigue. • None of this is particularly surprising, is it? Computer Memory Short Term Long Term Hard Disk (slow, large) RAM (fast, “small”) Cache Memory (faster, smaller) On Chip Memory (fastest, smallest) CPU Removable Storage (slowest, “largest”) Computers are designed to account for tradeoffs of speed and size. Fast memory tends to be more expensive. Files are not “erased.” Rather the information that points to them is deleted, allowing information to be overwritten. Programs can retrieve these files after deletion if they have not been (completely or partially) overwritten. Associative Memory Clock Piano Time Hands Music Fingers Score Metronome Toes Four score and seven years ago today, our forefathers … Uh … Dang! Metric Meters Lincoln’s Gettysberg Address Feet Length Gettysberg Corn Wheat Civil War Battles Codification of Memory • Same as “association.” • Organization of memories helps in retrieval. • Complex memories can be stored, but not immediately accessible. • My personal hypothesis on dreams – Random access of complex stored memories – Delete “links” to useless information – Similar to disk defragmentation? Role of Thalamus in Memory • Damage may cause retrograde amnesia without anterograde amnesia – Forget things – Forget recent things more than older things • Thalamus may affect memory “search.” Role of Hippocampus in Memory • Hippocampus output is reward/punishment • Removal of hippocampus – Inability to form new long term verbal/symbolic memories • Referred to as anterograde (ante=before) amnesia • You do not “forget” things. • You just cannot remember new things. – Can also cause some retrograde amnesia • Not important in reflexive learning (e.g. sports) Behavioral and Motivational Mechanisms of the Brain – The Limbic System and the Hypothalamus BIEN 500 Steven A. Jones Regulatory Functions • Cardiovascular • Body Temperature • Fluid regulation – Thirst – Excretion of water into the urine • Reproduction – Uterine contraction – Milk ejection • Feeding – Hunger (lateral hypothalamic area) – Satiety (in the ventromedial nucleus) – Licking lips, swallowing (mamillary bodies) • Control of pituitary gland Emotional Effects of Hypothalamus • Lateral hypothalamus – Thirst, eating – Increased activity level – Rage and fighting • Ventromedial nucleus – Reward centers – Opposite of lateral hypothalamus • Thin zone of periventricular nuclei & central gray area of mesencephalon – Fear and punishment • Anterior & Posterior regions (and others) – Sexual drive Hippocampus • Hippo – horse (e.g. hippopotumus=river horse). • Three nerve cell layers instead of six. • Filtering of incoming sensory information • Stimulation – rage, passivity & sex drive • Weak stimulation can lead to seizure – Localized – Olfactory, visual, auditory, tactile hallucinations Hippocampus and Learning • Role in learning discussed previously • Originally part of olfactory cortex. • Smells important to eating & sex drive. Amygdala & Hypothalamus • Bidirectional connections with hypothalamus. • Effects mediated through hypothalamus: – Control of arterial pressure, heart rate – Control of gastrointestinal motility & secretion – Defecation & micturation – Pupillary dilation and (rarely) constriction – Secretion of pituitary hormones, especially: • Gonadotropins • Adrenocortocopic hormone Amygdala Stimulation • Tonic movement (raising head, bending body) • Circling movments • Clonic, rhythmical movements • Olfaction/Eating movements (licking, chewing, swallowing) • Rage, escape, punishment, fear Similar to Hypothalamus • Reward & pleasure Amygdala Bilateral Ablation in Monkeys • • • • • • • • Klüver-Bucy syndrome Excessively examine objects orally Loss of fear Decreased aggressiveness Tameness Herbrivore may become carnivorous Sometimes psychic blindness Excessive sex drive Limbic Cortex • Surrounds the limbic structures. • Not well understood: – Stimulation yields little information – Ablation of anterior temporal cortex usually also damages amygdala. • Ablation of Posterior Orbital Frontal – Insomnia – Motor restlessness • Ablation of Anterior Cingulate & Subcallosal Gyri – Nothing to filter rage from hypothalamus • Regions of cortex appear to cause association of limbic functions with nearby regions of cerebral cortex. (e.g. connecting sensations with feelings) States of Brain Activity – Sleep; Brain Waves; Epilepsy; Psychoses BIEN 500 Steven A. Jones Types of Sleep • Slow-wave sleep – The largest proportion of a given sleep period – Deep, restful • Rapid-eye-movement (REM) sleep [Paradoxical or Desynchronized Sleep] – Occur periodically (ca. every 90 minutes for 5-30 minutes) – About 25% of time for young adult – Associated with vivid dreaming – Duration increases as person becomes less tired. Slow Wave Sleep • Decrease in body functions (10 – 30%) – Peripheral vascular tone – Blood pressure – Respiratory rate – Basal metabolic rate • Dreams & sometimes nightmares do occur. – Less likely to be remembered. – I.e. consolidation of dreams in memory does not occur. REM Sleep • • • • • • • • Usually associated with dreaming Harder to wake subject, yet … People usually awaken naturally during REM Strong suppression of muscle tone (but not the eye muscles, obviously). Irregular heart rate/respiratory rate Increase of brain metabolism of ~20%. EEG patterns similar to wakefulness Brain is active, just not connected to the world Theories of Sleep • Passive theory – Brain becomes fatigued and inactive • Active theory – Brain is actively inhibited – Transecting brain stem in midpontile region prevents sleep (based on EEG) – I.e., something below midpontile region causes sleep by inhibiting parts of the brain. Areas where stimulation causes sleep • Raphe nuclei in medulla & lower half of pons – Nerve fibers extend to reticular formation, thalamus, neocortex, hypothalamus & limbic system. – Can inhibit incoming pain signals – Release serotonin • If serotonin formation is blocked, no sleep but … • Serotonin levels are decreased during sleep. • Parts of nucleus of the tractus solitarius – Region of medulla & pons for visceral signals – Depends on presence of raphe nuclei • Regions of diencephalon Transmitter Substances & Sleep • Muramyl peptide – Low molecular weight – Accumulates in blood/cerebral spinal fluid in sleep deprivation • A nonapeptide (found in sleeping animals) • “A third factor” found in neuronal tissue after sleep-deprivation Causes of REM Sleep • Possibly associated with acetylcholine from upper brain stem reticular formation. • Cyclic behavior of sleep not well understood. Effects of Lack of Sleep • Sluggishness • Irritability • Psychosis Epilepsy EEG • Grand Mal Epilepsy (high voltage) • Petit Mal Epilepsy (spike & dome pattern) • Focal Epilepsy 100 m Grand Mal Epilepsy • • • • • • • Neural discharges in all areas of the brain Tonic seizures of the body Tonic-clonic seizures (near the end) Swallowing tongue, difficulty breating Sometimes defication, urination Duration: seconds to 4 minutes May lead to stupor/fatigue/sleep afterwards. Petit Mal Epilepsy • Probably involves thalamocortical brain activating system • 3 to 30 seconds of unconsciousness or diminished consciousness • Twitching, blinking • Remains conscious afterwards • Onset in late childhood, disappear at ~ age of 30. • May trigger Grand Mal Focal Epilepsy • Can occur anywhere in the brain • Usually caused by localized lesion • E.g. when occurs in motor cortex, causes “progressive march of muscle contractions on contralateral side (jacksonian epilepsy) • May trigger Grand Mal • Interesting case – psychomotor seizure Psychomotor Seizure • May involve – – – – – Short period of amnesia Attack of abnormal rage Sudden anxiety, fear, discomfort Moment of incoherent speech, mumbling trite phrase. Motor act (attack, rub face) • Subject may not remember activities • May remember he/she did them, but not know why. Depression • Depression – Diminished formation of norepinephrine, serotonin or both? (probably others) – Depression may involve • • • • Grief, unhappiness, despair, misery Loss of appetite, sex drive Insomnia Psychomotor agitation (despite the depression) • Manic Depression (biopolar disorder) – Alternating between depression & mania – Lithium diminishes norepinephrine/serotonin Treatments for Depression • Drugs – Monoamine oxidase inhibitors • Block destruction of norepi and serotonin – Tricyclic antidepressants • E.g. imipramine, amitriptyline, many newer ones. • Block norepi & serotonin reuptake – Drugs that enhance effect of serotonin • Electroconvulsive therapy – Enhances norepinephrine transmission efficiency – “Here. This ought to snap him out of it!” Schizophrenia • Symptoms (not “split personality”): – Hearing voices – Delusions of grandeur – Paranoia, sense of persecution – Incoherent speech – Dissociation of ideas, abnormal thought sequences – Witdrawn, abnormal posture, rigidity Possible causes of schizophrenia • Multiple prefrontal lobe areas where signals are blocked or dysfunctional • Excessive excitement of dopamine secreting neurons in behavioral centers – Parkinsons patients given L-Dopa can develop schizophrenia – Drug treatments reduce dopamine release • Abnormal function of limbic behavioral control system (hippocampus) – Hippocampus is often reduced in size – Regions connected to hippocampus are hyperfunctional Alzheimer’s Disease • Premature aging of the brain • Loss of memory function • Patients have amyloid plaques (10 – several hundred m) in cortex, hippocampus, basal ganglia, thalamus, cerebellum • Metabolic degenerative disease • Consistently find loss of limbic memoryrelated neurons The Autonomic Nervous System; the Adrenal Medulla BIEN 500 Steven A. Jones Cerebral Blood Flow, the Cerebrospinal Fluid, and Brain Metabolism BIEN 500 Steven A. Jones