Chapter 11 Dr Tamily Weissman, Department of Molecular and Cellular Biology, Harvard University

Functions of the Nervous System Master controller and communicator for the body Sensory input (to brain) Sensors External or internal info Integration Immediate context Experience Motor output (from brain) Effector organs Muscle or gland response See yellow light Foot to brake or gas Process options

Nervous System Central Nervous System (CNS) BrainSpinal Cord Peripheral Nervous System (PNS) Motor (efferent) Autonomic (involuntary) Sympathetic (Fight or flight)- adrenergic Parasympathetic (Rest and digest)- cholinergic Somatic (voluntary) Sensory (afferent) Human Nervous System Divisions Integration & command Info in & out

Neuroglia CNS Astrocytes Most abundant & versatile Exchange b/w blood & neurons; migration; environment control Microglia Immune cells of CNS Ependymal cells Circulate CSF & cushioning Oligodendrocytes Produce multiple myelin sheaths PNS Schwann cells Produce a single myelin sheath Satellite cells Similar to astrocytes u/instructors/rbwollaston/Nervous_system/neuroglia_of_CNS.gi f&imgrefurl= apter_8_Nervous_System.htm&usg=__2YxucQKrJmUKtfkBty- PZGw_y1A=&h=386&w=371&sz=9&hl=en&start=1&sig2=zDo9CPo P08kpEikUtueyXw&um=1&tbnid=7Kr6pqq0qPkVQM:&tbnh=123 &tbnw=118&prev=/images%3Fq%3Dneuroglia%26hl%3Den%26s a%3DG%26um%3D1&ei=NGVTSvmkE8yjmQels_CgCQ

Neurons tfolio/BBM/Audio/saraf/Neuron%208.gif Structural unit of the nervous system Cell body (soma) Nissl bodies (rough ER) Nuclei vs ganglia Processes Dendrites Input; dendritic spines; graded potentials Axons Axon hillock (trigger zone) Myelin sheath and nodes of Ranvier Axon terminals (secretory region) and Lack Nissl bodies and Golgi Anterograde and retrograde transport Axolemma and axoplasm Tracts vs nerves White vs gray matter

Classification of Neurons Structural classification Multipolar: 3+ processes; 99% of all neurons, major in CNS Bipolar: 2 processes; rare, located in sense organs Unipolar : short, divided process (peripheral and central processes); mainly in PNS Functional classification Sensory (afferent): message to CNS Motor (efferent): message from CNS Interneurons %201g.gif

Neurophysiology Resting membrane potential Positive charge outside, negative charge inside Separation of charges creates PE Measured in millivolts (mV) -70 mV in the plasma membrane of neurons Cell said to be polarized Flow of charge (ions) is the current K + flows out, Cl - flows in more readily than Na + in Na + /K + pump maintains negative intracellular environment Plasma membrane provide resistance Ohm’s law: current = (voltage/resistance) More volts (potential difference) = more movement Greater resistance = less movement

Ion Channels Proteins spanning PM controlling flow Leak channels Gated channels Chemical respond to NT Voltage respond to potential change Mechanical respond to physical change/deformation Ions move down an electrochemical gradient Concentration Charge

Graded Potentials Short lived and local Depolarizations or hyperpolarizations Decrease in magnitude w/distance, decremental Varies with strength of stimuli Point of stimulus only place ions can pass (+) ions toward (-) areas and (-) ions to (+) areas Inside (+) ions move from stimuli site to neighboring (-) areas Outside (+) ions move toward stimuli site

Action Potentials Rapid reversal of membrane potential All-or-nothing Graded until threshold reached Magnitude independent of strength Carry information Depolarization Positive feedback maintains Repolarization Hyperpolarization Returning electrical conditions Na+/K+ pump Returns ionic conditions Refractory periods Absolute vs relative

Propagation of an AP Local currents depolarize membrane at stimuli site and disperse Origin enters a refractory period Local changes can produce another AP Myelinated axons allow speed conduction and allow regeneration Saltatory conduction at nodes of Ranvier Axon diameter Larger = faster Degree of myelination w/o = continuous conduction; AP immediately = slow w/ = prevents leaks; faster change

Synapses Types Presynaptic neuron sends Postsynaptic neuron receives Classification Axodendritic Axosomatic Axoaxonic Function Electrical synapses allow ion flow b/w gap junctions Electrical only Chemical synapses release and receive NT’s b/w pre- and postsynaptic neurons Electrical  chemical  electrical

Transmission at a Synapse AP opens Ca 2+ channels in presynaptic neuron Ca 2+ influx causes synaptic vesicle fusion and NT exocytoic release Binds to postsynaptic neuron Postsynaptic ion channels change EPSP or IPSP Temporal summation Spatial summation Actions of NT in synaptic cleft ended Degradation Reuptake Diffusion

Neurotransmitter Classes Acetylcholine (Ach): skeletal muscles (excitatory) Biogenic amines Dopamine (DA): movement (both) Norepinephrine (NE) & epinephrine (Epi): feel good NT’s (both) Serotonin (5-HT): mood, sleep, appetite & anger (inhibitory) Histamine: immune response & wakefulness (both) Amino acids GABA (inhibitory) Glutamate (excitatory) Peptides Endorphins and enkephalins: natural opiates (inhibitory) Substance P: perception of pain (excitatory) Dissolved gases NO: synthesized on demand; relaxation of smooth muscle (Viagra)

Nervous System Disorders Polio: destroys motor neurons in CNS Rabies: inflames the brain Multiple sclerosis: destruction of myelin slows AP conduction, axons unaffected Tay-Sachs: harmful accumulation in brain tissue Shingles: viral infection in skin sensory neurons Numbing and prickling: slowed blood flow to areas impair nerve impulses