Fundamentals of the Nervous System

Fundamentals of the Nervous System 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 Process options Foot to brake or gas

Human Nervous System Divisions Info in & out Integration & command

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 http://images.google.com/imgres?imgurl=http://www.dmacc.edu/instructors/rbwollaston/Nervous_system/neuroglia_of_CNS.gif&imgrefurl=http://www.dmacc.edu/instructors/rbwollaston/Chapter_8_Nervous_System.htm&usg=__2YxucQKrJmUKtfkBty-PZGw_y1A=&h=386&w=371&sz=9&hl=en&start=1&sig2=zDo9CPoP08kpEikUtueyXw&um=1&tbnid=7Kr6pqq0qPkVQM:&tbnh=123&tbnw=118&prev=/images%3Fq%3Dneuroglia%26hl%3Den%26sa%3DG%26um%3D1&ei=NGVTSvmkE8yjmQels_CgCQ

Neurons • 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 http://www.pspnperak.edu.my/biologit5/Abd%20Razak%20b.%20Yaacob/Portfolio/BBM/Audio/saraf/Neuron%208.gif http://www.mind.ilstu.edu/curriculum/neurons_intro/imgs/neuron_types.gif

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 (Sensory Neuron) http://www.unisanet.unisa.edu.au/Resources/101766/Online%20Brain%20Development%20course/Pics/Photo%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 Ca2+ channels in presynaptic neuron • Ca2+ 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 http://anthropologynet.files.wordpress.com/2008/01/neuron-synapse.png

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

Fundamentals of the Nervous System