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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 of 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
• CNS– Astrocytes
• Maintaining blood-brain barrier• Guide growing neurons
– Microglia• Remove cellular debris and
foreign material– Ependymal cells
• Circulate and produce CSF– 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
Neuroglia
Neurons
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
• 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) • 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
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 – Polarity creates potential energy
• Measured in millivolts (mV)• -70 mV in the plasma membrane of neurons
• Flow of charge (ions) is the current– K+ flows out more readily than Na+ flows in– Na+/K+ pump maintains concentrations of Na + (3 out) and K + (2 in)
• Plasma membrane provides 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 (ligand) respond to NT• Voltage respond to change in polarization• Mechanical respond to physical change/deformation
• Ions move down an electrochemical gradient– Charge– Concentration
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– Intensity coded by frequency
• 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
Unmyleinated
Myleinated
• Stimuli site is depolarized and local ion movement disperses the signal (graded)
• Origin enters a refractory period– Local changes can produce another AP– Depolarization followed by repolarization
• Myelinated axons allow conduction spread and regeneration– Saltatory conduction at nodes of Ranvier– Axon diameter
• Larger = faster– Degree of myelination
• w/o = continuous conduction; AP immediately = slow
• w/ = prevents leaks; fasterchange
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
• NT 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); acetylcholinesterase (AChE)• Biogenic amines
– Dopamine (DA): movement (both)– Norepinephrine (NE) & epinephrine (Epi): feel good NT’s (both)
• Common pathway from AA tyrosine– Serotonin (5-HT): mood, sleep, appetite & anger (inhibitory); AA tryptophan– Histamine: immune response & wakefulness (both); AA histidine
• Amino acids– GABA (inhibitory)– Glutamate (excitatory)
• Neuropeptides– 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 of lipids in
brain tissue• Shingles: viral infection in skin sensory neurons• Numbing and prickling: slowed blood flow to
areas impair nerve impulses