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

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

Neuroglia CNS PNS Astrocytes Microglia Ependymal cells 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

Neurons Structural unit of the nervous system Cell body (soma) 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 (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 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 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; faster change Unmyleinated Myleinated

Synapses Types Classification Function 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