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 CNS)– External or internal sensors

• Integration– Immediate context– Experience

• Motor output (from CNS)– Muscle or gland response

See yellow light

Foot to brake or gas

Process options

Human Nervous System Divisions

Integration & command Info in & out

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

• CNS– Astrocytes

• Connect neurons to capillaries• Guide growing neurons

– Microglia: immune functions– Ependymal cells: circulate CSF– Oligodendrocytes: produce

multiple myelin sheaths• PNS– Schwann cells: produce myelin – Satellite cells: similar to

astrocytes

Neurons

http://www.pspnperak.edu.my/biologit5/Abd%20Razak%20b.%20Yaacob/Portfolio/BBM/Audio/saraf/Neuron%208.gif

• Structural unit of the nervous system

• Cell body – Nissl bodies – Nuclei vs ganglia

• Processes– Dendrites

• Graded potentials– Axons

• Axon hillock • Axon terminals • Axolemma and axoplasm• Myelin sheath and nodes of Ranvier

• White vs gray matter

– Tracts vs nerves

Classification of Neurons

http://www.unisanet.unisa.edu.au/Resources/101766/Online%20Brain%20Development%20course/Pics/Photo%201g.gif

• Structural classification– Multipolar: mainly in

CNS– Bipolar: sense organs– Unipolar : mainly in PNS

• Functional classification– Sensory are afferents– Motor are efferents– Interneurons

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

Resting Membrane Potential

• Polarization creates potential energy

• Changes allow cellular communication– Current from flow of

charge (ions) • K+ leaks out and Na+ leaks in• Na+/K+ pump maintains

– Resistance regulates

Graded Potentials

• Varies with stimuli– Short, localized, and decremental• Depolarization (-70mV to -55mV)• Hyperpolarization (-70mV to -100mV)

– Summation can initiate action potentials (APs)

http://www.colorado.edu/intphys/Class/IPHY3430-200/image/figure4-8.jpg

Action Potentials (AP’s)

• Depolarization – Positive feedback maintains– Threshold = all or nothing response

• Repolarization – Returning electrical conditions

• Hyperpolarization– Na+/K+ pump returns ionic

conditions• Refractory periods– Absolute– Relative

Propagation of an Action Potential

Unmyelinated

Myelinated

• Self-generating • Refractory period ensures

one-way movement• Conduction speed– Axon diameter– Degree of myelination• Continuous conduction w/o

myelin• Saltatory conduction at

nodes of Ranvier• Multiple sclerosis

Synapses• Types– Presynaptic neuron sends– Postsynaptic neuron

receives

• Classification– Axodendritic– Axosomatic– Axoaxonic

• Function– Electrical synapses – Chemical synapses

Chemical Transmission

• AP reaches axon terminal

• Ca2+ signals NT release• NT binds postsynaptic

neuron– EPSP (depolarize)– IPSP (hyperpolarize)

• Ending transmission– Degradation– Reuptake– Diffusion http://anthropologynet.files.wordpress.com/2008/01/neuron-synapse.png

Neurotransmitter Classes• Acetylcholine (ACh): skeletal muscles• Biogenic amines

– Dopamine (DA)– Norepinephrine (NE) & epinephrine (Epi)– Serotonin (5-HT)– Histamine

• Amino acids– GABA – Glutamate

• Neuropeptides– Endorphins– Substance P

• Dissolved gases– NO: synthesized on demand; relaxation of smooth muscle (Viagra)