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

Integration & command Info in & out

• 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