Download Nervous Tissue

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)
See yellow light
– Sensors
– External or internal info
• Integration
– Immediate context
– Experience
• Motor output (from brain)
– Effector organs
– Muscle or gland response
Foot to brake or gas
Process options
Human Nervous System Divisions
Nervous System
Info in & out
Integration & command
Peripheral
Nervous
System (PNS)
Central Nervous
System (CNS)
Brain
Spinal Cord
Motor (efferent)
Autonomic
(involuntary)
Sympathetic
(Fight or flight)adrenergic
Sensory (afferent)
Somatic
(voluntary)
Parasympathetic
(Rest and digest)cholinergic
Neuroglia
• 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.ed
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Neurons
• Structural unit of the nervous
system
• Cell body (soma)
– Nissl bodies (rough ER)
– Nuclei vs ganglia
• Processes
– Dendrites
• Input; dendritic spines; graded potentials
http://www.pspnperak.edu.my/biologit5/Abd%20Razak%20b.%20Yaacob/Por
tfolio/BBM/Audio/saraf/Neuron%208.gif
– Axons
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•
•
•
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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
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
http://www.unisanet.unisa.edu.au/Resources/101766/Online%20Brain%20Development%20course/Pics/Photo
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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
Unmyleinated
– Degree of myelination
• w/o = continuous conduction; AP immediately =
slow
• w/ = prevents leaks; faster
change
Myleinated
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