1. Fundamentals of the Nervous System
Chapter 11
Dr Tamily Weissman, Department of Molecular and Cellular Biology, Harvard University

2. 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

3. 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

4. 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

5. Neurons Structural unit of the nervous system Cell body (soma)
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

6. 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)

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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)

15. 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