This is a comprehensive and detailed note on Neurons, Glia, and Nervous Systems for Biol 105.
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Neurons and glia are unique cells of nervous systems
nervous systems have 2 types of cells:
o neurons (nerve cells) generate and conduct electric signals
o glia
macroglia – modulate neuron activity and provide support
microglia – small phagocytic cells; major immune defense mechanism in
the nervous system
vertebrates have a central nervous system (CNS)
o includes brain, spinal cord; sites of information processing and storage
peripheral nervous system (PNS)
o provides communication between central division and all the rest of the body
neurons and macroglia originate from neural stem cells in the neural tube in the early
embryo
when the stem cells divide, one daughter cell remains a stem cell, the other becomes a
neuroblast or gliablast (progenitor cells for neurons and glia)
neuron structure:
o cell body – contains nucleus and organelles
o dendrites – bring information to the cell body
o axon – carries information away from the cell body
o axon terminals – at the tip of the axon
neuron form reflects function
o number of dendrites reflects the number of sources of information coming to the
neuron
o some neurons communicate over very short distances; others communicate over
long distances and have very long axons
neurons process and communicate information through changes in electric potential
across their membranes
o at rest – K- inside of cell, Na+ outside of cell; interior of cell is negative, and
exterior is positive
o during depolarization, Na+ channels open, allowing influx of Na+ and reversal of
polarity; interior of cell is now more positively charged than exterior
small changes in membrane electric potential generate large, rapidly reversed changes in
membrane potential called “action potentials” (APs)
axon terminals come extremely close to the membrane of a target cell (another neuron,
muscle, or a gland), forming a synapse
when the action potential reaches the axon terminals, the synapse transfers the
information from presynaptic cell to postsynaptic cell
in vertebrates, most synapses are chemical:
, o an action potential causes the terminal to release neurotransmitter chemicals
which diffuse to receptors on the postsynaptic cell
o binding of the neurotransmitter to the postsynaptic cell may excite or inhibit it
in brain and spinal cord, glia called oligodendrocytes wrap around neuron axons, forming
concentric layers of insulating cell membrane (myelin)
o glia called Schwann cells wrap the axons of other nerves (peripheral nervous
system)
oligodendrocytes and Schwann cells produce myelin that covers axons
o multiple sclerosis is an autoimmune disease; antibodies to proteins in myelin in
the brain and spinal cord are produced, breaks down myelin
Neurons generate and transmit electric signals
sodium-potassium pumps in all animal cells create gradients of Na+ and K+ across the
cell membrane
o voltage-gated Na+ channel
o chemical-gated K+ channel
the inside of the cell is usually negative relative to the outside because “leaky channels”
allow some K+ ions to diffuse out
membrane potential - electrical charge difference across a cell membrane; due to a
balance between the tendency of K+ ions to diffuse down their concentration gradient
and the electrical potential that holds them back
resting potential – steady state membrane potential of a neuron (nothing is happening)
o only leaky K+ channel is open
voltage (electric potential difference) – force that causes charged particles to move
between two points
o in solutions and across cell membranes, electric current is carried by ions
o major ions in neurons: Na+, K+, Ca2+, Cl-
membrane potentials are measured with electrodes
the resting potential of an axon is -60 to -70 millivolts (mV)
the inside of the cell is negative at rest; a stimulus that changes the permeability of the
membrane allows ions to move quickly across
an action potential is a sudden, rapid reversal in the voltage across a portion of the cell
membrane
o for 1-2 milliseconds, positively charged ions flow into the cell, making the inside
of the cell more positive than the outside
o depolarization
o repolarization
o back to resting potential
ion transporters and ion channels are responsible for the distribution of charges across
the membrane that determine membrane potential
the sodium-potassium pump moves Na+ to the outside and K+ to the inside; requires
energy (ATP)
o establishes concentration gradients
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