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I.
II.
What Cells Are Unique to the Nervous System?
A. Nervous systems have two categories of cells:
1. Neurons, or nerve cells, are excitable—they generate and transmit electrical
signals, called action potentials.
2. Glia, or glial cells, provide support and maintain extracellular environment.
B. Neurons are organized into neural networks.
1. Afferent neurons carry sensory info into the nervous system from sensory
neurons that convert stimuli into action potentials.
2. Efferent neurons carry commands to effectors such as muscles, glands.
3. Interneurons store info and communicate between neurons.
C. Networks vary in complexity.
1. Nerve net—simple network of neurons.
2. Ganglia—neurons organized into clusters, sometimes in pairs.
3. Brain—the largest pair of ganglia.
D. Central nervous system (CNS)—consists of cells found in brain and spinal cord.
E. Peripheral nervous system (PNS)
F. Neurons pass info at synapses:
1. The presynaptic neuron sends the message.
2. The postsynaptic neuron receives the message.
G. Structure of a nerve cell most neurons have four regions:
1. Cell body—contains nucleus and organelles
2. Dendrites—bring info to the cell
3. Axon—carries info away from the cell
4. Axon terminal—synapse at tip of axon release neurotransmitters.
H. Glial cells outnumber neurons in the human brain.
1. Glia do not transmit electrical signals but can release neurotransmitters.
2. Glia also give support during development, supply nutrients, and maintain
extracellular environment.
3. Others insulate axons.
I. In the CNS oligodendrocytes produce myelin and insulate axons.
J. Schwann cells insulate axons in the PNS.
K. Astrocytes contribute to the blood-brain barrier, which protects the brain.
L. Microglia provide the CNS with immune defenses.
How Do Neurons Generate and Transmit Electrical Singlas?
A. General: Ions move according to the differences in concentration gradients and
electrical charge, electrochemical gradients.
1. Leak currents allow only certain ions to pass in or out.
2. Membrane potential is the electrical potential, the charge difference, across the
membrane.
3. Resting potential is the membrane potential of a resting neuron.
4. An action potential or nerve impulse, is a rapid large change in membrane
potential.
III.
5. Action potentials are generated by openings and closing of ion channels.
6. Voltage causes electrically charged particles, ions, to move across cell
membranes.
7. Major ions in neurons
a) Sodium
b) Potassium
c) Calcium
d) Chloride
B. Neuron membrane potentials are measured in
1. Ion channels and ion transporters in the membrane create the resting and
action potentials.
a) Sodium-potassium pump—moves sodium ions from inside and
exchanges for potassium from outsides. Establishes concentration
gradients.
b) The pump is an antiporter and uses ATP.
c) Ion channels in the membrane are selective.
2. The potassium equilibrium potential is the membrane potential at which the
net movement of potassium ceases.
3. Some ion channels are “gated”
a) Voltage gated channels
b) Chemically gated channels
c) Mechanically gated channels
4. Gated ion channels change the resting potential when they open and close.
a) The membrane is depolarized when sodium enters the cell.
5. Graded membrane potentials are changes from the resting potential.
a) A threshold is reached at 5-10mV above resting potential, this triggers
an action potential.
6. Voltage gated Na channels cannot open during the refractory period.
a) Activation gate – closed at rest, opened at threshold.
b) Inactivation gate—open at rest, closed at threshold.
C. Myelination by Glial cells increases the conduction velocity of axons.
D. The nodes of Ranvier are regularly spaced gaps where myelin is absent.
How do neurons communicate with other cells”
A. Neurons communicate with other neurons or target cells at synapses.
1. In a chemical synapse
2. In an electrical synapse the action potential spreads directly to the postsynaptic
cell.
3. The neuromuscular junction is a chemical synapse between motor neurons and
skeletal muscle cells.
a) The motor neuron releases acetylcholine (Ach) from its axon terminals.
b) Post synaptic membrane of the muscle cells is the motor end plate.
c) The synaptic cleft is the space between the presynaptic and
postsynaptic membranes.
d) An action potential causes release of the neurotransmitter Ach when
voltage gated calcium channels open and calcium enters the axon
terminal.
e) Vesicles release Ach
B. To decide whether or not to respond, the postsynaptic cell must sum the input.
1. Summation occurs at the axon hillock.