Individual nerve cells, called neurons, are the basic units of the nervous system. The neurons form the
communication network of the nervous system. They can generate electrical signals at their cell bodies, and relay
them to distant regions of the cell. They can also relay signals to other cells by releasing chemical messengers. It
is therefore quite fitting that we should start our discussion of the nervous system with the neuron.
All cells have a voltage or potential difference across their plasma membranes; the inside of cells has a
negative potential (or is negatively charged) with respect to the outside (potential of 0 mV). This potential
difference is called the membrane potential. Certain cells possess special properties that enable them to change
their permeabilities to certain ions in response to a stimulus. These cells are called excitable cells and make up
what is called excitable tissue. The neurons of the nervous system are excitable cells that can change their
permeabilities to ions such as sodium and potassium. A change in permeability to these ions causes a change in
membrane potential. These changes in membrane potential are the electrical signals that are used to communicate
with other parts of the nervous system.
After a brief look at the organization of the nervous system, this unit focusses on membrane potentials,
and on how certain changes in membrane potentials can be used to transmit information from one part of the
nervous system to another. Then you will learn how electrical activity in one neuron can be transmitted to other
neurons. Central to this process is the synapse, a specialized junction between two neurons. At the synapse, the
electrical activity of one neuron, the presynaptic neuron, can influence the electrical activity of a second neuron,
the postsynaptic neuron. The connecting link between the two is a chemical transmitter. Chemical transmission at
synapses allows electrical signals that are generated in one part of the nervous system to affect electrical activity
in another part. Chemical transmission also occurs between nerve cells and effector cells. The classic example is
neuromuscular transmission. It occurs at the neuromuscular junction between motor nerve endings and skeletal
muscle cells, and will be studied in Module 7.
ORGANIZATION OF THE NERVOUS SYSTEM:
• PNS sends information to the CNS through afferent sensory neurons
• PNS takes information from the CNS to target cells via efferent neurons
• Enteric nervous system can act autonomously
o Or can be controlled by the CNS through the autonomic division of the PNS
• Functional units of the nervous system
Neurons make up about 10% of the cells in the central nervous system. The other cells are glial cells
(neuroglia). The majority of the neurons are interneurons, and some of these interneurons receive synaptic input
from as many as 100,000 other neurons. The importance of this will become more apparent as you study this unit.
Figure 82 shows the basic structure of a neuron. In figure 84 you can see the location of organelles within
the neuron and the transport process between the cell body and axon terminal.
Neuron A nerve cell, capable of generating & transmitting electrical signals.
Cell Body (Soma) Part of the cell that contains the nucleus and many organelles.
Axon An extension of a neuron that carries signals to the target cell.
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Dendrite Thin, branched processes that receive and transfer incoming information to an
integrating region within the neuron.
Axon Hillock Region of the axon where it joins the cell body; often contains the trigger zone.
Initial Segment The axon hillock and first part of an axon; often the location of the neuron’s trigger zone.
Collateral Branch of an axon.
Axon Terminal The distal end of a neuron where NT is released into a synapse.
Synapse Region where a neuron meets its target cell.
Afferent Neurons Efferent Neurons Interneurons
Sensory neuron Associated neuron
A neuron that transmits sensory Peripheral neuron that carries Neuron that is completely
information to the CNS signals from CNS to target cells contained within the CNS
Presynaptic Neuron Postsynaptic Neuron
The neuron that delivers the signal to the synapse The cell that receives the signal
Glial cells and their functions are found in figure 85. Pay particular attention to Figures 85 and 86 they
show how Schwann cells and oligodendrocytes form the myelin covering of axons. Myelin is a fatty membranous
sheath that speeds up the passage of electrical signals along the axon.
• Nonexcitable support cells of the CNS
o Provide physical & biochemical support for neurons
• PNS has 2 types (Schwann cells & satellite cells)
• CNS has 4 types (oligodendrocytes, microglia, astrocytes, ependymal cells)
TYPE OF GLIAL PNS OR CNS? FUNCTION
Satellite Cells PNS Support cell bodies
Schwann Cells PNS Secrete neurotrophic factors
Form myelin sheaths
Oligodendrocytes CNS Form myelin sheaths
Microglia CNS Act as scavengers
Astrocytes CNS Provide substrates for ATP production
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Help form the bloodbrain barrier
Secrete neurotrophic factors
Take up K , water & NT
Source of neural stem cells
Ependymal Cells CNS Source of neural stem cells
Create barriers between compartments
• Movement of material between the axon terminal and the cell body
• Fast or slow
o Fast axonal transport has the ability to:
Move forward (anterograde) material from cell body to the axon terminal
Move backward (retrograde) material from the axon terminal to the cell body for
• All living cells have a resting membrane potential that results from the uneven distribution of ions across
the cell membrane
• 2 factors influence membrane potential
o Concentration gradients of ions across the membrane
o Membrane permeability to ions
• The Nernst Equation describes the membrane potential that a single ion would produce if the membrane
were only permeable to that one ion
o For any one ion, this membrane potential is called the equilibrium potention (E ) of the ion
Major Ions Contributing to Resting Membrane Potentials:
• Major ions are Na , K & Cl
Ion Equilibrium Potential (E ion Where is Concentration Higher?
K + 90 mV ICF
Na + + 60 mV ECF
Cl 63 mV ECF
Resting membrane potentials exist due to ion concentration differences across the membrane. Table 82
lists the distribution of major ions for a typical nerve cell. Graded potentials and action potentials are rapid
changes in membrane potentials that function as electrical signals. Note the millisecond time course of the action
potential. Also note the changes in permeability to sodium and potassium, and observe how these changes in
permeability change the membrane potential from its resting state into the transient action potential. The
upstroke (depolarizing phase) of the action potential is caused by the movement of sodium ions into the cell when
the sodium channels open. The repolarizing phase of the action potential is caused by (i) closure of the sodium
channels, and (ii) opening of potassium channels and movement of potassium ions out of the cell. Consider the
allornone concept, threshold and refractory periods. Know how action potential are generated and propagated.
Finally, learn why myelin is important for nerve signal transmission. Figures 89, 10, 15 and 18 describe the
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generation and propagation of action potentials. Figure 812 gives a good description of refractory periods. In
Table 83 you will find a comparison of graded and action potentials.
• Variable strength signals that travel over short distances & lose strength as they travel through the cell
• Used for shortdistance communication
• Lose strength as they move from the point of origin through the cytoplasm due to:
o Ionic leaks
o Cytoplasmic resistance
Spread of Graded Potentials:
• Starts above threshold at its initiation point
o Decreases in strength as it travels through the body
• When it reaches the trigger zone it EITHER is:
o Subthreshold ▯Graded potential is below threshold at trigger zone ▯NO action potential
o Suprathreshold ▯Graded potential above threshold at trigger zone ▯ACTION POTENTIAL
Inhibitory Postsynaptic Potential (IPSP) Excitatory Postsynaptic Potential (EPSP)
HYPERPOLARIZING graded potentials that make a DEPOLARIZING graded potentials that make a
neuron LESS LIKELY to fire an action potential neuron MORE LIKELY to fire an action potential
• Very brief, large depolarizations that travel for long distances through a neuron without losing strength
• Used for rapid signaling over long distances
• Allornone (occur to maximum depolarization or not at all)
Na and K Move Across the Membrane During Action Potentials:
1. Membrane depolarizes to threshold
i. Voltage gated Na channels open ▯Rapid Na entry DEPOLARIZES the cell
ii. K channels begin to open slowly
2. Na channe+s close & slower K channels open
i. K moves from cell to ECF ▯HYPERPOLARIZES the cell
3. Voltagegated K channels close ▯Less K leaks out of the cell
4. Cell returns to resting ion permeability & resting membrane potential
Threshold: The minimum depolarization that will initiate an action potential in the trigger zone.
Rising Phase • Due to sudden temporary increase in cell’s permeability to Na+
• Addition of positive charge to cell membrane causes depolarization
o Na channels close once cell peaks at + 30 mV
Falling Phase +
• Corresponds to an increase in K permeability
• K channels fully open when Na start closing
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o K moves out of cell ▯Cell becomes more negative causing
hyperpolarization as it reachs 90 mV (EK)
Depolarization • Decrease in the membrane potential difference of a cell
o Addition of Na (positive charge) to cell
• Membrane potential that is more negative than the resting potential
o K moves out of the cell ▯Cell membrane becomes more negative
to 90 mV
After Hyperpolarization • AKA undershoot
• When cell reaches 90 mV
Overshoot • The portion of the action potential above 0 mV
• Phase during which the depolarized membrane returns to its resting
Relative Refractory Period Absolute Refractory Period
A period of time immediately following an action The time required for the Na channel gates to reset to