Chapter 11
I. Organization of the nervous system
A. The two main branches:
Central Nervous System (CNS)- brain and spinal cord
Peripheral Nervous System (PNS)- the rest. Originates from major nerves
(bundles of neurons) coming out of the brain (cranial nerves) and spinal
cord (spinal nerves)
B. The PNS branches
1. Afferent vs. Efferent Divisions
Afferent- "sensory." Brings information to the CNS about
sensation (olfaction, audition, etc), "state of the body" (position of
joints & muscles, stretch of organs, etc), pressure, pain, etc.
Efferent- Brings messages out of the CNS to muscles and glands
(called "effectors")
2. The Efferent Division
a. Somatic vs. Autonomic Systems
Somatic (SNS)- messages sent to skeletal muscle (that's
what we talked about in muscle physiology)
Autonomic (ANS)- messages sent to cardiac & smooth
muscle, and glands
b. The Autonomic Nervous System
Parasympathetic- "normal" state of being; really- conserve
energy, digest and excrete
Sympathetic- "emergency"; really- mobilize energy,
prepare for exercise or fasting
* The para- and sympathetic systems have opposing effects to one another (ex. paradecreases heart rate, sympathetic increases heart rate). The quick & dirty introduction to
them makes it sound like either one or the other is functioning, depending on whether
there's an emergency or not. Really, both are typically influencing different parts of the
body to varying extents at any given time.
II. Histology of nervous tissue
A. Neurons
1. Some characteristics
With few exceptions (olfactory neurons and hippocampus), cannot
divide, so are very long-lived
High metabolic rate; that is, they use a lot of energy. Prefer glucose
as the energy source.
Consist of a cell body, dendrites, and an axon
2. Structure
a. Cell body- large nucleus & nucleoli. Elaborate cytoskeleton
which extend into axon. This is where neurotransmitters are made,
and are sent down the axon to synaptic terminals. Lots of
ribosomes and extensive ER: Nissl bodies (clumps of cell bodies
appear gray because of Nissl bodies- "gray matter")
b. Dendrites- highly branched extensions (branches- "dendritic
spines"). Primary area of reception from other neurons (receive
neurotransmitters).
c. Axon- long extension that branches at the end (branches"telodendria," end at synaptic terminals)
-axoplasm, axolemma
-axon hillock- thickened area where axon merges with cell
body
-no Golgi or Nissl bodies
-materials move between the cell body and synaptic
terminals along microtubules ("neurotubules"). ATP is
required (review microtubules from Chapter 3).
Anterograde & retrograde flow.
-the axon carries action potentials from the axon hillock to
synaptic terminals. An action potential arriving at the
synaptic terminals triggers the release of neurotransmitters.
-axons of many neurons are myelinated (see "neuroglia,"
below).
3. Classification
a. By structureBipolar- cell body between dendrites and axon.
Rare, in sense organs.
Unipolar- cell body bulges out to the side, axon and
dendrites are continuous. Common in PNS, especially
sensory.
Multipolar- the "representative" neuron. The most common
type.
b. By function
1) Sensory/Afferent neurons- most are unipolar; brings info
to the CNS; types-Exteroceptors- info about the outside world: touch,
temperature, senses
-Proprioceptors- info about position/movement of
skeletal muscles & joints
-Interoceptors- info about organ systems & pain
2) Motor neurons- efferent, multipolar
3) Interneurons- in CNS. Coordinate information, for
instance, between sensory and motor neurons; multipolar
B. Neuroglia- support cells of the nervous system
1. Support cells in the CNS
a. Ependymal cells- line central cavities of brain and spinal cord,
produce and circulate Cerebrospinal Fluid.
b. Astrocytes
-Wrap around capillaries and control which substances
enter the CNS: blood-brain barrier
-Extensive cytoskeleton gives support and structure to the
CNS
-Aid in repair of neural tissue
-Absorb substances from interstitium (ex.,
neurotransmitters)
c. Oligodendrocytes- numerous extensions "reach out" and wrap
around axons, myelinating them. One oligodendrocyte adds
myelination to many neurons. Myelinated CNS appears whitish
and constitutes white matter.
d. Microglia- derived from immune cells. Phagocytize debris and
any pathogens that happen to invade.
2. PNS support cells
a. Satellite cells- surround cell bodies, regulate interstitial fluid
(help to maintain ions, etc).
b. Schwann cells- surround axons
-Some Schwann cells completely myelinate axons. The
entire cell wraps itself around a segment of the axon.
Unlike oligodendrocytes, each Schwann cell surrounds
only one axon. Each axon has several Schwann cells
wrapped around its length.
-Some Schwann cells send a whimpy covering around
many axons, and don't really wrap around them completely.
Axons with this whimpy covering are considered
unmyelinated. Many sensory neurons are unmyelinated.
*Myelination affects the speed of conduction: myelinated neurons conduct signals
MUCH faster (~250 miles/hour vs. ~2 mph).
III. Neurophysiology
A. Background: types of membrane channels we’ll be looking at:
1. Chemically regulated (“ligand-gated”): open/close in response to
the binding of a chemical, like the Na+ channels of a muscle cell at
the motor end plate. Facilitated diffusion.
2. Voltage regulated: open/close in response to a specific change in
voltage, like the Na+ channels of a muscle cell along the rest of its
membrane, and the Ca2+ channels in the SR. Facilitated diffusion
3. Mechanically regulated: open/close in response to some sort of
mechanical stimulus, like pressure or vibration. We'll get to these
later on. Facilitated diffusion.
4. Leak channels- always open; there are many more K+ leak
channels than Na+, and this helps to create the electrochemical
gradients that exist. Facilitated diffusion.
5. Sodium-potassium exchange pump- pumps 3 Na+ out and 2 K+
in; constantly working; helps to establish and maintain the
electrochemical gradients that exist. Active transport.
B. The nerve cell at rest
1. Leak channels: the membrane of a neuron contains leak channels for
both Na+ and K+. However, it contains MANY more K+ leak channels.
Since more K+ is in the cell than out, K+ passively diffuses down its
concentration gradient through these leak channels to the exterior of the
cell (the opposite is true for Na+). Since there are more K+ channels, more
K+ ions leave the cell than Na+ ions enter. Therefore, more positive
charges leave the cell than enter. This is partially responsible for creating
the slight negative interior of the cell compared with the exterior.
2. Sodium-Postassium Exchange pumps- constantly work to pump Na+
out and K+ in, against their concentration gradients. It's not an even trade
though. 3 Na+ are pumped out for every 2 K+ pumped in. Again, more
positive charge goes out than comes in, helping to create the relative
negative interior of the cell.
3. The resting potential of the average nerve cell is -70mV.
*Keep in mind that the electrochemical gradient of sodium (the combination of electrical
attraction and a concentration gradient) means sodium REALLY "wants" to get in, if it
could.
*Take a minute to draw a cell with several K+ leak channels, a few Na+ leak channels,
and some Na+-K= exchange pumps. Draw an ion at each of these channels, and draw an
arrow to indicate which direction each ion travels through. This will help you to visualize
the uneven movement of positive charges.
C. Introduction to action potential generation/propagation, and release of
neurotransmitter
1. Local/graded potential- Neurotransmitters from the presynaptic cell
binds to receptors in the postsynaptic cell, causing chemically-gated Na+
channels to open, and Na + rushes in. The interior of the cell becomes less
negative, or depolarized (the voltage increases). If enough Na+ enters that
the voltage around the axon hillock reaches threshold (-60 - -55 mV), an
action potential will be generated.
2. Action potential
a. Voltage-regulated Na+ channels at the base of the axon
(adjacent to the hillock) respond to threshold voltage. At resting
potential, the activation gate of these channels is closed, while the
inactivation gate is open. At threshold, the activation gate opens.
Now the channel is completely open and Na+ ions enter the cell,
following their electrochemical gradient.
As Na+ enter, the voltage increases (becomes less negative). When
voltage reaches +30 mV, the inactivation gate closes, preventing
further movement by Na+. Inactivation gates will reopen when
voltage returns to around resting potential by the efflux of K+ (see
below).
*Side note: Na+ continues to enter the cell even after the voltage has hit zero because a
concentration gradient for Na+ still exists. That is, even though the total charges have
been balanced, there are still more Na+ outside than inside the cell, so Na+ will continue
to move down its concentration gradient.
b. Around the same time (+30mV), voltage-regulated K+ channels
open. K+ then rushes out, following its electrochemical gradient.
(Remember that there are more K+ inside than outside the cell, and
at +30 mV, the exterior of the cell is actually negative compared
with the interior). We are now in a phase where voltage is being
restored by K+: repolarization.
c. When enough K+ have left the cell that the voltage hits -70
again, K+ channels start to close, but some K+ still sneak through
and voltage will peak around -90mV. At this point the cell is
hyperpolarized. A hyperpolarized cell is less sensitive to stimuli,
because a larger change in voltage is required to hit threshold.
d. Sodium-Potassium exchange pumps will return the cell to its
normal ion concentrations.
*Keep in mind that each of these changes in voltage is a local event; that is,
depolarization occurs first at the base of the axon. It causes an influx of Na+ through
neighboring channels. That influx affects neighboring channels, which open; that effects
the next neighbors down, and so-on. So depolarization (and all subsequent events) occurs
sequentially down the axon, toward the synaptic terminals.
e. Absolute refractory period: From the time the Na+ activation
gate opens until the time the INactivation gate reopens, the cell
cannot fire another action potential.
Relative refractory period: After the inactivation gate reopens until
the cell has restored resting ion distributions. The cell CAN fire
another action potential during this time.
3. An action potential reaches a synaptic terminal
-Voltage-gated Ca++ channels in the membrane open, Ca++ pours
in from the extracellular fluid.
-Ca++ triggers the release of Nt (neurotransmitters) from synaptic
vesicles
-Ca++ is actively pumped back out, and Nt are broken down by
enzymes and the remnants are reabsorbed into the presynaptic cell
(as in Ach); or, the Nt are taken up by the presynaptic cell whole
(“reuptake,” as in serotonin).
D. Other factors involved with Action Potentials and Synapses
1. Graded potentials
a. Typically, one delivery of Nt onto a postsynaptic cell is not
enough to trigger an AP. It will cause some depolarization, but not
to threshold. More than one depolarization events are typically
required to reach threshold and generate an AP. What can bring a
cell to threshold?
Spatial summation- many stimuli (ex. Nt) arrive at once
Temporal summation- many stimuli arrive in rapid
succession
b. Excitatory Post-Synaptic Potentials (EPSP) and Inhibitory PostSynaptic Potentials (IPSP)
An EPSP is a change in membrane potential that will make a
neuron more likely to reach threshold. That is, it is a depolarized
potential. A neuron that experiences an EPSP is "excited," or needs
less stimulation to reach threshold. For example, in an EPSP, a few
chemically-gated Na+ channels are opened.
An IPSP is a change in membrane potential that will make a
neuron less likely to reach threshold. That is, a hyperpolarized
potential. A neuron that experiences an IPSP is "inhibited," or
needs more stimulation to reach threshold. For example, in an
IPSP, chemically-gated K+ or Cl- channels are opened.
*Please read "Summation by the Post-Synaptic Neuron”
*Take a minute to draw a neuron with chemically gated Na+, K+, and Cl- channels.
Draw where these ions are in a resting cell (in or out of the cell). Now show yourself,
with arrows, which direction each would travel if its channels were open. What effect
will the movement of each have on the voltage (will the interior become more positive or
more negative)?
2. Synaptic Potentiation-please read
*Take a minute to draw three neurons: the presynaptic cell and the postsynaptic cell as
usual. Now, draw the third neuron. This third neuron should have its synaptic terminal
synapse with the synaptic terminal of the presynaptic neuron. Use arrows to indicate
serotonin coming from that 3rd cell onto the presynaptic cell. Now draw Ca++ channels
opening on the presynaptic cell, and Nt being released from it to the postsynaptic cell.
4. Speed of AP conduction (how fast an Action Potential travels down an
axon) affected by two factors
a. Diameter of axon- larger diameter = faster
b. Myelination- myelinated axons = faster. In a myelinated axon,
only the nodes of Ranvier contain ion channels, so the movement
of ions seems to "jump" from node to node. This type of
conduction is called saltatory (saltation- jumping).
*Conduction in non-myelinated axons called continuous
c. Axon types based on diameter & myelination
Type A- large, myelin; up to ~300mph; ex. motor fibers to skeletal
muscle
Type B- medium, myelin; up to ~40 mph; ex. some fibers to
smooth muscle, some from sensory
Type C- small, no myelin; ~2 mph; ex. many sensory, many
interneurons, some motor to smooth muscle
III. Neurotransmitters
A. Actions- *Keep in mind that the action of a neurotransmitter (whether it's
excitatory or inhibitory, and whether it's direct or indirect) depends on the
receptors of the postsynaptic cell. For instance, skeletal muscle contains receptors
that open Na+ channels (excited) in response to Ach; while cardiac muscle
contains receptors that open K+ or Cl- channels (inhibited) in response to Ach.
*Please read "chemical synapses" under heading “The Synapse”
1. Direct- cause Na+, K+, or Cl- channels to open in response to binding
of Nt. These have fast, short-term effects.
2. Indirect- cause the postsynaptic cell to change in some way, making it
more or less sensitive to direct Nt (for example, could cause the
postsynaptic cell to build more Na+ channels, so that each EPSP would be
stronger because more sodium would enter with ONE stimulus). Indirectly
acting Nt initiate a cascade of chemical reactions that include "secondmessenger" chemicals: when the Nt binds, chemicals within the cell
"respond" and "send messages" to the nucleus (ex, we need more Na+
channels).
B. Specific Nt; know structural class, whether it acts directly, indirectly or both;
whether it is excitatory, inhibitory or both; whether it is used by the CNS, PNS or
both; and general function. Keep in mind the function/s listed are oversimplified
and often not the only function/s.
1. Ach- direct & indirect, excitatory(E) to skeletal muscle, inhibitory(I) to
cardiac. Used in PNS and CNS.
2. Biogenic Amines (amino acid derivatives)
a. NorEpinephrine (NE)- indirect, usually E, PNS & CNS. Among
other functions- emergency response.
b. Serotonin- indirect & direct, I, CNS. Mood regulation.
c. Dopamine- indirect, I or E, CNS & PNS. Mood regulation,
postural muscle control.
4. Glutamate- an amino acid. direct, E, CNS. Learning and memory.
Overrelease kills neurons and is associated with stroke.
5. Opioids- a peptide. (incl. endorphins)- indirect, I, CNS. Pain reduction.
6. CO (carbon monoxide)-a gas. not well understood, may be involved
with memory and concentration.
*This is a VERY short list, and not all Nt have been discovered.
IV. Neuronal Pools- Groups of interconnected interneurons. Can communicate in a
number of different ways, which are not necessarily mutually exclusive:
A. Divergence
B. Convergence
C. Serial processing
D. Parallel Processing
E. Reverberation