Chapter 12: Neural Tissue
I. An Overview of the Nervous System

The nervous system includes all the ______________________________ in the body.

Neural tissue contains 2 kinds of cells:
1. ______________________________: the cells that send and receive signals
2. ______________________________ (glial cells): the cells that support and protect the neurons

The organs of the nervous system include:
- the ______________________________
- the ______________________________
- ______________________________receptors of sense organs (eye, ears, etc.)
- the ______________________________that connect the nervous system with other systems

The nervous system has 2 major anatomical divisions:
1. the ______________________________ (CNS)
2. the ______________________________ (PNS)
The Anatomical Divisions of the Nervous System

The CNS is responsible for processing and coordinating:
1. ______________________________data from inside and outside the body.
2. ______________________________commands that control activities of peripheral organs such as the
skeletal muscles.
3. ______________________________functions of the brain such as intelligence, memory, learning and
emotion.

The peripheral nervous system (PNS) includes all neural tissue ______________________________.

The PNS is responsible for:
1. ______________________________sensory information to the CNS
2. ______________________________motor command to peripheral tissues and systems

Sensory information and motor commands in the PNS are carried by bundles of axons (with their
associated connective tissues and blood vessels) called peripheral nerves (nerves):
1. ______________________________nerves are connected to the brain
2. ______________________________nerves are attached to the spinal cord
The Functional Divisions of the Nervous System

The PNS is separated into 2 divisions:
1. the ______________________________division, which carries
______________________________information from sensory receptors of the PNS to the CNS.
2. the ______________________________division, which carries
______________________________commands from the CNS to muscles and glands of the PNS.

The efferent division is divided into 2 parts:
1. the ______________________________nervous system (SNS), which controls skeletal muscle
contractions
a. ______________________________muscle contractions
b. ______________________________muscle contractions (______________________________)
2. the ______________________________nervous system (ANS), which controls subconscious actions
such as contractions of smooth muscle and cardiac muscle, and glandular secretions.

The ANS is separated into 2 divisions:
1. the ______________________________division, which has a stimulating effect
2. the ______________________________division, which has a relaxing effect

Neurons are the basic ______________________________units of the nervous system.
II. Neurons
The Structure of Neurons
1. The cell body includes:
- a relatively large
______________________________and
______________________________
- the ______________________________,
called the
______________________________, which
contains mitochondria that provide
energy, and dense areas of RER and
______________________________that
produce neurotransmitters. These dense areas, called ______________________________, make neural
tissues appear gray (the gray matter).
- the cytoskeleton with ______________________________and ______________________________ (in place
of microfilaments and microtubules)
- most nerve cells do not contain ______________________________and ___________________divide
2. Dendrites are highly branched, with many fine ______________________________that receive information
from other neurons. Dendritic spines make up ______ % of the neuron’s surface area.
3. The long axon carries the electrical signal (______________________________) to its target. The structure
of an axon is critical to its function.
- ______________________________: the
cytoplasm of the axon
- ______________________________: a
specialized cell membrane, covers the
axoplasm
- the initial segment of the axon attaches to
the cell body at a thick section called the axon
______________________________
- ______________________________are
branches of a single axon
- ______________________________are the fine extensions at the synaptic terminal of the axon

Within the synapse, the 2 cells do not actually touch. A small gap called the synaptic cleft separates the
______________________________membrane and the ______________________________membrane.

The expanded area of the axon, called the ______________________________, contains synaptic vesicles
filled with chemical messengers called ______________________________.

The neurotransmitter chemical is then broken down and reassembled at the synaptic knob. The transport
of raw materials between the cell body and the synaptic knob by neurotubules within the axon is called
______________________________transport
The Classification of Neurons

There are 4 classifications of neurons
based on structure:
1. ______________________________
neurons:
- small
- all cell processes look
______________________________
- found in
______________________________and
______________________________organs
2.
______________________________neurons:
- small
- one dendrite and one axon
- found in special ______________________________ (sight, smell, hearing)
3. ______________________________neurons:
- very long axons
- dendrites and axon are fused, with cell body to one side
- found in sensory neurons of the ______________________________system
4. ______________________________neurons:
- very long axons
- 2 or more dendrites and 1 axon
- ______________________________in the ______________________________
- includes all motor ______________________________of skeletal muscles

There are 3 classifications of neurons based on function:
1. Sensory neurons or ______________________________neurons, (the afferent division of the PNS):
-
Cell bodies of sensory neurons are grouped in sensory ganglia.
-
Sensory neurons collect information about our internal environment (visceral sensory
neurons) and our relationship to the external environment (somatic sensory neurons).
-
Sensory receptors are categorized as:
a. ______________________________:
- monitor digestive, respiratory, cardiovascular, urinary and reproductive
systems
- provide internal senses of taste, deep pressure and pain
b. ______________________________:
- external senses of touch, temperature, and pressure
- distance senses of sight, smell and hearing
c. ______________________________:
- monitor position and movement of skeletal muscles and joints
2. Motor neurons or ______________________________neurons (the efferent division of the PNS):
-
carry instructions from the CNS to peripheral effectors of tissues and organs via axons called
efferent fibers.
-
the 2 major efferent systems are:
1. the ______________________________nervous system (SNS), including all the
somatic motor neurons that innervate skeletal muscles.
2. the ______________________________nervous system (ANS), including the visceral
motor neurons that innervate all other peripheral effectors (smooth muscle,
cardiac muscle, glands and adipose tissue).
-
signals from CNS motor neurons to visceral effectors pass through synapses at _________________
ganglia, dividing efferent axons into 2 groups:
1. ______________________________fibers
2. ______________________________fibers
3. ______________________________or association neurons:
-
located in the brain, spinal cord and some autonomic ganglia, between sensory neurons and motor
neurons
-
responsible for distribution of sensory information and coordination of motor activity
-
involved in higher functions such as memory, planning and learning
III. Neuroglia

Neuroglia make up
______________________________of the nervous
system. There are many types of neuroglia,
with more variety in the CNS than in the
PNS.
Neuroglia of the Central Nervous System

The central nervous system has 4 types of
neuroglia:
1. ______________________________Cells
-
The central canal of the spinal cord
and ventricles of the brain, filled with
circulating cerebrospinal fluid (CSF),
are lined with ependymal cells which
form an epithelium called the ______________________________.
-
Some ependymal cells secrete cerebrospinal fluid, and some have cilia or microvilli that help
______________________________CSF. Others monitor the CSF or contain stem cells for repair.
2. ______________________________
-
Astrocytes are large and have many functions, including:

______________________________the blood-brain barrier that isolates the CNS

creating a ______________________________framework for the CNS

repairing ______________________________neural tissue

guiding ______________________________development

controlling the ______________________________environment
3. ______________________________
-
Oligodendrocytes have smaller cell bodies and fewer processes than astrocytes. Processes may
contact other neuron cell bodies, or wrap around axons to ______________________________
sheaths.
-
Myelinated segments of an axon are called
internodes. The gaps between internodes, where
axons may branch, are called nodes (nodes of
Ranvier).
-
Because myelin is white, regions of the CNS
that have many myelinated nerves are called
white matter, while unmyelinated areas are
called gray matter.
4. ______________________________
-
Microglia are small, with many fine-branched
processes. They migrate through neural tissue, cleaning up cellular debris, waste products and
pathogens.
Neuroglia of the Peripheral Nervous System

There are 2 types of neuroglia in the PNS: satellite cells and Schwann cells.
1. ______________________________cells (amphicytes) surround ganglia and regulate the environment
around the neuron.
2. ______________________________cells (neurilemmacytes) form a myelin sheath called the neurilemma
around peripheral axons. One Schwann cell encloses only one segment of an axon, so it takes many
Schwann cells to sheath an entire axon.
Neural Responses to Injuries

In the PNS, peripheral nerves can regenerate after
injury. Scwann cells assist in a process called
______________________________. As the axon distal to
the injury site degenerates, Schwann cells form a line
along the path of the original axon, and wrap the new
axon as it grows.

In the CNS, nerve regeneration is limited because
astrocytes ______________________________ by
releasing chemicals and producing scar tissue.
IV. Ion Movements and Electrical Signals

The main membrane processes involved in neural
activities are:
1. ______________________________potential: the
transmembrane potential of a resting cell
2. ______________________________potential: a
temporary localized change in the resting
potential, caused by a stimulus
3. ______________________________potential: an electrical impulse (produced by the graded
potential) that propagates along the surface of an axon to a synapse.
4. ______________________________activity: the release of neurotransmitters at the presynaptic
membrane, which produce graded potentials in a postsynaptic membrane.
5. ______________________________processing: the response (integration of stimuli) of a postsynaptic
cell.
The Transmembrane Potential

The 3 main requirements for a transmembrane
potential are:
1. A ______________________________of ions
(Na+, K+) across the cell membrane
2. The membrane be
______________________________through
membrane channels
3. ______________________________
mechanisms maintain a difference in
charge across the membrane (resting potential = -70 mV)

Passive forces acting across the membrane are
chemical and electrical.
1. ______________________________gradients:
- concentration gradients of ions
(Na+, K+) across the membrane
2. ______________________________gradients:
- the charges of positive and
negative ions are separated across
the membrane, resulting in a
potential difference.
- positive and negative charges attract one another
- if charges are not separated, they will move to eliminate potential difference, resulting in
an electrical current
- how much current a membrane can restrict is
called its resistance

______________________________gradient:
1. the sum of chemical and electrical forces acting on an
ion (Na+, K+) across a cell membrane is the
electrochemical gradient for that ion.
2. chemical gradient of potassium tends to move
potassium out of the cell, but the electrical gradient of
the cell membrane opposes this movement (Figure 129a)
3. the transmembrane potential at which there is no net
movement of a particular ion across the cell
membrane is the equilibrium potential for that ion
(K+ = -90 mV, Na+ = +66 mV).
4. the electrochemical gradient is a form of potential
energy

Active forces maintain the cell membrane’s
______________________________ (-70 mV). The cell actively
pumps out ______________________________ and pumps in
______________________________. The sodium-potassium exchange pump (the carrier protein sodiumpotassium ATPase), powered by ATP, exchanges 3 Na+ for each 2 K+, balancing the passive forces of
diffusion.
Changes in the Transmembrane Potential

The transmembrane potential rises or falls in response to temporary changes in membrane permeability
resulting from opening or closing specific ______________________________channels.

It is primarily the membrane’s permeability to sodium and potassium ions that determines
transmembrane potential. Sodium and potassium channels are either passive or active.
1. ______________________________channels (________________channels) are always open, but their
permeability changes according to conditions.
2. ______________________________channels (________________channels) open and close in response
to stimuli. At the resting potential, most gated channels are closed.

Gated channels can be in one of 3 conditions:
1. closed, but capable of opening
2. open (activated)
3. closed, and not capable of opening (inactivated)

There are 3 classes of gated channels:
1. ______________________________regulated channels:
-
open in response to the presence of specific
chemicals (e.g. ACh) at a binding site
-
found on the dendrites and cell body of a
neuron
2. ______________________________channels:
-
respond to changes in the transmembrane
potential
-
characteristic of
______________________________
membrane
-
found in axons of neurons, sarcolemma of
skeletal muscle fibers, and cardiac muscle
cells
-
have an activation gate (opens) and an inactivation gate (closes)
3. ______________________________regulated channels:
-
open in response to distortion of the membrane
-
found in sensory receptors for touch, pressure and vibration
Graded Potentials

Graded potentials (______________________________) are changes in transmembrane potential that
______________________________spread far from the site of stimulation.

Any stimulus that opens a gated channel produces a graded potential.

Opening a sodium channel produces a graded potential:
1. The resting membrane is exposed to a chemical that opens the sodium channel. Sodium ions enter
the cell, raising the transmembrane potential. A shift in transmembrane potential toward 0 mV is
called ______________________________.
2. The movement of sodium ions through the channel produces a local current that depolarizes
nearby parts of the cell membrane (the graded potential).

When the stimulus is removed, the transmembrane potential returns to normal (_____________________).

Opening a potassium channel has the opposite effect of opening a sodium channel (since positive ions
move out of, instead of into the cell), increasing the negativity of the resting potential
(______________________________).

Local graded potentials, originating on cell dendrites or cell bodies, trigger specific cell functions (such as
exocytosis of glandular secretions or ACh at a motor end plate) at the synaptic terminals of axons. The
link between the two actions is the action potential.
Action Potentials

Action potentials are ______________________________changes in transmembrane potential that affect an
______________________________membrane.

The stimulus that initiates an action potential is a ______________________________of the axon hillock,
large enough (10 to 15 mV) to reach the threshold level required to open voltage-regulated sodium
channels (usually -60 to -55 mV).
o

A smaller stimulus will produce only a local, graded depolarization.
As long as a stimulus exceeds the threshold amount, the action potential is the same, no matter how
large the stimulus may be (much like the pressure on the trigger of a gun). Either an action potential is
triggered, or it is not. This is called the ______________________________.

An action potential is
generated in 4 steps:
1. ____________________
__________ to
threshold.
2. ___________________
___________ of sodium
channels and rapid
depolarization:
- sodium ions
rush into the
cytoplasm
- the inner
membrane
surface changes from negative to positive
3. ______________________________of sodium channels and activation of potassium channels:
- at +30 mV, inactivation gates of sodium channels close (sodium channel inactivation), and
potassium channels open, beginning repolarization
4. ______________________________ permeability:
- potassium channels begin to close when the membrane reaches normal resting potential (70 mV)
- when potassium channels finish closing, the membrane is hyperpolarized to -90 mV
- transmembrane potential then returns to resting level, and the action potential is over
The Refractory Period:

During the time period from the beginning of the action potential to the return to resting state (the
refractory period), the membrane ______________________________ to additional stimuli.

The refractory period is divided into:
1. the ______________________________period, when sodium channels are either open or inactivated
and no action potential is possible
2. the ______________________________period, when the membrane potential is almost normal, and a
very large stimulus can initiate an action potential.
The Sodium-Potassium Exchange Pump:

A stimulated neuron can produce ______________________________action potentials per second.
Maintaining the concentration gradients of Na+ and K+ over time requires the sodium potassium pump,
which in turn requires energy in the form of ATP (1 ATP for each exchange of 2K+ for 3 Na+). If a cell ran
out of ATP, neurons would stop functioning.
Propagation of Action Potentials:

Once an action potential has been generated in the axon hillock, it must be
______________________________along the entire length of the axon. The term propagation is used
because it is a series of ______________________________rather than a passive flow.

Action potentials travel along axons by:
1. ______________________________propagation (unmyelinated axons)
2. ______________________________propagation (myelinated axons)

Action potentials move along an unmyelinated axon by continuous propagation, in which the moving
action potential affects one segment of the axon at a time:
1. The action potential in segment 1 depolarizes the membrane to ________________.
2. A local current depolarizes the 2nd segment to ______________________________.
3. The 2nd segment develops an ______________________________, and the 1st segment enters the
______________________________period.
4. A local current depolarizes the next segment to threshold, and the cycle repeats, propagating the
action potential along the axon in ______________________________, at a speed of about
______________________________.

An action potential moves along a myelinated axon by ______________________________propagation,
which is ______________________________ energy than continuous propagation. (Continuous propagation
cannot occur in myelinated axons because of the insulating effect of the myelin.) In saltatory propagation,
the local current produced by the action potential ______________________________node to node along the
axon, so depolarization occurs only at the nodes.
Axon Diameter and Propagation Speed

Since ion movement is related to the cytoplasm, the diameter of an axon affects the speed of an action
potential -- the ______________________________the diameter, the ______________________________the
resistance.
There are 3 groups of axons, depending on diameter, myelination and speed of their action potentials:
1. Type ______________________________:
-
______________________________
-
large diameter
-
high speed (______________________________)
2. Type B fibers:
-
______________________________
-
medium diameter
-
medium speed (______________________________)
3. Type C fibers:

-
______________________________
-
small diameter
-
slow speed (______________________________)
Type A fibers carry the most ______________________________information to and from the CNS (senses of
position, balance and touch; and motor impulses to skeletal muscles). All other signals (sensory
information, peripheral effectors, and involuntary muscle and gland controls) travel by
______________________________ fibers.
V. Synaptic Activity

To be effective, action potentials (also called ______________________________) must be transmitted across
a synapse, from a ______________________________neuron to a ______________________________neuron or
other type of postsynaptic cell.

There are 2 types of synapses:
1. ______________________________synapses in which there is direct physical contact between cells,
and
2. ______________________________synapses in which the signal is transmitted across a gap by a
chemical neurotransmitter.
Electrical Synapses

Electrical synapses are locked together at ______________________________held together by
______________________________which allow ions to pass between the cells, producing a continuous local
current and action potential propagation (Figure 4-2). Electrical synapses are found in some areas of the
brain, eye, and ciliary ganglia.
Chemical Synapses

Most synapses between neurons and all synapses between neurons and other types of cells are chemical
synapses.

At a chemical synapse, the cells are ______________________________. An arriving action potential may or
may not be propagated to the postsynaptic cell, depending on the amount of neurotransmitter released
and the sensitivity of the postsynaptic cell.

There are 2 classes of neurotransmitters, based on their effects on postsynaptic membranes:
1. ______________________________neurotransmitters cause depolarization and promote action
potentials.
2. ______________________________neurotransmitters cause hyperpolarization and suppress action
potentials.

However, the effect of a neurotransmitter on a postsynaptic membrane depends on the properties of the
receptor, not on the nature of the neurotransmitter.

For example, the neurotransmitter acetylcholine (ACh) usually promotes action potentials, but inhibits
neuromuscular junctions in the heart.

Synapses that release acetylcholine are ______________________________synapses. ACH is released at:
1. all ______________________________junctions
involving skeletal muscle fibers
2. many ______________________________in the CNS
3. all ______________________________synapses in
the PNS
4. all ______________________________and
______________________________junctions within
the parasympathetic division of the ANS

Events at a Cholinergic Synapse
1. An action potential
______________________________ the synaptic
knob.
2. ______________________________ions enter the
synaptic knob, triggering the exocytosis of ACh.
3. ACH binds to
______________________________and depolarizes
the postsynaptic membrane.
4. AChE breaks down ACh into molecules of
______________________________
Synaptic Delay

A ______________________________occurs between
the arrival of the action potential at the synaptic knob and the effect on the postsynaptic membrane -- a
long time relative to the speed of the action potential along the axon. The fewer synapses involved in
relaying a message, the faster the response. Reflexes are important to survival because they may involve
only one synapse.
Synaptic Fatigue

When a neurotransmitter cannot be recycled fast enough to meet the demand of an intense stimulus,
synaptic fatigue occurs. The synapse becomes inactive until ______________________________.
The Activities of Other Neurotransmitters

At least 50 neurotransmitters have been identified so far, including certain amino acids, peptides,
prostaglandins, ATP, and some dissolved gases. Other than acetylcholine, some of the most important
neurotransmitters are:
- ______________________________ (NE):
- found in the brain and portions of the ANS
- effect is excitatory and depolarizing
- synapses that release NE are called adrenergic synapses
- ______________________________:
- a CNS neurotransmitter
- may be excitatory or inhibitory
- involved in Parkinson’s disease and cocaine use
- ______________________________:
- a CNS neurotransmitter
- affects attention and emotional states
- ______________________________ (GABA):
- has an inhibitory effect
- its functions in the CNS are not well understood
Neuromodulators

In addition to neurotransmitters, synaptic knobs may release other chemicals, called neuromodulators.
(There may be little functional difference between neurotransmitters and neuromodulators.) Some
characteristics of ______________________________are:
1. Their effects are ______________________________to appear.
2. They trigger responses that involve ______________________________and intermediary
compounds.
3. They may affect the ______________________________, postsynaptic membrane, or both.
4. They can be ______________________________or with a neurotransmitter.

Neuropeptides are ______________________________that act by binding to receptors and activating
enzymes.

Opioids are ______________________________that bind to the same receptors as opium or morphine, and
probably function to relieve pain. There are 4 classes of opioids in the CNS:
1. ______________________________
2. ______________________________
3. ______________________________
4. ______________________________
How Neurotransmitters and Neuromodulators Work

There are 3 groups of neurotransmitters and neuromodulators:
1. Compounds that have a direct effect on membrane potential:
- ______________________________effects
- open or close gated ion channels
2. Compounds that have an indirect effect on membrane potential:
-
work through ______________________________
-
the link between the neurotransmitter (first messenger) and the second messenger may be a G
protein (enzyme complex) which binds GTP.
-
binding may activate the enzyme ______________________________, which produces the second
messenger cyclic AMP.
3. Lipid soluble gases that affect the inside of the cell:
- ______________________________ (NO) and ______________________________ (CO)
- bind to ______________________________in brain cells
VI. Information Processing by Individual Neurons

A single neuron has many dendrites and can receive many neurotransmitter messages at the same time - some excitatory, some inhibitory. The net effect on the axon hillock, which determines whether
or not an action potential will be produced, is the simplest level of information processing.
Postsynaptic Potentials,

The graded potentials that develop in a postsynaptic cell in response to neurotransmitters are called
postsynaptic potentials. The 2 major types of postsynaptic potentials are:
1. ______________________________postsynaptic potential (EPSP):
-
a graded depolarization of the postsynaptic membrane
2. ______________________________postsynaptic potential (IPSP):

a graded hyperpolarization of the postsynaptic membrane
A neuron that receives many IPSPs is inhibited from producing an action potential, because it increases
the amount of ______________________________to reach threshold.

A single EPSP is not enough to trigger an action potential. EPSPs (and IPSPs) must combine through a
process called ______________________________. If the resulting depolarization is large enough, an action
potential is triggered.

There are 2 forms of summation:
1. ______________________________ (multiple times):
-
rapid, repeated stimuli at a single synapse
2. ______________________________ (multiple locations):

many stimuli, arriving simultaneously at multiple synapses
As EPSPs accumulate, raising the transmembrane potential closer to threshold, the neuron becomes
facilitated, making it easier for even a small stimulus to trigger an action potential.

Neuromodulators and hormones can change a membrane’s sensitivity to excitatory or inhibitory
neurotransmitters, shifting the balance between EPSPs and IPSPs.
Presynaptic Inhibition and Presynaptic
Facilitation

Synapses may be found not only between
cell bodies and dendrites, but also between
the axons of 2 neurons (an
______________________________ synapse).

An axoaxonal synapse at a synaptic knob
may either
______________________________the
amount of neurotransmitter released by
the presynaptic membrane (presynaptic
inhibition) or
______________________________the
amount of neurotransmitter released by
the presynaptic membrane (presynaptic
facilitation).
The Rate of Generation of Action Potentials

The information received by a
postsynaptic cell is often interpreted only
in terms of the frequency or rate of action
potentials received. The frequency of
action potentials in turn depends on the
degree of depolarization above threshold.
Holding the membrane above threshold
for an extended period of time has the
same effect as applying a second, larger
stimulus, reducing the relative refractory period.