NERVOUS TISSUE
The nervous system is the body’s control center and communication network. What
three functions does it serve?
1.
2.
3.
senses changes in the environment, both internal and external
integrates and interprets the sensory input for understanding
responds by initiating muscular contractions or glandular secretions
How does the nervous system accomplish its homeostatic role?
These reactions, carried out by electrical messages called nerve impulses (action
potentials), allow for second-to-second adjustments in homeostasis.
How does the role of the endocrine system compare with that of the nervous system?
The endocrine system, using blood-borne chemical messengers called
hormones, controls long-term homeostasis. Rather than making second-tosecond adjustments, the endocrine system controls processes over days, weeks,
months, and years.
A.
NERVOUS SYSTEM DIVISIONS
Name the two principal divisions of the nervous system.
central nervous system (CNS)
peripheral nervous system (PNS)
Describe the central and peripheral nervous systems.
The CNS consists of the brain and the spinal cord, within which incoming
sensory information is processed, thoughts and emotions are generated,
and memories are stored. Most nerve impulses that stimulate muscle
contraction and glandular secretion originate in the CNS.
The PNS consists of 12 pairs of cranial nerves associated with the brain
and 31 pairs of spinal nerves associated with the spinal cord.
What is the relationship between the two systems?
Cranial and spinal nerves of the PNS carry sensory information from the
CNS to effectors (muscles and glands) in the periphery.
87
What are sensory neurons?
The input component of the PNS consists of nerve cells called sensory
(afferent) neurons that conduct nerve impulses from sensory receptors to
the CNS and end within the CNS.
What are motor neurons?
The output component of the PNS consist of nerve cells called motor
(efferent) neurons that originate in the CNS and conduct nerve impulses
away from the CNS to the effectors.
Compare the somatic with the autonomic nervous system.
Based upon the body part that responds, the PNS is further subdivided
into a somatic and autonomic nervous system.
The somatic nervous system is concerned with sensory information from
the skin, skeletal muscles, and special senses, and motor information to
the skeletal muscle only.
The autonomic nervous system carries sensory information from the
viscera to the CNS and motor information from the CNS to cardiac
muscle, smooth muscles, and glands.
Name the autonomic subdivisions.
The motor portion of the autonomic nervous system is divided into two
portions: the sympathetic nervous system and the parasympathetic
nervous system.
B.
FUNCTIONAL ANATOMY OF NERVOUS TISSUE
1.
NEUROGLIA
a.
TYPES OF NEUROGLIA
What are neuroglial cells?
Neuroglial cells are the supportive, nurturing, and protective
cells for the neurons. They occupy only half of the CNS, are
much smaller than neurons and outnumber them. They
remain mitotic throughout life and tend to fill in spaces of
injured neurons after disease and injury.
Name and then briefly describe the six types of neuroglial cells.
88
Astrocytes -- Astrocytes attach blood vessels to neurons,
helping to form the blood-brain barrier. They help
maintain the proper balance of K+ for the neurons and
participate in the metabolism of neurotransmitters.
They are responsible for forming scars in the CNS
after injury.
Oligodendrocytes -- Oligodendrocytes give support to
neurons of the CNS. They produce the myelin sheath
found around axons of the CNS. Each
oligodendrocyte uses its processes to wrap several
axons.
Microglia -- Microglia are small phagocytic cells that engulf
and destroy microbes and cellular debris in the CNS.
They migrate to areas of injured nervous tissue and
help to clean the area.
Ependyma -- Ependymal cells form a continuous epithelial
lining for the ventricles of the brain and the central
canal of the spinal cord. They probably assist in the
circulation of cerebrospinal fluid, but their role is
mostly unknown.
Schwann cells -- Schwann cells, also known as neurolemmocytes, produce the myelin sheath around the
axons of motor neurons and the dendrites of sensory
neurons in the PNS.
Satellite cells -- Satellite cells support neurons found in
ganglia in the PNS; function is obscure.
b.
MYELINATION
What is the myelin sheath?
Most nerve fibers are surrounded by a multilayered
lipoprotein produced by the neuroglia (Schwann cells in the
PNS and oligodendrocytes in the CNS) called the myelin
sheath.
What is its function?
The sheath electrically insulates the nerve fiber, greatly
increasing the speed of nerve impulse conduction.
Nerve processes with such a covering are said to be
myelinated while those without are unmyelinated. Therefore,
there are neurons with different speeds of transmission.
89
How is it formed in the PNS?
Schwann cells form the myelin sheath around motor axons
and sensory dendrites during fetal life and the first postnatal
year. In this process, Schwann cells line up along the length
of the nerve fiber, attach to it, then begin to spiral around it,
leaving behind multiple layers of glial cell membrane.
What is the neurilemma?
As the multiple layers of membrane are formed, the
cytoplasm and organelles of the Schwann cells are pushed
to the outside. This portion of the Schwann cell is known as
the neurilemma. It is found only around neurons of the PNS;
oligodendrocytes do not form a neurilemma.
What are the nodes of Ranvier?
At intervals along the length of a nerve process, between the
individual Schwann cells (PNS) or pieces of
oligodendrocytes (CNS), are gaps in the myelin sheath
called the nodes of Ranvier (neurofibral nodes).
2.
NEURONS
What is the function of neurons?
Nerve cells, called neurons, are responsible for conducting
impulses from one part of the body to another and are therefore the
structural and functional units of the nervous system.
a.
PARTS OF A NEURON
Give a brief description of each of the following:
Cell body -- The cell body (perikaryon, soma) contains
typical cellular organelles surrounded by cytoplasm.
There is a large nucleus with a very prominent
nucleolus and neurofibrils, elements of the
cytoskeleon that give the neuron structure and shape.
Nissl substance -- Scattered throughout the cytoplasm of the
cell body are structures called Nissl bodies
(chromatophilic substance), orderly arrays of rough
endoplasmic reticulum used for protein synthesis.
90
Dendrite -- The dendrite is usually a short, tapering, and
highly branched process extending from the cell body;
usually unmyelinated (sensory dendrites are the
exception). It is ALWAYS a process that carries the
nervous message towards the cell body.
Axon -- The axon is a single, long, thin, cylindrical projection
from the cell body that moves toward an effector or
another neuron. It ALWAYS carries the nervous
message away from the cell body.
Describe each of the following axonal structures:
Axon hillock -- The axon hillock, a cone-shaped elevation, is
that region of the neuronal cell body from which the
axon arises.
Trigger zone -- Just distal to the axon hillock is an area
called the trigger zone where nerve impulses arise for
propagation along the axon. This area of membrane
is rich with voltage-gated sodium channels (to be
described later).
Axon collateral -- Along the axon’s length, side branches
called axon collaterals may depart from the main axon
to innervate other structures.
Telodendrion -- At their terminations with effectors, the axon
and axon collaterals end by dividing into many fine
processes called axon terminals or telodendria.
End bulbs -- The tips of the axon terminals swell into bulbshaped synaptic end-bulbs that contain synaptic
vesicles filled with a chemical known as a
neurotransmitter. Neurons utilize a single type of
neurotransmitter.
b.
CLASSIFICATION OF NEURONS
A nerve fiber is …?
a general term for any nerve process (sensory dendrite or
motor axon).
A nerve is …?
a bundle of many nerve fibers, both sensory and motor, that
course along the same path in the PNS.
91
A tract is …?
a bundle of related nerve fibers in the CNS that connects
different areas of the CNS.
Describe each of the following neuronal types:
Multipolar -- Multipolar neurons usually have several
dendrites and a single axon; most neurons are of this
type.
Bipolar -- Bipolar neurons have a single dendrite and a
single axon extending from the cell body; associated
with the special senses.
Unipolar -- Unipolar neurons (sensory only) have a single
process extending from the cell body called the
central process that divides into two parts. The
axonal portion carries an impulse away from the cell
body; the dendritic portion is attached to a receptor
distally and carries an impulse to the cell body.
3.
GRAY AND WHITE MATTER
Distinguish between the following:
gray matter vs. white matter -- In a section of fresh brain and spinal
cord, some regions appear white and glistening while others
are grayish. The gray areas are the gray matter of the CNS,
areas of nerve cell bodies, dendrites, axon terminals, and/or
unmyelinated axons, and neuroglia.
The whitish areas are the white matter of the CNS; this
refers to aggregations of the myelinated processes of many
neurons, arranged into tracts.
nucleus vs. ganglion -- A nucleus is a collection of similar neuronal
cell bodies and dendrites within the gray matter of the CNS
that perform a specific function. These collections of
neurons are often called centers.
A ganglion is a collection of similar neuronal cell bodies
outside the CNS, lying in the periphery. Ganglia contain the
cell bodies of sensory neurons or the second neuronal cell
body of an autonomic pathway.
92
C.
NEUROPHYSIOLOGY
Communication by neurons depends upon two basic properties of their cell
membranes. List them.
1.
2.
There is an electrical voltage, called the resting membrane potential
(RMP), across the cell membrane.
Their cell membranes contain a variety of ion channels (pores) that
may be open or closed.
What is a membrane channel?
A membrane channel (pore) allows a specific substance to move through
a water-filled passageway to either enter or leave the cell. In neuronal
membranes, sodium and potassium channels are of utmost importance.
Assuming that there is a diffusion gradient, what happens when a channel
opens?
When the channels are open, specific ions in the intracellular fluid or the
extracellular fluid flow through them, according to their diffusion gradients.
Describe the concept of the channel as a gate or door.
Part of the integral protein that forms such a channel may act as a gate or
door, opening and closing on demand, to alter the flow of ions along their
diffusion gradient.
Depending on the types of channels that are present, a portion of a neuron may
be able to produce either a gradient potential or and action potential (nerve
impulse).
1.
RESTING MEMBRANE POTENTIAL
What is the resting membrane potential? How is it related to the formation
of a polarized membrane?
The resting membrane potential (RMP) occurs because there is a
small build up of negative charges just inside the cell membrane of
the neuron and an equal build up of positive charges outside.
Such a separation of charges by the membrane is a form of
potential energy that is measured in milliVolts (mV).
The greater the difference in charge across the membrane, the
larger the membrane potential (voltage).
93
Note that this buildup in charges is very close to the cell membrane;
elsewhere, there are equal numbers of positive and negative
charges.
In neurons the RMP ranges from -40 to -90 mV with a typical value
of -70 mV. The minus sign indicates that the inside of the neuron is
negative by 70 mV relative to the outside.
A cell that exhibits a membrane potential is called a polarized cell
or has a polarized membrane.
What is current? What are the paths for ion flow through the neuronal
membrane?
The resting membrane potential (RMP) serves as a type of battery.
Flow of the electrical charges is called current.
In living cells, this current is created when the ions flow across the
membrane.
Since the lipid bilayer is a good insulator, the main paths for current
flow across the membrane are the ion channels.
Thus, when the ion channels are open in the membrane, current
flows and this changes the membrane potential.
Describe the two main factors related to ions that contribute to the resting
membrane potential.
1.
2.
Distribution of ions across the cell membrane.
--extracellular fluid is rich in Na+ and Cl--intracellular fluid is rich in K+ and anions such as
organophosphates and proteins
Relative permeability of the cell membrane to Na+ and K+
--membrane is moderately permeable to K+ and Cl-; slightly
permeable to Na+; impermeable to intracellular
anions.
Why does the inside of the neuronal cell membrane become negativelycharged?
Since the membrane is moderately permeable to potassium, there
is always a slow diffusion of positively-charged potassium ions out
of the cell. The intracellular fluid just next to the inner surface of the
neuronal membrane becomes more and more negatively-charged
as potassium diffuse out.
94
Why does the outside of the neuronal cell membrane become positivelycharged?
Since the neuronal membrane is only slightly permeable to sodium,
the sodium ions accumulate outside the cell. They are attracted to
the anions within the cell and because of the large diffusion
gradient for Na+ into the cell, the extracellular fluid just next to the
outer surface of the neuronal membrane becomes more and more
positively-charged.
What is the net result of this ion distribution?
The net result is that neuronal membrane at rest tends to have
positive charges lined up along its outer surface and negative
charges lined up along its inner surface.
It is important that the membrane potential be maintained and that the ions
remain in their respective resting positions. With such great diffusion
gradients, how is this accomplished?
In the membrane are Na/K-ATPase pumps that expel 3 Na+ for 2
K+ imported. Such pumps are said to be electrogenic since they
contribute to the negativity of the resting membrane potential.
2.
ION CHANNELS
Describe the different types of ion channels and how they work?
An ion channel (pore) is a specific membrane protein structure that
allows only a specific substance to pass through the membrane
while excluding others.
There are two basic types of ion channels :
1.
Leakage (non-gated) channels are always open
(glucose channels, for example)
2.
Gated channels open and close in response to
some sort of stimulus: voltage, chemical,
mechanical, light.
Voltage-gated (or regulated) channels open in response to a
direct change in the membrane potential (voltage).
The presence of these channels in nerve and muscle cell
membranes makes them excitable (irritable); that is, the
ability to respond to certain types of stimuli by producing
impulses.
95
The trigger zone for a particular neuron is the place where
the voltage-gated ion channels are clustered most densely.
Describe the following:
Chemically-gated channel -- Chemically-gated ion channels open
and close in response to specific chemical stimuli, such as
neurotransmitters and hormones.
Mechanically-gated channel -- Mechanically-gated ion channels
open or close in response to mechanical stimuli such as
touch, pressure, or vibration (sensory receptors).
Light-gated channel -- Light-gated ion channels close in response
to light energy (found only in the photoreceptors of the eyes).
Graded response -- The presence of chemical-,mechanical-, or
light-gated channels allows for a graded response, an
electrical response that varies in size, depending on how
many channels are opened and for how long.
In general, describe the ion channel events that occur during
depolarization and repolarization
During an action potential (nerve impulse), two types of voltagegated ion channels open and then close, first the channels for Na+,
then those for K+.
Rapid opening of the voltage-gated Na+ channels results in
depolarization, the loss and then reversal of the membrane
polarization by the neuron.
The slower opening voltage-gated K+ channels and the closing of
the previously open Na+ channels result in repolarization, the
recovery of the resting membrane potential.
3.
ACTION POTENTIAL (IMPULSE)
a.
DEPOLARIZATION
b.
REPOLARIZATION
c.
REFRACTORY PERIOD
d.
PROPAGATION (CONDUCTION) OF ACTION POTENTIALS
Describe the specifics of the process of depolarization. Why is
depolarization a positive feedback mechanism?
If a graded potential causes the membrane to depolarize to a
critical level, called the threshold (about -55 mV), the
voltage-gated Na+ channels open.
96
Na+ ions rush through the channel, driven by both the
electrical and concentration gradients formed during rest,
and the membrane potential changes from -70 mV towards 0
mV, then up to +30 mV.
Throughout depolarization, Na+ ions continue to diffuse into
the neuron until the membrane potential reverses so that the
inside of the cell becomes +30 mV.
Each voltage-gated Na+ channel has two separate gates, an
activation gate and an inactivation gate.
In a resting neuron, the inactivation gate is open and the
activation gate is closed. As a result, Na+ ions cannot
diffuse into the cell.
At threshold, many voltage-gated Na+ channels suddenly
change from the resting state to the activated state, allowing
Na+ ions to now diffuse into the cell.
As more channels open, more Na+ moves into the cell and
the membrane depolarizes further. This positive feedback
mechanism allows current created by Na+ influx at one
channel to activate adjacent Na+ channels.
Voltage-gated Na+ channels are open only a few 10/1000ths
of a second, so that only about 20,000 Na+ ions move into
the cell. Since this is only a small fraction of the total Na+,
the Na-K pump can move them back out.
Describe the specifics of the process of repolarization.
A threshold depolarization not only opens the voltage-gated
Na+ channels, but it also opens the voltage-gated K+
channels, thus initiating repolarization.
The K+ channels open more slowly than the Na+ channels,
however, so that their opening coincides with the closing of
the Na+ channels.
Na+ channel inactivation slows the influx of the Na+ ions into
the neuron and is coupled with the efflux of K+ as it flows
down its concentration gradient to the outside. The
membrane potential moves back towards -70 mV.
97
Repolarization, therefore, restores the resting membrane
potential and allows the Na+ channels to return to their
inactivated state.
While the K+ channels are open, outflow of the K+ is so
great that there is hyperpolarization of the membrane,
meaning that the membrane becomes even more negative
than at rest (-90 mV).
As the voltage-gated K+ channels close, however, the
membrane potential returns to the normal resting levels as
the Na-K pumps continue to move ions.
Describe the refractory period of a neuronal membrane.
The period of time during which an excitable cell (muscle or
neuron) cannot generate another action potential is called
the refractory period.
The absolute refractory period refers to the time period
during which a second stimulus cannot initiate a second
action potential. It coincides with Na+ channel activation and
inactivation.
Since inactivated Na+ channels cannot reopen, they must
first return to the resting state before they can be activated
again.
The relative refractory period is the period of time when a
second action potential can be initiated, but only by a
suprathreshold stimulus.
The relative refractory period coincides with the period of
time when voltage-gated K+ channels are still open after the
inactivated Na+ channels have returned to their resting state.
Work through these diagrams that describe the positive feedback
nature of an action potential.
Condition -- A stimulus or stress disrupts membrane
homeostasis by causing a threshold depolarization.
Receptors -- The receptors in this case are voltage-gated
Na+ channels in their resting state.
Control center - -The shape of the voltage-gated Na+
channel depends on membrane voltage.
98
Effectors -- Voltage-gated Na+ channels are also the
effectors. Threshold depolarization causes shape
changes in the channel.
Response -- Opening of the voltage-gated Na+ channels
causes depolarization in adjacent membrane, opening
more voltage-gated Na+ channels. This is positive
feedback.
e.
THE ALL-OR-NONE PRINCIPLE
What is the all-or-none principle of neuron function?
A single neuron, like a single muscle fiber, generates an
action potential according to the all-or-none principle.
If depolarization reached threshold, voltage-gated Na+
channels open, the positive feedback mechanism is initiated,
and an action potential arises.
Each time an action potential is formed, it has a constant
and maximum strength for the existing conditions.
f.
CONTINUOUS CONDUCTION
Describe continuous conduction.
Nerve impulses communicate from one part of the body to
another. To do this, they must travel from there they arise,
at a trigger zone, to axon terminals at a synapse.
The special mode of impulse travel across a neuron is called
propagation (conduction) and is dependent upon positive
feedback.
As Na+ flows into the neuron, depolarization occurs and
adjacent Na+ channels are opened. In this way, the nerve
impulse self-propagates along the membrane.
Also, since the membrane is refractory just behind the
leading edge of the impulse, an impulse normally travels in
one direction only from where it arises (the trigger zone).
This type of conduction, in which each piece of neuronal
membrane becomes depolarized during propagation, is
called continuous conduction. It is common in muscle
membranes and unmyelinated neuronal membranes.
99
g.
SALTATORY CONDUCTION
Describe salutatory conduction.
In myelinated membranes conduction is somewhat different
because the myelin sheath acts as an electrical insulator to
block ionic currents across the membranes.
At the nodes of Ranvier, however, the myelin sheath is
interrupted and the membrane has a high density of Na+
channels. It is here that depolarization occurs and current is
carried into the neuron.
In this matter, ionic current flows into the nerve fiber. The
Na+ ions then follow their diffusion gradient, moving along
the inside of the nerve fiber to the next node of Ranvier.
This movement produces just enough voltage at the next
node to cause voltage-gated Na+ channels to open there,
thus self-propagating the message.
As a result, the nerve impulse appears to “jump” from node
to node as each area depolarizes and so conducts the
impulse as it arises anew at each node (saltare = to leap).
This is saltatory conduction.
Since the impulse “jumps” long intervals of membrane without depolarizing it, the impulse travels much faster than by
continuous conduction (0.5 m/sec vs 130 m/sec).
In addition, only small regions of the membrane must be
repolarized, so that there is much less work involved by the
Na-K pumps and therefore energy is conserved.
4.
TRANSMISSION AT SYNAPSES
What is a synapse?
The point of communication between cells, either neuron-neuron or
neuron-effector (“synapsis” = connection)
Why are synapses essential for homeostasis?
Because they allow information to be integrated and filtered; some
signals are transmitted while others are inhibited.
Distinguish between presynaptic and postsynaptic neurons.
100
At a synapse, the neuron sending the signal is the presynaptic
neuron, and the neuron receiving the message is the postsynaptic
neuron.
a.
CHEMICAL SYNAPSES
Describe conduction across the synapse.
Presynaptic and postsynaptic neurons or effectors (muscle
and glands) do not touch. They are separated by the
synaptic cleft (20-50 nm), a space filled with extracellular
fluid.
Within the axon terminals the presynaptic neurons are
synaptic vesicles filled with a particular neurotransmitter.
Arrival of the action potential in the membrane of he axon
terminal opens Ca++ channels in the membrane. This, in
turn, opens calcium channels, allowing extracellular calcium
to diffuse into the axon terminal.
Elevated intracellular Ca++ causes some of the synaptic
vesicles to fuse with the membrane of the axon terminal and
release into the synaptic cleft the stored neurotransmitter.
Neurotransmitter then diffuses across the synaptic cleft
through the extracellular fluid to the adjacent membrane of
the postsynaptic neuron or effector.
Interaction between neurotransmitter and its specific
receptor protein in the membrane of the postsynaptic neuron
initiates the formation of a postsynaptic potential.
The time required for this process, called the synaptic delay,
is about 0.5 msec.
At the chemical synapse there can only be one-way
information transfer, from presynaptic neuron to postsynaptic
neuron or effector.
As a result, graded potentials and action potentials must
move forward over their pathways. Action potentials cannot
back up into another presynaptic neuron.
b.
EXCITATORY AND INHIBITORY POSTSYNAPTIC POTENTIALS
Describe the following:
101
Excitatory neurotransmitter -- If the neurotransmitter
released at a synapse causes depolarization, it is
excitatory because it brings the membrane of the
postsynaptic neuron closer to threshold (more
positive). Such an event is called the excitatory
postsynaptic potential (EPSP).
Facilitation and summation -- During the time that the
postsynaptic membrane is brought closer to its
threshold, it is said to be facilitated. If a series of
facilitations brings the membrane to threshold, then
the effect is called summation and an action potential
is generated.
Inhibitory neurotransmitter -- If the neurotransmitter causes
hyperpolarization of the postsynaptic membrane, it is
said to be inhibitory because it moves the membrane
potential further from threshold (more negative).
Such an event is called the inhibitory postsynaptic
potential (IPSP) and the neuron is inhibited.
c.
SPATIAL AND TEMPORAL SUMMATION OF PSPS
Describe the following:
Spatial summation -- When the summation is the result of
several presynaptic neurons releasing their
neurotransmitter into the trigger zone within the same
time frame, it is called spatial summation.
Temporal summation - -When summation results from
buildup of neurotransmitter released by a single
presynaptic neuron firing several times in rapid
succession, the result is called temporal summation.
The sum of all the effects on a postsynaptic neuron, both
excitability and inhibitory, determines the action taken by the
neuron. Therefore, one of three events must occur. Name them.
1.
If the excitatory effect is greater than the inhibitory
effect, but less than threshold, the result is an EPSP
and the process is facilitation.
2.
If the excitatory effect is greater than the inhibitory
effect and reaches threshold, the result is summation
and an action potential in the postsynaptic neuron or
effector.
102
3.
d.
If the inhibitory effect is greater than the excitatory
effect, the result is membrane hyperpolarization and
inhibition of the postsynaptic neuron.
REMOVAL OF NEUROTRANSMITTER
Removal of neurotransmitter from the synaptic cleft is essential to
homeostasis because if it lingered, its influence on the effector
would linger also. List, then describe, the three ways neurotransmitter is removed from the synapse.
Diffusion -- The neurotransmitter may diffuse through the
extracellular fluid and away from the synapse.
Enzymatic degradation -- There may be specific enzymes
present in the synaptic cleft or on the postsynaptic
membrane to degrade the neurotransmitter.
Uptake into the cell -- In some cases the neurotransmitter is
actively transported back into the presynaptic neuron
and reused.
5.
NEURONAL CIRCUITS
What is a neuronal pool?
The CNS contains billions of neurons organized into complex
patterns called neuronal pools, each with its own role in
homeostasis. Neuronal pools are arranged into patterns called
circuits over which nerve impulses are conducted. There are five
common types of these circuits.
Describe the following circuits:
Simple series -- The simplest circuit is the simple series circuit in
which a single presynaptic neuron synapses with a single
postsynaptic neuron, which stimulates another, which
stimulates another, etc.
Diverging -- In a diverging circuit, a single presynaptic neuron
synapses with more than one postsynaptic neuron, in a
process called divergence (Ex: a sensory signal is relayed to
several different parts of the brain.)
Parallel after-discharge -- Parallel after-discharge circuits have a
single synaptic neuron that diverges, then each neuron in
the pathway synapses on a common postsynaptic neuron
(Ex: precise mental activities such as math.)
103
Reverberating -- In a reverberating circuit, branches from neurons
later in a pathway synapse with neurons found earlier in the
pathway (Ex: breathing, sleep-wake cycles, short-term
memory.)
Converging -- In a converging circuit, several presynaptic neurons
with a single postsynaptic neuron, in a process known as
convergence (Ex: a single motor neuron receives motor
information from several sources.)
104