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The Nervous System
An Overview of Nervous Systems:
The nervous system performs sensory input, integration (the process by which input is
interpreted and associated with appropriate responses of the body), and motor output. The
nervous system consists of a central nervous system (the brain and spinal cord) and a
peripheral nervous system (all the nervous system besides the CNS). These two work together
to keep the organism functioning. Electrical and chemical signals are how the system functions.
Networks of Neurons with Intricate Connections Form Nervous Systems:
Neurons are the basic units of structure and function of the nervous system. These cells
look like blobs of jelly, with one long branch called an axon, which takes outgoing signals, and
many small branches called dendrites which take signals to the cell.
A reflex arc is the simplest nerve circuit, and regulates a reflex. A sensory neuron
receives information, such as a stretch in a muscle, and conveys this information to the spinal
cord. Motor neurons convey the signals to effector cells, which carry out the actual reflex (like
kicking when the patellar tendon is hit).
Interneurons help to mediate reflexes, and, in the case of a knee-jerk reaction, inhibit
muscles besides the quadriceps so that the reaction takes place. Interneurons are constantly
“talking”, and this is what keeps sensory input processed and directs responses.
There are three basic patterns for nerve circuits: one that takes information from one
source to several parts of the brain (1 presynaptic neuron  several postsynaptic neurons); one
where information from several presynaptic neurons goes to one postsynaptic neuron; and one
where information flows circularly, from one neuron to another and back again. This last one
may be how memories are processed and stored.
Supporting cells called glia are the “helpers” of the nervous system. They provide
structural integrity to the nervous system and outnumber neurons tenfold to fiftyfold. They
even have some synaptic interactions with neurons. Glia perform many functions, such as for
tracks along which neurons will migrate and specialized glia called astrocytes provide metabolic
support and a blood-brain barrier to neurons in the CNS. This keeps the ambiance of the CNS
strictly controlled. Another type of glia, oligodendrocytes (CNS) and Schwann cells (PNS), form
myelin sheaths like jelly rolls around the axons of some neurons to insulate them, which
increases the speed of propagation of nerve impulses.
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THE NATURE OF NERVE SIGNALS
Every Cell has a Voltage, or Membrane Potential, Across its Plasma Membrane:
The plasma membranes of cells are polarized, so there is more negative charge on one
side than the other. This potential difference is the membrane potential.
Typically the charge separation across a membrane is about –50 to –100 mV (millivolts)
in animal cells. There is more negative charge inside than outside the cell. This is because there
are more anions inside the cell than out. The fact that K+ ions diffuse more easily out of the cell
than Na+ ions diffuse in, along with the sodium-potassium pump, keeps it this way
Main Ions Inside the Cell
Potassium (K+)
Proteins (-)
Amino acids(-)
Sulfate (-)
Phosphate (-)
Other negatively charged ions (A-)
Main Ions Outside the Cell
(Na+)
Sodium
Chloride (Cl-)
Selective ion channels control the passage of these ions across the membrane. The
sodium-potassium pump transports potassium in and sodium out (both against their
concentration gradients).
Changes in the Membrane Potential of a Neuron Give Rise to Nerve Impulses:
Only neurons and muscle cells can generate large membrane potential changes. They
are called excitable cells, and have specialized gated ion channels (chemically-gated and
voltage-gated for both sodium and potassium). They achieve this increased/decreased
membrane potential through increases in the outflow of potassium ions or inflow of sodium
ions. Graded potentials are voltage changes across a membrane whose magnitude depends
upon the strength of the stimulus.
Depolarization is only graded to a certain voltage, which is called the threshold
potential. If this specific voltage is reached, the cell will then reach its action potential, where
extra sodium gates open, but potassium gates stay closed, so that many sodium ions enter the
cell’s interior, which becomes drastically more positive. After action potential is reached,
repolarization occurs: inactivation gates close sodium channels and potassium channels open so
that sodium flows out, restoring more than the original negative charge inside the cell (slight
hyperpolarization). This is called the undershoot, and occurs because potassium channels have
fairly slow gates which do not respond immediately to the repolarization. Finally, after the
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undershoot, resting potential is restored. This whole process takes very few milliseconds. Note
that hyperpolarization NEVER causes action potential to be reached.
The number of action potentials per second communicates how strong the stimulus is to
the nervous system.
Nerve Impulses Propagate Themselves Along an Axon:
Action potentials travel along axons with a wave-like motion, but only in one direction.
The sodium influx into the cell’s interior triggers a sodium influx further along the axon (but
never in the direction from which it came, since the undershoot, where depolarization cannot
occur, keeps it from happening). Thicker axons can transmit action potentials faster than
thinner ones. In vertebrates, action potential speed is helped by myelin and oligodendrocytes
and Schwann cells, which allow salutatory conduction, where the depolarization “jumps” from
one node of Ranvier to the next, since this is the only place where extracellular fluid contacts
the axon if it is myelinated.
Chemical or Electrical Communication Between Cells Occurs at Synapses:
Where Synapses Can be Found
neuron  neuron
sensory receptor  sensory neurons
motor neuron  muscle cells
neuron  gland cells
Electrical synapses transmit action potentials directly from the presynaptic to the
postsynaptic neuron through a gap junction. More common in vertebrates and most
invertebrates, chemical synapses turn action potentials into chemical signals (neurotransmitter)
which travel across synapses and are converted back into electrical impulses in the postsynaptic
cell.
What happens when an action potential depolarizes the membrane of a presynaptic terminal:
1. Calcium ions (Ca2+) flow into the cell from outside.
2. This signals synaptic vesicles to fuse with the membrane.
3. Through exocytosis, the vesicles release neurotransmitters into the synaptic cleft. These
diffuse across the cleft and bind to the receptors of ion channels in the postsynaptic
membrane.
4. This opens specific ion channels, depolarizing the postsynaptic membrane.
5. Neurotransmittes are degraded quickly or taken in by other neurons so that they don’t
run amok in the extracellular fluid.
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Nerve impulse are only transmitted in a single direction along neural pathways, and
communication only occurs between one postsynaptic membrane and one other neuron.
Neural Integration Occurs at the Cellular Level:
There are both excitatory and inhibitory postsynaptic potentials. Excitatory ones
transmit action potential if there is enough depolarization. Inhibitory ones transmit
hyperpolarization, so there is no chance of action potential being reached, since that takes
depolarization.
Each neuron is on the receiving end of thousands of synapses, and whether or not the
axon of the neuron fires depends on the summation of the input from all these synapses
(though synapses closer to the axon hillock usually have more effect than others). In temporal
summation, chemical transmissions from some synaptic terminals occur almost simultaneously
so that the postsynaptic potential affects the membrane before the voltage has returned to
resting potential from the previous stimulation. In spatial stimulation, several presynaptic
terminals act on one postsynaptic cell, so there is an additive effect on membrane potential.
The Same Neurotransmitter Can Produce Different Effects on Different Types of Cells:
There are many different kinds of neurotransmitters, and many different receptors
present on postsynaptic cells, so the results of various neurotransmitters varies enormously.
Here are some of the common ones:
Neurotransmitter
Acetylcholine
Biogenic Amines:
Norepinephrine
Dopamine
Serotonin
Amino Acids:
Gamma aminobutyric acid
(GABA)
Glycine
Glutamate
Asparate
Neuropeptides (only two of
many shown):
Functional Class
Excitatory to vertebrate
skeletal muscles; excitatory or
inhibitory at other sites
Secretion Sites
CNS; PNS; vertebrate
neuromuscular junction
Excitatory or inhibitory
Usually excitatory; can be
inhibitory at some sites
Usually inhibitory
CNS; PNS
CNS; PNS
Inhibitory
CNS; invertebrate
neuromuscular junction
CNS
CNS; invertebrate
neuromuscular junction
CNS
Inhibitory
Excitatory
Excitatory
CNS
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Substance P
Met-enkephalin (an
endorphin)
Excitatory
Usually inhibitory
CNS; PNS
CNS
In addition, some neurons in vertebrates use gas molecules as local regulators. Cells
synthesize these molecules as they are needed and they diffuse into nearby target cells and
break down all within a couple of seconds. Gasses often work like hormones, signaling to an
enzyme in the membrane which synthesizes a second messenger inside the cell.
EVOLUTION AND DIVERSITY OF NERVOUS SYSTEMS
The Ability of Cells to Respond to the Environment has Evolved Over Billions of Years:
Excitability developed billions of years ago, when single-celled organisms began to be
able to sense some aspects of their environment and react in ways which increased their
Darwinian fitness. By the Cambrian explosion, systems of nerve cells had practically evolved
into their modern forms (though they weren’t nearly as complex).
Nervous Systems Show Diverse Patterns of Organization:
Though the basic function of nervous systems are mostly the same, there is much
diversity when it comes to how it is organized and its complexity. The simplest organisms with
nervous systems, cnidarians, have nerve nets.
More complex nervous systems developed along with cephalization and nerve cords.
VERTEBRATE NERVOUS SYSTEMS
Vertebrate Nervous Systems Have Central and Peripheral Components:
All vertebrates have distinct peripheral and central nervous systems and are cephalized
to a high degree. The brain provides the integrative power that allows the complex behaviors of
vertebrates. The vertebrate central nervous system comes from the dorsal hollow nerve cord of
the embryo. The central canal of the spinal cord is a space filled with cerebrospinal fluid, which
is basically filtered blood from the brane. This is essential as it conveys nutrients, hormones, etc
and also is a shock absorber, cushioning the CNS.
The Divisions of the Peripheral Nervous System Interact in Maintaining Homeostasis:
The peripheral nervous system of humans is composed of 12 pairs of cranial nerves and
31 pairs of spinal nerves. The PNS can be divided into a sensory division, which is composed of
sensory neurons that give signals to the CNS, and a motor division, which is composed of
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outgoing or efferent, neurons, which convey signals from the CNS to effector cells. This system
is divided into the somatic and autonomic nervous systems. The somatic nervous system carries
signals to skeletal muscles mostly in response to external stimuli. It is subject to conscious
control, bus is also controlled by reflexes. The autonomic nervous system conveys signals in
response mostly to the internal environment. It is mostly involuntary. The autonomic nervous
system is further divided into sympathetic and parasympathetic divisions. The sympathetic
division correlates with arousal and the secretion of adrenaline from the adrenal medulla. The
parasympathetic division is associated with calming and the functions of self-maintenance
(keeping in homeostasis). These two systems cooperate.
Embryonic Development of the Vertebrate Brain Reflects its Evolution from Three Anterior
Bulges of the Neural Tune:
In vertebrates, there are three bilaterally symmetrical anterior bulges in the dorsal
hollow nerve cord: the forebrain, midbrain, and hindbrain. The cerebrum is an outgrowth of the
forebrain. As an embryo develops these three bulges are transform into various parts of the
brain, with the most significant changes occurring in the telecephalon which forms the cerebral
cortex.
Evolutionarily Older Structures of the vertebrate Brain Regulate Essential Automatic and
Integrative Functions:
Portions of the brainstem are the medulla oblongata, the pons, and the midbrain; these
are derived from the embryonic hindbrain and midbrain. The medulla and Pons contain nerve
cell bodies which have axons extending to many places throughout the cerebral cortex and
cerebellum where they release neurotransmitters. The medulla oblongata controls automatic
and homeostatic functions, like breathing and swallowing and digestion.
Vocabulary:
sensory receptors:
function: to collect information about the physical world outside the body and processes inside
the organism
ex: light-detecting cells in the eyes
sensory input: the information gathered from sensory receptors
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central nervous system (CNS): the brain and spinal cord in vertebrates
function: performs integration; conducts motor output to effector cells.
motor output: the conduction of signals from the CNS to effector cells
effector cells: the muscle cells or gland cells which carry out the body’s responses to stimuli
nerves: bundles of individual nerve fibers held together by connective tissue
peripheral nervous system (PNS): the nerves which communicate motor and sensory signals
between the CNS and the rest of the body (basically any nerves besides the brain and spinal
cord)
neuron: a nerve cell, composed of a cell body (with a nucleus and other organelles), fiberlike
processes called dendrites, and axons
dendrite: (from the Greek dendron, tree) a short, branched extensions of a neuron
function: to receive incoming messages from other cells and carry the information as an
electrical impulse to the cell body
axon: extensions of nerve cells (a nerve cell only has one axon); axons may be branched and
may have hundreds or thousands of synaptic terminals
function: to convey outgoing messages to other cells
axon hillock: the conical region of the axon where it joins the cell body
function: helps transmit and integrate nerve signals
myelin sheath: an insulating layer that encloses some axons (where there is white matter)
synaptic terminals: specialized axon endings
function: to relay signals from the neuron to other cells by releasing neurotransmitters
neurotransmitters: chemical messengers
synapse: the site of contact between a synaptic terminal and a target cell, which can be either
another neuron, or an effector, like a muscle or gland cell
presynaptic cell: the transmitting cell
postsynaptic cell: the target cell
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reflex: an automatic response
reflex arc: the simplest type of nerve circuit, regulating a reflex
sensory neuron: nerve cell receiving information from a sensory receptor about a change in
stimulus and passes this information to a motor neuron
motor neuron: nerve cell which transmits signals from the brain or spinal cord to muscles or
glands
effector cell: a muscle or gland cell which carries out a response and gets information from a
motor neuron
ganglion (plural ganglia): a cluster of nerve cell bodies, often with similar functions, located in
the peripheral nervous system
nuclei: clusters of neuron bodies in vertebrate brains similar to ganglia (NOT the same as nuclei
inside a cell where the DNA is)
supporting cells or glia (from the Greek, glue):
function: to contribute to the structural integrity of the nervous system and have some synaptic
interactions with neurons
astrocyte: a type of glia which provides structural and metabolic support for neurons and
induces the formation of tight junctions between cells lining the capillaries of the brain,
resulting in a blood-brain barrier
blood-brain barrier:formed by astrocytes, this is what restricts the passage of most substances
into to the brain so that the CNS environment is strictly controlled
oligodendrocytes (in the CNS) and Schwann cells (in the PNS): glia which form myelin sheaths
around the axons of many neurons
membrane potential: the potential difference across a cell membrane
excitable cells: cells which can generate large changes in their membrane potentials
gated ion channels: ion channels in a cell membrane that open/close in response to stimuli
chemically-gated ion channels: ion channels which open/close in response to chemical
stimulation, like the release of a neurotransmitter from a synaptic terminal
voltage-gated ion channels: ion channels which open/close in response to changing membrane
potential
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hyperpolarization: an increase in membrane potential
depolarization: a decrease in membrane potential
graded potentials: voltage changes across a membrane whose magnitude depends upon the
strength of the stimulus
threshold potential: the voltage at which an action potential is triggered in a neuron
action potential: triggered by graded depolarization, an all-or-nothing event
refractory period: when a neuron is insensitive to depolarization because it is still in the
undershoot phase of an action potential, so both the activation and inactivation gates of the
sodium channels are closed (this sets the maximum frequency with which action potentials can
be generated)
nodes of Ranvier: successive gaps between Schwann cells along an axon (the spaces between
the myelin jelly rolls)
saltatory conduction: the mechanism where the sodium current generated at one node
“jumps” to the next node, stimulating depolarization there
synaptic cleft: in chemical synapses, a narrow gap which separates the postsynaptic and
presynaptic cells
synaptic vesicles: sacs in a presynaptic axon which contain neurotransmitters, which are
expelled at specific times through exocytosis
presynaptic/postsynaptic membrane: the surfaces of the terminals facing the cleft between
presynaptic and postsynaptic cells
excitatory postsynaptic potential (EPSP): the voltage change (depolarization only) that is
caused by the binding of a neurotransmitter to the receptor in an excitatory synapse
inhibitory postsynaptic potential (IPSP): the voltage change (hyperpolarization only) associated
with chemical signaling at an inhibitory synapse
summation: the additive effect of postsynaptic potentials (ex: several synaptic terminals actin
on one postsynaptic cell