The human brain contains 100 billion neurons connected by trillions of synapses. Each neuron connects to thousands of other neurons, making the brain incredibly complex. The document provides an overview of neuroscience topics including the basic structure and function of neurons, glial cells, the blood-brain barrier, how neurons communicate through action potentials and synaptic transmission. It describes in detail the anatomy and physiology of neurons, synaptic transmission, and various cell types and structures in the nervous system.

1. INTRODUCTION TO NEUROSCIENCE

2. The human brain has 100 billion neurons, each neuron
connected to 10,000 other neurons. Sitting on your shoulders is
the most complicated object in the known universe.
-Michio Kaku, American Physicist.

3. The Basic Animal Cell
• The surface of a cell is its membrane (or plasma
membrane), a structure that separates the inside of the
cell from the outside environment. Most chemicals
cannot cross the membrane, but protein channels in
the membrane permit a controlled flow of water,
oxygen, sodium, potassium, calcium, chloride, and
other important chemicals.
• Except for mammalian red blood cells, all animal cells
have a nucleus, the structure that contains the
chromosomes.
• A mitochondrion (plural: mitochondria) is the structure
that metabolic activities, providing the energy that the
cell uses for all activities. Mitochondria have genes
separate from
those in the nucleus of a cell, and mitochondria differ from
one another genetically. People with overactive
mitochondria tend to burn their fuel rapidly and
overheat, even in a cool environment. People whose
mitochondria are less active than normal are
predisposed to depression and pains. Mutated
mitochondrial genes are a possible cause of autism
(Aoki & Cortese, 2016).
• Ribosomes are the sites within a cell that synthesize

4. The Neuron

5. • All neurons include a soma (cell body), and most also have dendrites, an axon, and
presynaptic terminals. The tiniest neurons lack axons, and some lack well-defined
dendrites.
• A motor neuron, with its soma in the spinal cord, receives excitation through its dendrites
and conducts impulses along its axon to a muscle.
• A sensory neuron is specialized at one end to be highly sensitive to a particular type of
stimulation, such as light, sound, or touch.
• Dendrites are branching fibers that get narrower near their ends. The dendrite’s surface
is lined with specialized synaptic receptors, at which the dendrite receives information
from other neurons. The greater the surface area of a dendrite, the more information it
can receive.
• Many dendrites contain dendritic spines, short outgrowths that increase the surface area
available for synapses.
• The axon is a thin fiber of constant diameter. The axon conveys an impulse toward other
neurons, an organ, or a muscle. Axons can be more than a meter in length, as in the case
of axons from your spinal cord to your feet. The length of an axon is enormous in
comparison to its width, and in comparison.

6. • Many vertebrate axons are covered with an insulating material
called a myelin sheath with interruptions known as nodes of
Ranvier (RAHN-vee-ay).
• Although a neuron can have many dendrites, it can have only one
axon, but the axon may have branches. The end of each branch
has a swelling, called a presynaptic terminal, also known as an
end bulb or bouton (French for “button”). At that point the axon
releases chemicals that cross through the junction between that
neuron and another cell.
• Other terms associated with neurons are afferent, efferent, and
intrinsic.
• An afferent axon brings information into a structure; an efferent
axon carries information away from a structure.
• Every sensory neuron is an afferent to the rest of the nervous
system, and every motor neuron is an efferent from the
nervous system.
• Within the nervous system, a given neuron is an efferent from one
structure and an afferent to another. You can remember that
efferent starts with e as in exit; afferent starts with a as in admit.
• If a cell’s dendrites and axon are entirely contained within a single
structure, the cell is an interneuron or intrinsic neuron of that
structure. For example, an intrinsic neuron of the thalamus has its

7. Varying Types of Neurons
• Neurons vary enormously
in size, shape, and
function.
• The shape of a neuron
determines its connections
with other cells and
thereby determines its
function. For example, the
widely branching dendrites
of the Purkinje cell in the
cerebellum enable it to
receive input from up to
200,000 other neurons.
• By contrast, bipolar
neurons in the retina have
only short branches, and
some receive input from as
few as two other cells.

8. Glia

9. Neuroglial cells—usually referred to simply as glial cells or glia—are quite different
from nerve cells.
The major distinction is that glia do not participate directly in synaptic interactions and
electrical signaling, although their supportive functions help define synaptic contacts and
maintain the signaling abilities of neurons.
Although glial cells also have complex processes extending from their cell bodies, they are
generally smaller than neurons, and they lack axons and dendrites.
Glial roles that are well-established include:
1. Maintaining the ionic environment of nerve cells,
2. Modulating the rate of nerve signal propagation,
3. Modulating synaptic action by controlling the uptake of neurotransmitters,
4. Providing a scaffold for some aspects of neural development,
5. Aiding in (or preventing, in some instances) recovery from neural injury.

10. There are three types of glial cells in the mature central
nervous system: astrocytes, oligodendrocytes,
and microglial cells.
1. Astrocytes, which are restricted to the brain
and spinal cord, have elaborate local processes that
give these cells a starlike appearance. The major
function of astrocytes is to maintain, in a variety of
ways, an appropriate chemical environment for
neuronal signaling.
2. Oligodendrocytes, which are also restricted to the
central nervous system, lay down a laminated, lipid-rich
wrapping called myelin around some, but not all,
axons. Myelin has important effects on the speed
of action potential conduction.
3. In the peripheral nervous system, the cells that
elaborate myelin are called Schwann cells.
4. Microglial cells are primarily scavenger cells that
remove cellular debris from sites of injury or normal cell
turnover.

11. The Blood-Brain Barrier
• The brain, like any other organ, needs to receive nutrients from the blood, many
chemicals cannot cross from the blood to the brain (Hagenbuch, Gao, & Meier,
2002). The mechanism that excludes most chemicals from the vertebrate brain
is known as the blood–brain barrier.
• WHY DO WE NEED IT?
• When a virus invades a cell, mechanisms within the cell extrude virus particles
through the membrane so that the immune system can find them. When the
immune system cells discover a virus, they kill it and the cell that contains it.
• This plan works fine if the virus-infected cell is, for example, a skin cell or a
blood cell, which the body replaces easily. However, with few exceptions, the
vertebrate brain does not replace damaged neurons. To minimize the risk of
irreparable brain damage, the body lines the brain’s blood vessels with tightly
packed cells that keep out most viruses, bacteria, and harmful chemicals.

12. How the Blood-Brain
Barrier works
The blood–brain barrier depends on the endothelial cells that
form the walls of the capillaries (Bundgaard, 1986; Rapoport &
Robinson, 1986).
Outside the brain, such cells are separated by small gaps, but
in the brain, they are joined so tightly that they block viruses,
bacteria, and other harmful chemicals from passage.
If the blood–brain barrier is such a good defense, then
why don’t we have similar walls around all our other
organs?
The barrier keeps out useful chemicals as well as harmful ones.
Those useful chemicals include all fuels and amino acids, the
building blocks for proteins. For these chemicals to cross the
blood–brain barrier, the brain needs special mechanisms not
found in the rest of the body.

13. • No special mechanism is required for small, uncharged molecules such as
oxygen and carbon dioxide that cross through cell walls freely. Also,
molecules that dissolve in the fats of the membrane cross easily.
Examples include vitamins A and D and all the drugs that affect the brain—from
antidepressants and other psychiatric drugs to illegal drugs such as heroin.
How fast a drug takes effect depends largely on how readily it dissolves in fats
and therefore crosses the blood–brain barrier.
• Water crosses through special protein channels in the wall of the endothelial
cells (Amiry-Moghaddam & Ottersen, 2003). For certain other chemicals, the
brain uses active transport, a protein-mediated process that expends energy
to pump chemicals from the blood into the brain.
• Chemicals that are actively transported into the brain include glucose (the
brain’s main fuel), amino acids (the building blocks of proteins), purines,
choline, a few vitamins, and iron (Abbott, Rönnback, & Hansson, 2006;
Jones & Shusta, 2007).

14. COMMUNICATION WITHIN NEURONS

15. The Nerve Impulse
• Messages in a neuron develop from disturbances of the
resting potential.
• All parts of a neuron are covered by a membrane about 8
nanometers (nm) thick. The membrane is composed of
two layers (free to float relative to each other) of
phospholipid molecules.
• Embedded among the phospholipids are cylindrical
protein molecules through which certain chemicals can
pass.
• When at rest, the membrane maintains an electrical
gradient, also known as polarization—a difference in
electrical charge between the inside and outside of the
cell.
• The electrical potential inside the membrane is slightly
negative with respect to the outside, mainly because of
negatively charged proteins inside the cell.
• This difference in voltage is called the resting potential.

16. Forces Acting on Sodium and Potassium Ions
• If charged ions could flow freely across the membrane, the membrane would depolarize, eliminating the
negative potential inside.
• However, the membrane has selective permeability. That is, some chemicals pass through it more freely
than others do.
• When the membrane is at rest, the sodium and potassium channels are closed, permitting almost no flow of
sodium and only a small flow of potassium.
• Certain types of stimulation can open these channels, permitting freer flow of either or both ions.
• The sodium–potassium pump, a protein complex, repeatedly transports three sodium ions out of the cell
while drawing two potassium ions into it. The sodium–potassium pump is an active transport that requires
energy. membrane than inside, and potassium ions are more concentrated inside than outside.
• The sodium–potassium pump is effective only because of the selective permeability of the membrane,
which prevents the sodium ions that were pumped out of the neuron from leaking right back in again.
• When sodium ions are pumped out, they stay out. However, some of the potassium ions in the neuron
slowly leak out, carrying a positive charge with them.

17. The Neuron at Rest: Resting Potential
• If charged ions could flow freely across the membrane, the membrane would
depolarize at once.
– SInce the membrane is selectively permeable; that is, some chemicals can
pass through it more freely than others can.
• Due to its selective permeability, the membrane of a neuron maintains an
electrical gradient, a difference in electrical charge between the inside and
outside of the cell.
• This positive outside/negative inside electric polarization is called resting
potential (approximately -70mV).

18. Why is a Resting Potential Necessary
• The body invests much energy to operate the sodium–potassium pump,
which maintains the resting potential. Why is it worth so much energy? The
resting potential prepares the neuron to respond rapidly.
• Because the membrane did its work in advance by maintaining the
concentration gradient for sodium, the cell is prepared to respond
vigorously to a stimulus.
• Compare the resting potential of a neuron to a poised bow and arrow: An
archer who pulls the bow in advance is ready to fire at the appropriate
moment. The neuron uses the same strategy. The resting potential
remains stable until the neuron is stimulated.

19. Action Potential
• The message conducted down an
axon is called an “action potential.”
• Whereas, the primary means of
communication between neurons is
“synaptic transmission.”
– The transmission of messages from
one neuron to another through a
synapse.

20. • The cytosol in the neuron at rest is negatively
charged with respect to the extracellular fluid.
• The action potential is a rapid reversal of this
situation such that, for an instant, the inside of
the membrane becomes positively charged
with respect to the outside.
• The action potential is also often called a
spike, a nerve impulse, or a discharge.

21. • The action potentials generated by a cell are
all similar in size and duration, and they do
not diminish as they are conducted down the
axon.
• The frequency and pattern of action potentials
constitute the code used by neurons to
transfer information from one location to
another.

22. Action Potential
• In a sensory neuron, an action potential begins on the
axon hillock, a swelling where the axon exits the soma.
Example: Breaking of the skin by a nail/sharp object
• The perception of sharp pain when a nail enters your
foot is caused by the generation of action potentials in
certain nerve fibers in the skin.
• The membrane of these fibers is believed to possess a
type of gated sodium channel that opens when the
nerve ending is stretched.

23. Action Potential
1) The nail enters the skin.
2) The membrane of the nerve fibers in the skin is stretched.
3) Na+ permeable channels open.
4) Because of the large concentration gradient and the negative
charge of the cytosol, Na+ ions enter the fiber through these
channels.
5) The entry of Na+ depolarizes the membrane.
– the cytoplasmic (inside) surface of the membrane
becomes less negative.

24. Action Potential
6) If this depolarization achieves a critical level, the
membrane will generate an action potential.
7) The critical level of depolarization that must be
crossed in order to trigger an action potential is
called threshold (threshold of excitation)
8) After depolarization potassium (K+) moves out
restoring the inside to a negative voltage – this is
called repolarization.

25. Action Potential
Action potentials are caused by depolarization of the
membrane beyond threshold.
The rapid depolarization and repolarization produce a
pattern called a spike.

26. Action Potential

27. Absolute Refractory Period
• The period immediately after an action potential
when another action potential cannot occur.
• The refractory period is the time during which a
neuron resists further action potentials while it
recharges.

28. Relative Refractory Period
• The period following absolute refractory period
when a neuron will only respond to a stronger
than normal impulse.

29. All-or-No Law
• A neuron either fires or it does not.
• When it does fire, it will always produce an impulse of the same strength.
• Intensity of a stimulus is coded by the frequency of action potentials.
– The size, amplitude and velocity of an action potential are independent of
the intensity of the stimulus that initiated it.
– Neurons don’t fire faster instead more neurons are fired and fired more
often.

30. The Neural Impulse
• Each point along the membrane regenerates the action potential in much the
same way that it was generated initially.
• During the action potential, sodium ions enters a point on the axon.
• Temporarily, that location is positively charged in comparison with neighboring
areas along the axon.
• The positive ions flow down the axon and across the membrane.
• The positive charges now inside the membrane slightly depolarize the adjacent
areas of the membrane, causing the next area to reach its threshold and open
the voltage-gated sodium channels.

31. The Neural Impulse

32. The Neural Impulse
• Therefore, the membrane regenerates the action potential at that point.
• In this manner, the action potential travels like a wave along the axon.
• The term propagation of the action potential describes the transmission of
an action potential down an axon.

33. Communication
Between Neurons:
Synapses

34. Overview
• Synaptic transmission is a comprehensive and fascinating topic.
• The actions of psychoactive drugs, the causes of mental disorders, the
neural bases of learning and memory – indeed, all the operations of the
nervous system – cannot be understood without knowledge of synaptic
transmission.

35. Concept of Synapse
• In the late 1800s, Cajal anatomically demonstrated a narrow gap
separating one neuron from another.
• In 1906, Sherrington physiologically demonstrated that communication
between one neuron and the next differs from communication along a
single axon.
– He inferred a specialized gap between neurons and introduced the term
synapse to describe it.

36. Synapse
• The synapse is where the nerve impulse passes from one cell to the next.
• The electrical signal (the action potential) stops and a chemical signal takes
over to cross the gap between the cells – the chemical messenger is called a
neurotransmitter.
• The neurotransmitter crosses the gap by diffusion, which creates a small delay.
• The advantage of using neurotransmitter is that the nerve impulse can be given
some more specificity.
• Neurotransmitters can also control the operation of the nervous system by
inhibition or excitation.

37. Properties of a Synapse
• Sherrington conducted his research on reflexes (automatic muscular
responses to stimuli) and observed several properties of reflexes
suggesting special processes at the junctions between neurons:
a) Reflexes are slower than conduction along an axon.
b) Several weak stimuli presented at slightly different times (temporal summation)
or slightly different locations (spatial summation) produce a stronger reflex than
a single stimulus does .
c) When one set of muscles becomes excited, a different set becomes relaxed .

39. Structure of Synapses

40. Structure of Synapses
• Synapses are junctions between the terminal buttons at the ends of the
axonal branches of one neuron and the membrane of another.
• Synapses can occur in three places: on dendrites, on the soma, and on
other axons.
– These synapses are referred to as a axodendritic, axosomatic, and axoaxonic.
– Axodendritic synapses can occur on the smooth surface of a dendrite or on
dendritic spines (small protrusions that stud the dendrites of several types of
large neurons in the brain).

41. Structure of Synapses
• Synaptic cleft – The space between the presynaptic membrane and the
postsynaptic membrane. This is approximately 20 to 40 nm wide. (A
nanometer (nm) is one billionth of a meter.)
• Synaptic vesicle – A small, hollow, bead-like structure found in terminal
buttons: contains molecules of a neurotransmitter
• Release zone – A region of the interior of the presynaptic membrane of a
synapse to which synaptic vesicles attach and release their
neurotransmitter into the synaptic cleft.

43. Sequence of Events at a
Synapse
1) The neuron synthesizes chemicals that serve as neurotransmitters. It
synthesizes the smaller neurotransmitters in the axon terminals and
neuropeptides in the cell body.
2) The neuron transports the neuropeptides that were formed in the cell body
to the axon terminals or to the dendrites. (Neuropeptides are released from
multiple sites in the cell.)
3) Action potentials travel down the axon. At the presynaptic terminal, an
action potential enables calcium to enter the cell. Calcium releases
neurotransmitters from the terminals (exocytosis) and into the synaptic
cleft.

44. Sequence of Events at a
Synapse
4) The released molecules diffuse across the cleft, attach to receptors, and
alter the activity of the postsynaptic neuron.
5) The neurotransmitter molecules separate from their receptors. Depending
on the neurotransmitter, it may be converted into inactive chemicals.
6) The neurotransmitter molecules may be taken back into the presynaptic
neuron for recycling or may diffuse away. In some cases, empty vesicles
are returned to the cell body.
7) Some postsynaptic cells send reverse messages to control the further
release of neurotransmitter by presynaptic cells.

45. Sequence of Events at a
Synapse

46. Neurotransmitters
• At a synapse, a neuron releases chemicals that affect another neuron. Those chemicals are known
as neurotransmitters. A hundred or so chemicals are known or suspected to be neurotransmitters,

47. Neurotransmitters

48. Neurotransmitters
• Each pathway above begins with substances found in the diet.
• Acetylcholine is synthesized from choline, which is abundant in milk, eggs, and
peanuts.
• The amino acids phenylalanine and tyrosine, present in virtually any protein, are
precursors of dopamine, norepinephrine, and epinephrine.
• The amino acid tryptophan, the precursor to serotonin, crosses the blood-brain
barrier by a special transport system that it shares with other large amino acids.

49. Neurotransmitters
• The amount of tryptophan in the diet controls the amount of serotonin in the brain, so your
serotonin levels rise after you eat foods rich in tryptophan, such as soy, and fall after
something low in tryptophan, such as maize (corn).
• Another way to increase tryptophan entry to the brain is to decrease consumption of
phenylalanine or increase consumption of carbohydrates.
• Carbohydrates increase the release of insulin, a hormone, which takes several of the
competing amino acids out of the bloodstream and into body cells, thus decreasing the
competition against tryptophan.