types of neurons, structure and functions, types of glia cells, their structure and function, functioning of a neuron - resting potential, action potential, graded potential, absolute and relative refractory period.

1. NEURONS AND THE GLIA

2.  Neurons are cells that are specialized for the reception, conduction,
and transmission of electrochemical signals.
 They come in an incredible variety of shapes and sizes
 You must be familiar with the External anatomy
Internal anatomy
Cell membrane

3. Cell membrane. The semipermeable membrane that encloses
the neuron.
Dendrites. The short processes emanating from the cell body,
which receive most of the synaptic contacts from other neurons.
Cell body. The metabolic center of the neuron; also called the
soma.
Axon hillock. The cone-shaped region at the junction between
the axon and the cell body.
Axon. The long, narrow process that projects from the cell body.
Myelin. The fatty insulation around many axons.
Nodes of Ranvier (pronounced “RAHN-vee-yay”).
The gaps between sections of myelin.
Buttons. The buttonlike endings of the axon branches, which
release chemicals into synapses.
Synapses. The gaps between adjacent neurons across which
chemical signals are transmitted.

4. Endoplasmic reticulum. A system of folded membranes in
the cell body; rough portions (those with ribosomes) play a
role in the synthesis of proteins; smooth portions (those without
ribosomes) play a role in the synthesis of fats.
Mitochondria. Sites of aerobic (oxygen-consuming) energy release.
Nucleus. The spherical DNA-containing structure of the
cell body.
Cytoplasm. The clear internal fluid of the cell.
Ribosomes. Internal cellular structures on which proteins are
synthesized; they are locatedon the endoplasmic reticulum.
Golgi complex. A connected system of membranes that
packages molecules in vesicles.
Microtubules. Tubules responsible for the rapid transport of material
throughout neurons.
Synaptic vesicles. Spherical membrane packages that store
neurotransmitter molecules ready for release near synapses.
Neurotransmitters. Molecules that are released from active neurons and
influence the activity of other cells.

5. NEURON CELL MEMBRANE. The neuron cell
membrane is composed of a lipid bilayer, or
two layers of fat molecules Embedded in
the lipid bilayer are numerous protein
molecules that are the basis of many of the
cell membrane’s functional properties.
Some membrane proteins are channel
proteins, through which certain molecules
can pass; others are signal proteins,which
transfer a signal to the inside of the neuron
when particular molecules bind to them on
the outside of the membrane.

6. There are two kinds of gross neural structures in the nervous
system: those composed primarily of cell bodies and those
composed primarily of axons.
In the central nervous system, clusters of cell bodies are called
nuclei (singular nucleus); in the peripheral nervous system, they
are called ganglia (singular ganglion).
In the central nervous system, bundles of axons are called tracts;
in the peripheral nervous system, they are called nerves.

7. Glia Cells
In the human brain, there are roughly equal numbers of neurons
and glia.
There are several kinds of glia : Oligodendrocytes
Schwann cells
Microglia
Astrocytes

8. are glial cells with extensions that
wrap around the axons of some
neurons of the central nervous system.
These extensions are rich in myelin, a
fatty insulating substance, and the
myelin sheaths they form increase the
speed and efficiency of axonal
conduction

9. SCHWANN CELLS
A similar function is performed in the peripheral nervous system by Schwann
cells, a second class of glia. Each Schwann cell constitutes one myelin
segment, whereas each oligodendrocyte provides several myelin segments,
often on more than one axon. Another important difference between Schwann
cells and Oligodendrocytes is that only Schwann cells can guide axonal
regeneration ( regrowth) after damage. That is why effective axonal
regeneration in the mammalian nervous system is restricted to the PNS

10. MICROGLIA
Microglia make up a third class of glia. Microglia are smaller than other glial cells —
thus their name. They respond to injury or disease by multiplying, engulfing cellular
debris or even entire cells and triggering inflammatory responses.

11. Astrocytes constitute a fourth class of glia. They are the largest glial cells, and they are so
named be cause they are star-shaped (astron means “star”).
• The extensions of some astrocytes cover the outer surfaces of blood vessels that course
through the brain
• they also make contact with neurons.
• they appear to play a role in allowing the passage of some chemicals from the blood into CNS
neurons and in blocking other chemicals and they have the ability to contract or relax blood
vessels based on the blood flow demands of particular brain regions .
• For decades, it was assumed that the function of glia was mainly to provide support for
neurons— providing them with nutrition, clearing waste, and forming a physical matrix to hold
neural circuits together (glia means “glue”).
• astrocytes, have been shown to exchange chemical signals with neurons and other astrocytes
to control the establishment and maintenance of synapses between neuron to modulate neural
activity.

12. • to form functional networks with neurons and
other astrocytes, to control the blood–brain
barrier and to respond to brain injury.
• Microglia have also been shown to play more than
just a supportive role; for example, they
have recently been shown to play a role in the
regulation of cell death, synapse formation and
synapse eliminatio
• There is now substantial evidence that the
physiological effects of glia are numerous, but the
exact nature of their functions is still largely a
matter of conjecture.

13. BLOOD-BRAIN BARRIER
In most of the body the cells that line the capillaries do not fit tightly together
small gaps are found in between that permit the free exchanges of most
substances between the blood plasma and the fluid outside the capillaries
that surrounds the cells of the body. In the CNS the capillaries lack these
gaps so many substances cannot leave the blood.

14. RESTING POTENTIAL
• the potential inside the resting neuron is about 70 mV less than that outside the neuron. This
steady membrane potential of about −70 mV is called the neuron’s resting potential
• In its resting state, with the −70 mV charge built up across its membrane, a neuron is said to be
polarized.
• positively and negatively charged salt particles are called ions.
• In resting neurons, there are more Na+ ions outside the cell than inside and more K+ ions inside
than outside.
• neural membranes have specialized pores, called ion channels through which ions can pass.
• Each type of ion channel is specialized for the passage of particular ions (e.g., Na+ or K+).
• There is substantial pressure on Na+ ions to enter the resting neurons. This pressure is of two
types.
• First is the electrostatic pressure from the resting membrane potential - Because opposite
charges attract, the -70 mV charge at_x0002_tracts the positively charged Na+ ions into resting
neurons.
• Second is the pressure from random motion for Na+ ions to move down their concentration
gradient - particles are more likely to move from areas of high concentration to areas of low

15. • why then do Na+ ions not come rushing into neurons?
• answer - The sodium ion channels in resting neurons are closed and K+ ions are largely
held inside by the negative resting membrane potential.
• In the 1950s, Alan Hodgkin and Andrew Huxley discovered that the rate at whichNa+ ions
leaked into resting neurons, other Na+ ions were actively transported out at the same
rate; and at the same rate that K+ ions leaked out of resting neurons, other K+ ions were
actively transported in.
• Such ion transport is performed by mechanisms in the cell membrane that continually
exchange three Na+ ions inside the neuron for two K+ ions outside. These transporters
are commonly referred to as sodium–potassium pumps. (bouncers)

17. GRADED POTENTIALS
• When neurons fire, they release from their terminal buttons chemicals called
neurotransmitters, molecules of which bind to postsynaptic receptors. They typically have
one of two effects - They may depolarize the receptive membrane (decrease the resting
membrane potential, from −70 to −67 mV, for example), or they may hyperpolarize it
(increase the resting membrane potential, from −70 to −72 mV, for example).
• Postsynaptic depolarizations are called excitatory postsynaptic potentials (EPSPs)
Postsynaptic hyperpolarizations are called inhibitory postsynaptic potentials (IPSPs).
• Both EPSPs and IPSPs are graded responses. Weak signals elicit small postsynaptic
potentials, and strong signals elicit large ones.
• the transmission of postsynaptic potentials has two important characteristics
1. it is rapid, very rapid (INSTANTANEOUS)
2. the transmission of EPSPs and IPSPs is decremental: EPSPs and IPSPs decrease in
amplitude as they travel through the neuron, Most EPSPs and IPSPs do not travel more than
a couple of millimeters from their site of generation before they fade out; few travel very
far along an axon.

18. ACTION POTENTIAL
• most neurons are covered with thousands of synapses, and whether a neuron fires is
determined by the net effect of their activity. More specifically, whether a neuron fires
depends on the balance between the excitatory and inhibitory signals reaching its axon.
• Action potentials are generated in the adjacent section of the axon, called the axon initial
segment. If the sum of the depolarizations and hyperpolarizations reaching the axon
initial segment at any time is sufficient to depolarize the membrane to a level referred to
as its threshold of excitation—usually about −65 mV—an action potential is generated.
• The action potential (AP) is a massive but momentary —lasting for 1 millisecond—reversal
of the membrane potential from about −70 to about +50 mV. Unlike postsynaptic
potentials, action potentials are not graded responses; their magnitude is not related in
any way to the intensity of the stimuli that elicit them. To the contrary, they are all -or-
none responses; that is, they either occur to their full extent or do not occur at all.

20. • Adding or combining a number of individual signals into one overall signal is called
integration.
• Neurons integrate incoming signals in two ways - over space(spatial summation) and over
time (temporal summation)

21. CONDUCTION OF ACTION POTENTIAL
 Action potentials are conducted through the action of voltage -activated ion channels—ion
channels that open or close in response to changes in the level of the membrane potential.
 when the membrane potential of the axon is depolarized to the threshold of excitation by an
EPSP. The voltage-activated sodium channels in the axon membrane open wide, and Na+ ions
rush in, suddenly driving the membrane potential from about −70 to about +50 mV.
 The rapid change in the membrane potential associated with the influx of Na+ ions then triggers
the opening of voltage-activated potassium channels.
 At this point, K+ ions near the membrane are driven out of the cell through these channels —
first by their relatively high internal concentration and then, when the action potential is near
its peak, by the positive internal charge.
 After about 1 millisecond, the sodium channels close. This marks the end of the rising phase of
the action potential and the beginning of repolarization by the continued efflux of K+ ions.
 Once repolarization has been achieved, the potassium channels gradually close. Because they
close gradually, too many K+ ions flow out of the neuron, and it is left hyperpolarized for a brief
period of time.

22. The conduction of action potentials along an axon differs from the conduction of EPSPs and IPSPs
in two important ways.
• First, the conduction of action potentials along an axon is nondecremental; action potentials do
not grow weaker as they travel along the axonal membrane.
• Second, action potentials are conducted more slowly than postsynaptic potentials.
• The reason for these two differences is that the conduction of EPSPs and IPSPs is passive,
whereas the axonal conduction of action potentials is largely active.
REFRACTORY PERIOD
• There is a brief period of about 1 to 2 milliseconds after the initiation of an action potential
during which it is impossible to elicit a second one. This period is called the -----
absolute refractory period.
• The absolute refractory period is followed by the relative refractory period—the period during
which it is possible to fire the neuron again but only by applying higher -than- normal levels of
stimulation.

23. CONDUCTION IN MYELINATED AXONS - In myelinated axons, ions can pass through the axonal membrane
only at the nodes of Ranvier—the gaps between adjacent myelin segments.
In myelinated axons, axonal sodium channels are concentrated at the nodes of Ranvier.
Myelination increases the speed of axonal conduction. Because conduction along the myelinated
segments of the axon is passive, it occurs instantly, and the signal thus “jumps” along the axon from
node to node.
The transmission of action potentials in myelinated axons is called saltatory conduction.
Conduction is faster in large-diameter axons, and it is faster in those that are myelinated. Mammalian
motor neurons are large and myelinated; thus, some can conduct at speeds of 100 meters per second
(about 224 miles per hour). In contrast, small, unmyelinated axons conduct action potentials at about 1
meter per second.
The maximum velocity of conduction in human motor neurons is about 60 meters per second.
Many neurons in mammalian brains either do not have axons or have very short ones, and many of
these neurons do not normally display action potentials. Conduction in these interneurons is typically
passive and decremental.

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