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Atlantic University School of MedicineNeuroscience
Neurobiology of the Neuron
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Characteristics of Neurons
Neurons are excitable cellsspecialized to transmit stimulivia nerve impulses.
They vary considerably in size
and structure.
Neurons consist of a cell
body, neurites (which extendfrom the cell body) and anaxon (a long tubular neurite).
Neurites responsible for
receiving information and
conducting it TOWARD thecell body are called dendrites.
Dendrites and axons are
often referred to as nervefibers.
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Types of Neurons
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Types of Neurons
Unipolar neurons: cell bodyhas a single neurite thatdivides a short distance fromthe cell body into two
branches.
One usually goes to aperipheral structure theother to the CNS.
This type of neuron is found inthe posterior (dorsal) rootganglion.
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Types of Neurons
Bipolar neurons: have
an elongated cell body
from which two long
dendrites extend from
each end.
This type of neuron is
found among retinalbipolar cells and the
cells of the cochlearand vestibular ganglia.
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Types of Neurons
Multipolar neurons:neurons that have anumber of neuritesarising from the cell
body. Aside from the axon, the
rest of the neurites aredendrites.
These are thepredominate type of cellin the brain and spinalcord.
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Golgi type I Neurons
Silver-stained Purkinje cells of the cerebellar cortex
Classification by Size
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Golgi type II Neurons
Silver stained section of the cerebral cortex.
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Classification by Size
Golgi type I neurons: have a long axonthat may be 1 meter or more in length.
The axons form long fiber tracts of the
brain and spinal cord and nerve fibers ofthe peripheral nerves.
Examples ofGolgi type I neurons include
pyramidal cells of the cerebral cortex,Purkinje cells of the cerebellar cortexand motor cells of the spinal cord.
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Classification by Size
Golgi type II neurons : have short axons
that terminate in the region of the cell ofthe neighboring cell body or is totallyabsent.
They greatly outnumber Golgi type Ineurons.
They usually have a star-shapedappearance.
These are found in the cerebral andcerebellar cortex and are ofteninhibitory in function.
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Golgi type I Neurons
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Golgi type I Neurons
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Different Types of Neurons
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Structure of the NeuronNucleus
nuclear envelope
nuclear poresCytoplasm
Nissl substance (rough ER-protein synthesis)
chromatolysis
Golgi complex
Mitochondria
Neurofibrilsneurofilaments
Microfilaments
Microtubules
Lysosomes
Centrioles
Lipofuscin (pigment material)Melanin granules
Plasma Membrane & Excitation of the Plasma Membrane
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Anterior gray column of the spinal cord
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Structure of the NeuronNucleus
nuclear envelope
nuclear poresCytoplasm
Nissl substance (rough ER-protein synthesis)
chromatolysis
Golgi complex
Mitochondria
Neurofibrilsneurofilaments
Microfilaments
Microtubules
Lysosomes
Centrioles
Lipofuscin (pigment material)Melanin granules
Plasma Membrane & Excitation of the Plasma Membrane
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Structure of the NeuronNucleus
nuclear envelope
nuclear poresCytoplasm
Nissl substance (rough ER-protein synthesis)
chromatolysis
Golgi complex
Mitochondria
Neurofibrilsneurofilaments
Microfilaments
Microtubules
Lysosomes
Centrioles
Lipofuscin (pigment material)Melanin granules
Plasma Membrane & Excitation of the Plasma Membrane
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Posterior root ganglion lipofuscin granules within sensory neurons
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Longitudinal Section Transverse section
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Structure of the NeuronNucleus
nuclear envelope
nuclear pores
Cytoplasm
Nissl substance (rough ER-protein synthesis)
chromatolysis
Golgi complex
Mitochondria
Neurofibrilsneurofilaments
Microfilaments
Microtubules
Lysosomes
Centrioles
Lipofuscin (pigment material)Melanin granules
Plasma Membrane & Excitation of the Plasma Membrane
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Plasma Membrane
About 8nm thick
Composed of an inner and outer layer separatedby a middle layer of lipid (phospholipidbilayer).
Certain protein molecules lie within thephospholipid layer and span the entire width ofthe lipid layer(channels).
Carbohydrate molecules attached to the outside
of the membrane form the cell coat orglycocalyx.
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Resting potential
In the resting state (unstimulated) K+ ionsdiffuse through the plasma membrane from the
cell cytoplasm to the tissue fluid.
K+permeability is much greater than to Na+ ions
so that passive efflux of K+ is greater than theinflux of Na+.
This results in a steady potential difference of
about -80mV (inside is more negative relative
to outside).
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Action Potential
Stimulation of a nerve cell via electrical,chemical or mechanical stimulation results ina rapid change in membrane permeability toNa+ ions.
Na+ ions diffuse through the plasma membrane
into the cell cytoplasm from the tissue fluid. Results in the membrane becomingdepolarized.
This sudden influx of Na+ ions followed by the
altered polarityproduces the action potential(+40mV).
Na+ permeability ceases whereas K+increases returning the cell back to the restingstate.
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Refractory Period
Once a nerve impulse has spread over the
plasma membrane another action
potential cannot be elicited
immediately.
The duration of this non-excitable state is
called the refractory period.
S i d I hibi i
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Summation and Inhibition
The greater the strength of the initial stimulus, the largerthe initial depolarization and the greater the spread into thesurrounding areas of the plasma membrane.
If multiple excitatory stimuli are applied to the surface of aneuron then the effect can be summated.
Inhibition, orhyperpolarization, is produced by an influx ofCl- ions through the plasma membrane into the neuron.
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Ion Channels
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Ion Channels
Ion channels exist in at least two conformational states: open andclosed.
Gating involves the twisting or distortion of the various sub-
units of a channel protein producing a wider or a more narrowlumen.
Gating occurs in response to voltage change, presence of a
ligand, stretch or pressure.
I Ch l
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Ion Channels
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Nerve Cell: Axon and Dendrites
Dendrites: the short processes of the cellbody.
In various neurons finer branches have
large numbers of small projections calleddendritic spines.
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Dendritic spines on pyramidal neurons of the cerebral cortex.
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Nerve Cell: Axon and Dendrites
Axon: the name given to the longest
process of the cell body.
It arises from a small conical elevation on
the cell body, devoid of Nissle granules
called the axon hillock.
Distal ends of the terminal branches of the
axons that are often large are calledterminals.
Th A
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The Axon
The plasma membrane bounding the axon
is the axolemma. The cytoplasm of the axon is termed the
axoplasm.
Axoplasm does not possess Nisslgranules or Golgi complex.
The initial segment of the axon (first 50 to100um) after it leaves the axon hi l lockisthe most excitable part of the axon andis the site at which an action potentialalways originates.
Axon Hillock
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Axon Hillock
Longitudinal section
of a neuron from thecerebral cortex.
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Axon Transport
Anterograde transport: materials that aretransported from the cell body to theaxon terminals.
Fast anterograde transport (100 to 400mm
per day) refers to the transport ofproteinsand transmitter substances or theirprecursors.
Slow anterograde transport (0.1 to 3mm per
day) refers to the transport ofaxoplasm andmicrofilaments and microtubules.
A T t
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Axon Transport
Retrogradetransport: materials
that are transported
from the terminals to
the cell body.
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Synapses
Where two neurons come into closeproximity and functional interneuronal
communication occurs is called a
synapse. Most neurons make synaptic connections
to 1000 or more neurons and may
receive up to 10,000 connections from
other neurons.
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Synapses
Communication at a
Synapse takes place in
one direction only.
Synapses occur in a
number of forms:
AxodendriticAxosomatic
Axoaxonic
f
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Types of Synapses
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Synapses
There are two types of synapses:
chemical and electrical.
Most synapses are chemical and utilize
a neurotransmitter which passes across
the narrow space between the cells and
attaches to a protein molecule called a
receptor.
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Neurotransmitter Action
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Neurotransmitter Action
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Neurotransmitter Action
Transmitter is released
Receptor on postsynaptic cell bind transmitter ligand
Produce an EPSP or IPSP
Ligand Gated (fast, i.e. nicotinic acetylcholine, glutamate) or
G-protein linked (slow, Dopamine)
Synapses
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Synapses
The apposedsurfaces of theterminal axon andanother neuron arecalled thepresynaptic andpostsynapticmembranes and areseparated by thesynaptic cleft (20-30nm wide).
Chemical Synapses
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Chemical Synapses
Presynapticvesicles,
mitochondria, and
occasional lysosomes
are present in the
cytoplasm close to
the presynaptic
membrane.
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Chemical Synapses
The presynaptic
terminal contains
many small
presynaptic vesicles
that contain
molecules of various
neurotransmitters or
one specificneurotransmitter.
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Chemical Synapses
Once released the
vesicles fuse with the
presynaptic
membrane and
discharge the
neurotransmitter into
the synaptic cleft by a
process calledexocytosis.
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Neurotransmitter Action
Depending on the summation of variousinputs into the primary cell, the cell can beexcited orDEPOLARIZED, and an action
potential will be initiated at the initialsegment and travel down the axon.
If the overall effect results in aHYPERPOLARIZED cell, the neuron will
be inhibited and no nerve impulse willarise.
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I ti ti f N t itt
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Inactivation of Neurotransmitter
N d l t
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Neuromodulators
Neuromodulators are neurotransmitters either
co-released with the primary transmitter orpackaged separately in other vesicles andreleased.
They are capable ofmodulating and modifying
the activity of the postsynaptic neuron. They may enhance, prolong, inhibit or limit
the effect of the principle neurotransmitter onthe postsynaptic membrane.
Neuromodulators act through a secondmessenger system (G-proteins).
Electrical Synapses
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Electrical Synapses
Electrical synapses are gap junctions.
There is no chemical transmitter.
They are fast.
Electrical Synapses
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Electrical Synapses
The rapid spread of activity from one neuron toanother ensures that a group of neuronsperforming an identical function act together.
Electrical synapses are bidirectional, chemical
synapses are not.
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Adios