Download Cellular Neuroanatomy II

Cellular Neuroanatomy II
The Prototypical Neuron: Neurites
Reading:
BCP Chapter 2
Major Internal Features of a Neuron
The neuron is the functional
unit of the nervous system.
A typical neuron has a soma
(which contains a nucleus that
holds genetic information and
organelles that support the life
of the cell) and neurites,
including dendrites and an
axon, that are specialized
structures designed to receive
and transmit information.
Neurites
Neurites are protoplasmic protrusions that extrude from the cell body of a neuron;
there are two types: dendrites and axons.
Axons can be distinguished from dendrites by several features including shape, length,
myelination and function. Dendrites often taper off in shape, are shorter (usually <200 mm)
and branch profusely at all angles. They do not have myelin sheaths, and receive electrochemical signals. In contrast, axons tend to maintain a constant radius, be long (up to 2 m)
and branch relatively sparingly at right angles. They have myelin sheaths, and transmit
electro-chemical signals.
The Cytoskeleton
Cytoskeleton is the scaffolding
that give a neuron its shape. The
“bones” of the cytoskeleton are
microtubules, microfilaments and
neuro-filaments. Largest of the
three types, microtubules run
longitudinally down neurites.
Elements of the cytoskeleton are
dynamically regulated (assembled
and disassembled) thus neuron
shape changes continually.
In addition to giving shape to
neurons, the cytoskeleton also
provides mechanical support, aids
in transport of substances, allows
cells to migrate, and plays a role
in segregating chromosomes
during cell division.
The Axon 1
The axon, found only in neurons, is a highly specialized
structure for the transmission of information (in the
form of electrical activity) over long distances.
The axon begins at a region called the axon hillock,
which tapers away from the soma to form the initial
segment of the axon. Two features distinguish the axon
from the soma:
•
•
there are few, if any, ribosomes in the axon (bound or free)
thus there is no protein synthesis;
the protein composition of the axonal membrane is
fundamentally different than that of the soma.
The diameter of an axon proper (constant radius) is
variable, ranging from 1 to 25 mm; the thicker the axon,
the faster the transfer of information.
Axons may give off branches (called axon collaterals)
(some of which may return to contact the cell itself,
called recurrent) allowing neurons to communicate with
many parts of the nervous system.
The Axon 2
The axon ends at the axon terminal or
terminal button. The terminal is a site where
the axon comes in close proximity to other
neurons (usually their dendrites or cell body)
and passes information on to them (i.e.,
provides innervation to them). The point of
near-contact is called the synapse.
The cytoplasm of the axon terminal differs
from that of the axon proper in several
respects:
•
•
•
microtubules do not extend into the terminal;
the terminal contains numerous small bubbles
(50 nm in diameter) of membrane called
synaptic vesicles;
the terminal contains many mitochondria
indicating high energy needs.
Information (in the form of chemicals) flows in
a direction from pre- to post-synaptic sides of
the synapse (i.e., from axon terminal of one
neuron to dendrite or soma of the next cell).
Axoplasmic Transport
Axons lack ribosomes, thus proteins
must be synthesized in the soma and
then shipped down the axon. Axoplasmic transport can be slow (up to
10 mm/day; diffusion) or fast (up to
1000 mm/day).
Fast axoplasmic transport is via the
microtubules. Material is enclosed in
vesicles, which is then “walked” along
the microtubules at the expense of
ATP. The “legs” differ depending on
direction of movement:
• anterograde ( terminal): kinesin
• retrograde ( soma): dynein
Tract Tracing
Tract tracing techniques: used to
trace the paths of axons
Anterograde: to trace axons
projecting away from cell bodies
Retrograde: to trace axons
projecting into an area of cell
bodies
cmbn-approd01.uio.no
www.seriousmadscience.com
Dendrites 1
The term dendrite is derived
from the Greek word for tree
reflecting the fact that these
neurites resemble the branches
of a tree (called the dendritic
tree).
Dendrites function as the
antennae of the neuron, thus are
covered with thousands of
synapses (stained red at right).
Dendritic trees have a large
variety of shapes and sizes to
enhance this functionality. In
addition, the dendritic membrane
has many specialized protein
molecules called receptors that
detect the chemicals released at
the synapse.
cell bodies: blue
microtubules: green
axon terminals: red
Dendrites 2
The dendrites of some neurons
are covered with specialized
structures called dendritic
spines. A lack (or abnormal
shape) of these structures can
lead to cognitive disabilities.
Dendritic spines are thought to
isolate various chemical
reactions that are triggered by
some types of synaptic activity.
For the most part, the cytoplasm
of dendrites resembles that of
axons. However, free ribosomes
have been observed at the base
of dendritic spines suggesting
that protein synthesis occurs
here (memory storage).
free
ribosomes
Classification of Neurons
Neurons are classified based on the
morphology of dendrites, axons and
the structures they innervate. There
are four common schemes.
Unipolar
Number of neurites
1 (unipolar), 2 (bipolar), and 3 or
more (multipolar)
Multipolar
Bipolar
Dendritic tree shape and/or presence
of spines (spiny vs aspinous)
Stellate
cell
Connections
sensory, motor, interneurons
Axon length
Golgi type I: projection neurons, long
Golgi type II: local circuit, short
Pyramidal
cell
Non-Neuronal Cells: The Glia
Helper cells (glia = “glue”)
Outnumber neurons up to 5:1
Provide structural/metabolic
support to neurons
Recent evidence for glial
communication and modulatory
effects of glia on neuronal
communication
www.jaynejubb.com
Classes of Glial Cells
There are four types of glial cells:
Oligodendrocytes: extensions rich in
myelin; create myelin sheaths around
axons in CNS
Schwann cells: similar to function of
oligodendrocytes, but in PNS; can guide
axonal regeneration
Microglia: involved in response to injury
or disease
Astrocytes: largest glia; star-shaped;
many functions
blogs.scientificamerican.com
Myelination of Neurons
Microglia
Microglia exist in a
ramified state at rest.
When activated,
these cells retract
their processes, then
move towards and
engulf injured or
diseased tissue.
ramified
motile
ucsf.edu
Astrocytes
Astrocytes control and communicate
widely with many neurons:
• form a barrier to unwanted
substances entering the brain
• control blood flow to neurons
• maintain the proper chemical state
outside of neurons/remove waste
• surround synapses and can modify
neuronal signals
• send nutrients (glucose) to neurons
• digest old neuronal parts
• secrete neurotransmitters and glialtransmitters
www.nature.com