Textbook notes-Chapter 8 Neurons-Cellular and Network Properties

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11 Aug 2010
Chapter 8 ± Neurons: Cellular and Network Properties
Emergent Properties: Complex processes, such as consciousness, intelligence, emotion, that
cannot be predicted from the properties of individual nerve cells
The nervous system can be divided into two parts: The central nervous system (the brain and spinal
cord; CNS) and the peripheral nervous system (afferent/sensory neurons and efferent neurons; PNS)
Efferent neurons can also be divided into two subgroups: somatic motor division (controls skeletal
muscles) and autonomic division (controls smooth/cardiac muscles, exocrine and some endocrine
glands, as well as adipose tissue)
The autonomic division, also called the visceral nervous system, is further subdivided into the
sympathetic and parasympathetic branches
The nervous system is composed primarily of two types of cells: neurons and glial cells
Neurons are the functional unit of the nervous system; they consist of dendrites (which receive
incoming signals), a cell body (called the soma where the nucleus of the cell is found), and the axon
(which carries outgoing signals)
Neurons can be classified structurally, based on the presence or lack of axons and dendrites
1. Pseudounipolar: A neuron where the axon and the dendrite fuse during development.
They are both found on the same single body. The soma is to the side of
the dendrite/axon fuse
2. Bipolar: There is a single, separate axon and a separate dendrite on the neuron.
Both are about the same length and are separated by the cell body
3. Multipolar: There are many branched dendrites and axons on the neuron
4. Anaxonic: There is no distinguishable axon on the neuron
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Axons can also be functionally classified:
1. Sensory (afferent): Neurons that carry information about temperature, pressure, light, etc.
From sensory receptors to the CNS; usually they are found close to the
CNS with long bodies reaching the receptor in the limbs and organs
Most sensory neurons have a pseudounipolar structure (the cell body is out of the direct path of
signals along the axon/dendrite). However for smell and vision, they have a bipolar structure
(signal travels through the cell body)
2. Interneurons: Neurons that lie completely within the CNS. They have complex
branching systems that allow them to communicate with other neurons
3. Efferent neurons: sÇ]u]o}ZZu}ovµ}v[~(]PXô-2, page 247). They usually are
In terms of afferent and efferent peripheral (sensory) neurons, many axons are bundled
together with connective tissue to form a nerve. Some nerves carry afferent signals only, some
only carry efferent signals, some carry both
Dendrites receive incoming information for other cells. They increase the surface area of the neurons
allowing them to communicate at different locations
In the PNS, dendrites are used primarily to receive information and relay it to other areas of the neuron
that can process the information (usually the soma)
However in the CNS dendrites are more complex, sometimes acting as an independent compartment
and communicating back and forth with other cells
Peripheral neurons usually have one axon which branches off the soma at the axon hillock (area of the
soma where the soma stops and the axon starts)
The primary function of axons is to transmit outgoing electrical signals from the integrating center (axon
hillock) to the end of the axon (axon terminal)
The region where the axon terminal meets its target cell is called the synapse; the space between the
two neurons is called the synaptic cleft
The cytoplasm of the axon (called the axoplasm) lacks ribosomes and endoplasmic reticulum therefore it
cannot produce proteins.
Instead, proteins must be synthesized in the rough endoplasmic reticulum in the soma and transported
down the axon via axonal transport. There are 2 types of axonal transport:
1) Slow Axonal Transport: Moves the materials through the axoplasm (axoplasmic flow)
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from the soma to the axon terminal very slowly (about 0.2 - 2.5
mm/day). Therefore it is used only for the transport of material
not rapidly absorbed by cells
2) Fast Axonal Transport: Moves proteins at a rate of 400 mm/day. Uses stationary
microtubules and ATP-powered motor neurons to move the
material through the axon
Fast axonal transport can go 2 ways: forward (anterograde: from the soma to the axon) or backwards
(retrograde: transports old material from the axon terminal to the soma for recycling)
Glial Cells
Glial cells are much more numerous than neurons; outnumbering them about 10-50:1; they help
provide physical (wrapping around cells i.e. myelination) and biochemical (metabolic support,
maintenance of homeostasis) support for neurons
There are 2 types of glial cells in the PNS: Schwann cells and Satellite cells
There are 4 types of glial cells in the CNS: Oligiodendrocytes, Microglia, Astrocytes, and
Ependymal cells
Glial Cells of the PNS
Schwann cells and Oligodendrocytes (CNS) both support/insulate neurons by forming myelin sheaths
around the axon (multiple layers of phospholipids membrane). Each time it wraps around the cell, it
forms two layers.
The difference between Schwann cells and Oligodendrocytes is the number of cells they wrap around. In
the CNS, Oligiodendrocytes wrap around portions of many different cells whereas in the PNS, each
Schwann cell is associated with an individual axon (one axon may have 100s of Schwann cells)
Not the entire portion of the axon is myelinated, a tiny portion between myelination called the nodes of
Ranvier is important for electrical transference down the axon
Satellite Cells are unmyelinated Schwann cells. They form supportive capsules around cell bodies in the
ganglia (ganglion = clusters of nerve body cells found outside the CNS. Equivalent to cell bodies in the
Glial cells of the CNS
Microglia are specialized immune cells that remove damaged cells and foreign invaders
Astrocytes are highly branched cells that connect to both blood vessels and neurons, allowing them to
transfer nutrients. Also helps maintain homeostasis by absorbing additional K in extracellular fluid.
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