Chapter 2: Neurons and Glia

Transcription

The process whereby the genes encoded by the genomic DNA are converted to messenger RNA (mRNA)

• occurs in the cellular nucleus

Transcription is the process of converting the genetic template in DNA to messenger RNA.

True or False

True

Neurons are similar to other cells

Neuron cells also have

• Organelles

• Cell membranes

• Cytoplasm

• Genetic material

• Basic functions like metabolism, protein synthesis

What do all cells including neurons derive from?

Stem Cells and Precursor cells

Stem cells

cells that are able to produce cells identical to themselves (self-renew) and may produce daughter cells that differentiate into a more mature cell type

During development

• Each cell starts as an egg that has been fertilized

• then divides and forms a group of identical embryonic stem cells

• form an inner mass of cells in a developing zygote that are totipotent or capable of forming any type of cell in the human body

Neural Stem cells

cellular source of the central nervous system (CNS) and peripheral nervous system (PNS).

Neural Restricted Progenitors

NRP

can differentiate into a limited range of neuron types

Totipotent

Embryonic stem cells form an inner mass of cells in a developing zygote

• capable of forming any type of cell in the human body

• Early-stage embryo

How important are neurons for normal human functioning?

They control most of our major functions, including regulating systems that are integral to life like heart rate and respiration

also provide a means for us to experience our world through sensory systems (somatosensory and special senses) and allow us to move through and manipulate our environment through motor systems

Neurons are polarized

meaning that the different ends of a neuron are specialized to perform certain functions. The functional poles of a neuron include the dendritic trees and axons.

Soma

The cell body

• generally lies between the dendrites and the axon and contains the nucleus and all the major cell organelles

• functions as the organizing center for the neuron

Dendrites

are the receiving end of a neuron

• They receive messages from other neurons on specialized structures called dendritic spines.

Axon

Each neuron has one, and only one

the sending end of the neurons and the site of action potential propagation

• may extend for long distances, making neurons the largest cells in the body

• may be myelinated, which speeds up action potential propagation

Cytoskeleton

The unique shape of neurons is provided for by the neuronal..

Neurons need to generate significant amounts of cellular energy

so they have many mitochondria

Nucleus

Located in the soma, contains the cell’s DNA,
involved in gene expression

Myelin Sheath

A fatty, insulating layer that wraps around the
axon

Node of Ranvier

Gaps in myelin sheath that allow for rapid,
saltatory conduction of action potentials

Axon terminal

Transmits signals to other neurons or the target
cells, contain synaptic vesicles that store
neurotransmitters

Synapses

Essential for the transmission of neuronal
impulses from one neuron to the next

Difference between Grey matter and White Matter

Grey Matter→unmyelinated

White Matter→ myelinated

Specialized neuron functions

• Detect environment
  ● Relay neural impulses
  ● Process neural
  information
  ● Execute response

Unipolar Neuron

Single elongated process, with the cell body located off to the side

Bipolar Neuron

Two processes separated by the cell body

• often situated within the special senses (seeing, hearing, balance, olfaction, taste), where a one-to-one correlation with incoming sensory stimuli is needed

• ex: are located in the retina, where they transfer information coming from the light-sensing rods and cones to the neurons that make up the optic nerve

Multipolar Neuron

Have more than two processes, a single axon, and multiple dendrites

• They are the most numerous type of neuron shape and many neuron subtypes

There are three types of cytoskeleton scaffolds:

microfilaments, intermediate filaments, and microtubules

Microfilaments

the smallest type of cytoskeletal elements

• 6 nanometers (nm) in diameter

• composed of polymerized filaments of the protein, actin

• function both in maintaining structures and in transporting cellular components with the cooperation of myosins

Intermediate Filaments

• 10 nm in diameter, but these filaments may be composed of different proteins.

• referred to as neurofilaments, of which there are several types (e.g., neurofilament light, medium, and heavy chains, internexins, peripherin

• proteins are specific for the type of cell where they are expressed

• only known function is to maintain structure

Microtubules

• largest cytoskeletal element, having a diameter of about 25 nm

• spiral-shaped polymers of the dimeric protein, tubulin

• are found in the shafts of axons and dendrites, where they function both in maintaining structure and in the transport of cellular components

• second function, it is important to note that microtubules are directional. They have a plus (+) end, where tubulin monomers are preferentially added during polymerization, and a minus (−) end, where tubulin monomers are preferentially removed during depolymerization.

Glia

provide vital support, insulation, nourishment, and defense for neurons, ensuring the overall health and function of the nervous system. Their key roles include insulating nerve fibers with myelin, controlling ion and water balance, clearing debris, producing cerebrospinal fluid, and providing an immune response against pathogens

Glia of the CNS

• Astrocytes

• oligodendrocytes

• microglia

• ependymal

Glia of the PNS

• Satellite cells

• Schwann cells

astrocytes

Glia cell of the CNS

• star-shaped cells that have several processes that extend from the cell bodies

• found in gray matter and white matter

• forms the blood-brain barrier to separate brain tissues from blood vessels

• Maintains chemical concentrations; links neurons and blood vessels to help
  with nutrient transfer and remove wastes
  • Aids in the repair of damaged areas of the brain; glial scar formation

guides neurons during embryonic development

• Cellular communication
  • Neuronal survival
  • Nutrient supply
  • The formation of barriers
  • Development of the brain

How do Astrocytes communicate?

Through Gap Junctions that facilitate rapid signaling between astrocytes

Composed of channel proteins called connexins

Hemichannels function to secrete molecules into the environment
(e.g. ATP, glutamate)

Gap Junctions

What Astrocytes are connected to each other by

protein-based channels that adhere adjacent astrocytes to each other and produce small pores where the cytoplasm and small molecules can easily pass from one astrocyte to another

ions can easily pass through gap junctions, which allows signals to move rapidly across whole fields of astrocytes

End feet

How Astrocytes can also deliver nutrients to neurons

specialized structures that form at the ends of the astrocytic cellular extensions

can connect to blood vessels and to neuron cell bodies

can shuttle nutrients from blood by absorbing these substances through end feet on the blood vessels and then transferring these nutrients to neurons through the astrocyte end feet on the neurons

How do Astrocytes regulate transmission?

Tripartite Synapse

• a synapse where astrocyte processes surround pre- and
  postsynaptic neurons, actively sensing and
  influencing communication

Gliotransmission

• he process by which astrocytes release signaling molecules (ATP,
  glutamate, D-serine) that modulate neuronal activity.

blood–brain barrier

keeps blood contained within vessels to both regulate the transfer of blood nutrients and to keep white blood cells that mediate immunity separated from brain tissue, which they may react against

• The first layer consists of the endothelial cells that make up the blood vessel wall. connected by tight junctions, making the endothelial cells resistant to diffusion of blood components

• basement membrane that is secreted by the endothelial cells. This is composed mainly of interconnected extracellular matrix proteins like laminin and fibronectin

• Finally, blood vessels in the CNS are lined by astrocyte end feet

Glia limitans

is the superficial layer of the cortex
composed of astrocyte endfeet.

Diseases of the Astrocyte

Astrocytoma- a tumor originating from an astrocyte

Glioblastoma multiforme is the most aggressive form, grade IV
glioblastoma

Oligodendrocytes

• Which are the myelinating cells for the CNS

• wrap axons creating the myelin sheath

  Supports CNS and extends processes that form myelin sheath around axons, which increases the speed of nerve impulses.

•

• Propagating neural
  impulses

• Increase speed of action
  potentials

• Can interact with multiple
  neurons

Myelin Sheath

formed from cellular extensions from myelinating cells that wrap around segments of axons to provide a layer of insulating material

Myelin speeds up action potential propagation, and the thicker the myelin sheath, the faster the action potential is propagated. In general, thicker axons are more heavily myelinated

Diseases of the oligodendrocytes

Oligodendrogliomas are tumors that arise from oligodendrocytes

• are tumors that arise from oligodendrocytes

Multiple Sclerosis - degeneration of the myelin sheath

• caused by the immune system targeting myelin protein, women diagnosed more, motor/vision impairment,

Microglia

Small cells are the resident immune cells for the CNS

do not originate from the common neural stem cell

Closely related to macrophages that are part of the innate immune system that operates in the periphery

cells act as macrophages to engulf necrotic tissue and pathogens;
proliferate during infection.
• Considered to be the immune system of CNS; clean the neural environment.

Synaptic Pruning

facilitated by Microglia

a natural process that occurs during brain development, where unnecessary or weak connections between neurons are eliminated

Ependymal Cells

Specialized glial cells that form the lining of the brain’s
ventricles and the central canal of the spinal cord

Play crucial roles in:
- propelling the circulation of
cerebral spinal fluid (CSF)
- regulating the migration of
neurblasts

Types of Ependymal Cells

E1 Cells

E2 Cells

E3 Cells

Tanycytes

Glia cells of the PNS

Satellite cells

Schwann cells

Schwann Cells

myelinating cells for the PNS

extend cellular processes to wrap layers around axon segments, the way in which they myelinate axons differs from the way oligodendrocytes wrap myelin in the CNS. Each Schwann cell wraps myelin around one segment of an axon

Myelination helps insulation, increase speed of action potentials, and can help with regeneration of a damaged axon

Guillain- Barre Syndrome

ccurs after a viral infection and occurs in one episode, from which the patient either recovers or does not. There is no gender preference in the incidence of Guillain-Barré syndrome, which is treated with plasmapheresis and/or immune suppression therapies including high doses of steroids.

Satellite Cells

glial cell of the PNS

usually found in ganglia, groups of neuronal cell bodies

is to protect and regulate the environment around the dorsal root
ganglia (cell bodies) including ion balance and homeostasis
• Research continues into their role in neuron injury and pain modulation

• responds to injury, forms gap junction, synthesize glutamate, express NT receptors