AHDOct/22/08

Amal Al-Hashmi

R4

Quiz

• List the different parts of a neuron?

• What are the functions of a neuron?

• List the different types of glia cells and name one function for each?

Out line

1. Structure of neurons

2. Classifications of neurons

3. Axonal transport and its clinical applications

4. Electrical properties of neurons

5. Glial cell functions and clinical application

6. Conclusion/quiz

The structure of neurons

• Dendrites

• Cell body

• Axons and axons terminals

Dendrites

• *Usually brch extensively in the vicinity of the cell body

• *receive signals either from other neurons or from the environment via specialized receptors

• *Info travels from distal to proximal along dendrites to converge at the cell body

• *Small bud like extension (dendrite spines) of variety of shapes is frequently seen on the more distal brch of dendrite contact

Cell body

• Cell body of neuron is also called the soma or the perikaryon

• Varies widely in shape depending on the neuron type

• Active protein synthesis is indicated by the large size of the nucleus and its content of diffuse chromatin and at least one prominent nucleolus (site of ribosomal RNA synthesis),

Cell body

The cell body of multipolar neurons vary widely in shape so their profiles in tissue sections may appear fusiform , flask shaped , triangular , polygon or stellate

Variations of stellate polygon are most common

Bipolar have a round or oval perikaryon, with a single large process emanating from each end of the cell body

The pseudo unipolar has a spherical cell body with centrally placed nucleus

Axons and Axons Terminals• The axon arises from the cell body at a small elevation called axon hillock.

• The proximal part of the axon, adjacent to the axon hillock, is the initial segment

• The cytoplasm of the axon (axoplasm) contains dense bundles of microtubules and neurofilaments

• Axons are typically devoid of ribosomes

• Axon in CNS often end with fine terminals called terminal arobes which are capped with small terminal boutons

• The surface area of axon can be several 1000 times the surface area of the parent cell body

Classification of neurons

• Multipolar 99%

• Bipolar

• Pseudounipolar/unipolar

Type of neuronLocation of cell bodies

Unipolar Post root or CN ganglion

bipolar Retina, olfactory epithelium,vestibular

ganglion , auditory ganglion Multipolar

Stellate

Fusiform

Pyriform

Pyramidal

Purkinje

Mitral

Chandelier

Granule

amacrine

Many areas of CNS

Hippocampal

Cerebellar cortex

Olfactory bulb

Visual area of cerebral cortex

Cerebral and cerebellar cortex

retina

Axonal transport

Anterograde

Slow1mm/day

Not clearly understood

Fast @speed 400mm/day,

driven by the protien Kinesin

Retrograde

Allows neurons to respond to molecules

Continual recycling of components of axon terminal. driven by the protien dyneinFast 50-250mm/day

Implication in of axonal transport in pathogenesis of some human neurological diseases e.g.

Rabies virus

Clostridium tetani ( Tetanus)

Axonal transport as a research tool

Injecting a fluorescent substance into region containing axons terminals ( retrograde)

Injecting radioactively labeled a. a into cell body ( anterograde)

Electrical properties of neuron

Neurons carry a negative electrical charge relative to the extra cellular fluid bathing them

The plasma membrane is a semi permeable because certain ions can cross at certain times but there is not a free exchange

The opening and closing of specific ion channels can be controlled by chemical signals including neurotransmitter

Neurons as info receivers

• Primary sensory neurons

photoreceptors ( cones. rods)

chemoreceptors (taste/smell, PH receptor

mechanoreceptors ( vestibular, auditory, somatosensory system)

thermoreceptors ( pain/temp)

Neurons as info transmitters

• Synapses

• Location at which a process of one neuron communicates with a second neuron or a effector cell

• Chemical synapse consists of presynaptic elements , postsynaptic elements and synaptic cleft

• Unidirectional • The strength of effect on postsynaptic membrane is

variable

Glial cells

• Major types in CNS are: astrocytes oligodendrocytes microglia• Major types in PNS: satellite schwann cells MQ

Astrocytes

• Occur through out CNS• Highly branched cells with processes that

contact most of of the surface of neuronal dendrites and cell bodies as well as axonal surface

• Role in structural support and response to injury• Secrete growth factors, Cholesterol and

lipoprotein , • Contribute to the regulation of inflammatory

processes ( cytokines and IL-1, IL6)

Astrocytes

• Environmental modulation

• Metabolism

• Regional heterogeneity

• Role at the BBB

Oligodendrocytes

Arise during development from oligodendrocte precursor cells

Occurs in both grey and white matter

Main function is myelination–One oligodendrocyte can myelinates 30 to 50

nearby axons

–Oligodendrogial cell body located at some distance form myelin sheath, linked by thin cytoplasmic bridge

oligodendrocytes

Microglia

Immune effector cells of CNS

Predominant cells involved in CNS inflammation

Makes up to 1% of CNS cell populationThey have phagocytic roleThey secrete IL-IB, TNF, PG

microglia

Clinical implications

Excessive secretion of Cytokines IL-IB &TNF induces endothelial cells to open the BBB allowing leukocytes

infiltration into the brain parenchyma )bacterial meningitis(

some molecules secreted by activated microglial cells are neurotoxin( including glutamate, oxygen radicals and TNF ) and neurons may be directly damaged by inflammatory rxn in the CNS

thus the microglial rxn maybe a 2nd cause of neuronal damage following stroke or tumor

Clinical implications

• release of O2 radicals by microglia and astrocytes can lead to oxidative damage

• in MS , demylination results from inflammatory signaling that allows T lymphocytes to extravasate and enter CNS tissue

• HIV virus leads to neuronal damage through targeting microglia cells. ?!dementia

Glia-derived tumors

Astrocytomas

the most frequently encountered glial cell tumor

Grade I

astrocytoma are slowly growing, arising from white matter

Grade II

Slow, but more infiltrative

Glia-derived tumors

Grade IIIRapid growing with mitotic figuresgrade IVAlso called glioblastoma multiformHighly malignant , rapidly growing invasive

tumor• Oligodendrogliomas slow growing tumors

Summery

• Neurons are excitable cells that are specialized for the perception of stimuli and conduction of nerve impulses

• They vary in shape and size• There are two ways of axonal

transportation • Chemical synapse consists of presynaptic

elements , postsynaptic elements and synaptic cleft

Summery

Glial cells provide structural support for the nerve cells and control the environment within the CNS

quiz

• Define the term synapse?

• List the different types of glia cells and name one function for each.

• What is the role of microglial cells in CNS truma or inflammation?

Thank you

questions