Neuro-Cytology & Histology

Unit 4

What are the cellular elements of the nervous system?

1. neurons
2. glia

Role of neurons

* building blocks of NS
* responsible for processing information by **electrical conduction and chemical transmission**

How many neurons are estimated to be in the adult brain?

100 billion neurons

Describe the axons of the neurons

long

\
ie: motor neuron from cerebral cortex to sacral spinal cord can be **1m**

Role of glia

* glue
* **neuronal support**

Are there more glia or neurons in the system?

outnumber neurons 1:10

How do neurons connect to each other?

Synapses

Parts of a basic neuron

1. dendrites
2. soma
3. axons

Describe how messages are communicated in neurons

1. dendrites **receive** signals
2. perikaryon **receives** signals
3. axons **transmits** signals

Synapse

the electrochemical communication site

In a synapse, what initiates the release of neurotransmitter?

electrical impulse

What organelles are found in the perikaryon?

* (1 or more) nucleolus
* ER
* ribosome
* golgi apparatus
* mitochondria
* lysosomes and vacuoles
* microtubules

What is the function of the Nucleolus

* contains cell DNA
* involved with transcription of ribosomal RNA
* responsible for mitosis and cell growth
* synthesizes proteins, hormones, neurotransmitters

Functions of the endoplasmic reticuli?

Smooth ER:

* continuous with outer nuclear membrane

Rough ER:

* involved in water-soluble protein synthesis
* Nissl substance

Function of Ribosomal clumps

responsible for steroid metabolism, drug detoxification, carbohydrate metabolism

What color do the Nissl bodies stain?

^^Blue, due to RNA’s basophilic nature^^

Where can you find Nissl bodies abundantly?

gonadal cells and liver

Function of Golgi apparatus

* process and package synthesized proteins
* neurotransmitters bud off from GA and are shipped to axon terminal

Function of mitochondria

provide energy for protein synthesis

Functions of lysosomes and vacuoles

hold whatever has been packaged

Function of microtubules

* anterograde transport
* retrograde transport

Anterograde transport

* transport the molecules produced in the **soma to the synapses**
* neurotransmitters via this transport

Retrograde transport

* transport the molecules from the **synapse to the soma**
* trophins, viral infections through this pathway

Example of infection that travels via retrograde transport

herpes virus

\
enters the soma after being transported from the dendrites, dormant on ganglionic neuron until activated

Capsid

icosahedral protein cage

Envelope

lipid bilayer

What does chickenpox reactivate into? Where does the dormant state hang out?

1. Shingles
2. Dorsal root ganglion

What is occurs during neurogensis?

1. nucleus disappears
2. reduction in active protein synthesis

Adult __ replenishes some cerebral cortical neurons

mitosis

What disorder is observed when neurogenesis does not occur?

dementia

What happens when neurons do not regenerate?

they are lost

Describe the neuronal membrane

Fluid phospholipid bilayer

* hydrophobic core
* hydrophilic inner and outer surfaces
* proteins can move within the plane of the membrane

What does protein synthesis in neuronal cells create?

* cytoplasmic protein
* membrane bound protein
* secretory protein

Transcription

Translation

Steps of Protein synthesis

1. ribosomes enter the cytoplasm
2. transcription
3. translation

Prion

protein infectious particle

What can happen when there is an error in protein synthesis?

* misfolding of the proteins
* introduces infection prions that cause neurodegenerative disease

Creutzfeld-Jakob Disease

* AKA **spongiform encephalopathy**
* causes holes in the brain
* occurs in only humans

What stain is used to visualize nerve tissue?

Silver nitrate stain

What type of stain is used to visualize neurons?

golgi stain

Function of sensory neurons

receive direct connections from sensory receptor cells (non-neuronal cells)

Function of motor neurons

end on muscles, glands for movements

Functions of interneurons

connect to a small area of the CNS

Functions of projection neurons

relay information to different areas through long axons

What type of neuron is this?

Unipolar

What type of neuron is this?

pseudo-unipolar neuron

What type of neuron is this?

Bipolar neuron

What type of neuron is this?

Multipolar

Axon hillock

* initial segment of the axon at the soma
* most electrically excitable part of the cell
* decision to propagate an action potential happens here
* all input (excitatory/inhibitory) is summed at the hillock

Are RNA and Nissl bodies present on axons?

some RNA, but NO nissl

What increase the speed of signals on axons?

myelin

Axoplasm

cell cytoplasm found in an axon

Axonal transport

transport of mitochondria, lipids, synaptic vesicles through the cytoplasm of the axon

Anterograde axoplasmic transport

transport along the axon towards the presynaptic terminal

Retrograde axoplasmic transport

transport neurotrophic back to the soma

Examples of retrograde axoplasmic transport

1. damaged organelles and recycled plasma membrane
2. trophins

SLOW anterograde axoplasmic transport

1-10mm per day

FAST anterograde axoplasmic transport

50-400mm per day

what do we use to transport vesicles from the axon to the soma

dynein

What is an ocular example of interrupted axonal transport?

Swollen optic nerve, disc edema

\
disc margins blur, cup becomes smaller, disc appears elevated

Parts of a synapse

1. axon terminal of sending cell
2. synaptic cleft
3. dendrite of the receiving cell

Axosomatic synapse

axon to cell body

\
NOT common in the CNS

Axodendritic synapse

axon to dendrite

Axoaxonic synapse

axon to axon

What is special about axodendritic synapses

dendritic tree generates action potential, important for temporo-spatial summation of a signal

What is special about a axoaxonic synapse

cause powerful effects

Temporo-spatial summation

convergence of signal in time and space

Simple and facilitated diffusion

maintain Na+ and K+ gradients

leak channels

channels open during resting potential

resting potential range

\-30mV to -90mV

What ion has a higher concentration inside the cell at equilibrium?

K+

What ion concentration is at a higher concentration outside the cell at equilibrium?

Na+ and Cl-

At rest, what is the extracellular charge? What is the intracellular charge?

1. positive
2. negative

equilibrium potential

stable concentration gradient across the membrane

is the resting membrane polarized? why?

yes, because of the ionic concentration gradient

Depolarized

more positive membrane potential, more Na+ inside

Hyperpolarized

more negative membrane potential, more K+ outside

What are the different ion-specific channels in the neuronal membrane

1. leak
2. mechanical distortion channels
3. protein ligand gated channels
4. voltage gated channels
5. phosphorylation gated channels

Mechanical distortion channels

found in skin, stretches membrane of the sensory ending

protein ligand gated channels

* chemical messenger, neurotransmitter is the ligand
* activates G-protein
* releases a second messenger that augments ligand binding

Voltage gated channels

open when membrane potential reaches its action potential of about -50 to -55mv

Phosphorylation gated channels

intracellular enzyme mediated

If more K+ channels are open

* K+ can cross membrane easier
* closer to the K+ equilibrium potential

If more Na+ channels open

* Na+ can cross membrane easier
* closer to the Na+ equilibrium potential

What is important about *Na-K pump using ATP?*

controls osmolarity

\
without it, water would rush in and **lyse the cell**

Resting potential for glia is closer to what ion?

K+

Resting potential of neurons is what?

more positive that K+

What happens if there is a malfunction in the **second messenger ligand system**?

* depression
* migraine
* diabetes
* osteoporosis

absolute refractory period

occurs after depolarization, when summation does not occur

depolarization produces an __ __ spike

action potential

Saltatory conduction

myelinated nerves allow for current to jump from a **node of ranvier** to another, allowing for faster conduction velocity

What ion channels are found densely on Nodes of Ranvier?

Na+ channels

dendritic spines

extensions of dendrites that sometimes act as preferred sites for some types of synapses

macroglia

astrocytes and oligodendrocytes

What embryonic tissue are macroglia derived from?

neuro-ectoderm

What are the different types of astrocytes and where are they found?

1. fibrous - white matter
2. protoplasmic - gray matter
3. mueller cells - retina

What are the functions of astrocytes?

* structural support to developing neurons
* metabolic support/homeostasis
* recycle excess glutamate
* blood brain barrier
* tripartite synapse

How do astrocytes help with the blood-brain barrier

* astrocytes feet form a barrier
* separates neutrophil and blood

How do astrocytes form tripartite synapse

join additional neurons to modulate a signal