Neuron Structure and Function

Lexy Adams and Zach HutchinsonSecond Year Medical Students

Penn State Hershey College of Medicine

Learning Objectives

1. Students will explain the structures and functions of the cellular features of a neuron including: the neuronal membrane, the cytoskeleton (especially microtubules), the axon, the axon terminal (including the synaptic vesicles), and dendrites.

2. Students will justify why the special electrochemical properties of the neuronal membrane are essential to neuron function.

MOVIE!!!!!!!!!!!!!!!!!!!!

https://www.youtube.com/watch?v=vyNkAuX29OU

• The human brain is an incredibly complicated calculating machine. It receives, processes, and sends millions of messages every minute to help you sense things, make decisions, and control your body.

• Nerves transmit information to and from the brain and spinal cord.

Fun Facts about Neurons

• There are approximately 100 BILLION neurons in the human brain (That’s 100,000,000,000)!

• If you were to line up all the neurons in the brain in a row, they would extend ~600 miles.– Neurons range in length from less than 1 mm up

to 3 feet in length. – Neurons are about 10-100 micrometers wide

(0.0001 meters).

1. Cell body 2. Nucleus3. Dendrites 4. Axon5. Axon terminals 6. Schwann cell7. Myelin sheath8. Nodes of Ranvier

1. Cell body 2. Nucleus3. Dendrites 4. Axon5. Axon terminals 6. Schwann cell7. Myelin sheath8. Nodes of Ranvier

Cell Body• The metabolic center of the neuron that

produces the energy to keep the cell alive.• Contains the nucleus, the site of DNA storage, as

well as:endoplasmic reticulum, ribosomes, and Golgi apparatus

1. Cell body 2. Nucleus3. Dendrites 4. Axon5. Axon terminals 6. Schwann cell7. Myelin sheath8. Nodes of Ranvier

Dendrites

• The branching extensions of neurons that RECEIVE the electrical signals and carry them to the cell bodies.

1. Cell body 2. Nucleus3. Dendrites 4. Axon5. Axon terminals 6. Schwann cell7. Myelin sheath8. Nodes of Ranvier

Axons• Extension of the neuron that carries electrical

impulses AWAY from the cell body (“the tail”).• The axon terminal is the end of the axon,

where neurotransmitters are released to send the signal along to the next neuron.– What part of the neuron senses the

neurotransmitters that are released from the preceding neuron?

What other structures can neurotransmitters stimulate or inhibit?

Axon Terminal

Neurotransmitter

Dendrite

What other structures can neurotransmitters stimulate or inhibit?Neurons can stimulate muscle cells, glands, or other neurons.

Axon Terminal

Neurotransmitter

Dendrite

Each neuron can have thousands of dendrites but only ONE axon.

Axon Terminal

Neurotransmitter

Dendrite

The Synaptic Cleft

The binding of the neurotransmitter to the receptor on the postsynaptic neuron (dendrite) opens various ion channels. This changes the permeability of the cell membrane and themembrane potential. This can either be an inhibitory or stimulatory effect.

Step 2 (which got cut off): Vesicles fuse with the plasma membrane (exocytosis).

Neuronal Membrane

• Serves as a barrier to enclose the cytoplasm inside the neuron, and keep unwanted substance out.

• Contains receptors on the outer surface that bind neurotransmitters (lock and key mechanism). This allows for great specificity.

• Contains ion channels that allow some ions to enter the cell while blocking others.

• This establishes an electrical potential along the cell membrane (a difference between positive and negative charges inside the cell vs outside the cell).

• This serves as the basis for ion flow along the cell membrane = action potentials

(More to come on thisnext week…)

Cytoskeleton

• Network of tiny fibers within cells. These tiny fibers are called microtubules, microfilaments, and intermediate fibers.

• Gives cells shape and stability. • Provides a scaffold for intracellular transport

of vesicles (such as neurotransmitter containing vesicles in neurons).

• Also involved in endocytosis, cell division, cell movement (flagella), and others.

Cytoskeleton in green

1. Cell body 2. Nucleus3. Dendrites 4. Axon5. Axon terminals 6. Schwann cell7. Myelin sheath8. Nodes of Ranvier

• Myelin: a fatty substance that coats long nerve fibers to make the movement of nerve impulses faster.

• Schwann cells make myelin.

• “Myelin”https://www.youtube.com/watch?v=TFnwPJxV_VA• “No Myelin”https://www.youtube.com/watch?

v=mMCKyhOzNU4“Schwann Cells”https://www.youtube.com/watch?v=DEaUQ_Zg24c

1. Cell body 2. Nucleus3. Dendrites 4. Axon5. Axon terminals 6. Schwann cell7. Myelin sheath8. Nodes of Ranvier

Nodes of Ranvier

• Gaps along the myelin sheath that speed up the nerve impulses.

https://www.youtube.com/watch?v=lN4_CMj41JQ

Can you label me?

Learning Objective #3

3. Students will list the different types of glial cells and describe the functions of each.

Glial cells (“Neuroglia”)

• GLIA = GLUE …plus some other stuff

• Neurons’ “partners” with important functions:1. Support – hold neurons in place2. Homeostasis – food & O23. Insulation – myelin formation4. Protection – destroy pathogens

Ependymal Cells (CNS)

• Line the spinal cord and ventricles of the brain• Create cerebrospinal fluid (CSF)• Beat cilia and keep CSF circulating• Make the blood-brain barrier

Astrocytes (CNS)

• Most abundant glial cell in CNS• Anchors neurons to blood supply & regulates

blood flow• Remove excess environmental ions and

neurotransmitters

Microglia (CNS)

• Macrophage protectors• Direct the immune response to damage

Oligodendrocytes (CNS) & Schwann Cells (PNS)

• Produce the myelin sheath • Provide insulation for electrical signals

Satellite Cells (PNS)

• External surroundings sensors• Regulate ion concentrations in response to

chemical environment• Highly sensitive to injury and inflammation -

play a role in chronic pain

Putting it all Together

Quick Review!

What are the four functions of glial cells, and an example cell for each function?

Quick Review!

What are the four functions of glial cells, and an example cell for each function?

1. Support – astrocytes, ependymal cells2. Homeostasis – satellite cells, astrocytes3. Insulation – Schwann cells,

oligodendrocytes4. Protection – microglia, ependymal cells

More Learning Objectives

4. Students will describe how the Nissl staining technique is used to distinguish between neural cells and glial cells.

5. Students will describe how the Golgi stain is used to identify unique components of the neuron, including the soma, the axon, and dendrites.

Nissl Bodies

• Large granular bodies in the neuron• Composed of rough endoplasmic reticulum

and ribosome rosettes• Function: manufacture and release proteins

Nissl Stain

Aniline Stain for extranuclear RNA granules – can see neuronal body and dendrites clearly!

Chromatolysis

• Degeneration of Nissl bodies• Triggers: ischemia, toxicity, cell exhaustion,

viral infection

Why do we care?

• Chromatolysis shown in amyotrophic lateral sclerosis (ALS), Alzheimer’s, and other disease

Golgi Stain

• Silver stain used to visualize tiny details of neuron and glia, including axons and dendrites

• Four day process – fix with potassium dichromate and then immerse in silver nitrate, allowing crystallization of silver chromate

Structure and connectivity details are revealed!!

Final Question

• Which stain is used to reveal neuronal cells, but not glial cells?

Final Question

• Which stain is used to reveal neuronal cells, but not glial cells?

Nissl Stain!